Sunlight-Assisted Degradation of Dye Pollutants in Ag3PO4

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Sunlight-Assisted Degradation of Dye Pollutants in Ag3PO4 Suspension Ming Ge,†,§ Na Zhu,†,§ Yaping Zhao,*,‡ Jing Li,*,⊥ and Lu Liu*,† †

Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin 300071, China Department of Environmental Science, East China Normal University, Shanghai, 200062, China ⊥ General English Language Department, Nankai University, Tianjin, 300071, China ‡

S Supporting Information *

ABSTRACT: Sunlight-induced photodegradation of rhodamine B over Ag3PO4 has been observed. Nanosized Ag3PO4 was synthesized by a facile ion-exchange route. X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, the Brunauer−Emmett−Teller surface area, UV−vis diffuse reflectance spectroscopy and photoluminescence spectra were employed to investigate the phase structure, morphology and optical property of the Ag3PO4 product. Nearly 100% of rhodamine B was degraded after a very short irradiation time using simulated sunlight in Ag3PO4 suspension, and the total organic carbon measurement revealed that a high degree of mineralization was achieved in the present photocatalytic system. Ag3PO4 catalyst has an excellent photocatalytic performance due to the high separation efficiency of electron and hole pairs. In the neutral pH solution, Ag3PO4 catalyst exhibited the best photoactivity under simulated sunlight. The photoinduced holes were considered to be the dominant active species in the photodegradation process.

1. INTRODUCTION With the industrial development, a large number of various toxic pollutants were released into the environment that were hazardous to human health. Dye pollutants were inevitably released into the aquatic environments due to their huge production from the textile industry.1 Most of dyes are resistant to biodegradation, and N-containing dyes undergo natural reductive anaerobic degradation to yield potentially carcinogenic aromatic amines.2 Biological oxidation and physical chemical treatments cannot usually remove dyes efficiently.3 In the past decades, heterogeneous photocatalysis has been considered as a cost-effective alternative for the purification of dye-containing wastewater.4,5 TiO2-based photocatalysts have been widely used for photocatalytic degradation of dyes;6−8 however, TiO2 with a relatively wide bandgap of 3.2 eV limits its efficient utilization of sunlight. Although dye-sensitized TiO2 photocatalytic processes can utilize visible light to degrade dyes,9 photoefficiency of these systems and mineralization degree of dyes are limited due to the slower interfacial electron transfer.10 Therefore, it is highly desirable to design new visible-light-induced photocatalysts from the viewpoint of using solar light. In recent years, visible-light-driven photocatalysts such as TiO2‑xNx,11In1‑xNixTaO4,12 CaBi2O413, and Ag@AgCl14 have been developed. In this area, a breakthrough was made to find silver ortho-phosphate (Ag3PO4) as an active visible-light-induced photocatalyst.15,16 Yi et al. reported that Ag3PO4 crystal possessed an excellent photocatalytic ability due to the high separation of photoexcited electrons and holes.15 The work reported by Bi and co-workers revealed a facet effect of single-crystalline Ag3PO4 products on their photocatalytic performances.16 Dinh et al. discussed the size-dependent photocatalytic activities of the Ag3PO4 product.17 However, no systemic study about pH-dependent photocatalytic performance, reaction kinetics, and the photocatalytic mechanism in a Ag3PO4 suspension was investigated. © 2012 American Chemical Society

In this contribution, a facile method was employed to synthesize Ag3PO4 product at room temperature. The photocatalytic ability of the as-obtained Ag3PO4 product was investigated by photodecomposition of different dyes, especially for Rhodamine B (RhB) dye. In addition, the reaction kinetics, pH-dependent photocatalytic experiments, and possible photodegradation mechanism in a Ag3PO4 suspension were also discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. Isopropyl alcohol, benzoquinone, and EDTA were purchased from Acros Corp (Belgium). Rhodamine B, methylene blue (MB), and methyl orange (MO) were obtained from Shanghai Aladdin Chemistry Corp (China). All of the other reagents were purchased from Tianjin Chemical Company (China) and used as received. 2.2. Synthesis. Ag3PO4 was synthesized by a simple ionexchange method. In a typical process, 0.003 mol of AgNO3 was dissolved in 20 mL of H2O, then 30 mL of (NH4)2HPO4 (0.001 mol) aqueous solution was added by drops into the above solution under vigorous stirring. After stirring for another 30 min, the yellow precipitate from the reaction was collected by filtration, washed with distilled water and absolute ethanol for several times, and then dried at room temperature in the air for further use. N-doped TiO2 was synthesized according to the previous report.18 2.3. Characterization. The phase of the as-obtained product was characterized by X-ray diffraction (XRD) under a Rigaku D/Max-2500 X-ray diffractometer employing Cu Kα Received: Revised: Accepted: Published: 5167

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radiation, λ = 1.54056 Å. The morphology and size of the asprepared product were characterized by using a field emission scanning electron microscope (JEOL JSM-6700). The elemental composition was detected by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD). UV−vis diffuse reflectance spectra (DRS) were recorded with a UV−vis spectrophotometer by using BaSO4 as the reference (Shimadzu, UV-3600). The Brunauer− Emmett−Teller (BET) surface area was obtained on a Quantachrome NOVA 2000e sorption analyzer. The Fourier transform infrared (FTIR) spectrum was recorded with a GX spectrophotometer (Perkin-Elmer) with the KBr wafer technique. The photoluminescence (PL) spectrum was conducted on a Fluorolog-3 Jobin Yvon fluorescence spectroscope. 2.4. Photocatalytic Evaluation. The photocatalytic activities of the as-obtained samples were monitored through the photodegradation of dye pollutants under simulated sunlight irradiation. The photocatalytic degradation experiments were carried out in a photochemical reactor (Supporting Information, Figure S1). The as-synthesized Ag3PO4 particles (0.200 g) were added to an aqueous dye solution (5 ppm, 200 mL), which was stored in the dark for 20 min under stirring. A 350 W Xe lamp (190 nm < wavelength < 1100 nm) was employed as simulated sunlight source. About 3 mL of the suspension was taken at given time intervals and separated through centrifugation (2000 rpm, 10 min). Nitric acid and sodium hydroxide were used to adjust the initial pH of the solutions. The concentration of RhB was analyzed at 553 nm19 on a UV−vis spectrophotometer (UV2550, Shimadzu). Total organic carbon (TOC) was conducted on a Shimadzu TOC-V CPH analyzer.

can be attributed to Ag 3d3/2 and Ag 3d5/2 binding energies, respectively (Figure 2a).20 In Figure 2b, the P 2p peak of the Ag3PO4 product appears at ca. 132 eV, which corresponded to P5+ according to the previous results.21 The O 1s binding energy (Figure 2c) of ∼530 eV is in agreement with O2‑ anion.22 Thus, the XPS analysis further confirmed the purity for Ag3PO4 product. The shape and size of the as-prepared Ag3PO4 product were investigated by SEM technique. The images at different magnifications reveal that the as-obtained sample is composed of quasisphere particles with diameters in the range of 70−480 nm (Supporting Information, Figure S2). The UV−vis diffuse reflectance spectra of the as-obtained N-doped TiO2 and Ag3PO4 product is shown in Figure 3. It can be seen that both N-TiO2 and Ag3PO4 products exhibit absorption in the visible range in addition to the UV range. The band gap (Eg) of the semiconductor can be obtained from the equation below (eq 1): αhν = A(hν − Eg )n /2

(1)

where α, ν, and A are absorption coefficient, light frequency, and proportionality constant, respectively. When plotting (αhν)2, that is, the square of the absorption coefficient multiplied by the photon energy (hν), versus hν a direct band gap of 2.34 eV was obtained, while (αhν)1/2 versus hν indicated an indirect band gap of 2.35 eV(Supporting Information, Figure S3).17 Photoluminescence (PL) is an extremely useful tool for obtaining information about the photoelectric property of materials. The PL spectra of the asprepared Ag3PO4 product was recorded at room temperature with excitation wavelength of 415 nm as shown in Figure 4. Obviously, three sharp emission peaks were observed in the visible-light range. 3.2. Photocatalytic Activity of the As-Prepared Ag3PO4 Product. The photocatalytic performance of the assynthesized Ag3PO4 product was evaluated by degradation of RhB under simulated sunlight irradiation. The degradation of RhB as a function of time in different systems is shown in Figure 5a. The blank experiment revealed that RhB was slightly degraded using simulated sunlight in the absence of Ag3PO4 catalyst, and the removal of RhB in the dark was almost negligible. It is clearly seen that, Ag3PO4 particles exhibited superior photocatalytic activity in the photodecomposition of RhB under simulated sunlight, as compared to the N-doped TiO2. This can be explained by weak absorption in the visible region for N-doped TiO2.17 Nearly 100% of RhB was photocatalytically degraded over Ag3PO4 product after 12 min under simulated sunlight. Figures 5b demonstrates the evolution of RhB absorption spectra at different irradiation times, from which we can see that the concentration of RhB decreased rapidly, and the total decomposition of RhB was achieved at about 12 min in the case of the as-synthesized Ag3PO4 particles (Figure 5a). The photocatalytic efficiency of the as-obtained Ag3PO4 product in this work for dye degradation was as well as other reported Ag3PO4 product in previous reports.16 To further investigate the photocatalytic activity of the as-obtained Ag3PO4 sample, the TOC experiment was also performed, shown in Figure 5c. The TOC value decreased by 76.5% after 90-min exposure to simulated sunlight, indicating that the mineralization of RhB occurred. In addition, it is noteworthy that, under UV light irradiation, the photocatalytic efficiency of Ag3PO4 catalyst for RhB degradation in a very short irradiation time was as high as P25 (Supporting Information, Figure S4). For the obtained Ag3PO4 catalyst, the BET surface area is only 2 m2/g, much less than that of P25 (50 m2/g). It can be concluded that the Ag3PO4 catalyst exhibits much higher photocatalytic activity per unit surface area than P25. This result demonstrates that the recombination of photoexcited electrons and holes of Ag3PO4 catalyst is very weak.23

3. RESULTS AND DISCUSSION 3.1. Characterization. The phase and purity of the assynthesized sample were characterized by XRD measurement, as shown in Figure 1. All of the peaks of the product can be

Figure 1. XRD pattern of the as-synthesized Ag3PO4 particles.

readily indexed to the cubic structure of Ag3PO4 (JCPDS No. 06-0505). No characteristic peak was observed for other types, indicating the high purity of the product formed via the simple ion-exchange process. The X-ray photoelectron spectroscopy (XPS) was carried out to investigate the surface compositions and chemical state of the as-obtained Ag3PO4 particles (Figure 2). The binding energies obtained from the XPS analysis were corrected by referencing C 1s to 284.50 eV. The Ag 3d spectra of Ag3PO4 is composed of two individual peaks at ∼374 and ∼368 eV, which 5168

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Figure 2. The XPS spectra of the as-synthesized Ag3PO4 particles: (a) Ag 3d; (b) P 2p; (c) O 1s.

Figure 4. Room temperature photoluminescence spectra of the asprepared Ag3PO4.

Figure 3. UV−vis diffuse reflectance spectra of the products: (a) the as-synthesized Ag3PO4 particles; (b) N-doped TiO2.

concentration of pollutants is very low.24 The relevant equations are listed as follows: Figure 6 shows the influence of the initial RhB concentration on the photodegradation rate over Ag3PO4 catalyst. The Langmuir− Hinshelwood model is well established for photocatalysis when the

r=− 5169

k KC dC = r dt 1 + KC

(2)

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Figure 5. (a) The photodegradation efficiencies of RhB as a function of irradiation time in different systems; (b) absorption spectra of a solution of RhB in the presence of the as-obtained Ag3PO4 catalyst under simulated sunlight; (c) change of TOC with irradiation time.

ln

C0 = kt C

(3)

where r is the reaction rate, kr is the reaction rate constant, K is the adsorption coefficient, and C is the reactant concentration (eq 2). If C is very small, eq 3 is obtained. C0 and C are the concentration of reactant at time 0 and t, respectively, and k is the apparent first order rate constant. The k value is obtained from the gradient of the graph of ln(C0/C) versus time. The apparent reaction rate constant was 0.4518, 0.3324, and 0.1163 min−1 for the initial RhB concentrations of 5, 10, and 20 ppm, respectively. It can be found that the photocatalytic degradation rate of RhB is much higher when the initial concentration is lower in the Ag3PO4/RhB/ sunlight system. This result is consistent with those previous works for RhB degradation by other photocatalysts.25,26 The recycle experiments were carried out to investigate the reuse of Ag3PO4 particles under simulated sunlight. After every run of photocatalytic reaction, the concentrated RhB solution was injected and the separated photocatalysts were washed back into the reactor in order to keep the initial concentration of RhB and photocatalysts constant.25 It can be observed that after three cycling runs of photodegradation of RhB, the photocatalytic activity of the Ag3PO4 product did not show any

Figure 6. First-order plots for the photodegradation of RhB by Ag3PO4 particles at various initial concentrations.

significant loss (Figure 7a) . However, in the acid solution (pH = 4) or in the basic solution (pH = 10), the photocatalytic 5170

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Figure 7. (a) Cycling runs in the photocatalytic degradation of RhB by Ag3PO4 without pH adjustment; (b) photodegradation of RhB over Ag3PO4 at different pH values (Ag3PO4 loading, 1 g/L; initial concentration of RhB, 5 ppm).

efficiency of Ag3PO4 product in RhB degradation was greatly decreased (Figure 7b). The Ag3PO4 catalyst could be dissolved in the acid solution to reduce the catalyst dosage, which led to the slow degradation. Supporting Information, Figure S5a shows that Ag3PO4 catalyst was not transformed to other materials in the acid solution. However, in the basic solution, the lower photocatalytic activity was possible due to the Ag produced in the photocatalytic process on the surfaces of the Ag3PO4 particles to cover the active sites (Supporting Information, Figure S5b).27 Hence, one can deduce that Ag3PO4 catalysts exhibite the best photocatalytic performance for RhB degradation in the neutral condition. In additional to the RhB removal, MB and MO were also chosen as representative model dyes to evaluate the photocatalytic activity of the as-prepared Ag3PO4 product. The structures of RhB, MB, and MO are illustrated in Supporting Information, Figure S6, obviously, RhB and MB are cationic dyes, and MO is anionic dye. Note that the concentration of MB and MO, was determined according to the characteristic peak at 663 and 464 nm, respectively. Figure 8 shows the photodegradation rates of three kinds of dyes as a function of irradiation time in a Ag3PO4 suspension. From the degradation process, it can be found that the degradation rate is higher for the cationic dyes (RhB and MB) than for the anionic dye (MO). The FT-IR spectra of the Ag3PO4 product is shown in Supporting Information, Figure S7. The peak at 1650 cm−1 is attributed to bending vibration of OH− ions, which reveals that the as-obtained Ag3PO4 product inherits negative surface charge.28 Thus the cationic dye (RhB or MB) could be easily absorbed onto the catalyst surface by the electrostatic field force, and charge transfer is facilitated. However, for the anionic dye (MO) this effect is not operative as such. Hence, Ag3PO4 catalyst is good at removing the cationic dyes efficiently. 3.3. Possible Photocatalytic Mechanism in Ag3PO4 System. The reactive oxygen species trapping experiments were designed to investigate the main reactive oxygen species in the photocatalytic process of Ag3PO4. It is well-known that OH· radicals have been deemed to be the major reactive oxygen species in the photocatalytic process. Isopropyl alcohol has been reported as the best OH· radicals scavenger due to its high-rate constant reaction with OH· radicals.29,30 Here, 1 M isopropyl alcohol was added to the RhB solution; no apparent change was found in the degradation rate of RhB (lines a and b of Figure 9). Even though

Figure 8. Photocatalytic degradation of three kinds of dyes in Ag3PO4 suspension without pH adjustment (Ag3PO4 loading, 1 g/L; initial concentration of dyes, 5 ppm).

the concentration of isopropyl alcohol was increased to 4 M, no obvious inhibition for RhB degradation was observed (Supporting Information, Figure S8). This indicated that the OH· radicals could not be the main active oxygen species in this photocatalytic process. In the photodegradation system, the O2·− radicals are effective reactive oxygen species, which are produced by the reduction of O2 molecules adsorbed on the catalyst surface by the photogenerated electrons. Under irradiation the photocatalytic degradation of RhB was not greatly suppressed by the addition of a O2·− radical scavenger, benzoquinone (lines a and c of Figure 9).30 This means that O2·− radicals do not play a important role in the RhB degradation in Ag3PO4 suspension. From the theoretical viewpoint, the roles of OH· and O2·− radicals in the present photodegradation system are almost negligible. For the Ag3PO4 catalyst, the valence band edge potential (2.45 eV)15 is less positive than E0 (OH·/H2O) (2.68 eV),31 which indicated that holes cannot directly oxidize the adsorbed H2O molecules into OH· radicals. The E0 (O2/ O2·−) (0.13 eV) is more negative than the conduction band edge potential of Ag3PO4 (0.45 eV), which indicated that the yield of O2·− radicals via the reduction of O2 by 5171

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-22-23504015. Fax: +86-22-23500557. E-mail: liul@ nankai.edu.cn. Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by 2011 Science Foundation of Tianjin (Grant No. 11JCZDJC24800) and China−US Center for Environmental Remediation and Sustainable Development.

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Figure 9. Photodegradation of RhB by Ag3PO4 catalyst under different solutions (Ag3PO4 loading, 1 g/L; initial concentration of RhB, 5 ppm).

CB electrons was impossible. However, the photodegradation rate of RhB was greatly inhibited when a hole scavenger, EDTA, was added into the photocatalytic system (lines a and d of Figure 9). This indicated that the degradation of RhB was achieved by direct hole oxidation in Ag3PO4 suspension. Hence, RhB and MB could be efficiently degraded because they are easily adsorbed by Ag3PO4 and they can be oxidized by holes on Ag3PO4 surfaces, while MO has a different case. Just like the Ag3PO4 catalyst, the photocatalytic process in BiVO4 suspension was also dominated by direct hole attack.32 The photocatalytic activity of pure BiVO4 was low due to the low separation efficiency of electron and hole pairs;33 however, the pure Ag3PO4 catalyst has a excellent photocatalytic performance (line a of Figure 9). First, both the highly dispersive valence bands and conduction bands of Ag3PO4 should be beneficial for the transport of photoexcited electrons and holes.15Second, PO43− ions in Ag3PO4 have a large negative charge which maintains a large dipole, which promotes the separation of photogenerated electrons and holes.34

4. CONCLUSION Nanosized Ag3PO4 has been fabricated via a simple ion-exchange method at room temperature. Under simulated sunlight irradiation, Ag3PO4 catalyst exhibited an excellent photocatalytic ability for dyes degradation. pH-dependent photocatalytic experiments indicated that the highest photoactivity of Ag3PO4 was observed in the netural solution. The degradation of RhB was mainly achieved by the direct hole oxidation in Ag3PO4 suspension. This study provided a potential approach to the removal of organic pollutants using sunlight.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The photocatalytic apparatus; SEM images of the as-obtained Ag3PO4 particles; plots for determining the band gap of Ag3PO4 product; photodecomposition of RhB under UV light irradiation by different catalysts (300 W, mercury lamp) for 3 min; XRD patterns of catalysts gained after photocatalytic reactions at pH 4 (a) and pH 10 (b); structures of RhB, MB, and MO dyes; the FT-IR spectra of the Ag3PO4 product; photodegradation of RhB by Ag3PO4 catalyst with the different isopropyl alcohol concentration. This material is available free of charge via the Internet at http://pubs.acs.org. 5172

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