ABSTRACT:

Photocatalysis, a promising technology platform to address the environmental  problems, has been attracted considerable attentions. In this paper, Ag/AgxH3-xPMo12O40 (simplified as Ag/AgHPMo12) nanowires have been synthesized by a facile solid reaction route  and in-situ photo-deposited method. The results of SEM and TEM indicate that the diameters of  AgHPMo12 nanowires are about 45±10 nm. And Ag nanoparticles (NPs) with diameters in the  range of 5-15 nm are uniformly anchored on the surface of AgHPMo12 nanowires. The Ag  content in the Ag/AgHPMo12 composite was manipulated by the light irradiation time  (Ag/AgHPMo12-x; x stands for the irradiation time; x = 2, 4, 6, 8 h, respectively). With  increasing irradiation time, the light absorption of as-synthesized samples in the visible region  was gradually enhanced. The Ag/AgHPMo12-4 exhibits the best photocatalytic performance for the degradation of methyl orange and reduction of Cr2O7 2- under visible light (λ > 420 nm) irradiation. The study of photocatalytic mechanism reveals that both Ag and AgHPMo12 can be  excited by visible light. The photoinduced electrons were transferred from AgHPMo12 to  metallic Ag, and combined with the Ag plasmonic holes. The Ag plasmonic electrons were  trapped by O2 to form •O2- , or directly reduced Cr2O7 2- to Cr3+. Meanwhile, the •O2 - species and  the photogenerated holes of AgHPMo12 were used to oxidize MO or i-PrOH, thus they showed  highly efficient and recyclable photocatalytic performance for removing the organic and  inorganic pollutants.  

1. INTRODUCTION

Nowadays, the environmental problems have become one of the top and challenging issues for  humanity. Among various kinds of technologies for environmental remediation, semiconductor  photocatalysis has been proved to be one of the most promising technologies because it provides  a facile way to utilize the inexhaustible light energy for the highly efficient and green removal of  environmental pollutants.1-6 In recent years, productive efforts have been made to synthesize Agbased photocatalysts because of their considerably good photocatalytic response, such as  Ag/AgX (X=Cl, Br, I),7-10 Ag3PO4, 11 AgVO3, 12 Ag/(BiO)2CO3, 13 Ag/SnNb2O6, 14 Ag/TiO2, 15-18 Au/CeO2  19,20 and so on. However, most of these Ag-based photocatalysts have a relatively low  capability to separate photogenerated charge carriers. Moreover, the anions of these Ag-based  composites have neither light response nor redox. Therefore, these Ag-based photocatalysts are  light instability, which might be gradually depleted under visible light (λ > 420 nm) irradiation,  thus resulting in the deactivation of catalysts in the photocatalysis.  

Polyoxometalates (POMs) are a fascinating class of typical transition metal-oxygen clusters,  which have plenty of compositions, structures, and functionalities.21,22 They possess reversible redox properties, which can undergo a stepwise multi-electron redox without any significant  structural alteration.23,24 They can serve as photocatalysts, and possess many advantages such as  optical and chemical stability, adjustable oxidizability and photochemical activity.25,26 Therefore,  they have been successfully utilized in photocatalysis, including water splitting and degradation  of pollutants.22, 26-28 However, most of POMs have good water solubility, which are usually used  as the homogenous catalysts, thus resulting in difficult separation of catalysts. The photocatalytic  performance of POMs is originated from the photoexcitation of the oxygen-to-metal charge  transfer of POMs to photogenerate the charge carriers (e- -h+ ). The fast recombination of  photogenerated e- -h+  pairs in POMs causes the low photocatalytic efficiency. Moreover, most  previous works related to POMs-based photocatalyst are limited to UV light (λ < 380 nm), which  is only about 4% of the solar spectra.29-32 After thoroughly literature survey, we noticed that  reports on the high efficacy of POMs-based photocatalysts under visible-light irradiation by  heterogeneous catalytic reaction are rare.  

Therefore, to design a suitable POM-based photocatalyst with enhanced visible-light driven  photocatalytic activity is still a challenging task. One effective way to construct heterogeneous  POMs-based catalyst is to couple POMs with the large counter cations. Ag+  with the ionic radius  about 0.115 nm has been proved as one of the large cations that can be used to regulate the  solubility of POMs. A few Ag+ -POMs compounds have been prepared and exhibit good photocatalytic performance.33-37 More importantly, these compounds are usually composed of  wide band gap polyoxoanions, such as [PW12O40] 3- and [SiW12O40] 4-, possessing irregular  morphology. Its formation mechanism, the photocatalytic process and mechanism are still not  exactly known. Moreover, the enhancement of photocatalytic performance and the further  control of such kind of catalysts are scarcely investigated.34  

In this paper, we successfully fabricated Ag/AgxH3-xPMo12O40 (simplified as Ag/AgHPMo12)  nanowires composites through a facile solid reaction route and in-situ photoreduction method.  H3PMo12O40 with the band gap of 2.4 eV, which possesses obvious visible light absorption in the  range of 400-516 nm, were selected to react with AgNO3 by ball-milling to form insoluble  AgPMo12 nanowires. The diameters of AgPMo12 nanowires are about 45±10 nm. After  irradiation under UV-Vis light, part of Ag+  in AgPMo12 nanowires were in-situ photoreduced to  Ag NPs forming Ag/AgHPMo12 composites. Ag NPs exhibit surface plasmon resonance (SPR)  absorption, thus enhancing the visible light absorption of Ag/AgHPMo12. And in addition, it also  causes intense local electromagnetic fields by SPR, which accelerates the charge separation of  photogenerated e-  and h+ in Ag/AgHPMo12. Moreover, the relative narrow band gap and strong reversible redox properties of polyoxoanion [PMo12O40] 3- ensure the photochemical stability of  the composites. Therefore, the as-synthesized Ag/AgHPMo12 exhibits highly efficient and  recyclable photocatalytic performance for the degradation of MO and reduction of Cr2O7 2- under  visible light (λ > 420 nm) irradiation.  

22. EXPERIMENTAL SECTIONS

2.1. Chemicals and Reagents. 

Phosphomolybdic acid (H3PMo12O40), Silver nitrate (AgNO3),  Sodium sulfate (Na2SO4), ethanol (CH3CH2OH), Isopropanol (C3H7OH), 4-Hydroxy-TEMPO,  Triethanolamine (C6H15NO3), Methyl orange (MO), K4[Fe(CN)6], KCl, Barium sulfate (BaSO4),  K3[Fe(CN)6], K2Cr2O7 were purchased from Aladdin Chemical Co., Ltd., China. All chemicals  were used without any further purification.  

2.2. The Preparation of Ag/AgHPMo12 Nanowires. 

The phosphomolybdic acid was dried at  150oC for 12h to remove the crystallization water. 5g phosphomolybdic acid and 5g silver nitrate were ball-milled at the rate of 400 rpm for 5 h on the QM-QX04 planetary ball mill  (Nanjing NanDa Instrument Plant). The obtained samples were washed successively by  deionized water for three times and then dried in air. Afterwards, the dry samples were  illuminated under 300 W (CEL-HXF300, AULIGHT) Xe lamp for different time to form  Ag/AgHPMo12-x composite (x stands for the irradiation time; x = 2, 4, 6, 8 h, respectively). The  obtained samples were again washed with deionized water and dried in air. By this method,  Ag/AgHPMo12 nanowires with different contents of Ag have been fabricated. For comparison, a  series of samples with different stoichiometric ratio of Ag substituted H3PMo12O40 have been  prepared according to reference38 by traditional liquid reaction (abbreviated as Ag3PMo12(L),  Ag2HPMo12(L) and AgH2PMo12(L), respectively) .

2.3. Characterization Methods. 

The surface morphology of the photocatalysts has been  characterized using a JEOL JSM 4800F SEM. Transmission electron microscopy (TEM) and  HRTEM images were performed on a JEM-2100F microscope at an acceleration voltage of 200  kV. X-Ray diffraction data were collected on a Bruker AXS D8 Focus by filtered Cu Ka  radiation (λ= 1.54056 Å). X-ray photoelectron spectra were performed using an ESCALABMKII  spectrometer with an Al-Kα achromatic X-ray source. The UV-Vis diffuse reflectance spectra  (DRS) measurements were carried out on a UV-2600 UV-Vis spectrophotometer (Shimadzu),  and BaSO4 was employed as a reference. The PL spectra were measured on a Hitachi F-7000  spectrophotometer with the excitation wavelength of 400 nm.

22.4. Photoelectrochemical Measurements.

Photocurrent measurements were conducted on a  CHI660E Electrochemical Workstation in a conventional three-electrode configuration including  a counter electrode, reference electrode and working electrode in a quartz cell. The working  electrode prepared with the sample has an active area of ca. 3cm2 . A Pt foil and Hg/Hg2Cl2 electrode were used as the counter electrode and reference electrode, respectively. A 300 W  (CEL-HXF300, AULIGHT) Xe lamp was applied as the light source. The electrolyte was 0.5 M  Na2SO4 aqueous solution. Typically, the working electrodes were prepared as follows: 40 mg of  as-prepared samples were suspended in 5ml ethanol with sonication for 30 minutes to obtain  slurry. Next, 1 mL solution was uniformly dropped onto a 1×5 cm2  FTO glass substrate. At last,  the prepared electrodes were dried at room temperature to obtain the working electrodes.  

2.5. Electrochemical Impedance Spectroscopy (EIS) Measurements. 

EIS was performed  using a Model CS350 (Wuhan CorrTest Instrument Corporation) electrochemistry station in 0.1  M KCl solution containing 5 mM Fe(CN)6 3−/ 4− with a frequency range from 0.01 Hz to 10 kHz at  0.2 V. The EIS data were recorded using a conventional three-electrode system, where samples  on FTO glass with an active area of ca. 1.0 cm2 were prepared as the working electrode, Pt wire  as a counter electrode, and Ag/AgCl as a reference electrode, respectively.

2.6. Photocatalytic Tests.

The photocatalytic activities of the as-obtained products were  evaluated by the photodegradation of MO and photoreduction of K2Cr2O7. A glass vessel with a  water-cooling jacket was applied as reactor and a 300 W Xe lamp with a 420 nm cut-off filter  was employed as illuminant, respectively. The distance between the lamp and the mixture  solution was about 12 cm. In a typical process, 20 mg of samples was dispersed into a solution  containing 20 mL of MO solution (20 mg·L-1; pH=1) or 40 mL of K2Cr2O7 (80 mg·L-1; VH2O:  Visopropanol =1). Before irradiation, the mixture solution was stirred in darkness for a period of  time to attain absorption-desorption equilibrium. Afterwards, the above suspension was  continually stirred and exposed to the visible-light irradiation. And then a certain amount of  suspension was taken out and centrifuged to separate solid particles at given time intervals. The  remaining concentration of MO and K2Cr2O7 were determined by a Shimadzu UV-2600 UV-Vis spectrophotometer. For comparison, the photodegradation reactions were also evaluated in the  absence of any catalyst. The degradation efficiency was calculated by C/C0, where C is the  concentration of remaining MO or K2Cr2O7 solution at each irradiated time, and C0 is the initial  concentration.  

22.7. Active Species Trapping Experiment.

To explore the major active species in the  photocatalytic degradation process of MO, we conducted the radical-trapping experiments. The  triethanolamine (TEOA), 4-Hydroxy-TEMPO and isopropanol (IPA) were employed as hole (h+ )  scavenger, superoxide radical (·O2- ) scavenger and hydroxyl radical (·OH) scavenger,  respectively. Typically, 20 mg of photocatalyst together with different scavenger was dispersed  in 20 mL (20 ppm) MO aqueous solution, and the following process was similar to the MO  degradation test.

3. RESULTS AND DISCUSSION

3.1. Compositional and Structural Characterization.

The surface morphology of the  AgPMo12 and Ag/AgHPMo12 samples were visualized by SEM and TEM images. As Figure 1a  shows, the AgPMo12 samples are comprised of nanowires with uniform shape and size. The  average diameter of the nanowires is about 45 ± 10 nm, and its length is as long as several  micrometers in size. After irradiation under UV-Vis light, Ag/AgHPMo12 has been formed. As  shown in Figure 1b and 1c, it could be seen that many small Ag nanoparticles are in-situ deposited, which are uniformly anchored on the surface of AgHPMo12 nanowires. The size of Ag  NPs ranges from 5 to 15 nm. And the morphology of AgHPMo12 nanowires is well maintained  after irradiation. In Figure 1d, the HRTEM image reveals that two independent crystal lattices,  Ag and AgHPMo12 are co-existed. The lattice fringe corresponding to the interplanar distance of 0.243 nm is assigned to the lattice spacing of Ag (101) plane (JCPDS NO. 41-1402), and the  other lattice diffraction fringe with the lattice plane distance 0.292 nm can be ascribed to  AgHPMo12, which confirm that Ag nanoparticles have been successfully deposited on the  surface of AgHPMo12. The corresponding elemental mapping of Ag/AgHPMo12 is illustrated in  Figure 1e-1h, which demonstrates that the Ag, P, Mo elements are well-arranged over the  Ag/AgHPMo12 nanowires (Figure S1).  

Figure 1. (a) Scanning electron microscopy (SEM) image of AgPMo12 nanowires; (b) SEM, (c)  TEM (Insert: the particle size distribution of Ag NPs.) and (d) HRTEM photographs of  Ag/AgHPMo12-4; (e) TEM-EDS elemental mapping of Ag/AgHPMo12-4, the corresponding  elemental mappings of (f) P, (g) Mo, and (h) Ag elements.  

Figure 2 a displays the XRD patterns of the as-synthesized AgPMo12 and Ag/AgHPMo12 samples. The XRD pattern of AgPMo12 (b) is different from that of pure HPMo12 (a), because H+ cations in H3PMo12O40 have been completely replaced by Ag+  cations forming Ag3PMo12O40  (abbreviated as AgPMo12) compared to XRD patterns of AgxH3-xPMo12 (x=1; 2; 3) samples  prepared by liquid reaction (Figure S2). The peaks of 17.80o , 27.10o , 28.51o , 30.56o , 32.50o ,  35.12o , 47.91o , 56.32o  (marked with *) can be assigned to the diffraction peaks of AgPMo12.  After Ag was in-situ photoreduced, part of Ag+  in or adsorbed on AgPMo12 nanowires could be  deposited. And in view of the charge balance, the obtained samples can be expressed as  Ag/AgxH3-xPMo12O40 (abbreviated as Ag/AgHPMo12). The XRD patterns of Ag/AgHPMo12 show slight changes compared with that of AgPMo12 (Figure S3). Especially, in Ag/AgHPMo12- 8, a series of characteristic diffraction peaks of metal silver have been observed (Figure S4). The  peaks at 37.0o , 45.3o , 64.5o , 76.8o，82.3o  ( marked with #), can be assigned to the diffraction  planes of Ag (JCPDS NO. 41-1402) (101), (103), (110), (201) and (203), respectively.

The optical properties of the as-prepared samples were investigated by UV-Vis diffuse  reflectance spectra in the range of 200-800 nm. As shown in Figure 2b, HPMo12 exhibits a  spectrum absorption onset at 516 nm, which is consistent with the band gap of HPMo12 (2.4 eV).  As H+  cations were exchanged with Ag+ , the absorption band (440 nm) of AgPMo12 nanowires  shows an obvious blue shift in comparison with that of HPMo12, which indicates the counter  cations of POMs have a significant influence on the band gap of POMs-based photocatalysts.  After Ag was photoreduced, the absorption of Ag/AgHPMo12 composites in the visible light  region has been greatly enhanced. A wide absorption peak at about 480 nm has been clearly  observed, which can be attributed to the surface plasmon resonance (SPR) absorption of Ag  NPs.39-41 With the increase of irradiation time, the absorbance of composites in visible light  region was also gradually enhanced. It means the amount of Ag NPs gradually increased. As  shown in Figure 2d and 2e, the colour of samples changed from Kelly green (HPMo12) via palegreen (AgPMo12) to gray (Ag/AgHPMo12). The band gap energies of AgPMo12 and  Ag/AgHPMo12 could be estimated from the Tauc plot42 (Figure S5). It shows that the energy gap  for AgPMo12 and Ag/AgHPMo12-x (x=2, 4, 6, 8) is 2.87, 2.82, 2.75, 2.73, 2.72 eV, respectively.  The band gap of Ag/AgHPMo12 shows a slight shift compared to AgPMo12. That might be  caused by the partial reduction of Ag+  ions, thus leading to the substitution of Ag+  ions with H+ ions in AgHPMo12 nanowires. And according to previous studies, 43, 44 it also might be related to the Ag metallic clusters that introduce localized energy levels into the AgHPMo12 band gap, thus  reducing the energy gap of AgHPMo12 nanowires in Ag/AgHPMo12.