Phosphate Heterostructured

Sep 2, 2009 - Synopsis. We report a facile hydrothermal route to the ZnS/phosphate heterostructured hemispheres. The equilibrium controlled growth ...
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DOI: 10.1021/cg9003643

Preparation and Usage of ZnS/Phosphate Heterostructured Hemispheres in Enhanced Photocatalytic Activities

2009, Vol. 9 4384–4390

Jun Liu, Baoyou Geng,* and Shaozhen Wang College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Laboratory of Molecular-Based Materials, Anhui Normal University, Wuhu 241000, P. R. China. Received April 2, 2009; Revised Manuscript Received August 20, 2009

ABSTRACT: Novel rough ZnS/slick hopeite heterostructured hemispheres are fabricated through a facile hydrothermal route. By controlling the reaction conditions, pure ZnS microspheres, Zn3(PO4)2 3 4H2O panels and concave heterostructured microspheres also can be obtained. According to experimental results, the equilibrium controlled growth mechanism is proposed for the formation of products. The as-synthesized Zn3(PO4)2 3 4H2O panels have one Zn-terminated surface and one PO4 tetrahedral group terminated surface. The ZnS nanostructures form on the Zn-terminated surface, resulting in different heterostructures. The obtained ZnS/Zn3(PO4)2 3 4H2O heterostructured hemispheres show strongly structure-induced enhancement of photocatalytic performance for the photodegradation of Rhodamine B. The superiority of photocatalytic performance is attributed to the high specific surface area, larger size and the heterostructured architectures. This work not only gives insight into understanding the heterostructured growth behavior of microstructures in a solution-phase synthetic system, which may extend to synthesizing other controllable morphology and heterostructured materials but also provides an efficient route to optimize some properties of semiconductors. Introduction Heterostructural nanomaterial is now becoming a promising and challenging area of research because of its unique properties and applications that are better than that of the single material. Especially, semiconductor-based heterostructures with desired compositions and/or morphologies can modulate the properties of materials and find potential applications in biomedicine, photocatalysis, and nanodevices.1-12 Because of these potential applications, various kinds of semiconductor-based heterostructures have been designed in recent years, such as anisotropic and core/shell heterostructures.13-17 Semiconductor/semiconductor hetero/homojunction (such as ZnO/ZnS,18-20 Ga/ZnS,21 TiO2/Cu2O,22 ZnO/Cu2O,23 etc.) is one of the most popular heterostructures and has been studied extensively because of its excellent catalytic and photovoltaic activity. In addition, the metal/ semiconductor heterostructure has also attracted much research attention. For instance, Au/PbS,24 Ag/ZnO,25 Pt/ CdSe,26 etc., heterostructures have been reported for their excellent optoelectronic performance. As an important compound semiconductor, ZnS is usually used to make heterostructure materials. It is an important material with band gap energy (Eg) is 3.72 eV for cubic phase and 3.77 eV for hexagonal wurtzite phase at room temperature.27,28 Tremendous efforts have been made to synthesize different dimensional ZnS nanostructures, such as quantum dots (QDs), nanorods, nanowires, nanobelts, nanotubes, solid nanospheres, hollow nanospheres, nanoporous particles, etc.29 Much research has revealed that ZnS nanostructures displayed excellent photocatalytic activity toward the photodegradation of different dyes under the irradiation of UV.30-33 For example, Matthias Batzill reported that the modified ZnO with submonolayer ZnS films could change the band gap energy, which reduced the photoexcitation threshold energy and improved surface properties.20 The result *Corresponding author. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 09/02/2009

enhanced photocatalytic activity and potentially improved the solar energy conversion capabilities. It is obvious that ZnO/ZnS heterostructures have more ascendant and special properties than single ZnO or ZnS. Therefore, to design and synthesize some extraordinary heterostructures is a valuable and available study. In this paper, for the first time, novel ZnS/Zn3(PO4)2 3 4H2O heterostructured hemispheres are synthesized by a facile onepot hydrothermal method. Zn3(PO4)2 3 4H2O is one of the important metal phosphates and has found widespread application in coating, electric motors, and transformers and in the automotive and biomedical industries.34-36 Here, heterostructured materials were made and applied in a photocatalytic field for the first time. These as-synthesized heterostructured hemispheres are confirmed to possess high photodegradation compared with that of ZnS microspheres (asprepared in this experiment) and commercial TiO2 powders. Herein, an equilibrium-controlled growth mechanism is proposed. And the important role of the H3PO4 has been discussed in detail. The photocatalytic activity of ZnS/Zn3(PO4)2 3 4H2O heterostructured hemispheres was tested by photodegradation of Rhodamine B under UV irradiation. The obtained ZnS/Zn3(PO4)2 3 4H2O heterostructured hemispheres show strongly structure-induced enhancement of photocatalytic performance and exhibit much better photocatalytic property than that of ZnS microspheres and commercial TiO2 powders. This work gives insight into understanding the growth behaviors of heterostructured micro/nanomaterials in a solution-phase synthetic system. It may extend to the synthesis of other controllable morphology and heterostructured materials and provides an efficient route to optimize some properties of semiconductors. Experimental section Preparation. All chemicals were of analytic grade and used without further treatment. In a typical, 0.5 mmol (0.1093 g) Zn(Ac)2 3 2H2O was dissolved in 25 mL of distilled water under r 2009 American Chemical Society

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Figure 1. SEM images of the products prepared by the hydrothermal reaction at 180 °C for 24 h, with VH3PO4 = 1.00 mL. (a) Lowmagnification image of the products. (b, c) High-magnification images of hemispheres. (d) SEM image of single hemisphere. The arrows in b and c indicate the interface of the rough and slick surface. strong stirring, and then 1.00 mL of 1.5 mol/L H3PO4 was added in the solution dropwise. Afterward, 1 mmol (0.0320 g) of sulfur powder was added to the above solution. The final mixture was transferred to a Teflon-lined stainless autoclave (30 mL capacity). The autoclave was sealed and maintained at 180 °C for 24 h hours. The system then cooled to room temperature naturally. The precipitate was filtered off, washed with distilled water and absolute ethanol several times, and then dried in a vacuum at 60 °C for 12 h. Characterization. X-ray powder diffraction (XRD) was carried out on an XRD-6000 (Japan) X-ray diffractometer with Cu-KR radiation (λ = 1.54060 A˚) at a scanning rate of 0.05° s-1. Scanning electron microscopy (SEM) micrographs were taken using a Hitachi S-4800 scanning electron microscope attached with energy-dispersive X-ray spectroscopy. Transmission electron microscopy (TEM) micrographs were performed using JEM 2010 F microscopes operated at optimum defocus with accelerating voltages of 200 kV. Infrared spectra was taken on a Fourier transform infrared spectrophotometer (SHIMSDZU 8400 S), and thermogravimetric/differential thermal analysis was taken on the SHIMADZU DTG-60A thermal analysis system. Specific surface areas were computed from the results of N2 physisorption at 77 K (model BECKMAN SA3100 COULTER) using the BET (Brunauer-Emmet-Teller) formalism. Photocatalytic Activity. The photocatalytic activity experiments on the as-obtained products for the decomposition of Rhodamine B (RB, Sigma-Aldrich Chemical Co.) were performed at ambient temperature in air. The reaction system contained 75 mL of 1  10-5 M RB and 10 mg of ZnS/Zn3(PO4)2 3 4H2O powder. The mixture was magnetically stirred in the dark for 15 min to reach the adsorption equilibrium of RB with the photocatalyst before exposure to UV irradiation (250 W). As a comparison, the photocatalytic activities of the as-prepared ZnS microspheres and commercial TiO2 powders (Degussa P25, Degussa Co. the surface area is ca. 45 m2 g-1) were also tested at the same experimental conditions.

Results and Discussion Morphology, Structure, and Component. Figure 1 shows the SEM images of the as-synthesized products with the

Figure 2. XRD patterns of the products obtained under different volumes of H3PO4: (a) 0.30, (b) 1.00, and (c) 1.80 mL. The products were fabricated with the same initial reactant mass and reaction for 24 h at 180 °C.

different magnification and visual angle. The results reveal that the obtained products are large-scale hemispheres (see Figure 1a). The average diameter of the hemisphere is about 800 nm. It is interesting that the hemisphere possesses the different external surfaces. The high-magnification SEM images (Figure 1b and 1c) show that the hemisphere possesses a slick plane and a rough hemispherical face. The interface is clearly marked with the arrows in Figure 1b and 1c. The rough surface seemed to be composed of lots of interconnected nanosheets which growth on the slick face (Figure 1c). Figure 1d shows a single hemisphere image of the hemisphere, which reveals that the planar face is fairly slick. Figure 2 shows the corresponding X-ray diffractions (XRD) patterns of the as-synthesized products at different

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Figure 3. EDX spectrum of as-prepared heterostructured hemispheres: (a) slick plane, (b) rough hemispherical face. The corresponding SEM images are showed on the right upper part.

Figure 4. (a) Typical TEM image and (b) SAED pattern of asprepared heterostructured hemispheres.

amount of H3PO4 with 0.1093 g of Zn(Ac)2 3 2H2O and 0.0320 g of sulfur powder. Compared with the diffraction of standard Zn3 (PO4)2 3 4H2O (JCPDS 74-1778) and ZnS powders (JCPDS 772100), all the diffraction peaks of curves a and c are in good agreement with the standard literature values, which can be indexed to the orthorhombic R-Zn3 (PO4)2 3 4H2O (R-hopeite) and cubic ZnS, respectively. Curve b is the XRD pattern of ZnS/Zn3 (PO4)2 3 4H2O heterostructured hemispheres. It contains not only ZnS typical diffraction peaks of (111), (200), (220), and (311) planes at 2θ value of 28.5, 33.6, 47.5, and 56.6° but also Zn3 (PO4)2 3 4H2O typical diffraction peaks of (020), (040), (311), (351), and (0120) planes at 2θ value of 9.61, 19.27, 31.21, 39.36, and 60.69°. Corresponding FTIR spectrum and DTA/TGA results of the as-synthesized Zn3(PO4)2 3 4H2O panels further demonstrated the products are orthorhombic R-hopeite (as shown in Figure SI 1 and SI 2). To reveal the component of the different parts of the hemispheres, the energy dispersion spectroscopy (EDS) analysis has been performed on the SEM. We choose the different rough surface and slick surface as the collection areas. The EDS result in Figure 3a reveals that the slick plane of the hemispheres mainly composed with elements Zn, P, and O. The quantitative analysis shows that the atomic ratio of Zn, P, and O is 2.98:2.02:11.95. Figure 3b confirms the rough face is mainly composed of Zn and an S element with the atomic ratio is 1.08:1. Element P among rough is nearly neglected. The results in panels a and b in Figure 3 are close to the stoichiometry of Zn3(PO4)2 3 4H2O and ZnS, respectively. The EDS results demonstrate that the obtained hemispheres are ZnS/Zn3(PO4)2 3 4H2O heterostructures. Figure 4a presents a TEM image of typical ZnS/Zn3(PO4)2 3 4H2O heterostructured hemispheres obtained by ultrasonic dispersion of the as-prepared sample in ethanol. The TEM image clearly exhibits the slick faces and rough faces of the hemispheres as well as shows that the rough faces

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of the products are composed of lots of interconnected nanosheets. The arrows mark the slick and rough faces of the hemispheres, respectively. The SAED pattern (Figure 4b) which was taken on the rough surface of the products is consistent with cubic structure ZnS with ring patterns instead of sharp spots as a result of the polycrystalline nature of the products. To further confirm the component of the heterostructured hemispheres, X-ray photoelectron spectroscopy (XPS) was performed. The overview XPS spectrum in Figure 5a shows peaks corresponding to Zn 2p1 (1045.2 eV), Zn 3s (135.0 eV), Zn 3p (91.6 eV), Zn 2p3 (1022.8 eV), P 2p (133.3 eV), P 2s (190.0 eV), S 2p (161.8 eV), and O 1s (531.6 eV). The fine spectra of the Zn 2p3, S 2p and P 2p binding energies were located at 1022.8, 161.8, and 133.3 eV, as shown in Figure 5b-d. All the data demonstrate that the heterostructured hemispheres mainly contain Zn, O, P, and S elements. Growth Mechanism. During the experiment, we found that the amount of the H3PO4 played an important role in the formation of the obtained structures. The growth process can be described in Scheme 1. As illustrated in the top right corner of the Scheme 1, the growth process should be controlled by two equilibrium systems as follows. Equilibrium system 1 (eq 1): 3ZnðCH3 COOÞ2 3 2H2 O þ 2H3 PO4 f 3Zn3 ðPO4 Þ2 3 4H2 OV þ 6CH3 COOH þ 2H2 O CH3 COOH f CH3 COO - þ Hþ

ð1Þ ð2Þ

H3 PO4 f Hþ þ H2 PO4 - f Hþ þ HPO4 2 - f Hþ þ PO4 3 -

ð3Þ

Hþ þ Zn3 ðPO4 Þ2 3 4H2 O f Zn2þ þ H2 PO4 - or HPO4 2 -

ð4Þ

Equilibrium system 2 (eq 2): 4S þ 3H2 O f 2H2 Sv þ H2 S2 O3

ð5Þ

H2 S f Hþ þ HS - f Hþ þ S2 -

ð6Þ

Zn2þ þ S2 - f ZnSV

ð7Þ

In eq 1, the corresponding reactions depend on the amount of H3PO4. At low amounts of H3PO4 (VH3PO4 = 0.30 mL), the products are slick Zn3 (PO4)2 3 4H2O panels (see Figure 6a). If the amount of H3PO4 exceeds 0.30 mL, there will be free Zn2þ ions on its surfaces, followed by reaction 4. However, eq 2 occurs only at high temperature (from 130 to 180 °C). In eq 2, the H2S gas is produced by sulfur powder react with the water steam (reaction 5), and then, the H2S ionize to Hþ ions and S2- ions (reaction 6). The S2- ions form ZnS precipitation with the free Zn2þ surrounded the Zn3 (PO4)2 3 4H2O panels surface (reaction 7). The unit-cell diagram crystal structure of Zn3 (PO4)2 3 4H2O has researched by R. J. Hill and J. B. Jones37 (see Figure SI 3a in the Supporting Information) indicates that the formed Zn3(PO4)2 3 4H2O panels possess two different surfaces. One surface of the panel is Zn-terminated and the opposite surface is PO4 tetrahedral group terminated (see Figure SI 3b in the Supporting Information). A PO4 tetrahedral-group-terminated surface prevents Hþ, S2-, and other ions from

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Figure 5. XPS spectra of as-synthesized heterostructured hemispheres. (a) Overview XPS spectrum of the products; (b) S 2p, spectrum; (c) Zn 2p3 spectrum; (d) P 2p spectrum.

Scheme 1. Schematic Illustration of the Growth Process for the Formation of Various ZnS/Zn3(PO4)2 3 4H2O Heterostructures and ZnS Microspheres

entering the panel. Thus, ZnS can be obtained only on the Zn-terminated surface of the hopeite panels when VH3PO4 exceeds 0.30 mL. A similar situation has been reported by Wang and co-workers.19 When the VH3PO4 was increased to 0.50 mL, a loose ZnS film formed on Zn3 (PO4)2 3 4H2O panels (as shown in

Figure 6b). At a higher concentration of H3PO4, more Hþ ions are produced and they can permeate the loose film to produce more Zn2þ. Also, the S2- ions can permeate the film to react with Zn2þ ions to form ZnS. As more and more ZnS accumulate under the film, some cracks and protrudes appear on the surface of the film. These cracks and protrudes

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Figure 6. Typical SEM images of samples prepared at 180 °C for 24 h with different amounts of H3PO4: (a-h) 0.3, 0.5, 0.7, 0.9, 1.0, 1.4, and 1.8 mL, respectively.

can even decrease the high surface energy of the loose and rough film (Figure 6c). As VH3PO4 is 0.9 mL, most of the Zn3(PO4)2 3 4H2O transform to ZnS. The convexities turn to hemispherical surfaces and the cracks are large enough to break the panels into small pieces. Images d and e in Figure 6 show the rough and slick face of the heterostructured ZnS/ Zn3 (PO4)2 3 4H2O panels with many cracks in them. When the amount of the H3PO4 increased to 1.0 mL, the heterostructured panels thoroughly split to some single heterostructured hemispheres (Figure 6f). As VH3PO4 continues to increase, heterostructured hemispheres, concave microspheres, and ZnS microspheres can be obtained. Increasing VH3PO4 to 1.4 mL, more ZnS are formed on Zn3 (PO4)2 3 4H2O, which makes the panels turned bend and the hemispheres transform to concave microspheres (Figure 6g). Adding VH3PO4 to1.8 mL, the products are microspheres (Figure 6h). The corresponding EDS result (see Figure SI 4 in the Supporting Information) shows that the obtained microspheres are mainly composed of Zn and S elements. Scarcely any P and O elements can be detected. When we used HNO3 or CH3COOH to adjust the same pH value, we obtained only ZnS microspheres assembled by nanoparticles (see Figure SI 5 in the Supporting Information) because the HNO3 and CH3COOH could not supply PO43- to form Zn3(PO4)2 3 4H2O panels.

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Figure 7. (a) Absorption spectrum of a solution of Rhodamine B (1.0  10-5 M, 75 mL) in the presence of ZnS/Zn3(PO4)2 3 4H2O hemispheres (10 mg) under exposure to UV light. (b) Photodegradation of Rhodamine B (1.0  10-5 M, 75 mL) under UV light with different photocatalyst: (1) without any photocatalyst; (2) with Degussa P25 titania (10 mg); (3) with ZnS microspheres (10 mg); (4) with ZnS/Zn3(PO4)2 3 4H2O hemispheres (10 mg).

Photocatalytic Activity. To demonstrate the potential applicability in photocatalysis of the obtained heterostructured ZnS/Zn3(PO4)2 3 4H2O hemispheres, we investigated their photocatalytic activity by choosing photocatalytic degradation of Rhodamine B. Figure 7a indicates the absorption spectra of an aqueous solution of Rhodamine B (initial concentration is 1.0  10-5 M, 75 mL) in the presence of 10 mg heterostructured hemispheres under exposure to the ultraviolet (UV) (250 W). The main absorption peak at 550 nm corresponds to the Rhodamine B molecules. The intensity of absorption diminishes rapidly with extension of the exposure time and the absorption peak completely disappears after about 40 min. A series of color changes is shown in the inset of Figure 7a, which corresponds to the sequential changes of the absorption measurements. To further understand the photocatalytic properties of heterostructured hemispheres, we used, as a comparison, 10 mg of the synthesized ZnS microspheres and the commercial photocatalyst Degussa P25 titania to degrade Rhodamine B at the same conditions. The results are shown in Figure 7b. Without any catalyst, only a slow decrease in the concentration of Rhodamine B was detected under UV irradiation (curve 1). The activity increases in turn for Degussa P25 titania (curve 2), ZnS microspheres (curve 3), and

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heterostructured ZnS/Zn3(PO4)2 3 4H2O hemispheres (curve 4). The Rhodamine B solution is decolorized completely by using the heterostructured ZnS/Zn3(PO4)2 3 4H2O hemispheres after UV irradiation more than 40 min, which is apparently shorter than that of ZnS microspheres and Degussa P25 titania. The superiority of photocatalytic performance of the heterostructured ZnS/Zn3(PO4)2 3 4H2O hemispheres should be attributed to their special structural features. As mentioned above, heterostructured materials may lead to the change of the band gap energy compare with the single component materials. Here, to understand the band gap energy of samples, the diffuse reflectance spectra of the ZnS/Zn3(PO4)2 3 4H2O heterostructured hemispheres and ZnS microspheres have been taken (as shown in Figure SI 6 in the Supporting Information). The diffuse reflectance experiments reveal that the absorption peak of the heterostructured hemispheres presents blue shift. The above result indicates that the heterostructured hemispheres exhibit quantum size effects, which may enhance the photocatalytic activity of the products. In addition, the rough surface of ZnS/Zn3(PO4)2 3 4H2O hemispheres should possess high specific surface area. To evaluate the surface area of the obtained products, full nitrogen sorption isotherms were measured (see Figure SI 7 in the Supporting Information). The specific surface area was thus evaluated to be 78.2 m2 g-1 from data points in this pressure range by the BET equation. The result shows that the obtained hemispheres possess higher specific surface area than that of Degussa P25 powders (∼45 m2 g-1), which is obviously beneficial for the enhancement of photocatalytic performance. Moreover, good dispersity and uniformity also can provide a large active surface area. Comparatively, unwanted aggregation during the reaction usually leads to a significant decrease in the active surface area and thus the photocatalytic performance of the reference samples, such as Degussa P25 TiO2 nanopowders.30,33 On the basis of the above results, compared with the P25 TiO2 powders, the advantages of heterostructured ZnS/Zn3(PO4)2 3 4H2O hemispheres are (1) a high surface-to-volume ratio with effective prevention from further aggregation to maintain the high catalytic activity area arising from the rough surface structure; (2) easier separation and recycling than the common nanocrystals because of the larger size of the products; (3) heterostructured interface between ZnS and Zn3(PO4)2 3 4H2O, leading to the quantum size effects. Conclusions In summary, novel ZnS/Zn3(PO4)2 3 4H2O heterostructured hemispheres are synthesized through a facile one-pot hydrothermal route. By adjusting the amount of H3PO4, a series of products with different morphologies and components have been prepared, such as heterostructured hemispheres, concave microspheres and ZnS microspheres. An equilibrium-controlled growth mechanism is proposed to explain the formation of the products. The obtained ZnS/ Zn3(PO4)2 3 4H2O heterostructured hemispheres display very high photocatalytic activity and are much more efficient than that of assynthesized ZnS microspheres and Degussa P25 titania powders. The good photocatalytic performance of the heterostructured ZnS/Zn3(PO4)2 3 4H2O hemispheres should be attributed to their special structures and implied potential applications in other fields, such as solar cell and so on.

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Acknowledgment. This work was supported by the National Natural Science Foundation of China (20671003), the Key Project of Chinese Ministry of Education (209060), the Education Department of Anhui Province (2006KJ006TD), and the Program for Innovative Research Team in Anhui Normal University. Supporting Information Available: FTIR spectrum of Zn3 (PO4)2 3 4H2O panels (Figure SI 1); DTA/TG analysis for Zn3 (PO4)2 3 4H2O panels (Figures SI 2); unit-cell diagram crystal structure and scheme of two surfaces of the Zn3(PO4)2 3 4H2O panels (Figures SI 3); EDS pattern of the obtained ZnS microspherers (Figures SI 4); SEM images of the samples synthesized with CH3COOH and HNO3 (Figures SI 5); diffuse reflectance spectra of the obtained structures (Figures SI 6); BET measurement of heterostructured hemispheres (Figures SI 7) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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