This journal is c The Royal Society of Chemistry 2011 Catal. Sci. Technol., 2011, 1, 1399–1405 1399

Cite this: Catal. Sci. Technol., 2011, 1, 1399–1405

Effects of distortion of PO4 tetrahedron on the photocatalytic

performances of BiPO4w

Chengsi Pan,aDi Li,

aXinguo Ma,

aYi Chen

band Yongfa Zhu*

a

Received 11th July 2011, Accepted 17th August 2011

DOI: 10.1039/c1cy00261a

Three kinds of crystal phase BiPO4 (HBIP, nMBIP and mMBIP) were selectively synthesized by a

hydrothermal method. The governed factors for the formation of three crystal phase of BiPO4

were both on the acidity of the solution and reaction temperature. The BiPO4 with nMBIP phase

structure showed higher activity for degradation of MB solution under UV irradiation than HBIP

and mMBIP. The catalytic activity per surface area activity of nMBIP (3.8�105 h�1 m�2) wasabout ten times higher than that of P25(1.4�104 h�1 m�2). The BiPO4 with nMBIP structure

exhibited the highest activity due to the most distorted PO4 tetrahedron. The induce effect of the

dipole moment derived from the distorted PO4 tetrahedron promoting the separation of e�/h+

pairs and further benefited for the photocatalytic reaction. This correlation may help to design

and develop other oxoacid salt photocatalysts with high activity.

1. Introduction

Photocatalysis attracts much attention these days due to the

potential application of removing pollutants in water and air

by the direct absorption of light. This property has been

discovered mainly on metal oxide and composite metal oxide

semiconductors, such as the most widely used TiO2.1 Never-

theless, the photocatalytic activity of TiO2 is not high enough

to be used for industrial purposes, because of the limitation of

the absorption edge and the recombination of photogenerated

charges.2 Therefore, many efforts have been made to develop

new semiconductor photocatalysts in order to overcome the

drawbacks of TiO2 and achieve the maximum activity.

Oxoacid salts like Ti3(PO4)4,3 Cu2(OH)PO4,

4 Bi2SiO5,5

Ag3PO4,6,7 Bi2O2CO3

8 are a new kind of photocatalysts that

exhibit high photocatalytic activities, due to their unique

structure, such as highly crystalline, good stability, easily

combining with H2O, and the high negative energy of anions

to promote electron and hole separation.9 Although much

attention has been paid to this new kind of photocatalysts,

only a few studies on the relationships between the structure

and photocatalytic activity have been made. Lee et al.4 has

reported that the variety of lattice parameters influenced the

band gap of Cu2(OH)PO4 and further their activities.

Ye et al.6,7 reported the narrow band gap together with the

(110) surface of Ag3PO4 was in response to its high activity.

Besides the band gap and surface, photocatalytic activity may

first be determined by the crystal phase of the photocatalysts.

The crystal phase may influence the band gap, the separation

of photogenerated electron-holes and the positions of the

valence and conduction bands in metal oxide and composite

metal oxide photocatalysts. For example, for TiO2, it was

recently discovered that brookite had markedly high photo-

catalytic activity for H2 production as compared to those of

rutile and anatase due to a high conductive band.10,11 Also,

monoclinic BiVO4 has an obviously higher activity than the

trigonal one due to the higher efficiency for the separation of

photogenerated electron-holes derived from its distortion of

V–O and Bi–O polyhedrons.12,13 However, the relationship

between the phase structure and photocatalytic properties in

oxoacid salt photocatalysts has not been fully studied.

Recently, our group found that monoclinic BiPO4 (space

group: P21/n), as one of oxoacid salt photocatalysts, had a

superior photocatalytic performance to P25 (TiO2) but its

surface area is only one-sixteenth.14 It was noted that BiPO4

had three main crystal phases: hexagon BiPO4 (space group:

P3121, HBIP), monoclinic BiPO4 (space group: P21/n,

nMBIP), and monoclinic BiPO4 (space group: P21/m,

mMBIP). So, it is important to evaluate the photocatalytic

activity in other BiPO4 crystal phases (HBIP and mMBIP)

with the same oxoacid salt structure and make further under-

standing on relationship between this unique structure and

photocatalytic activity. This requires a controlled synthesis of

three crystal phases of BiPO4 in the analogous condition. Up

to now, the crystal phase with a hexagon structure (HBIP)

could be easily obtained by a direct deposition method15 or electro-

chemical deposition16 at room temperature in aqueous system.

aDepartment of Chemistry, Tsinghua University, Beijing, 100084,P.R. China. E-mail: zhuyf@mail.tsinghua.edu.cn;Fax: +86-10-62787601; Tel: +86-10-62783586

bResearch Center of Nano-Science and Nano-Technology,Shanghai University, Shanghai 200444, PR China

w Electronic supplementary information (ESI) available: Additionalfigures of crystals and catalysis data. See DOI: 10.1039/c1cy00261a

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1400 Catal. Sci. Technol., 2011, 1, 1399–1405 This journal is c The Royal Society of Chemistry 2011

But the monoclinic structure (nMBIP and mMBIP)

needed high temperature solid state reaction.15,17,18 The

controlled preparation method of three phases of BiPO4

especially for mMBIP by an aqueous process has not been

developed. On the other hand, it is difficult to compare the

properties among the different crystal phases of BiPO4

obtained by various methods because the chemical history of

the preparation determine the properties.19

In this paper, crystalline HBIP, nMBIP and mMBIP

powders were selectively synthesized in aqueous media by a

hydrothermal reaction. The correlation between the photo-

catalytic activity and PO4 tetrahedral distortion was demon-

strated for the three BiPO4 crystal phases. This correlation

may help to the synthesis of other non-metal oxoacid salt

photocatalysts with high activity.

2. Experimental section

2.1 Synthesis

Three crystal phases of BiPO4 powders were synthesized

through a hydrothermal process. All chemicals used were

analytic grade reagents without further purification. 3 mmol

Bi(NO3)3�6H2O and 30 mL, a given concentration of H3PO4

were put into a beaker and then magnetically stirred to form a

homogeneous solution at room temperature. The resulting

precursor suspension was transferred into a Teflon-lined stain-

less steel autoclave. The autoclave was sealed and maintained

at the given temperature for 72 h, without shaking or stirring,

then allowed to cool naturally to room temperature. The

products were filtered off, washed several times with distilled

water, and dried at 80 1C for 24 h, subsequently.

2.2 Characterization

The products were characterized by powder X-ray diffraction

(XRD) on Bruker D8-advance X-ray diffractometer at 40 kV

and 40 mA for monochromatized Cu Ka (l = 1.5406 A)

radiation. Morphologies and structures of the prepared

samples were further examined with JSM 6301 electron scan-

ning microscope (SEM) and transmission electron microscopy

(TEM) by a JEM 1010 electron microscope operated at an

accelerating voltage of 100 kV. UV–vis diffuse reflectance

spectra (DRS) of the samples were measured by using Hitachi

U-3010 UV–vis spectrophotometer. The Brunauer–Emmett–

Teller (BET) specific surface area of the samples was

characterized by nitrogen adsorption at 77 K with Micro-

meritics 3020. PL spectra were obtained using an Edinburgh

Analytical Instruments FL/FSTCSPC920 coupled with a time-

correlated single-photo counting system.

2.3 Photocatalytic evaluation

Photocatalytic activities of BiPO4 were evaluated by degrada-

tion of methylene blue (MB) under ultraviolet light irradiation

of 11W low-pressure lamp with 254 nm, respectively. The

average light intensity was 1.5 mW cm�2. The radiant flux was

measured with a power meter from Institute of Electric Light

Source (Beijing). MB solutions (200 ml, 10�5 mol L�1)

containing 0.100 g of BiPO4 were put in a glass beaker. Before

the light was turned on, the solution was first ultrasonicated

for 10 min, and then stirred for 10 min to ensure equilibrium

between the catalysts. Three millilitres of sample solution were

taken at given time intervals and separated through centri-

fugation (4000 rpm, 10 min). The supernatants were analyzed

by recording variations of the absorption band maximum

(664 nm) in the UV–vis spectra of MB using a U-3010

spectrophotometer (Hitachi).

Photoelectrochemical measurements were carried out in a

conventional three-electrode, single-compartment glass cell,

fitted with a synthesized quartz window, using a potentiostat.

The quartz electrolytic cell was filled with 0.1 M Na2SO4. The

ITO/BiPO4 electrodes served as the working electrode. The

counter and the reference electrodes were platinum black wire

and saturated calomel electrode (SCE), respectively. An 11 W

germicidal lamp were used as the excitation light source for

ultraviolet irradiation.

3. Results and discussion

3.1 Formation and transformation of HBIP, nMBIP andmMBIP

The three crystal phases of BiPO4 could be controlled

synthesis through hydrothermal reactions. By studying these

reactions in the system Bi3+-H3PO4-H2O, the formation of

different BiPO4 crystal phases was found depending both on

the hydrothermal temperature and phosphorus-containing

reagents in the reaction mixture and on the acidity of the

solution. Regulation of the acidity of the reaction system was

achieved by adding H3PO4. Fig. 1 shows the ranges of con-

centration used for the preparation of individual crystalline

phases. At lower H3PO4 concentration and temperature,

the formation of HBIP as the sole product takes place. The

increasing of temperature to the reaction system favors the

synthesis of nMBIP. The compound mMBIP as a pure phase

is formed when the acidity of the solution and temperature is

higher. It also indicated that the formation of mMBIP and

nMBIP costs lower temperature with the increasing of the

acidity of the solution. As previous study,14,16 all the three

structures had an analogous structure but a little difference

due to the symmetries and the choice of coordinate systems, as

shown in Supporting Information Figure S1.w Therefore, the

transformation between the three BiPO4 crystal phases only

requires a small rotation of the tetrahedron to adopt

appropriate symmetric arrangement, with no change of the

topological features.

Fig. 1 Phase diagram for various BiPO4 crystal phases of (J) HBIP,

(n) nMBIP, (&) mMBIP.

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More detailed information on the phase purity and crystal

structure of the products synthesized at 12 M H3PO4 obtained

by XRD measurement is shown in Fig. 2. In this concentra-

tion, the transformation of BiPO4 from hexagon to monoclinic

occurs with the temperature increasing. All the diffraction

peaks of HBIP can be indexed to the pure hexagonal phase

of BiPO4 (JPCDS 45–1370) with lattice constants a = 6.986,

b = 6.986 and c = 6.475 A. While at higher temperature HBIP

is converted into monoclinic phase: nMBIP (below 160 1C)

and then mMBIP (above 160 1C). nMBIP is consistent with

JPCDS 80–0209 with lattice constants a = 6.762, b = 6.951

and c = 6.482 A. While the lattice constants of

mMBIP(JPCDS 43–0637) are a = 4.882, b = 7.068 and

c = 4.704 A. During the transformation, the peak at 14.61,

20.11, 29.51 belongs to (100), (101), (200) of HBIP is

disappeared while the peak at 17.01, 21.31, 34.41 belongs to

(�101), (�111), (�202) of nMBIP emerged. Finally, at above

160 1C, the crystal phase is completely transformed into

mMBIP and the peak at 18.31, 22.81, 37.01 attributed to the

(100), (011), and (200) surfaces can be recognized as the unique

peak of mMBIP.

The size and morphology of the products were examined by

SEM and TEM. Fig. 3 shows that the obtained HBIP and

nMBIP samples exhibit 1D structure but with large difference

in size, while the mMBIP has a lamellar structure. The

as-prepared nMBIP rods are several micrometres long and

about 1 mmwide. On the other hand, HBIP rods are 100–400 nm

in length and about 50 nm in width. The mMBIP plates are

larger than 10 mm2 in size and about 1 mm in thickness. It is

well-known that when the particle size is smaller than 100 nm,

the nano-size effect mainly influences the photocatalytic

activity. In our case, the large size of the three BiPO4 struc-

tures ensures that the photocatalytic activities of three phases

are mainly influenced by their crystal structures, not by the

nano-size effect.

3.2 Photo absorption properties and photocatalytic

performance of HBIP, nMBIP, and mMBIP

Diffuse reflectance spectra of three BiPO4 crystal phases

synthesized at 12 M H3PO4 are shown in Fig. 4. According

to the plots, the absorption edges of HBIP, nMBIP, and

mMBIP occur at about 360nm, 322 nm, and 296nm, respectively,

due to the excitation of electrons from valence gap to

conductive gap. The inset profiles show that the plots of

absorption1/2 (or absorption2]) versus energy, which may helps

to distinguish the direct or indirect transition of BiPO4 crystal

phases. In semiconductors, the square of absorption coefficient

is linear with energy for direct optical transitions in the

absorption edge region; whereas the square root of absorption

coefficient is linear with energy for indirect transitions.20

According to the plots inset Fig. 4, the monoclinic

BiPO4 is indirect, while the hexagon BiPO4 is direct. The

band-gap energies are estimated to be 4.6, 3.8 eV, and

4.2 eV for HBIP, nMBIP, and mMBIP, respectively. The

difference may be due to their electronic structural difference

limited by the crystal phase, as also reported in brookite and

anatase TiO211

Fig. 2 X-ray-diffraction patterns of BiPO4 crystal phases obtained in

12 M H3PO4 for 72 h at various hydrothermal temperature: at 20 1C

for HBIP; at 40 1C for HBIP and nMBIP; at 100 1C for nMBIP; at

160 1C for nMBIP and mMBIP; and at 200 1C for mMBIP.

Fig. 3 SEM (a, c, e) and TEM (b, d, f) images of BiPO4 crystal phases

obtained in 12 M H3PO4 for 72 h at various hydrothermal tempera-

ture: (a, b) HBIP at 20 1C, (c, d) nMBIP at 100 1C, (e, f) mMBIP at

200 1C.

Fig. 4 UV-DRS patterns of various BiPO4 crystal phases obtained in

12 M H3PO4 for 72 h at different hydrothermal temperatures:

(a) HBIP at 20 1C, (b) nMBIP at 100 1C, (c) mMBIP at 200 1C.

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1402 Catal. Sci. Technol., 2011, 1, 1399–1405 This journal is c The Royal Society of Chemistry 2011

Photocatalytic activities for MB degradation catalyzed by

HBIP, nMBIP, and mMBIP are shown in Fig. 5 and Table 1.

Although the absorption bands, pore volumes and average

pore diameters of HBIP, nMBIP, and mMBIP are almost the

same, nMBIP shows much higher photocatalytic activity than

the other two. The degradation of MB without catalysts can be

ignored, and thus the activities for three crystal phases

decrease in the following order: nMBIP > mMBIP > HBIP.

According to the BET surface area listed in Table 1, the

catalytic activity per surface area of HBIP, nMBIP, and

mMBIP can be calculated as 6.1 � 102 h�1m�2, 3.8 �105 h�1m�2, 3.0 � 104 h�1 m�2. Among the three crystal

phases the the catalytic activity per surface area of nMBIP is

about ten times higher than that of P25 (1.4 � 104 h�1 m�2)14

but competing to the nanosized BiPO4 (3.9 � 105 h�1 m�2) as

we reported before.14 The apparent quantum efficiency was

also calculated according to ref. 1, that is, 0.06%, 0.54% and

0.15% for HBIP, nMBIP and mMBIP, respectively. This is

consistent with the order of photocatalytic activity. A similar

photodegrading behavior was also found in the methyl

orange (MO) degrading as shown in Supporting Information

Figure S2.w The MO is known as a kind of cation dye

molecular(Figure S2 inset), while MB is regarded as an anion

dye(Figure 5 inset). Furthermore, the degradation activity of

colorless 4-chlorophenol (4-CP) on three BiPO4 crystal phases

has been given in Supporting Information Figure S3, which

shows the same order.w As a result, the activity differences for

three BiPO4 crystal phase are supposed not due to the effect of

dye adsorption, but due to the change of crystal phase

structures.

To further confirm this conclusion, the transient photo-

current responses were measured via three on-off cycles of

irradiation as shown in Fig. 6. As previous studies, the

photocatalytic reactions could be regarded as an electro-

chemical process and thus, photocurrent was regarded as

equivalent to photocatalytic activity.21,22 Furthermore, this

magnitude of the photocurrent directly reflected the number of

photogenerated electrons and holes but with less regards to the

degrading substrate difference. From Figure 6 a prompt

generation of photocurrents is observed and with good repro-

ducibility when samples are irradiated by UV light. This

indicates that our electrode is stable and photoresponsive

phenomenon is entirely reversible. The photocurrent of HBIP,

nMBIP, and mMBIP is 0.6, 2.1, and 0.2 mA, respectively. This

order is consistent with their photocatalytic activities. There-

fore, the decrease in photocatalytic activity is mainly due to

the concentration reduction in photogenerated carriers, which

is probably caused by e�/h+ recombination in three BiPO4

crystal phases discussed in the next paragraph.

3.3 The influence of the crystal phases on the photocatalytic

activity

The photocatalytic activity is governed by various factors such

as surface area, photo absorption, the oxidation potential of

photogenerated holes, and the separation efficiency of photo-

induced electrons and holes. In our case, it is noted that

activity decreasing order is just opposite to BET surface area

of the three structures, while the photo absorption is all

around 300 nm, as shown above. Therefore, it may be due

to other structure factors that influence the photocatalytic

activity, such as the oxidation potential of photogenerated

holes, and the transfer and separation of the photogenerated

electron-hole pairs.

In general, the high potential of photogenerated holes in the

valence band benefits for the production of active �OH

radicals, as reported in In(OH)3 (Evb = 4.2eV)23 and

CaSb2O5(OH)2 (Evb = 4.1eV).24 To indentify the influence

of the oxidation potential of holes, the position of the valence

band is estimated by the position of the conductive band and

the band gap. The former one can be reflected by the flat band

in n-typed semiconductors.25,26 As shown in Fig. 7, the

flat band potential for HBIP, nMBIP, mMBIP, is �0.60 V,

�0.70 V, �0.68 V vs. SCE, respectively. In addition, the

differences (DE = ECB � Efb) between the bottom of conduc-

tion band and the flat band potential for three BiPO4 crystal

Fig. 5 Photocatalytic degradation curves of MB for various BiPO4

crystal phases obtained in 12 M H3PO4 for 72 h at different hydro-

thermal temperatures: (a) HBIP at 20 1C, (b) nMBIP at 100 1C,

(c) mMBIP at 200 1C. Photocatalyst, 0.5 g L�1; MB concentration,

10�5 mol/L. Inset is the structure of MB.

Table 1 The BET surface area, pore volumes, pore diameters andrate constants for the reaction for various BiPO4 crystal phases

Sample kMB/h�1

BET surfacearea/m2 g�1

Pore Volume/cm3 g�1

Average PoreDiameter/nm

HBIP 0.05 0.82 0.008 5.6nMBIP 1.13 0.03 0.002 4.7mMBIP 0.18 0.06 0.003 8.7

Fig. 6 Transient photocurrent responses for various crystal phases

BiPO4 electrodes. Electrolyte: 0.1 M Na2SO4.

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phases were assumed to be �0.3 V due to the similar composi-

tion and insulating n-type semiconducting properties.26,27 As a

result, the potential of conductive bands of HBIP, nMBIP and

mMBIP is the �0.9 V, �1.0 V, �0.98 V vs. SCE, respectively.

Combining with the band gap, the oxidation potential in the

valence band for HBIP, nMBIP and mMBIP is estimated to be

0.37 V, 0. 28 V, 0.32 V vs. SCE. It can be seen that the HBIP

may have the highest the oxidation potential of the holes in the

valence band, while nMBIP has lowest oxidation potential.

However, the photocatalytic activity of HBIP is much less

than the nMBIP as discussed above. This implies that the

photocatalytic activity difference among HBIP, nMBIP, and

mMBIP is not due to the oxidation potential of

photogenerated holes.

On the other hand, the electrochemical impedance spectro-

scopy was performed in order to evaluate the differences of

transfer and separation of the photogenerated electron-hole

pairs between the photocatalysts. (Fig. 8) The diameters of the

EIS Nyquist plots under UV illumination are supposed to

indicate the charge separation and transfer process in the

electrode-electrolyte interface region.28–30 Under illumination,

the diameters tend to decrease from HBIP to mMBIP, and

nMBIP, significantly. This indicates that the activity difference

in HBIP, mMBIP, nMBIP is due to variation of the charge

transfer and the recombination of e�/h+ pairs. The recombi-

nation of e�/h+ pairs is also characterized by the lifetime of

the carriers. It can be reflected by the decays of PL transition

centered on 550 nm excited at 254 nm as shown in Fig. 9.

Moreover, the PL lifetime of three samples calculated by the

exponential analysis was also shown in Fig. 9 inset. The PL

lifetime decreases in the following order: nMBIP > mMBIP >

HBIP. A longer PL lifetime means lower recombination rate of

the electron–hole pairs, and thus higher photocatalytic activity.

This order is quit consistent with EIS results.

In a word, the nMBIP phase favors the generation and

separation of the photo-excited electron-hole pairs and thus

enhances the photocatalytic activity.

3.4 Relationship between structure and photocatalytic activity

According to the above discussion, the photocatalytic activity

difference between three BiPO4 structures is mainly due to the

difference of their separation efficiency of the photogenerated

electron-hole pairs. This difference in reactivity among the

BiPO4 polymorphs might be related to their geometric and

electronic structures.

Using the crystallographic data regarding the atom posi-

tions, three structures of BiPO4 are shown in Fig. 10. The data

of the crystal is obtained from ref. 17 (HBIP and nMBIP) and

ICSD No. 060522 (mMBIP), and further optimized by the

CASTEP. In these calculations, the energy cutoff was chosen

at 380 eV.31,32 In all three crystals, one bismuth atom is

surrounded by eight oxygen atoms and one phosphorus atom

is surrounded by four oxygen atoms. However, the distortion

of Bi–O polyhedron and P–O tetrahedron are quit different

due to the limitation by the symmetry of the crystal phase. The

Bi–O and P–O bond length in nMBIP are dispersed mostly

broadly while the slightest distortion occurs in HBIP. As

discussed before,14 the high photocatalytic activity of BiPO4

was mainly derived from the induced effect of phosphate,

compared with the Bi2O3 structure.33 The PO4

3� was assumed

to motivate the separation of photogenerated electron-hole

pairs and then improve the photocatalytic activity of the

catalysts. It was also reported that not only the PO43� itself

Fig. 7 Mott-Schottky plots for various BiPO4 crystal phases

obtained in 12MH3PO4 for 72 h at different hydrothermal temperatures:

(a) HBIP at 20 1C, (b) nMBIP at 100 1C, (c) mMBIP at 200 1C.

Counter electrode: Pt. Electrolyte: 0.1 M Na2SO4. Frequency: 1 kHz.

Fig. 8 EIS spectra for various BiPO4 crystal phases obtained in 12 M

H3PO4 for 72 h at different hydrothermal temperatures: (a) HBIP at

20 1C, (b) nMBIP at 100 1C, (c) mMBIP at 200 1C.

Fig. 9 PL decay curves measured at lex = 254 nm and lem =

550 nm for various BiPO4 crystal phases. Inset is the lifetime of

carriers for three BiPO4 crystal phases.

Fig. 10 Crystal structures of (a) HBIP, (b) nMBIP, (c) mMBIP.

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1404 Catal. Sci. Technol., 2011, 1, 1399–1405 This journal is c The Royal Society of Chemistry 2011

but also the dipole moment derived from the distorted PO4

tetrahedron would yield the induce effect.34 Therefore, the

distortion of it would change the distribution of the electronic

cloud between P–O, which may further change the electron-

hole separation.

To understand this feature deeply, we calculated the dipole

moment of the three BiPO4 structures as shown in Fig. 11. The

dipole moment of PO4 tetrahedron was calculated according

to the center of gravity of oxygen ions in the tetrahedron.

According to Fig. 11, the dipole moment in HBIP is just

0.05D, while in nMBIP it is 10.6D. At the same time the

photocatalytic activities for the above two structures are

0.05 h�1 and 1.13 h�1, respectively. This obviously shows that

BiPO4 with large dipole moments are photocatalytically

active, whereas the slight distortion BiPO4 exhibited negligible

activity, demonstrating that a correlation exists between the

photocatalytic activity and the dipole moment. The one who

has the most distortion P–O tetrahedron has the highest

activity.

In previous studies, a good correlation between the photo-

catalytic activity and the dipole moment has been demon-

strated.35–37 It is known that the dipole moment induces the

formation of local fields in the distorted polyhedron. The fields

are considered to promote the separation of photogenerated

e�/h+. Such as in a simple metal oxide Ga2O3, the distorted

tetrahedral and octahedral in b-Ga2O3 with dipole moment

showed high photocatalytic activity.35 While in composite

metal oxides, it has been reported that the induce effect of

alkaline earth metal will influence the dipole moment in

distorted SbO6 polyhedron and further make the catalysts

photocalytically active for water decomposition.37 It has also

reported that the dipole moment in V–O tetrahedron in BiVO4

will facilitates the electron excitation.13 In our case, the Bi–O

bonds contain not only valence bonds but also ionic bonds,

like in BiVO4, so dipole moment in Bi–O polyhedron can’t

describe the influence of the electron separation exactly.

On the contrary the dipole moment derived from the

distorted PO4 tetrahedron will induce a dipole moment in

Bi–O polyhedron, which is further useful for electron-

hole separation. In a word, it should be noted that the

correlation between the photocatalytic activity and PO4

tetrahedron distortion is true for both three BiPO4 crystal

phases and other inorganic salt photocatalysts of non-metal

oxoacid.

4. Conclusion

Crystalline HBIP, nMBIP and mMBIP powders in aqueous

media by a hydrothermal reaction have been synthesized

through the control of the acidity of the solution and

temperature in the reaction. The nMBIP who has the most

distortion PO4 tetrahedron exhibits the highest activity for

degradation of MB solution among three structures. The

correlation between the photocatalytic activity and P–O tetra-

hedron distortion is demonstrated for the three BiPO4 crystal

phases. We also assume that this correlation may be useful in

other oxoacid salt photocatalysts. In this regard, it is an

important discovery for the design and synthesis of other

oxoacid salt photocatalysts with a high activity.

Acknowledgements

This work was partly supported by the National Natural

Science Foundation of China (20925725 and 50972070) and

National Basic Research Program of China (2007CB613303)

and Key Subject of Shanghai Municipal Education Commission

(J50102).

Notes and references

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Fig. 11 Correlation between dipole moments in various BiPO4

crystal phases and photocatalytic activities.

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