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Facile preparation of three-dimensionally ordered macroporous Bi2WO6 withhigh photocatalytic activity

Songmei Sun, Wenzhong Wang* and Ling Zhang

Received 29th June 2012, Accepted 1st August 2012

DOI: 10.1039/c2jm34211a

Three-dimensionally ordered macroporous Bi2WO6 (3DOM Bi2WO6) with mesoporous walls was

firstly realized by a hard template synthesis method. The commercially available bismuth nitrate and

phosphotungstic acid were used as the precursors to prepare the 3DOM Bi2WO6 sample without

introducing other complex agents. The as-prepared 3DOM Bi2WO6 was composed of periodically

arranged Bi2WO6 hollow spheres with a diameter of about 90 nm. Because of its particular ordered

macroporous and mesoporous structure, the 3DOMBi2WO6 exhibited excellent photocatalytic activity

on the degradation of phenol and aqueous ammonia under irradiation from simulated sunlight. Its

photocatalytic activity is much higher than that of the Bi2WO6 samples prepared by hydrothermal

synthesis (HR) and solid-state reaction (SSR) methods. This work provides a simple way to prepare

bismuth-based 3DOM multiple metal oxides with excellent performance.

Introduction

Environmental pollution is bringing acute health problems to

human beings. Heterogeneous photocatalysis appears to be one

of the most efficient and economical techniques for remediation

of organics contaminated environment.1–3 Recently, the devel-

opment of porous photocatalysts has become a research hotspot

in order to obtain higher photocatalytic activity.4–7

Ordered macroporous structure with pore size in the optical

wavelength range is of particular interest in photocatalysts, for

their periodic structures could interact with light to enhance light

energy conversion efficiency and their continuous pore channels

could facilitate the transfer of reactant molecules. It has been

reported that TiO2 photocatalyst with periodic structures

exhibited significantly increased photocatalytic efficiencies as

compared to conventional TiO2.8–12 As TiO2 can only be acti-

vated by UV light which greatly limits its practical application,

considerable attention has been given to the development of

visible-light-active photocatalyst in recent years.13–16 Among the

various studied visible-light-active photocatalysts, Bi2WO6 is

one of the most attractive materials because of its high stability,

non-toxicity, wide solar response and good photocatalytic

activity.17–20 Many studies have been carried out on the develop-

ment of highly active Bi2WO6 photocatalysts with particular

structures, such as nanoplates,21,22 nanofibres,23 nanospheres,24

hierarchical nanostructures,25–27 porous structures,28–31 etc. The

three dimensionally ordered macroporous Bi2WO6 (3DOM

State Key Laboratory of High Performance Ceramics and SuperfineMicrostructure, Shanghai Institute of Ceramics, Chinese Academy ofSciences, 1295 Dingxi Road, Shanghai 200050, P. R. China. E-mail:wzwang@mail.sic.ac.cn; Fax: +86-21-5241-3122

19244 | J. Mater. Chem., 2012, 22, 19244–19249

Bi2WO6) may exhibit much enhanced photocatalytic activity as

compared with other porous structures, as in the case of TiO2.8

The light harvesting efficiency of an ordered macroporous pho-

tocatalyst is affected by the direction of irradiated light. It attains

its maximum efficiency at normal incidence. To the best of our

knowledge, 3DOM Bi2WO6, which could significantly improve

light harvesting at normal incidence has not been achieved up to

the present day.

The colloidal crystal template method is a promising process

for the synthesis of an ordered macroporous structure.32,33 In this

manuscript, we present a SiO2 colloidal template assisted process

to prepare 3DOM Bi2WO6 photocatalyst with high photo-

catalytic performance. For 3DOMBi2WO6, a complex oxide, the

preparation of the desired precursor is essential to obtain the

final ordered structure. Usually, metal alkoxides were needed to

fill the interstices of the colloidal crystal in a template-assisted

preparation method of 3DOM structure.34 However, the

alkoxide precursors of bismuth and tungsten are expensive and

they are difficult to prepare. Here, we use commercially available

bismuth nitrate and phosphotungstic acid as precursors, without

introducing other complex agents. Our approach for the prepa-

ration of 3DOM Bi2WO6 utilizes our recently developed tech-

nique for the synthesis of ordered mesoporous Bi2WO6.35 This

technique, which involves the infiltration of an acidic ethanol

solution containing bismuth nitrate and phosphotungstic acid

into the SiO2 template, was shown to be an effective method to

uniformly infiltrate Bi2WO6 precursors into the interstices of the

template. After evaporation, ammonia alkalization and thermal

treatment process, we present the first example of 3DOM

Bi2WO6 with mesoporous walls which exhibits a high photo-

catalytic performance.

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Experimental

Firstly, monodisperse SiO2 spheres were prepared using

the St€ober method from the sol–gel process of TEOS (AR,

Sinopharm) under base catalysis. The colloidal SiO2 crystal

template was prepared by a centrifugation method under 10 000

rpm rotate speed for 20 min. The as-prepared 3-D bulk SiO2

template was dried in an electric thermostatic drier at 50 �C for

12 h and then the colloidal SiO2 crystal template was obtained.

For the preparation of 3DOM Bi2WO6 photocatalyst, 2 mmol

of Bi(NO3)3$5H2O (AR, Sinopharm) was dissolved in 4 mL of

4 M HNO3. The stoichiometric amount of 12-phosphotungstic

acid (AR, Sinopharm) was dissolved in 10 mL of ethanol. These

two solutions were mixed together and infiltrated into the as-

prepared colloidal SiO2 crystal template by the impregnation

technique in an electric thermostatic drier at 50 �C for 24 h. After

the solvent was gradually evaporated, white bulk solid was

obtained. The bulk solid was then put into a sealed reactor filled

with NH3$H2O vapor for 12 h. Subsequently, the bulk solid was

dried and calcined at 450 �C for 2 h to give a decomposed

product of bismuth tungstate inside the SiO2 crystal template.

The SiO2 colloidal crystal template was removed with 2 M

NaOH solution. After washing with enough distilled water and

drying at room temperature, the 3DOM Bi2WO6 sample was

obtained.

For comparison, HR-Bi2WO6 was prepared by traditional

hydrothermal synthesis. In a typical procedure, 5 mmol

Bi(NO3)3$5H2O and 2.5 mmol Na2WO4 were mixed together,

and 40 mL of de-ionized water was added under vigorous stir-

ring. After being stirred for 20 min, the suspension was trans-

ferred to a 50 mL Teflon-lined autoclave. Then, the autoclave

was sealed in a stainless steel tank and heated at 160 �C for 24 h.

Subsequently, the autoclave was cooled to room temperature

naturally. The obtained HR-Bi2WO6 sample was collected,

washed with de-ionized water and dried at 50 �C in air. The

as-prepared HR-Bi2WO6 sample has a BET surface area of

9.8 m2 g�1. The bulk Bi2WO6 was prepared by traditional solid-

state reaction (SSR) according to a previous study.36

The purity and the crystallinity of the as-prepared samples

were characterized by powder X-ray diffraction (XRD) on a

Japan Rigaku Rotaflex diffractometer using Cu Ka radiation

while the voltage and electric current were held at 40 kV and

100 mA. The scanning electron microscope (SEM) character-

izations were performed on a JEOL JSM-6700F field emission

scanning electron microscope. The transmission electron micro-

scope (TEM) analyses were performed by a JEOL JEM-2100F

field emission electron microscope. The N2 sorption measure-

ment was performed using Micromeritics Tristar 3000 at 77 K.

The specific surface area and the pore size distribution were

calculated using the Brunauer–Emmett–Teller (BET) and Bar-

rett–Joyner–Halenda (BJH) methods, respectively. UV-Vis

absorbance spectra of the samples were measured by using a

Hitachi U-3010 UV-Vis spectrophotometer.

The photocatalytic activities of the samples were evaluated by

the degradation of phenol and aqueous ammonia under irradi-

ation of a 500WXe lamp. The reaction cell was placed in a sealed

black box the top of which was opened. In each experiment,

0.05 g of photocatalyst was added into 100 mL of phenol solution

(20 mg L�1) or 100 mL of NH4Cl solution. The pH value of the

This journal is ª The Royal Society of Chemistry 2012

NH4Cl solution was adjusted to 10.5 by aqueous NaOH. Before

illumination, the solution was stirred for 120 min in the dark in

order to reach adsorption–desorption equilibrium. At certain

min intervals, a 4 mL suspension was sampled and centrifuged to

remove the photocatalyst particles. The adsorption UV-visible

spectrum of the centrifugated phenol solution was recorded using

a Hitachi U-3010 UV-visible spectrophotometer. Total organic

carbon (TOC) analysis of the phenol solution was carried out

with an Elementar’s Liqui TOC II analyzer. The concentration of

NH4+/NH3 was estimated before and after the treatment using

Nessler’s reagent colorimetric method.

Results and disscussion

The 3DOM Bi2WO6 was synthesized via a hard template

synthesis method. The synthesis process involves seven basic

steps, as illustrated in Fig. 1. First, monodisperse SiO2 spheres

were centrifuged to prepare a colloidal crystal template. The

template was dried and then immersed in a precursor solution

containing bismuth nitrate, 12-phosphotungstic acid, ethanol

and nitric acid. The precursor infiltrated the interstices of the

SiO2 colloidal crystal in this process. After that, the obtained

blocky Bi2WO6 precursor/SiO2 was put into a sealed reactor

filled with ammonia vapor. By this procedure, the ammonia

vapor permeated the Bi2WO6 precursor and the acid staying in

the precursor was neutralized. Subsequently, the prepared

Bi2WO6 precursor/SiO2 block was dried and calcined to produce

the crystalline Bi2WO6. The 3DOM Bi2WO6 with mesoporous

walls was finally obtained after the SiO2 template was removed

by aqueous NaOH. It was found that the ammonia permeation

process is the key step to get the final pure Bi2WO6. If the acidic

precursor was directly calcined, only a small amount of the final

product is Bi2WO6 phase.

XRD was used to characterize the phase structure and crys-

tallinity of the obtained 3DOM Bi2WO6 product. As shown in

Fig. 2, all diffraction peaks can be well indexed to the ortho-

rhombic phase of Bi2WO6 (space group B2ab (41), JCPDS

73-2020). There is no indication of the presence of Bi2O3 or WO3

phase in the XRD profile. The results indicate the formation of

homogeneous Bi2WO6 solid product with high crystallinity. The

crystal size of the 3DOM Bi2WO6 is calculated to be about 8 nm

based on the Scherrer equation.

Fig. 3 shows SEM images of the obtained 3DOM Bi2WO6.

From the low magnified SEM image, it was found that well-

ordered structures were obtained in high yield (Fig. 3a). The

Bi2WO6 sample had a highly ordered porous structure in three

dimensions over a range of micrometers. A highly magnified

SEM image (Fig. 3b) showed that the material is macroporous,

containing periodic spherical voids with an average diameter of

90 nm. The next layer is visibly clearly in the highly magnified

SEM image. Further information about the 3DOM Bi2WO6

product was obtained from the TEM images (Fig. 4). The low-

magnification TEM image confirms the large scale of the 3DOM

structure (Fig. 4a). The high-magnification TEM (Fig. 4b) image

indicates that the walls of the macroporous Bi2WO6 are

composed of crystalline nanoparticles, which was confirmed by

the selected area electron diffraction (SAED) pattern (inset in

Fig. 4b). The size of the Bi2WO6 nanoparticles which constitute

the macroporous wall is estimated as about 6–8 nm from the

J. Mater. Chem., 2012, 22, 19244–19249 | 19245

Fig. 1 Synthesis steps for the 3DOM Bi2WO6 with mesoporous walls.

Fig. 2 XRD pattern of the as-prepared 3DOM Bi2WO6 sample. The

vertical lines at the bottom correspond to the standard XRD pattern of

orthorhombic Bi2WO6 (JCPDS 73-2020).

Fig. 3 (a) Low-magnification SEM and (b) high-magnification SEM

images of 3DOM Bi2WO6.

Fig. 4 (a) Low-magnification TEM and (b) high-magnification TEM

image of 3DOM Bi2WO6, the inset in (b) is the corresponding SAED

pattern.

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TEM image. This value is close to the estimated value obtained

from the XRD pattern. These small nanoparticles were con-

nected to form mesopores in the macroporous wall.

Nitrogen cryosorption studies were conducted to investigate

the porosity of the sample, which clearly revealed the existence of

mesopores and macropores in the 3DOM Bi2WO6 sample. The

nitrogen adsorption–desorption isotherm and pore size distri-

bution plots for the 3DOM Bi2WO6 were shown in Fig. 5. The

nitrogen adsorption–desorption isotherm shows a type-IV

isotherm according to IUPAC recommendations (Fig. 5a),37

19246 | J. Mater. Chem., 2012, 22, 19244–19249

which is representative of mesoporous solids. The amount of

adsorbed N2 gradually increased in the region of middle P/P0 and

markedly increased again in the region of high P/P0 (>0.8). This

adsorption behavior can be attributed to the capillary conden-

sation of N2 in the macropores (and mesopores) and multilayer

adsorption on the macropores and mesopores. The type of

hysteresis loop of N2 isotherm is intermediate between H1 and

H3 (IUPAC classification). Type H1 is often associated with

porous materials to consist of agglomerates of approximately

uniform spheres in a fairly regular array. The H1 hysteresis loop

here is originated from the uniform macropores in the 3DOM

Bi2WO6 sample. The observed hysteresis extended to P/P0 z 1

indicates the presence of large pores, which are not being filled.

The pore size distribution for the 3DOM Bi2WO6 was calculated

by the BJHmethod from the desorption branches (Fig. 5b).38 It is

obvious that the pore size distribution is bimodal centered at

3.8 nm and 30 nm as shown in Fig. 5b. The smaller pores are the

mesopores in the 3DOM Bi2WO6 macroporous walls. The larger

pores are the macropores between the Bi2WO6 hollow spheres,

the size of which is very similar to the calculated values as shown

in the inset of Fig. 5b. The BET surface area of the 3DOM

Bi2WO6 is 36 m2 g�1, which is in the range of typical macro-

porous oxide.39,40

The light utilization efficiency, which greatly influences the

photocatalytic performance, could be enhanced around the

photonic band-gap in a 3DOM structure by the interactions

between light and the periodic structure. The photonic band-gap

This journal is ª The Royal Society of Chemistry 2012

Fig. 5 (a) Nitrogen adsorption–desorption isotherms of the 3DOM

Bi2WO6, (b) the corresponding pore-size-distribution curve of 3DOM

Bi2WO6 calculated from the desorption branch of the isotherms by the

BJH method. The inset in (b) shows the calculated distance between

adjacent Bi2WO6 hollow spheres.

Fig. 6 UV-vis absorption spectra for the as-prepared 3DOM Bi2WO6

and HR-Bi2WO6 samples.

Fig. 7 (a) UV spectral changes of phenol aqueous solution in 3DOM

Bi2WO6 suspension as a function of irradiation time. The inset shows the

variation of TOC of phenol aqueous solutions with irradiation time, (b)

photodegradation efficiencies of phenol as a function of irradiation time

by different photocatalysts.

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of 3DOM Bi2WO6 can be determined by the modified Bragg’s

law for normal incidence:

lmax ¼ 2d(111)(fnBi2WO6+ (1 � f)nair)

where nBi2WO6(z2.2) and nair are the refractive indexes of

Bi2WO6 and air, respectively; f is the volume percentage of

Bi2WO6 phase; d(111) is associated with the pore size. In 3DOM

Bi2WO6, the pore size is around 90 nm. For such a pore distri-

bution, the modified Bragg’s law foresees the existence of a

photonic band-gap at 228 nm in water for normal incidence of

light to the (111) plane. Fig. 6 shows the UV-vis absorbance

spectrum of 3DOM Bi2WO6. It exhibits light absorption from

UV light to visible light within a wavelength shorter than about

450 nm. It is obvious that the photonic band-gap of 3DOM

Bi2WO6 is in the range of the absorption band of Bi2WO6. For

this reason, it is difficult to find an obvious peak of a photonic

crystal in Fig. 6 because it was hidden by the strong intrinsic

absorption of Bi2WO6 when measuring the UV-Vis absorbance

spectrum. The photonic band-gap located in the absorption band

of Bi2WO6 avails light energy utilization to obtain high photo-

catalytic activity.

This journal is ª The Royal Society of Chemistry 2012

The photocatalytic activity of 3DOM Bi2WO6 was tested

towards the degradation of phenol under Xe lamp. Although

phenol is quite toxic and slowly degradable in the natural envi-

ronment, the 3DOM Bi2WO6 could efficiently remove this

J. Mater. Chem., 2012, 22, 19244–19249 | 19247

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organic contaminant from water. Fig. 7a displays the temporal

evolution of the spectral changes during the photodegradation of

phenol over the 3DOM Bi2WO6 sample. A rapid decrease of

phenol absorption at a wavelength of 269 nm was observed. The

sharp decrease of the major absorption band within 20 min

indicates that the as-prepared 3DOM Bi2WO6 sample exhibits

high photocatalytic activity in the degradation of phenol. The

mineralization of phenol was investigated by measuring the

decrease of TOC during the photodegradation process. As

shown in the inset of Fig. 7a, the TOC concentration decreased

by about 85% after irradiation for 60 min. This confirms that

most of the phenol molecules were mineralized, further demon-

strating the high photocatalytic activity of 3DOM Bi2WO6.

When compared with the HR-Bi2WO6 and SSR-Bi2WO6, the

photocatalytic activity of the 3DOMBi2WO6 exhibits an obvious

advantage. Phenol degradation with different Bi2WO6 samples

with otherwise identical conditions was measured and the result

is shown in Fig. 7b, where Ct is the absorption of phenol at

wavelength 269 nm and C0 is the absorption of phenol after the

adsorption equilibrium on Bi2WO6 samples before irradiation.

Blank test (phenol without any catalyst) under the lamp exhibi-

ted no degradation of phenol, which indicates the photolysis of

phenol could be ignored. However, if 3DOM Bi2WO6 was used

as the photocatalyst, 96% of phenol was degraded after 60 min

under Xe lamp, showing the excellent photocatalytic activity of

3DOM Bi2WO6. The photocatalytic degradation of phenol with

HR-Bi2WO6 and SSR-Bi2WO6 samples was also tested under

the same conditions. After 60 min, the degraded phenol by

SSR-Bi2WO6 and HR-Bi2WO6 were only about 10 and 30%,

respectively. It is obvious that the 3DOM Bi2WO6 sample

possesses much higher photocatalytic activity.

Besides the phenol decomposition, the photocatalytic degra-

dation of aqueous ammonia, a major nitrogen-containing

pollutant, on the as-prepared 3DOM Bi2WO6 sample was per-

formed at pH 10.5. The final product of ammonia degraded

through a photocatalytic process is NO2�/NO3

� or N2.41,42 An

initial concentration of 20 mg L�1 NH4+/NH3 was used

throughout this study. As shown in Fig. 8, the concentration of

NH4+/NH3 decreases from an initial 20 mg L�1 to approximately

2.6 mg L�1 in the presence of the as-prepared 3DOM Bi2WO6

Fig. 8 NH4+/NH3 photocatalytic degradation in the presence of

different Bi2WO6 photocatalysts.

19248 | J. Mater. Chem., 2012, 22, 19244–19249

photocatalyst under irradiation for 2 h. About 87% of the

ammonia was degraded by this 3DOM Bi2WO6 photocatalyst.

The photocatalytic performances of the SSR-Bi2WO6 and

HR-Bi2WO6 on the ammonia removal were also tested. Only 12

and 41% of the ammonia were degraded after 2 h in the presence

of SSR-Bi2WO6 and HR-Bi2WO6, respectively, indicating the

excellent photocatalytic performance of the 3DOM Bi2WO6

photocatalyst. To make sure the photocatalytic degradation of

aqueous ammonia by the photocatalyst is not ascribed to a

photolysis process or the volatilization of NH3, a blank ammonia

removal experiment was conducted under light irradiation

without the photocatalyst. The result indicated the concentration

of ammonia only decreased 5% under irradiation for 2 h, further

proving the excellent photocatalytic activity of the as-prepared

3DOM Bi2WO6 photocatalyst.

The above experiments have shown the excellent photo-

catalytic activity of the as-prepared 3DOM Bi2WO6 on the

degradation of the widely used contaminant phenol and the

major nitrogen-containing pollutant aqueous ammonia. The

excellent photocatalytic activity of the 3DOM Bi2WO6 may be

ascribed to its particularly ordered macroporous and meso-

porous structure. The ordered macroporous structure with

mesoporous spherical walls could not only produce a photonic

band-gap to increase the light-harvesting efficiency but also

afford a lot of pore channels to facilitate the migration of

contaminants, which is advantageous for the photocatalytic

reaction.

Conclusion

A novel 3DOM Bi2WO6 with mesoporous walls was successfully

synthesized through a SiO2 colloidal template assisted method,

using bismuth nitrate and phosphotungstic acid as precursors.

The as-prepared 3DOM Bi2WO6 exhibits an excellent photo-

catalytic decomposition of phenol and aqueous ammonia under

irradiation from simulated sunlight. Comparative studies indi-

cated that the photocatalytic performance of the 3DOMBi2WO6

sample is much higher than that of SSR-Bi2WO6 and

HR-Bi2WO6 samples under the same conditions. The high

photocatalytic activity is ascribed to its particularly ordered

macroporous and mesoporous structure which could increase

the light-harvesting efficiency and facilitate the migration of

pollutants. This work provides a principal method to produce

bismuth-based 3DOM multiple metal oxides, and also a route to

obtain an efficient photocatalyst for environmental purification.

Acknowledgements

We acknowledge financial support from the National Natural

Science Foundation of China (51102262, 50972155), National

Basic Research Program of China (2010CB933503) and the

Innovation Research of the Shanghai Institute of Ceramics

(Y11ZCE1E0G).

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