- Email: [email protected]
Bi3.64Mo0.36O6.55 nanospheres with enhanced photocatalytic activity under visible light irradiation
Separation and Purification Technology 156 (2015) 875–880
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Short Communication
Heterostructured Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanospheres with enhanced photocatalytic activity under visible light irradiation Xue Lin, Jing Hou, Xiaoyu Guo, Yushuang Wang, Jia Zheng, Chang Liu, Yang Yang, Guangbo Che ⇑ Key Laboratory of Preparation and Application Environmentally Friendly Materials of Ministry of Education, Jilin Normal University, Siping 136000, PR China
a r t i c l e
i n f o
Article history: Received 16 September 2015 Received in revised form 15 October 2015 Accepted 20 October 2015 Available online 21 October 2015 Keywords: Ag3PO4/Ag/Bi3.64Mo0.36O6.55 Heterostructure Photocatalysis Visible light
a b s t r a c t Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanoheterostructure with enhanced photocatalytic activity was successfully synthesized for the first time via a simple and practical low-temperature solution-phase route. The as-prepared Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanoheterostructure included Ag3PO4 quantum dots (QDs) as well as Ag QDs assembling on the surface of Bi3.64Mo0.36O6.55 nanospheres. Ag QDs were photodeposited on the surface of Bi3.64Mo0.36O6.55 nanospheres to increase visible-light absorption via the surface plasmon resonance. The interface between Ag/Ag3PO4 and Bi3.64Mo0.36O6.55 facilitates the separation and transfer of photoinduced charges. The Ag3PO4/Ag/Bi3.64Mo0.36O6.55 hybrid photocatalyst degraded rhodamine B (Rh B) efficiently and displayed much higher photocatalytic activity than pure Bi3.64Mo0.36O6.55, Ag3PO4, Ag/Ag3PO4, Ag/Bi3.64Mo0.36O6.55 or Ag3PO4/Bi3.64Mo0.36O6.55 under visible light irradiation (k > 420 nm). It indicates that the heterostructured combination of Ag3PO4, Ag, and Bi3.64Mo0.36O6.55 provides synergistic photocatalytic activity through an efficient electron transfer process. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The exploration and construction of new photocatalysts with high catalytic efficiency under visible light is an important issue in photocatalysis all the time, and is also significant in solving current environment and energy problems [1–4]. Among these works, Ag-based photocatalysts had attracted much interest for their excellent photocatalytic performances, such as AgX (X = Cl, Br and I) [5–7], Ag2O [8], Ag2CO3 [9], Ag6Si2O7 [10] and Ag3PO4 [11,12]. Ag3PO4 is extremely attractive due to that it can achieve quantum efficiency up to 90% for oxidize water (O2 evolution) as well as photodegradation of organic contaminant under visible light irradiation. However, Ag3PO4 is slightly soluble in aqueous solution, and it will be photo-corroded and decompose to weakly active Ag during the photocatalytic process. Moreover, the by-product Ag particles usually generate from the photodecomposition of Ag3PO4 in the absence of electron acceptor during the photocatalytic process, which would decrease its visible light absorption and photocatalytic activity [13–16]. In order to improve the photocatalytic performance of Ag3PO4, several kinds of materials have been cooperated with Ag3PO4 to generate hybrid photocatalysts, for instance semiconductor materials (TiO2, ZnO, SnO2, BiMoO4, Cr–SrTiO3, Bi2MoO6, g-C3N4 [17–27]) and carbon ⇑ Corresponding author. E-mail address: [email protected] (G. Che). http://dx.doi.org/10.1016/j.seppur.2015.10.049 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
materials (oxidized graphene, graphene and carbon quantum dots [28–31]). The Ag3PO4 mixed with carbon materials can facilitate electron transport and hinder the electron–hole recombination due to the excellent electrical conductivity of carbon materials. However, the carbon materials generally have a more negative conduction band potential than that of Ag3PO4, which prevents the photogenerated electrons transfer from Ag3PO4 to the carbon materials. Thus, the hybrids with semiconductors are considered to be ideal photocatalysts because they can improve the optical absorption property and enhance the photogenerated charge separation. But the obtained heterostructures usually suffer from the less reducibility and oxidability of the remaining electrons and holes. Recently, with more and more attention has been paid to the mechanism of Ag3PO4-based hybrid photocatalytic systems, the by-product Ag has been found to be a electron transmission bridge in the Ag/Ag3PO4-based systems [32,33]. It has been reported that the Ag/Ag3PO4-based photocatalysts can not only facilitate the charge separation but also retain the high reducibility and oxidability of the remaining electrons and holes for the corresponding photocatalysts. Thus, it is important to search for suitable components that can construct novel Ag/Ag3PO4-based photocatalysts and largely enhance their photocatalytic performances. Bi3.64Mo0.36O6.55, with a band gap of around 2.60 eV, has been reported to be a visible-light induced photocatalyst [34]. There are a few investigations concerning Bi3.64Mo0.36O6.55 as an active
876
X. Lin et al. / Separation and Purification Technology 156 (2015) 875–880
photocatalyst [34,35]. However, similar to other semiconductor photocatalysts, the high-rate recombination of electron/hole (e/ h+) pairs and poor visible light absorption efficiency are still great challenges to enhance the photocatalytic performance of pure Bi3.64Mo0.36O6.55 in order to meet the practical application requirements [36,37]. Herein, Bi3.64Mo0.36O6.55 was chosen to combine with Ag3PO4, and a novel high-efficiency Ag3PO4/Ag/Bi3.64Mo0.36O6.55 hybrid photocatalyst was demonstrated for the first time. The photocatalytic activity of the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 hybrid photocatalyst was evaluated by degradation rhodamine B (Rh B) under visible light (k > 420 nm). It is shown that a heterostructure composed of Ag3PO4, Ag and Bi3.64Mo0.36O6.55 contributes to the enhanced performance of the hybrid photocatalyst. 2. Experimental 2.1. Preparation of photocatalysts 2.1.1. Preparation of Bi3.64Mo0.36O6.55 nanospheres All reagents for synthesis and analysis were commercially available and used without further treatments. Bi3.64Mo0.36O6.55 was prepared based on a facile hydrothermal method. A typical synthesis process is as follows: Bi(NO3)35H2O (2 mmol) was dissolved in 10 mL distilled water under magnetic stirring for 30 min. (NH4)6Mo7O244H2O (1 mmol) was dissolved in 5 mL distilled water, and added dropwise into the Bi(NO3)3 solution, and the pH of the mixed solution was adjusted to 13 through the adding of 2 mol L1 NH3H2O solution. After being stirred for 30 min at room temperature, the mixture was subject to ultrasonic treatment for 30 min and then to hydrothermal treatment in a 20 mL Teflon-lined stainless steel autoclave at 160 °C for 24 h. At last, the obtained particles were collected and washed with ethanol and distilled water several times, and dried at 100 °C for 2 h. 2.1.2. Preparation of Ag3PO4/Ag/Bi3.64Mo0.36O6.55 photocatalyst The Ag3PO4/Ag/Bi3.64Mo0.36O6.55 hybrid photocatalyst was synthesized through an in-situ precipitation method at room temperature. In a typical process, Bi3.64Mo0.36O6.55 nanospheres (100 mg) were dispersed in 100 mL deionized water by ultrasound for 30 min, and then 50 mL 0.0075 M AgNO3 aqueous solution was dropped into the Bi3.64Mo0.36O6.55 dispersed solution. After stirring for 15 min, a 50 mL of 0.0025 M Na3PO4 aqueous solution was added to the above solution drop by drop under magnetic stirring. The pH value was adjusted to 3 by adding 1.0 M NaOH. The resulting suspension was stirred in the dark for another 30 min. Then, the suspension was irradiated by a 300 W Xe lamp equipped with an optical cut-off filter (k > 420 nm) for 30 min. Finally, the precipitate was washed with deionized water for 3 times and collected by centrifugation, and then dried at 60 °C in the vacuum drying oven. The theoretical value of Ag3PO4 loading amount was 20 wt%. For comparison, pure Ag3PO4, Ag/Bi3.64Mo0.36O6.55, Ag/Ag3PO4, and Ag3PO4/Bi3.64Mo0.36O6.55 samples were prepared under the same conditions.
England) using 300 W Al Ka radiation. The photoluminescence (PL) spectra of the photocatalysts were obtained by a F4500 (Hitachi, Japan) photoluminescence detector with an excitation wavelength of 325 nm. The UV–vis diffuse reflectance spectra (DRS) were recorded using a scan UV–vis spectrophotometer (UV-2550). 2.3. Photocatalytic activities studies The photocatalytic properties of the as-prepared samples were evaluated using Rh B as a model compound. The Rh B is a very stable compound, which has been used widely as a representative reaction for examining the performance of numerous visible light active catalysts. In experiments, the Rh B solution (0.01 mmol L1, 100 mL) containing 0.02 g of photocatalyst was mixed in a Pyrex reaction glass. A 300 W Xe lamp (k > 420 nm) was employed to provide visible light irradiation. A 420 nm cut-off filter was inserted between the lamp and the sample to filter out UV light (k < 420 nm). Prior to visible light illumination, the suspension was strongly stirred in the dark for 40 min. Then the solution was exposed to visible light irradiation under magnetic stirring. At given time intervals, 4 mL of the suspension was periodically collected and analyzed after centrifugation. The Rh B concentration was analyzed by a UV-2550 spectrometer to record intensity of the maximum band at 552 nm in the UV–vis absorption spectra. 3. Results and discussion Fig. 1 displays the XRD pattern of the Ag3PO4/Ag/Bi3.64Mo0.36 O6.55 hybrid photocatalyst, together with those of Ag3PO4, Bi3.64Mo0.36O6.55, Ag/Bi3.64Mo0.36O6.55, Ag/Ag3PO4, and Ag3PO4/Bi3.64 Mo0.36O6.55. All the peaks in the XRD pattern of Bi3.64Mo0.36O6.55 can be readily indexed into specific crystal planes of Bi3.64Mo0.36O6.55 phase (JCPDS No. 43-0446), which is in accordance with the reported works [34,35]. For Ag3PO4, all the diffraction peaks can be indexed to specific crystal planes of Ag3PO4 phase (JCPDS No. 06-0505). It is observed that no peaks assigned to Ag0 or Ag3PO4 species were detected in the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 sample due to the small crystal size or low percentage of Ag and Ag3PO4. Herein, the successful loading of Ag and Ag3PO4 was verified by XPS, as revealed in Fig. 2. The Ag 3d peaks of Ag3PO4/Ag/Bi3.64Mo0.36O6.55 have separated as Ag+ peaks and Ag0 peaks (Fig. 2a). The weak peaks at 368.40 and 374.66 eV are attributed to Ag0 [32], indicating the existence of metallic Ag on the surface of Bi3.64Mo0.36O6.55 nanospheres. The strong peaks at 368.00 and 374.00 eV are assigned to Ag+ of Ag3PO4 [32]. A broad peak in the range of 130–134 eV of the P 2p spectrum (Fig. 2b) is observed for the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 sample
2.2. Characterization of photocatalysts The crystal structures of the samples were characterized by X-ray diffraction (XRD) on a Rigaku (Japan) D/max 2500 X-ray diffractometer (Cu Ka radiation, k = 0.154 18 nm). The morphologies and structure details of the as-synthesized samples were detected using field emission scanning microscopy (SEM, JSM6510) and transmission electron microscopy (TEM, JEM-2100F). X-ray photoelectron spectroscopy (XPS) analysis was performed with an ESCALa-b220i-XL electron spectrometer (VGScientific,
Fig. 1. XRD patterns of the as-prepared Ag3PO4, Bi3.64Mo0.36O6.55, Ag/Bi3.64Mo0.36O6.55, Ag/Ag3PO4, Ag3PO4/Bi3.64Mo0.36O6.55, and Ag3PO4/Ag/Bi3.64Mo0.36O6.55 samples.
X. Lin et al. / Separation and Purification Technology 156 (2015) 875–880
877
Fig. 2. XPS spectra of the as-obtained Ag3PO4/Ag/Bi3.64Mo0.36O6.55 sample: (a) Ag 3d spectrum, (b) P 2p spectrum, (c) Bi 4f spectrum, (d) Mo 3d spectrum.
which is corresponding to the phosphorus of Ag3PO4 [32]. The Bi 4f fine XPS spectrum of the sample is shown in Fig. 2c. XPS signals of Bi 4f are observed at binding energies at about 164.53 eV (Bi 4f7/2) and 159.18 eV (Bi 4f5/2), ascribed to Bi3+ [27]. The characteristic spin– orbital splitting photoelectrons for Mo 3d (235.31 and 232.20 eV) indicate a six-valent oxidation state for Mo6+ [27] (Fig. 2d). The SEM images of the as-prepared Bi3.64Mo0.36O6.55 and Ag3PO4/Ag/Bi3.64Mo0.36O6.55 samples are shown in Fig. 3(a) and (b). The bare Bi3.64Mo0.36O6.55 crystals are composed of nanospheres with an average diameter of tens of nanometers. The Ag3PO4/Ag/ Bi3.64Mo0.36O6.55 sample shows a similar morphology to pure Bi3.64Mo0.36O6.55, which indicates that low amount Ag3PO4 and Ag loading didn’t have any influence on the morphology of Bi3.64Mo0.36O6.55 crystals. In order to further ascertain the heterostructure among Ag3PO4, Ag, and Bi3.64Mo0.36O6.55, Ag3PO4/ Ag/Bi3.64Mo0.36O6.55 sample as well as pure Bi3.64Mo0.36O6.55 was investigated by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM), as revealed in Fig. 3(c–e). Fig. 3c shows the spherical structure of the assynthesized Bi3.64Mo0.36O6.55 sample. From Fig. 3d, it can be observed that Ag3PO4 QDs as well as Ag QDs were loaded on the surface of Bi3.64Mo0.36O6.55 nanospheres. HRTEM image further confirm the formation of a novel ternary heterostructure (Fig. 3e). An interconnected fine particulate morphology observed indicates the existence of the ternary heterostructure. By measuring the lattice fringes, the resolved interplanar distances are about 0.19, 0.213, and 0.325 nm, which correspond to the (2 0 2) plane of Ag, the (2 1 0) plane of Ag3PO4, and the (1 1 1) plane of Bi3.64Mo0.36O6.55, respectively.
The optical absorption properties of the photocatalysts were studied by UV–vis DRS spectra. As shown in Fig. 4(a), bare Bi3.64Mo0.36O6.55 exhibits strong absorbance in wavelengths shorter than 460 nm, and bare Ag3PO4 shows strong absorbance in wavelengths shorter than 550 nm. After Ag3PO4 is wrapped on Bi3.64Mo0.36O6.55 nanospheres, the optical absorption of Ag3PO4/Bi3.64Mo0.36O6.55 hybrid photocatalyst is enhanced, which is due to the loading of Ag3PO4. Ag/Bi3.64Mo0.36O6.55 and Ag/Ag3PO4 samples show obvious visible-light absorption, which can be attributed to the surface Plasmon resonance (SPR) of the loading Ag, further confirming the formation of Ag. The absorption curve of the heterostructured Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanospheres shows distinctly enhanced visible-light absorption compared to the pure Bi3.64Mo0.36O6.55 and Ag3PO4/Bi3.64Mo0.36O6.55 samples. The band gap energies of semiconductors can be estimated by Kubelka–Munk transformation, ahv = A(hv Eg)n/2, where a represents the absorption coefficient, t is the light frequency, Eg is the band gap energy, A is a constant and n depends on the characteristics of the transition in a semiconductor (n = 1 for a direct transition and n = 4 for an indirect transition). For Ag3PO4 and Bi3.64Mo0.36O6.55, the value of n is 1 for the direct transition. Thus, as shown in Fig. 4b, the band gap energies (Eg) of pure Ag3PO4 and Bi3.64Mo0.36O6.55 are calculated to be 2.00 and 2.60 eV, respectively. The potentials of VB and CB of a semiconductor material can be estimated according to the following empirical equations:
EVB ¼ X Ee þ 0:5Eg
ð1Þ
ECB ¼ EVB Eg
ð2Þ
878
X. Lin et al. / Separation and Purification Technology 156 (2015) 875–880
Fig. 3. SEM images of Bi3.64Mo0.36O6.55 (a), Ag3PO4/Ag/Bi3.64Mo0.36O6.55 (b), TEM images of Bi3.64Mo0.36O6.55 (c), Ag3PO4/Ag/Bi3.64Mo0.36O6.55 (d), HRTEM image of Ag3PO4/Ag/ Bi3.64Mo0.36O6.55 (e).
where EVB is the valence band edge potential, ECB is the conduction band edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, and the values of X for Ag3PO4 and Bi3.64Mo0.36O6.55 are ca. 6.72 and 5.70 eV, respectively, Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV), Eg is the band gap energy of the semiconductor. Using the Eqs. (1) and (2), the EVB potentials of Ag3PO4 and Bi3.64Mo0.36O6.55 are calculated to be 2.47 and 2.50 eV, respectively. The ECB potentials of Ag3PO4 and Bi3.64Mo0.36O6.55 are determined to be 0.47 and 0.10 eV, respectively. The energy band structure diagram of Ag3PO4, Ag, and Bi3.64Mo0.36O6.55 is thus schematically illustrated, as shown in Fig. 4c. Under visible light irradiation, Bi3.64Mo0.36O6.55, as well as Ag3PO4 can be excited to produced h+ and e. The electrons on the CB of Ag3PO4 can effectively migrate to metal Ag through the Schottky barrier (electron transfer I: Ag3PO4 CB ? Ag) because the CB potential of Ag3PO4 is more negative than the Fermi level of the loaded metal Ag, and electron transfer in this process is faster
than the electron–hole recombination between the VB and CB of Ag3PO4. Therefore, the electrons in the CB of Ag3PO4 can be stored in the Ag component. In addition, because the energy level of Ag is above the VB of Bi3.64Mo0.36O6.55, the holes in the VB of Bi3.64Mo0.36O6.55 can also flow easily into the Ag metal (hole transfer II: Bi3.64Mo0.36O6.55 VB ? Ag), and this process is faster than the electron–hole recombination between the VB and CB of Bi3.64Mo0.36O6.55. As a result, the holes in the VB of Ag3PO4 with strong oxidation power can participate in photocatalytic reactions to degrade organic pollutants directly, and moreover, the electrons in the CB of Bi3.64Mo0.36O6.55 with strong reduction power can be transferred to the adsorbed O2, which acts as an electron acceptor to form first superoxide radical anions O 2 , and finally OH radicals [38]. These radicals are strong oxidants that can completely oxidize Rh B molecules to water and carbon dioxide [39]. Therefore, synchronous electron transfer (vectorial electron transfer of Ag3PO4 ? Ag ? Bi3.64Mo0.36O6.55) should take place under visible light excitation of both Bi3.64Mo0.36O6.55 and Ag3PO4, in agreement with
X. Lin et al. / Separation and Purification Technology 156 (2015) 875–880
879
Fig. 4. (a) UV–vis DRS of as-synthesized samples. (b) the plots of (ahv)2 versus photon energy (hm) for the band gap energies of Ag3PO4 and Bi3.64Mo0.36O6.55. (c) Schematic diagram of the separation and transfer of photogenerated charges in the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 heterostructure under visible light irradiation.
previous reports [32,33]. In this vectorial electron-transfer process, the Ag metal in the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanojunction system acts as a storage and/or recombination center for CB electrons of Ag3PO4 and VB holes of Bi3.64Mo0.36O6.55, and contributes to the enhancement of interfacial charge transfer and separation. The photocatalytic activities of the as-prepared samples were evaluated by degradation of Rh B under visible light, as shown in Fig. 5a. Before light irradiation, the solution of Rh B and photocatalyst was magnetically stirred in dark for 40 min to establish an adsorption desorption equilibrium. The decrease of the Rh B concentration due to the adsorption of Ag3PO4/Ag/Bi3.64Mo0.36O6.55 (about 20%) is much higher than that of pure Bi3.64Mo0.36O6.55 sample (around 4%), indicating that Ag3PO4 QD and Ag QD loading can enhance the absorption of organic molecules on the surface of photocatalyst. From the catalytic experiments, Ag3PO4/Ag/ Bi3.64Mo0.36O6.55 was detected to be more photoactive toward Rh B solution than the pure Bi3.64Mo0.36O6.55, Ag3PO4, Ag/Bi3.64Mo0.36 O6.55, Ag/Ag3PO4, and Ag3PO4/Bi3.64Mo0.36O6.55 samples; almost 100% was eliminated from the solution in 80 min. The Ag3PO4 QD and Ag QD coating can improve the visible light absorption efficiency (Fig. 4a), which is beneficial for the nanoheterostructure to photolyze Rh B. In addition, efficient heterostructure interface between two or three components can restrain the recombination of photoinduced charges effectively, thus enhancing the photocatalytic performance of the Ag3PO4/Ag/ Bi3.64Mo0.36O6.55. The comparison of PL spectra of the as-prepared samples under the excitation wavelength of 325 nm is shown in Fig. 5b. It can be
observed that the PL peak intensity of Ag3PO4/Ag/Bi3.64Mo0.36O6.55 decrease obviously. This result shows that the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanoheterostructure contributes to the effective electron–hole pair separation, which may be a reason for the Ag3PO4/ Ag/Bi3.64Mo0.36O6.55 sample showing enhanced photocatalytic efficiency under visible light irradiation. In addition to photocatalytic activity, the stability of photocatalyst is another important issue. Four runs of cycling photodegradation experiments have been carried out for Ag3PO4/Ag/Bi3.64Mo0.36O6.55 (Fig. 5c) to investigate the stability of the photocatalyst. The Ag3PO4/Ag/Bi3.64Mo0.36O6.55 sample still maintained a high level of degradation performance in the repeated experiments, revealing that the photocatalyst showed good stability during the photocatalytic reaction.
4. Conclusions In summary, a novel Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanoheterostructure, in which Bi3.64Mo0.36O6.55 nanospheres was decorated with Ag3PO4 QD and Ag QD, has been synthesized by a facile low-temperature solution-phase method. The Ag3PO4/Ag/ Bi3.64Mo0.36O6.55 nanospheres exhibited superior photocatalytic activity to bare Bi3.64Mo0.36O6.55, Ag3PO4, Ag/Bi3.64Mo0.36O6.55, Ag/ Ag3PO4, and Ag3PO4/Bi3.64Mo0.36O6.55 in degradation of Rh B under visible light irradiation. This improvement is attributed to the migration efficiency of the photoinduced carriers at the interface of the heterostructure and the enhanced efficiency of photoharvesting owing to the stable deposition of Ag and Ag3PO4.
880
X. Lin et al. / Separation and Purification Technology 156 (2015) 875–880
Fig. 5. (a) Photodegradation efficiencies of Rh B as a function of irradiation time for different photocatalysts. (b) Room temperature PL spectra of as-synthesized samples. (c) Cycling runs for the photocatalytic degradation of Rh B over the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 sample under visible light irradiation.
Acknowledgment This work was supported by National Natural Science Fundation of China (21407059, 21576112), the Science Development Project of Jilin Province (20130522071JH, 20130521019JH), and the Science and Technology Research Project of the Department of Education of Jilin Province (2015220). References [1] S. Fukuzumi, T. Kobayashi, T. Suenobu, Angew. Chem. Int. Edit. 50 (2011) 728– 731. [2] H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Adv. Mater. 24 (2012) 229–251. [3] X. Li, R. Huang, Y. Hu, Y. Chen, W. Liu, R. Yuan, Z. Li, Inorg. Chem. 51 (2012) 6245–6250. [4] Y.B. Mao, Stanislaus S. Wong, J. Am. Chem. Soc. 128 (2006) 8217–8226. [5] P. Wang, B.B. Huang, X.Y. Qin, X.Y. Zhang, Y. Dai, J.Y. Wei, M.H. Whangbo, Angew. Chem. – Int. Edit. 47 (2008) 7931–7933. [6] P. Wang, B.B. Huang, X.Y. Zhang, X.Y. Qin, H. Jin, Y. Dai, Z.Y. Wang, J.Y. Wei, J. Zhan, S.Y. Wang, J.P. Wang, M.H. Whangbo, Chem. – Eur. J. 15 (2009) 1821– 1824. [7] C. Hu, T.W. Peng, X.X. Hu, Y.L. Nie, X.F. Zhou, J.H. Qu, H. He, J. Am. Chem. Soc. 132 (2010) 857–862. [8] X.F. Wang, S.F. Li, H.G. Yu, J.G. Yu, S.W. Liu, Chem. – Eur. J. 17 (2011) 7777– 7780. [9] C.W. Xu, Y.Y. Liu, B.B. Huang, H. Li, X.Y. Qin, X.Y. Zhang, Y. Dai, Appl. Surf. Sci. 257 (2011) 8732–8736. [10] Z. Lou, B. Huang, Z. Wang, X. Ma, R. Zhang, X. Zhang, X. Qin, Y. Dai, M.H. Whangbo, Chem. Mater. 26 (2014) 3873–3875. [11] Y.P. Bi, S.X. Ouyang, N. Umezawa, J.Y. Cao, J.H. Ye, J. Am. Chem. Soc. 133 (2011) 6490–6492. [12] C.T. Dinh, T.D. Nguyen, F. Kleitz, T.O. Do, Chem. Commun. 47 (2011) 7797– 7799. [13] Z. Yi, J. Ye, N. Kikugawa, T. Kako, S. Ouyang, H. Stuart-Williams, H. Yang, J. Cao, W. Luo, Z. Li, Y. Liu, R.L. Withers, Nat. Mater. 9 (2010) 559–564. [14] H.G. Yu, G.Q. Cao, F. Chen, X.F. Wang, J.G. Yu, M. Lei, Appl. Catal. B 160–161 (2014) 658–665.
[15] H.L. Lin, H.F. Ye, B.Y. Xu, J. Cao, S.F. Chen, Catal. Commun. 37 (2013) 55–59. [16] B.Q. Lu, N. Ma, Y.P. Wang, Y.W. Qiu, H.H. Hu, J.H. Zhao, D.Y. Liang, S. Xu, X.Y. Li, Z.Y. Zhu, C. Cui, J. Alloy. Compd. 630 (2015) 163–171. [17] W. Yao, B. Zhang, C. Huang, C. Ma, X. Song, Q. Xu, J. Mater. Chem. 22 (2012) 4050–4055. [18] X. Ma, H. Li, Y. Wang, H. Li, B. Liu, S. Yin, T. Sato, Appl. Catal. B Environ. 158 (2014) 314–320. [19] W. Liu, M. Wang, C. Xu, S. Chen, X. Fu, Mater. Res. Bull. 48 (2013) 106–113. [20] L. Zhang, H. Zhang, H. Huang, Y. Liu, Z. Kang, New J. Chem. 36 (2012) 1541– 1544. [21] B. Cao, P. Dong, S. Cao, Y. Wang, J. Am. Ceram. Soc. 96 (2013) 544–548. [22] G. Fu, G. Xu, S. Chen, L. Lei, M. Zhang, Catal. Commun. 40 (2013) 120–124. [23] J. Guo, S. Ouyang, P. Li, Y. Zhang, T. Kako, J. Ye, Appl. Catal. B Environ. 134 (2013) 286–292. [24] S. Kumar, T. Surendar, A. Baruah, V. Shanker, J. Mater. Chem. A 1 (2013) 5333– 5340. [25] C.J. Li, P. Zhang, R. Lv, J.W. Lu, T. Wang, S.P. Wang, H.F. Wang, J.L. Gong, Small 9 (2013) 3951–3956. [26] H. Yu, Z. Jiao, H. Hu, G. Lu, J. Ye, Y. Bi, Cryst. Eng. Commun. 15 (2013) 4802– 4805. [27] Y.S. Xu, W.D. Zhang, Dalton Trans. 42 (2013) 1094–1101. [28] H.C. Zhang, H. Huang, H. Ming, H.T. Li, L.L. Zhang, Y. Liu, Z.H. Kang, J. Mater. Chem. 22 (2012) 10501–10506. [29] G.D. Chen, M. Sun, Q. Wei, Y.F. Zhang, B.C. Zhu, B. Du, J. Hazard. Mater. 244– 245 (2013) 86–93. [30] Q.H. Liang, Y. Shi, W.J. Ma, Z. Li, X.M. Yang, Phys. Chem. Chem. Phys. 14 (2012) 15657–15665. [31] L. Liu, J.C. Liu, D.D. Sun, Catal. Sci. Technol. 2 (2012) 2525–2532. [32] Z.H. Cheng, F. Bing, Q. Liu, Z.G. Zhang, X.M. Fang, J. Mater. Chem. A 00 (2014) 1–3. [33] W. Teng, X.Y. Li, Q.D. Zhao, G.H. Chen, J. Mater. Chem. A 1 (2013) 9060–9068. [34] L.J. Xie, Z.M. Liu, J.B. Zhang, J.F. Ma, J. Alloy. Compd. 503 (2010) 159–162. [35] J. Ren, W.Z. Wang, M. Shang, S.M. Sun, E.P. Gao, ACS Appl. Mater. Interfaces 3 (2011) 2529–2533. [36] J.P. Zou, J. Ma, Q. Huang, S.L. Luo, J. Yu, X.B. Luo, W.L. Dai, J. Sun, G.C. Guo, C.T. Au, S.L. Suib, Appl. Catal. B: Environ. 156–157 (2014) 447–455. [37] J.P. Zou, S.L. Luo, L.Z. Zhang, J. Ma, S.L. Lei, L.S. Zhang, X.B. Luo, Y. Luo, G.S. Zeng, C.T. Au, Appl. Catal. B: Environ. 140–141 (2013) 608–618. [38] G.H. Tian, Y.J. Chen, X.Y. Meng, J. Zhou, W. Zhou, K. Pan, C.G. Tian, Z.Y. Ren, H.G. Fu, Chem. Plus Chem. 78 (2013) 117–123. [39] D.L. Jiang, L.L. Chen, J.J. Zhu, M. Chen, W.D. Shi, J.M. Xie, Dalton Trans. 42 (2013) 15726–15734.
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Short Communication
Heterostructured Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanospheres with enhanced photocatalytic activity under visible light irradiation Xue Lin, Jing Hou, Xiaoyu Guo, Yushuang Wang, Jia Zheng, Chang Liu, Yang Yang, Guangbo Che ⇑ Key Laboratory of Preparation and Application Environmentally Friendly Materials of Ministry of Education, Jilin Normal University, Siping 136000, PR China
a r t i c l e
i n f o
Article history: Received 16 September 2015 Received in revised form 15 October 2015 Accepted 20 October 2015 Available online 21 October 2015 Keywords: Ag3PO4/Ag/Bi3.64Mo0.36O6.55 Heterostructure Photocatalysis Visible light
a b s t r a c t Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanoheterostructure with enhanced photocatalytic activity was successfully synthesized for the first time via a simple and practical low-temperature solution-phase route. The as-prepared Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanoheterostructure included Ag3PO4 quantum dots (QDs) as well as Ag QDs assembling on the surface of Bi3.64Mo0.36O6.55 nanospheres. Ag QDs were photodeposited on the surface of Bi3.64Mo0.36O6.55 nanospheres to increase visible-light absorption via the surface plasmon resonance. The interface between Ag/Ag3PO4 and Bi3.64Mo0.36O6.55 facilitates the separation and transfer of photoinduced charges. The Ag3PO4/Ag/Bi3.64Mo0.36O6.55 hybrid photocatalyst degraded rhodamine B (Rh B) efficiently and displayed much higher photocatalytic activity than pure Bi3.64Mo0.36O6.55, Ag3PO4, Ag/Ag3PO4, Ag/Bi3.64Mo0.36O6.55 or Ag3PO4/Bi3.64Mo0.36O6.55 under visible light irradiation (k > 420 nm). It indicates that the heterostructured combination of Ag3PO4, Ag, and Bi3.64Mo0.36O6.55 provides synergistic photocatalytic activity through an efficient electron transfer process. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The exploration and construction of new photocatalysts with high catalytic efficiency under visible light is an important issue in photocatalysis all the time, and is also significant in solving current environment and energy problems [1–4]. Among these works, Ag-based photocatalysts had attracted much interest for their excellent photocatalytic performances, such as AgX (X = Cl, Br and I) [5–7], Ag2O [8], Ag2CO3 [9], Ag6Si2O7 [10] and Ag3PO4 [11,12]. Ag3PO4 is extremely attractive due to that it can achieve quantum efficiency up to 90% for oxidize water (O2 evolution) as well as photodegradation of organic contaminant under visible light irradiation. However, Ag3PO4 is slightly soluble in aqueous solution, and it will be photo-corroded and decompose to weakly active Ag during the photocatalytic process. Moreover, the by-product Ag particles usually generate from the photodecomposition of Ag3PO4 in the absence of electron acceptor during the photocatalytic process, which would decrease its visible light absorption and photocatalytic activity [13–16]. In order to improve the photocatalytic performance of Ag3PO4, several kinds of materials have been cooperated with Ag3PO4 to generate hybrid photocatalysts, for instance semiconductor materials (TiO2, ZnO, SnO2, BiMoO4, Cr–SrTiO3, Bi2MoO6, g-C3N4 [17–27]) and carbon ⇑ Corresponding author. E-mail address: [email protected] (G. Che). http://dx.doi.org/10.1016/j.seppur.2015.10.049 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
materials (oxidized graphene, graphene and carbon quantum dots [28–31]). The Ag3PO4 mixed with carbon materials can facilitate electron transport and hinder the electron–hole recombination due to the excellent electrical conductivity of carbon materials. However, the carbon materials generally have a more negative conduction band potential than that of Ag3PO4, which prevents the photogenerated electrons transfer from Ag3PO4 to the carbon materials. Thus, the hybrids with semiconductors are considered to be ideal photocatalysts because they can improve the optical absorption property and enhance the photogenerated charge separation. But the obtained heterostructures usually suffer from the less reducibility and oxidability of the remaining electrons and holes. Recently, with more and more attention has been paid to the mechanism of Ag3PO4-based hybrid photocatalytic systems, the by-product Ag has been found to be a electron transmission bridge in the Ag/Ag3PO4-based systems [32,33]. It has been reported that the Ag/Ag3PO4-based photocatalysts can not only facilitate the charge separation but also retain the high reducibility and oxidability of the remaining electrons and holes for the corresponding photocatalysts. Thus, it is important to search for suitable components that can construct novel Ag/Ag3PO4-based photocatalysts and largely enhance their photocatalytic performances. Bi3.64Mo0.36O6.55, with a band gap of around 2.60 eV, has been reported to be a visible-light induced photocatalyst [34]. There are a few investigations concerning Bi3.64Mo0.36O6.55 as an active
876
X. Lin et al. / Separation and Purification Technology 156 (2015) 875–880
photocatalyst [34,35]. However, similar to other semiconductor photocatalysts, the high-rate recombination of electron/hole (e/ h+) pairs and poor visible light absorption efficiency are still great challenges to enhance the photocatalytic performance of pure Bi3.64Mo0.36O6.55 in order to meet the practical application requirements [36,37]. Herein, Bi3.64Mo0.36O6.55 was chosen to combine with Ag3PO4, and a novel high-efficiency Ag3PO4/Ag/Bi3.64Mo0.36O6.55 hybrid photocatalyst was demonstrated for the first time. The photocatalytic activity of the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 hybrid photocatalyst was evaluated by degradation rhodamine B (Rh B) under visible light (k > 420 nm). It is shown that a heterostructure composed of Ag3PO4, Ag and Bi3.64Mo0.36O6.55 contributes to the enhanced performance of the hybrid photocatalyst. 2. Experimental 2.1. Preparation of photocatalysts 2.1.1. Preparation of Bi3.64Mo0.36O6.55 nanospheres All reagents for synthesis and analysis were commercially available and used without further treatments. Bi3.64Mo0.36O6.55 was prepared based on a facile hydrothermal method. A typical synthesis process is as follows: Bi(NO3)35H2O (2 mmol) was dissolved in 10 mL distilled water under magnetic stirring for 30 min. (NH4)6Mo7O244H2O (1 mmol) was dissolved in 5 mL distilled water, and added dropwise into the Bi(NO3)3 solution, and the pH of the mixed solution was adjusted to 13 through the adding of 2 mol L1 NH3H2O solution. After being stirred for 30 min at room temperature, the mixture was subject to ultrasonic treatment for 30 min and then to hydrothermal treatment in a 20 mL Teflon-lined stainless steel autoclave at 160 °C for 24 h. At last, the obtained particles were collected and washed with ethanol and distilled water several times, and dried at 100 °C for 2 h. 2.1.2. Preparation of Ag3PO4/Ag/Bi3.64Mo0.36O6.55 photocatalyst The Ag3PO4/Ag/Bi3.64Mo0.36O6.55 hybrid photocatalyst was synthesized through an in-situ precipitation method at room temperature. In a typical process, Bi3.64Mo0.36O6.55 nanospheres (100 mg) were dispersed in 100 mL deionized water by ultrasound for 30 min, and then 50 mL 0.0075 M AgNO3 aqueous solution was dropped into the Bi3.64Mo0.36O6.55 dispersed solution. After stirring for 15 min, a 50 mL of 0.0025 M Na3PO4 aqueous solution was added to the above solution drop by drop under magnetic stirring. The pH value was adjusted to 3 by adding 1.0 M NaOH. The resulting suspension was stirred in the dark for another 30 min. Then, the suspension was irradiated by a 300 W Xe lamp equipped with an optical cut-off filter (k > 420 nm) for 30 min. Finally, the precipitate was washed with deionized water for 3 times and collected by centrifugation, and then dried at 60 °C in the vacuum drying oven. The theoretical value of Ag3PO4 loading amount was 20 wt%. For comparison, pure Ag3PO4, Ag/Bi3.64Mo0.36O6.55, Ag/Ag3PO4, and Ag3PO4/Bi3.64Mo0.36O6.55 samples were prepared under the same conditions.
England) using 300 W Al Ka radiation. The photoluminescence (PL) spectra of the photocatalysts were obtained by a F4500 (Hitachi, Japan) photoluminescence detector with an excitation wavelength of 325 nm. The UV–vis diffuse reflectance spectra (DRS) were recorded using a scan UV–vis spectrophotometer (UV-2550). 2.3. Photocatalytic activities studies The photocatalytic properties of the as-prepared samples were evaluated using Rh B as a model compound. The Rh B is a very stable compound, which has been used widely as a representative reaction for examining the performance of numerous visible light active catalysts. In experiments, the Rh B solution (0.01 mmol L1, 100 mL) containing 0.02 g of photocatalyst was mixed in a Pyrex reaction glass. A 300 W Xe lamp (k > 420 nm) was employed to provide visible light irradiation. A 420 nm cut-off filter was inserted between the lamp and the sample to filter out UV light (k < 420 nm). Prior to visible light illumination, the suspension was strongly stirred in the dark for 40 min. Then the solution was exposed to visible light irradiation under magnetic stirring. At given time intervals, 4 mL of the suspension was periodically collected and analyzed after centrifugation. The Rh B concentration was analyzed by a UV-2550 spectrometer to record intensity of the maximum band at 552 nm in the UV–vis absorption spectra. 3. Results and discussion Fig. 1 displays the XRD pattern of the Ag3PO4/Ag/Bi3.64Mo0.36 O6.55 hybrid photocatalyst, together with those of Ag3PO4, Bi3.64Mo0.36O6.55, Ag/Bi3.64Mo0.36O6.55, Ag/Ag3PO4, and Ag3PO4/Bi3.64 Mo0.36O6.55. All the peaks in the XRD pattern of Bi3.64Mo0.36O6.55 can be readily indexed into specific crystal planes of Bi3.64Mo0.36O6.55 phase (JCPDS No. 43-0446), which is in accordance with the reported works [34,35]. For Ag3PO4, all the diffraction peaks can be indexed to specific crystal planes of Ag3PO4 phase (JCPDS No. 06-0505). It is observed that no peaks assigned to Ag0 or Ag3PO4 species were detected in the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 sample due to the small crystal size or low percentage of Ag and Ag3PO4. Herein, the successful loading of Ag and Ag3PO4 was verified by XPS, as revealed in Fig. 2. The Ag 3d peaks of Ag3PO4/Ag/Bi3.64Mo0.36O6.55 have separated as Ag+ peaks and Ag0 peaks (Fig. 2a). The weak peaks at 368.40 and 374.66 eV are attributed to Ag0 [32], indicating the existence of metallic Ag on the surface of Bi3.64Mo0.36O6.55 nanospheres. The strong peaks at 368.00 and 374.00 eV are assigned to Ag+ of Ag3PO4 [32]. A broad peak in the range of 130–134 eV of the P 2p spectrum (Fig. 2b) is observed for the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 sample
2.2. Characterization of photocatalysts The crystal structures of the samples were characterized by X-ray diffraction (XRD) on a Rigaku (Japan) D/max 2500 X-ray diffractometer (Cu Ka radiation, k = 0.154 18 nm). The morphologies and structure details of the as-synthesized samples were detected using field emission scanning microscopy (SEM, JSM6510) and transmission electron microscopy (TEM, JEM-2100F). X-ray photoelectron spectroscopy (XPS) analysis was performed with an ESCALa-b220i-XL electron spectrometer (VGScientific,
Fig. 1. XRD patterns of the as-prepared Ag3PO4, Bi3.64Mo0.36O6.55, Ag/Bi3.64Mo0.36O6.55, Ag/Ag3PO4, Ag3PO4/Bi3.64Mo0.36O6.55, and Ag3PO4/Ag/Bi3.64Mo0.36O6.55 samples.
X. Lin et al. / Separation and Purification Technology 156 (2015) 875–880
877
Fig. 2. XPS spectra of the as-obtained Ag3PO4/Ag/Bi3.64Mo0.36O6.55 sample: (a) Ag 3d spectrum, (b) P 2p spectrum, (c) Bi 4f spectrum, (d) Mo 3d spectrum.
which is corresponding to the phosphorus of Ag3PO4 [32]. The Bi 4f fine XPS spectrum of the sample is shown in Fig. 2c. XPS signals of Bi 4f are observed at binding energies at about 164.53 eV (Bi 4f7/2) and 159.18 eV (Bi 4f5/2), ascribed to Bi3+ [27]. The characteristic spin– orbital splitting photoelectrons for Mo 3d (235.31 and 232.20 eV) indicate a six-valent oxidation state for Mo6+ [27] (Fig. 2d). The SEM images of the as-prepared Bi3.64Mo0.36O6.55 and Ag3PO4/Ag/Bi3.64Mo0.36O6.55 samples are shown in Fig. 3(a) and (b). The bare Bi3.64Mo0.36O6.55 crystals are composed of nanospheres with an average diameter of tens of nanometers. The Ag3PO4/Ag/ Bi3.64Mo0.36O6.55 sample shows a similar morphology to pure Bi3.64Mo0.36O6.55, which indicates that low amount Ag3PO4 and Ag loading didn’t have any influence on the morphology of Bi3.64Mo0.36O6.55 crystals. In order to further ascertain the heterostructure among Ag3PO4, Ag, and Bi3.64Mo0.36O6.55, Ag3PO4/ Ag/Bi3.64Mo0.36O6.55 sample as well as pure Bi3.64Mo0.36O6.55 was investigated by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM), as revealed in Fig. 3(c–e). Fig. 3c shows the spherical structure of the assynthesized Bi3.64Mo0.36O6.55 sample. From Fig. 3d, it can be observed that Ag3PO4 QDs as well as Ag QDs were loaded on the surface of Bi3.64Mo0.36O6.55 nanospheres. HRTEM image further confirm the formation of a novel ternary heterostructure (Fig. 3e). An interconnected fine particulate morphology observed indicates the existence of the ternary heterostructure. By measuring the lattice fringes, the resolved interplanar distances are about 0.19, 0.213, and 0.325 nm, which correspond to the (2 0 2) plane of Ag, the (2 1 0) plane of Ag3PO4, and the (1 1 1) plane of Bi3.64Mo0.36O6.55, respectively.
The optical absorption properties of the photocatalysts were studied by UV–vis DRS spectra. As shown in Fig. 4(a), bare Bi3.64Mo0.36O6.55 exhibits strong absorbance in wavelengths shorter than 460 nm, and bare Ag3PO4 shows strong absorbance in wavelengths shorter than 550 nm. After Ag3PO4 is wrapped on Bi3.64Mo0.36O6.55 nanospheres, the optical absorption of Ag3PO4/Bi3.64Mo0.36O6.55 hybrid photocatalyst is enhanced, which is due to the loading of Ag3PO4. Ag/Bi3.64Mo0.36O6.55 and Ag/Ag3PO4 samples show obvious visible-light absorption, which can be attributed to the surface Plasmon resonance (SPR) of the loading Ag, further confirming the formation of Ag. The absorption curve of the heterostructured Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanospheres shows distinctly enhanced visible-light absorption compared to the pure Bi3.64Mo0.36O6.55 and Ag3PO4/Bi3.64Mo0.36O6.55 samples. The band gap energies of semiconductors can be estimated by Kubelka–Munk transformation, ahv = A(hv Eg)n/2, where a represents the absorption coefficient, t is the light frequency, Eg is the band gap energy, A is a constant and n depends on the characteristics of the transition in a semiconductor (n = 1 for a direct transition and n = 4 for an indirect transition). For Ag3PO4 and Bi3.64Mo0.36O6.55, the value of n is 1 for the direct transition. Thus, as shown in Fig. 4b, the band gap energies (Eg) of pure Ag3PO4 and Bi3.64Mo0.36O6.55 are calculated to be 2.00 and 2.60 eV, respectively. The potentials of VB and CB of a semiconductor material can be estimated according to the following empirical equations:
EVB ¼ X Ee þ 0:5Eg
ð1Þ
ECB ¼ EVB Eg
ð2Þ
878
X. Lin et al. / Separation and Purification Technology 156 (2015) 875–880
Fig. 3. SEM images of Bi3.64Mo0.36O6.55 (a), Ag3PO4/Ag/Bi3.64Mo0.36O6.55 (b), TEM images of Bi3.64Mo0.36O6.55 (c), Ag3PO4/Ag/Bi3.64Mo0.36O6.55 (d), HRTEM image of Ag3PO4/Ag/ Bi3.64Mo0.36O6.55 (e).
where EVB is the valence band edge potential, ECB is the conduction band edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, and the values of X for Ag3PO4 and Bi3.64Mo0.36O6.55 are ca. 6.72 and 5.70 eV, respectively, Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV), Eg is the band gap energy of the semiconductor. Using the Eqs. (1) and (2), the EVB potentials of Ag3PO4 and Bi3.64Mo0.36O6.55 are calculated to be 2.47 and 2.50 eV, respectively. The ECB potentials of Ag3PO4 and Bi3.64Mo0.36O6.55 are determined to be 0.47 and 0.10 eV, respectively. The energy band structure diagram of Ag3PO4, Ag, and Bi3.64Mo0.36O6.55 is thus schematically illustrated, as shown in Fig. 4c. Under visible light irradiation, Bi3.64Mo0.36O6.55, as well as Ag3PO4 can be excited to produced h+ and e. The electrons on the CB of Ag3PO4 can effectively migrate to metal Ag through the Schottky barrier (electron transfer I: Ag3PO4 CB ? Ag) because the CB potential of Ag3PO4 is more negative than the Fermi level of the loaded metal Ag, and electron transfer in this process is faster
than the electron–hole recombination between the VB and CB of Ag3PO4. Therefore, the electrons in the CB of Ag3PO4 can be stored in the Ag component. In addition, because the energy level of Ag is above the VB of Bi3.64Mo0.36O6.55, the holes in the VB of Bi3.64Mo0.36O6.55 can also flow easily into the Ag metal (hole transfer II: Bi3.64Mo0.36O6.55 VB ? Ag), and this process is faster than the electron–hole recombination between the VB and CB of Bi3.64Mo0.36O6.55. As a result, the holes in the VB of Ag3PO4 with strong oxidation power can participate in photocatalytic reactions to degrade organic pollutants directly, and moreover, the electrons in the CB of Bi3.64Mo0.36O6.55 with strong reduction power can be transferred to the adsorbed O2, which acts as an electron acceptor to form first superoxide radical anions O 2 , and finally OH radicals [38]. These radicals are strong oxidants that can completely oxidize Rh B molecules to water and carbon dioxide [39]. Therefore, synchronous electron transfer (vectorial electron transfer of Ag3PO4 ? Ag ? Bi3.64Mo0.36O6.55) should take place under visible light excitation of both Bi3.64Mo0.36O6.55 and Ag3PO4, in agreement with
X. Lin et al. / Separation and Purification Technology 156 (2015) 875–880
879
Fig. 4. (a) UV–vis DRS of as-synthesized samples. (b) the plots of (ahv)2 versus photon energy (hm) for the band gap energies of Ag3PO4 and Bi3.64Mo0.36O6.55. (c) Schematic diagram of the separation and transfer of photogenerated charges in the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 heterostructure under visible light irradiation.
previous reports [32,33]. In this vectorial electron-transfer process, the Ag metal in the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanojunction system acts as a storage and/or recombination center for CB electrons of Ag3PO4 and VB holes of Bi3.64Mo0.36O6.55, and contributes to the enhancement of interfacial charge transfer and separation. The photocatalytic activities of the as-prepared samples were evaluated by degradation of Rh B under visible light, as shown in Fig. 5a. Before light irradiation, the solution of Rh B and photocatalyst was magnetically stirred in dark for 40 min to establish an adsorption desorption equilibrium. The decrease of the Rh B concentration due to the adsorption of Ag3PO4/Ag/Bi3.64Mo0.36O6.55 (about 20%) is much higher than that of pure Bi3.64Mo0.36O6.55 sample (around 4%), indicating that Ag3PO4 QD and Ag QD loading can enhance the absorption of organic molecules on the surface of photocatalyst. From the catalytic experiments, Ag3PO4/Ag/ Bi3.64Mo0.36O6.55 was detected to be more photoactive toward Rh B solution than the pure Bi3.64Mo0.36O6.55, Ag3PO4, Ag/Bi3.64Mo0.36 O6.55, Ag/Ag3PO4, and Ag3PO4/Bi3.64Mo0.36O6.55 samples; almost 100% was eliminated from the solution in 80 min. The Ag3PO4 QD and Ag QD coating can improve the visible light absorption efficiency (Fig. 4a), which is beneficial for the nanoheterostructure to photolyze Rh B. In addition, efficient heterostructure interface between two or three components can restrain the recombination of photoinduced charges effectively, thus enhancing the photocatalytic performance of the Ag3PO4/Ag/ Bi3.64Mo0.36O6.55. The comparison of PL spectra of the as-prepared samples under the excitation wavelength of 325 nm is shown in Fig. 5b. It can be
observed that the PL peak intensity of Ag3PO4/Ag/Bi3.64Mo0.36O6.55 decrease obviously. This result shows that the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanoheterostructure contributes to the effective electron–hole pair separation, which may be a reason for the Ag3PO4/ Ag/Bi3.64Mo0.36O6.55 sample showing enhanced photocatalytic efficiency under visible light irradiation. In addition to photocatalytic activity, the stability of photocatalyst is another important issue. Four runs of cycling photodegradation experiments have been carried out for Ag3PO4/Ag/Bi3.64Mo0.36O6.55 (Fig. 5c) to investigate the stability of the photocatalyst. The Ag3PO4/Ag/Bi3.64Mo0.36O6.55 sample still maintained a high level of degradation performance in the repeated experiments, revealing that the photocatalyst showed good stability during the photocatalytic reaction.
4. Conclusions In summary, a novel Ag3PO4/Ag/Bi3.64Mo0.36O6.55 nanoheterostructure, in which Bi3.64Mo0.36O6.55 nanospheres was decorated with Ag3PO4 QD and Ag QD, has been synthesized by a facile low-temperature solution-phase method. The Ag3PO4/Ag/ Bi3.64Mo0.36O6.55 nanospheres exhibited superior photocatalytic activity to bare Bi3.64Mo0.36O6.55, Ag3PO4, Ag/Bi3.64Mo0.36O6.55, Ag/ Ag3PO4, and Ag3PO4/Bi3.64Mo0.36O6.55 in degradation of Rh B under visible light irradiation. This improvement is attributed to the migration efficiency of the photoinduced carriers at the interface of the heterostructure and the enhanced efficiency of photoharvesting owing to the stable deposition of Ag and Ag3PO4.
880
X. Lin et al. / Separation and Purification Technology 156 (2015) 875–880
Fig. 5. (a) Photodegradation efficiencies of Rh B as a function of irradiation time for different photocatalysts. (b) Room temperature PL spectra of as-synthesized samples. (c) Cycling runs for the photocatalytic degradation of Rh B over the Ag3PO4/Ag/Bi3.64Mo0.36O6.55 sample under visible light irradiation.
Acknowledgment This work was supported by National Natural Science Fundation of China (21407059, 21576112), the Science Development Project of Jilin Province (20130522071JH, 20130521019JH), and the Science and Technology Research Project of the Department of Education of Jilin Province (2015220). References [1] S. Fukuzumi, T. Kobayashi, T. Suenobu, Angew. Chem. Int. Edit. 50 (2011) 728– 731. [2] H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Adv. Mater. 24 (2012) 229–251. [3] X. Li, R. Huang, Y. Hu, Y. Chen, W. Liu, R. Yuan, Z. Li, Inorg. Chem. 51 (2012) 6245–6250. [4] Y.B. Mao, Stanislaus S. Wong, J. Am. Chem. Soc. 128 (2006) 8217–8226. [5] P. Wang, B.B. Huang, X.Y. Qin, X.Y. Zhang, Y. Dai, J.Y. Wei, M.H. Whangbo, Angew. Chem. – Int. Edit. 47 (2008) 7931–7933. [6] P. Wang, B.B. Huang, X.Y. Zhang, X.Y. Qin, H. Jin, Y. Dai, Z.Y. Wang, J.Y. Wei, J. Zhan, S.Y. Wang, J.P. Wang, M.H. Whangbo, Chem. – Eur. J. 15 (2009) 1821– 1824. [7] C. Hu, T.W. Peng, X.X. Hu, Y.L. Nie, X.F. Zhou, J.H. Qu, H. He, J. Am. Chem. Soc. 132 (2010) 857–862. [8] X.F. Wang, S.F. Li, H.G. Yu, J.G. Yu, S.W. Liu, Chem. – Eur. J. 17 (2011) 7777– 7780. [9] C.W. Xu, Y.Y. Liu, B.B. Huang, H. Li, X.Y. Qin, X.Y. Zhang, Y. Dai, Appl. Surf. Sci. 257 (2011) 8732–8736. [10] Z. Lou, B. Huang, Z. Wang, X. Ma, R. Zhang, X. Zhang, X. Qin, Y. Dai, M.H. Whangbo, Chem. Mater. 26 (2014) 3873–3875. [11] Y.P. Bi, S.X. Ouyang, N. Umezawa, J.Y. Cao, J.H. Ye, J. Am. Chem. Soc. 133 (2011) 6490–6492. [12] C.T. Dinh, T.D. Nguyen, F. Kleitz, T.O. Do, Chem. Commun. 47 (2011) 7797– 7799. [13] Z. Yi, J. Ye, N. Kikugawa, T. Kako, S. Ouyang, H. Stuart-Williams, H. Yang, J. Cao, W. Luo, Z. Li, Y. Liu, R.L. Withers, Nat. Mater. 9 (2010) 559–564. [14] H.G. Yu, G.Q. Cao, F. Chen, X.F. Wang, J.G. Yu, M. Lei, Appl. Catal. B 160–161 (2014) 658–665.
[15] H.L. Lin, H.F. Ye, B.Y. Xu, J. Cao, S.F. Chen, Catal. Commun. 37 (2013) 55–59. [16] B.Q. Lu, N. Ma, Y.P. Wang, Y.W. Qiu, H.H. Hu, J.H. Zhao, D.Y. Liang, S. Xu, X.Y. Li, Z.Y. Zhu, C. Cui, J. Alloy. Compd. 630 (2015) 163–171. [17] W. Yao, B. Zhang, C. Huang, C. Ma, X. Song, Q. Xu, J. Mater. Chem. 22 (2012) 4050–4055. [18] X. Ma, H. Li, Y. Wang, H. Li, B. Liu, S. Yin, T. Sato, Appl. Catal. B Environ. 158 (2014) 314–320. [19] W. Liu, M. Wang, C. Xu, S. Chen, X. Fu, Mater. Res. Bull. 48 (2013) 106–113. [20] L. Zhang, H. Zhang, H. Huang, Y. Liu, Z. Kang, New J. Chem. 36 (2012) 1541– 1544. [21] B. Cao, P. Dong, S. Cao, Y. Wang, J. Am. Ceram. Soc. 96 (2013) 544–548. [22] G. Fu, G. Xu, S. Chen, L. Lei, M. Zhang, Catal. Commun. 40 (2013) 120–124. [23] J. Guo, S. Ouyang, P. Li, Y. Zhang, T. Kako, J. Ye, Appl. Catal. B Environ. 134 (2013) 286–292. [24] S. Kumar, T. Surendar, A. Baruah, V. Shanker, J. Mater. Chem. A 1 (2013) 5333– 5340. [25] C.J. Li, P. Zhang, R. Lv, J.W. Lu, T. Wang, S.P. Wang, H.F. Wang, J.L. Gong, Small 9 (2013) 3951–3956. [26] H. Yu, Z. Jiao, H. Hu, G. Lu, J. Ye, Y. Bi, Cryst. Eng. Commun. 15 (2013) 4802– 4805. [27] Y.S. Xu, W.D. Zhang, Dalton Trans. 42 (2013) 1094–1101. [28] H.C. Zhang, H. Huang, H. Ming, H.T. Li, L.L. Zhang, Y. Liu, Z.H. Kang, J. Mater. Chem. 22 (2012) 10501–10506. [29] G.D. Chen, M. Sun, Q. Wei, Y.F. Zhang, B.C. Zhu, B. Du, J. Hazard. Mater. 244– 245 (2013) 86–93. [30] Q.H. Liang, Y. Shi, W.J. Ma, Z. Li, X.M. Yang, Phys. Chem. Chem. Phys. 14 (2012) 15657–15665. [31] L. Liu, J.C. Liu, D.D. Sun, Catal. Sci. Technol. 2 (2012) 2525–2532. [32] Z.H. Cheng, F. Bing, Q. Liu, Z.G. Zhang, X.M. Fang, J. Mater. Chem. A 00 (2014) 1–3. [33] W. Teng, X.Y. Li, Q.D. Zhao, G.H. Chen, J. Mater. Chem. A 1 (2013) 9060–9068. [34] L.J. Xie, Z.M. Liu, J.B. Zhang, J.F. Ma, J. Alloy. Compd. 503 (2010) 159–162. [35] J. Ren, W.Z. Wang, M. Shang, S.M. Sun, E.P. Gao, ACS Appl. Mater. Interfaces 3 (2011) 2529–2533. [36] J.P. Zou, J. Ma, Q. Huang, S.L. Luo, J. Yu, X.B. Luo, W.L. Dai, J. Sun, G.C. Guo, C.T. Au, S.L. Suib, Appl. Catal. B: Environ. 156–157 (2014) 447–455. [37] J.P. Zou, S.L. Luo, L.Z. Zhang, J. Ma, S.L. Lei, L.S. Zhang, X.B. Luo, Y. Luo, G.S. Zeng, C.T. Au, Appl. Catal. B: Environ. 140–141 (2013) 608–618. [38] G.H. Tian, Y.J. Chen, X.Y. Meng, J. Zhou, W. Zhou, K. Pan, C.G. Tian, Z.Y. Ren, H.G. Fu, Chem. Plus Chem. 78 (2013) 117–123. [39] D.L. Jiang, L.L. Chen, J.J. Zhu, M. Chen, W.D. Shi, J.M. Xie, Dalton Trans. 42 (2013) 15726–15734.