Photocatalytic Activities of Heterojunction Semiconductors Bi2O3

Nov 20, 2007 - The obviously increased performance of Bi2O3/BaTiO3 is ascribed mainly to the electric-field-driven electron−hole separations both at...
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J. Phys. Chem. C 2007, 111, 18288-18293

Photocatalytic Activities of Heterojunction Semiconductors Bi2O3/BaTiO3: A Strategy for the Design of Efficient Combined Photocatalysts Xinping Lin, Jingcheng Xing, Wendeng Wang, Zhichao Shan, Fangfang Xu, and Fuqiang Huang* State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, P R China ReceiVed: May 22, 2007

The heterojunction semiconductors Bi2O3/BaTiO3 were prepared by a milling-annealing method. The powders were characterized by X-ray diffraction (XRD), the Brunauer-Emmett-Teller (BET) method, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and UV-vis diffuse reflection spectroscopy (DRS). Their UV-induced photocatalytic activities were evaluated by the degradations of methyl orange and methylene blue. The results generally show that the heterojunction semiconductors Bi2O3/BaTiO3 exhibit better photocatalytic properties than the single-phase BaTiO3 or Bi2O3. The obviously increased performance of Bi2O3/BaTiO3 is ascribed mainly to the electric-field-driven electron-hole separations both at the interface and in the semiconductors. A strategy for the design of efficient heterojunction photocatalysts was proposed. That is, an electron-accepting semiconductor and a hole-accepting semiconductor with matching band potentials, which respectively possess high electron and hole conduction abilities, are tightly chemically bonded to construct the efficient heterojunction structure.

1. Introduction Since Fujishima and Honda reported a TiO2 photochemical electrode for splitting water in 1972,1 photocatalysis has drawn increasing attention. It is regarded as a potential solution to the recent severe problems of energy shortages and environment crises. The general photocatalytic process of a semiconductor involves the formation of photoinduced electrons at the conduction band and holes at the valence band, and the subsequent chemical reactions with the surrounding media after photostimulated charges move to the powder surface. In this way, water can be split into hydrogen and oxygen gases, and pollutants in water or gas can be degraded or purified effectively. Thus, high mobility for photoinduced electron-hole separation and their transportation in the crystal lattice and the corresponding low efficiency of electron-hole recombination are demanded in order to achieve the high photocatalytic performance of a semiconductor. As one of effective methods for photostimulated electronhole separation, semiconductor combination is of increasing interest. The method constructs a heterojunction interface between the semiconductors with matching band potentials. In this way, the electric-field-assisted charge transport from one particle to the other via interfaces is positive for the electronhole separations in the coupled materials, and for the consequent electron or/and hole abundance on the surfaces of the two combined semiconductors, respectively. The reported heterojunction semiconductors are mainly TiO2-based photocatalysts and some limited non-TiO2 based systems.2-5 The extensively researched TiO2-based combined photocatalysts include WO3/ TiO2,6,7 MoO3/TiO2,8 ZrO2/TiO2,9 Fe2O3/TiO2,10,11 ZnO/TiO2,12 SnO2/TiO2,13 Cu2O/TiO2,14 Bi2O3/TiO2,14 CdS/TiO2,6 Bi2S3/ TiO2,15 and so forth. * Corresponding author. Tel: +86-21-52411620. Fax: +86-21-52413903. E-mail address: [email protected].

In the reported TiO2-based systems, the phase of TiO2, which has a good ability for hole transport, is not rutile but anatase; and the combined materials are mainly n-type semiconductors, such as WO3, MoO3, Fe2O3, SnO2, Bi2O3, and CdS, which have a good ability for electron transport. Such a phenomenon gives us an implication for a design scheme for the efficient heterojunction photocatalysts. As we have known, these n-type semiconductors possess a good electron conducting ability. Compared to the rutile phase with the density of 4.25 g/cm3, the anatase TiO2 with a loosely packed structure and a density of 3.89 g/cm3 has a higher structural openness degree,16 which favors the hole transport in the crystal lattice by the available displacement of the O atoms through the strong vibration model (associated with O-). Therefore, it may be a good combination system that a good n-type semiconductor with fair electron conductivity cooperates with a material with an open structure and the fair mobility for the hole conduction. Such an idea will be proven experimentally and discussed in the present paper. Bi2O3 is a good n-type semiconductor, and BaTiO3 is a high dielectric and ferroelectric material, where some atoms in the lattice are movable. On the basis of the above discussion, we chose Bi2O3/BaTiO3 as an example. The mechanism of the combined system with a higher catalytic activity was discussed. Particularly, besides the demand of a commonsensible matching ability of band potentials between semiconductors for the design of highly efficient combined photocatalyst, we found that the high electron and hole mobility of the electron-accepting and hole-accepting semiconductors, respectively, are significant to increase the photocatalytic activity. Furthermore, an ideal construction scheme for an efficient combined photocatalyst was proposed. 2. Experimental Section The combined semiconductors Bi2O3/BaTiO3 were prepared by a milling-annealing method. BaTiO3 synthesized by a high-

10.1021/jp073955d CCC: $37.00 © 2007 American Chemical Society Published on Web 11/20/2007

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Figure 1. XRD patterns and the value of the c lattice parameter of BaTiO3 for the prepared powders (a) BaTiO3, (b) 25% Bi2O3/BaTiO3, (c) 50% Bi2O3/BaTiO3, (d) 75% Bi2O3/BaTiO3, and (e) 100% Bi2O3/BaTiO3.

temperature solid-state reaction between BaCO3 (Sinoreg., 99.5%) and TiO2 (Sinoreg., 99.5%) at 1350 °C for 24 h was used as a raw material. Bi2O3 (Sinoreg., 99.5%) and the prepared BaTiO3 were weighed according to Bi2O3: BaTiO3 ) 25%, 50%, 75%, 100% (mass ratio) and subsequently ball-milled in a planetary ball miller with a rotating rate of 200 r/min for 12 h. Correspondingly, the composite powders are labeled as x% Bi2O3/BaTiO3 (x ) 25, 50, 75, 100), respectively. The mixed composite powders were finally annealed at 700 °C for 2 h. The purpose of the final annealing is to provide enough thermal energy to achieve a tight chemical binding between the semiconductor powders, which could provide a spatial condition for charge transport from one particle to the other via the interfaces. The references, pure Bi2O3 and BaTiO3 powders, were also treated by the same milling-annealing process, respectively. To elucidate the importance of chemical bonding formation at interfaces for the enhanced photocatalytic activity of the heterojunction semiconductors, we used the Bi2O3 and BaTiO3 composite produced by the direct mixing method as a reference. Here, pure Bi2O3 and BaTiO3 powders (both from the millingannealing process) were mixed physically for the subsequent photocatalytic reaction, without any further treatment. X-ray diffraction (XRD) patterns of as-prepared powders were obtained on a Bruker D8 Advance diffractometer using Cu KR radiation (λ ) 0.15406 nm). The morphological image, X-ray elemental line scans, and elemental maps were obtained on a JEOL JEM-2100F transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectrometer (EDS). The Brunauer-Emmett-Teller (BET) specific surface areas (SSA) were determined through nitrogen adsorption at 77 K using a Micromeritics ASAP2010 instrument. Diffuse reflectance ultraviolet-visible light (UV-vis) spectra (DRS) were measured at room temperature on a HITACHI U-3010 spectrophotometer by using BaSO4 as a reference and were converted from reflectance to absorbance by the Kubelka-Munk method. The photocatalytic reactor consists of two parts, a quartz cell with a circulating water jack and a 500-W high-pressure mercury lamp with a maximum emission at 365 nm placed inside the quartz cell. In all experiments, the reaction temperature was kept at room temperature to prevent any thermal catalytic effect by using a circulating water jack. Before photocatalytic reaction, 0.6 g of catalyst powder was added in 300 mL methyl orange (MO) solution (pH ) 6.88) or methlene blue (MB) solution (pH ) 6.82), both with the same concentration of 10 mg/L. UV illumination was conducted after the suspension was magnetically stirred in the dark for 50 min to reach the adsorption-desorption equilibrium of MO or MB on catalysts.

During irradiation, about 5 mL of suspension was continually taken from the reaction cell at given time intervals for subsequent MO concentration analysis after centrifuging. 3. Results and Discussion Figure 1 shows the XRD patterns of as-prepared powders. The pure-phase BaTiO3 is a tetragonal structure, and the corresponding JCPDS-ICDD number is 05-0626. The diffraction peaks of Bi2O3 appear and are intensified gradually as an increasing amount of Bi2O3 from 0% to 100% is dispersed over BaTiO3. Thus, we can confirm that the composite powders have a two-phase composition: BaTiO3 + Bi2O3. It is worth mentioning that the samples were prepared by a ballingannealing process, other than the commonly used direct-mixing one. The purpose of such a technique is to construct a chemical bonding at the interface of the two materials, which would provide a spatial condition for the charge transfer. During the initial ball milling, the powders were mixed homogeneously and the strong mechanical collision between the two powder particles might induce their metallurgically chemical contacts. During the final annealing treatment, the Bi3+ ions can thermally diffuse into the BaTiO3 crystal lattice at the surfaces. As a result, the c value of the unit cell of BaTiO3 in the combined system is increased, as compared to that of the single-phase BaTiO3. The variation of c is evidence of the thermal diffusion of ions between the two materials and for the consequent formation of tight chemically bonded interfaces. Additionally, the X-ray elemental line scans and elemental maps for Bi2O3 bonded BaTiO3 powders in Figure 2 give an intuitionistic image for the formation of chemically bonded interfaces between Bi2O3 and BaTiO3 particles. Because of the fact that cationic ions in AMO3 perovskite materials are exchangeable by other ions,17 the increasing c value in the present experiment is caused mainly by the substitution of Ti4+ with larger Bi3+ ions. However, the c value does not monotonically rise as an increasing amount of Bi2O3 is dispersed over BaTiO3 (see Figure 1), and a fluctuation exists between 50% and 75% Bi2O3 combined samples. This means that Bi3+ can also substitute for the larger Ba2+ ions. Generally, the milling-annealing technique applied in the present experiment leads to a tight chemically bonded interface between BaTiO3 and Bi2O3, and the incorporation of Bi3+ into BaTiO3 can enhance the mobility for charge transport in the interfacial lattice due to the effect of charge defects of BiTi- and BiBa+. The UV-vis diffuse reflectance spectra of the as-prepared powders are shown in Figure 3. The band gaps of BaTiO3 and Bi2O3 were estimated to be 3.14 and 2.75 eV, respectively. The

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Figure 2. X-ray elemental line scans and elemental maps for Bi2O3 chemically bonded BaTiO3 composite powders.

Figure 3. UV-vis diffuse reflectance spectra of BaTiO3, Bi2O3, and Bi2O3/BaTiO3.

Figure 4. Photocatalytic properties of BiTiO3, Bi2O3, and Bi2O3/BaTiO3 for degrading methyl orange.

optical absorptions of Bi2O3/BaTiO3 composite powders start at about 450 nm, corresponding to the absorption edge of Bi2O3. Figure 4 exhibits the photocatalytic activities of BaTiO3, Bi2O3, and Bi2O3/BaTiO3. MO is a kind of chemically stable and difficultly decomposed dye. In our experiments, the MO photolysis upon UV-light irradiation in the blank experiment is not observable. The equilibrium adsorptions of MO over

catalysts before the photocatalytic reactions listed in Table 1 are in good agreement with the BET surface areas of the asprepared powders. The slight difference of initial equilibrium adsorptions is believed not to affect the subsequent comparative discussion on the intrinsic activity discrepancy of different materials. Generally, the composite powders show better photocatalytic properties than the single-phase BaTiO3 or Bi2O3. After 50 min of UV illumination, the MO removal over BaTiO3 is as low as 6.9% and the single-phase Bi2O3 shows a relatively high photocatalytic performance with a degradation ratio of 89.8%. The MO decompositions over the combined systems are as high as 92.0-98.9%. It is worth noting that the combination of a small amount of Bi2O3 (25%) over BaTiO3 results in a sharp increase of MO degradation from 6.9% to 92.0%. As the Bi2O3 dispersion increases to 75%, the highest photocatalytic efficacy is achieved for the combined powders. The 100% Bi2O3 combined sample is less photocatalytically active than the 75% Bi2O3 loaded specimen. Such a phenomenon is ascribed mainly to the effect of charge defects. As stated above, as a larger amount of Bi2O3 powders were combined over BaTiO3, more Bi3+ ions were incorporated into the structure of BaTiO3 at the surfaces. Bi3+ can substitute for both Ti4+ and Ba2+ to form BiTi- and BiBa+ defects. As a result, holes and electrons are captured by the two charge defects, respectively, as described in eqs 1 and 2. Additionally, the stereochemically active Bi 6s2 lone electron pair, with a spatial extending distance of about 1.8 Å,18,19 can induce a polarized field in the Bi-O local structure,20-25 which is also favorable for an efficient electron-hole separation. Alternatively, too much amount of charge defects, however, will work as electron-hole recombination centers because of the well-known effect of concentration quenching. For the MB dye photodegradation, the same activity sequence of the combined powders as that obtained from MO degradation, that is, 75% Bi2O3 > 100% Bi2O3 > 50% Bi2O3 > 25% Bi2O3, was also found (see Figure 5).

BiTi- + hV f (BiTi- + h+) + e-

(1)

BiBa+ + hV f (BiBa+ + e-) + h+

(2)

To elucidate the significance of chemical bonding formation for the photocatalytic activity of combined semiconductors, we used a direct mixing method to prepare composite powders as comparison with the milling-annealing technique. The asprepared 75% Bi2O3/BaTiO3 powders show a rather low photocatalytic performance for degrading MO, as compared to

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TABLE 1: BET Surface Areas, MO Equilibrium Adsorptions (D) before Reactions, and Photocatalytic Reaction Rate Constants (k) of As-Prepared Powders catalyst

BET (m2/g)

D (%)

k (min-1)

Ra

Bi2O3 BaTiO3 25% Bi2O3/BaTiO3 50% Bi2O3/BaTiO3 75% Bi2O3/BaTiO3 100% Bi2O3/BaTiO3

0.15 0.28 0.24 0.22 0.21 0.19

3.1 4.8 4.3 4.2 4.2 3.8

0.034 0.0016 0.040 0.059 0.11 0.091

0.983 0.989 0.981 0.986 0.985 0.982

a

R is the correlation to the pseudo-first-order reaction kinetics.

Figure 6. Comparison of photocatalytic activities of 75% Bi2O3/BaTiO3 prepared by direct mixing and milling-annealing methods.

Figure 5. Photocatalytic properties of BiTiO3, Bi2O3, and Bi2O3/BaTiO3 for degrading methylene blue.

the ones prepared by the “ball milling + annealing” technique (see Figure 6). This implies the superiority of the millingannealing method. The main advantage of the method is the formation of tight chemically bonded interfaces between the two materials (see Figure 2). As a consequence, the charge transfer between two semiconductors is spatially smooth and the presence of charge defects in the interfacial crystal lattice may also benefit the charge transportation. To quantitatively understand the reaction kinetics of dye degradation in our experiments, we selected the results on MO degradation as an example and applied the pseudo-first-order model as expressed by eq 3, which is generally used for photocatalytic degradation process if the initial concentration of pollutant is low26

ln(C0/C) ) kt

(3)

where C0 and C are the concentrations of dye in solution at time 0 and t, respectively, and k is the pseudo-first-order rate constant. The rate constants originally obtained from data plotted in Figure 4 are listed in Table 1. As can be seen, a rather good correlation to the pseudo-first-order reaction kinetics (R > 0.98) was found. It is shown that the MO removal rates over Bi2O3/ BaTiO3 (0.04-0.11 min-1) are amazingly about 25-70 times as much as that over BaTiO3 (0.0016 min-1). Though the presence of charge defects on the surfaces of BaTiO3 has a positive influence on the charge transportation in the crystal lattice, it is believed that the enhancement of photocatalytic performance of combined system is attributed mainly to electric-field-assisted charge transfer at the heterojunction interfaces between BaTiO3 and Bi2O3 with matching band potentials, which consequently favors an effective photoexcited electron-hole separation in the two materials. The conduction band (CB) and valence band (VB) potentials of the

Figure 7. Conduction band and valence band potentials of BaTiO3 and Bi2O3.

two semiconductors at the point of zero charge can be calculated by the following empirical equation27,28

EVB ) X - E e + 0.5Eg

(4)

where EVB is the VB edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, E e is the energy of free electrons on the hydrogen scale (∼4.5 eV), Eg is the band gap energy of the semiconductor, and ECB can be determined by ECB ) EVB - Eg. The X values for BaTiO3 and Bi2O3 are ca. 5.242 and 5.986 eV, respectively. The calculated CB and VB edge potentials of BaTiO3 and Bi2O3 are shown in Figure 7. The CB edge potential of BaTiO3 (-0.83 eV) is more active than that of Bi2O3 (0.11 eV); hence, photoinduced electrons on the BaTiO3 particle surface transfer easily to Bi2O3 via interfaces; similarly, photoinduced holes on the Bi2O3 surface migrate to BaTiO3 owing to the different VB edge potentials, as shown in Figure 8. Alternatively, the electron-hole separations are also driven by the internal rebuilt electric fields in the two semiconductors. In other words, the effective electric field inducing the separation of photostimulated electron-hole pairs both at the interface and in the individual semiconductors reduces the probability of electron-hole recombination. As a result, a larger amount of electrons on the Bi2O3 surface and holes on the BaTiO3 surface, respectively, can participate in photocataltyic reactions to directly or indirectly decompose MO. Besides the commonsensible matching ability of band potentials between semiconductors described above, there other

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Figure 8. Schematic diagram for electric-field-driven electron-hole separations at the hetero interface and in both semiconductors. ED is the contact electric field induced by the differences of band potentials of two materials; EB is the potential barrier in the interfacial depletion layer (EB < ED during the nonequilibrium process of a photocatalytic reaction); E1 and E2 are the internal electric fields induced by the redistribution of the spatial charges in BaTiO3 and Bi2O3 particles, respectively.

considerations exist for the design of highly efficient heterojunction photocatalysts. A heterojunction system can be divided into three parts, electron-accepting semiconductor, hole-accepting semiconductor, and the interface, respectively. In other words, an ideal combined photocatalyst construction scheme can be described below: (1) the coupled semiconductors should have matching band potentials; (2) charge (electron and hole) transfer at the interface is smooth and spatially available; (3) the electron-accepting semiconductor possesses a fair electron transport ability after interparticle charge transfer, assuring a large amount of electrons on the surface to participate in chemical reactions; (4) the hole-accepting semiconductor has fair mobility for the hole transport in the crystal lattice. To the best of our knowledge, eyes were mostly focused on point 1 in reported literature about combined photocatalysts, and only little attention was paid to points 2-4. We have proven the positive influence of the presence of spatial chemically bonded interface on the enhanced photocatalytic performance of combined Bi2O3/ BaTiO3 by comparing samples prepared by the physical mixing and the milling-annealing methods. Points 3 and 4 suggest the significance of electron and hole conductivity in the coupled semiconductors, respectively. This is believed to be the main reason that noble metals and their oxides such as Pt, Ag, Pd, Rh, Au, RuO2, and IrO2, which have strong electron affinity and high electron conductivity, can be used universally as highly efficient cocatalysts for most photocatalysts. When some n-type semiconductors were combined over SrNb2O6 to improve the photocatalytic activity by the milling-annealing process, it is found that SnO2 and In2O3 combined samples show better performances than Nb2O5, WO3, and MoO3 combined systems.29 This is caused mainly by the higher electron conductivity of SnO2 and In2O3 semiconductors. As known, both SnO2 and In2O3, which are electron acceptors in the combined systems, have s-type conduction bands, and the dispersion of s bands is very large at the bottom of the conduction band. The strong dispersion of the s-type conduction band gives rise to an excellent electron-conducting performance, which is the original reason that doped SnO2 and In2O3 can be used as excellent

Lin et al. n-type transparent conducting materials from the viewpoint of band structure.30,31 There are differences for the conductions of an electron and a hole, although a hole can be commonly regarded as a positive electron and the two charges transport in crystal lattices mainly via the [M-O-M]∞ network in metal oxides. From the viewpoint of band structure, in a metal oxide (M/O) semiconductor, electrons occur at the conduction band that is composed primarily of valence orbitals of M, and holes appear at the valence band that mainly consists of O 2p orbitals. Therefore, the conductions of an electron and a hole, respectively, are related primarily to individual inherent attributes of the two ions and structurally correlated to their crystalline environments as well. In the present combined system Bi2O3/BaTiO3, Bi2O3 with the Bi 6p conduction band is a well-known good n-type electronconducting material. This means that it possesses fair mobility for the electron transport. The hole-accepting semiconductor in the system, BaTiO3, with a polarized dipole moment of ca. 2.32 D (Debye) in the [TiO6] local structure, is an excellent dielectric and ferroelectric material used commonly for the capacitor use, whose atoms in crystal structure are movable or vibratile. Compared to the heavier Ti atoms, the vibratility or movability of O atoms is larger. Therefore, it is believed that BaTiO3 has fair hole conductivity structurally induced by the O atom vibration (associated with O-). Similarly, the SrNb2O6 semiconductor mentioned above is also an open structure material with a dipole moment of ca. 6.56 D in the [NbO6] octahedron, and the combination of n-type semiconductors such as MoO3, WO3, Nb2O5, In2O3, and SnO2 over SrNb2O6 is an effective way to improve its photocatalytic property. In the reported TiO2based combined systems, TiO2 is used mainly as a holeaccepting semiconductor and the phase is not rutile but anatase structure. As we know, the anatase phase (density ) 3.89 g/cm3) is a more loosely packed structure than the rutile type (density ) 4.25 g/cm3). The more open structure in anatase TiO2 and the corresponding higher hole conductivity can partly account for the reported anatase-TiO2 combined photocatalysts with higher performances. Additionally, the commercial product Degussa P25 consisting of both anatase and rutile TiO2 is additional evidence of our proposed idea described above in which the anatase phase functions as a hole acceptor and the rutile structure is an electron acceptor. Therefore, the above facts, in this sense, may imply an ideal construction scheme for the highly efficient combined photocatalysts. That is, an electron-accepting semiconductor and a hole-accepting semiconductor with matching band potentials, which respectively possess high electron and hole conduction abilities, are tightly bonded to construct the efficient heterojunction structure. During photocatalysis over effective binary combined semiconductors, the interparticle charge transfer leads to electron and hole abundance in two respective components in composite. Therefore, one semiconductor in the heterojunction shows an n-type electron conducting behavior, and the other shows a p-type hole conducting behavior.32 However, most of the known heterojunction systems are the combinations of n-type semiconductors. The hole conductivity in the n-type semiconductor used as the hole accepting component in the composite is apparently lower than that in the p-type semiconductor. Therefore, the strategy for designing highly efficient heterojunction photocatalysts, as described above, may imply a superior way to combine p-type and n-type semiconductors to form the photocatalytic heterojunction with a better activity. To the best of our knowledge, the reported p-n-type combined semiconductor systems are rare, probably because most metal oxides

Photocatalytic Activities are n-type semiconductors. A typical p-n type combined photocatalyst system is Cu2O/TiO2.33,34,35 In this heterojunction, Cu2O as the hole-accepting component is not photocatalytically stable, and Cu(I) can be oxidized by the hole to form Cu(II). However, generally, the Cu2O/TiO2 binary semiconductors show excellent photocatalytic activities due to the high hole conductivity of p-type Cu2O and the fair electron conductivity of n-type TiO2, which is in good agreement with our proposed construction scheme for designing highly efficient combined photocatalysts. The excellent hole conductivity in synthesized Cu2O mainly results from the inevitable existence of a Cu(II) defect in the Cu2O structure, and the hybridization of Cu 3d and O 2p orbitals decreases the effective mass of holes in the valence band. Other reported p-n-type heterojunction systems with enhanced performances, to the best of our knowledge, are p-CuAlO2/n-TiO2,36 p-CuBi2O4/n-WO3,37 p-CuO/n-SnO2,38 p-Co3O4/n-BiVO4,39 p-CoOx/n-TiO2,40 p-TiO2-xNx/n-WO3,41 p-ZnMn2O4/n-TiO2,33 and p-CaFe2O4/n-PbBi2Nb0.9W0.1O9.42 In these examples, p-type semiconductors are all used as holeaccepting components in composites, and n-type semiconductors all function as electron acceptors. Generally, the principles (e.g., matching band potentials and fair electron/hole conductivity in respective semiconductors) of our proposed strategy can be also found in these p-n-type heterojunction systems. 4. Conclusions In summary, a milling-annealing technique was applied to prepare heterojunction photocatalysts Bi2O3/BaTiO3. This technique shows the advantage over the direct mixing method mainly in that it can construct a tight chemically bonded interface between the coupled materials. The heterojunction semiconductors Bi2O3/BaTiO3 show better photocatalytic activities than single-phase BaTiO3 or Bi2O3 for degrading methyl orange and methylene blue. The remarkable enhancement in the photocatalytic performance of Bi2O3/BaTiO3 is ascribed mainly to the electric-field-driven electron-hole separations at the interface and in the two semiconductors. Besides, the fair mobility for electron and hole transportation in Bi2O3 and BaTiO3, respectively, are also favorable for the high photocatalytic property. An ideal construction scheme for highly efficient combined photocatalysts can be that an electron-accepting n-type semiconductor with a high electron conduction ability cooperates with a hole-accepting semiconductor with a fair structureopenness degree and the corresponding fair hole conductivity. Acknowledgment. This research was financially supported by National 973 Program of China Grant 2007CB936704, National Science Foundation of China Grant 20471068, and Shanghai Research Grant 05JC14080. References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Zhang, M.; An, T.; Hu, X.; Wang, C.; Sheng, G.; Fu, J. Appl. Catal., A 2004, 260, 215.

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18293 (3) Spanhel, L.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 6632. (4) Rabani, J. J. Phys. Chem. 1989, 93, 7707. (5) Zhang, H.; Ouyang, S.; Li, Z.; Liu, L.; Yu, T.; Ye, J.; Zou, Z. J. Phys. Chem. Solids 2006, 67, 2501. (6) Serpone, N.; Maruthamuthu, P.; Pichat, P.; Pelizzetti, E.; Hidaka, H. J. Photochem. Photobiol., A 1995, 85, 247. (7) Shiyanovskaya, I.; Hepel, M. J. Electrochem. Soc. 1998, 145, 3981. (8) Song, K. Y.; Park, M. K.; Kwon, Y. T.; Lee, H. W.; Chung, W. J.; Lee, W. I. Chem. Mater. 2001, 13, 2349. (9) Yu, J. C.; Lin, J.; Kwok, R. W. M. J. Phys. Chem. B 1998, 102, 5094. (10) Pal, B.; Sharon, M.; Nogami, G. Mater. Chem. Phys. 1999, 59, 254. (11) Pal, B.; Hata, T.; Goto, K.; Nogami, G. J. Mol. Catal. A: Chem. 2001, 169, 147. (12) Marci, G.; Augugliaro, V.; Lopez-Munoz, M. J.; Martin, C.; Palmisano, L.; Rives, V.; Schiavello, M.; Tilley, R. J. D.; Venezia, A. M. J. Phys. Chem. B 2001, 105, 1026. (13) Bedja, I.; Kamat, P. V. J. Phys. Chem. 1995, 99, 9182. (14) Bessekhouad, Y.; Robert, D.; Weber, J.-V. Catal. Today 2005, 101, 315. (15) Bessekhouad, Y.; Robert, D.; Weber, J.-V. J. Photochem. Photobiol., A 2004, 163, 569. (16) Lin, X.; Huang, F.; Wang, W.; Wang, Y.; Xia, Y.; Shi, J. Appl. Catal., A 2006, 313, 218. (17) Yin, J.; Zou, Z.; Ye, J. J. Phys. Chem. B 2003, 107, 4936. (18) Radaev, S. F.; Simonov, V. I. SoV. Phys. Crystallogr. 1992, 37, 484. (19) Matjaz, V.; Danilo, S. J. Am. Ceram. Soc. 2001, 84, 2900. (20) Chen, A.; Yu, Z.; Vilarinho, P. M. Phys. ReV. B 1998, 57, 7403. (21) Chen, A.; Scott, J. F.; Ledbetter, H. Phys. ReV. B 1999, 59, 6661. (22) Chen, A.; Yu, Z. Phys. ReV. B 2000, 61, 11363. (23) Chen, A.; Yu, Z. Phys. ReV. B 2000, 61, 3922. (24) Chen, A.; Yu, Z.; Scottb, J. J. Phys. Chem. Solids 2000, 61, 191. (25) Noguchi, Y. Phys. ReV. B 2001, 63, 214102. (26) Herrmann, J. M.; Tahiri, H.; Ait-Ichou, Y.; Lassaletta, G.; GonzalezElipe, A. R.; Fernandez, A. Appl. Catal., B 1997, 13, 219. (27) Butler, M. A.; Ginley, D. S. J. Electrochem. Soc. 1978, 125, 228. (28) Tang, J.; Ye, J. Chem. Phys. Lett. 2005, 410, 104. (29) Lin, X. P.; Huang, F. Q.; Wang, W. D. Unpublished data. (30) Hamberg, I.; Granqvist, C. G. J. Appl. Phys. 1986, 60, R123. (31) Chopra, K. L.; Major, S.; Pandya, D. K. Thin Solid Films 1983, 102, 1. (32) Chen, Y.; Crittenden, J. C.; Hackney, S.; Sutter, L.; Hand, D. W. EnViron. Sci. Technol. 2005, 39, 1201. (33) Bessekhouad, Y.; Robert, D.; Weber, J.-V. Catal. Today 2005, 101, 315. (34) Siripala, W.; Ivanovskaya, A.; Jaramillo, T. F.; Baeck, S.-H.; Mcfarland, E. W. Sol. Energy Mater. Sol. Cells 2003, 77, 229. (35) Li, J.; Liu, L.; Yu, Y.; Tang, Y.; Li, H.; Du, F. Electrochem. Commun. 2004, 6, 940. (36) Brahimi, R.; Bessekhouad, Y.; Bouguelia, A.; Trari, M. J. Photochem. Photobiol. A 2007, 186, 242. (37) Long, M.; Cai, W.; Cai, J.; Zhou, B.; Chai, X.; Wu, Y. J. Phys. Chem. B 2006, 110, 20211. (38) Xia, H. L.; Zhuang, H. S.; Zhang, T.; Xiao, D. C. J. EnViron. Sci. 2007, 19, 1141. (39) Arai, T.; Yanagida, M.; Konishi, Y.; Iwasaki, Y.; Sugihara, H.; Sayama, K. J. Phys. Chem. C 2007, 111, 7574. (40) Wu, Y. Q.; Lu, G. X.; Li, S. B. Acta Chim. Sin. 2005, 63, 671. (41) Gao, B.; Ma, Y.; Cao, Y.; Yang, W.; Yao, J. J. Phys. Chem. B 2006, 110, 14391. (42) Kim, H. G.; Borse, P. H.; Choi, W.; Lee, J. S. Angew. Chem. Int. Ed. 2005, 44, 4585.