Journal of Hazardous Materials 293 (2015) 72–80
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Co(II)–grafted Ag3 PO4 photocatalysts with unexpected photocatalytic ability: Enhanced photogenerated charge separation efficiency, photocatalytic mechanism and activity Shuna Zhang a , Shujuan Zhang b,∗ , Limin Song c,∗ a
College of Textile Engineering, Zhejiang Industry Polytechnic College, Shaoxing 312000, PR China College of Science, Tianjin University of Science & Technology, Tianjin 300457, PR China c College of Environment and Chemical Engineering & State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, PR China b
h i g h l i g h t s • Co–Ag3 PO4 with higher photodegradation ability was synthesized. • • OH was the main active species in the oxidation of MO. • The synergy of Co(II) and Ag3 PO4 greatly enhanced the separation efficiency.
a r t i c l e
i n f o
Article history: Received 3 December 2014 Received in revised form 20 March 2015 Accepted 22 March 2015 Available online 24 March 2015 Keywords: Co(II)–grafted Ag3 PO4 Photodegradation Methyl orange Photogenerated charges separation and transmission efficiency Enhanced mechanism
a b s t r a c t Since the photocatalytic capability is determined by the separation and transmission efficiency of photoinduced charges, its improvement remains a challenge for development of efficient photocatalysts. Here, we made large improvement on the surface of Ag3 PO4 using Co(II)–grafted Ag3 PO4 by a hydrothermal method. During the photocatalytic process, Co(II) was oxidized to Co(III) by the photogenerated holes under visible light radiation, which enhanced the separation efficiency of photogenerated charges. Meanwhile, the Co(III) as-formed could oxidize dye molecules, which recovered the Co(II). The synergy of Co(II) and Ag3 PO4 greatly promoted the separation and transmission efficiency of the photogenerated charges, and severely improved the photocatalytic activity of Ag3 PO4 . The surface grafted Co(II) on Ag3 PO4 is responsible for the enhancement of photocatalytic activity. © 2015 Published by Elsevier B.V.
1. Introduction As environmental pollution is getting worse with the rapid development of industrial production, photocatalytic technology has become an increasingly popular method for effective treatment of wastewater. Finding suitable photocatalytic materials is one effective way to improve the photocatalytic efficiency. Since the high photocatalytic capability of Ag3 PO4 was reported in 2010 [1], Ag3 PO4 is gradually known as a novel photocatalytic material [2–4]. Ag3 PO4 can absorb sunlight at less than 520 nm [1], and its quantum yield under visible light radiation is up to 90% [1]. Ag3 PO4 under visible light radiation shows strong
∗ Corresponding authors. Tel.: +86 22 83955458; fax: +86 22 83955458. E-mail addresses: [email protected] (S. Zhang), [email protected] (L. Song). http://dx.doi.org/10.1016/j.jhazmat.2015.03.047 0304-3894/© 2015 Published by Elsevier B.V.
oxidation ability [5–7], and its ability to oxidize organic dye is much higher than other materials such as TiO2 [1]. Therefore, Ag3 PO4 exhibits a great potential as photocatalytic materials. As reported, the strong binding ability of the P O bonds of PO4 tetrahedrons in Ag3 PO4 significantly weakens the strength of the Ag O covalent bonds and prevents the hybrid of the Ag4d and O2p orbits, which leads to the deviation of the 4d orbit from the conduction band bottom [1,3]. Thus, the interaction between the delocalized charges on the conduction band bottom and the electrons is very weak, which helps the charges to transport the particle surface, and improves the redox capacity [1,3]. Zhu Yongfa’s and co-workers used the first-principle calculation to demonstrate the reasons for the superior oxidative capacity and the quantum efficiency of Ag3 PO4 . One reason is the relatively low valence band level of Ag3 PO4 , which improves its oxidizing ability [2]. The other reason is that the * orbit in the conduction band in Ag3 PO4 improves the electron migration rate compared with the holes. This improvement
S. Zhang et al. / Journal of Hazardous Materials 293 (2015) 72–80
together with the static induction effect on PO4 3− help to more effectively separate the photogenerated electrons and holes, and therefore, can significantly enhance the photocatalytic ability [2]. As is known, the introduction of additional ions can improve the photocatalytic oxidation ability of photocatalytic materials [8–10]. The doping of ions is able to change the electronic structure and band structure of photocatalytic materials, which can enhance their photocatalytic ability [11–14]. In addition to the doping ions, the ions grafted on the surface of the photocatalyst can also improve the photocatalytic ability of the materials [15,16]. So far, Cu(II)–grafted Ti1−3x Wx Ga2x O2 [17], Fe(III)–grafted TiO2 [18], Cu(II)–grafted TiO2 [19], Fe(III)–grafted AgBr [20], and Cu(II)–grafted Mo-doped SrTiO3 [21] have been reported. Photogenerated electrons in these iongrafted photocatalysts under visible light radiation can efficiently transfer to the surface grafting ions as co-catalysts. The electrons in the surface grafting ions can efficiently reduce the multielectron of the adsorbed O2 . Thus, the ion-grafted photocalysts exhibit the highest photocatalytic oxidation ability. In the present paper, the photocatalytic ability of Ag3 PO4 was enhanced through grafting with Co(II). Ag3 PO4 grafted by ions has not been reported. The grafted Co(II) obviously enhanced the separation and transmission efficiency of photogenerated charges on the surface of Ag3 PO4 , which severely improved the photocatalytic performance. It is noteworthy that the grafted ions in our study are cobalt ions with low valence, which is different from the grafted high valence ions previously reported [18]. The oxidation mechanism of organic dyes is also largely different. The enhanced separation and transmission efficiency of photogenerated charges, photocatalytic mechanism, and activity will be discussed later.
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solution was measured by a Shimadzu-Toc-Vcph instrument. Electron spin resonance (ESR) spectra were recorded by an electron paramagnetic resonance spectrometer (JES FA200). 2.3. Activity measurement In a typical experiment, 20 mg/L methyl orange (MO) and a 1.0 g/L catalyst were used. About 100 mL of the MO and catalyst suspension was circulated in a home-made photoreactor, and then photocatalytic oxidation proceeded with continuous stirring under a 300 W Xe lamb with a filter to remove UV components ( > 420 nm). Before the photocatalytic reaction, the mixture was stirred in dark for 30 min until reaching the adsorption equilibrium. An O2 flow was provided bubbling additional air, and 3 mL of the residual solution was extracted every 2 min. The residual concentration was measured by the UV–vis spectrophotometer. 2.4. Photoelectric measurement The photoelectric properties were studied in a three-electrode cell using a CHI 650D electrochemical workstation (reference elec-
Ag3PO4
2. Experimental 2.1. Synthesis of samples
1-Co-Ag3PO4
Intensity (a.u.)
Ag3 PO4 was synthesized as follows: First, 0.41 g of (NH4 )3 PO4 and 1.02 g of AgNO3 were dissolved in 20 mL of distilled water separately. Then, the (NH4 )3 PO4 solution was dripped into the AgNO3 solution under constant stirring. The mixture as-obtained was washed with distilled water, and dried at 60 ◦ C in a vacuum oven for 6 h. Co(II)–grafted Ag3 PO4 (marked as Co–Ag3 PO4 ) was synthesized as follows: typically, different amounts of Co(NO3 )2 ·6H2 O were dissolved separately in 60 mL of distilled water to the final concentrations of 0.0005, 0.001, 0.002, 0.003, 0.004, and 0.005 mol/L Co(II). The obtained Co–Ag3 PO4 samples were named as 0.5-, 1, 2-, 3-, 4-, and 5-Co–Ag3 PO4 , respectively. About 0.5 g of Ag3 PO4 was dispersed into each of the above solutions under stirring for 30 min. Then the suspensions were transferred into a 100 mL stainless teflonlined autoclave and heated at 80 ◦ C for 3 h. The resulting precipitates were collected, centrifuged, washed with deionized water three times, and dried at 60 ◦ C in a vacuum oven for 6 h.
05-Co-Ag3PO4
2-Co-Ag3PO4
3-Co-Ag3PO4
2.2. Characterization of samples X-ray diffraction (XRD) patterns were recorded by a diffractometer (Rigaku D/max 2500, CuK␣, = 1.5406 Å, 40 kV, 40 mA). The particle morphology and size of the photocatalysts were analyzed using a transmission electron microscope (TEM, Hitachi H-7650, 80 kV). A UV–vis spectrophotometer equipped with an integrating sphere assembly was used in this study (HP8453). The binding energy (BE) was measured by an X-ray photoelectron spectrometer (XPS, PerkinElmer PHI5300). The standard peak of adventitious carbon (C1s ) was used to calibrate BE. The Brunauer–Emmett–Teller (BET) surface area of the samples was measured by a Micromeritics ASAP2020 automatic surface area analyzer. The total organic carbon content (TOC) in the residual
4-Co-Ag3PO4
5-Co-Ag3PO4
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20
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2Theta/degree Fig. 1. X-ray diffraction patterns of pure and Co–Ag3 PO4 .
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trode electrode: saturated calomel electrode; counter-electrode: Pt wire; electrolyte: 0.1 M Na2 SO4 aqueous solution). Specifically, 50 mg of a catalyst was dispersed in 2 mL of ethanol under 30 min of ultrasonic scattering. Then, 0.5 mL of the suspension was dripped onto a 1 × 4 cm fluorine-doped tin oxide (FTO) glass. After evaporation in the air, the FTO glass was calcined at 150 ◦ C for 1 h. A 300 W Xe lamp with > 420 nm was used as the light source. The applied voltage was 0.4 V in the photoelectric experiments.
3. Results and discussion 3.1. Characterization of photocatalysts Fig. 1 shows the XRD patterns of pure Ag3 PO4 and Co–Ag3 PO4 . The pure Ag3 PO4 exhibited an cubic structure (JCPDS 06-0505, ¯
space group: P43n[218], a = b = c = 6.013 Å). Both the position and shape of XRD peaks of each Co–Ag3 PO4 sample are similar to those of pure Ag3 PO4 (Fig. 1), indicating that the cubic structure of Ag3 PO4 was reserved after the grafting with Co(II). Therefore, the grafted Co(II) did not enter the crystal lattice of Ag3 PO4 , or form new cobalt compounds. The low-resolution TEM images of pure Ag3 PO4 and 1Co–Ag3 PO4 (Fig. 2) reveal that the particles are globularly-shaped with mean size of 436and 427 nm, respectively. These results indicate that the shape and size of Ag3 PO4 did not change much after grafting with Co(II). We studied the electronic band structures of pure Ag3 PO4 and Co–Ag3 PO4 by analysis of their optical absorption spectra. Both the position and shape of absorption peaks of Co–Ag3 PO4 samples are similar to those of pure Ag3 PO4 (Fig. 3A). No other peaks were found except those of Ag3 PO4 , which indicated that the grafted Co(II) species did not form any absorption from band to band. However, the UV–vis absorption spectra in Fig. 3A show that the intrinsic absorption edge in Co–Ag3 PO4 obviously red-shifts relative to pure Ag3 PO4 . The derived bandgap from the plots of “n (KM × energy, n = 0.5 for the indirect bandgap) vs. the energy of the adsorbed light” is 2.30 eV for pure Ag3 PO4 (Fig. 3B). The derived
2.0
A 5-Co-Ag3PO4 Absorbtion (a.u.)
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0.00 1.5
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hv (eV) Fig. 2. TEM images of (a) Ag3 PO4 and (b) 1-Co–Ag3 PO4 .
Fig. 3. UV–vis absorption spectra of Ag3 PO4 and Co–Ag3 PO4 .
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S. Zhang et al. / Journal of Hazardous Materials 293 (2015) 72–80 180000
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a hydrothermal method 770
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Fig. 4. XPS spectra of Ag3 PO4 and 1-Co–Ag3 PO4 . (A) Valence, (B) P2P , (C) Ag3d , (D) Co2p .
bandgaps are 2.18, 2.21, 2.25, 2.26, and 2.26 eV for 0.5-, 1-, 3-, 4-, and 5-Co–Ag3 PO4 , respectively (Fig. 3B). The bandgaps of Co–Ag3 PO4 slightly decrease relative to pure Ag3 PO4 . Regarding the reason for the smaller bandgaps, the valance values of pure Ag3 PO4 and 1Co–Ag3 PO4 were confirmed by XPS, which are similarly 2.30 eV (Fig. 4A). Therefore, the lower conduction band bottom may be responsible for the smaller bandgap because additional cations in semiconductors only changed the conduction band position, but not the position of the valence band. The XPS spectra of the P2p , Ag3d , and Co2p regions for pure Ag3 PO4 and 1-Co–Ag3 PO4 are shown in Fig. 4. As reported, the P2p and Ag3d binding energies are 133.5, 368.8 (3d5/2) and 374.8 (3d3/2) eV for Ag3 PO4 [22], which are consistent with the results in the present study (Fig. 4B and C). For 1-Co–Ag3 PO4 , the P2p and Ag3d binding energies are 132.8, 367.9 (3d5/2), and 373.9 (3d3/2) eV, respectively. The low binding energies originated from the strong interaction between the grafted Co(II) and Ag3 PO4 . To study the Co(II) species, the Co2p XPS spectra of 1-Co–Ag3 PO4 obtained by a hydrothermal and mixed route were recorded (Fig. 4D). The Co2p peaks at 781.9 and 797.5 eV in Co–Ag3 PO4 obtained by a mixed route are assigned to the Co(II) in Co(NO3 )2 ·6H2 O, and are larger than the Co2p binding energies of Co–Ag3 PO4 obtained by a hydrothermal method (781.6 and 796.2 eV, respectively). In contrast to the Co(II) from Co(NO3 )2 ·6H2 O, the strong interaction between Co(II) and PO4 3− results in a slight transfer of electron density of Co(II). 3.2. Photocatalytic activity of photocatalysts The photocatalytic activities of TiO2 (commercial P25), pure Ag3 PO4 and Co–Ag3 PO4 with different amounts of Co(II) were compared and the results are presented in Fig. 5. The
concentrations of the MO and the catalyst solution are 20 mg/L and 1.0 g/L, respectively. The mixed MO and catalyst solution was stirred for 30 min until reaching the adsorption equilibrium between the MO molecules and the catalyst in dark. In Fig. 5A, the concentrations of the MO did not change largely over P25 under the same experimental conditions, while the as-prepared samples showed higher photocatalytic efficiency. The adsorption ratios after 30 min are 3.3%, 7.8%, 6.9%, 2.8%, 7.5%, and 2% for 0-, 0.5-, 1-, 3-, 4-, and 5-Co–Ag3 PO4 , respectively (Fig. 5A). The adsorption ratio at less than 10% cannot significantly improve the photocatalytic efficiency. During the decomposition of MO, the conversion ratio of pure Ag3 PO4 after 6 min was 24.8%, while the results of the Co–Ag3 PO4 catalysts (0.0005, 0.001, 0.003, 0.004, and 0.005 mol/L) are 72.7%, 89.2%, 84.3%, 89.7%, and 80.9%, respectively (Fig. 5A), which indicates the Co–Ag3 PO4 catalysts are more efficient. The photocatalytic abilities of all the Co–Ag3 PO4 samples are obviously higher than that of pure Ag3 PO4 . The rate constants derived from the plots of “ln C/C0 (C and C0 : original and residual MO concentrations, respectively) vs. photocatalytic reaction time” are 0.059, 0.203, 0.381, 0.304, 0.366, and 0.268 min−1 for pure Ag3 PO4 and Co–Ag3 PO4 (0.0005, 0.001, 0.003, 0.004, and 0.005 mol/L Co(II)) in Fig. 5B. The rate constant of 1-Co–Ag3 PO4 is 6.5 times larger than that of Ag3 PO4 , suggesting that the grafted Co(II) in Ag3 PO4 can significantly improve the photocatalytic efficiency of Ag3 PO4 . To study the effects of high concentration MO on Co–Ag3 PO4 , the 1-Co–Ag3 PO4 was used to photodegrade 30 and 40 mg/L MO solutions separately (Fig. 5C). The photocatalytic activities are 88% and 74% (30 and 40 mg/L MO solutions, respectively) after 6 min for 1-Co–Ag3 PO4 , while the conversion ratios are similar after 10 min (95.6% vs. 94.5%) (Fig. 5C). Their rate constants are 0.307 and 0.284 min−1 , respectively (Fig. 5D), which do not decrease obviously relative
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C
A In dark
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Recycling time Fig. 5. (A) and (B) The photocatalytic activity over P25, Ag3 PO4 and Co–Ag3 PO4 . (C) and (D) The relationship between ln (C/C0 ) and irradiation time of Ag3 PO4 and Co–Ag3 PO4 . (E) Recycling tests of degradation of 20 mg/L MO over 1-Co–Ag3 PO4 . The concentration of catalysts is 1.0 g/L.
to the 10 mg/L MO solution, suggesting that the 1-Co–Ag3 PO4 asprepared can treat the high concentration MO solutions under visible light radiation. The recycle runs indicate high stability of 1-Co–Ag3 PO4 (Fig. 5E). After 5 consecutive tests, the photodegradation percentages of 20 mg/L MO were still similar to that of the first run for 1-Co–Ag3 PO4 . These results indicate that the 1-Co–Ag3 PO4 samples can photodegrade MO stably.
3.3. Photocatalytic process and mechanism To study the process of MO photodegradation over the as-synthesized 1-CoAg3 PO4 , we monitored the change of MO concentration by recording the characteristic UV–vis absorption spectra of MO at 465 nm. The strong absorption peak at 465 nm is assigned to the n → * electronic transition from the hydrazone
S. Zhang et al. / Journal of Hazardous Materials 293 (2015) 72–80 12000
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Fig. 6. (A) The UV–vis absorption spectra of solution in the photocatalytic degradation of 20 mg/L MO on 1.0 g/L 1-Co–Ag3 PO4 . (B) TOC evolution vs. irradiation time in the photocatalytic reaction.
structure of MO. Fig. 6A shows the evolution of absorption during the photocatalytic degradation. The absorption peak at 465 nm was rapidly weakened with the prolonged irradiation time, which indicates the occurrence of MO photodegradation. After 8 min, the peak at 465 nm almost disappeared, suggesting that the MO molecular structure was destroyed. To clarify the final products after decomposition, we measured the total organic carbon (TOC) in the residual solution after the photodegradation of 20 mg/L MO. Fig. 6B shows that the TOC of residual solution decreased with time, which indicates that many MO molecules were destroyed to form CO2 , H2 O, and other small molecules. However, the TOC conversion ratios are not consistent with the photodegradation ratios of MO, indicating that a part of MO molecules were decomposed to CO2 and H2 O, and the other part of MO molecules only transferred to intermediate products. As is well-known, the main species during photocatalytic oxidation of organic dyes under visible light radiation are • OH, • O2 − , and • OOH. To investigate the oxidation species over 1-Co–Ag3 PO4 , several radicals in the photocatalytic process were monitored by ESR, and the results are shown in Fig. 7A. Clearly, only • OH was observed during the photocatalytic process, while O2 − and • OOH were not found, suggesting that OH was the main oxidation species over 1-Co–Ag3 PO4 under visible light radiation. In addition, the photocatalytic activity of 1-Co–Ag3 PO4 was weakened significantly
Fig. 7. DMPO spin-trapping ESR spectra for DMPO-OH in the presence of Ag3 PO4 and 1-Co–Ag3 PO4 under visible light irradiation ( > 420 nm) and in dark.
if the photocatalytic reaction proceeded without air bubble. Therefore, O2 is also involved in the photocatalytic reaction. The possible photocatalytic mechanism is as follows: Co–Ag3 PO4 + h → h+ + e− O2 + e− → O2 − → OOH → H2 O2 → OH OH + MO → products During the process, the generated O2 − and OOH are immediately converted to OH and could not be detected, and thus, a large amount of OH was found. Compared with the typical signals of DMPO-OH in pure Ag3 PO4 and 1-Co–Ag3 PO4 , the signals of DMPO-OH in pure Ag3 PO4 obviously attenuate with time (Fig. 7B), while those from 1-Co–Ag3 PO4 do not change after 16 min (Fig. 7A). The above results exhibit that the Co(II)–grafted Ag3 PO4 maintains a better and stable lifetime than that of pure Ag3 PO4 . In addition, the amplitude of the DMPO-OH signals for 1-Co–Ag3 PO4 is up to 8.5 times higher than that of Ag3 PO4 (Fig. 7), indicating that the grafted Co(II) can largely improve the amount of OH and the photocatalytic ability. At the beginning of the ESR measurement, abundant DMPOOH signals for 1-Co–Ag3 PO4 were detected, and did not decay with time. However, we did not find the DMPO-O2 − signal of 1Co–Ag3 PO4 throughout the reaction. • OH was generated by two paths under visible light radiation. In one way, OH− reacted with the
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100
MO concentration removal (%)
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Ag3PO4 1-Co-Ag3PO4 2+
mixture of Ag3PO4 and 0.001 mol/L Co ions
50 40 30 20
In dark
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10 0 -10
Fig. 8. Schematic diagrams showing the possible photocatalytic mechanism over Co–Ag3 PO4 .
-30
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Irradiation time (min) Fig. 9. The photocatalytic activity over Ag3 PO4 , 1-Co–Ag3 PO4 and mixture of Ag3 PO4 and 0.001 mol/L Co2+ ions.
photogenerated holes to form OH. In the other way, the reaction of O2 and the photogenerated electrons formed O2 − , which was then O2 − converted to OH. In the photocatalytic reaction, O2 played an important role in promoting the photodegradation MO. Therefore, we concluded that OH mainly resulted from the second step. The action of additional Co(II) and a portion of photogenerated holes can promote the separation efficiency of photogenerated charges, and more photogenerated electrons will produce a large amount of OH. Therefore, the DMPO-OH signal for 1-Co–Ag3 PO4 is higher than pure Ag3 PO4 in Fig. 7. The specific surface areas of the pure Ag3 PO4 and 1-Co–Ag3 PO4 are 0.81 and 0.73 m2 /g, respectively. The non-porous structure of powder samples led to very small specific surface areas. The above result indicates that the grafted Co(II) did not obviously influence the specific surface areas of the samples. Therefore, the specific surface areas of the samples do not significantly impact the photodegradation of MO. The photocatalytic materials grafted by metal ions have been reported extensively, but the grafted ions generally have a high valence [19–21]. The high-valence ions can accept photogenerated electrons from the photocatalysts, which enhances the separation efficiency of the photogenerated charges. On the other hand, after accepting a photogenerated electron, the highvalence ions transfer into low-valence ions. The high-valence ions can be well recovered via the effective oxidation of the low-valence ions by O2 in air. The high-valence ions possibly reduce O2 via a multi-electron transfer route if their positive potential is higher than those of O2 /H2 O2 (+0.682 eV, vs. SHE) and O2 /H2 O (+1.23 eV vs. SHE). Therefore, the grafted ions can largely enhance the photocatalytic performance [19–21]. However, in our study, Co(II) did not accept the photogenerated electrons from Ag3 PO4 because of its low valence. As reported [23], Co(II) can be oxidized to Co(III) by the photogenerated holes under visible light radiation. The reaction between Co(II) and the photogenerated holes can improve the separation efficiency of the photogenerated charges. Meanwhile, the strong interaction between Co(II) and PO4 3− helps to transport the photogenerated charge [24]. The positive potential of Co(III)/Co(II) (1.808 eV vs. SHE) is higher than those of MO. Therefore, Co(III)/Co(II) can oxidize MO molecules. At the same time, Co(II) can be recovered from the oxidization of MO. Fig. 8 shows a possible photocatalytic mechanism over Co–Ag3 PO4 . Therefore, the synergy of Co(II) and Ag3 PO4 greatly enhanced the separation and transmission efficiency of the photogenerated charges and largely improved the photocatalytic activity of Ag3 PO4 . In order to illustrate the synergy of Co(II) and Ag3 PO4 , we conducted the experiment involving 0.001 mol/L Co(II) in MO solution with Ag3 PO4 and compared with 1-Co–Ag3 PO4 in Fig. 9. The photacatalytic ability
only reaches 31.7% after 10 min, which is importantly lower those of pure and 1-Co–Ag3 PO4 . Co(II) is a dissociative state if Co(II) is added directly to the MO solution. The dissociative Co(II) is very unable to contact and react with the photogenerated holes with very short life on the surface of Ag3 PO4 . In 1-Co–Ag3 PO4 , there exists a strong interaction of Co(II) and its surface. The strong interaction between Co(II) and PO4 3− was proved by XPS in Fig. 4D. Therefore, the photogenerated holes migrating to the surface of Ag3 PO4 can react quickly with the surface Co(II) to form Co(III). The Co(III) can further oxidate MO molecules. Therefore, the surface synergy of grafted Co(II) and Ag3 PO4 is responsible for the enhanced photocatalytic activity. To validate the electronic properties of Co–Ag3 PO4 , the electrochemical analysis of pure Ag3 PO4 and 1-Co–Ag3 PO4 was conducted (Fig. 10). The response of generated photocurrents over Ag3 PO4 and 1-Co–Ag3 PO4 under visible light irradiation is illustrated in Fig. 10A. Clearly, the induced photocurrent of 1-Co–Ag3 PO4 increases rapidly and reaches a plateau within 60 s after the switchon of light, while the photocurrent induced by pure Ag3 PO4 slightly increases only under the same condition (Fig. 10A). The different trend of photocurrent production relies on the amount of photoelectrons transferred from the semiconductor to the substrate of FTO glass. Therefore, the grafted Co(II) can largely improve the separation efficiency of photogenerated charges and electron density over Ag3 PO4 under visible light irradiation. This trend helps to enhance the photocatalytic ability of Ag3 PO4 . In order to check other electrochemical reactions during the measurements. We have done the linear sweep voltammograms of Ag3 PO4 and 1-Co–Ag3 PO4 . Compared with Fig. 10B and C, the linear sweep voltammograms of Ag3 PO4 and 1-Co–Ag3 PO4 are similar, which shows that their electrochemical reactions are similar during the measurements. Fig. 10D shows the alternating current (AC) impedance spectra of pure Ag3 PO4 and 1-Co–Ag3 PO4 , which show a semicircular form. Therefore, the rate-determining step is the charge transfer on the surface of samples (Fig. 10D). The semicircle of the AC impedance spectrum from 1-Co–Ag3 PO4 becomes smaller than that of Ag3 PO4 . The above results indicate that the space-charge layer resistance in the 1-Co–Ag3 PO4 electrode is significantly reduced, which rapidly accelerates the photoelectron transmission and photocatalytic reaction. This fact proves that the presence of Co(II) in the electrochemical system can reduce the interface charge transfer resistance of the 1-Co–Ag3 PO4 electrode, and effectively enhance the separation and transmission efficiency of photogenerated charges. This result is consistent with
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Fig. 10. The photo-electrochemical analysis of pure Ag3 PO4 and 1-Co–Ag3 PO4 . (A) Photocurrent, (B) AC impedance, (C) and (D) the linear sweep voltammograms.
the above mechanism in Fig. 8. Therefore, the grafted Co(II) can effectively promote the separation and transmission of photogenerated charges over Ag3 PO4 , which is also responsible for the improvement of the photocatalytic ability.
4. Conclusions Co(II) was successfully grafted onto the surface of Ag3 PO4 by a hydrothermal method. Additional grafted low-valence Co(II) can severely improve the separation and transmission efficiency of photogenerated charges on the surface of Ag3 PO4 , which obviously enhances the photocatalytic ability of Ag3 PO4 . This work is different from previous research reporting the grafting of high-valence ions, and may provide a new insight for the potential applications of Ag3 PO4 . Further work by our group is in process. Acknowledgement This work was supported by Natural Science Foundation of Tianjin of China (Grant 14JCYBJC20500).
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Co(II)–grafted Ag3 PO4 photocatalysts with unexpected photocatalytic ability: Enhanced photogenerated charge separation efficiency, photocatalytic mechanism and activity Shuna Zhang a , Shujuan Zhang b,∗ , Limin Song c,∗ a
College of Textile Engineering, Zhejiang Industry Polytechnic College, Shaoxing 312000, PR China College of Science, Tianjin University of Science & Technology, Tianjin 300457, PR China c College of Environment and Chemical Engineering & State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, PR China b
h i g h l i g h t s • Co–Ag3 PO4 with higher photodegradation ability was synthesized. • • OH was the main active species in the oxidation of MO. • The synergy of Co(II) and Ag3 PO4 greatly enhanced the separation efficiency.
a r t i c l e
i n f o
Article history: Received 3 December 2014 Received in revised form 20 March 2015 Accepted 22 March 2015 Available online 24 March 2015 Keywords: Co(II)–grafted Ag3 PO4 Photodegradation Methyl orange Photogenerated charges separation and transmission efficiency Enhanced mechanism
a b s t r a c t Since the photocatalytic capability is determined by the separation and transmission efficiency of photoinduced charges, its improvement remains a challenge for development of efficient photocatalysts. Here, we made large improvement on the surface of Ag3 PO4 using Co(II)–grafted Ag3 PO4 by a hydrothermal method. During the photocatalytic process, Co(II) was oxidized to Co(III) by the photogenerated holes under visible light radiation, which enhanced the separation efficiency of photogenerated charges. Meanwhile, the Co(III) as-formed could oxidize dye molecules, which recovered the Co(II). The synergy of Co(II) and Ag3 PO4 greatly promoted the separation and transmission efficiency of the photogenerated charges, and severely improved the photocatalytic activity of Ag3 PO4 . The surface grafted Co(II) on Ag3 PO4 is responsible for the enhancement of photocatalytic activity. © 2015 Published by Elsevier B.V.
1. Introduction As environmental pollution is getting worse with the rapid development of industrial production, photocatalytic technology has become an increasingly popular method for effective treatment of wastewater. Finding suitable photocatalytic materials is one effective way to improve the photocatalytic efficiency. Since the high photocatalytic capability of Ag3 PO4 was reported in 2010 [1], Ag3 PO4 is gradually known as a novel photocatalytic material [2–4]. Ag3 PO4 can absorb sunlight at less than 520 nm [1], and its quantum yield under visible light radiation is up to 90% [1]. Ag3 PO4 under visible light radiation shows strong
∗ Corresponding authors. Tel.: +86 22 83955458; fax: +86 22 83955458. E-mail addresses: [email protected] (S. Zhang), [email protected] (L. Song). http://dx.doi.org/10.1016/j.jhazmat.2015.03.047 0304-3894/© 2015 Published by Elsevier B.V.
oxidation ability [5–7], and its ability to oxidize organic dye is much higher than other materials such as TiO2 [1]. Therefore, Ag3 PO4 exhibits a great potential as photocatalytic materials. As reported, the strong binding ability of the P O bonds of PO4 tetrahedrons in Ag3 PO4 significantly weakens the strength of the Ag O covalent bonds and prevents the hybrid of the Ag4d and O2p orbits, which leads to the deviation of the 4d orbit from the conduction band bottom [1,3]. Thus, the interaction between the delocalized charges on the conduction band bottom and the electrons is very weak, which helps the charges to transport the particle surface, and improves the redox capacity [1,3]. Zhu Yongfa’s and co-workers used the first-principle calculation to demonstrate the reasons for the superior oxidative capacity and the quantum efficiency of Ag3 PO4 . One reason is the relatively low valence band level of Ag3 PO4 , which improves its oxidizing ability [2]. The other reason is that the * orbit in the conduction band in Ag3 PO4 improves the electron migration rate compared with the holes. This improvement
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together with the static induction effect on PO4 3− help to more effectively separate the photogenerated electrons and holes, and therefore, can significantly enhance the photocatalytic ability [2]. As is known, the introduction of additional ions can improve the photocatalytic oxidation ability of photocatalytic materials [8–10]. The doping of ions is able to change the electronic structure and band structure of photocatalytic materials, which can enhance their photocatalytic ability [11–14]. In addition to the doping ions, the ions grafted on the surface of the photocatalyst can also improve the photocatalytic ability of the materials [15,16]. So far, Cu(II)–grafted Ti1−3x Wx Ga2x O2 [17], Fe(III)–grafted TiO2 [18], Cu(II)–grafted TiO2 [19], Fe(III)–grafted AgBr [20], and Cu(II)–grafted Mo-doped SrTiO3 [21] have been reported. Photogenerated electrons in these iongrafted photocatalysts under visible light radiation can efficiently transfer to the surface grafting ions as co-catalysts. The electrons in the surface grafting ions can efficiently reduce the multielectron of the adsorbed O2 . Thus, the ion-grafted photocalysts exhibit the highest photocatalytic oxidation ability. In the present paper, the photocatalytic ability of Ag3 PO4 was enhanced through grafting with Co(II). Ag3 PO4 grafted by ions has not been reported. The grafted Co(II) obviously enhanced the separation and transmission efficiency of photogenerated charges on the surface of Ag3 PO4 , which severely improved the photocatalytic performance. It is noteworthy that the grafted ions in our study are cobalt ions with low valence, which is different from the grafted high valence ions previously reported [18]. The oxidation mechanism of organic dyes is also largely different. The enhanced separation and transmission efficiency of photogenerated charges, photocatalytic mechanism, and activity will be discussed later.
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solution was measured by a Shimadzu-Toc-Vcph instrument. Electron spin resonance (ESR) spectra were recorded by an electron paramagnetic resonance spectrometer (JES FA200). 2.3. Activity measurement In a typical experiment, 20 mg/L methyl orange (MO) and a 1.0 g/L catalyst were used. About 100 mL of the MO and catalyst suspension was circulated in a home-made photoreactor, and then photocatalytic oxidation proceeded with continuous stirring under a 300 W Xe lamb with a filter to remove UV components ( > 420 nm). Before the photocatalytic reaction, the mixture was stirred in dark for 30 min until reaching the adsorption equilibrium. An O2 flow was provided bubbling additional air, and 3 mL of the residual solution was extracted every 2 min. The residual concentration was measured by the UV–vis spectrophotometer. 2.4. Photoelectric measurement The photoelectric properties were studied in a three-electrode cell using a CHI 650D electrochemical workstation (reference elec-
Ag3PO4
2. Experimental 2.1. Synthesis of samples
1-Co-Ag3PO4
Intensity (a.u.)
Ag3 PO4 was synthesized as follows: First, 0.41 g of (NH4 )3 PO4 and 1.02 g of AgNO3 were dissolved in 20 mL of distilled water separately. Then, the (NH4 )3 PO4 solution was dripped into the AgNO3 solution under constant stirring. The mixture as-obtained was washed with distilled water, and dried at 60 ◦ C in a vacuum oven for 6 h. Co(II)–grafted Ag3 PO4 (marked as Co–Ag3 PO4 ) was synthesized as follows: typically, different amounts of Co(NO3 )2 ·6H2 O were dissolved separately in 60 mL of distilled water to the final concentrations of 0.0005, 0.001, 0.002, 0.003, 0.004, and 0.005 mol/L Co(II). The obtained Co–Ag3 PO4 samples were named as 0.5-, 1, 2-, 3-, 4-, and 5-Co–Ag3 PO4 , respectively. About 0.5 g of Ag3 PO4 was dispersed into each of the above solutions under stirring for 30 min. Then the suspensions were transferred into a 100 mL stainless teflonlined autoclave and heated at 80 ◦ C for 3 h. The resulting precipitates were collected, centrifuged, washed with deionized water three times, and dried at 60 ◦ C in a vacuum oven for 6 h.
05-Co-Ag3PO4
2-Co-Ag3PO4
3-Co-Ag3PO4
2.2. Characterization of samples X-ray diffraction (XRD) patterns were recorded by a diffractometer (Rigaku D/max 2500, CuK␣, = 1.5406 Å, 40 kV, 40 mA). The particle morphology and size of the photocatalysts were analyzed using a transmission electron microscope (TEM, Hitachi H-7650, 80 kV). A UV–vis spectrophotometer equipped with an integrating sphere assembly was used in this study (HP8453). The binding energy (BE) was measured by an X-ray photoelectron spectrometer (XPS, PerkinElmer PHI5300). The standard peak of adventitious carbon (C1s ) was used to calibrate BE. The Brunauer–Emmett–Teller (BET) surface area of the samples was measured by a Micromeritics ASAP2020 automatic surface area analyzer. The total organic carbon content (TOC) in the residual
4-Co-Ag3PO4
5-Co-Ag3PO4
10
20
30
40
50
60
70
2Theta/degree Fig. 1. X-ray diffraction patterns of pure and Co–Ag3 PO4 .
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trode electrode: saturated calomel electrode; counter-electrode: Pt wire; electrolyte: 0.1 M Na2 SO4 aqueous solution). Specifically, 50 mg of a catalyst was dispersed in 2 mL of ethanol under 30 min of ultrasonic scattering. Then, 0.5 mL of the suspension was dripped onto a 1 × 4 cm fluorine-doped tin oxide (FTO) glass. After evaporation in the air, the FTO glass was calcined at 150 ◦ C for 1 h. A 300 W Xe lamp with > 420 nm was used as the light source. The applied voltage was 0.4 V in the photoelectric experiments.
3. Results and discussion 3.1. Characterization of photocatalysts Fig. 1 shows the XRD patterns of pure Ag3 PO4 and Co–Ag3 PO4 . The pure Ag3 PO4 exhibited an cubic structure (JCPDS 06-0505, ¯
space group: P43n[218], a = b = c = 6.013 Å). Both the position and shape of XRD peaks of each Co–Ag3 PO4 sample are similar to those of pure Ag3 PO4 (Fig. 1), indicating that the cubic structure of Ag3 PO4 was reserved after the grafting with Co(II). Therefore, the grafted Co(II) did not enter the crystal lattice of Ag3 PO4 , or form new cobalt compounds. The low-resolution TEM images of pure Ag3 PO4 and 1Co–Ag3 PO4 (Fig. 2) reveal that the particles are globularly-shaped with mean size of 436and 427 nm, respectively. These results indicate that the shape and size of Ag3 PO4 did not change much after grafting with Co(II). We studied the electronic band structures of pure Ag3 PO4 and Co–Ag3 PO4 by analysis of their optical absorption spectra. Both the position and shape of absorption peaks of Co–Ag3 PO4 samples are similar to those of pure Ag3 PO4 (Fig. 3A). No other peaks were found except those of Ag3 PO4 , which indicated that the grafted Co(II) species did not form any absorption from band to band. However, the UV–vis absorption spectra in Fig. 3A show that the intrinsic absorption edge in Co–Ag3 PO4 obviously red-shifts relative to pure Ag3 PO4 . The derived bandgap from the plots of “n (KM × energy, n = 0.5 for the indirect bandgap) vs. the energy of the adsorbed light” is 2.30 eV for pure Ag3 PO4 (Fig. 3B). The derived
2.0
A 5-Co-Ag3PO4 Absorbtion (a.u.)
1.5
3-Co-Ag3PO4 05-Co-Ag3PO4
1.0
1-Co-Ag3PO4 Ag3PO4
0.5
4-Co-Ag3PO4
0.0 300
400
500
600
700
800
Wavelength (nm) 0.30
1-Co-Ag3PO4
B 4-Co-Ag3PO4
0.25
05-Co-Ag3PO4
(Ahv)
1/2
0.20
5-Co-Ag3PO4 3-Co-Ag3PO4
0.15
0.10
Ag3PO4
0.05
0.00 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
hv (eV) Fig. 2. TEM images of (a) Ag3 PO4 and (b) 1-Co–Ag3 PO4 .
Fig. 3. UV–vis absorption spectra of Ag3 PO4 and Co–Ag3 PO4 .
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18000
Valence
16000
A
1-Co-Ag3PO4
C
160000
14000
140000
12000
120000
Ag3d 367.9 eV
10000
Intensity (a.u.)
Intensity (a.u.)
75
Ag3PO4
8000 6000 4000
100000
368.8 eV
80000
Ag3PO4
373.9 eV 374.8 eV
1-Co-Ag3PO4
60000 40000
2000
20000 0
0 -2000 -10
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10
20
30
40
360
Bing energy (eV)
375
380
28000
781.9 eV
B
P2p
27000
133.5 eV
8000
132.8 eV 7000
Ag3PO4 1-Co-Ag3PO4
6000
D 797.5 eV a mixed method
26000
Intensity (a.u.)
9000
Intensity (a.u.)
370
Bing energy (eV)
11000 10000
365
25000
781.6 eV 796.2 eV
24000 23000
5000 22000
4000 21000
3000 120
125
130
135
Bing energy (eV)
140
145
a hydrothermal method 770
780
790
Co2p 800
810
820
Bing energy (eV)
Fig. 4. XPS spectra of Ag3 PO4 and 1-Co–Ag3 PO4 . (A) Valence, (B) P2P , (C) Ag3d , (D) Co2p .
bandgaps are 2.18, 2.21, 2.25, 2.26, and 2.26 eV for 0.5-, 1-, 3-, 4-, and 5-Co–Ag3 PO4 , respectively (Fig. 3B). The bandgaps of Co–Ag3 PO4 slightly decrease relative to pure Ag3 PO4 . Regarding the reason for the smaller bandgaps, the valance values of pure Ag3 PO4 and 1Co–Ag3 PO4 were confirmed by XPS, which are similarly 2.30 eV (Fig. 4A). Therefore, the lower conduction band bottom may be responsible for the smaller bandgap because additional cations in semiconductors only changed the conduction band position, but not the position of the valence band. The XPS spectra of the P2p , Ag3d , and Co2p regions for pure Ag3 PO4 and 1-Co–Ag3 PO4 are shown in Fig. 4. As reported, the P2p and Ag3d binding energies are 133.5, 368.8 (3d5/2) and 374.8 (3d3/2) eV for Ag3 PO4 [22], which are consistent with the results in the present study (Fig. 4B and C). For 1-Co–Ag3 PO4 , the P2p and Ag3d binding energies are 132.8, 367.9 (3d5/2), and 373.9 (3d3/2) eV, respectively. The low binding energies originated from the strong interaction between the grafted Co(II) and Ag3 PO4 . To study the Co(II) species, the Co2p XPS spectra of 1-Co–Ag3 PO4 obtained by a hydrothermal and mixed route were recorded (Fig. 4D). The Co2p peaks at 781.9 and 797.5 eV in Co–Ag3 PO4 obtained by a mixed route are assigned to the Co(II) in Co(NO3 )2 ·6H2 O, and are larger than the Co2p binding energies of Co–Ag3 PO4 obtained by a hydrothermal method (781.6 and 796.2 eV, respectively). In contrast to the Co(II) from Co(NO3 )2 ·6H2 O, the strong interaction between Co(II) and PO4 3− results in a slight transfer of electron density of Co(II). 3.2. Photocatalytic activity of photocatalysts The photocatalytic activities of TiO2 (commercial P25), pure Ag3 PO4 and Co–Ag3 PO4 with different amounts of Co(II) were compared and the results are presented in Fig. 5. The
concentrations of the MO and the catalyst solution are 20 mg/L and 1.0 g/L, respectively. The mixed MO and catalyst solution was stirred for 30 min until reaching the adsorption equilibrium between the MO molecules and the catalyst in dark. In Fig. 5A, the concentrations of the MO did not change largely over P25 under the same experimental conditions, while the as-prepared samples showed higher photocatalytic efficiency. The adsorption ratios after 30 min are 3.3%, 7.8%, 6.9%, 2.8%, 7.5%, and 2% for 0-, 0.5-, 1-, 3-, 4-, and 5-Co–Ag3 PO4 , respectively (Fig. 5A). The adsorption ratio at less than 10% cannot significantly improve the photocatalytic efficiency. During the decomposition of MO, the conversion ratio of pure Ag3 PO4 after 6 min was 24.8%, while the results of the Co–Ag3 PO4 catalysts (0.0005, 0.001, 0.003, 0.004, and 0.005 mol/L) are 72.7%, 89.2%, 84.3%, 89.7%, and 80.9%, respectively (Fig. 5A), which indicates the Co–Ag3 PO4 catalysts are more efficient. The photocatalytic abilities of all the Co–Ag3 PO4 samples are obviously higher than that of pure Ag3 PO4 . The rate constants derived from the plots of “ln C/C0 (C and C0 : original and residual MO concentrations, respectively) vs. photocatalytic reaction time” are 0.059, 0.203, 0.381, 0.304, 0.366, and 0.268 min−1 for pure Ag3 PO4 and Co–Ag3 PO4 (0.0005, 0.001, 0.003, 0.004, and 0.005 mol/L Co(II)) in Fig. 5B. The rate constant of 1-Co–Ag3 PO4 is 6.5 times larger than that of Ag3 PO4 , suggesting that the grafted Co(II) in Ag3 PO4 can significantly improve the photocatalytic efficiency of Ag3 PO4 . To study the effects of high concentration MO on Co–Ag3 PO4 , the 1-Co–Ag3 PO4 was used to photodegrade 30 and 40 mg/L MO solutions separately (Fig. 5C). The photocatalytic activities are 88% and 74% (30 and 40 mg/L MO solutions, respectively) after 6 min for 1-Co–Ag3 PO4 , while the conversion ratios are similar after 10 min (95.6% vs. 94.5%) (Fig. 5C). Their rate constants are 0.307 and 0.284 min−1 , respectively (Fig. 5D), which do not decrease obviously relative
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Recycling time Fig. 5. (A) and (B) The photocatalytic activity over P25, Ag3 PO4 and Co–Ag3 PO4 . (C) and (D) The relationship between ln (C/C0 ) and irradiation time of Ag3 PO4 and Co–Ag3 PO4 . (E) Recycling tests of degradation of 20 mg/L MO over 1-Co–Ag3 PO4 . The concentration of catalysts is 1.0 g/L.
to the 10 mg/L MO solution, suggesting that the 1-Co–Ag3 PO4 asprepared can treat the high concentration MO solutions under visible light radiation. The recycle runs indicate high stability of 1-Co–Ag3 PO4 (Fig. 5E). After 5 consecutive tests, the photodegradation percentages of 20 mg/L MO were still similar to that of the first run for 1-Co–Ag3 PO4 . These results indicate that the 1-Co–Ag3 PO4 samples can photodegrade MO stably.
3.3. Photocatalytic process and mechanism To study the process of MO photodegradation over the as-synthesized 1-CoAg3 PO4 , we monitored the change of MO concentration by recording the characteristic UV–vis absorption spectra of MO at 465 nm. The strong absorption peak at 465 nm is assigned to the n → * electronic transition from the hydrazone
S. Zhang et al. / Journal of Hazardous Materials 293 (2015) 72–80 12000
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Fig. 6. (A) The UV–vis absorption spectra of solution in the photocatalytic degradation of 20 mg/L MO on 1.0 g/L 1-Co–Ag3 PO4 . (B) TOC evolution vs. irradiation time in the photocatalytic reaction.
structure of MO. Fig. 6A shows the evolution of absorption during the photocatalytic degradation. The absorption peak at 465 nm was rapidly weakened with the prolonged irradiation time, which indicates the occurrence of MO photodegradation. After 8 min, the peak at 465 nm almost disappeared, suggesting that the MO molecular structure was destroyed. To clarify the final products after decomposition, we measured the total organic carbon (TOC) in the residual solution after the photodegradation of 20 mg/L MO. Fig. 6B shows that the TOC of residual solution decreased with time, which indicates that many MO molecules were destroyed to form CO2 , H2 O, and other small molecules. However, the TOC conversion ratios are not consistent with the photodegradation ratios of MO, indicating that a part of MO molecules were decomposed to CO2 and H2 O, and the other part of MO molecules only transferred to intermediate products. As is well-known, the main species during photocatalytic oxidation of organic dyes under visible light radiation are • OH, • O2 − , and • OOH. To investigate the oxidation species over 1-Co–Ag3 PO4 , several radicals in the photocatalytic process were monitored by ESR, and the results are shown in Fig. 7A. Clearly, only • OH was observed during the photocatalytic process, while O2 − and • OOH were not found, suggesting that OH was the main oxidation species over 1-Co–Ag3 PO4 under visible light radiation. In addition, the photocatalytic activity of 1-Co–Ag3 PO4 was weakened significantly
Fig. 7. DMPO spin-trapping ESR spectra for DMPO-OH in the presence of Ag3 PO4 and 1-Co–Ag3 PO4 under visible light irradiation ( > 420 nm) and in dark.
if the photocatalytic reaction proceeded without air bubble. Therefore, O2 is also involved in the photocatalytic reaction. The possible photocatalytic mechanism is as follows: Co–Ag3 PO4 + h → h+ + e− O2 + e− → O2 − → OOH → H2 O2 → OH OH + MO → products During the process, the generated O2 − and OOH are immediately converted to OH and could not be detected, and thus, a large amount of OH was found. Compared with the typical signals of DMPO-OH in pure Ag3 PO4 and 1-Co–Ag3 PO4 , the signals of DMPO-OH in pure Ag3 PO4 obviously attenuate with time (Fig. 7B), while those from 1-Co–Ag3 PO4 do not change after 16 min (Fig. 7A). The above results exhibit that the Co(II)–grafted Ag3 PO4 maintains a better and stable lifetime than that of pure Ag3 PO4 . In addition, the amplitude of the DMPO-OH signals for 1-Co–Ag3 PO4 is up to 8.5 times higher than that of Ag3 PO4 (Fig. 7), indicating that the grafted Co(II) can largely improve the amount of OH and the photocatalytic ability. At the beginning of the ESR measurement, abundant DMPOOH signals for 1-Co–Ag3 PO4 were detected, and did not decay with time. However, we did not find the DMPO-O2 − signal of 1Co–Ag3 PO4 throughout the reaction. • OH was generated by two paths under visible light radiation. In one way, OH− reacted with the
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MO concentration removal (%)
90 80 70 60
Ag3PO4 1-Co-Ag3PO4 2+
mixture of Ag3PO4 and 0.001 mol/L Co ions
50 40 30 20
In dark
20 mg/L MO
10 0 -10
Fig. 8. Schematic diagrams showing the possible photocatalytic mechanism over Co–Ag3 PO4 .
-30
-20
-10
0
10
Irradiation time (min) Fig. 9. The photocatalytic activity over Ag3 PO4 , 1-Co–Ag3 PO4 and mixture of Ag3 PO4 and 0.001 mol/L Co2+ ions.
photogenerated holes to form OH. In the other way, the reaction of O2 and the photogenerated electrons formed O2 − , which was then O2 − converted to OH. In the photocatalytic reaction, O2 played an important role in promoting the photodegradation MO. Therefore, we concluded that OH mainly resulted from the second step. The action of additional Co(II) and a portion of photogenerated holes can promote the separation efficiency of photogenerated charges, and more photogenerated electrons will produce a large amount of OH. Therefore, the DMPO-OH signal for 1-Co–Ag3 PO4 is higher than pure Ag3 PO4 in Fig. 7. The specific surface areas of the pure Ag3 PO4 and 1-Co–Ag3 PO4 are 0.81 and 0.73 m2 /g, respectively. The non-porous structure of powder samples led to very small specific surface areas. The above result indicates that the grafted Co(II) did not obviously influence the specific surface areas of the samples. Therefore, the specific surface areas of the samples do not significantly impact the photodegradation of MO. The photocatalytic materials grafted by metal ions have been reported extensively, but the grafted ions generally have a high valence [19–21]. The high-valence ions can accept photogenerated electrons from the photocatalysts, which enhances the separation efficiency of the photogenerated charges. On the other hand, after accepting a photogenerated electron, the highvalence ions transfer into low-valence ions. The high-valence ions can be well recovered via the effective oxidation of the low-valence ions by O2 in air. The high-valence ions possibly reduce O2 via a multi-electron transfer route if their positive potential is higher than those of O2 /H2 O2 (+0.682 eV, vs. SHE) and O2 /H2 O (+1.23 eV vs. SHE). Therefore, the grafted ions can largely enhance the photocatalytic performance [19–21]. However, in our study, Co(II) did not accept the photogenerated electrons from Ag3 PO4 because of its low valence. As reported [23], Co(II) can be oxidized to Co(III) by the photogenerated holes under visible light radiation. The reaction between Co(II) and the photogenerated holes can improve the separation efficiency of the photogenerated charges. Meanwhile, the strong interaction between Co(II) and PO4 3− helps to transport the photogenerated charge [24]. The positive potential of Co(III)/Co(II) (1.808 eV vs. SHE) is higher than those of MO. Therefore, Co(III)/Co(II) can oxidize MO molecules. At the same time, Co(II) can be recovered from the oxidization of MO. Fig. 8 shows a possible photocatalytic mechanism over Co–Ag3 PO4 . Therefore, the synergy of Co(II) and Ag3 PO4 greatly enhanced the separation and transmission efficiency of the photogenerated charges and largely improved the photocatalytic activity of Ag3 PO4 . In order to illustrate the synergy of Co(II) and Ag3 PO4 , we conducted the experiment involving 0.001 mol/L Co(II) in MO solution with Ag3 PO4 and compared with 1-Co–Ag3 PO4 in Fig. 9. The photacatalytic ability
only reaches 31.7% after 10 min, which is importantly lower those of pure and 1-Co–Ag3 PO4 . Co(II) is a dissociative state if Co(II) is added directly to the MO solution. The dissociative Co(II) is very unable to contact and react with the photogenerated holes with very short life on the surface of Ag3 PO4 . In 1-Co–Ag3 PO4 , there exists a strong interaction of Co(II) and its surface. The strong interaction between Co(II) and PO4 3− was proved by XPS in Fig. 4D. Therefore, the photogenerated holes migrating to the surface of Ag3 PO4 can react quickly with the surface Co(II) to form Co(III). The Co(III) can further oxidate MO molecules. Therefore, the surface synergy of grafted Co(II) and Ag3 PO4 is responsible for the enhanced photocatalytic activity. To validate the electronic properties of Co–Ag3 PO4 , the electrochemical analysis of pure Ag3 PO4 and 1-Co–Ag3 PO4 was conducted (Fig. 10). The response of generated photocurrents over Ag3 PO4 and 1-Co–Ag3 PO4 under visible light irradiation is illustrated in Fig. 10A. Clearly, the induced photocurrent of 1-Co–Ag3 PO4 increases rapidly and reaches a plateau within 60 s after the switchon of light, while the photocurrent induced by pure Ag3 PO4 slightly increases only under the same condition (Fig. 10A). The different trend of photocurrent production relies on the amount of photoelectrons transferred from the semiconductor to the substrate of FTO glass. Therefore, the grafted Co(II) can largely improve the separation efficiency of photogenerated charges and electron density over Ag3 PO4 under visible light irradiation. This trend helps to enhance the photocatalytic ability of Ag3 PO4 . In order to check other electrochemical reactions during the measurements. We have done the linear sweep voltammograms of Ag3 PO4 and 1-Co–Ag3 PO4 . Compared with Fig. 10B and C, the linear sweep voltammograms of Ag3 PO4 and 1-Co–Ag3 PO4 are similar, which shows that their electrochemical reactions are similar during the measurements. Fig. 10D shows the alternating current (AC) impedance spectra of pure Ag3 PO4 and 1-Co–Ag3 PO4 , which show a semicircular form. Therefore, the rate-determining step is the charge transfer on the surface of samples (Fig. 10D). The semicircle of the AC impedance spectrum from 1-Co–Ag3 PO4 becomes smaller than that of Ag3 PO4 . The above results indicate that the space-charge layer resistance in the 1-Co–Ag3 PO4 electrode is significantly reduced, which rapidly accelerates the photoelectron transmission and photocatalytic reaction. This fact proves that the presence of Co(II) in the electrochemical system can reduce the interface charge transfer resistance of the 1-Co–Ag3 PO4 electrode, and effectively enhance the separation and transmission efficiency of photogenerated charges. This result is consistent with
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Fig. 10. The photo-electrochemical analysis of pure Ag3 PO4 and 1-Co–Ag3 PO4 . (A) Photocurrent, (B) AC impedance, (C) and (D) the linear sweep voltammograms.
the above mechanism in Fig. 8. Therefore, the grafted Co(II) can effectively promote the separation and transmission of photogenerated charges over Ag3 PO4 , which is also responsible for the improvement of the photocatalytic ability.
4. Conclusions Co(II) was successfully grafted onto the surface of Ag3 PO4 by a hydrothermal method. Additional grafted low-valence Co(II) can severely improve the separation and transmission efficiency of photogenerated charges on the surface of Ag3 PO4 , which obviously enhances the photocatalytic ability of Ag3 PO4 . This work is different from previous research reporting the grafting of high-valence ions, and may provide a new insight for the potential applications of Ag3 PO4 . Further work by our group is in process. Acknowledgement This work was supported by Natural Science Foundation of Tianjin of China (Grant 14JCYBJC20500).
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