AgCl

Article pubs.acs.org/JPCC

Hierarchically Plasmonic Z‑Scheme Photocatalyst of Ag/AgCl Nanocrystals Decorated Mesoporous Single-Crystalline Metastable Bi20TiO32 Nanosheets Jungang Hou, Zheng Wang, Chao Yang, Weilin Zhou, Shuqiang Jiao,* and Hongmin Zhu State Key Laboratory of Advanced Metallurgy, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China S Supporting Information *

ABSTRACT: The hierarchical photocatalysts of Ag−AgCl@Bi20TiO32 composites have been successfully synthesized by anchoring Ag−AgCl nanocrystals on the surfaces of mesoporous single-crystalline metastable Bi20TiO32 nanosheets via a two-stage strategy for excellent visible-light-driven photocatalytic activities in the Z-scheme system. First, the single-crystalline metastable Bi20TiO32 nanosheets with tetragonal structures were prepared via a facile hydrothermal process in assistance with the post-heat-treatment route using benzyl alcohol. Especially, the mesoporous Bi20TiO32 nanosheets showed high photocatalytic activity for the degradation of rhodamine B dye under visible-light irradiation. Then, the as-prepared mesoporous Bi20TiO32 nanosheets were used as a support for loading Ag−AgCl nanocrystals using the deposition−precipitation method and irradiation−reduction process to fabricate the Ag−AgCl@Bi20TiO32 composites. Inspiringly, the hierarchical Ag−AgCl@Bi20TiO32 photocatalyst has the higher photocatalytic performance than Ag−AgCl nanocrystals and mesoporous Bi20TiO32 nanosheets over the degradation of rhodamine B and acid orange 7 dyes, which is attributed to the effective charge transfer from plasmon-excited Ag nanocrystal to Bi20TiO32 for the construction of a Z-scheme visible-light photocatalyst. This work could provide new insights into the fabrication of hierarchically plasmonic photocatalysts with high performance and facilitate their practical application in environmental issues.

1. INTRODUCTION Environmental purification and energy conversion on the basis of highly efficient photocatalysts and solar energy attract more and more attention.1−5 To date, a large number of metal oxides as photocatalysts have been explored for the purpose of efficient degradation of harmful organic substances and hydrogen production through splitting water.6−10 Especially, developing semiconductor photocatalysts with certain welldefined facets is becoming an important strategy to improve their photocatalytic activity and/or tune their reaction preferences toward different applications.11,12 The base of this strategy is that both the activity and the preference of photocatalytic reactions are sensitive to the surface atomic structure and electronic structure, which varies with different crystallographic orientations, of the photocatalyst crystals. Maximizing the surface of a photocatalyst to reactive facets is feasible to realize such a goal. For example, both theoretical and experimental studies have demonstrated that the {001} facets of anatase TiO 2 are much more reactive than the thermodynamically more stable {101} facets.13 Bi2O3 quantum dots decorated anatase TiO2 with exposed {001} high energy facets has been prepared on graphene sheets as Bi2O3/TiO2/ graphene hybrid photocatalysts.14 m-BiVO4 nanoplates with exposed {001} facets exhibit greatly enhanced activity in the visible-light photocatalytic degradation of organic contaminants and photocatalytic oxidation of water for O2 generation.15 © 2013 American Chemical Society

Therefore, it is of great interest to develop novel visible-light responsive photocatalysts with controllable surface. Many Bi- and Ti-containing compounds, such as Bi2Ti2O7, Bi2Ti4O11, Bi4Ti3O12, Bi12TiO20, and Bi20TiO32, have attracted much attention due to their layered structure and the resulting unique physical properties.15−20 Among these, bismuth titanates, a large family that includes several phases in the Bi−Ti−O system, are promising candidates for various technological applications.15−25 For example, Zhou et al.20 demonstrated the potential of Bi12TiO20, as a visible-light photocatalyst for the oxidation of methanol to CO2. Bi12TiO20 nanostructures and their photocatalytic activity under visiblelight irradiation have been systematically investigated in our group.21,22 However, it is hard to produce highly crystalline and single-phase bismuth titanate because different bismuth titanate phases are formed depending on different chemical compositions and processing conditions. Metastable Bi20TiO32 photocatalysts using α-Bi2O3 and anatase TiO2 as raw materials were synthesized by a high-temperature quenching method, which typically results in an irregular morphology and large agglomerated particles as well as a low surface area due to high temperatures.23 Further, highly crystalline metastable Received: December 5, 2012 Revised: January 18, 2013 Published: February 20, 2013 5132

dx.doi.org/10.1021/jp311996r | J. Phys. Chem. C 2013, 117, 5132−5141

The Journal of Physical Chemistry C

Article

bismuth titanate (Bi20TiO32) nanosheets were prepared via a wet chemical route in assistance with the pressure of 1 MPa of nitrogen.24 These studies revealed that bismuth titanate could perform as an excellent photocatalytic material and solarenergy-conversion material. However, to the best of our knowledge, there is no systematic study on shape control and associated visible-light photocatalytic activities of mesoporous Bi20TiO32 nanosheets. To improve the photocatalytic performance, a novel strategy has been applied via constructing hierarchical nanostructures for photocatalytic applications by anchoring functional species on semiconductors.25 The surface plasmon resonance of metal nanoparticles has been introduced to photocatalysts due to the enhanced absorption in the visible light region.26,27 Dai et al. precipitated Ag/AgCl on P25 to synthesize Ag/AgCl/TiO2 photocatalysts for visible-light-driven photoreduction of Cr(VI) and organic dyes.28 Ag/AgX/BiOX (X = Cl, Br) threecomponent visible-light-driven photocatalysts were synthesized by a low-temperature chemical bath method.29 Other systems, including Ag/AgCl/ZnO,30 Ag/AgCl/BiOCl,31 Ag/AgCl/titanate honeycomb,32 Ag/AgBr/TiO2,33 Ag/AgX/GO (X = Cl, Br),34 AgX/Ag3PO4 (X = Cl, Br, I),35 and Ag/AgCl@WO3,36 have also been investigated. Thus, there appears to be great untapped potential for exploring the possibility of using Ag− AgCl to help improve photocatalytic activity with Ag−AgCl@ Bi20TiO32 composites. In this work, the influences of synthesis parameters on the resulting products, the synthesis mechanism, the critical roles of treatment conditions, and catalyst compositions in determining catalytic performance, as well as the contribution of the work to the fields of visible-light photocatalytic activities, were elaborated in detail. There are several significant aspects of the work described in this paper. First, the synthesis of shapecontrolled mesoporous Bi20TiO32 nanosheets with exposed {001} facets have been found to be extremely evasive to date. Hence, a facile hydrothermal process in assistance with the postheat treatment route should be an important progress that may inspire subsequent catalytic materials synthesis. Second, the significant improvement of photocatalytic performance of mesoporous Bi20TiO32 nanosheets coupled with Ag−AgCl nanocrystals in the degradation of cationic rhodamine B (RhB) and acid orange 7 (AO7) under visible-light irradiation has been rarely reported. Hence, this work may be of interest to both materials scientists and those working in the area of catalyst design.

composites, 0.2 g of mesoporous Bi20TiO32 nanosheets and 0.3 g of CTAC were added to 100 mL of deionized water, and the suspension was stirred for 60 min. Then 2.0 mL of 0.1 M AgNO3 was quickly added to the above mixture. During this process, the excessive surfactant CTAC not only adsorbed onto the surface of Bi20TiO32 to limit the number of nucleation sites for AgCl to grow, resulting in homogenously dispersed AgCl, but also induced Cl− to precipitate Ag+ in the suspension. The resulting suspension was stirred for 1.0 h and then placed under irradiation of 300 W Xe lamp for the indicated lengths of time. The suspension was filtered, washed with deionized water, and dried at 80 ◦C for 12 h. Then the gray powder was calcined at 300 °C for 3 h. Depending on the duration of irradiation, the as-prepared catalysts were denoted as Ag−AgCl@Bi20TiO32 composites-m, where “m” represents 10, 30, 50, and 70 min of photoreduction. 2.3. Characterization. The obtained products were characterized by powder X-ray diffraction (XRD, MAC Science Co. Ltd. Japan) using Cu Kα (λ = 0.1546 nm), and XRD patterns were obtained at 10°−90° 2θ by step scanning with a step size of 0.02°. The lattice parameters were calculated with the least-squares method. The average crystal domain size of the Bi20TiO32 crystallite size was estimated with the Scherrer equation: D = 0.90λ/β cos θ, where θ is the diffraction angle of the (201) peak of the cubic phase and β is the full width at halfmaximum (fwhm) of the (201) peak in radians, which is calibrated from high-purity silicon. The morphology and size of the resultant products were observed using a field emission scanning electron microscope (FESEM, JEOL, JSM-6701F) with energy-dispersive spectra and a transmission electron microscope (TEM, JEM-2010). The optical properties of the samples were analyzed by UV−vis diffuse reflectance spectroscopy (UV−vis DRS) using a UV−vis spectrophotometer (UV2550, Shimadzu) in the range 190−900 nm. The surface areas of the samples were measured by a TriStar 3000-BET/BJH Surface Area. The chemical states of the sample were determined by X-ray photoelectron spectroscopy (XPS) in a VG Multilab 2009 system (UK) with a monochromatic Al Kα source and a charge neutralizer. 2.4. Photocatalytic Test. Photocatalysis reactions were performed in an air-free, closed gas circulation system with a quartz reaction cell. Photocatalytic activity was evaluated by the degradation of rhodamine B (RhB) and acid orange 7 (AO7) in aqueous solution under visible-light irradiation using a 300 W Xe lamp with a cutoff filter (λ > 420 nm). A cylindrical Pyrex flask (200 mL) was placed in a sealed black box of which the top was open and the cutoff filter was set on the window face of the reaction vessel to ensure the desired irradiation condition. In each experiment, the samples as catalysts were added into rhodamine B (RhB) and acid orange 7 (AO7) solution (1 × 10−4 M, 100 mL). For the evaluation of photocatalytic activity, the 0.2 g of Bi20TiO32 nanosheets as catalysts was chosen, and 0.1 g of Ag−AgCl@Bi20TiO32 composites as the catalysts was chosen for the evaluation of photocatalytic activity. Before illumination, the suspension between photocatalyst powders and rhodamine B (RhB) and acid orange 7 (AO7) at given time intervals (3 mL aliquots) were sampled and centrifuged to remove photocatalyst powders. The concentration of the organic dyes was determined by monitoring the height of the maximum of the absorbance in ultraviolet visible spectra (wavelength from 200 to 800 nm) by a a UV−vis/NIR spectrophotometer (Shimadzu 2550, Japan). As a comparison,

2. EXPERIMENTAL SECTION 2.1. Synthesis of Mesoporous Bi20TiO32 Nanosheets. All chemicals were analytical grade and used without further purification. In a typical procedure, 3.64 g of Bi(NO3)3·5H2O and 0.18 g of Ti(OC3H7)4 was dissolved in 15 mL of benzyl alcohol under vigorous stirring before being transferred to a Teflon-lined stainless autoclave (50 mL capacity). The hydrothermal synthesis was conducted at 90−150 °C for 24 h in an electric oven. The system was then cooled to ambient temperature naturally. The as-prepared samples as precursors were collected and washed with distilled water and absolute alcohol several times, vacuum-dried, and then heated at 300 °C to obtain Bi20TiO32 nanostuctures. In comparison, the Bi12TiO20 nanowires were also synthesized by the hydrothermal process according to the previous work.21 2.2. Synthesis of Hierarchical Ag−AgCl@Bi20TiO32 Composites. In a typical synthesis of Ag−AgCl@Bi20TiO32 5133



Article

the Bi12TiO20 nanowires were also tested under visible light experiment conditions.

3. RESULTS AND DISCUSSION 3.1. Characterization of Mesoporous Bi20TiO32 Nanosheets. The crystal structure of the as-prepared sample was analyzed by the XRD pattern, as shown in Figure 1. It is evident

Figure 1. XRD pattern of the as-prepared Bi20TiO32 nanostructures via the post-heat-treatment route in assistance with a facile hydrothermal process.

Figure 3. TEM and HRTEM images with different magnification (a− c) and the SAED pattern (d) of the as-prepared mesoporous metastable Bi20TiO32 nanosheets.

that the product calcined at 300 °C using the Bi−O−Ti precursor at the hydrothermal temperature of 120 °C can be indexed to well-crystallized Bi20TiO32 structure (PDF No. 42202) with the major peaks at 2θ = 28.02°, 31.62°, 32.88°, 46.26°, 54.16°, 55.77°, and 57.91°, corresponding to the diffractions of the (201), (002), (220), (222), (203), (421), and (402) planes of the tetragonal Bi20TiO32 structure with the lattice parameters a = 7.700 Å and c = 5.653 Å. The broadening of the reflections distinctly indicates the intrinsic nature of nanocrystals, and the average crystallite size of Bi20TiO32 nanocrystals is around 6 nm, calculated by the Sherrer formula. With the increase of the calcined temperature up to 500 °C using the Bi−O−Ti precursor, the Bi20TiO32 structure was transformed gradually into the cubic Bi12TiO20 phase (see Supporting Information, Figure S1). Thus, the controllable temperature played a very important role in the synthesis of single-phase Bi20TiO32. The morphologies of the as-prepared samples using transmission electron microscopy (TEM) are presented in Figures 2 and 3. The TEM images with low magnification of the as-synthesized Bi20TiO32 samples are shown in Figure 2, exhibiting that the products are composed of flat sheetlike mesostructures with a high degree of periodicity over large

domains. This kind of open mesoporous architecture, with a two-dimentional connected pore system, is a desirable feature of catalyst design due to its ability to improve the molecular transport of reactants and products.37 With the increase of the calcined temperature up to 500 °C, the cubic Bi12TiO20 sample possesses the dispersed nanocrystals with the average particle size above 100 nm (see Supporting Information, Figure S2), indicating that ordered mesostructure cannot be retained after the high calcined temperature. Combined the XRD and TEM analysis, the appropriate temperature played a pivot role in the determination of the mesoporous single-phase Bi20TiO32 nanosheets. In addition, the TEM images with high magnification and HRTEM images are presented in Figure 3, also demonstrating that the synthesized mesoporous Bi20TiO32 possesses highly ordered mesostructure. The high-resolution TEM (HRTEM) images (Figure 3c,d) show that the walls are highly crystalline with a lattice spacing of 0.28 nm for the (002) planes of tetragonal Bi20TiO32 phase. The selective area electron diffraction (SAED) pattern of ordered mesostructure domains further confirms that the mesoporous wall is crystalline (as shown in Figure 3d), corresponding with a spot pattern consistent with Bi20TiO32 nanosheets exposed {001} facets, which agrees well with the previous works.38 In order to reveal the very essential part of the bismuth titanate, the chemical states of as-prepared Bi20TiO32 nanosheets were carefully checked by the X-ray photoelectron spectroscopy (XPS), as shown in Figure 4. The Bi 4f fine XPS spectrum of the Bi20TiO32 nanosheets is presented in Figure 4b. XPS signals of Bi 4f were observed at binding energies at around 164.2 eV (Bi 4f5/2) and 158.9 eV (Bi 4f7/2) ascribed to Bi3+, which are consistent with the data of Bi2O3 powders.38 The Ti 2p XPS spectrum of the Bi20TiO32 nanosheets is shown in Figure 4c. XPS signals of Ti 2p were observed at binding energies at around 465.5 eV (Ti 2p1/2) and 458.5 eV (Ti 2p3/2), which is in good agreement with the previous report.38

Figure 2. TEM images with different magnification of the as-prepared mesoporous metastable Bi20TiO32 nanosheets. 5134



Article

Figure 4. XPS spectra of (a) survey spectrum, (b) Bi 4f, and (c) Ti 2p for the as-prepared mesoporous metastable Bi20TiO32 nanosheets.

Full nitrogen sorption isotherms of the mesoporous Bi20TiO32 nanosheets were measured to gain the information about the specific surface area, as shown in Figure 5. The

Figure 6. UV−vis spectra of the as-prepared mesoporous metastable Bi20TiO32 nanosheets and P25. The inset is plots of (ahv)2 vs energy (hv) for the band gap energies.

gap energy of the as-prepared Bi20TiO32 samples is determined from a plot of (ahv)2 vs energy (hv) and is found to be about 2.36 eV. In comparison, the absorption spectra onset of P25 is just about 387 nm, and the relative band gap of P25 TiO2 is about 3.2 eV. Thus, it is considered that utilizing visible light for driving photocatalytic reactions is a key challenge and visible light absorption of a material is a prerequisite for visible light activity. The photocatalytic activities of the as-prepared Bi20TiO32 samples were evaluated by monitoring the decomposition of rhodamine B (RhB) in an aqueous solution under visible light irradiation. Figure 7 shows the photodegradation of RhB dye as a function of irradiation time. The results show that after irradiation for 60 min about 99.3% of RhB dye molecules were degraded for as-prepared mesoporous metastable Bi20TiO32 nanosheets. In comparison, only 30.0% of RhB dye molecules were degraded for as-prepared Bi12TiO20 nanowires.21 In addition, the blank experiments were also carried out in the presence of Bi20TiO32 without irradiation or in the presence of irradiation without Bi20TiO32, from which we can observe that the RhB dye cannot be degraded under visible light irradiation in the absence of photocatalysts. Thus, the as-prepared mesoporous metastable Bi20TiO32 nanosheets are promising photocatalysts for environmental purification. 3.3. Characterization of Ag−AgCl@Bi20TiO32 Composites. There has been an explosion of interest in plasmonic photocatalysts, such as Ag/AgCl/ZnO,30 Ag/AgCl/BiOCl,31 Ag/AgCl/titanate,32 Ag/AgBr/TiO2,33 Ag/AgX/GO (X = Cl, Br),34 AgX/Ag3PO4 (X = Cl, Br, I),35 and Ag/AgCl@WO3,36 due to its potential applications in the physical, chemical,

Figure 5. Full nitrogen sorption isotherms of as-prepared mesoporous metastable Bi20TiO32 nanosheets. The inset is the corresponding poresize distribution of the mesoporous Bi20TiO32 nanosheets.

specific surface area was calculated to be 26 m2 g−1 by the BET equation, as shown in Figure 5. The corresponding Barrett− Joyner−Halenda (BJH) analyses (the inset in Figure 5) exhibit that most of the pores fall into the size range from 3 to 55 nm. These pores presumably arise from the spaces among the mesoporous Bi20TiO32 nanosheets. The high surface area and mesoporous structure of the mesoporous Bi20TiO32 nanosheets provide the possibility for the efficient diffusion and transportation of the degradable organic molecules and hydroxyl radicals in photochemical reaction, which will lead to the enhanced photocatalytic performance of the mesoporous Bi20TiO32 nanosheets. 3.2. Light Absorption and Photocatalytic Activities of Mesoporous Bi20TiO32 Nanosheets. The optical absorption of the mesoporous Bi20TiO32 nanosheets was measured by UV−vis diffuse reflection spectroscopy, as shown in Figure 6. The mesoporous single-phase metastable Bi20TiO32 nanosheets presented the wavelength range up to 550 nm for the visible light absorption. For a crystalline semiconductor, the optical absorption near the band edge follows the equation39 ahv = A(hv − Eg )n/2

where a, n, Eg, and A are absorption coefficient, light frequency, band gap, and a constant, respectively. According to the equation above, the value of n for Bi20TiO32 is 1. The band 5135



Article

detect by the XRD method for the sample, which is in agreement with the recent reports.40 Typical FE-SEM and TEM observations with different magnification of the as-prepared hierarchical Ag−AgCl@ Bi20TiO32 composites are shown in Figure 3S and Figure 9.

Figure 7. Visible-light photocatalytic degradation of RhB solution for the various samples: RhB without catalyst, Bi12TiO20 nanowires, and as-prepared mesoporous metastable Bi20TiO32 nanosheets.

biological, photoelectric, and catalytic fields. Figure 8 shows the XRD patterns of the as-prepared Ag/AgCl nanocrystals

Figure 9. TEM images with the various magnification of the asprepared hierarchical Ag−AgCl@Bi20TiO32-50 photocatalysts.

From Figure 3S, the FE-SEM images of the as-prepared Ag− AgCl@Bi20TiO32 composites are shown, which indicates that the Ag/AgCl nanoparticles are uniformly anchored on the surfaces of the mesoporous Bi20TiO32 nanosheets. Besides, the corresponding EDS spectrum (Figure 4S) obtained in the FESEM observation shows that the mesoporous Bi20TiO32 nanosheets are composed of Bi, Ti, and O as the component elements, and the as-prepared Ag−AgCl nanocrystals consist of Ag, O, and Cl as the major elements. According to the EDS result, the surface atomic ratio of silver to chlorine was 1.4 times higher than the stoichiometric ratio in AgCl (1:1), indicating the existence of excessive Ag nanocrystals on the surface. Furthermore, the TEM images of the as-prepared hierarchical Ag−AgCl@Bi20TiO32 composites in Figure 9 confirm that the AgCl nanoparticles are firmly attached on the surfaces of the mesoporous Bi20TiO32 nanosheets. Considering the fact that there was metallic silver in the asprepared Ag−AgCl@Bi20TiO32 sample from the XRD, FESEM, EDS, and TEM results, this implied that the excessive amount of silver might have been produced from the photoreduction of AgCl on the surface. Thus, this hierarchical nanostructure is especially favorable for the enhancement of photocatalytic performance. 3.4. Light Absorption and Photocatalytic Activities of Ag−AgCl@Bi20TiO32 Composites. The UV−vis diffusereflectance spectra of the as-prepared catalysts are shown in Figure 10. The series of Ag−AgCl@Bi20TiO32 photocatalysts exhibited broad absorption in the region of visible light, which is attributed to the surface plasmon resonance effect of the Ag species formed in situ on the surfaces of the AgCl nanoparticles.36,42 Thus, the hybridization of Ag−AgCl nanocrystals and the mesoporous Bi20TiO32 nanosheets is effective for the visible-light response of the composite. The photocatalytic activities of the as-prepared Ag−AgCl@ Bi20TiO32 composites samples were evaluated by monitoring

Figure 8. XRD patterns of Ag-AgCl@Bi20TiO32 with different photoreduction times: (a) mesoporous metastable Bi20TiO32 nanosheets, (b) Ag-AgCl@Bi20TiO32-10, (c) Ag-AgCl@Bi20TiO32-30, (d) Ag-AgCl@Bi20TiO32-50, and (e) Ag−AgCl@Bi20TiO32-70.

decorated the mesoporous metastable Bi20TiO32 nanosheets catalysts. All the diffraction lines can be indexed as the tetragonal Bi20TiO32, cubic phase Ag, and cubic phase AgCl, which are marked clearly in the XRD patterns. Especially, after photoreducing AgCl in UV−vis light for 10−30 min, three major diffraction peaks at 28.06°, 32.48°, and 46.54° in the 2θ range of 10°−80°, which can be indexed to the (111), (200), and (220) reflections of a cubic silver chloride phase (space group: Fm3m [225]) according to the JCPDS card (No. 311238). Currently, as the irradiation time increased, the intensity of peaks ascribed to AgCl decreased, indicating that some AgCl was photoreduced, and the corresponding Ag nanoparticles were generated on the surface of Ag−AgCl@Bi20 TiO 32 composites. However, there are no obvious reflections belonging to metal Ag species during this photoreducing process for 10−30 min. Though the photoreduction for 50−70 min, the Ag−AgCl@Bi20TiO32 composites show a recognizable reflection at 2θ = 38.1° (as shown in Figure 8de) belonging to the metal Ag species in their XRD patterns. It is considered that the amount of the Ag species photoinduced is too small to 5136



Article

Figure 12. Visible-light photocatalytic degradation of RhB solution for various samples: (a) Ag−AgCl, (b) mesoporous Bi20TiO32 nanosheets, (c) Ag−AgCl@Bi20TiO32-10, (d) Ag−AgCl@Bi20TiO32-30, (e) Ag− AgCl@Bi20TiO32-50, and (f) Ag−AgCl@Bi20TiO32-70.

Figure 10. UV−vis spectra of the as-prepared mesoporous metastable Bi20TiO32 nanosheets coupled with different amount of Ag−AgCl nanocrystals: (a) mesoporous Bi20TiO32 nanosheets, (b) Ag−AgCl@ Bi20TiO32-10, (c) Ag−AgCl@Bi20TiO32-30, (d) Ag−AgCl@Bi20TiO3250, and (e) Ag−AgCl@Bi20TiO32-70.

min). As the duration of photoreduction increased, the Ag nanoparticles content increased correspondingly, changing the ratios of Ag0/Ag+. There should be an optimum ratio related to the best photocatalytic activity. For a fair comparison of all the Ag−AgCl@Bi20TiO32 samples, the ratio of the Ag−AgCl@ Bi20TiO32-50 sample may best approach the optimum one. In order to examine the elemental composition, chemical status and the content of elements of the Ag−AgCl@Bi20TiO32-50 composite after photocatalytic reduction were examined by Xray photoelectron spectroscopy. In the XPS spectra in Figure 13a of the Ag 3d regions of the Ag−AgCl@Bi20TiO32-50 sample, two peaks are observed at approximately 367.4 and 373.6 eV in each spectrum, which are ascribed to the Ag 3d5/2 and Ag 3d3/2 binding energies, respectively. These two peaks could be further deconvoluted into two peaks, at about 367.3/ 368.4 eV and 373.6/374.6 eV, respectively. The peaks at 367.3 and 373.6 eV are attributed to Ag+ of AgCl, and those at 368.4 and 374.6 eV are ascribed to the metal Ag0. According to XPS results, the ratio of Ag0/Ag+ increased as the duration of photoreduction increased from 10 to 70 min, indicating that the excessive amount of silver might have been produced from the photoreduction of AgCl on the surface. That is why the Ag− AgCl@Bi20TiO32-70 sample shows lower efficiency in photocatalytic activity. Besides, it is generally recognized the

the decomposition of rhodamine B (RhB) and acid orange 7 (AO7) in an aqueous solution under visible light irradiation, respectively. The time-dependent UV−vis absorption spectra of RhB and AO7 dyes during the irradiation are displayed in Figures 11 and 14. It can be seen clearly that the maximum absorbance decreases greatly after visible-light irradiation within 30 min for the as-prepared mesoporous metastable Bi20TiO32 nanosheets and the hierarchical Ag−AgCl@Bi20TiO32-50 photocatalysts. Figures 12 and 15 show the photodegradation of RhB and AO7 dyes as a function of irradiation time for the various Ag−AgCl@Bi20TiO32 samples. The obtained results for RhB degradation show that after visible-light irradiation the photocatalytic activities for the Ag−AgCl@Bi20TiO32 samples increased with the increase of photoreduction time among 0− 50 min. However, the photocatalytic performance decreased with the increase of photoreduction time up to 70 min. In addition, the photocatalytic performance of Ag or AgCl nanocrystals supporting Bi20TiO32 alone was conducted in comparison (see Supporting Information, Figure S5). Among these samples, the Ag−AgCl@Bi20TiO32-50 photocatalysts exhibited the most pronounced photocatalytic activity with the highest RhB and AO7 degradation efficiencies of about 98.9% (irradiation for 25 min) and 97.5% (irradiation for 30

Figure 11. UV−vis spectra changes in the RhB degradation using different samples: (a) as-prepared mesoporous metastable Bi20TiO32 nanosheets and (b) the hierarchical Ag−AgCl@Bi20TiO32-50 photocatalysts under visible-light irradiation. 5137



Article

Figure 13. XPS spectra of (a) Ag 3d and (b) Cl 2p for the Ag−AgCl@Bi20TiO32-50 composite.

Figure 14. UV−vis spectra changes in the acid orange 7 degradation with (a) the mesoporous metastable Bi20TiO32 nanosheets and (b) the hierarchical Ag−AgCl@Bi20TiO32-50 photocatalysts under visible-light irradiation.

tion results indicated that the introduction of Ag/AgCl had little effect on the structural properties of the Bi20TiO32 support. Thus, the enhancement of photocatalytic performance of the Ag−AgCl@Bi20TiO32-50 composite is ascribed to the roles of Ag−AgCl nanocrystals. The recycling properties of the hierarchical Ag−AgCl@ Bi20TiO32 photocatalyst for RhB and AO7 solutions under visible-light irradiation (λ > 420 nm), as shown in Figure S7, demonstrating a fairly stable photocatalytic performance for RhB and AO7 photodegradation. After a recycling application of four times, there is almost no obvious changes for the photodegrading performance of the Ag−AgCl@Bi20TiO32 photocatalyst. The XRD pattern of the Ag−AgCl@Bi20TiO32 sample after the recycling use indicates that the Ag content slightly increases, but the major phases are still the AgCl and Bi20TiO32 nanosheets (see Supporting Information, Figure S8). We further understand the mechanism for the remarkably enhanced photocatalytic performance of the Ag−AgCl@ Bi20TiO32 photocatalysts from the following aspects. First, the existence of Ag/AgCl nanocrystals on the surfaces of the Bi20TiO32 nanosheets forms an uniquely hierarchical nanostructure, which provides a high surface area and a large number of interfaces between the Ag/AgCl and Bi20TiO32 species. The high surface areas and profuse interfaces are accessible to the outer environment and provide numerous active sites for the photodegradation of dye molecules. Second, the metal Ag clusters formed in situ on the semiconductors

Figure 15. Visible-light photocatalytic degradation of the acid orange 7 solution using various samples: (a) mesoporous metastable Bi20TiO32 nanosheets and (b) the hierarchical Ag−AgCl@Bi20TiO32-50 photocatalysts under visible-light irradiation.

importance of surface area on the catalytic activity. After the BET measurement (see Figure S6), it is found that there was no obvious difference before and after the deposition of Ag/ AgCl on Bi20TiO32 structures. The N2 adsorption−desorption isotherms curves of photocatalysts are type IV isotherms. The pore-size distribution was also very similar for Bi20TiO32 and Ag−AgCl@Bi20TiO32-50 materials. N2 adsorption and desorp5138



Article

electron to be separated from recombination with the hole. Therefore, the Ag−AgCl@Bi20TiO32 photocatalysts exhibit excellent photocatalytic performance.

AgCl and the Bi20TiO32 nanocrystals remarkably enhance the absorption in the visible light region due to the surface plasmonic resonance effect, as shown in Figure 16, which

4. CONCLUSION The hierarchical photocatalysts of Ag/AgCl/Bi20TiO32 composites have been successfully synthesized by anchoring Ag/AgCl nanocrystals on the surfaces of mesoporous single-crystalline metastable Bi20TiO32 nanosheets via a two-stage strategy for excellent visible-light-driven photocatalytic activities. The asprepared samples were characterized by a series of techniques, such as X-ray diffraction (XRD), electron microscopy (EM), Brunauer−Emmett−Teller analysis (BET), X-ray photoelectron spectroscopy (XPS), and UV−vis diffuse reflectance absorption spectra (UV−vis). First, the single-crystalline metastable Bi20TiO32 nanosheets with dominant {001} facets and tetragonal structures were prepared via the post-heattreatment route in assistance with a facile hydrothermal process using benzyl alcohol as solvent without the use of any template. Especially, the mesoporous single-crystalline metastable Bi20TiO32 nanosheets showed high photocatalytic activity for the degradation of rhodamine B dye under visible-light irradiation. Then, the as-prepared mesoporous tetragonal Bi20TiO32 nanosheets were used as a support for loading AgCl nanocrystals using the deposition−precipitation method, and the deposited AgCl was partially reduced to Ag via the irradiation process to fabricate the Ag/AgCl/Bi 20 TiO 32 composites. Inspiringly, the hierarchical Ag/AgCl/Bi20TiO32 photocatalyst has the higher photocatalytic performance than Ag/AgCl and Bi20TiO32 nanosheets over the degradation of rhodamine B and acid orange 7 dyes, which is attributed to the effective charge transfer from plasmon-excited Ag nanocrystal to Bi20TiO32 for the construction of a Z-scheme visible-light photocatalyst. This work could provide new insights into the fabrication of hierarchically plasmonic photocatalysts with high performance and facilitate their practical application in environmental issues.

Figure 16. Schematic illustration of the charge separation and transfer in the Ag−AgCl@Bi20TiO32 composites under visible-light irradiation.

photogenerates transient holes that can oxidize the dye molecules.36,42 Last but not least, the positively synergistic effects of the coupling of Ag/AgCl and the Bi 20 TiO32 nanosheets improve the effective separation of the photogenerated electron−hole pairs. The possible transfer routes of the photogenerated electrons and holes are also introduced to explain the enhanced photocatalytic activity using the conduction band minimum (CBM) and valence band maximum (VBM) potentials of Ag/ AgCl and Bi20TiO32. For a semiconductor, according to the following empirical equation,39 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, Ee 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 = X − Ee − 0.5Eg. The X value for Bi20TiO32 is ca. 5.913 eV. The conduction band (CB) and valence band (VB) potentials of Bi20TiO32 are ca. +0.22 and 2.60 V (vs SHE), respectively. Besides, the CB and VB energy levels of AgCl are calculated to be ca. −0.06 and +3.2 V (vs SHE), respectively. On the basis of the above experimental results and band structure analysis of Ag/AgCl and Bi20TiO32, a plasmonic Z-scheme mechanism of Ag−AgCl@Bi20TiO32 composite is proposed and shown in Figure 16. Under visible-light irradiation, the Bi20TiO32 nanosheets can absorb visible-light photons to produce photogenerated electrons and holes. Because of the surface plasmonic resonance effect and dipolar character of metallic Ag, metallic Ag can also absorb visible light, and the absorbed photon would be efficiently separated to an electron and a hole.43,44 The plasmon-induced electrons of Ag nanoparticles are transported to the CB of AgCl to reduce oxygen, while the holes remain on the Ag nanoparticles. Currently, the photogenerated electrons of Bi20TiO32 nanosheets transfer to the Ag nanoparticles to recombine with the plasmon-induced holes produced by plasmonic absorption of Ag nanoparticles, while the VB holes remain on Bi20TiO32 to oxidize organic substances. So, such electron transfer from Ag to a semiconductor should be expected to facilitate the photoexcited

■

ASSOCIATED CONTENT

S Supporting Information *

Synthesis details of Bi12TiO20 nanowires, XRD pattern and TEM image of bismuth titanate using the precursors calcined at 500 °C; SEM images, EDS sepectra, BET measurement, the efficiency, and XRD patterns of the recycling photocatalytic tests for Ag−AgCl@Bi20TiO32-50 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86 10 62334204. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 51102015, 21071014, and 51004008), the Fundamental Research Funds for the Central Universities (No. FRF-AS-11-002A and FRF-TP-12-023A), China Postdoctoral Science Foundation (No. 20110490009), Research Fund for the Doctoral Program of Higher Education of China (No. 20110006120027), National High Technology Research and Development Program of China (863 Program, 5139



Article

(18) Hou, J. G.; Wang, Z.; Jiao, S. Q.; Zhu, H. M. Bismuth Titanate Pyrochlore Microspheres: Directed Synthesis and Their Visible Light Photocatalytic Activity. J. Solid State Chem. 2011, 184, 154−158. (19) Hou, J. G.; Jiao, S. Q.; Zhu, H. M.; Kumar, R. V. Facile Synthesis And Visible-Light Photocatalytic Activity of Bismuth Titanate Nanorods. J. Nanopart. Res. 2011, 13, 5557−5564. (20) Zhou, J. K.; Zou, Z. G.; Ray, A. K.; Zhao, X. S. Preparation and Characterization of Polycrystalline Bismuth Titanate Bi12TiO20 and Its Photocatalytic Properties under Visible Light Irradiation. Ind. Eng. Chem. Res. 2007, 46, 745−749. (21) Hou, J. G.; Qu, Y. F.; Krsmanovic, D.; Ducati, C.; Eder, D.; Kumar, R. V. Hierarchinal Synthesis of Bismuth Titanate Complex Architectures and Their Visible-Light Photocatalytic Activities. J. Mater. Chem. 2010, 20, 2418−2423. (22) Hou, J. G.; Qu, Y. F.; Krsmanovic, D.; Ducati, C.; Eder, D.; Kumar, R. V. Solution-Phase Synthesis of Single-Crystalline Bi12TiO20 Nanowires with Photocatalytic Property. Chem. Commun. 2009, 26, 3937−3939. (23) Cheng, H. F.; Huang, B. B.; Dai, Y.; Qin, X. Y.; Zhang, X. Y.; Wang, Z. Y.; Jiang, M. H. Visible-Light Photocatalytic Activity of the Metastable Bi20TiO32 Synthesized by a High-Temperature Quenching Method. J. Solid State Chem. 2009, 182, 2274−2280. (24) Zhou, T. F.; Hu, J. C. Mass Production and Photocatalytic Activity of Highly Crystalline Metastable Single-Phase Bi20TiO32 Nanosheets. Environ. Sci. Technol. 2010, 44, 8698−8703. (25) Hou, J. G.; Wang, Z.; Jiao, S. Q.; Zhu, H. M. 3D Bi12TiO20/TiO2 Hierarchical Heterostructure: Synthesis and Enhanced Visible-Light Photocatalytic Activities. J. Hazard. Mater. 2011, 192, 1772−1779. (26) Murray, W. A.; Barnes, W. L. Plasmonic Materials. Adv. Mater. 2007, 19, 3771−3782. (27) Tsukamoto, D.; Shiraishi, Y.; Sugano, Y.; Ichikawa, S.; Tanaka, S.; Hirai, T. Gold Nanoparticles Located at the Interface of Anatase/ Rutile TiO2 Particles as Active Plasmonic Photocatalysts for Aerobic Oxidation. J. Am. Chem. Soc. 2012, 134, 6309−6315. (28) Guo, J. F.; Ma, B.; Yin, A.; Fan, K.; Dai, W. L. Highly Stable and Efficient Ag/AgCl@TiO2 Photocatalyst: Preparation, Characterization, and Application in the Treatment of Aqueous Hazardous Pollutants. J. Hazard. Mater. 2012, 211−212, 77−82. (29) Ye, L. Q.; Liu, J. Y.; Gong, C. Q.; Tian, L. H.; Peng, T. Y.; Zan, L. Two Different Roles of Metallic Ag on Ag/AgX/BiOX (X = Cl, Br) Visible Light Photocatalysts: Surface Plasmon Resonance and ZScheme Bridge. ACS Catal. 2012, 2, 1677−1683. (30) Xua, Y.; Xu, H.; Li, H.; Xia, J.; Liu, C.; Liu, L. Enhanced Photocatalytic Activity of New Photocatalyst Ag/AgCl/ZnO. J. Alloys Compd. 2011, 509, 3286−3292. (31) Xiong, W.; Zhao, Q.; Li, X.; Zhang, D. One-Step Synthesis of Flower-Like Ag/Agcl/BiOCl Composite with Enhanced Visible-Light Photocatalytic Activity. Catal. Commun. 2011, 16, 229−233. (32) Tang, Y.; Subramaniam, V. P.; Lau, T. H.; Lai, Y.; Gong, D.; Kanhere, P. D.; Cheng, Y. H.; Chen, Z.; Dong, Z. In Situ Formation of Large-Scale Ag/AgCl Nanoparticles on Layered Titanate Honeycomb by Gas Phase Reaction for Visible Light Degradation of Phenol Solution. Appl. Catal., B 2011, 106, 577−585. (33) Tian, G.; Chen, Y.; Bao, H. L.; Meng, X.; Pan, K.; Zhou, W.; Tian, C.; Wang, J. Q.; Fu, H. Controlled Synthesis of Thorny Anatase TiO2 Tubes for Construction of Ag−AgBr/TiO2 Composites as Highly Efficient Simulated Solar-Light Photocatalyst. J. Mater. Chem. 2012, 22, 2081−2088. (34) Zhu, M.; Chen, P.; Liu, M. Graphene Oxide Enwrapped Ag/ AgX (X = Br, Cl) Nanocomposite as a Highly Efficient Visible-Light Plasmonic Photocatalyst. ACS Nano 2011, 5, 4529−4536. (35) Bi, Y. P.; Ouyang, S. X.; Cao, J. Y.; Ye, J. H. Facile Synthesis of Rhombic Dodecahedral AgX/Ag3PO4 (X = Cl, Br, I) Heterocrystals with Enhanced Photocatalytic Properties and Stabilities. Phys. Chem. Chem. Phys. 2011, 13, 10071−10075. (36) Chen, D. L.; Li, T.; Chen, Q. Q.; Gao, J. B.; Fan, B. B.; Li, J.; Li, X. J.; Zhang, R.; Sun, J.; Gao, L. Hierarchically Plasmonic Photocatalysts of Ag/AgCl Nanocrystals Coupled with SingleCrystalline WO3 Nanoplates. Nanoscale 2012, 4, 5431−5439.

No. 2012AA062302), National Basic Research Program of China (973 Program, No. 2007CB613301), and the Program for New Century Excellent Talents in University (NCET-110577).

■

REFERENCES

(1) Chen, X.; Mao, S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107, 2891−2959. (2) Kozhummal, R.; Yang, Y.; Güder, F.; Hartel, A.; Lu, X.; Kücu̧ ̈kbayrak, U. M.; Mateo-Alonso, A.; Elwenspoek, M.; Zacharias, M. Homoepitaxial Branching: An Unusual Polymorph of Zinc Oxide Derived from Seeded Solution Growth. ACS Nano 2012, 6, 7133− 7141. (3) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253−278. (4) Hou, J. G.; Cao, R.; Wang, Z.; Jiao, S. Q.; Zhu, H. M. Hierarchical Nitrogen Doped Bismuth Niobate Architectures: Controllable Synthesis and Excellent Photocatalytic Activity. J. Hazard. Mater. 2012, 217−218, 177−186. (5) Hou, J. G.; Wang, Z.; Jiao, S. Q.; Zhu, H. M. Bi2O3 Quantum-Dot Decorated Nitrogen-Doped Bi3NbO7 Nanosheets: In Situ Synthesis and Enhanced Visible-Light Photocatalytic Activity. CrystEngComm 2012, 14, 5923−5928. (6) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Photocatalyst Releasing Hydrogen from Water. Nature 2006, 440, 295−295. (7) Hou, J. G.; Cao, R.; Wang, Z.; Jiao, S. Q.; Zhu, H. M. Preparation of Polyaniline Modified TaON with Enhanced Visible Light Photocatalytic Activities. Dalton Trans. 2011, 40, 4038−4041. (8) Yi, Z. G.; Ye, J. H.; Kikugawa, N.; Kako, T.; Ouyang, S. X.; StuartWilliams, H.; et al. An Orthophosphate Semiconductor with Photooxidation Properties under Visible-Light Irradiation. Nat. Mater. 2010, 9, 559−564. (9) Hou, J. G.; Wang, Z.; Kan, W. B.; Jiao, S. Q.; Zhu, H. M.; Kumar, R. V. Efficient Visible-Light-Driven Photocatalytic Hydrogen Production Using CdS@TaON Core-Shell Composites Coupled with Graphene Oxide Nanosheets. J. Mater. Chem. 2012, 22, 7291−7299. (10) Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76−82. (11) Hameed, A.; Montini, T.; Gombac, V.; Fornasiero, P. Surface Phases and Photocatalytic Activity Correlation of Bi2O3/Bi2O4‑x Nanocomposite. J. Am. Chem. Soc. 2008, 130, 9658−9659. (12) Zhang, J.; Xu, Q.; Feng, Z. C.; Li, M. J.; Li, C. Importance of the Relationship between Surface Phases and Photocatalytic Activity of TiO2. Angew. Chem., Int. Ed. 2008, 47, 1766−1769. (13) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature 2008, 453, 638−641. (14) Hou, J. G.; Yang, C.; Wang, Z.; Jiao, S. Q.; Zhu, H. M. Bi2O3 Quantum Dots Decorated Anatase TiO2 Nanocrystals with Exposed {001} Facets on Graphene Sheets for Enhanced Visible-Light Photocatalytic Performance. Appl. Catal., B 2013, 129, 333−341. (15) Hou, J. G.; Cao, R.; Wang, Z.; Jiao, S. Q.; Zhu, H. M.; Kumar, R. V. PANI/Bi12TiO20 Complex Architectures: Controllable Synthesis and Efficient Visible Light-Driven Photocatalytic Activities. Appl. Catal., B 2011, 104, 399−406. (16) Wei, W.; Dai, Y.; Huang, B. B. First-Principles Characterization of Bi-based Photocatalysts: Bi12TiO20, Bi2Ti2O7, and Bi4Ti3O12. J. Phys. Chem. C 2009, 113, 5658−5663. (17) Hou, J. G.; Qu, Y. F.; Krsmanovic, D.; Kumar, R. V. Peroxide Based Route Assisted with an Inverse Microemulsion Process to WellDispersed Bi4Ti3O12 Nanocrystals. J. Nanopart. Res. 2010, 12, 1797− 1805. 5140



Article

(37) Rolison, D. R. Catalytic Nanoarchitectures the Importance of Nothing and the Unimportance of Periodicity. Science 2003, 299, 1698−1701. (38) Hou, J. G.; Cao, R.; Wang, Z.; Jiao, S. Q.; Zhu, H. M. Chromium-Doped Bismuth Titanate Nanosheets as Enhanced VisibleLight Photocatalysts with a High Percentage of Reactive {110} Facets. J. Mater. Chem. 2011, 11, 7296−7301. (39) Butler, M. A.; Ginley, D. S. Prediction of Flatband Potentials at Semiconductor-Electrolyte Interfaces from Atomic Electronegativities. J. Electrochem. Soc. 1978, 125, 228−232. (40) Jones, M. R.; Osberg, K. D.; Macfarlane, R. J.; Langille, M. R.; Mirkin, C. A. Templated Techniques for the Synthesis and Assembly of Plasmonic Nanostructures. Chem. Rev. 2011, 111, 3736−3827. (41) Wu, T.; Liu, S.; Luo, Y.; Lu, W.; Wang, L.; Sun, X. Surface Plasmon Resonance-Induced Visible Light Photocatalytic Reduction of Graphene Oxide: Using Ag Nanoparticles as a Plasmonic Photocatalyst. Nanoscale 2011, 3, 2142−2144. (42) Lan, J.; Zhou, X.; Liu, G.; Yu, J.; Zhang, J.; Zhi, L.; Nie, G. Enhancing Photocatalytic Activity of One-Dimensional KNbO3 Nanowires by au Nanoparticles under Ultraviolet and Visible-Light. Nanoscale 2011, 3, 5161−5167. (43) Linic, S.; Christopher, P.; Ingram, D. B. Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911−921. (44) Liu, R.; Wang, P.; Wang, X. F.; Yu, H. G.; Yu, J. G. UV- and Visible-Light Photocatalytic Activity of Simultaneously Deposited and Doped Ag/Ag(I)-TiO2 Photocatalyst. J. Phys. Chem. C 2012, 116, 17721−17728.

5141


AgCl

Recommend Documents