Materials Chemistry A

Journal of Materials Chemistry A

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Novel Ag3PO4/CeO2 composite with high eﬃciency and stability for photocatalytic applications† Cite this: J. Mater. Chem. A,2014,2, 1750 Zheng-Mei Yang, Gui-Fang Huang,* Wei-Qing Huang,* Jia-Mou Wei, Xin-Guo Yan, Yue-Yang Liu, Chao Jiao, Zhuo Wan and Anlian Pan

A novel Ag3PO4/CeO2 composite was fabricated by in situ wrapping CeO2 nanoparticles with Ag3PO4

through a facile precipitation method. The photocatalytic properties of Ag3PO4/CeO2 were evaluated by the photocatalytic degradation of MB and phenol under visible light and UV light irradiation. The

photocatalytic activity of the composite is much higher than that of pure Ag3PO4 or CeO2. The rate

constant of MB degradation over Ag3PO4/CeO2 is more than 2 times and 20 times than those of pure

Ag3PO4 and CeO2 under visible light irradiation, respectively. The Ag3PO4/CeO2 composite photocatalyst

also shows higher photocatalytic activity for the colorless phenol degradation compared to pure Ag3PO4.

Moreover, the Ag3PO4/CeO2 sample has almost no loss of photocatalytic activity after ﬁve recycles under the irradiation of visible light and UV light, indicating that the composite has good photocatalytic

stability. The excellent photocatalytic activity of the Ag3PO4/CeO2 composite is closely related to the fast Received 23rd October 2013 transfer and efficient separation of electron–hole pairs at the interfaces of the two semiconductors Accepted 5th November 2013 derived from the matching band positions between CeO2 and Ag3PO4. This newly constructed Ag3PO4/ DOI: 10.1039/c3ta14286h CeO2 composite, with promising and fascinating visible light-driven photocatalytic activity as well as www.rsc.org/MaterialsA good stability, could find potential applications in environmental purification and solar energy conversion.

1. Introduction dyes. The coupled semiconductor with a narrow band gap usually acts as a visible light sensitizer, and the photogenerated Semiconductor-based photocatalysis is a promising avenue to electrons or holes excited by visible light irradiation will trans- 21 ﬃ solve the worldwide energy shortage and environmental pollu- fer to the CeO2. Thus the high e ciency of the interfacial tion using the abundant solar light.1–7 Of the well-known pho- charge transfer as well as the stronger visible light absorption

tocatalysts, cerium dioxide (CeO2), as a fascinating rare earth capacity results in the enhanced activity of the composite – material, has attracted much attention owing to its high activity, photocatalysts.21 25 However, the limited photocatalytic activity low cost and environmentally friendly properties.8–11 It shows is still a barrier for practical applications. Therefore, seeking an promising photoactivity for the degradation of organic pollut- appropriate sensitizer to form CeO2-based composites is a ants and water splitting for hydrogen generation.12 Neverthe- highly crucial task for achieving superior photocatalytic activity. 26,27 less, pristine CeO2 can only be excited by ultraviolet light (UV) Recently, Bi et al. have presented pioneering work on because of its wide band gap (about 3.2 eV), limiting its further exploring the photocatalytic properties of Ag3PO4, which 13 application in the visible light region. In order to highly utilize exhibits extremely high photooxidative capabilities for O2 solar energy, various methods, such as doping, noble metal evolution from water and the decomposition of organic dyes deposition and forming composites, have been designed to under visible-light irradiation. For example, the photo-

enhance the absorption of CeO2 photocatalysts in the visible degradation rate of organic dyes over Ag3PO4 is dozens of times 14–17 light region. Among them, the most eﬀective strategy is the faster than the rate over BiVO4 and commercial TiO2xNx.

coupling of two semiconductors, CeO2 and another semi- Moreover, most interestingly, this novel photocatalyst can 18 ﬃ conductor, to form a composite. So far, various CeO2-based achieve a quantum e ciency of up to 90% at wavelengths 14  composites with a visible light response, such as Cu2O–CeO2, greater than 420 nm, which is signi cantly higher than the 15 19 20 26–28  Bi2O3–CeO2, ZnO–CeO2 and BiVO4–CeO2, have been previously reported values. This nding potentially opens an reported to be eﬃcient for the photodecomposition of organic avenue for solving the current energy crisis and environmental problems using the abundant solar light. Unfortunately, the low

Department of Applied Physics, Key Laboratory for Micro-Nano Physics and Technology structural stability of pure Ag3PO4 strongly limits its practical of Hunan Province, Hunan University, Changsha 410082, China. E-mail: guang@ environmental applications. hnu.edu.cn; [email protected]; [email protected] Considering the unique photocatalytic activity and its † Electronic supplementary information (ESI) available. See DOI: limitations, coupling Ag3PO4 with other photocatalysts could be 10.1039/c3ta14286h

1750 | J. Mater. Chem. A,2014,2,1750–1756 This journal is © The Royal Society of Chemistry 2014 Paper Journal of Materials Chemistry A regarded as a good strategy to design efficient and stable pho- 2.3 Photocatalytic activity test tocatalysts. Moreover, beneting from the relatively small band The photocatalytic behavior for the degradation of methylene gap (the indirect and direct band gaps are 2.36 eV and 2.43 eV, 1 blue (MB, 10 mg L , 80 mL) with 30 mg of the photocatalysts 29 respectively), Ag3PO4 is an ideal candidate to be a sensitizer (CeO2,Ag3PO4 or Ag3PO4/CeO2) under irradiation was explored. which absorbs visible light to improve the photocatalytic A low-power 5 W compact uorescent lamp equipped with a UV activity in the composite system. In this paper, we report a facile cutoff lter (l > 400 nm) and a 300 W UV lamp were chosen as process to synthesize a newly constructed Ag PO /CeO 3 4 2 the visible light and UV light source, respectively. Prior to composite by an in situ precipitation method. The results show irradiation, solutions suspended with photocatalysts were that the novel composite photocatalyst displays much higher sonicated in the dark for 10 min to ensure the adsorption– activity than that of single Ag PO or CeO under the irradiation 3 4 2 desorption equilibrium of MB on the surface of the photo- of visible light as well as UV light. Moreover, the prepared catalysts. During irradiation, the samples were withdrawn at Ag PO /CeO composite shows good stability. This work 3 4 2 regular time intervals and centrifuged to remove the catalysts. provides a possible way to develop new visible light-responsive The photodegradation efficiency was monitored by measuring photocatalysts with excellent activity and good stability, thus the absorbance of the solution samples at its characteristic meeting the requirements of future environmental and energy absorption wavelength of 663 nm (MB) with a UV-Vis spectro- technologies. photometer at room temperature. To further investigate the visible light photocatalytic activity, a colorless compound, phenol, was also chosen as a model 2. Experimental section pollutant since phenol shows no absorption in the visible 2.1 Photocatalyst synthesis region. A 55 W compact uorescent lamp equipped with a UV cutoff lter (l > 400 nm) was used as the light source. The visible CeO2 nanoparticles were synthesized by the low-temperature light photocatalytic activity of the Ag PO or Ag PO /CeO solution combustion method using citric acid as the fuel, 3 4 3 4 2 samples for the degradation of phenol was tested with a UV-Vis followed by annealing at 500 C. The preparation of the spectrophotometer. Ag3PO4/CeO2 composite structures was carried out by an in situ precipitation method. All of the reagents were of analytical grade and used without further purication. In a 3. Results and discussion typical synthesis process, 0.1 g of as-prepared CeO2 nano- 3.1 Morphology and structure characterization of catalyst particles were dispersed in 150 mL deionized water and sonicated for 30 minutes. Immediately aer sonication, SEM was used to investigate the morphology and size of the 1 AgNO3 aqueous solution (100 mL, 0.012 mol L ) was added prepared samples. The typical SEM images of pure CeO2,Ag3PO4 to the white CeO2 dispersed solution, followed by magnetic and the Ag3PO4/CeO2 composite are displayed in Fig. 1. The 1 stirring. Na2HPO4 aqueous solution (200 mL, 0.003 mol L ) morphology of pure CeO2 (Fig. 1(a)) shows spherical-shaped was then added dropwise, accompanied with thorough stir- nanoparticles with diameters of about 40 nm, whereas pure ring until the color of the solution changed from white to Ag3PO4 (Fig. 1(b)) is found to consist of agglomerated grains yellow. The precipitate was centrifuged and washed several composed of irregular particles with approximately 200 nm times with deionized water and absolute ethanol, and dried dimensions. The SEM image in Fig. 1(c) and the TEM observation at 60 C. For comparison, Ag3PO4 particles were also prepared under the same conditions without the presence of CeO2 nanoparticles.

2.2 Characterization The crystallographic structures of the samples were charac- terized by power X-ray diﬀraction (XRD, Siemens D-5000 diﬀractometer with Cu Ka irradiation) and high-resolution transmission electron microscopy (HRTEM, FEI Tecai F20). The morphological details of the prepared samples were pro- bed by eld emission scanning electron microscopy (FESEM, S-4800) and TEM. The optical absorption spectra of the samples were recorded using a UV-Vis spectrometer (UV-2450, Shimadzu). The Brunauer–Emmett–Teller (BET) specic surface area of the samples was analyzed by nitrogen adsorption using a Micromeritics ASAP 2020 nitrogen adsorption apparatus. The photophysics of the excited states generated by absorption was investigated by surface photovoltage spec- Fig. 1 SEM images of (a) CeO2, (b) Ag3PO4, (c) Ag3PO4/CeO2 troscopy (SPS) measurements. composite, and (d) HRTEM image of the Ag3PO4/CeO2 composite.

This journal is © The Royal Society of Chemistry 2014 J. Mater. Chem. A,2014,2,1750–1756 | 1751 Journal of Materials Chemistry A Paper

shown in the inset of Fig. 1(c) reveal that the Ag3PO4/CeO2 composite is composed of two distinct phases. The spherical- shaped spots are CeO2. It is noticeable that the CeO2 nanoparticles are uniformly distributed in the Ag3PO4 crystallites and form a composite structure. To further conrm the crystallographic structure of the Ag3PO4/CeO2 composite, high-resolution TEM (HRTEM) measurements was carried out. The HRTEM images of

Ag3PO4/CeO2 composite in Fig. 1(d) and S1† clearly show two distinct sets of lattice fringes. The uniform lattice fringes have spacings corresponding to the (111) plane of cubic uorite-type

CeO2 and the (210) plane of cubic Ag3PO4, respectively. Fig. 2 illustrates the XRD patterns of the as-prepared pure Fig. 3 UV-Vis absorption spectra of (a) Ag3PO4, (b) Ag3PO4/CeO2 and Ag3PO4, CeO2 and Ag3PO4/CeO2 composite photocatalysts. It is (c) pure CeO2. observed that all of the diﬀraction peaks of pure Ag3PO4 corre- spond to the body-centered cubic structure of Ag3PO4 (JCPDS no.

06-0505), while those of CeO2 can be indexed to the cubic uo- a small fraction (4%) of solar radiation is in the UV region, while rite-type CeO2 structure (JCPDS no. 81-0792). As is expected, the visible light is far more abundant (46%), thus enhancing the

XRD pattern of the Ag3PO4/CeO2 composite clearly matches with photocatalytic capability of semiconductors under visible as well the polycrystalline structures of Ag3PO4 and CeO2, and rules out as UV light irradiation has become an imperative topic to solve the possibility of any third phase formation, indicating that the the worldwide energy shortage and environmental pollution, in

Ag3PO4/CeO2 composite has been successfully prepared. This is order to highly utilize solar energy. In our study, both UV and consistent with the SEM and TEM images. visible light serve as the irradiation source. MB, with a characteristic absorption at 663 nm, is chosen as a typical organic 3.2 UV-Vis absorption spectra pollutant for testing the photocatalytic activity of the as-prepared products. The absorption spectra of MB (ESI, Fig. S2†), with The UV-Vis absorption spectra of CeO2,Ag3PO4 and the Ag3PO4/ 30 mg of the Ag3PO4/CeO2 composite photocatalyst under visible CeO2 composite are displayed in Fig. 3. It can be seen that the light irradiation, clearly show that the characteristic absorption Ag3PO4 sample can absorb UV and visible light with wavelengths less than 500 nm, corresponding to a band gap energy peaks corresponding to MB decrease rapidly as the exposure time increases, indicating the decomposition of MB and the of 2.45 eV, which agrees with the light-absorption properties of 30 signicant reduction in the MB concentration. In this experi- Ag3PO4 powders reported by other groups. The absorption ment, the photodegradation process is studied by monitoring band edge of pure CeO2 is located at approximately 390 nm, the change in MB concentration. The degradation eﬃciency of corresponding to a band gap energy of 3.2 eV. In the case of the MB over pure Ag3PO4, CeO2 and the Ag3PO4/CeO2 composite Ag3PO4/CeO2 composite, except for the characteristic absorp- photocatalyst, or without the photocatalyst, under visible light tion band edge (about 500 nm) of Ag3PO4, a feature band edge irradiation is presented in Fig. 4(a). The results show that the (390 nm) of pure CeO2 appears in the UV light range based on its decrease in the concentration of MB without a photocatalyst is UV-Vis spectrum. This result also suggests that Ag3PO4 and extremely slow; only 0.27% and 1.7% of MB is decolorized in the CeO2 have been composited successfully. dark for 10 min and aer 60 min irradiation, respectively. The negligible decrease in the concentration of MB without a 3.3 Photocatalytic behavior photocatalyst both in the dark and under visible light irradiation 3.3.1 Photocatalytic activity. It is known that solar light is emphasizes the stability of MB. Meanwhile, in the presence of an inexhaustible and unlimited supply of energy in nature. Only photocatalysts, the MB concentration decreases steadily with increasing irradiation time. It can be seen that 98.0% of the MB is photocatalytically degraded aer 60 min irradiation for the

Ag3PO4/CeO2 composite. However, for the pure Ag3PO4 and CeO2 samples, the MB is degraded by only 82.1% and 20.8%, respectively. The kinetic data curves for MB photocatalytic degradation with photocatalysts in the inset of Fig. 4(a) show that the

relationship between ln(C0/Ct)(C0 and Ct are the initial concentration and the concentrations of MB aer irradiation for t min, respectively) and irradiation time is almost linear, suggesting that the photocatalytic reaction follows pseudo-rst- order kinetics. According to the Langmuir–Hinshelwood 31 ¼ model ( ln(Ct/C0) kt), the rate constant k of MB decompo-

sition over the Ag3PO4/CeO2 composite is estimated to be about 1 1 Fig. 2 XRD patterns of the prepared CeO2,Ag3PO4 and Ag3PO4/CeO2 0.0627 min , faster than that with Ag3PO4 (0.0276 min ) and 1 composite. pure CeO2 (0.0031 min ) by a factor of 2.27 and 20.23,

1752 | J. Mater. Chem. A,2014,2,1750–1756 This journal is © The Royal Society of Chemistry 2014 Paper Journal of Materials Chemistry A

photocatalyst under visible light irradiation for various dura- tions. It can be seen that the absorption peaks corresponding to phenol decrease rapidly as the exposure time increases, indicating the decomposition of phenol. Thus, the characteristic absorption of phenol at 270 nm is chosen as the monitored parameter for the photocatalytic degradation process. The

degradation of phenol with Ag3PO4 or Ag3PO4/CeO2 photocatalysts under visible light irradiation is presented in Fig. 5(b). It is clear that the concentration of phenol decreases with the

irradiation time. Moreover, the Ag3PO4/CeO2 composite photocatalyst shows slightly higher photocatalytic activity for

colorless phenol degradation compared to pure Ag3PO4. It is demonstrated that the photocatalytic activity is mainly governed by structural features, light absorption ability and the charge-carrier dynamics. As is discussed above, the crystal

phase structures of CeO2 and Ag3PO4 in the Ag3PO4/CeO2 composite show no change, while the absorption ability of the

Ag3PO4/CeO2 composite is lower than that of pure Ag3PO4.In  addition, the speci c surface area of pure Ag3PO4 is evaluated to be about 2.8 m2 g 1 by BET measurement, which is a little larger 2 1 than that of the Ag3PO4/CeO2 composite (about 2.1 m g ).

Thus the possible reason for the high performance of Ag3PO4/

CeO2 could be attributed to the effective transfer and separation Fig. 4 MB concentration changes over photocatalyst-free solution, of the electron–hole pairs due to the composite formation CeO2,Ag3PO4 and the Ag3PO4/CeO2 composite: (a) under visible light irradiation, (b) under UV light irradiation. between Ag3PO4 and CeO2, which greatly enhances the photocatalytic activity. This mechanism can be conrmed by the results of the SPS response. As the direct result of the separation respectively. The results demonstrate that the degradation of electron–hole pairs, SPS is regarded as an effective approach ffi e ciency of the Ag3PO4/CeO2 composite to MB is much higher to evaluate the separation capacity of the photoinduced charges than those of pure Ag3PO4 and CeO2 under visible light irradi- with the aid of light. Moreover, SPS signals can not only result ation. Therefore, we can conclude that the formation of the from band-to-band transitions, but also from sub-band transi-

Ag3PO4/CeO2 composite structure facilitates the enhanced tions. The SPS responses of pure CeO2,Ag3PO4 and the Ag3PO4/ photocatalytic activity. CeO2 composite are shown in Fig. 6. We observe a relatively low

Although CeO2 has little capacity for MB degradation under visible light irradiation due to its higher band gap energy, it shows high photocatalytic capability in the UV region.8,13 To ﬀ investigate the e ect of the coupling Ag3PO4 sensitizer on the photocatalytic capability of CeO2, the photocatalytic degradation of MB over CeO2,Ag3PO4 and the Ag3PO4/CeO2 composite under 300 W UV irradiation has also been performed. As is shown in Fig. 4(b), CeO2 and Ag3PO4 show only 40.6% and

92.4% MB degradation, whereas the Ag3PO4/CeO2 composite renders 98.9% MB degradation aer 6 min of photocatalytic reaction. Thus the Ag3PO4/CeO2 composite exhibits the highest photocatalytic degradation eﬃciency, followed by the pure

Ag3PO4 and CeO2 photocatalysts. Additionally, the apparent  rst-order rate constant k for MB decomposition by the Ag3PO4/ CeO2 composite under UV light irradiation is about 0.7255 1 1 min , faster than that with Ag3PO4 (0.4091 min ) and pure 1 CeO2 (0.1381 min ) by a factor of 1.77 and 5.25, respectively. As it is well known that MB may absorb visible light, the sensitization possibility for samples should be considered. Therefore, the photocatalytic activity of the samples was also evaluated by colorless phenol degradation to ensure the visible Fig. 5 (a) Absorption spectra of phenol solution in the presence of the light photocatalytic activity and exclude the dye sensitization Ag3PO4/CeO2 composite photocatalyst under visible light irradiation. under visible light. Fig. 5(a) shows the absorption spectra of an (b) Comparison of the photocatalytic activities of Ag3PO4 and Ag3PO4/ aqueous solution of phenol with 30 mg of the Ag3PO4/CeO2 CeO2 photocatalyst samples for the photodegradation of phenol.

This journal is © The Royal Society of Chemistry 2014 J. Mater. Chem. A,2014,2,1750–1756 | 1753 Journal of Materials Chemistry A Paper

photovoltaic response for pure CeO2, which features a strong peak located at about 325 nm (P1) and a weak peak located at about 360 nm (P2). The P1 peak can be attributed mainly to the electron transition from O 2p in the bulk to Ce 4f, while the P2 peak can be assigned to the electron transition from O 2p on the 32 surface to Ce 4f. The SPS of the Ag3PO4 sample gives a strong and broad SPS response band in the range 300–500 nm with peaks appearing at about 390 and 450 nm. The Ag3PO4/CeO2 composite sample displays the SPS characteristic response of

Ag3PO4 and CeO2, suggesting the coexistence of Ag3PO4 and

CeO2. Moreover, the combining of Ag3PO4 and CeO2 results in the slight shi of the peaks to longer wavelengths, indicating the heterojunction formation in the prepared Ag3PO4/CeO2 composite instead of a simple mixture of Ag3PO4 and CeO2.In addition, the characteristic P2 peak of CeO2 is hardly detectable, suggesting that CeO2 is studded in Ag3PO4 crystallites and mainly acts as a core material in the prepared Ag3PO4/CeO2 composite sample, which is consistent with the SEM and TEM images (see Fig. 1(c)). Moreover, it is noticeable that there is an obviously broader and stronger photovoltaic response for the

Ag3PO4/CeO2 composite than that of Ag3PO4 or CeO2 alone. In general, the stronger SPS response corresponds to the higher separation rate of photoinduced charge carriers on the basis of Fig. 7 Cycling runs of the Ag3PO4/CeO2 composite and Ag3PO4 in the the SPS principle. Therefore, the stronger SPS intensity of the photodegradation of MB: (a) under visible light irradiation, (b) under UV Ag3PO4/CeO2 composite indicates that the photogenerated light irradiation. – electron hole pairs are easily separated in the Ag3PO4/CeO2 composite than in the pure CeO2 and Ag3PO4 samples. As a ﬃ consequence, more e cient carriers will participate in the Ag3PO4 photocatalyst is unstable under visible light illumina- photocatalytic reaction and enhance the photocatalytic activity. tion. Fig. 7(b) illustrates cycling runs of the Ag3PO4/CeO2

3.3.2 Photocatalytic stability. To study the photocatalytic composite and Ag3PO4 in the photodegradation of MB under UV stability of the prepared Ag3PO4/CeO2 composite, recycle light irradiation. As shown in Fig. 7(b), the rate of MB degra- degradation experiments of MB were performed under visible dation over pure Ag3PO4 decreases signicantly aer four light irradiation, as shown in Fig. 7(a). It is clear that the high degradation cycles, while the photocatalytic activity of the photocatalytic performance of the as-prepared Ag3PO4/CeO2 as-prepared Ag3PO4/CeO2 composite still maintains a high level, composite has no detectable loss aer ve recycles, indicating even aer it has been used 5 times. Therefore, the photo- the high photocatalytic stability of the Ag3PO4/CeO2 composite catalytic activity of Ag3PO4/CeO2 is more stable than that of under visible light irradiation. On the contrary, the photo- Ag3PO4 for the photocatalytic degradation of MB solutions catalytic performance of Ag3PO4 shows an obvious loss, and the under irradiation. These results demonstrate that the Ag3PO4/  photocatalytic activity decreases to 74.66% and 66.54% a er CeO2 composite, formed by the coupling of CeO2 and Ag3PO4, four and ve recycles respectively, suggesting that the pure shows excellent photocatalytic performance, as well as good

stability. The origin of the improved stability of the Ag3PO4/

CeO2 composite under visible light irradiation may result from metallic Ag0 species that are produced in the initial stage of the

photocatalytic oxidation process, due to the reduction of Ag3PO4

and the formation of an Ag/Ag3PO4/CeO2 sandwich-like structure. As photons from the visible-light irradiation are absorbed

by Ag3PO4, electrons are excited to the conduction bands (CB), resulting in the generation of electron–hole pairs. The photo-

induced holes in the Ag3PO4 particles may quickly transfer to

the valence band (VB) of the CeO2 particles, whereas photoin-

duced electrons migrate to the surface of the Ag3PO4 particles 0 and lead to the reduction of Ag3PO4 and the formation of Ag species. The formed Ag0 species deposit on the surface of the

Ag3PO4 catalyst and form the Ag/Ag3PO4/CeO2 sandwich-like structure. As is known, metallic Ag0 can serve as an excellent ﬃ Fig. 6 The surface photovoltage spectra of Ag3PO4, CeO2 and the electron acceptor and e ciently trap the photoinduced elec- Ag3PO4/CeO2 composite. trons. Therefore, the localized surface plasmon resonance

1754 | J. Mater. Chem. A,2014,2,1750–1756 This journal is © The Royal Society of Chemistry 2014 Paper Journal of Materials Chemistry A produced by the collective oscillations of surface electrons on the Ag0 species could induce an enhancement of the local inner electromagnetic eld. Due to the local electromagnetic eld and the excellent conductivity of the Ag0 species, the photoinduced electrons can be transferred quickly to the Ag0 species, instead + of remaining in the Ag ions of the Ag3PO4 lattice. These 0 electrons trapped by Ag species will further react with the O2 molecules adsorbed on the surface of the composite to form active cOH radicals. Therefore, the further reduction of Ag3PO4 could be inhibited. Similar results have been reported for

Ag3PO4/AgBr, Ag3PO4/TiO2 and Ag/Ag3PO4 composite materials.23,33,34 This provides a new way to design eﬃcient and stable visible light photocatalysts and thus meet the requirements for future environmental and energy technologies. Fig. 8 Schematic diagram of the photoexcited electron–hole sepa-

ration process in the Ag3PO4/CeO2 composite photocatalysts under 3.4 Mechanism of the photocatalytic activity of Ag3PO4/CeO2 visible-light irradiation. composite It is well known that the photocatalytic activity is largely aﬀected c ﬃ by the recombination of the photoinduced electrons and holes. OH radicals. The more e cient charge-carrier separation and increased cOH radical formation leads to the enhanced photo- In the case of pure Ag3PO4, the photoinduced electrons and holes can recombine rapidly because of its narrow band gap. catalytic activity of the Ag3PO4/CeO2 composite.

However, in the case of the Ag3PO4/CeO2 composite, the photoinduced carriers can transfer easily between CeO2 and 4. Conclusions Ag PO due to their matching band positions. The band edge 3 4 ﬃ positions of the CB and VB of a semiconductor can be deter- In conclusion, we have demonstrated a facile and e cient mined using the following equations: process for fabricating an Ag3PO4/CeO2 composite by an in situ precipitation method. The prepared Ag3PO4/CeO2 composite

EVB ¼ c Ee + 1/2 Eg exhibits excellent photocatalytic activity for organic contami- nant degradation under visible light and UV light irradiation.

ECB ¼ EVB Eg The rate constant of MB degradation over the Ag3PO4/CeO2

composite is much faster than those over pure Ag3PO4 and CeO2 where c is the absolute electronegativity of the semiconductor by a factor of 2.27 and 20.23 under visible-light irradiation, c ( is 5.57 eV and 6.2 eV for CeO2 and Ag3PO4, respectively), Ee is respectively. In addition, the Ag3PO4/CeO2 composite photo- the energy of free electrons on the hydrogen scale (4.5 eV) and Eg catalyst also shows higher photocatalytic activity for colorless is the band gap of the semiconductor. The calculated CB and VB phenol degradation compared to pure Ag3PO4. The enhanced of Ag3PO4 are 0.45 and 2.9 eV respectively, which are more photocatalytic activity can be attributed to the easier transfer positive than those of CeO2 ( 0.53 and 2.67 eV, respectively). and separation of photogenerated electron–hole pairs between

Based on the alignments of the levels of Ag3PO4 and CeO2,an Ag3PO4 and CeO2 due to their matching band positions. illustration of possible interparticle electron transfer behavior Moreover, the composite shows superior photocatalytic stability is proposed, as shown in Fig. 8. Under visible or UV light irra- to that of pure Ag3PO4. This work provides a simple strategy to diation, photon-generated holes in the Ag3PO4 particles quickly design novel photocatalysts with high eﬃciency and stability for transfer to the VB of the CeO2 particles, whereas photon- photocatalytic applications. generated electrons migrate to the surface of the Ag3PO4 particles. Subsequently, the holes in the VB of CeO2 can oxidize OH Acknowledgements or H2O to form cOH radicals, which are then involved in the photocatalytic degradation reaction of MB. Meanwhile, the This work is supported by the Science and Technology Plan accumulated electrons in the CB of Ag3PO4 can be transferred to Projects of Hunan Province (2013SK3148) and the Hunan O2 molecules adsorbed on the surface of the composite semi- Provincial Natural Science Foundation of China (Grant no. conductors and yield H2O2 because the CB level of Ag3PO4 is 12JJ3009). more negative than the standard redox equilibrium potential 35 EQ(O2/H2O2) (0.682 eV vs. NHE). H2O2 reacts with electrons in References succession to produce active cOH radicals. These formed hydroxyl radicals will further oxidize the MB molecules. One can 1 A. Fujishima and K. Honda, Nature, 1972, 238,37–38. conclude, therefore, that the coupling of CeO2 and Ag3PO4 to 2 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, form a composite is helpful to improve the transfer of photo- Science, 2001, 293, 269–271. excited electron–hole pairs, inhibit the recombination of 3 X. W. Lou, L. A. Archer and Z. C. Yang, Adv. Mater., 2008, 20, photoinduced carriers and facilitate the production of more 3987–4019.

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