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DOI: 10.1039/C5RA05063D (Paper) RSC Adv., 2015, 5, 47884-47891

Controlled synthesis of peony-shaped photocatalyst grains of Ag3PO4/Zn3(PO4)2 by coprecipitation and recrystallization technology†

Ma Peiyana, Yu Yonga, Yu Hongjianc, Che Liyuana, Wang Lingb and Xiong Yan*b
aSchool of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, Wuhan, Hubei 430070, PR China. E-mail: Xiongyan1980@hotmail.com
bSchool of Materials Science & Engineering, Hubei Province Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan, Hubei 430023, PR China
cState Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, Hubei 430070, PR China

Received 22nd March 2015 , Accepted 21st May 2015

First published on 22nd May 2015

Abstract

Peony-shaped grains of a Ag3PO4/Zn3(PO4)2 composite have been synthesized at room temperature through coprecipitation and recrystallization technology with the coexistence of Zn2+ and Ag+ in a aqueous solution. The simultaneous presence of high-concentration Ac− ions plays a key role in the initial formation of flower-shaped grains of Zn3(PO4)2 being the large-area host of this composite. Compared with pure Ag3PO4 crystals, the obtained composite exhibits enhanced visible-light-driven photocatalytic activity and stability for decomposition of Rhodamine B in aqueous solution. This is attributed to effective hole–electron pair transport and optimal Ag3PO4 dispersion achieved by confining Ag3PO4 to the as-prepared Zn3(PO4)2 framework.

1 Introduction

In order to resolve environmental and energy issues, great efforts have been devoted to the removal of pollutants from water by developing more active visible-light-driven photocatalysts.1 One such photocatalyst, Ag3PO4, has recently drawn attention due to its high efficiency in the photocatalytic degradation of dyes.2,3 It has been reported that Ag3PO4 crystals exhibit higher photocatalytic activity than N-doped TiO2 on the degradation of methylene blue aqueous solution under visible light irradiation. In addition, Ag3PO4 crystals with high-energy crystal faces, porous Ag3PO4,4 and Ag3PO4-based heterostructured photocatalysts possess excellent performance for dye degradation.5–9 However, the stability of Ag3PO4 suffers from the dissolution of active Ag+-ions in the water and consequently results in performance deterioration over repeated cycles.10 Hence, effective methods should be explored to improve the stability of Ag3PO4 in photocatalysis processes.

Immobilization of inorganic photocatalysts on inert supports has been attractive because it can help to stabilize the photocatalysts. Some suitable supports not only possess the ability of decreasing the recombination rate of electron–hole pairs but supplying a great amount of active sites. In addition, supports with excellent absorption performance can concentrate the pollutants from the solution to the photoactive sites of the photocatalysts. Different materials have been used as suitable supports for the photocatalysts, such as Al2O3,11 TiO2,12 graphene,13 SBA-1514 and C3N4.15 Those supports can reduce the contact area of water with the catalysts and contribute to improving the stability of the target materials. The structural and morphological characteristics of the supports play important roles in the catalytic behavior. Therefore, choosing suitable supports is necessary for the enhancement of photocatalytic activity of Ag3PO4.

Flower-shaped inorganic semiconductor materials have been widely applied in the area of solar cell16 and photocatalysis17 due to their high specific surface area and effective hole–electron pair transport through the interconnected network. Template molecules containing strong coordinating groups have been used to mediate the formation of flower-like materials with large surface area. It is worth noting that flower-shaped Au,18 CdS,19 Cu3(PO4)220,21 and ZnO22 crystal aggregates have been successfully synthesized with the participation of DNA, L-cysteine, BSA or ascorbate, respectively. However, the hybridization of inorganic materials with organic molecules would decrease the degree of crystallinity and material properties. Subsequent treatment of hybridized samples at temperature as high as 600–800 °C is inevitable. Until now, there have been no reports of flower-shaped inorganic crystals made from pure inorganic components by means of facile synthetic routes in water.

For the first time, we report a novel method for creating peony-shaped grains of Ag3PO4/Zn3(PO4)2 composite at room temperature in water without the addition of any organic molecules. The compound Zn3(PO4)2 is a common host lattice for fluorescent materials like Zn3(PO4)2:Hf,23 Zn3(PO4)2:Eu24 and Zn3(PO4)2:Dy.25 The environmentally friendly material will be applied in this study as the framework for Ag3PO4 crystals. In the preparation process, large-area Zn3(PO4)2 framework is firstly precipitated ruled by Ac−-ions and, secondly, Ag3PO4 is precipitated onto the surface of the Zn3(PO4)2 support. Lastly, Ag3PO4 nanoparticles self-aggregate to form hierarchically flower-like structure, leading to the formation of well-distributed Ag3PO4/Zn3(PO4)2 composite catalysts.

2 Results and discussion

2.1 Morphology and structure of the composites

The crystallinity and purity of the as-synthesized samples were investigated by XRD recordings. A XRD pattern of a typical Ag3PO4/Zn3(PO4)2 composite is shown in Fig. 1a. The diffraction peaks denoted by “#” and “*” symbols are assigned to cubic Ag3PO4 (JCPDS 06-0505) and orthorhombic Zn3(PO4)2·4H2O (JCPDS 74-1778), respectively. SEM images of the corresponding sample are displayed at different magnifications in Fig. 1b–d. It illustrates that the typical morphology of the sample grains is a well-defined peony-shaped three-dimension structure with a diameter in the range of 4–8 μm. The peony-shaped grains are assembled by many densely arranged nano-thin sheet-shaped “petals”. A close-up SEM view of this flower-like microstructure reveals that the sheets are about 100 nm in thickness, as seen in Fig. 1d. The microscopic characteristic of flower-shaped composite was further analyzed by TEM (Fig. 1S†). TEM image verifies the microstructure of flower-like Ag3PO4/Zn3(PO4)2 assembled with nano-thick plates (Fig. 1Sa†). Obvious contrast can be observed from the edge (Fig. 1Sb†). The microstructures indicate Ag3PO4 is covered by Zn3(PO4)2 and the optimal Ag3PO4 dispersion is achieved by confining Ag3PO4 to the as-prepared Zn3(PO4)2 framework.
image file: c5ra05063d-f1.tif
Fig. 1 The XRD pattern and SEM images of a typical Ag3PO4/Zn3(PO4)2 composite sample. (a) XRD pattern. The diffraction peaks denoted by “#” are assigned to cubic Ag3PO4 (JCPDS 06-0505). The other peaks denoted by signals of sea star are assigned to orthorhombic Zn3(PO4)2·4H2O (JCPDS 74-1778); (b)–(d) SEM images at different magnification.

EDX analysis of the thin sheets forming the typical composite grains register the major elements Zn, Ag, O and P, as seen from Fig. 2S.† The element signal of Al results from the sample support. The estimated atom ratio of Ag to Zn is 2.4. Elemental maps of the same sample clearly demonstrate uniform distribution of Zn, Ag, O and P elements over the flat surface, see Fig. 2. Hence, the typical sample is mainly composed of well-mixed Ag3PO4 and Zn3(PO4)2 phases. The formation of a Ag3PO4/Zn3(PO4)2 composite is further confirmed by measuring XPS spectra. The binding energy of Ag 3d5/2 and Ag 3d3/2 states corresponding to 368 and 374 eV peaks, respectively, are seen in Fig. 3a and are ascribed to the Ag-ion in Ag3PO4.26 The binding energy of Zn 2p1/2 and Zn 2p2/3 located at 1045 eV and 1022 eV, respectively, and seen in Fig. 3b are associated with Zn2+ in Zn3(PO4)2.27


image file: c5ra05063d-f2.tif
Fig. 2 Elemental maps of a flat surface of a flower from as-synthesized sample.

image file: c5ra05063d-f3.tif
Fig. 3 XPS analysis of the as-synthesized sample, see text.

2.2 Formation process of peony-shaped grains of the Ag3PO4/Zn3(PO4)2 composite

XRD patterns of the Ag3PO4/Zn3(PO4)2 composites prepared with different AgNO3 concentration (at a fixed mole ratio of Ag3PO4/Zn3(PO4)2/KH2PO4) are summarized in Fig. 4a. With the increase of AgNO3 concentration, the diffraction peaks of Ag3PO4 become stronger, indicating the crystalline degree of Ag3PO4 is improved gradually. The SEM morphology of Ag3PO4/Zn3(PO4)2 composite grains prepared with 0.05 M AgNO3 solution is seen in Fig. 4b. The obtained sample consists of a large number of hierarchical crystals with an average size of 10–20 μm. The crystal agglomerates are assembled by octahedron-shaped crystals with a size of 4 μm. For the sample prepared with 0.1 M AgNO3 solution, hierarchical crystal aggregates evolve into flower-like grains with a size of 5 μm and the building crystallites change from micro-sized octahedrons to thin plates, see Fig. 4c. Further increase of the AgNO3 solution concentration (0.2 M) results in the formation of peony-shaped grain aggregates. These are assembled by nano-thick plates or “petals”, as demonstrated earlier by SEM in Fig. 1b. The morphology of pure Zn3(PO4)2 crystals as a reference consist of open and cross-linked interiors, see Fig. 4d. With the increase of reactant concentration, more Zn3(PO4)2 and Ag3PO4 crystals are obtained resulting in that flower-shaped composite grains with more “petals”. Increasing petals bring out more exposed crystal faces, which exhibit the superiority in the process of photocatalysis.
image file: c5ra05063d-f4.tif
Fig. 4 XRD patterns and SEM images of the samples prepared with different AgNO3 concentration. (a) XRD patterns of Ag3PO4/Zn3(PO4)2 composites prepared with different AgNO3 concentration and a fixed mole ratio of Ag3PO4/Zn3(PO4)2/KH2PO4. The diffraction peaks denoted by “●” symbols are attributed to orthorhombic Zn3(PO4)2·4H2O (JCPDS 74-1778) and the other diffraction peaks are attributed to cubic Ag3PO4 (JCPDS 06-0505). (b) and (c) Corresponding SEM images of Ag3PO4/Zn3(PO4)2 composites prepared with 0.05 M and 0.1 M AgNO3 solution. (d) SEM image of pure Zn3(PO4)2 extracted from the solution before the formation of Ag3PO4.

To further elucidate the formation process of peony-shaped Ag3PO4/Zn3(PO4)2 crystal aggregates, the time dependent morphological evolution was examined by SEM. Corresponding XRD pattern and SEM images of the samples at different reaction periods are summarized in Fig. 3S† and 5a–d. SEM images of the formed precipitates show that band-shaped AgAc (saturated solution) seen initially (2 min) changes to porous Zn3(PO4)2 at a reaction time of 5 min, compare Fig. 4S† and 5a. After 20 min, tiny Ag3PO4 nanoparticles appear on the surface of these porous Zn3(PO4)2 grains (Fig. 5b). When the reaction time is prolonged to 1.5 h, more and larger Ag3PO4 crystals grow and the flower-like structure is further pronounced (Fig. 5c). After 4 h reaction time the image of a peony-like composite grains around 4–8 μm are seen assembled by separated nano-sheets, see Fig. 5d. With a further extension of the reaction time, the morphology of the composite remains nearly unchanged, as shown earlier for 5 h in Fig. 1d.


image file: c5ra05063d-f5.tif
Fig. 5 SEM images of samples extracted from the solution for preparing peony-shaped Ag3PO4/Zn3(PO4)2 composite at different reaction times. In (a) 5 min; (b) 20 min; (c) 1.5 h; and (d) 4 h.

To validate the function of Ac− for the formation of peony-shaped Ag3PO4/Zn3(PO4)2 composite grains, control experiments of two kinds of pure Ag3PO4 crystals were performed. Firstly, when Ag3PO4 crystals are prepared by the direct reaction of 20 ml of 0.2 M AgNO3 and 20 ml of 0.1 M KH2PO4 solution the Ag3PO4 crystals formed are composed of small particles with a size of 200–500 nm and some polyhedron particles with a size larger than 2 μm, as shown in Fig. 6a. Secondly, after replacing the AgNO3 solution with an AgAc solution, micron-sized Ag3PO4 particles with several morphologies form, e.g. spherical, triangular pyramid or cubic as seen in Fig. 6b. These experiments indicate that flower-like Ag3PO4 could not create without Zn3(PO4)2 framework. It is also shown that the morphology of Zn3(PO4)2 grains extracted from the solution before the formation of Ag3PO4 are open and porous frame construction, see Fig. 4d. However, irregular sheet-shaped Zn3(PO4)2 crystals with micro-scaled thickness are obtained by the direct reaction of Zn(NO3)2 solution (0.15 M, 20 ml) and KH2PO4 solution (0.1 M, 20 ml), as shown in Fig. 6c. It is clear that the open and porous Zn3(PO4)2 crystals form in the presence of Ac−. For further analysis, an Ag3PO4/Zn3(PO4)2 composite was prepared with Zn(NO3)2 as Zn2+ source and with other experimental conditions unchanged. The obtained Ag3PO4/Zn3(PO4)2 sample consists of a majority of tetrahedral particles with a size of 15 μm and a small quantity of cubic particles with a size of 5 μm, as seen in Fig. 6d. The observed morphology is obviously different from that of the Ag3PO4/Zn3(PO4)2 composite obtained in the presence of Ac−. Therefore, the interaction between Ac− and Zn2+ on the surface of Zn3(PO4)2 is the main driving force for forming porous Zn3(PO4)2 framework. Secondary nucleation and growth of Zn3(PO4)2 crystals originating at these Zn2+-binding sites result in the formation of open crystal aggregates, which is essential for the formation of peony-shaped Ag3PO4/Zn3(PO4)2 composite grains with a large surface area.


image file: c5ra05063d-f6.tif
Fig. 6 SEM images of (a) pure Ag3PO4 crystals prepared with 0.2 M AgNO3 solution (20 ml) and 0.1 M KH2PO4 (20 ml); (b) pure Ag3PO4 crystals prepared with 0.2 M AgAc solution (20 ml) and 0.1 M KH2PO4 (20 ml); (c) Zn3(PO4)2 crystals prepared with same volume of 0.15 M Zn(NO3)2 solution and 0.1 M KH2PO4 solution; (d) Ag3PO4/Zn3(PO4)2 composite prepared with 0.2 M AgNO3 solution (20 ml), 0.15 M Zn(NO3)2 solution (20 ml) and 0.1 M KH2PO4 (20 ml).

In the reaction process, the following reactions (1) and (2) should be involved in the synthesis process of Ag3PO4/Zn3(PO4)2 composite:

 
3Zn2+(aq) + 2H2PO4−(aq) = Zn3(PO4)2(s) + 4H+ (1)
 
3Ag+(aq) + H2PO4−(aq) = Ag3PO4(s) + 2H+ (2)
 
6Ag+(aq) + Zn3(PO4)2(s) [left over right harpoons] 2Ag3PO4(s) + 3Zn2+(aq) (3)

Both Ag3PO4 and Zn3(PO4)2 contain phosphate anions, therefore we should consider whether the transformation reaction occur from Zn3(PO4)2 (A3B2 precipitate) to Ag3PO4 (A3B4 precipitate). The total equilibrium constant for the transformation reaction is calculated to be 0.459 according to the reaction (3) and the solubility product constants of Ag3PO4 (1.4 × 10−16) and Zn3(PO4)2 (9.0 × 10−33), which means that the spontaneous trend at room temperature is small. A supplementary experiment was conducted to verify it. At room temperature, 20 ml of 0.15 M Zn(NO3)2 solution was mixed with 20 ml of 0.1 M KH2PO4 solution to obtain pure Zn3(PO4)2 precipitate, then 20 ml of 0.2 M AgNO3 solution was added. After reacting for 2 h, the white precipitate did not change its color, which indicates that after Zn3(PO4)2 crystallizes, it is difficult to transform to Ag3PO4 by ion-exchange process even in the Ag+ saturated solution (see Fig. 5S†). It is believed that the formation of the peony-shaped Ag3PO4/Zn3(PO4)2 composite involves a coprecipitation process. In the reaction process, the color change of the precipitates can be observed from the photos taken at different time (Fig. 6S†). Slow release of Ag+ from oversaturated AgAc decreases the formation speed of Ag3PO4, therefore Ag3PO4 precipitates appear after Zn3(PO4)2 has precipitated completely.

On the basis of the above observations, the detailed formation process of peony-shaped Ag3PO4/Zn3(PO4)2 composite undergoes three stages: (1) oversaturated AgAc precipitates from the solution. (2) Ac−-ions induces the formation of an open Zn3(PO4)2 skeleton. (3) The slow release of Ag+ from supersaturated AgAc offers enough time for the initial formation of a Zn3(PO4)2 skeleton. After that, Ag3PO4 nucleates on the surface of Zn3(PO4)2 skeleton. Based on the above comparative experiments, we believe that the Ag3PO4 flowers involve a sequence of dissolution, recrystallization, and oriented attachment. With the presence of supersaturated AgAc, the tiny Ag3PO4 particles formed on the surface of Zn3(PO4)2 skeleton undergo a crystal relocation process, which is a thermodynamic process. The larger Ag3PO4 particles continue to grow and become thinner depending on the dissolution of the smaller Ag3PO4 particles. The dissolved ions will initiate anisotropic regrowth of nano Ag3PO4 particles on the surface of the preexisted sheets to form peony-shaped structures. As the process continues and the solution keep supersaturated, nanosheets continue to grow until small Ag3PO4 particles disappear. Thereby the uniform flower-like composite were fabricated via a dissolution–recrystallization process, which is the driving force for the assembling of the hierarchical structure, such as flower-like α-Ni(OH)2 microspheres28 and anatase TiO2,29 3D-network polyaniline,30 porous ZnO–ZnSe nanocomposites,31 and porous CoFe2O4 microspheres.32

2.3 Photocatalytic activity

The UV-Vis diffusive absorption spectra of as-prepared peony-shaped Ag3PO4/Zn3(PO4)2 composite grains and the Zn3(PO4)2 support are shown in Fig. 7. Both display an absorption in the region ranging from 250 to 400 nm, corresponding to the absorption of Zn3(PO4)2.33 Besides the absorption in the UV region, the Ag3PO4/Zn3(PO4)2 composite can also absorb visible light with a wavelength range from 420 to 530 nm, which corresponds to the absorption range of pure Ag3PO4 (Fig. 7Sa†). Thus, valence electrons of Ag3PO4 confined in the framework of Zn3(PO4)2 can still be excited to the conduction band state by absorbing visible light, which suggests that the Ag3PO4/Zn3(PO4)2 composite could be used as visible light photocatalysts. Fig. 7Sb† indicates the bandgap of Zn3(PO4)2 is 4.5 eV based on the (αhν)2–hν curve.
image file: c5ra05063d-f7.tif
Fig. 7 UV-Vis diffusive absorption spectra of porous Zn3(PO4)2 support and as-prepared peony-shaped grains of the Ag3PO4/Zn3(PO4)2 composite.

Photoluminescence (PL) spectra of pure Zn3(PO4)2, pure Ag3PO4 and peony-shaped Ag3PO4/Zn3(PO4)2 composite were compared to inquire into the migration and separation efficiency of the photo-generated electrons and holes, as well as the surface vacancies. The emission peaks of Zn3(PO4)2 and flower-like Ag3PO4/Zn3(PO4)2 composite show similar characteristic (Fig. 8Sa†). For Ag3PO4/Zn3(PO4)2 composite, the intensity of the peaks is weaker probably due to the covering of defect sites by Ag3PO4. For pure Ag3PO4, the broad PL peak centered at 557 nm is originated from the recombination of photoexcited electrons with holes34 (Fig. 8Sb†). The PL peak of Ag3PO4 disappears after the presence of Zn3(PO4)2, which indicates that the combination of photogenerated electrons and holes occurred over Ag3PO4 is inhibited.

The photodegradation of RhB aqueous solution in the presence of peony-shaped grains of Ag3PO4/Zn3(PO4)2 composite under visible light illumination is shown in Fig. 8a. The absorption peak of RhB at 554 nm decreases gradually and disappears in less than 60 min, suggesting a complete cleavage of RhB chromophores. Furthermore, the photodegradation curves of RhB in the form of C/C0 as a function of irradiation time are utilized to compare the photocatalytic performance of Ag3PO4/Zn3(PO4)2 with that of pure Ag3PO4, as shown in Fig. 8b. The photocatalytic reaction rate of Ag3PO4/Zn3(PO4)2 is faster than that of pure Ag3PO4. The complete degradation of RhB dye over pure Ag3PO4 crystals takes about 170 min. Close to 100% of the dyes is removed over the Ag3PO4/Zn3(PO4)2 composite within 60 min, whereas only 50% of the dyes are degraded for pure Ag3PO4 catalyst during the same time duration.


image file: c5ra05063d-f8.tif
Fig. 8 (a) The photodegradation of RhB aqueous solution in the presence of peony-shaped grains of Ag3PO4/Zn3(PO4)2 composite under visible light illumination. (b) The photodegradation curves of RhB in the form of C/C0 as a function of irradiation time. (c) Cycling runs in the photodegradation of RhB in the presence of used Ag3PO4/Zn3(PO4)2 composite under visible-light irradiation, cf. text.

To study the stability of the photocatalyst, Ag3PO4/Zn3(PO4)2 composite grains used in above degradation process was collected and reused for additional RhB degradation cycles. After another 4 cycles of photodegradation of RhB solution under same visible light exposure time, the photocatalytic activity of Ag3PO4/Zn3(PO4)2 composite shows a slight loss, see Fig. 8c. Therefore, the composite catalyst could be used more than 200 min, which exceeds the stability of pure Ag3PO4.35

In order to explore the essential reasons for the enhanced performance produced by Zn3(PO4)2 framework for Ag3PO4, active species trapping experiments were carried out. Three substances, including isopropanol, benzoquinone (BZQ), and EDTA-2Na, were added at the beginning of the catalytic process as the scavengers of ˙OH, ˙O2− and h+, respectively. Fig. 9S† shows the photocatalytic curves of RhB solution over peony-shaped Ag3PO4/Zn3(PO4)2 with the presence of different quenchers. The results show that the degradation trend of RhB has been completely inhibited by the addition of BZQ. While about 20 percent of degradation efficiency was lost after isopropanol and EDTA-2Na were added, respectively. Consequently, ˙O2−, ˙OH and h+ involve in the process of degrading dyes. Based on the previous reports, the oxidization of RhB dyes over Ag3PO4 mainly depends on the h+. Two reactions will occur after the formation of photogenerated electrons. O2 molecules trap the electrons to produce ˙O2− and photoreduction reaction (Ag+ + e− → Ag) also consumes photogenerated electrons. For common Ag3PO4-based materials, the second reaction dominates, resulting in the failure of producing ˙O2− radicals. For the peony-shaped Ag3PO4/Zn3(PO4)2, the effective combination of Ag3PO4 with Zn3(PO4)2 by the coprecipitation technology separates the electrons from the Ag+, leading to the occurrence of ˙O2−-related reaction. On the other hand, with the assistance of Zn3(PO4)2, partial holes on the surface of Ag3PO4 react with OH− to produce ˙OH species, which has a strong oxidizing ability to decompose organic chemicals.

The analysis of band structure of Zn3(PO4)2 and Ag3PO4 is illustrated in Fig. 9. The ECB and EVB of Zn3(PO4)2 are calculated to be 0.59 and 5.09 eV, respectively (the calculation was described in ESI†), therefore Zn3(PO4)2 can accept the photogenerated electrons from Ag3PO4.


image file: c5ra05063d-f9.tif
Fig. 9 Schematic diagram of the photodegradation mechanism of peony-shaped Zn3(PO4)2/Ag3PO4 under visible light.

According to the detailed investigation, three factors are accounted for the excellent performance of peony-shaped Ag3PO4/Zn3(PO4)2. Firstly, the specific surface area of peony-shaped Ag3PO4/Zn3(PO4)2 is 28.20 m2 g−1, which is higher than that of pure Ag3PO4 (13.71 m2 g−1) and helps to increase the photocatalytic reaction sites. Secondly, three active species with high oxidizing ability, concluding ˙O2−, ˙OH and h+, involve in the process of degrading dyes. Lastly, photogenerated electrons can transferred from the conduction band of Ag3PO4 to that of Zn3(PO4)2. The special microstructure regulated by Zn3(PO4)2 promotes the quick migration of carriers, leading to the improvement of the separation efficiency of photo-generated electrons and holes.

3 Experimental

3.1 Materials

All the reagents utilized in the present work were of analytical grade and were used without further purification.

3.2 Synthesis

The preparation procedure takes place in water solution. 20 ml of 0.2 M AgNO3 was added drop-wise to 20 ml of 0.15 M Zn(Ac)2 at room temperature. The obtained mixture (containing supersaturated AgAc) was stirred for 1 h. Thereafter 20 ml of 0.1 M KH2PO4 solution was added and a great amount of white precipitate formed immediately. The color of the precipitate changed slowly from white to yellow during prolonged reaction time for 3 h. Afterwards, the solution was transferred to a closed steel autoclave and heated at 110 °C for 2 h. Subsequently, the precipitate was collected by centrifugation, thoroughly rinsed with distilled water and absolute ethanol before drying at 60 °C for 6 h. As a reference pure Ag3PO4 crystals were prepared in a similar way by mixing 20 ml of 0.1 M KH2PO4 with 20 ml of 0.2 M AgAc.

For exploring the formation of the obtained peony-shaped Ag3PO4/Zn3(PO4)2 composite grains different AgNO3 concentrations were used (0, 0.05, 0.1 and 0.2 M) while keeping the same mole ratio of AgNO3, Zn(Ac)2 and KH2PO4.

3.3 Characterization

Powder X-ray diffraction (XRD) was carried out with a Rigaku D/max-2400 powder X-ray diffractometer. The morphologies of the as-synthesized samples were observed using JSM-5610LV scanning electron microscope (SEM) and Hitachi H-800 transmission electron microscope (TEM). X-ray photoelectron spectroscopy (XPS) was performed using an ESCALABMK II X-ray photoelectron spectrometer. UV-Vis diffusive absorption spectra were recorded on a Shimadzu UV2550 spectrophotometer. PL study was carried out on an Opticon 2 Luminescence Spectrometer with the excitation wavelength at 325 nm.

3.4 Photocatalytic experiments

The photocatalytic activity was investigated through the decomposition of a Rhodamine B (RhB) aqueous solution at room temperature. The reaction system, containing 100 ml of RhB solution with an initial concentration of 10−5 M and 50 mg of the tested photocatalysts, were placed in a cylindrical quartz vessel to record the absorption spectrum. Prior to irradiation, the reaction system was magnetically stirred in darkness for 0.5 h to reach equilibrium. A visible light irradiation (λ > 400 nm) through the quartz vessel was provided by a 150 W Xe-lamp with UV-light filter. A sequence of UV-Vis spectra were collected at fixed time intervals to measure the RhB degradation by time.

4 Conclusions

In this paper, we present the first report on coprecipitation and recrystallization technology for the fabrication of peony-shaped Ag3PO4/Zn3(PO4)2 composite grains. The Ac− ion is the key factor for ruling the initial precipitation of peony-shaped grains as it induce the formation of an open Zn3(PO4)2 support. The support also reacts with excess of free Ag+-ions in water from AgAc precipitate that slows the formation rate of Ag3PO4, leading to the formation of well-distributed Ag3PO4 upon the framework surfaces of the host. Compared with pure Ag3PO4 crystals, the obtained Ag3PO4/Zn3(PO4)2 composite exhibits excellent visible-light-driven photocatalytic activity for the decomposition of RhB aqueous solution. This work illustrates a new way for fabrication of Ag3PO4 containing composites from pure inorganic components and promotes the practical applications of this catalyst in addressing environmental and energy issues.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (no. 51402097), Natural Science Foundation of Hubei Province of China (112-45111811) and Fundamental Research Funds for the Central Universities (WUT: 2015IB002). The authors are grateful to Prof. Thommy Eirkssion for his careful work on revising and polishing this paper.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05063d
This journal is © The Royal Society of Chemistry 2015
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