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Facile synthesis of polypod-like Ag3PO4 particles and its application in pollutant degradation under natural indoor weak light irradiation Fei Teng, Zailun Liu, and An Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00735 • Publication Date (Web): 25 Mar 2015 Downloaded from http://pubs.acs.org on April 1, 2015

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Facile Synthesis of Polypod-like Ag3PO4 Particles and Its Application in

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Pollutant Degradation under Natural Indoor Weak Light Irradiation

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FEI TENG,* ZAILUN LIU, AN ZHANG

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Jiangsu Engineering and Technology Research Center of Environmental Cleaning Materials

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(ECM), Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control

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(AEMPC), Jiangsu Joint Laboratory of Atmospheric Pollution Control (APC), Collaborative

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Innovation Center of Atmospheric Environment and Equipment Technology (AEET), School of

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Environmental Science and Engineering, Nanjing University of Information Science & Technology

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,219 Ningliu Road, Nanjing 210044, China; Email: tfwd@ 163.com (F. Teng); Phone/Fax:

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+86-25-58731090

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Abstract

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Today, it is still a big challenge for Ag3PO4 to be applied in practices mainly due to the low

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stability resistant to light irradiation, although it is an efficient photocatalyst. Herein, Ag3PO4

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polypods are prepared by a facile precipitation method, and we have also investigated the

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degradation reaction of pollutants driven under indoor weak light. It is amazing that under indoor

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weak light irradiation, rhodamine B (RhB) can be completely degraded by Ag3PO4 polypods after

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36 h, but only 18% of RhB by N-doped TiO2 after 120 h; and that under indoor weak light

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irradiation, the degradation rate (0.08099 h-1) of RhB by the polypods are 46 times higher than that

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(0.00173 h-1) of N-doped TiO2. The high activity of Ag3PO4 polypods are mainly attributed to the ACS Paragon Plus Environment 1

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three-dimensional branched nanostructure and high-energy {110} facets exposed. Surprisingly,

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after three cycles, Ag3PO4 polypods show a higher stability under indoor weak light than under

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visible light irradiation, while Ag3PO4 have been decomposed into Ag under visible light

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irradiation.

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Keywords: Nanostructures; Polypod; Crystal growth; Ag3PO4

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Introduction

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Semiconductor photocatalysis is one of the most promising ways to solve current energy and

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environment problems (1-3). From energy-saving and environmental protection viewpoint, it is still

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a big challenge for photocatalysis to utilize indoor natural weak light to cleaning indoor

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environmental pollution, without needing an extra artificial optical condenser system. Up to now,

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nevertheless, there is still short of efficient photocatalysts to meet this requirement. Recently, silver

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orthophosphate (Ag3PO4) photocatalyst has attracted considerable interest, which has been

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demonstrated to be an highly active photocatalyst for the degradation of organic pollutants and the

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oxidation of water under visible light irradiation (4,5). Strikingly, Ag3PO4 photocatalyst is reported

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to have a high quantum efficiency of 90% at the wavelengths longer than 420 nm (6). Thus, Ag3PO4

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could be expected to be a promising catalyst driven by indoor natural weak light-driven for the

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cleaning of environmental pollutants, but not dependent on artificial light source.

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To date, many efforts have been devoted to improving their photoelectric and photocatalytic

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properties, e.g., semiconductor coupling (6-9) and polymer composites (10). It is well known that

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the photocatalytic and photo-electric properties of photocatalysts are greatly affected by the

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morphology and the active facets exposed (5,11-16). For example, Ye et al. (5) have synthesized

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Ag3PO4 rhombic dodecahedronsACS andParagon cubes, Plus and Environment they have demonstrated that the rhombic 2

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dodecahedrons with the active {110} facets exposed have a much higher activity than the cubes

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with the exposed {100} facets for the degradation of rhodamine B (RhB). Moreover, they have

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reported that Ag3PO4 concave trisoctahedrons with the high-index facets exposed have the

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improved photocatalytic activity (14). Besides, Wang et al. (15) have reported that Ag3PO4 crystals

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with the exposed {111} facets have the improved photocatalytic properties. Recently, our groups

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have also reported that the Ag3PO4 tetrapods with the high-energy {110} facets exposed have a

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higher photocatalytic activity than the irregular one (16). To the best of our knowledge,

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nevertheless, only the limited micro/nanostructures have been reported for Ag3PO4 so far (5,11-16).

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It still remains a big challenge to acquire Ag3PO4 with the other novel micro/nanostructures to

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further improve its photocatalytic activity. The present question is that whether we can obtain the

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other new nanostructures for Ag3PO4. Numerous studies (17) on Cu2O could provide us with the

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positive answer, because both Ag3PO4 and Cu2O have the same bulk centre cubic (bcc) structures.

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It is well known that various Cu2O micro/nanostructures have been achieved by the chemical

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methods (17), for example, cubes, octahedrons, dodecahedrons, polyhedrons, nanowires,

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nanocages, multipods, hierarchical and hollow structures, and so on. Recently, we have reported the

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synthesis of the non-overlapped Ag3PO4 tetrapods (NOT), which are acquired under hydrothermal

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conditions (16). The preparation is time-consuming and energy intensive. Therefore, we hold that it

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is feasible to achieve novel Ag3PO4 nanostructures with greatly improved performances through an

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innovative approach. On the other hand, it is still a big challenge for Ag3PO4 to be applied in

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practices mainly due to the low stability resistant to light irradiation, although it is a highly efficient

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photocatalyst.

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Herein, the samples are synthesized by a simple precipitation reaction in a mixture containing

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both tetrahydrofuran (THF) and water (W). Furthermore, we have mainly investigated the photo

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degradation properties under indoor under weak light and visible light irradiation. It could be

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expected that a green and energy-saving cleaning approach could be developed using Ag3PO4 for

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the cleaning of environmental pollutants under indoor natural weak light.

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Experimental Section

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Chemicals. All the chemicals are purchased from Shanghai Chemical Company, are of analytical

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grade and used without further purification. The samples are synthesized by a simple precipitation

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reaction, in which a mixture containing both tetrahydrofuran (THF) and water (W) is employed as

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the solvent, phosphoric acid is used as the phosphorus source, and hexamethylenetetramine (HMT)

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is added to adjust the pH value of the system.

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Threefold-overlapped tetrapods (TOTs). Typically, 32 mL of deionized water was placed in a

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breaker, and 8 mL THF was then added. 0.318 g Ag3NO4 was added into the mixed solvent above

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under stirring. Then, 41 µL of 85 Wt.% H3PO4 was added drop wise to the solution above. Finally,

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0.197 g of hexamethylenetetramine (HMT) was introduced into the above solution. The whole

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process was carried out at room temperature under stirring. The color of the reaction mixture

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changed from silvery white to golden yellow after injection of the HMT. After stirring for 5 min,

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the yellow precipitation was collected, washed with deionized water for several times, and dried at

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room temperature.

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Three-dimensional towers (TDTs) and highly-branched tetrapods (HBTs). The same

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procedures as above were taken, but the THF/W volumetric ratios were changed to 0:1 and 0.13:1

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while the solvent volumes were kept same (40 mL) for TDT and HBT, respectively.

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Non-overlapped tetrapods (NOTs). NOTs sample is synthesized as our previously reported

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(16). Typically, 3 mmol of 85 Wt.% H3PO4 was dissolved in 80 mL of deionized water and 2.5

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mmol of AgNO3 was added under stirring. Then, 37.5 mmol of urea were put into above solution.

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The resulting precursor was transferred into a Teflon-lined stainless steel autoclave and maintained ACS Paragon Plus Environment 4

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at 80 oC for 24 h. After cooling to room temperature, the yellow precipitation was collected, washed

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with deionized water several times, and dried overnight at 60 oC.

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Bulk Ag3PO4. Bulk Ag3PO4 was synthesized as previously reported. Specifically, the

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appropriate amounts of Na2HPO4 and AgNO3 powders were thoroughly ground until the initial

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white changed to yellow. The sample is obtained by washing and drying as same above.

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N-doped TiO2 (NTs). Nitrogen doping was conducted as described previously (18). Typically,

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0.5 g of Degauss P25 TiO2 powders was suspended in ethanol (5 mL). Then, urea (1 g) dissolved in

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a mixture solvent of both 2.5 mL ethanol and 0.5 mL H2O was added into the suspension above.

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The mixture was stirred and heated to completely evaporate the solvent, followed by calcination at

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400 oC for 4 h in air.

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Characterization. The crystal structures of the samples were determined by X-ray powder

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polycrystalline diffractometer (Rigaku D/max-2550VB), using graphite monochromatized CuKα

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radiation (λ= 0.154 nm), operating at 40 kV and 50 mA. The XRD patterns were scanned in the

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range of 20-80o (2θ) at a scanning rate of 5o min-1. The samples were characterized on a scanning

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electron microscope (SEM, Hitachi SU-1510) with an acceleration voltage of 15 keV. The samples

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were coated with 5-nm-thick gold layer before observations. The texture properties of the samples

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were measured by nitrogen sorption isotherms. The surface areas the samples were calculated by

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the Brunauer-Emmett-Teller (BET) method. UV-Vis diffuse reflectance spectra (UV-DRS) of the

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samples were obtained using a UV-vis spectrophotometer (UV-2550, Shimadzu, Japan).

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Photocatalytic degradation reaction. Photocatalytic activities of the samples were evaluated

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by photocatalytic decomposition of rhodamine B (RhB). Typically, the suitable amounts of powders

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were put into a solution of RhB (100 mL, 10 mg L-1), which was irradiated with a 300W Xe arc

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lamp equipped with an ultraviolet cut off filter to provide visible light (λ ≥ 420 nm). Because the

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BET areas of HBTs, TDTs, TOTs, NOTs and Plus NTsEnvironment are 4.9, 3.2, 3.3, 3.1, 32.2 m2 g-1, 64 mg of ACS Paragon 5

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HBTs, 97 mg of TDTs, 94 mg of TOTs, 100 mg of NOTs and 96 mg of NTs are used in the

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degradation, respectively. According to reference (19), the aim is to keep their surface areas same.

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The degradation reactions are also carried out under indoor weak light irradiation, while keeping the

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other conditions constant.

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Results and Discussion

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Effect of THF/W volumetric ratio. Figure 1 shows the typical scanning electron microscopy

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(SEM) images of the samples prepared at different THF/W volumetric ratios. When the volume

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ratio of THF/W is increased, the morphology of Ag3PO4 changes from three-dimensional towers

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(TDTs) to highly-branched tetrapods (HBTs), threefold-overlapped tetrapods (TOTs), irregular

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Ag3PO4. At 0:1, Three-dimensional towers (TDT) form, whose bifurcated angle between the shaft

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and secondary branch is about 109o28' (Figure 1a). A few of fractured HBT are found in TDT

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sample. At 0.13:1, the formed HBT sample has four stretched shafts, which further grow through

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the secondary branching growth (Figure 1b). From the SEM images, we can observe that the four

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shafts of HBT stretch along four [111] directions (16) and they have further branched through a

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secondary growth process.

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angles are also 109°28′. Nevertheless, these TOTs are distinct from the previously reported

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unoverlapped tetrapods (13,15,16). The threefold-overlapped branches are parallel to one another

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and their twelve arms are 5-8 µm × 500 nm. The twelve branches are about 5-8 µm in length and

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500 nm in diameter. At too higher THF/W ratios (0.5:1, 1:1 and 1:0), however, the irregular samples

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are obtained (Figure S1, seeing electronic supporting information (ESI)). It is clear that the THF/W

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ratio plays a key role in the growth of Ag3PO4.

Figure 1c shows the uniform TOTs formed at 0.25:1, whose bifurcated

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Figure 2A shows X-ray diffraction (XRD) patterns of the samples prepared at THF/W =0/1,

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0.13/1 and 0.25/1. All diffraction peaks can be well indexed to the bcc Ag3PO4 (JCPDS No.

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06-0505) and no impurities phases are found, Plus confirming the formation of phase-pure Ag3PO4. ACS Paragon Environment 6

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Furthermore, we have calculated the intensity ratios of (222)/(110) and (222)/(200) peaks of TDTs

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(Figure 2B). The peak intensity ratios of (222)/(110) and (222)/(200) are 2.86 and 2.83,

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respectively; whereas 1.43 and 1.47 for the bulk one (Figure S2, seeing ESI). The results mean that

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{111} crystal facets may preferentially grow. We have tried to perform the high-resolution

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transmission electron microscopy to determine its growth direction. However, our attempt is

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unsuccessful because the Ag3PO4 crystals are too large and unstable under irradiation with

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high-energy electrons. Nevertheless, several researchers have demonstrated that the branches of

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Ag3PO4 tetrapods grow preferentially along the [111] direction (13,15,16). Herein, we could only

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assume that the crystals grow preferentially along the [111] direction. Moreover, Yu et al. have

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revealed the correlation of the ZnO tetrapods with the growth orientation (20).

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Effects of pH value and reaction temperature. First, the HMT-dependent experiments have been

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performed to understand the role of HMT, while keeping the THF/W volumetric ratio at 0.25:1 at

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30 oC.

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values. Under strong acidic conditions, we hold that at pH values low than 0.21, phosphorous

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source mainly exists as the molecular form of H3PO4. Thus, Ag3PO4 can not form without adding

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HMT. We have found that with increasing the HMT amount, the pH value of the system increases

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from 0.21 to 4.82 due to the hydrolysis of HMT (Table S1 of ESI). As a result, Ag3PO4 can form

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only in a certain molar ratio range of HMT/Ag(I) (Figure 4). At 0.5:1 (HMT/Ag(I)), TOTs form, on

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which a few nanoparticles attaches (Figure 4A). Although TOTs can also form at 0.75:1, almost no

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any nanoparticles attach on their surfaces (Figure 4B). At 1:1, the pitted tetrahedrons form (Figure

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4C); and at 1.25:1 and 1.5:1, the formed samples are poly-armed Ag3PO4 (Figure 4D). It is clear

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that the HMT amount added has also a great influence on the morphology of Ag3PO4.

It is obviously observed from Figure 3 that H3PO4 has four existing forms at different pH

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On base of the results above, we hold that it is the pH value that has changed the existing form

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and distribution of phosphate ions, namely, more PO43- ions exist at high pH values (21). Hence the

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nucleation and growth rates of Ag3PO4 greatly increase with the HMT amount added. As a result, ACS Paragon Plus Environment 7

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the branching growth of Ag3PO4 becomes pronounced, leading to the formation of poly-armed

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sample.

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Moreover, the temperature-dependent experiments are also performed (Figure 5). The results

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show that the non-overlapped Ag3PO4 tetrapods (NOT) form at 15 and 20 oC, which have the wide

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leaves (Figure 5(A,B)). At 30 and 45 oC, the threefold-overlapped Ag3PO4 tetrapods (TOTs) form

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(Figure 5(C,D)). It seems that TOTs form from the NOTs with wide leaves. It is reasonable that the

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wide leaves are metastable due to the high surface energy. At high temperatures, the wide leaves

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may split and grow into the overlapped branches through the dissolution and re-crystallization

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processes. At low temperatures, the tetrapods with wide leaves form; with increasing the

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temperature, each wide leaf of tetrapods gradually transform into three independent branches, and

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finally become overlapped tetrapods.

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Generally, crystal formation is determined by the thermodynamics and kinetics of growth. Thus

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various parameters, including intrinsic crystal structure, nature and concentration of precursor,

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molecule adsorption, temperature, time, and the other moieties present in solution such as counter

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ions and foreign metal ions, can greatly affect the final crystal (22). Obviously, these factors are

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closely interdependent and cannot be considered independently. It seems that the crystals form

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through the complicated process of dissolution – recrystallization – splitting growth. It is reasonable

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that reaction temperature generally has a significant influence on the kinetics and thermodynamics

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of nucleation and growth, leading to different morphologies.

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Formation mechanism of Ag3PO4. On base of the results above, a plausible growth mechanism is

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proposed as follows. First, THF, as an aprotic, polar solvent, can strongly solvate Ag(I) ions,

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resulting in a slow diffusion rate of Ag(I) ion. Moreover, a large spatial obstacle will exist by the

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solvating shells of THFs, which can reduce the collision reaction of both Ag(I) with PO43-. As a

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result, the secondary branching growth of the shaft is refrained, as demonstrated by the fact that the

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sub-branches of HBTs are shorter than those of TDTs (Figures 1C vs. 1A). The sub-branches of ACS Paragon Plus Environment 8

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TDT can fully grow without THF. Limited by small space close to the center of a tetrapod, the

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fully-grown sub-branches of the four shafts will intersect and squeeze one another and finally the

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shafts snap off, leading to the formation of TDTs with long sub-branches (Figure 1A).

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We hold that the adsorption of THF on the surface of Ag3PO4 is another important factor. Since

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the oxygen atom of THF molecule has two pairs of unshared electrons, THF molecules are easy to

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adsorb on Ag(I)-enriched facets of Ag3PO4 nanocrystals through the coordination of Ag(I) with the

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oxygen of THF (5). As a result, the growth rate of Ag(I) ion-enriched facets, i.e. {110}, may be

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reduced greatly. With further increasing the THF amount, the growth of sub-branches is inhibited

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significantly. As a result, the uniform TOTs form at 0.25:1 (Figures 1C and 4). At too high THF/W

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ratios (0.5:1, 1:1, 1:0), AgNO3 can not completely dissolve in the system, leading to the formation

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of irregular particles (Figure S1, seeing ESI). To conclude, THF plays a vital role in the growth of

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Ag3PO4, which needs extensive research in future.

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Degradation performance of Ag3PO4 polypods under visible light irradiation The

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photocatalytic activities of the typical samples have been investigated using the degradation of

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rhodamine B (RhB) as the probe reaction (Figure 6). Herein, N-doped TiO2 (NT) is also prepared to

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compare with Ag3PO4. Because the BET areas of HBTs, TDTs, TOTs, NOTs and NTs are measured

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to be 4.9, 3.2, 3.3, 3.1, 32.2 m2 g-1, their amounts used in the degradation of RhB are 64, 97, 94, 100

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and 9.6 mg, respectively. The aim is to keep their surface areas same according to the reference (19).

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Moreover, we have measured the absorption spectra of RhB with irradiation time. It is obvious that

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RhB has been destroyed, characterized by the disappearance of the maximum absorbance of RhB at

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553 nm (Figure 7). The apparent reaction rate constants are 1.1335, 0.6936, 0.4008, 0.2199 and

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0.0078 min-1 for HBT, TDT, TOT, NOT and NT, respectively (Figure 6B). The apparent rate

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constants of HBT, TDT, TOT and NOT are 144.3, 87.9, 50.4 and 27.2 times higher than that of NT,

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respectively, indicating the high photocatalytic activity of Ag3PO4 (4). For the Ag3PO4 samples,

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their activity orders are HBT > TDT > TOT > NOT. On one hand, the percentages of the {110}

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facets exposed are calculated to be 99%, 97%, 90% and 87% for HBT, TDT, TOT and NOT,

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respectively. Taking their same surface areas into account, their different activities can be mainly

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attributed to the active {110} facets exposed. Many researchers (4,5,13,14,16) have reported that the

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surface energies of {110} and {100} facets are 1.31 and 1.12 J/m2; respectively; and further

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demonstrated that the high-energy {110} facets have a higher photocatalytic activity than the {100}

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and {111} facets for the degradation of RhB. On the other hand, the ultraviolet-visible diffuse

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reflectance spectra (UV-DRS) reveal that three Ag3PO4 samples can absorb the visible light with a

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wavelength shorter than 510 nm, which have the same absorption edge, indicating their same band

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gap energy (Figure 8). However, the absorbencies of both polypods are about 2.5 times higher than

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the bulk, which may favor to improve the photo activity. The high absorbencies of the former two

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samples can be attributed to their three-dimensional microstructures, which favors for the

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repeatedly reflections and absorptions of irradiation light (the inset of Figure 8) (17,23).

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Degradation performance of Ag3PO4 polypods under natural indoor weak light We have

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investigated the indoor weak light-driven degradation performances of Ag3PO4 polypod structures

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(Figure 9). It is amazing that under indoor weak light irradiation, 100% RhB has been degraded

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after 36 h, while only 18% can be degraded by N-doped TiO2 after 120 h (Figure 9a). The apparent

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reaction kinetic constant (0.08099 h-1) of Ag3PO4 polypods is 46.8 times higher than that (0.00173

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h-1) of N-doped TiO2 (Figure 9b). Besides, the complete mineralization of RhB dye by Ag3PO4 has

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been confirmed by using the organic carbon (TOC) test, infrared spectroscopy (IR), UV-Vis

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absorption spectra and mass spectrum (MS) (Figures S3-S6, seeing ESI). Moreover, the cycle

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stability of Ag3PO4 polypod structures have been investigated (Figure 10). After three cycles under

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indoor weak light irradiation, Ag3PO4 polypods nearly maintain activity within 36 h (Figure 10a).

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Under visible light irradiation, however, the degradation ratios of RhB by Ag3PO4 polypods are

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63% and 34% for the second and third cycles, respectively (Figure 10b). XRD patterns (Figure 10c)

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irradiation; while more Ag3PO4 have decomposed into Ag under visible light. Also we observed

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with SEM that under visible light irradiation, the polypods have broken into fractures (Figure 10d);

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while the polypods are still maintained (not showing). Summarily, Ag3PO4 polypods show a higher

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cycling stability under natural weak light irradiation than under visible light irradiation.

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It is important that Ag3PO4 photocatalyst is discovered with good photocatalytic activity under

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indoor natural weak sunlight. Compared with conventional photocatalysis, the degradation reaction

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is carried out under indoor natural weak sunlight that does not needs extra optical system. It is a

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green and energy-saving technology to utilize Ag3PO4 as catalyst for the degradation of

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environmental pollutants driven by indoor natural weak light-driven, not depending on artificial

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light source. It could be expected that Ag3PO4 polypod structures can be conveniently applied in

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indoor air cleaning under weak light irradiation.

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The Ag3PO4 polypod structures are discovered with an efficient degradation activity under

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indoor weak light without needing an extra, complicate artificial optical condenser system. The

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efficient Ag3PO4 polypods could be promising to be conveniently applied in indoor air cleaning.

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Acknowledgements

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This work is financially supported by National Science Foundation of China (21377060,

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21103049), the Project Funded by the Science and Technology Infrastructure Program of Jiangsu

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(BM201380277, 2013139), Jiangsu Science Foundation of China (BK2012862), Six Talent Climax

259

Foundation of Jiangsu (20100292), Jiangsu Province of Academic Scientific Research

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Industrialization Projects (JHB2012-10, JH10-17), the Key Project of Environmental Protection

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Program of Jiangsu (2013016, 2012028), A Project Funded by the Priority Academic Program ACS Paragon Plus Environment 11

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Development of Jiangsu Higher Education Institutions (PAPD) sponsored by SRF for ROCS, SEM

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(2013S002) and “333” Outstanding Youth Scientist Foundation of Jiangsu (2011015).

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Electronic Supporting Information (ESI)

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This supporting material is available free of charge via the internet at http://www.acs.org

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References

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(6) Bi, Y.; Ouyang, S.; Cao, J.; Ye, J. Facile synthesis of rhombic dodecahedral AgX/Ag3PO4 (X =

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nanocomposites with enhanced photocatalytic activity and stability. New J. Chem. 2012, 36,

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characterization of high efficiency and stable Ag3PO4/TiO2 visible light photocatalyst for the

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(9) Li, G. P.; Mao, L. Magnetically separable Fe3O4–Ag3PO4 sub-micrometre composite: facile

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synthesis, high visible light-driven photocatalytic efficiency, and good recyclability. RSC Adv.

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(10) Zhang, H.; Huang, H.; Ming, H.; Li, H.; Zhang, L.; Liu, Y.; Kang, Z. Carbon quantum

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dots/Ag3PO4 complex photocatalysts with enhanced photocatalytic activity and stability under

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visible light. J. Mater. Chem. 2012, 22, 10501-10506.

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(11) Dinh, C. T.; Nguyen, T. D.; Kleitz, F.; Do, T.-O. Large-scale synthesis of uniform silver

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orthophosphate colloidal nanocrystals exhibiting high visible light photocatalytic activity.

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ChemComm. 2011, 47, 7797-7799.

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(12) Liang, Q. H.; Ma, W. J.; Shi, Y.; Li, Z.; Yang, X. M. Hierarchical Ag3PO4 porous microcubes

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with enhanced photocatalytic properties synthesized with the assistance of trisodium citrate.

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(13) Wang, H.; He, L.; Wang, L. H.; Hu, P. F.; Guo, L.; Han, X. D.; Li, J. H. Facile synthesis of

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Ag3PO4 tetrapod microcrystals with an increased percentage of exposed {110} facets and

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highly efficient photocatalytic properties. CrystEngComm. 2012, 14, 8342-8344.

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(14) Jiao, Z. B.; Zhang, Y.; Yu, H. C.; Lu, G. X.; Ye, J. H.; Bi, Y. P. Concave trisoctahedral Ag3PO4

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microcrystals with high-index facets and enhanced photocatalytic properties. Chem. Commun.

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2013, 49, 636-638.

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Ag3PO4 as a recyclable highly efficient photocatalyst. Chem. Eur. J. 2012, 18, 5524-5529.

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Ag3PO4 tetrapods and the {110} facets-dominated photocatalytic activity. CrystEngComm.

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(17) Sun, S. D.; Song, X. P.; Kong. C. C.; Yang, Z. M. Selective-etching growth of urchin-like Cu2O architectures. CrystEngComm. 2011, 13, 6616-6620. (18) Mitoraj, D.; Kisch, H. The nature of nitrogen-modified titanium dioxide photocatalysts active in visible light. Angew. Chem., Int. Ed. 2008, 47, 9975-9978. (19) Xu, L.; Jiang, L.-P.; Zhu, J.-J. Sonochemical synthesis and photocatalysis of porous Cu2O nanospheres with controllable structures. Nanotechnol. 2009, 20, 045605-045610.

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329

Title list of Figures and Tables

330

FIGURE 1 Scanning electron microscopy (SEM) macrographs of the samples

331

synthesized at different volumetric ratios of tetrahydrofuran (THF) to water (W): (a)

332

0:1, Three-dimensional towers (TDT); (b) 0.13:1, Highly-branched tetrapods (HBT);

333

(c) 0.25:1, Threefold-overlapped tetrapods (TOT); Scale bar = 5 µm; Reaction

334

temperature: 30 oC; Hexamethylenetetramine (HMT)/Ag(I) = 0.75 (molar ratio); the

335

arrow direction is along [111]

336

FIGURE 2 (A) X-ray diffraction (XRD) patterns of the samples and (B) Intensity

337

ratios of (222)/(110) and (222)/(200) peaks of (a) TDT and (b) bulk Ag3PO4

338

FIGURE 3 Distribution diagram of various existing forms of H3PO4 at different pH

339

values (the dotted line marked by arrow, indicating the pH value for our system)

340

FIGURE 4 SEM images of the samples synthesized at different molar ratios of

341

HMT/Ag(I): (A) 0.5:1; (B) 0.75:1; (C) 1:1 and 1.25:1; (D) 1.5:1. Reaction

342

temperature: 30 oC; THF/W = 0.25:1

343

FIGURE 5 SEM images of the samples synthesized at different reaction

344

temperatures: (A) 15 oC; (B) 20 oC; (C) 30 oC; (D) 45 oC. HMT/Ag(I) = 0.75/1;

345

THF/W = 0.25/1

346

FIGURE 6 The degradation curves (A) and apparent reaction kinetic constants (B)

347

of the samples for the degradation of rhodamine B under visible light irradiation (λ >

348

420 nm): HBT (64 mg); TDT (97 mg); TOT (94 mg); Non-overlapped tetrapods

349

(NOT, 100 mg) and N-doped TiO2 (NT, 9.6 mg)

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FIGURE 7 The absorption spectra of RhB at different irradiation times

351

FIGURE 8 UV-vis diffuse reflectance spectra (UV-DRS) of the Ag3PO4 samples

352

FIGURE 9 Degradation and reaction kinetic curves of rhodamine B over Ag3PO4

353

polypod structures and N-doped TiO2 under indoor weak light

354

FIGURE 10 Cycle stability of Ag3PO4 polypod structures: (a) under indoor natural

355

dim light; (b) under visible light irradiations; (c) XRD patterns; (d) SEM images

356

after reaction under visible light irradiation

17

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357 358 359

FIGURE 1

360 361 362 9µm

363 364 365

(a)

(c)

(b)

366 367

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368 369 FIGURE 2

210

320 321

400 411 420 421 332

(a)

310

222

211 200

110

Intensity (a.u.)

(A)

220

370

(b) (c)

20

30

40 50 60 2Theta (Degree)

70

80

371

3.0

Intensity ratio

2.5

(B)

TDT Bulk 2.86

2.83

2.0 1.5 1.0

1.47

1.43

0.5 0.0 (222)/(110)

(222)/(200)

372

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373 374

FIGURE 3

375 376 377 378 379 380 381 382 383 384 385 386

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387 388

FIGURE 4

389 390 391 392

5 µm

393 394 395 396

20 µm

(A)

3 µm

(B)

397 398 399

10 µm

5 µm

400 401 402 403

20 µm

(C)

30 µm

404 405 406

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(D)

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407 408

FIGURE 5

409 410

A

B

C

D

411 412 413 414 415 416 417 418 419 420 421 422

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423 FIGURE 6

1.0

5

0.8

4

0.6

ln(C0/C)

C/C0

424

HBT TDT TOT NOT NT

0.4 0.2

3

-1

HBT: ka=1.1335 min

2

TDT: ka=0.6936 min

-1

TOT: ka=0.4008 min

-1 -1

NOT: ka=0.2199 min

1

-1

NT: ka=0.0078 min

0.0 0

2

4

6 8 10 (A) t (min)

12

14

0

16

0

4

425 426

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8

12 16 (B) t (min)

20

24

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427 428

FIGURE 7

TDT Absorbance

Absorbance

HBT

0min 1min 2min 3min

300

400

500

600

700

0 min 2 min 3 min 4 min

300

800

400

500

600

700

800

700

800

700

800

Light length /nm

Length / nm

429

NOT Absorbance

Absorbance

TOT

0 2min 4min 6min 10min

300

400

500

600

700

0 2min 4min 6min 10min 15min

300

800

400

500

600

Light length /nm

Light length (nm)

430

Blank Absorbance

Absorbance

NT

0min 10min 15min 20min

300

400

500

600

700

0mim 20mim 40mim 60mim 80mim 300

800

400

500

600

Light length /nm

Light length (nm)

431 432

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433 434

FIGURE 8

Absorbance (a.u.)

435

HBT TOT B ulk

300

400

500

600

700

W avelength (nm )

436 437

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438 439

FIGURE 9

3.5

1.0 0.8 ln(C0/C)

2.5

0.6 C/C0

(b)

3.0

0.4 0.2

0

1.5 HBT:

1.0

ka=0.08099 h

-1 -1

N-TiO2: ka=0.00173 h

0.5

HBT NT

0.0 (a)

2.0

0.0

15 30 45 60 75 90 105 120

0

15 30 45 60 75 90 105 120

t /h

t /h

440 441

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442 443

FIGURE 10

444 445

1.0

1.0

1st cycle 2nd cycle 0.8

0.8

0.6

0.6

C/C0

C/C0

446

1st cycle

3rd cycle

0.4

0.4

0.2

0.2

447 448

0.0

449

0

15 30 45 60 75 90 105 120 Reaction time /h

(c)

Under indoor weak light Under visible light irradiation

0.0

(b) 0 1 2 3 4 5 6 7 8 9 10 11 12 Time /min

(d)

Intensity /a.u.

450

(a)

2nd cycle 3rd cycle

451 452

10

20

30

40

50

60

70

80

2Theta /degree

453 454

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