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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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nanocomposites with enhanced photocatalytic activity and stability. New J. Chem. 2012, 36,
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1041-1044.
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(8) Yao, W. F.; Zhang, B.; Huang, C. P.; Ma, C.; Song, X. L.; Xu, Q. J. Synthesis and
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characterization of high efficiency and stable Ag3PO4/TiO2 visible light photocatalyst for the
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degradation of methylene blue and rhodamine B solutions. J. Mater. Chem. 2012, 22,
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4050–4055
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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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2012, 2, 5108-5011.
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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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(15) Wang, H.; Bai, Y. S.; Yang, J. T.; Lang, X. F.; Li, J. H.; Guo L. A facile way to rejuvenate
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Ag3PO4 as a recyclable highly efficient photocatalyst. Chem. Eur. J. 2012, 18, 5524-5529.
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(16) Wang, J.; Teng, F.; Chen, M. D.; Xu, J. J.; Song, Y. Q.; Zhou X. L. Facile synthesis of novel
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Ag3PO4 tetrapods and the {110} facets-dominated photocatalytic activity. CrystEngComm.
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2013, 15, 39-42.
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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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(20) Yu, W. D.; Li, X. M.; Gao, X.D. Catalytic synthesis and structural characteristics of
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high-quality tetrapod-like ZnO nanocrystals by a modified vapor transport process. Cryst.
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Growth & Des. 2005, 5, 151-155.
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(21) M.Z. Cai, Y.P. Hang, Q. Yu, Anal. Chem. South China University of Technology, Beijing, Chem. Ind. Press, 2009. ACS Paragon Plus Environment 14
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(22) L. M. Liz-Marzan, Increasing complexity while maintaining a high degree of symmetry in nanocrystal growth, Angew. Chem. Int. Ed. 2015, 54, 3860-3861 (23) Li, G. ; Zhang, D. ; Yu, J. A new visible-light photocatalyst: CdS quantum dots embedded mesoporous TiO2. Environ. Sci. Technol. 2009, 43, 7079-7085.
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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
18
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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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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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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
27
ACS Paragon Plus Environment
Article
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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TOC 56x45mm (96 x 96 DPI)
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Facile Synthesis of Polypod-like Ag3PO4 Particles and Its Application in
2
Pollutant Degradation under Natural Indoor Weak Light Irradiation
3 4
FEI TENG,* ZAILUN LIU, AN ZHANG
5
Jiangsu Engineering and Technology Research Center of Environmental Cleaning Materials
6
(ECM), Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control
7
(AEMPC), Jiangsu Joint Laboratory of Atmospheric Pollution Control (APC), Collaborative
8
Innovation Center of Atmospheric Environment and Equipment Technology (AEET), School of
9
Environmental Science and Engineering, Nanjing University of Information Science & Technology
10
，219 Ningliu Road, Nanjing 210044, China; Email: tfwd@ 163.com (F. Teng); Phone/Fax:
11
+86-25-58731090
12 13
Abstract
14
Today, it is still a big challenge for Ag3PO4 to be applied in practices mainly due to the low
15
stability resistant to light irradiation, although it is an efficient photocatalyst. Herein, Ag3PO4
16
polypods are prepared by a facile precipitation method, and we have also investigated the
17
degradation reaction of pollutants driven under indoor weak light. It is amazing that under indoor
18
weak light irradiation, rhodamine B (RhB) can be completely degraded by Ag3PO4 polypods after
19
36 h, but only 18% of RhB by N-doped TiO2 after 120 h; and that under indoor weak light
20
irradiation, the degradation rate (0.08099 h-1) of RhB by the polypods are 46 times higher than that
21
(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,
23
after three cycles, Ag3PO4 polypods show a higher stability under indoor weak light than under
24
visible light irradiation, while Ag3PO4 have been decomposed into Ag under visible light
25
irradiation.
26
Keywords: Nanostructures; Polypod; Crystal growth; Ag3PO4
27 28
Introduction
29
Semiconductor photocatalysis is one of the most promising ways to solve current energy and
30
environment problems (1-3). From energy-saving and environmental protection viewpoint, it is still
31
a big challenge for photocatalysis to utilize indoor natural weak light to cleaning indoor
32
environmental pollution, without needing an extra artificial optical condenser system. Up to now,
33
nevertheless, there is still short of efficient photocatalysts to meet this requirement. Recently, silver
34
orthophosphate (Ag3PO4) photocatalyst has attracted considerable interest, which has been
35
demonstrated to be an highly active photocatalyst for the degradation of organic pollutants and the
36
oxidation of water under visible light irradiation (4,5). Strikingly, Ag3PO4 photocatalyst is reported
37
to have a high quantum efficiency of 90% at the wavelengths longer than 420 nm (6). Thus, Ag3PO4
38
could be expected to be a promising catalyst driven by indoor natural weak light-driven for the
39
cleaning of environmental pollutants, but not dependent on artificial light source.
40
To date, many efforts have been devoted to improving their photoelectric and photocatalytic
41
properties, e.g., semiconductor coupling (6-9) and polymer composites (10). It is well known that
42
the photocatalytic and photo-electric properties of photocatalysts are greatly affected by the
43
morphology and the active facets exposed (5,11-16). For example, Ye et al. (5) have synthesized
44
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
46
with the exposed {100} facets for the degradation of rhodamine B (RhB). Moreover, they have
47
reported that Ag3PO4 concave trisoctahedrons with the high-index facets exposed have the
48
improved photocatalytic activity (14). Besides, Wang et al. (15) have reported that Ag3PO4 crystals
49
with the exposed {111} facets have the improved photocatalytic properties. Recently, our groups
50
have also reported that the Ag3PO4 tetrapods with the high-energy {110} facets exposed have a
51
higher photocatalytic activity than the irregular one (16). To the best of our knowledge,
52
nevertheless, only the limited micro/nanostructures have been reported for Ag3PO4 so far (5,11-16).
53
It still remains a big challenge to acquire Ag3PO4 with the other novel micro/nanostructures to
54
further improve its photocatalytic activity. The present question is that whether we can obtain the
55
other new nanostructures for Ag3PO4. Numerous studies (17) on Cu2O could provide us with the
56
positive answer, because both Ag3PO4 and Cu2O have the same bulk centre cubic (bcc) structures.
57
It is well known that various Cu2O micro/nanostructures have been achieved by the chemical
58
methods (17), for example, cubes, octahedrons, dodecahedrons, polyhedrons, nanowires,
59
nanocages, multipods, hierarchical and hollow structures, and so on. Recently, we have reported the
60
synthesis of the non-overlapped Ag3PO4 tetrapods (NOT), which are acquired under hydrothermal
61
conditions (16). The preparation is time-consuming and energy intensive. Therefore, we hold that it
62
is feasible to achieve novel Ag3PO4 nanostructures with greatly improved performances through an
63
innovative approach. On the other hand, it is still a big challenge for Ag3PO4 to be applied in
64
practices mainly due to the low stability resistant to light irradiation, although it is a highly efficient
65
photocatalyst.
66
Herein, the samples are synthesized by a simple precipitation reaction in a mixture containing
67
both tetrahydrofuran (THF) and water (W). Furthermore, we have mainly investigated the photo
68
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
70
the cleaning of environmental pollutants under indoor natural weak light.
71
Experimental Section
72
Chemicals. All the chemicals are purchased from Shanghai Chemical Company, are of analytical
73
grade and used without further purification. The samples are synthesized by a simple precipitation
74
reaction, in which a mixture containing both tetrahydrofuran (THF) and water (W) is employed as
75
the solvent, phosphoric acid is used as the phosphorus source, and hexamethylenetetramine (HMT)
76
is added to adjust the pH value of the system.
77
Threefold-overlapped tetrapods (TOTs). Typically, 32 mL of deionized water was placed in a
78
breaker, and 8 mL THF was then added. 0.318 g Ag3NO4 was added into the mixed solvent above
79
under stirring. Then, 41 µL of 85 Wt.% H3PO4 was added drop wise to the solution above. Finally,
80
0.197 g of hexamethylenetetramine (HMT) was introduced into the above solution. The whole
81
process was carried out at room temperature under stirring. The color of the reaction mixture
82
changed from silvery white to golden yellow after injection of the HMT. After stirring for 5 min,
83
the yellow precipitation was collected, washed with deionized water for several times, and dried at
84
room temperature.
85
Three-dimensional towers (TDTs) and highly-branched tetrapods (HBTs). The same
86
procedures as above were taken, but the THF/W volumetric ratios were changed to 0:1 and 0.13:1
87
while the solvent volumes were kept same (40 mL) for TDT and HBT, respectively.
88
Non-overlapped tetrapods (NOTs). NOTs sample is synthesized as our previously reported
89
(16). Typically, 3 mmol of 85 Wt.% H3PO4 was dissolved in 80 mL of deionized water and 2.5
90
mmol of AgNO3 was added under stirring. Then, 37.5 mmol of urea were put into above solution.
91
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
93
with deionized water several times, and dried overnight at 60 oC.
94
Bulk Ag3PO4. Bulk Ag3PO4 was synthesized as previously reported. Specifically, the
95
appropriate amounts of Na2HPO4 and AgNO3 powders were thoroughly ground until the initial
96
white changed to yellow. The sample is obtained by washing and drying as same above.
97
N-doped TiO2 (NTs). Nitrogen doping was conducted as described previously (18). Typically,
98
0.5 g of Degauss P25 TiO2 powders was suspended in ethanol (5 mL). Then, urea (1 g) dissolved in
99
a mixture solvent of both 2.5 mL ethanol and 0.5 mL H2O was added into the suspension above.
100
The mixture was stirred and heated to completely evaporate the solvent, followed by calcination at
101
400 oC for 4 h in air.
102
Characterization. The crystal structures of the samples were determined by X-ray powder
103
polycrystalline diffractometer (Rigaku D/max-2550VB), using graphite monochromatized CuKα
104
radiation (λ= 0.154 nm), operating at 40 kV and 50 mA. The XRD patterns were scanned in the
105
range of 20-80o (2θ) at a scanning rate of 5o min-1. The samples were characterized on a scanning
106
electron microscope (SEM, Hitachi SU-1510) with an acceleration voltage of 15 keV. The samples
107
were coated with 5-nm-thick gold layer before observations. The texture properties of the samples
108
were measured by nitrogen sorption isotherms. The surface areas the samples were calculated by
109
the Brunauer-Emmett-Teller (BET) method. UV-Vis diffuse reflectance spectra (UV-DRS) of the
110
samples were obtained using a UV-vis spectrophotometer (UV-2550, Shimadzu, Japan).
111
Photocatalytic degradation reaction. Photocatalytic activities of the samples were evaluated
112
by photocatalytic decomposition of rhodamine B (RhB). Typically, the suitable amounts of powders
113
were put into a solution of RhB (100 mL, 10 mg L-1), which was irradiated with a 300W Xe arc
114
lamp equipped with an ultraviolet cut off filter to provide visible light (λ ≥ 420 nm). Because the
115
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
117
degradation, respectively. According to reference (19), the aim is to keep their surface areas same.
118
The degradation reactions are also carried out under indoor weak light irradiation, while keeping the
119
other conditions constant.
120
Results and Discussion
121
Effect of THF/W volumetric ratio. Figure 1 shows the typical scanning electron microscopy
122
(SEM) images of the samples prepared at different THF/W volumetric ratios. When the volume
123
ratio of THF/W is increased, the morphology of Ag3PO4 changes from three-dimensional towers
124
(TDTs) to highly-branched tetrapods (HBTs), threefold-overlapped tetrapods (TOTs), irregular
125
Ag3PO4. At 0:1, Three-dimensional towers (TDT) form, whose bifurcated angle between the shaft
126
and secondary branch is about 109o28' (Figure 1a). A few of fractured HBT are found in TDT
127
sample. At 0.13:1, the formed HBT sample has four stretched shafts, which further grow through
128
the secondary branching growth (Figure 1b). From the SEM images, we can observe that the four
129
shafts of HBT stretch along four [111] directions (16) and they have further branched through a
130
secondary growth process.
131
angles are also 109°28′. Nevertheless, these TOTs are distinct from the previously reported
132
unoverlapped tetrapods (13,15,16). The threefold-overlapped branches are parallel to one another
133
and their twelve arms are 5-8 µm × 500 nm. The twelve branches are about 5-8 µm in length and
134
500 nm in diameter. At too higher THF/W ratios (0.5:1, 1:1 and 1:0), however, the irregular samples
135
are obtained (Figure S1, seeing electronic supporting information (ESI)). It is clear that the THF/W
136
ratio plays a key role in the growth of Ag3PO4.
Figure 1c shows the uniform TOTs formed at 0.25:1, whose bifurcated
137
Figure 2A shows X-ray diffraction (XRD) patterns of the samples prepared at THF/W =0/1,
138
0.13/1 and 0.25/1. All diffraction peaks can be well indexed to the bcc Ag3PO4 (JCPDS No.
139
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
141
(Figure 2B). The peak intensity ratios of (222)/(110) and (222)/(200) are 2.86 and 2.83,
142
respectively; whereas 1.43 and 1.47 for the bulk one (Figure S2, seeing ESI). The results mean that
143
{111} crystal facets may preferentially grow. We have tried to perform the high-resolution
144
transmission electron microscopy to determine its growth direction. However, our attempt is
145
unsuccessful because the Ag3PO4 crystals are too large and unstable under irradiation with
146
high-energy electrons. Nevertheless, several researchers have demonstrated that the branches of
147
Ag3PO4 tetrapods grow preferentially along the [111] direction (13,15,16). Herein, we could only
148
assume that the crystals grow preferentially along the [111] direction. Moreover, Yu et al. have
149
revealed the correlation of the ZnO tetrapods with the growth orientation (20).
150
Effects of pH value and reaction temperature. First, the HMT-dependent experiments have been
151
performed to understand the role of HMT, while keeping the THF/W volumetric ratio at 0.25:1 at
152
30 oC.
153
values. Under strong acidic conditions, we hold that at pH values low than 0.21, phosphorous
154
source mainly exists as the molecular form of H3PO4. Thus, Ag3PO4 can not form without adding
155
HMT. We have found that with increasing the HMT amount, the pH value of the system increases
156
from 0.21 to 4.82 due to the hydrolysis of HMT (Table S1 of ESI). As a result, Ag3PO4 can form
157
only in a certain molar ratio range of HMT/Ag(I) (Figure 4). At 0.5:1 (HMT/Ag(I)), TOTs form, on
158
which a few nanoparticles attaches (Figure 4A). Although TOTs can also form at 0.75:1, almost no
159
any nanoparticles attach on their surfaces (Figure 4B). At 1:1, the pitted tetrahedrons form (Figure
160
4C); and at 1.25:1 and 1.5:1, the formed samples are poly-armed Ag3PO4 (Figure 4D). It is clear
161
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
162
On base of the results above, we hold that it is the pH value that has changed the existing form
163
and distribution of phosphate ions, namely, more PO43- ions exist at high pH values (21). Hence the
164
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
166
sample.
167
Moreover, the temperature-dependent experiments are also performed (Figure 5). The results
168
show that the non-overlapped Ag3PO4 tetrapods (NOT) form at 15 and 20 oC, which have the wide
169
leaves (Figure 5(A,B)). At 30 and 45 oC, the threefold-overlapped Ag3PO4 tetrapods (TOTs) form
170
(Figure 5(C,D)). It seems that TOTs form from the NOTs with wide leaves. It is reasonable that the
171
wide leaves are metastable due to the high surface energy. At high temperatures, the wide leaves
172
may split and grow into the overlapped branches through the dissolution and re-crystallization
173
processes. At low temperatures, the tetrapods with wide leaves form; with increasing the
174
temperature, each wide leaf of tetrapods gradually transform into three independent branches, and
175
finally become overlapped tetrapods.
176
Generally, crystal formation is determined by the thermodynamics and kinetics of growth. Thus
177
various parameters, including intrinsic crystal structure, nature and concentration of precursor,
178
molecule adsorption, temperature, time, and the other moieties present in solution such as counter
179
ions and foreign metal ions, can greatly affect the final crystal (22). Obviously, these factors are
180
closely interdependent and cannot be considered independently. It seems that the crystals form
181
through the complicated process of dissolution – recrystallization – splitting growth. It is reasonable
182
that reaction temperature generally has a significant influence on the kinetics and thermodynamics
183
of nucleation and growth, leading to different morphologies.
184
Formation mechanism of Ag3PO4. On base of the results above, a plausible growth mechanism is
185
proposed as follows. First, THF, as an aprotic, polar solvent, can strongly solvate Ag(I) ions,
186
resulting in a slow diffusion rate of Ag(I) ion. Moreover, a large spatial obstacle will exist by the
187
solvating shells of THFs, which can reduce the collision reaction of both Ag(I) with PO43-. As a
188
result, the secondary branching growth of the shaft is refrained, as demonstrated by the fact that the
189
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
191
fully-grown sub-branches of the four shafts will intersect and squeeze one another and finally the
192
shafts snap off, leading to the formation of TDTs with long sub-branches (Figure 1A).
193
We hold that the adsorption of THF on the surface of Ag3PO4 is another important factor. Since
194
the oxygen atom of THF molecule has two pairs of unshared electrons, THF molecules are easy to
195
adsorb on Ag(I)-enriched facets of Ag3PO4 nanocrystals through the coordination of Ag(I) with the
196
oxygen of THF (5). As a result, the growth rate of Ag(I) ion-enriched facets, i.e. {110}, may be
197
reduced greatly. With further increasing the THF amount, the growth of sub-branches is inhibited
198
significantly. As a result, the uniform TOTs form at 0.25:1 (Figures 1C and 4). At too high THF/W
199
ratios (0.5:1, 1:1, 1:0), AgNO3 can not completely dissolve in the system, leading to the formation
200
of irregular particles (Figure S1, seeing ESI). To conclude, THF plays a vital role in the growth of
201
Ag3PO4, which needs extensive research in future.
202
Degradation performance of Ag3PO4 polypods under visible light irradiation The
203
photocatalytic activities of the typical samples have been investigated using the degradation of
204
rhodamine B (RhB) as the probe reaction (Figure 6). Herein, N-doped TiO2 (NT) is also prepared to
205
compare with Ag3PO4. Because the BET areas of HBTs, TDTs, TOTs, NOTs and NTs are measured
206
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
207
and 9.6 mg, respectively. The aim is to keep their surface areas same according to the reference (19).
208
Moreover, we have measured the absorption spectra of RhB with irradiation time. It is obvious that
209
RhB has been destroyed, characterized by the disappearance of the maximum absorbance of RhB at
210
553 nm (Figure 7). The apparent reaction rate constants are 1.1335, 0.6936, 0.4008, 0.2199 and
211
0.0078 min-1 for HBT, TDT, TOT, NOT and NT, respectively (Figure 6B). The apparent rate
212
constants of HBT, TDT, TOT and NOT are 144.3, 87.9, 50.4 and 27.2 times higher than that of NT,
213
respectively, indicating the high photocatalytic activity of Ag3PO4 (4). For the Ag3PO4 samples,
214
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,
216
respectively. Taking their same surface areas into account, their different activities can be mainly
217
attributed to the active {110} facets exposed. Many researchers (4,5,13,14,16) have reported that the
218
surface energies of {110} and {100} facets are 1.31 and 1.12 J/m2; respectively; and further
219
demonstrated that the high-energy {110} facets have a higher photocatalytic activity than the {100}
220
and {111} facets for the degradation of RhB. On the other hand, the ultraviolet-visible diffuse
221
reflectance spectra (UV-DRS) reveal that three Ag3PO4 samples can absorb the visible light with a
222
wavelength shorter than 510 nm, which have the same absorption edge, indicating their same band
223
gap energy (Figure 8). However, the absorbencies of both polypods are about 2.5 times higher than
224
the bulk, which may favor to improve the photo activity. The high absorbencies of the former two
225
samples can be attributed to their three-dimensional microstructures, which favors for the
226
repeatedly reflections and absorptions of irradiation light (the inset of Figure 8) (17,23).
227
Degradation performance of Ag3PO4 polypods under natural indoor weak light We have
228
investigated the indoor weak light-driven degradation performances of Ag3PO4 polypod structures
229
(Figure 9). It is amazing that under indoor weak light irradiation, 100% RhB has been degraded
230
after 36 h, while only 18% can be degraded by N-doped TiO2 after 120 h (Figure 9a). The apparent
231
reaction kinetic constant (0.08099 h-1) of Ag3PO4 polypods is 46.8 times higher than that (0.00173
232
h-1) of N-doped TiO2 (Figure 9b). Besides, the complete mineralization of RhB dye by Ag3PO4 has
233
been confirmed by using the organic carbon (TOC) test, infrared spectroscopy (IR), UV-Vis
234
absorption spectra and mass spectrum (MS) (Figures S3-S6, seeing ESI). Moreover, the cycle
235
stability of Ag3PO4 polypod structures have been investigated (Figure 10). After three cycles under
236
indoor weak light irradiation, Ag3PO4 polypods nearly maintain activity within 36 h (Figure 10a).
237
Under visible light irradiation, however, the degradation ratios of RhB by Ag3PO4 polypods are
238
63% and 34% for the second and third cycles, respectively (Figure 10b). XRD patterns (Figure 10c)
239
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irradiation; while more Ag3PO4 have decomposed into Ag under visible light. Also we observed
241
with SEM that under visible light irradiation, the polypods have broken into fractures (Figure 10d);
242
while the polypods are still maintained (not showing). Summarily, Ag3PO4 polypods show a higher
243
cycling stability under natural weak light irradiation than under visible light irradiation.
244
It is important that Ag3PO4 photocatalyst is discovered with good photocatalytic activity under
245
indoor natural weak sunlight. Compared with conventional photocatalysis, the degradation reaction
246
is carried out under indoor natural weak sunlight that does not needs extra optical system. It is a
247
green and energy-saving technology to utilize Ag3PO4 as catalyst for the degradation of
248
environmental pollutants driven by indoor natural weak light-driven, not depending on artificial
249
light source. It could be expected that Ag3PO4 polypod structures can be conveniently applied in
250
indoor air cleaning under weak light irradiation.
251
The Ag3PO4 polypod structures are discovered with an efficient degradation activity under
252
indoor weak light without needing an extra, complicate artificial optical condenser system. The
253
efficient Ag3PO4 polypods could be promising to be conveniently applied in indoor air cleaning.
254
255
Acknowledgements
256
This work is financially supported by National Science Foundation of China (21377060,
257
21103049), the Project Funded by the Science and Technology Infrastructure Program of Jiangsu
258
(BM201380277, 2013139), Jiangsu Science Foundation of China (BK2012862), Six Talent Climax
259
Foundation of Jiangsu (20100292), Jiangsu Province of Academic Scientific Research
260
Industrialization Projects (JHB2012-10, JH10-17), the Key Project of Environmental Protection
261
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
263
(2013S002) and “333” Outstanding Youth Scientist Foundation of Jiangsu (2011015).
264
Electronic Supporting Information (ESI)
265
This supporting material is available free of charge via the internet at http://www.acs.org
266
267
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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(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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(11) Dinh, C. T.; Nguyen, T. D.; Kleitz, F.; Do, T.-O. Large-scale synthesis of uniform silver
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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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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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Ag3PO4 as a recyclable highly efficient photocatalyst. Chem. Eur. J. 2012, 18, 5524-5529.
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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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(22) L. M. Liz-Marzan, Increasing complexity while maintaining a high degree of symmetry in nanocrystal growth, Angew. Chem. Int. Ed. 2015, 54, 3860-3861 (23) Li, G. ; Zhang, D. ; Yu, J. A new visible-light photocatalyst: CdS quantum dots embedded mesoporous TiO2. Environ. Sci. Technol. 2009, 43, 7079-7085.
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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)
16
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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
18
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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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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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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
27
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