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TiO2 mesoporous spheres with highly efficient visible light photocatalytic activity
Journal of Colloid and Interface Science 450 (2015) 246–253
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Heterostructures of Ag3PO4/TiO2 mesoporous spheres with highly efficient visible light photocatalytic activity Yanjuan Li a,b, Liangmin Yu a, Nan Li b, Wenfu Yan c, Xiaotian Li b,⇑ a
Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, PR China College of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China c State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China b
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 8 January 2015 Accepted 9 March 2015 Available online 20 March 2015 Keywords: Ag3PO4 nanoparticle Mesoporous TiO2 sphere Photocatalysis Methylene blue UV–visible light
⇑ Corresponding author. Fax: +86 431 85168444. E-mail address: [email protected] (X. Li). http://dx.doi.org/10.1016/j.jcis.2015.03.016 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
a b s t r a c t Heterostructured Ag3PO4/m-TiO2 (mesoporous sphere) visible-light photocatalyst has been synthesized via a facile method. The resultant composite consists of numerous Ag3PO4 nanoparticles with diameter less than 10 nm, and these nanoparticles deposit onto the TiO2 nanoparticles surface forming a heterostructure. N2 adsorption–desorption measurements have suggested that the composite was porous with relative high surface area. Studies of the photocatalytic activity and stability of heterostructured Ag3PO4/m-TiO2 for the degradation of methylene blue (MB) have indicated that its visible light photocatalytic performance was improved compared with pure Ag3PO4 and Ag3PO4/m-TiO2, and exhibited excellent photocatalytic stability. The performance was improved attributing to three aspects: (1) the large specific surface area enhanced the adsorption of MB; (2) numerous pores enlarged the contact area between photocatalyst and MB; (3) the most importantly, depositing Ag3PO4 onto the surface of TiO2 facilitated the separation of electron and hole pairs, which also elevates the photocatalytic performance. Furthermore, the photocatalytic mechanism also has been discussed. Compare with Ag3PO4, the Ag weight percent of Ag3PO4/m-TiO2 decreases from 77% to 20.8%, significantly reducing the cost of photocatalyst. Ó 2015 Elsevier Inc. All rights reserved.
Y. Li et al. / Journal of Colloid and Interface Science 450 (2015) 246–253
1. Introduction Photocatalysis has attracted considerable attention attributing to the fact that it provides a new way to meet the challenges of the environment, energy and sustainability [1,2]. With the development of efficient photocatalytic systems, significant efforts have been devoted to synthesize highly active photocatalysts that function with visible light, and which can be applied in a wide region such as water splitting for hydrogen production, removal and degradation of pollutants and water purification [3–5]. Of the well-known photocatalysts, titanium oxide (TiO2) has been proved to be a prospect due to its high photocatalytic efficiency, long-term stability, low cost and nontoxicity [6,7]. However, with a wide band gap (3.2 eV), TiO2 only absorbs light in UV region, resulting in a low-efficiency in the utilization of solar energy. During past decades, researchers have made much effort to explore and fabricate the composite of TiO2 with narrow band materials to realize their visible light photocatalytic performance [8–11]. Various TiO2-based composites such as Bi2O3/TiO2 [12], Bi2S3/ TiO2 [13], N-doped TiO2 [14], BiMoO3/TiO2 [15], Ag@TiO2 [16], Ag–AgBr/TiO2 [9], CdS/TiO2 [17], PbS/TiO2 [18], CNT/TiO2 [19], and TiO2@graphene [20] have been successfully fabricated and their visible-light photocatalytic properties have been extensively explored. These novel materials have greatly expand our ability to construct a wide range of photocatalytic systems, however, it is worth pointing out that their unsatisfactory activities, low efficiencies in utilization of solar irradiation and toxicity of heavy metals have limited their applications. Therefore, it is a challenge to develop highly visible-light active photocatalysts for solar light conversion. Over the past few years, silver orthophosphate (Ag3PO4) has been shown to be a superior visible-light photocatalyst for the degradation of organic dyes [21–24]. However, the particle size of Ag3PO4 remains relatively large (0.5–2 lm) [24,25], hindering their performance in photocatalytic processes. Furthermore, the high-cost of starting material, AgNO3, prohibits its large-scale production. Therefore, it is desirable to develop a sample, effective and inexpensive visible-light photocatalyst. As catalysts, the Ag3PO4 synthesized as nanoparticles with a higher surface area is critical. Very recently, it was reported that the heterostructures composed of Ag3PO4 nanoparticles on TiO2 surface enhanced the photocatalytic performance [5,26–29]. This hetero-photocatalyst facilitates the surface separation of photoexcited electron and hole pairs rather than bulk phase separation. Herein, we demonstrate a facile route using silver ammonia solution for loading Ag3PO4 nanoparticles in mesoporous TiO2 spheres. The mesoporous TiO2 materials have a high surface area and large amounts of pores with diameter distribution of 2–50 nm. Due to the restriction of the nano-sized pores, Ag3PO4 nanoparticles can be easily prepared by filling silver sources into these pores, which will facilitate the adsorption of dye and enlarge the contact of dyes toward photocatalysts. Additionally, TiO2 is combined with Ag3PO4 with a narrow band gap will decrease its band gap [5]. When the composites are irradiated by visible-range light, the hole on Ag3PO4 will transferred to the VB of TiO2 and reduce combine rate between photo-generated electron–hole pairs. Both advantages can enhance the export capabilities [5] and consequently improve the photocatalytic performance. For comparison, we also synthesize the sample using traditional impregnated method in AgNO3 aqueous solution. The visible-light-driven photocatalytic activity of Ag3PO4/m-TiO2 is investigated by degradation of methylene blue (MB). The photocatalytic experiments results indicated that this novel heterostructure photocatalyst exhibits much higher activities under visible light than both pure Ag3PO4 and the Ag3PO4/m-TiO2 synthesized by impregnation
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method. And after four cycles, Ag3PO4/m-TiO2 still manifests excellent photocatalytic activity with a degradation of 91%. 2. Experiment section 2.1. Fabrication of mesoporous TiO2 spheres The mesoporous TiO2 spheres were prepared via sol–gel method and solvothermal method. In a typical synthesis process of mesoporous TiO2 spheres, 0.4 g hexadecylamine (HDA) was dissolved in 40 ml ethanol and stirred to form a clear solution. Then 0.385 ml 0.1 M KCl was added into the solution under continuously stirring. To this solution 4.75 ml titanium isopropoxide (TIP) was added under vigorous stirring at ambient temperature. The milky white suspension was kept static for 18 h. And then the mixture was centrifuged and washed several times with ethanol, at last the sample was dried in air at room temperature. To prepare mesoporous TiO2 spheres, a mixture contains 1.0 g dried powder, 20 ml ethanol and 10 ml H2O was put into a 50 ml Teflon-lined autoclave. The autoclave was then sealed and heated at 160 °C for 20 h. After cooling down to room temperature by air quenching, the obtained white powders were collected by centrifuge, washed with ethanol, and then dried at room temperature. Finally the powders were calcined at 500 °C for 2 h in air, denoted as m-TiO2. 2.2. Fabrication of Ag3PO4 loading in mesoporous TiO2 spheres heterostructured photocatalyst In a typical synthesis, 80 mg m-TiO2 spheres were added into 50 ml silver-ammonia ([Ag(NH3)2]+) complex aqueous solution (the concentration is 1.2 mM, 3.6 mM and 7.2 mM) with ultrasonics for 30 min. Then Na2HPO4 aqueous solution was added with continuously ultrasonics for another 15 min. After ultrasonics, the solution was magnetically stirred for 5 h. The powder was then centrifuged, washed with water for five times and dried at room temperature, denoted as Ag-P/m-Ti-X, (X refers to the concentration of [Ag(NH3)2]+ solution). For comparison, pure Ag3PO4 were also prepared under the same conditions without the addition of mesoporous TiO2. In addition, we also synthesized the sample using traditional impregnation for comparison. 80 mg m-TiO2 spheres were impregnated in AgNO3 solution, then amount of Na3PO4 was added and continues stirring. Finally the powders were collected, denoted as Ag-P/m-Ti-tra. The Ag3PO4 cubes and m-TiO2 spheres were mixture by grind, denoted as Ag-P/m-Ti-gri. 2.3. Structrual characterization Powder X-ray diffractions (XRD) were measured on a D8 Tools X-ray diffractometer operating at 40 kV and 30 mA using Cu Ka radiation (k = 1.54056 Å). The morphologies of the products and elemental constitution were determined by a scanning electron microscope with energy dispersive spectrometer (SEM/EDS, JEOL JSM-6700F) as well as by a transmission electron microscope equipped with selected area electron diffraction (TEM/SAED, Tecnai G2 F20). Nitrogen adsorption–desorption isotherms were determined on a Quantochrome Autosorb 1 sorption analyzer at the temperature of liquid nitrogen. Before measurements, samples were degassed in vacuum at 100 °C for 6 h. Specific surface area calculations were made using Brunauer–Emmett–Teller (BET) method at the relative pressure (P/P0) between 0.07 and 0.21. UV–Vis diffuse reflectance spectroscopy of the samples was measured by a spectrophotometer Bws003. BaSO4 was used as a reference standard, and the spectra were recorded in the range 190–800 nm.
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Scheme 1. Illustration of growth process of Ag-P/m-Ti-X composites spheres.
2.4. Catalytic activity testing A 130 mL of MB solution with an initial concentration of 20 mg L1 in the presence of solid catalyst (65 mg) was filled in a photoreactor designed with an internal light source (150 W xenon lamp) surrounded by a water-cooling quartz jacket to cool the lamp. Before the photocatalytic reaction was initiated, the solution was stirred in dark for 30 min to obtain a good dispersion and reached an adsorption–desorption equilibrium between the organic molecules and the catalyst surface. At given intervals of illumination, aliquots of dispersion of the reaction system were taken out for investigation. The degradation of MB was monitored by UV/Vis spectroscopy (UV-2550PC). For the evaluation of the photocatalytic activities, C is the concentration of the MB molecule at a real time t, and C0 stands for the initial concentration of MB. 3. Results and discussions 3.1. Structural features and morphology The method developed to fabricate Ag-P/m-Ti-X is illustrated as Scheme 1. Monodispersed titania beads with wormhole-like mesostructure are formed by the structure-directing role of HDA
in the sol–gel process. Further solvothermal treatment and calcination lead to mesoporous TiO2 spheres, which are served as templates for sequent loading of Ag3PO4 nanoparticles. After [Ag(NH3)2]NO3 and Na2HPO4 added, Ag3PO4 nanoparticles are successfully loaded in the wormhole-like mesoporous spheres. The morphology and structure of mesoporous TiO2 spheres (m-TiO2) were observed by SEM and TEM (Fig. S1). The spheres are well dispersed with uniform diameter of 750 nm, which are composed of numerous nanoparticles as can be seen by the rough surface (Fig. S1a and b). Wormhole-like mesopores (interparticle pores) are abundant throughout the TiO2 spheres (Fig. S1c). An individual particle is about 15 nm and the lattice fringes are correlated to anatase titania (Fig. S1d). After loading of Ag3PO4 nanoparticles, the dimensions and morphologies have no significant changes compared with that of m-TiO2 (Fig. 1a). Fig. 1b represents a single sphere, which reveals the integral sphere morphology of Ag-P/m-Ti-3.6 complex. Observed at this low magnification, no Ag3PO4 particles was found at the sphere surface, attributing to very small size and uniform distribution of Ag3PO4 particles. Under close observation from HRTEM image (Fig. 1c), numerous small nanoparticles less than 10 nm can be found at the edge of the m-TiO2 sphere. Further observation reveals two sets of lattice fringes of 0.357 nm and 0.24 nm, which are attributed to TiO2
Fig. 1. SEM image (a), TEM images (b–d), XRD pattern (e) of Ag-P/m-Ti-3.6 composites. The elemental mapping with the distribution of individual elements is shown as (f–i).
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Fig. 2. (a) XPS survey spectrum of Ag-P/m-Ti-3.6 and XPS spectra of (b) Ti2p, (c) Ag3d, (d) P2p and (e) O1s.
and Ag3PO4 respectively (Fig. 1d). Simultaneously, it is obviously observed that the heterostructure has been formed between Ag3PO4 and TiO2. Crystallographic structure of this composite was further examined by powder XRD shown as Fig. 1e. Several well-resolved diffraction peaks centered at 2h = 20.8°, 29.8°, 33.2°, 47.9° and 54.9° can be readily indexed to body-centered cubic (BCC) structure of Ag3PO4, in well agreement with the reported data JCPDS 35-0736. The energy-dispersive X-ray (EDS) spectra (see Fig. S2) from the corresponding SEM image confirm that except for the element C from the support, only Ti, O, Ag, and P element are detected, and the weight percent of Ag3PO4 is 20.8%. The corresponding elemental mapping of an individual sphere is represented in Fig. 1f–i. It illustrates that all Ti, O, Ag and P elements are homogeneously distributed in the whole mesoporous sphere.
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XPS was used to qualitatively determine surface oxidation state and composition of Ag-P/m-Ti-3.6. Fig. 2a exhibits the fully scanned spectra in the range of 0–1100 eV. The peaks on the curve of Ag-P/m-Ti-3.6 composite are assigned to Ti, O, Ag, P and C elements. The C element is attributed to the calibration carbon. Therefore, it is concluded that Ti, O, Ag and P exist in Ag-P/m-Ti3.6 composite. The high resolution XPS spectra of the Ti 2p are analyzed in Fig. 2b. The peaks centered at 464.5 and 458.8 eV are well corresponded to Ti 2p1/2 and Ti 2p3/2 binding energies, respectively [28]. The splitting between Ti 2p1/2 and Ti 2p3/2 is 5.7 eV, indicating a normal state of Ti4+. The peaks of Ag3d appearing at the binding energy of 374.2 and 368.2 eV can be assigned to the 3d3/2 and 3d5/2 of Ag+ respectively. No peak is found at 369.2 and 375.8 eV, demonstrating there no Ag0 existed [29,30]. The XPS peak of P 2p centered at 133.4 eV is attributed to P5+ in Ag3PO4 [31]. Fig. 2d is the XPS spectrum of O 1s. The peaks at 529.6, 530.1 and 531.8 eV relate to oxygen bonded to contaminated carbon, Ti–O and P–O. The XPS results further demonstrated that Ag3PO4 has been loaded in the mesoporous TiO2 sphere. The effects of [Ag(NH3)2]+ concentration on the structure and compositions of Ag-P/m-Ti heterostructures also have been investigated, and the experimental results are shown in Fig. 3. When [Ag(NH3)2]+ concentration of 1.2 mM and 3.6 mM are used, the morphologies of these composites have no significant changes compared with that of m-TiO2. Due to the adsorption capacity of m-TiO2 limited, further increasing the [Ag(NH3)2]+ concentration to 7.2 mM, a part of [Ag(NH3)2]+ could not adsorb and residual in the solution, so no restrict about Ag3PO4 and isolated Ag3PO4 cubes of 1–2 lm apparent appeared. Their crystalline structures have been also investigated, and the results are shown as Fig. 3C. The crystallinity and content of Ag3PO4 were enhanced with increasing concentration of [Ag(NH3)2]+. TEM images of Ag-P/m-Ti-X sphere composites are given in Fig. 4a–c. It clearly reveals that a large number of Ag3PO4 nanoparticles with different sizes exist in these m-TiO2 sphere. The size distribution histograms of Ag3PO4 nanoparticles calculated from the corresponding TEM images are given as Fig. 4d–f, respectively. It is indicated that the size of Ag3PO4 on the m-TiO2 sphere is strongly dependent on the concentration of [Ag(NH3)2]+. As observed in Fig. 4d, when a smaller concentration of [Ag(NH3)2]+ was added with a concentration of 1.2 mM, the obtained product exhibits a narrow size distribution. The size distribution of Ag3PO4 nanoparticles broadened with increasing concentration of [Ag(NH3)2]+ (Fig. 4e and f). Meanwhile, the large isolate Ag3PO4 particles also exist in this product (inset in Fig. 4c), which is in agreement with SEM result shown as Fig. 3B. The representative magnified TEM images taken from the Ag3PO4 nanoparticles on the different products are displayed in the inset of Fig. 4d–f. The detailed information is summered in Table 1. For comparison, Ag-P/m-TiO2-tra was synthesized through conventional impregnation method, in which
Fig. 3. SEM images (A and B) and XRD (C) of Ag-P/m-Ti-X composites: (a) Ag-P/m-Ti-1.2, (b) Ag-P/m-Ti-7.2. The red box regions shown in figure represent Ag3PO4 cubes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Typical TEM images of Ag-P/m-TiO2 composites: (a) Ag-P/m-Ti-1.2, (b) Ag-P/m-Ti-3.6, and Ag-P/m-Ti-7.2. (d–f) The size distribution histograms of Ag3PO4 calculated from the corresponding TEM images. Inset in c is an isolate Ag3PO4 particle existed in Ag-P/m-Ti-7.2 composite.
Table 1 Physicochemical property of Ag-P/m-Ti-X composites. Sample
Ag3PO4 loading (wt%)
Ag3PO4 particle size (nm)
Average pore size (nm)
BET surface area (m2/g)
Pore volume (cm3/g)
Ag-P/m-Ti-1.2 Ag-P/m-Ti-3.6 Ag-P/m-Ti-7.2 m-TiO2 Ag-P/m-Ti-tra
7.7 20.8 36.8 – 23.9
3.41 ± 0.66 3.44 ± 0.92 4.60 ± 1.06 – 83.2 ± 23.3
11.4 10.7 13.9 11.2 13.0
89.0 64.4 59.3 107.0 40.0
0.29 0.21 0.22 0.37 0.14
a mixture of Ag3PO4 particles and m-TiO2 spheres was observed (Fig. S3a). This result gives further evidence that the method we reported is a more effective way for loading small and well dispersed Ag3PO4 nanoparticles into m-TiO2 spheres. In addition, in the absence of m-TiO2, large Ag3PO4 cubes of 0.5–2 lm are obtained (Fig. S3b). In order to further understand the internal structure, N2sorption isotherms and pore distribution curves are recorded to investigate the effect of Ag3PO4 loading amount on the pore structure of all samples (Fig. 5). It can be seen that all isotherms exhibit type IV with H1 hysteresis loop, which are typical adsorptions for mesoporous materials. This curve feature does not change after Ag3PO4 loading, indicating that the mesoporous structure of mTiO2 is well maintained. However, the specific surface area and pore volume significantly. Moreover, the higher the loading amount, the smaller the specific surface areas and pore volume (see Table 1). These results indicate that Ag3PO4 nanoparticles have been encapsulated into the mesopores. In contrast to surface area and pore volume, the pore size does not change much after Ag3PO4 loading, except for sample Ag-P/m-Ti-7.2, indicating the well dispersed and small-sized Ag3PO4 present in the mesoporous TiO2 spheres. As for Ag-P/m-Ti-7.2 sample, an additional pore is observed at 207.7 nm and the pore size increased slightly compared with that of m-TiO2, which may be caused by the existence of large Ag3PO4 cubes. While compared with the sample Ag-P/m-Ti-tra with a specific surface area of 40 cm2/g, Ag-P/m-Ti-X products prepared by this new method have much higher specific surface area and excellent mesoporous structure, which may be conducive to enlarge the contact area between the catalysts and dyes, consequently enhance the photocatalytic performance.
3.2. Photocatalytic performance The photocatalytic activities were evaluated by photo-oxidation of aqueous methylene blue (MB). Before studying and comparing the activities of the Ag3PO4/m-TiO2 hierarchical nanostructures, the control experiments were performed in the dark (Fig. 6A). These results highlight that the adsorption–desorption equilibrium of MB in the dark is established within 30 min and Ag-P/m-Ti-3.6 has a maximum absorption capacity which is beneficial to improve its photocatalytic performance. Fig. 6B shows the C/C0 versus reaction time for the degradation of MB over the Ag3PO4/m-TiO2 composites. For comparison, the photocatalytic tests of m-TiO2 and Ag3PO4 were also conducted. Their concentrations of MB only decrease 12% and 39%. However, addition of the Ag-P/m-Ti-X composites catalysts to the solution causes the decrease in MB concentration. At first, with the loading of Ag3PO4 increasing (Ag-P/m-Ti-1.2, and Ag-P/m-Ti-3.6), the photocatalytic activities are distinctly on the increases as plots Fig. 6Bc and d showed. At 28 min, nearly 100% MB was degraded by Ag-P/m-Ti-3.6. While further increasing the content of Ag3PO4 to 36.8 wt.% only 83% MB is degraded by Ag-P/m-Ti-7.2, the decrease of degradation may be due to the existence of isolate Ag3PO4 cubes reducing the contact area between dye and catalyst. The specific surface area of Ag-P/m-Ti-X is several ten times of Ag3PO4 cubes (2.32 m2/g). Simultaneously, the degradation yield of Ag-P/m-Ti-tra and Ag-P/ m-Ti-gir can only reach 78% and 68% (Fig. S4), which is maybe due to two aspects: the low specific surface area and less amount of heterostructures, and this confirms that the heterostructures can improved the degradation rate of dyes. Fig. 6C presents evolution of UV–vis spectra of MB solution catalyzed by Ag-P/m-Ti-3.6 over
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Fig. 5. N2-sorption isotherms and pore size distribution curves of m-TiO2 (a), Ag-P/m-Ti-1.2 (b), Ag-P/m-Ti-3.6 (c), and Ag-P/m-Ti-7.2 (d). The pore size distribution curves were calculated from the absorption branch of the nitrogen isotherms by BJH method.
time. The absorbance of major absorption band reduces step by step under UV–visible light, indicating MB is destroyed step by step. At the same time, the suspension faded gradually, as the inset shown in Fig. 6C, demonstrating that the chromophoric structure of the dye has been destroyed. The photocatalytic degradation of MB follows pseudo first-order kinetics, and the apparent degradation rate constant k could be defined by ln (C0/C) = kt. Linear relationships for all the photocatalysts between ln (C0/C) and reaction time are shown in Fig. 6D, and from which we can observe that the apparent reaction rate constant for the Ag-P/m-Ti-3.6 is about 20 and 7.5 times of m-TiO2, Ag3PO4 cubes, further confirming the high visible light photocatalytic activity of the Ag-P/m-Ti-3.6. The stability of a photocatalyst is very important in practical applications. Fig. 7 shows the photocatalytic activity of Ag-P/m-Ti3.6 for the degradation of MB under visible-light irradiation for four cycles. After four-cycling photocatalytic experiments, Ag-P/m-Ti3.6 still exhibits excellent photocatalytic performance with a degradation rate of 91%, revealing its high stability after multiple reuses.
3.3. Mechanism of the improved photocatalytic performance of the Ag3PO4/m-TiO2 composite In order to investigate the adsorption of samples under visible light and UV light, all samples were characterized by UV–vis diffuse reflectance. A comparison of the UV–vis diffuse reflectance spectra of different Ag-P/m-Ti-X samples along with pure m-TiO2 and Ag3PO4 is displayed in Fig. 8A. It can be seen that the
adsorption edges of pure m-TiO2 spheres is at about 390 nm and no adsorption in the visible-light region is observed. As for Ag3PO4 cubes, an adsorption edge occurred at 530 nm. The UV– vis spectrum of Ag-P/m-Ti-gri represents two regions from 200 to 540 nm. The first domain ‘‘R1’’ in the region of 200–400 nm is attributed to the absorption of m-TiO2. And the second domain ‘‘R2’’ is due to the absorption of Ag3PO4. The adsorption region is not extended indicating no heterostructure is formed. Therefore, the band gap does not change. While Ag-P/m-Ti-X composites display clearly narrowed band gap. Their absorptions are in the whole region of 200–760 nm, falling into the visible light range, further indicating the heterostructure is formed between Ag3PO4 and TiO2. As increasing the loading amount of Ag3PO4 in m-TiO2 (AgP/m-Ti-1.2 and Ag-P/m-Ti-3.6), the absorption intensity in the visible-region enhanced, indicating Ag-P/m-Ti-3.6 can make full use of visible light. While for Ag-P/m-Ti-7.2, there is a slight decrease in the absorption intensity and absorption edge compared with that of Ag-P/m-Ti-3.6, which may be due to the existence of large Ag3PO4 cubes. A similar curve as that of Ag3PO4 can be seen as the arrow showed. The absorption of visible light will lay the foundation for the visible light photocatalyst. þ
Ag3 PO4 =TiO2 þ hm ! Ag3 PO4 ðe þ h Þ=TiO2 þ
ð1Þ þ
Ag3 PO4 ðe þ h Þ=TiO2 ! Ag3 PO4 ðe Þ=TiO2 ðh Þ þ
þ
h þ H2 O ! OH þ H
e þ O2 !
O 2
ð2Þ ð3Þ ð4Þ
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Fig. 6. (A) Adsorption in darkness, (B) evolution of MB concentration versus UV–vis irradiation time; (C) time-dependent adsorption spectra of MB in the presence of Ag-P/mTi-3.6; (D) the ln(C0/C) versus irradiation time. Sample a-e represents m-TiO2, Ag3PO4 cubes, Ag-P/m-Ti-1.2, Ag-P/m-Ti-3.6, and Ag-P/m-Ti-7.2 respectively.
Fig. 7. Photocatalytic activities of Ag-P/m-Ti-3.6 for the degradation of MB under visible-light irradiation for four cycles.
e þ O2 þ Hþ ! H2 O2 H2 O2 þ þ
O 2
ð5Þ
! OH þ OH þ O2
ð6Þ
h þ OH ! OH
ð7Þ
ð8Þ
OH þ MB ! CO2 þ H2 O
Combining our experiment results with the related literatures [5,9,27], a mechanism for the enhanced photocatalysis of the Ag-P/m-Ti-X spheres heteroarchitectures under visible light irradiation is proposed (Fig. 8B). As literatures documented, thermodynamic conditions such as the position of semiconductor conduction and valence bands of composite semiconductors favor
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Fig. 8. (A) UV–vis diffuse reflectance spectra of m-TiO2, Ag3PO4, and Ag-P/m-Ti-X samples; and (B) mechanism illustration for enhanced visible-light photocatalytic activity.
the occurrence of phenomenon that efficient interparticle charge transfer between the semiconductors TiO2 (wide band gap) and Ag3PO4 (narrow band gap). When this Ag-P/m-Ti-X composite system is irradiated by visible-range light, the electrons in the VB of Ag3PO4 are excited to its CB. Thereby the VB of Ag3PO4 is rendered partially holes. Because the VB level of Ag3PO4 is lower than that of TiO2, the holes can transferred to the VB of TiO2. Simultaneously, mesoporous TiO2 with a loosely packed structure and large pores is a hole-accepting semiconductor. As a result, the semiconductors with matching band potentials are tightly bonded to construct the efficient heterostructure. The migration of photogenerated holes can be promoted by the internal field, so less of a barrier exists. Meanwhile, the electrons on Ag3PO4 CB accumulating on the surface of Ag3PO4 are then scavenged by dissolved oxygen molecules in water to yield highly oxidative species such as peroxide (H2O2), which can generate hydroxyl radical (OH) and decompose organic substrates effectively. The major reactions that occur in this study can be summarized in reactions (1)–(8). Therefore, the photocatalytic performance can be enhanced greatly owing to the special heteroarchitectures, which can reduce the recombination of electron–hole, thus a larger number of electrons on the Ag3PO4 surface and holes on the TiO2 surface, respectively, can participate in photocatalytic reactions to directly or indirectly mineralize organic pollution. Above results indicate that the semiconductor composites can be excited by visible light. The photogenerated holes in the Ag3PO4 nanoparticles can be effectively injected into the TiO2 due to the strong interfacial bonding between them. As known, the absorption of visible light is the basis of its photocatalytic activity under visible light. Hence the absorption property here implies that the nanocomposites could be promising in visiblelight photocatalysis compared with that of pure TiO2. 4. Conclusions In summary, we have demonstrated a facile method to synthesize Ag-P/m-Ti-X heterostructure photocatalysts. The Ag3PO4 nanoparticles homogeneously distribute in m-TiO2 spheres. The size and loading amount can be adjusted by changing [Ag(NH3)2]+ concentration. Moreover, they have large pores and high specific surface area, which enable them promising materials in adsorption, catalysts, etc. Photocatalytic measurements results suggest that the activities of all Ag-P/m-Ti-X are significantly improved compared with m-TiO2 or Ag3PO4. Especially, Ag-P/mTi-3.6 expresses higher photocatalytic activity than other Ag3PO4/ m-TiO2 composites including the one synthesized by traditional impregnation method. And it represents excellent stability with a degradation of 91% after four cycles. Furthermore, the Ag weight
percent of Ag3PO4/m-TiO2 decreases from 77% to 20.8%, significantly reducing the cost of pure Ag3PO4 photocatalyst. Acknowledgments This work was supported by Natural Science Foundation of China (Nos. 20903046, 21076094). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.03.016. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Heterostructures of Ag3PO4/TiO2 mesoporous spheres with highly efficient visible light photocatalytic activity Yanjuan Li a,b, Liangmin Yu a, Nan Li b, Wenfu Yan c, Xiaotian Li b,⇑ a
Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, PR China College of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China c State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China b
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 8 January 2015 Accepted 9 March 2015 Available online 20 March 2015 Keywords: Ag3PO4 nanoparticle Mesoporous TiO2 sphere Photocatalysis Methylene blue UV–visible light
⇑ Corresponding author. Fax: +86 431 85168444. E-mail address: [email protected] (X. Li). http://dx.doi.org/10.1016/j.jcis.2015.03.016 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
a b s t r a c t Heterostructured Ag3PO4/m-TiO2 (mesoporous sphere) visible-light photocatalyst has been synthesized via a facile method. The resultant composite consists of numerous Ag3PO4 nanoparticles with diameter less than 10 nm, and these nanoparticles deposit onto the TiO2 nanoparticles surface forming a heterostructure. N2 adsorption–desorption measurements have suggested that the composite was porous with relative high surface area. Studies of the photocatalytic activity and stability of heterostructured Ag3PO4/m-TiO2 for the degradation of methylene blue (MB) have indicated that its visible light photocatalytic performance was improved compared with pure Ag3PO4 and Ag3PO4/m-TiO2, and exhibited excellent photocatalytic stability. The performance was improved attributing to three aspects: (1) the large specific surface area enhanced the adsorption of MB; (2) numerous pores enlarged the contact area between photocatalyst and MB; (3) the most importantly, depositing Ag3PO4 onto the surface of TiO2 facilitated the separation of electron and hole pairs, which also elevates the photocatalytic performance. Furthermore, the photocatalytic mechanism also has been discussed. Compare with Ag3PO4, the Ag weight percent of Ag3PO4/m-TiO2 decreases from 77% to 20.8%, significantly reducing the cost of photocatalyst. Ó 2015 Elsevier Inc. All rights reserved.
Y. Li et al. / Journal of Colloid and Interface Science 450 (2015) 246–253
1. Introduction Photocatalysis has attracted considerable attention attributing to the fact that it provides a new way to meet the challenges of the environment, energy and sustainability [1,2]. With the development of efficient photocatalytic systems, significant efforts have been devoted to synthesize highly active photocatalysts that function with visible light, and which can be applied in a wide region such as water splitting for hydrogen production, removal and degradation of pollutants and water purification [3–5]. Of the well-known photocatalysts, titanium oxide (TiO2) has been proved to be a prospect due to its high photocatalytic efficiency, long-term stability, low cost and nontoxicity [6,7]. However, with a wide band gap (3.2 eV), TiO2 only absorbs light in UV region, resulting in a low-efficiency in the utilization of solar energy. During past decades, researchers have made much effort to explore and fabricate the composite of TiO2 with narrow band materials to realize their visible light photocatalytic performance [8–11]. Various TiO2-based composites such as Bi2O3/TiO2 [12], Bi2S3/ TiO2 [13], N-doped TiO2 [14], BiMoO3/TiO2 [15], Ag@TiO2 [16], Ag–AgBr/TiO2 [9], CdS/TiO2 [17], PbS/TiO2 [18], CNT/TiO2 [19], and TiO2@graphene [20] have been successfully fabricated and their visible-light photocatalytic properties have been extensively explored. These novel materials have greatly expand our ability to construct a wide range of photocatalytic systems, however, it is worth pointing out that their unsatisfactory activities, low efficiencies in utilization of solar irradiation and toxicity of heavy metals have limited their applications. Therefore, it is a challenge to develop highly visible-light active photocatalysts for solar light conversion. Over the past few years, silver orthophosphate (Ag3PO4) has been shown to be a superior visible-light photocatalyst for the degradation of organic dyes [21–24]. However, the particle size of Ag3PO4 remains relatively large (0.5–2 lm) [24,25], hindering their performance in photocatalytic processes. Furthermore, the high-cost of starting material, AgNO3, prohibits its large-scale production. Therefore, it is desirable to develop a sample, effective and inexpensive visible-light photocatalyst. As catalysts, the Ag3PO4 synthesized as nanoparticles with a higher surface area is critical. Very recently, it was reported that the heterostructures composed of Ag3PO4 nanoparticles on TiO2 surface enhanced the photocatalytic performance [5,26–29]. This hetero-photocatalyst facilitates the surface separation of photoexcited electron and hole pairs rather than bulk phase separation. Herein, we demonstrate a facile route using silver ammonia solution for loading Ag3PO4 nanoparticles in mesoporous TiO2 spheres. The mesoporous TiO2 materials have a high surface area and large amounts of pores with diameter distribution of 2–50 nm. Due to the restriction of the nano-sized pores, Ag3PO4 nanoparticles can be easily prepared by filling silver sources into these pores, which will facilitate the adsorption of dye and enlarge the contact of dyes toward photocatalysts. Additionally, TiO2 is combined with Ag3PO4 with a narrow band gap will decrease its band gap [5]. When the composites are irradiated by visible-range light, the hole on Ag3PO4 will transferred to the VB of TiO2 and reduce combine rate between photo-generated electron–hole pairs. Both advantages can enhance the export capabilities [5] and consequently improve the photocatalytic performance. For comparison, we also synthesize the sample using traditional impregnated method in AgNO3 aqueous solution. The visible-light-driven photocatalytic activity of Ag3PO4/m-TiO2 is investigated by degradation of methylene blue (MB). The photocatalytic experiments results indicated that this novel heterostructure photocatalyst exhibits much higher activities under visible light than both pure Ag3PO4 and the Ag3PO4/m-TiO2 synthesized by impregnation
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method. And after four cycles, Ag3PO4/m-TiO2 still manifests excellent photocatalytic activity with a degradation of 91%. 2. Experiment section 2.1. Fabrication of mesoporous TiO2 spheres The mesoporous TiO2 spheres were prepared via sol–gel method and solvothermal method. In a typical synthesis process of mesoporous TiO2 spheres, 0.4 g hexadecylamine (HDA) was dissolved in 40 ml ethanol and stirred to form a clear solution. Then 0.385 ml 0.1 M KCl was added into the solution under continuously stirring. To this solution 4.75 ml titanium isopropoxide (TIP) was added under vigorous stirring at ambient temperature. The milky white suspension was kept static for 18 h. And then the mixture was centrifuged and washed several times with ethanol, at last the sample was dried in air at room temperature. To prepare mesoporous TiO2 spheres, a mixture contains 1.0 g dried powder, 20 ml ethanol and 10 ml H2O was put into a 50 ml Teflon-lined autoclave. The autoclave was then sealed and heated at 160 °C for 20 h. After cooling down to room temperature by air quenching, the obtained white powders were collected by centrifuge, washed with ethanol, and then dried at room temperature. Finally the powders were calcined at 500 °C for 2 h in air, denoted as m-TiO2. 2.2. Fabrication of Ag3PO4 loading in mesoporous TiO2 spheres heterostructured photocatalyst In a typical synthesis, 80 mg m-TiO2 spheres were added into 50 ml silver-ammonia ([Ag(NH3)2]+) complex aqueous solution (the concentration is 1.2 mM, 3.6 mM and 7.2 mM) with ultrasonics for 30 min. Then Na2HPO4 aqueous solution was added with continuously ultrasonics for another 15 min. After ultrasonics, the solution was magnetically stirred for 5 h. The powder was then centrifuged, washed with water for five times and dried at room temperature, denoted as Ag-P/m-Ti-X, (X refers to the concentration of [Ag(NH3)2]+ solution). For comparison, pure Ag3PO4 were also prepared under the same conditions without the addition of mesoporous TiO2. In addition, we also synthesized the sample using traditional impregnation for comparison. 80 mg m-TiO2 spheres were impregnated in AgNO3 solution, then amount of Na3PO4 was added and continues stirring. Finally the powders were collected, denoted as Ag-P/m-Ti-tra. The Ag3PO4 cubes and m-TiO2 spheres were mixture by grind, denoted as Ag-P/m-Ti-gri. 2.3. Structrual characterization Powder X-ray diffractions (XRD) were measured on a D8 Tools X-ray diffractometer operating at 40 kV and 30 mA using Cu Ka radiation (k = 1.54056 Å). The morphologies of the products and elemental constitution were determined by a scanning electron microscope with energy dispersive spectrometer (SEM/EDS, JEOL JSM-6700F) as well as by a transmission electron microscope equipped with selected area electron diffraction (TEM/SAED, Tecnai G2 F20). Nitrogen adsorption–desorption isotherms were determined on a Quantochrome Autosorb 1 sorption analyzer at the temperature of liquid nitrogen. Before measurements, samples were degassed in vacuum at 100 °C for 6 h. Specific surface area calculations were made using Brunauer–Emmett–Teller (BET) method at the relative pressure (P/P0) between 0.07 and 0.21. UV–Vis diffuse reflectance spectroscopy of the samples was measured by a spectrophotometer Bws003. BaSO4 was used as a reference standard, and the spectra were recorded in the range 190–800 nm.
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Scheme 1. Illustration of growth process of Ag-P/m-Ti-X composites spheres.
2.4. Catalytic activity testing A 130 mL of MB solution with an initial concentration of 20 mg L1 in the presence of solid catalyst (65 mg) was filled in a photoreactor designed with an internal light source (150 W xenon lamp) surrounded by a water-cooling quartz jacket to cool the lamp. Before the photocatalytic reaction was initiated, the solution was stirred in dark for 30 min to obtain a good dispersion and reached an adsorption–desorption equilibrium between the organic molecules and the catalyst surface. At given intervals of illumination, aliquots of dispersion of the reaction system were taken out for investigation. The degradation of MB was monitored by UV/Vis spectroscopy (UV-2550PC). For the evaluation of the photocatalytic activities, C is the concentration of the MB molecule at a real time t, and C0 stands for the initial concentration of MB. 3. Results and discussions 3.1. Structural features and morphology The method developed to fabricate Ag-P/m-Ti-X is illustrated as Scheme 1. Monodispersed titania beads with wormhole-like mesostructure are formed by the structure-directing role of HDA
in the sol–gel process. Further solvothermal treatment and calcination lead to mesoporous TiO2 spheres, which are served as templates for sequent loading of Ag3PO4 nanoparticles. After [Ag(NH3)2]NO3 and Na2HPO4 added, Ag3PO4 nanoparticles are successfully loaded in the wormhole-like mesoporous spheres. The morphology and structure of mesoporous TiO2 spheres (m-TiO2) were observed by SEM and TEM (Fig. S1). The spheres are well dispersed with uniform diameter of 750 nm, which are composed of numerous nanoparticles as can be seen by the rough surface (Fig. S1a and b). Wormhole-like mesopores (interparticle pores) are abundant throughout the TiO2 spheres (Fig. S1c). An individual particle is about 15 nm and the lattice fringes are correlated to anatase titania (Fig. S1d). After loading of Ag3PO4 nanoparticles, the dimensions and morphologies have no significant changes compared with that of m-TiO2 (Fig. 1a). Fig. 1b represents a single sphere, which reveals the integral sphere morphology of Ag-P/m-Ti-3.6 complex. Observed at this low magnification, no Ag3PO4 particles was found at the sphere surface, attributing to very small size and uniform distribution of Ag3PO4 particles. Under close observation from HRTEM image (Fig. 1c), numerous small nanoparticles less than 10 nm can be found at the edge of the m-TiO2 sphere. Further observation reveals two sets of lattice fringes of 0.357 nm and 0.24 nm, which are attributed to TiO2
Fig. 1. SEM image (a), TEM images (b–d), XRD pattern (e) of Ag-P/m-Ti-3.6 composites. The elemental mapping with the distribution of individual elements is shown as (f–i).
Y. Li et al. / Journal of Colloid and Interface Science 450 (2015) 246–253
Fig. 2. (a) XPS survey spectrum of Ag-P/m-Ti-3.6 and XPS spectra of (b) Ti2p, (c) Ag3d, (d) P2p and (e) O1s.
and Ag3PO4 respectively (Fig. 1d). Simultaneously, it is obviously observed that the heterostructure has been formed between Ag3PO4 and TiO2. Crystallographic structure of this composite was further examined by powder XRD shown as Fig. 1e. Several well-resolved diffraction peaks centered at 2h = 20.8°, 29.8°, 33.2°, 47.9° and 54.9° can be readily indexed to body-centered cubic (BCC) structure of Ag3PO4, in well agreement with the reported data JCPDS 35-0736. The energy-dispersive X-ray (EDS) spectra (see Fig. S2) from the corresponding SEM image confirm that except for the element C from the support, only Ti, O, Ag, and P element are detected, and the weight percent of Ag3PO4 is 20.8%. The corresponding elemental mapping of an individual sphere is represented in Fig. 1f–i. It illustrates that all Ti, O, Ag and P elements are homogeneously distributed in the whole mesoporous sphere.
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XPS was used to qualitatively determine surface oxidation state and composition of Ag-P/m-Ti-3.6. Fig. 2a exhibits the fully scanned spectra in the range of 0–1100 eV. The peaks on the curve of Ag-P/m-Ti-3.6 composite are assigned to Ti, O, Ag, P and C elements. The C element is attributed to the calibration carbon. Therefore, it is concluded that Ti, O, Ag and P exist in Ag-P/m-Ti3.6 composite. The high resolution XPS spectra of the Ti 2p are analyzed in Fig. 2b. The peaks centered at 464.5 and 458.8 eV are well corresponded to Ti 2p1/2 and Ti 2p3/2 binding energies, respectively [28]. The splitting between Ti 2p1/2 and Ti 2p3/2 is 5.7 eV, indicating a normal state of Ti4+. The peaks of Ag3d appearing at the binding energy of 374.2 and 368.2 eV can be assigned to the 3d3/2 and 3d5/2 of Ag+ respectively. No peak is found at 369.2 and 375.8 eV, demonstrating there no Ag0 existed [29,30]. The XPS peak of P 2p centered at 133.4 eV is attributed to P5+ in Ag3PO4 [31]. Fig. 2d is the XPS spectrum of O 1s. The peaks at 529.6, 530.1 and 531.8 eV relate to oxygen bonded to contaminated carbon, Ti–O and P–O. The XPS results further demonstrated that Ag3PO4 has been loaded in the mesoporous TiO2 sphere. The effects of [Ag(NH3)2]+ concentration on the structure and compositions of Ag-P/m-Ti heterostructures also have been investigated, and the experimental results are shown in Fig. 3. When [Ag(NH3)2]+ concentration of 1.2 mM and 3.6 mM are used, the morphologies of these composites have no significant changes compared with that of m-TiO2. Due to the adsorption capacity of m-TiO2 limited, further increasing the [Ag(NH3)2]+ concentration to 7.2 mM, a part of [Ag(NH3)2]+ could not adsorb and residual in the solution, so no restrict about Ag3PO4 and isolated Ag3PO4 cubes of 1–2 lm apparent appeared. Their crystalline structures have been also investigated, and the results are shown as Fig. 3C. The crystallinity and content of Ag3PO4 were enhanced with increasing concentration of [Ag(NH3)2]+. TEM images of Ag-P/m-Ti-X sphere composites are given in Fig. 4a–c. It clearly reveals that a large number of Ag3PO4 nanoparticles with different sizes exist in these m-TiO2 sphere. The size distribution histograms of Ag3PO4 nanoparticles calculated from the corresponding TEM images are given as Fig. 4d–f, respectively. It is indicated that the size of Ag3PO4 on the m-TiO2 sphere is strongly dependent on the concentration of [Ag(NH3)2]+. As observed in Fig. 4d, when a smaller concentration of [Ag(NH3)2]+ was added with a concentration of 1.2 mM, the obtained product exhibits a narrow size distribution. The size distribution of Ag3PO4 nanoparticles broadened with increasing concentration of [Ag(NH3)2]+ (Fig. 4e and f). Meanwhile, the large isolate Ag3PO4 particles also exist in this product (inset in Fig. 4c), which is in agreement with SEM result shown as Fig. 3B. The representative magnified TEM images taken from the Ag3PO4 nanoparticles on the different products are displayed in the inset of Fig. 4d–f. The detailed information is summered in Table 1. For comparison, Ag-P/m-TiO2-tra was synthesized through conventional impregnation method, in which
Fig. 3. SEM images (A and B) and XRD (C) of Ag-P/m-Ti-X composites: (a) Ag-P/m-Ti-1.2, (b) Ag-P/m-Ti-7.2. The red box regions shown in figure represent Ag3PO4 cubes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Typical TEM images of Ag-P/m-TiO2 composites: (a) Ag-P/m-Ti-1.2, (b) Ag-P/m-Ti-3.6, and Ag-P/m-Ti-7.2. (d–f) The size distribution histograms of Ag3PO4 calculated from the corresponding TEM images. Inset in c is an isolate Ag3PO4 particle existed in Ag-P/m-Ti-7.2 composite.
Table 1 Physicochemical property of Ag-P/m-Ti-X composites. Sample
Ag3PO4 loading (wt%)
Ag3PO4 particle size (nm)
Average pore size (nm)
BET surface area (m2/g)
Pore volume (cm3/g)
Ag-P/m-Ti-1.2 Ag-P/m-Ti-3.6 Ag-P/m-Ti-7.2 m-TiO2 Ag-P/m-Ti-tra
7.7 20.8 36.8 – 23.9
3.41 ± 0.66 3.44 ± 0.92 4.60 ± 1.06 – 83.2 ± 23.3
11.4 10.7 13.9 11.2 13.0
89.0 64.4 59.3 107.0 40.0
0.29 0.21 0.22 0.37 0.14
a mixture of Ag3PO4 particles and m-TiO2 spheres was observed (Fig. S3a). This result gives further evidence that the method we reported is a more effective way for loading small and well dispersed Ag3PO4 nanoparticles into m-TiO2 spheres. In addition, in the absence of m-TiO2, large Ag3PO4 cubes of 0.5–2 lm are obtained (Fig. S3b). In order to further understand the internal structure, N2sorption isotherms and pore distribution curves are recorded to investigate the effect of Ag3PO4 loading amount on the pore structure of all samples (Fig. 5). It can be seen that all isotherms exhibit type IV with H1 hysteresis loop, which are typical adsorptions for mesoporous materials. This curve feature does not change after Ag3PO4 loading, indicating that the mesoporous structure of mTiO2 is well maintained. However, the specific surface area and pore volume significantly. Moreover, the higher the loading amount, the smaller the specific surface areas and pore volume (see Table 1). These results indicate that Ag3PO4 nanoparticles have been encapsulated into the mesopores. In contrast to surface area and pore volume, the pore size does not change much after Ag3PO4 loading, except for sample Ag-P/m-Ti-7.2, indicating the well dispersed and small-sized Ag3PO4 present in the mesoporous TiO2 spheres. As for Ag-P/m-Ti-7.2 sample, an additional pore is observed at 207.7 nm and the pore size increased slightly compared with that of m-TiO2, which may be caused by the existence of large Ag3PO4 cubes. While compared with the sample Ag-P/m-Ti-tra with a specific surface area of 40 cm2/g, Ag-P/m-Ti-X products prepared by this new method have much higher specific surface area and excellent mesoporous structure, which may be conducive to enlarge the contact area between the catalysts and dyes, consequently enhance the photocatalytic performance.
3.2. Photocatalytic performance The photocatalytic activities were evaluated by photo-oxidation of aqueous methylene blue (MB). Before studying and comparing the activities of the Ag3PO4/m-TiO2 hierarchical nanostructures, the control experiments were performed in the dark (Fig. 6A). These results highlight that the adsorption–desorption equilibrium of MB in the dark is established within 30 min and Ag-P/m-Ti-3.6 has a maximum absorption capacity which is beneficial to improve its photocatalytic performance. Fig. 6B shows the C/C0 versus reaction time for the degradation of MB over the Ag3PO4/m-TiO2 composites. For comparison, the photocatalytic tests of m-TiO2 and Ag3PO4 were also conducted. Their concentrations of MB only decrease 12% and 39%. However, addition of the Ag-P/m-Ti-X composites catalysts to the solution causes the decrease in MB concentration. At first, with the loading of Ag3PO4 increasing (Ag-P/m-Ti-1.2, and Ag-P/m-Ti-3.6), the photocatalytic activities are distinctly on the increases as plots Fig. 6Bc and d showed. At 28 min, nearly 100% MB was degraded by Ag-P/m-Ti-3.6. While further increasing the content of Ag3PO4 to 36.8 wt.% only 83% MB is degraded by Ag-P/m-Ti-7.2, the decrease of degradation may be due to the existence of isolate Ag3PO4 cubes reducing the contact area between dye and catalyst. The specific surface area of Ag-P/m-Ti-X is several ten times of Ag3PO4 cubes (2.32 m2/g). Simultaneously, the degradation yield of Ag-P/m-Ti-tra and Ag-P/ m-Ti-gir can only reach 78% and 68% (Fig. S4), which is maybe due to two aspects: the low specific surface area and less amount of heterostructures, and this confirms that the heterostructures can improved the degradation rate of dyes. Fig. 6C presents evolution of UV–vis spectra of MB solution catalyzed by Ag-P/m-Ti-3.6 over
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Fig. 5. N2-sorption isotherms and pore size distribution curves of m-TiO2 (a), Ag-P/m-Ti-1.2 (b), Ag-P/m-Ti-3.6 (c), and Ag-P/m-Ti-7.2 (d). The pore size distribution curves were calculated from the absorption branch of the nitrogen isotherms by BJH method.
time. The absorbance of major absorption band reduces step by step under UV–visible light, indicating MB is destroyed step by step. At the same time, the suspension faded gradually, as the inset shown in Fig. 6C, demonstrating that the chromophoric structure of the dye has been destroyed. The photocatalytic degradation of MB follows pseudo first-order kinetics, and the apparent degradation rate constant k could be defined by ln (C0/C) = kt. Linear relationships for all the photocatalysts between ln (C0/C) and reaction time are shown in Fig. 6D, and from which we can observe that the apparent reaction rate constant for the Ag-P/m-Ti-3.6 is about 20 and 7.5 times of m-TiO2, Ag3PO4 cubes, further confirming the high visible light photocatalytic activity of the Ag-P/m-Ti-3.6. The stability of a photocatalyst is very important in practical applications. Fig. 7 shows the photocatalytic activity of Ag-P/m-Ti3.6 for the degradation of MB under visible-light irradiation for four cycles. After four-cycling photocatalytic experiments, Ag-P/m-Ti3.6 still exhibits excellent photocatalytic performance with a degradation rate of 91%, revealing its high stability after multiple reuses.
3.3. Mechanism of the improved photocatalytic performance of the Ag3PO4/m-TiO2 composite In order to investigate the adsorption of samples under visible light and UV light, all samples were characterized by UV–vis diffuse reflectance. A comparison of the UV–vis diffuse reflectance spectra of different Ag-P/m-Ti-X samples along with pure m-TiO2 and Ag3PO4 is displayed in Fig. 8A. It can be seen that the
adsorption edges of pure m-TiO2 spheres is at about 390 nm and no adsorption in the visible-light region is observed. As for Ag3PO4 cubes, an adsorption edge occurred at 530 nm. The UV– vis spectrum of Ag-P/m-Ti-gri represents two regions from 200 to 540 nm. The first domain ‘‘R1’’ in the region of 200–400 nm is attributed to the absorption of m-TiO2. And the second domain ‘‘R2’’ is due to the absorption of Ag3PO4. The adsorption region is not extended indicating no heterostructure is formed. Therefore, the band gap does not change. While Ag-P/m-Ti-X composites display clearly narrowed band gap. Their absorptions are in the whole region of 200–760 nm, falling into the visible light range, further indicating the heterostructure is formed between Ag3PO4 and TiO2. As increasing the loading amount of Ag3PO4 in m-TiO2 (AgP/m-Ti-1.2 and Ag-P/m-Ti-3.6), the absorption intensity in the visible-region enhanced, indicating Ag-P/m-Ti-3.6 can make full use of visible light. While for Ag-P/m-Ti-7.2, there is a slight decrease in the absorption intensity and absorption edge compared with that of Ag-P/m-Ti-3.6, which may be due to the existence of large Ag3PO4 cubes. A similar curve as that of Ag3PO4 can be seen as the arrow showed. The absorption of visible light will lay the foundation for the visible light photocatalyst. þ
Ag3 PO4 =TiO2 þ hm ! Ag3 PO4 ðe þ h Þ=TiO2 þ
ð1Þ þ
Ag3 PO4 ðe þ h Þ=TiO2 ! Ag3 PO4 ðe Þ=TiO2 ðh Þ þ
þ
h þ H2 O ! OH þ H
e þ O2 !
O 2
ð2Þ ð3Þ ð4Þ
252
Y. Li et al. / Journal of Colloid and Interface Science 450 (2015) 246–253
Fig. 6. (A) Adsorption in darkness, (B) evolution of MB concentration versus UV–vis irradiation time; (C) time-dependent adsorption spectra of MB in the presence of Ag-P/mTi-3.6; (D) the ln(C0/C) versus irradiation time. Sample a-e represents m-TiO2, Ag3PO4 cubes, Ag-P/m-Ti-1.2, Ag-P/m-Ti-3.6, and Ag-P/m-Ti-7.2 respectively.
Fig. 7. Photocatalytic activities of Ag-P/m-Ti-3.6 for the degradation of MB under visible-light irradiation for four cycles.
e þ O2 þ Hþ ! H2 O2 H2 O2 þ þ
O 2
ð5Þ
! OH þ OH þ O2
ð6Þ
h þ OH ! OH
ð7Þ
ð8Þ
OH þ MB ! CO2 þ H2 O
Combining our experiment results with the related literatures [5,9,27], a mechanism for the enhanced photocatalysis of the Ag-P/m-Ti-X spheres heteroarchitectures under visible light irradiation is proposed (Fig. 8B). As literatures documented, thermodynamic conditions such as the position of semiconductor conduction and valence bands of composite semiconductors favor
Y. Li et al. / Journal of Colloid and Interface Science 450 (2015) 246–253
253
Fig. 8. (A) UV–vis diffuse reflectance spectra of m-TiO2, Ag3PO4, and Ag-P/m-Ti-X samples; and (B) mechanism illustration for enhanced visible-light photocatalytic activity.
the occurrence of phenomenon that efficient interparticle charge transfer between the semiconductors TiO2 (wide band gap) and Ag3PO4 (narrow band gap). When this Ag-P/m-Ti-X composite system is irradiated by visible-range light, the electrons in the VB of Ag3PO4 are excited to its CB. Thereby the VB of Ag3PO4 is rendered partially holes. Because the VB level of Ag3PO4 is lower than that of TiO2, the holes can transferred to the VB of TiO2. Simultaneously, mesoporous TiO2 with a loosely packed structure and large pores is a hole-accepting semiconductor. As a result, the semiconductors with matching band potentials are tightly bonded to construct the efficient heterostructure. The migration of photogenerated holes can be promoted by the internal field, so less of a barrier exists. Meanwhile, the electrons on Ag3PO4 CB accumulating on the surface of Ag3PO4 are then scavenged by dissolved oxygen molecules in water to yield highly oxidative species such as peroxide (H2O2), which can generate hydroxyl radical (OH) and decompose organic substrates effectively. The major reactions that occur in this study can be summarized in reactions (1)–(8). Therefore, the photocatalytic performance can be enhanced greatly owing to the special heteroarchitectures, which can reduce the recombination of electron–hole, thus a larger number of electrons on the Ag3PO4 surface and holes on the TiO2 surface, respectively, can participate in photocatalytic reactions to directly or indirectly mineralize organic pollution. Above results indicate that the semiconductor composites can be excited by visible light. The photogenerated holes in the Ag3PO4 nanoparticles can be effectively injected into the TiO2 due to the strong interfacial bonding between them. As known, the absorption of visible light is the basis of its photocatalytic activity under visible light. Hence the absorption property here implies that the nanocomposites could be promising in visiblelight photocatalysis compared with that of pure TiO2. 4. Conclusions In summary, we have demonstrated a facile method to synthesize Ag-P/m-Ti-X heterostructure photocatalysts. The Ag3PO4 nanoparticles homogeneously distribute in m-TiO2 spheres. The size and loading amount can be adjusted by changing [Ag(NH3)2]+ concentration. Moreover, they have large pores and high specific surface area, which enable them promising materials in adsorption, catalysts, etc. Photocatalytic measurements results suggest that the activities of all Ag-P/m-Ti-X are significantly improved compared with m-TiO2 or Ag3PO4. Especially, Ag-P/mTi-3.6 expresses higher photocatalytic activity than other Ag3PO4/ m-TiO2 composites including the one synthesized by traditional impregnation method. And it represents excellent stability with a degradation of 91% after four cycles. Furthermore, the Ag weight
percent of Ag3PO4/m-TiO2 decreases from 77% to 20.8%, significantly reducing the cost of pure Ag3PO4 photocatalyst. Acknowledgments This work was supported by Natural Science Foundation of China (Nos. 20903046, 21076094). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.03.016. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
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