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WO3 photocatalysts for visible light degradation of organic dyes
Journal of Molecular Catalysis A: Chemical 391 (2014) 12–18
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Highly effective and stable Ag3 PO4 /WO3 photocatalysts for visible light degradation of organic dyes Jiqiao Zhang a,b , Kai Yu a,∗ , Yifei Yu b , Lan-Lan Lou b , Zequn Yang a , Junwei Yang a , Shuangxi Liu b,c,∗∗ a
College of Environmental Science and Engineering, Nankai University, Tianjin 300071, People’s Republic of China Institute of New Catalytic Materials Science and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China c Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 14 February 2014 Received in revised form 3 April 2014 Accepted 4 April 2014 Available online 19 April 2014 Keywords: Ag3 PO4 /WO3 composites Visible light photocatalysis Organic dye degradation Improved activity and stability
a b s t r a c t The Ag3 PO4 /WO3 composites were prepared through a deposition–precipitation method and characterized by XRD, SEM, and DR UV–vis. These photocatalysts were evaluated in the degradation of rhodamine B (RhB) and methyl orange (MO) under visible light irradiation ( > 420 nm), and the synergistic effect of Ag3 PO4 and WO3 was confirmed by notably higher photocatalytic activity compared to pure Ag3 PO4 and WO3 catalysts. The effect of Ag3 PO4 :WO3 ratio on the catalytic activity was systemically studied and the catalyst AW6/4 was found to exhibit the highest catalytic activity. The degradation rates of RhB and MO could reach up to 97% under visible light irradiation for 6 min and 35 min, respectively. Moreover, the Ag3 PO4 /WO3 photocatalysts showed higher recyclability than pure Ag3 PO4 catalyst and could be recycled five runs in the degradation of RhB without any loss in the activity. The characterization of used catalysts proved that Ag3 PO4 was effectively protected and much less metallic Ag was formed on the surface of Ag3 PO4 /WO3 catalyst. The improvement of photocatalytic activity and stability is mainly attributed to the highly effective separation of photogenerated electron–hole pairs and special transfer pathway of electrons and holes in Ag3 PO4 /WO3 composites. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The visible light photocatalytic degradation of organic pollutants over semiconductors shows great promise for environmental remediation, which could completely decompose environmental pollutions to carbon dioxide and water through utilizing solar energy. Many attempts have been made to prepare excellent visible light photocatalysts with high catalytic activity and stability. Recently, Ye and co-workers [1–6] reported an efficient photocatalyst, silver orthophosphate (Ag3 PO4 ), that can efficiently oxidize water to release oxygen [7] as well as degrade organic contaminants [8–10] under visible light irradiation. It has a suitable band gap of 2.45 eV and can achieve 90% quantum efficiencies at
∗ Corresponding author. Tel.: +86 22 23509005; fax: +86 22 23509005. ∗∗ Corresponding author at: College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China. Tel.: +86 22 23509005; fax: +86 22 23509005. E-mail addresses: [email protected] (K. Yu), [email protected] (S. Liu). http://dx.doi.org/10.1016/j.molcata.2014.04.010 1381-1169/© 2014 Elsevier B.V. All rights reserved.
wavelengths greater than 420 nm, which is significantly higher than that of other semiconductors reported in the literature. However, Ag3 PO4 catalyst was expensive and always suffered from stability issue, which limited their practical application. And therefore, many researches have attempted to improve the activity and stability of Ag3 PO4 through surface modification [11–16] and Ag3 PO4 supporting [17–28]. Among these studies, the heterojunction catalysts intermixing Ag3 PO4 with other semiconductors have received much attention. Yao et al. [22] and Rawal et al. [23] reported the Ag3 PO4 /TiO2 composites and used them as catalysts for the photodecomposition of azo dyes and isopropyl alcohol under visible light irradiation, respectively. Guo et al. [24] prepared an Ag3 PO4 /Cr-SrTiO3 heterojunction photocatalyst that exhibited improved efficiency in isopropyl alcohol photodegradation compared with pure Ag3 PO4 and Cr-SrTiO3 . Kumar et al. [25] reported a g-C3 N4 -Ag3 PO4 organic–inorganic hybrid nanocomposite photocatalyst and studied the photocatalytic activity for methyl orange degradation under visible light irradiation. Other heterojunction photocatalysts such as Ag3 PO4 /Bi2 MoO6 [26], Ag3 PO4 /BiPO4 [27], and Ag3 PO4 /AgBr [28] were also prepared and these composites
J. Zhang et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 12–18
exhibited improved activity for the photodegradation of organic dyes. All the above-mentioned composite photocatalysts exhibited improved activity than pure Ag3 PO4 catalyst due to the effective separation of photogenerated electron–hole pairs. However, the redox potentials of conduction band (CB) and valence band (VB) of TiO2 , Cr-SrTiO3 , g-C3 N4 , Bi2 MoO6 , BiPO4 and AgBr are more negative than those of Ag3 PO4 . It means that under visible light irradiation the photogenerated electrons could move to the CB of Ag3 PO4 and photogenerated holes could transfer to the VB of TiO2 , Cr-SrTiO3 , g-C3 N4 , Bi2 MoO6 , BiPO4 and AgBr. The enriched electrons on the surface of Ag3 PO4 would be captured by Ag+ ions in Ag3 PO4 (Ag+ + e− → Ag0 , E0 = +0.80 V vs. NHE) [29,30] instead of combined with O2 , then the metallic Ag would be produced after photoreaction and Ag3 PO4 was corroded. Tungsten trioxide (WO3 ), as a semiconductor material with an optical band gap of 2.7 eV, has been widely used in the fields of photocatalysis [31–33], electronic device [34,35], gas sensor [36–38] and rechargeable lithium-ion battery [39,40]. Especially, the redox potentials (CB and VB) of WO3 are more positive than those of Ag3 PO4 . Therefore, the photogenerated electrons could move from the CB of Ag3 PO4 to that of WO3 under visible light irradiation if Ag3 PO4 and WO3 are combined as a photocatalyst, which would avoid the formation of metallic Ag on the surface of Ag3 PO4 , and thus increase the stability of Ag3 PO4 . At the same time, the enhanced photocatalytic activity could be expected using combined Ag3 PO4 /WO3 catalyst due to the effective separation of photogenerated electron–hole pairs. So, we wish to report here the synthesis, characterization and photocatalytic applications of Ag3 PO4 /WO3 composite catalysts. A facile deposition–precipitation method was applied to synthesize a series of Ag3 PO4 /WO3 composites with different mass ratios of Ag3 PO4 to WO3 . The effect of Ag3 PO4 :WO3 ratio on the catalytic activity was systemically studied. Furthermore, the advantages of these photocatalysts were demonstrated by the higher photocatalytic activities and better stability in the visible light degradation of rhodamine B (RhB) and methyl orange (MO), in comparison to pure Ag3 PO4 and WO3 catalysts.
2. Experimental 2.1. General Sodium tungstate dihydrate (Na2 WO4 ·2H2 O), ammonium chloride (NH4 Cl), silver nitrate (AgNO3 ), disodium hydrogen orthophosphate (Na2 HPO4 ), RhB and MO that used in this study were analytical grade and purchased from Tianjin Heowns Biochemical Technology Co., Ltd. N-doped TiO2 (N-TiO2 ) was synthesized by wet method using urea and titanium tetraisopropoxide, as described by Guo et al. [41]. All solutions throughout the experiments were prepared with distilled water. Powder X-ray diffraction (XRD) patterns of the catalysts were recorded on a Bruker D8 Focus diffractometer with Cu K␣ radiation at a scanning speed of 0.01◦ /s in a scan range of 10◦ < 2 < 80◦ operating at 40 kV and 40 mA. Diffuse-reflectance UV–vis (DR UV–vis) characterization was carried out on a Shimadzu UV-2550 UV–vis spectrophotometer with an integrating sphere attachment in the range of 220–800 nm. Scanning electron microscopy (SEM) images were obtained on a JEOL JSM-7500F field-emission scanning election microscope at an accelerating voltage of 5 kV. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Kratos Axis Ultra DLD with a Al K␣ X-ray source. The concentrations of RhB and MO in water were measured by a Shimadzu UV2550 UV–vis spectrophotometer at the wavelength of 552 nm and 463 nm, respectively.
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Table 1 The composition of reaction solution for the preparation of Ag3 PO4 /WO3 composites. Ag3 PO4 /WO3 composites
Amount of WO3 (g)
Volume of AgNO3 solution (mL)
Volume of Na2 HPO4 solution (mL)
AW2/8 AW4/6 AW6/4 AW8/2
0.0328 0.0246 0.0164 0.0082
2.5 5.0 7.5 10.0
1.0 2.0 3.0 4.0
2.2. Synthesis of WO3 WO3 was synthesized by a simple hydrothermal process. In brief, 0.003 mol of Na2 WO4 ·2H2 O was dissolved in 60 mL of distilled water and 0.01 mol of NH4 Cl was dissolved in 20 mL of distilled water, then the above two solutions were mixed in ultrasonic water bath for 1 h. The obtained solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and kept at 180 ◦ C for 48 h. After cooling down to room temperature, the precipitate was harvested by filtration, washed with distilled water, and dried in a vacuum oven at 65 ◦ C for 12 h. Then the sample was crystallized at 500 ◦ C for 2 h and then WO3 was obtained.
2.3. Preparation of Ag3 PO4 /WO3 composite photocatalysts The Ag3 PO4 /WO3 composite photocatalysts with different mass ratios of Ag3 PO4 to WO3 were prepared by a deposition–precipitation process. Firstly, WO3 was dispersed in distilled water, and then the required amount of AgNO3 solution (0.0235 M) and Na2 HPO4 solution (0.15 M) were successively dropped into this suspension under stirring, in which the volume of distilled water was kept constant at 20 mL. Table 1 lists the composition of various reaction solutions. The obtained precipitate was washed with distilled water for several times and dried in vacuum at 65 ◦ C for 12 h. According to the mass ratio of Ag3 PO4 to WO3 , the as-synthesized Ag3 PO4 /WO3 photocatalysts were marked as AW2/8 , AW4/6 , AW6/4 , and AW8/2 , respectively. For comparison, a Ag3 PO4 catalyst was prepared through an ionexchange method as follows. 7.5 mL of AgNO3 solution (0.0235 M) was dispersed in 9.5 mL of distilled water and 3.0 mL of Na2 HPO4 solution (0.15 M) was added drop by drop to the solution under stirring, then golden yellow precipitate was formed instantly. The obtained precipitate was washed with distilled water for several times and dried in vacuum at 65 ◦ C for 12 h.
2.4. Photodegradation of organic dyes The photodegradation experiments were conducted in a photochemical reactor, which is configured with a light source system (500 W Xenon lamp), a magnetic stirrer and 12 quartz glass test tubes. The Xenon lamp was cooled by a water jacketed condenser and equipped with an ultraviolet cutoff filter to provide visible light with > 420 nm. In a typical photodegradation process, 0.04 g of photocatalyst was dispersed in 30 mL of RhB or MO aqueous solution with an initial concentration of 5 g/mL. The suspension was stirred in dark for 30 min to establish adsorption–desorption equilibrium, and then it was exposed to Xenon lamp irradiation under stirring. In all the photodegradation experiments, the parameters, such as exposure to a constant photo flux, stirring rate and initial concentration were kept constant. After the photodegradation, the catalysts were separated from the reaction solutions by centrifugation and the concentrations of RhB and MO were determined by UV–vis spectrophotometer.
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Fig. 1. Powder XRD patterns of as-prepared WO3 , Ag3 PO4 and Ag3 PO4 /WO3 photocatalysts.
Fig. 2. DR UV–vis spectra of as-prepared WO3 , Ag3 PO4 and Ag3 PO4 /WO3 photocatalysts.
3. Results and discussion
(2 1 0) and (2 1 1), increased evidently, while the intensities of WO3 diffraction peaks gradually decreased. The DR UV–vis spectra of obtained WO3 , Ag3 PO4 and Ag3 PO4 /WO3 composites are shown in Fig. 2. It could be found that the optical absorption edge was estimated to be ∼460 nm for WO3 and ∼510 nm for Ag3 PO4 , respectively. Moreover, with the increase of Ag3 PO4 content, the absorption edge of Ag3 PO4 /WO3 composite gradually moved to longer wavelength. The morphologies of the as-synthesized photocatalysts were examined by SEM and the typical SEM images are shown in Fig. 3. It could be seen from Fig. 3(a) and (b) that the obtained Ag3 PO4 was spherical-like with a particle size of 200–500 nm, while WO3 had a rod-like crystal structure with a length of 1–3 m and a diameter of ∼250 nm. For the Ag3 PO4 /WO3 samples, as shown in Fig. 3(c)–(f), the grain-like Ag3 PO4 particles with a diameter of 200–500 nm covered the surface of rod-like WO3 . With growing content of Ag3 PO4 in the composite samples, more and more Ag3 PO4 particles existed on the surface of WO3 and these particles were partly
3.1. Characterization of the photocatalysts The phase purity and crystal structure of as-synthesized WO3 , Ag3 PO4 and Ag3 PO4 /WO3 composites were confirmed by XRD characterization. As shown in Fig. 1, the characteristic diffraction peaks of (1 1 0), (2 0 0), (2 1 0), (2 1 1), (3 1 0), (2 2 2), (3 2 0), (3 2 1), (4 0 0), (4 2 0) and (4 2 1) can be perfectly indexed as the body-centered cubic structure of Ag3 PO4 (JCPDS No. 06-0505). Moreover, the diffraction peaks of (0 0 1), (0 2 0), (2 0 0), (1 2 0), (1 1 1), (2 0 1), (2 2 0), (1 2 1), (2 2 1), (0 0 2), (0 4 0), (1 4 0) and (4 2 0) can be attributed to the orthorhombic phase WO3 (JCPDS No. 201324). There were no additional diffraction peaks observed in the XRD patterns of Ag3 PO4 /WO3 composites except for the characteristic peaks of Ag3 PO4 and WO3 . It also could be observed in Fig. 1, with the increase of Ag3 PO4 content in Ag3 PO4 /WO3 composites, the intensities of Ag3 PO4 diffraction peaks, such as (1 1 0), (2 0 0),
Fig. 3. SEM micrographs of as-prepared Ag3 PO4 (a), WO3 (b) and Ag3 PO4 /WO3 composites (c–f).
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gap of 2.7 eV [44,45]) and Ag3 PO4 (with an optical band gap of 2.45 eV [1]) were combined. Under visible light irradiation, the electron–hole pairs were generated on the surface of Ag3 PO4 and WO3 . Because the redox potentials of CB and VB of WO3 are more positive than those of Ag3 PO4 , the photogenerated electrons could move from the CB of Ag3 PO4 to that of WO3 , and the photogenerated holes could move from the VB of WO3 to that of Ag3 PO4 . The holes could directly oxidize the organic dyes adsorbed on Ag3 PO4 surface [10,25,26] and the electrons could be consumed through a multi-electron reaction with oxygen (O2 + 2H+ + 2e− → H2 O2 , E0 = +0.682 V vs. NHE) [23]. The produced H2 O2 reacts with additional electron to produce • OH (H2 O2 + e− → OH− + • OH) [43], which could accelerate the degradation of organic dyes. Moreover, the transfer of photogenerated electrons and holes in the Ag3 PO4 /WO3 composites could prevent the recombination of electron–hole pairs, which also led to the improvement of photocatalytic activity. It could be also noticed from Fig. 4(a) that among these Ag3 PO4 /WO3 composites, the catalyst AW6/4 exhibited the highest catalytic activity for RhB degradation. When AW6/4 was used as photocatalyst, the RhB degradation rate could reach up to 97% under visible light irradiation for 6 min. While only 72%, 83% and 87% of RhB could be degraded over the other catalysts AW2/8 , AW4/6 , and AW8/2 , respectively, under the same reaction conditions. These results indicated that more remarkable synergistic effect was exhibited when the mass ratio of Ag3 PO4 :WO3 was 6:4. Moreover, the catalyst AW6/4 also exhibited improved activity for the photodegradation of MO, as shown in Fig. 4(b). Notable higher degradation rate of MO could be obtained catalyzed by AW6/4 compared with pure Ag3 PO4 and WO3 catalysts. 3.3. Photocatalytic stability of the Ag3 PO4 /WO3 composites Fig. 4. Photocatalytic degradation of RhB (a) and MO (b) under visible light irradiation ( > 420 nm).
agglomerated. When the Ag3 PO4 :WO3 mass ratio was 8:2, most of the WO3 crystal surfaces were covered by Ag3 PO4 particles. 3.2. Visible light photocatalytic activity of the Ag3 PO4 /WO3 composites The as-synthesized WO3 , Ag3 PO4 , and Ag3 PO4 /WO3 composites were used as photocatalysts for the decomposition of organic dye RhB and MO under visible light irradiation. Fig. 4(a) shows the degradation curves of RhB catalyzed by Ag3 PO4 /WO3 composites with different mass ratio of Ag3 PO4 to WO3 . It could be found that the Ag3 PO4 /WO3 composites exhibited excellent activities for RhB degradation, which were notably higher than those of pure Ag3 PO4 , WO3 and N-TiO2 catalysts under the same reaction conditions. After 10 min of visible light irradiation, nearly 100% of RhB was decomposed in the presence of Ag3 PO4 /WO3 photocatalysts. However, only 54% and 15% of RhB could be removed within the same time using pure Ag3 PO4 and WO3 as catalyst, respectively. The significantly lower catalytic activity of WO3 was mainly attributed to the more positive CB potential of WO3 compared with that of the single-electron reduction of O2 to • O2 − [42,43], as well as its absorption edge (∼460 nm) which would lead to the less absorption of visible light. While for Ag3 PO4 catalyst, the slightly lower activity can be ascribed to the relatively lower catalyst dosage in this study compared with the literature [2]. The notably enhanced photocatalytic performances of Ag3 PO4 /WO3 composites are mainly attributed to the highly effective separation of photogenerated electron–hole pairs. As shown in Fig. 5, the semiconductors of WO3 (with an optical band
The stability is an important issue for Ag3 PO4 catalyst. So the recyclability of as-synthesized Ag3 PO4 and AW6/4 catalysts were investigated in this section. After each photodegradation cycle, the photocatalyst was collected by centrifugation and reused in the following run. Fig. 6(a) shows the degradation curves of RhB in five consecutive applications with Ag3 PO4 and AW6/4 catalysts. As shown in Fig. 6(a), the catalyst AW6/4 exhibited excellent recyclability for the visible light degradation of RhB and no obvious decrease in catalytic activity was found after five runs. At the fifth run, the degradation rate of RhB was still up to 97% over AW6/4 under visible light irradiation for 6 min, which was the same as in the first run. However, the stability of Ag3 PO4 was found to be lower than that of AW6/4 , and the catalytic activity was notably decreased after the second run due to the absence of sacrificial reagent (such as AgNO3 [1]). Only 25% degradation rate of RhB was obtained over Ag3 PO4 catalyst for the fifth run. Fig. 6(b) shows the degradation curves of MO in three consecutive applications with Ag3 PO4 and AW6/4 catalysts. It could be found that the catalytic activity of AW6/4 was maintained absolutely throughout the three runs of MO degradation, however, the activity of Ag3 PO4 obviously decreased after the first run. These results indicated that the stability of Ag3 PO4 /WO3 composite was obviously improved compared with pure Ag3 PO4 catalyst. To exposit the reasons for the difference of catalytic stability between Ag3 PO4 and AW6/4 , the two catalysts used five runs in the degradation of RhB were characterized by XRD, SEM and XPS. Fig. 7 shows the XRD patterns of the used Ag3 PO4 and AW6/4 catalysts. Compared with Fig. 1, it could be found that the characteristic diffraction peaks of Ag3 PO4 and WO3 were still maintained in the XRD patterns of used catalysts. No characteristic peaks of other materials were detected, except a weak diffraction peak at 38.1◦ corresponding to metallic Ag in the used Ag3 PO4 catalyst. The SEM characterization results of used catalysts are shown in Fig. 8, which
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Fig. 5. Schematic diagram for the enhanced activity and stability of Ag3 PO4 /WO3 photocatalysts.
Fig. 7. Powder XRD patterns of used Ag3 PO4 (a) and AW6/4 (b) catalysts.
Fig. 6. Recycling tests of Ag3 PO4 and AW6/4 for the degradation of RhB (a) and MO (b) under visible light irradiation ( > 420 nm).
indicated that both the catalysts Ag3 PO4 and AW6/4 showed no obvious change in morphology after five runs. The XPS characterizations of the used Ag3 PO4 and AW6/4 catalysts were carried out and the full XPS spectra are depicted in Fig. 9. The peaks of Ag3d, O1s, and P2p were detected in the XPS spectrum of used Ag3 PO4 catalyst. Besides these, an additional peak of W4f was presented in the XPS spectrum of used AW6/4 catalyst. Fig. 10 shows the Ag3d XPS spectra of used Ag3 PO4 and AW6/4 catalysts. The peaks near 368 and 374 eV were attributed to Ag3d5/2 and Ag3d3/2 , respectively, each of which could be fitted to two
separate peaks corresponding to Ag0 and Ag+ ions [27]. As shown in Fig. 10, the peaks at 374.6 eV (374.7 eV for AW6/4 ) and 368.5 eV (368.6 eV for AW6/4 ) could be attributed to Ag0 [46], and the peaks at 373.8 eV (374.2 eV for AW6/4 ) and 367.8 eV (368.2 eV for AW6/4 ) could be assigned to Ag+ ions [47]. The slight peak shift was mainly attributed to the interaction between WO3 and Ag3 PO4 (or Ag) [48]. The calculated mole content of Ag0 was 42.6% of total silver element on the surface of Ag3 PO4 catalyst after five runs. While, only 14.5% of Ag0 was found on the surface of AW6/4 catalyst after five runs. That means, much less metallic Ag was formed on the surface of Ag3 PO4 /WO3 catalyst during the photodegradation process than that on Ag3 PO4 catalyst, which was consistent with the results of XRD characterization of used catalysts (Fig. 7). As reported in the literature [15], the formation of metallic Ag on the Ag3 PO4 surface would lead to the decrease of catalytic activity. When the Ag3 PO4 /WO3 catalysts were irradiated by visible light, the electrons and holes were generated on the surface of photocatalyst. Because of the special band gap structure of Ag3 PO4 /WO3 composites, the photogenerated electrons could move from the CB of Ag3 PO4 to that of WO3 , instead of be captured by Ag+ ions in Ag3 PO4 to produce metallic Ag. Thus, Ag3 PO4 was protected to a large extent in the Ag3 PO4 /WO3 catalyst and the stability of such photocatalyst was notably improved compared with single Ag3 PO4 .
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Fig. 8. SEM micrographs of used Ag3 PO4 (a) and AW6/4 (b) catalysts.
Fig. 10. High-resolution Ag3d XPS spectra of used Ag3 PO4 (a) and AW6/4 (b) catalysts.
4. Conclusions A series of Ag3 PO4 /WO3 composites were prepared with different Ag3 PO4 :WO3 mass ratios and used as photocatalysts for the visible light ( > 420 nm) degradation of RhB and MO. These Ag3 PO4 /WO3 photocatalysts exhibited more excellent activity than single Ag3 PO4 and WO3 catalyst. The enhanced catalytic activity over Ag3 PO4 /WO3 composites was mainly attributed to the highly effective electron–hole separation. Among all these composite catalysts, AW6/4 with the Ag3 PO4 :WO3 mass ratio of 6:4 showed the highest catalytic activity and the degradation rate of RhB and MO could reached up to 97% under visible light irradiation for 6 min and 35 min, respectively. Furthermore, because of the special pathway of electron transfer, Ag3 PO4 was effectively protected in Ag3 PO4 /WO3 composites and thus their stability was significantly improved. Acknowledgements
Fig. 9. Full XPS spectra of used Ag3 PO4 (a) and AW6/4 (b) catalysts.
This work was supported by the National High Technology Research and Development Program of China (Grant No. 2012AA063008), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20100031120029), and the National Natural Science Foundation of China (Grant No. 21203102).
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Highly effective and stable Ag3 PO4 /WO3 photocatalysts for visible light degradation of organic dyes Jiqiao Zhang a,b , Kai Yu a,∗ , Yifei Yu b , Lan-Lan Lou b , Zequn Yang a , Junwei Yang a , Shuangxi Liu b,c,∗∗ a
College of Environmental Science and Engineering, Nankai University, Tianjin 300071, People’s Republic of China Institute of New Catalytic Materials Science and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China c Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 14 February 2014 Received in revised form 3 April 2014 Accepted 4 April 2014 Available online 19 April 2014 Keywords: Ag3 PO4 /WO3 composites Visible light photocatalysis Organic dye degradation Improved activity and stability
a b s t r a c t The Ag3 PO4 /WO3 composites were prepared through a deposition–precipitation method and characterized by XRD, SEM, and DR UV–vis. These photocatalysts were evaluated in the degradation of rhodamine B (RhB) and methyl orange (MO) under visible light irradiation ( > 420 nm), and the synergistic effect of Ag3 PO4 and WO3 was confirmed by notably higher photocatalytic activity compared to pure Ag3 PO4 and WO3 catalysts. The effect of Ag3 PO4 :WO3 ratio on the catalytic activity was systemically studied and the catalyst AW6/4 was found to exhibit the highest catalytic activity. The degradation rates of RhB and MO could reach up to 97% under visible light irradiation for 6 min and 35 min, respectively. Moreover, the Ag3 PO4 /WO3 photocatalysts showed higher recyclability than pure Ag3 PO4 catalyst and could be recycled five runs in the degradation of RhB without any loss in the activity. The characterization of used catalysts proved that Ag3 PO4 was effectively protected and much less metallic Ag was formed on the surface of Ag3 PO4 /WO3 catalyst. The improvement of photocatalytic activity and stability is mainly attributed to the highly effective separation of photogenerated electron–hole pairs and special transfer pathway of electrons and holes in Ag3 PO4 /WO3 composites. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The visible light photocatalytic degradation of organic pollutants over semiconductors shows great promise for environmental remediation, which could completely decompose environmental pollutions to carbon dioxide and water through utilizing solar energy. Many attempts have been made to prepare excellent visible light photocatalysts with high catalytic activity and stability. Recently, Ye and co-workers [1–6] reported an efficient photocatalyst, silver orthophosphate (Ag3 PO4 ), that can efficiently oxidize water to release oxygen [7] as well as degrade organic contaminants [8–10] under visible light irradiation. It has a suitable band gap of 2.45 eV and can achieve 90% quantum efficiencies at
∗ Corresponding author. Tel.: +86 22 23509005; fax: +86 22 23509005. ∗∗ Corresponding author at: College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China. Tel.: +86 22 23509005; fax: +86 22 23509005. E-mail addresses: [email protected] (K. Yu), [email protected] (S. Liu). http://dx.doi.org/10.1016/j.molcata.2014.04.010 1381-1169/© 2014 Elsevier B.V. All rights reserved.
wavelengths greater than 420 nm, which is significantly higher than that of other semiconductors reported in the literature. However, Ag3 PO4 catalyst was expensive and always suffered from stability issue, which limited their practical application. And therefore, many researches have attempted to improve the activity and stability of Ag3 PO4 through surface modification [11–16] and Ag3 PO4 supporting [17–28]. Among these studies, the heterojunction catalysts intermixing Ag3 PO4 with other semiconductors have received much attention. Yao et al. [22] and Rawal et al. [23] reported the Ag3 PO4 /TiO2 composites and used them as catalysts for the photodecomposition of azo dyes and isopropyl alcohol under visible light irradiation, respectively. Guo et al. [24] prepared an Ag3 PO4 /Cr-SrTiO3 heterojunction photocatalyst that exhibited improved efficiency in isopropyl alcohol photodegradation compared with pure Ag3 PO4 and Cr-SrTiO3 . Kumar et al. [25] reported a g-C3 N4 -Ag3 PO4 organic–inorganic hybrid nanocomposite photocatalyst and studied the photocatalytic activity for methyl orange degradation under visible light irradiation. Other heterojunction photocatalysts such as Ag3 PO4 /Bi2 MoO6 [26], Ag3 PO4 /BiPO4 [27], and Ag3 PO4 /AgBr [28] were also prepared and these composites
J. Zhang et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 12–18
exhibited improved activity for the photodegradation of organic dyes. All the above-mentioned composite photocatalysts exhibited improved activity than pure Ag3 PO4 catalyst due to the effective separation of photogenerated electron–hole pairs. However, the redox potentials of conduction band (CB) and valence band (VB) of TiO2 , Cr-SrTiO3 , g-C3 N4 , Bi2 MoO6 , BiPO4 and AgBr are more negative than those of Ag3 PO4 . It means that under visible light irradiation the photogenerated electrons could move to the CB of Ag3 PO4 and photogenerated holes could transfer to the VB of TiO2 , Cr-SrTiO3 , g-C3 N4 , Bi2 MoO6 , BiPO4 and AgBr. The enriched electrons on the surface of Ag3 PO4 would be captured by Ag+ ions in Ag3 PO4 (Ag+ + e− → Ag0 , E0 = +0.80 V vs. NHE) [29,30] instead of combined with O2 , then the metallic Ag would be produced after photoreaction and Ag3 PO4 was corroded. Tungsten trioxide (WO3 ), as a semiconductor material with an optical band gap of 2.7 eV, has been widely used in the fields of photocatalysis [31–33], electronic device [34,35], gas sensor [36–38] and rechargeable lithium-ion battery [39,40]. Especially, the redox potentials (CB and VB) of WO3 are more positive than those of Ag3 PO4 . Therefore, the photogenerated electrons could move from the CB of Ag3 PO4 to that of WO3 under visible light irradiation if Ag3 PO4 and WO3 are combined as a photocatalyst, which would avoid the formation of metallic Ag on the surface of Ag3 PO4 , and thus increase the stability of Ag3 PO4 . At the same time, the enhanced photocatalytic activity could be expected using combined Ag3 PO4 /WO3 catalyst due to the effective separation of photogenerated electron–hole pairs. So, we wish to report here the synthesis, characterization and photocatalytic applications of Ag3 PO4 /WO3 composite catalysts. A facile deposition–precipitation method was applied to synthesize a series of Ag3 PO4 /WO3 composites with different mass ratios of Ag3 PO4 to WO3 . The effect of Ag3 PO4 :WO3 ratio on the catalytic activity was systemically studied. Furthermore, the advantages of these photocatalysts were demonstrated by the higher photocatalytic activities and better stability in the visible light degradation of rhodamine B (RhB) and methyl orange (MO), in comparison to pure Ag3 PO4 and WO3 catalysts.
2. Experimental 2.1. General Sodium tungstate dihydrate (Na2 WO4 ·2H2 O), ammonium chloride (NH4 Cl), silver nitrate (AgNO3 ), disodium hydrogen orthophosphate (Na2 HPO4 ), RhB and MO that used in this study were analytical grade and purchased from Tianjin Heowns Biochemical Technology Co., Ltd. N-doped TiO2 (N-TiO2 ) was synthesized by wet method using urea and titanium tetraisopropoxide, as described by Guo et al. [41]. All solutions throughout the experiments were prepared with distilled water. Powder X-ray diffraction (XRD) patterns of the catalysts were recorded on a Bruker D8 Focus diffractometer with Cu K␣ radiation at a scanning speed of 0.01◦ /s in a scan range of 10◦ < 2 < 80◦ operating at 40 kV and 40 mA. Diffuse-reflectance UV–vis (DR UV–vis) characterization was carried out on a Shimadzu UV-2550 UV–vis spectrophotometer with an integrating sphere attachment in the range of 220–800 nm. Scanning electron microscopy (SEM) images were obtained on a JEOL JSM-7500F field-emission scanning election microscope at an accelerating voltage of 5 kV. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Kratos Axis Ultra DLD with a Al K␣ X-ray source. The concentrations of RhB and MO in water were measured by a Shimadzu UV2550 UV–vis spectrophotometer at the wavelength of 552 nm and 463 nm, respectively.
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Table 1 The composition of reaction solution for the preparation of Ag3 PO4 /WO3 composites. Ag3 PO4 /WO3 composites
Amount of WO3 (g)
Volume of AgNO3 solution (mL)
Volume of Na2 HPO4 solution (mL)
AW2/8 AW4/6 AW6/4 AW8/2
0.0328 0.0246 0.0164 0.0082
2.5 5.0 7.5 10.0
1.0 2.0 3.0 4.0
2.2. Synthesis of WO3 WO3 was synthesized by a simple hydrothermal process. In brief, 0.003 mol of Na2 WO4 ·2H2 O was dissolved in 60 mL of distilled water and 0.01 mol of NH4 Cl was dissolved in 20 mL of distilled water, then the above two solutions were mixed in ultrasonic water bath for 1 h. The obtained solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and kept at 180 ◦ C for 48 h. After cooling down to room temperature, the precipitate was harvested by filtration, washed with distilled water, and dried in a vacuum oven at 65 ◦ C for 12 h. Then the sample was crystallized at 500 ◦ C for 2 h and then WO3 was obtained.
2.3. Preparation of Ag3 PO4 /WO3 composite photocatalysts The Ag3 PO4 /WO3 composite photocatalysts with different mass ratios of Ag3 PO4 to WO3 were prepared by a deposition–precipitation process. Firstly, WO3 was dispersed in distilled water, and then the required amount of AgNO3 solution (0.0235 M) and Na2 HPO4 solution (0.15 M) were successively dropped into this suspension under stirring, in which the volume of distilled water was kept constant at 20 mL. Table 1 lists the composition of various reaction solutions. The obtained precipitate was washed with distilled water for several times and dried in vacuum at 65 ◦ C for 12 h. According to the mass ratio of Ag3 PO4 to WO3 , the as-synthesized Ag3 PO4 /WO3 photocatalysts were marked as AW2/8 , AW4/6 , AW6/4 , and AW8/2 , respectively. For comparison, a Ag3 PO4 catalyst was prepared through an ionexchange method as follows. 7.5 mL of AgNO3 solution (0.0235 M) was dispersed in 9.5 mL of distilled water and 3.0 mL of Na2 HPO4 solution (0.15 M) was added drop by drop to the solution under stirring, then golden yellow precipitate was formed instantly. The obtained precipitate was washed with distilled water for several times and dried in vacuum at 65 ◦ C for 12 h.
2.4. Photodegradation of organic dyes The photodegradation experiments were conducted in a photochemical reactor, which is configured with a light source system (500 W Xenon lamp), a magnetic stirrer and 12 quartz glass test tubes. The Xenon lamp was cooled by a water jacketed condenser and equipped with an ultraviolet cutoff filter to provide visible light with > 420 nm. In a typical photodegradation process, 0.04 g of photocatalyst was dispersed in 30 mL of RhB or MO aqueous solution with an initial concentration of 5 g/mL. The suspension was stirred in dark for 30 min to establish adsorption–desorption equilibrium, and then it was exposed to Xenon lamp irradiation under stirring. In all the photodegradation experiments, the parameters, such as exposure to a constant photo flux, stirring rate and initial concentration were kept constant. After the photodegradation, the catalysts were separated from the reaction solutions by centrifugation and the concentrations of RhB and MO were determined by UV–vis spectrophotometer.
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Fig. 1. Powder XRD patterns of as-prepared WO3 , Ag3 PO4 and Ag3 PO4 /WO3 photocatalysts.
Fig. 2. DR UV–vis spectra of as-prepared WO3 , Ag3 PO4 and Ag3 PO4 /WO3 photocatalysts.
3. Results and discussion
(2 1 0) and (2 1 1), increased evidently, while the intensities of WO3 diffraction peaks gradually decreased. The DR UV–vis spectra of obtained WO3 , Ag3 PO4 and Ag3 PO4 /WO3 composites are shown in Fig. 2. It could be found that the optical absorption edge was estimated to be ∼460 nm for WO3 and ∼510 nm for Ag3 PO4 , respectively. Moreover, with the increase of Ag3 PO4 content, the absorption edge of Ag3 PO4 /WO3 composite gradually moved to longer wavelength. The morphologies of the as-synthesized photocatalysts were examined by SEM and the typical SEM images are shown in Fig. 3. It could be seen from Fig. 3(a) and (b) that the obtained Ag3 PO4 was spherical-like with a particle size of 200–500 nm, while WO3 had a rod-like crystal structure with a length of 1–3 m and a diameter of ∼250 nm. For the Ag3 PO4 /WO3 samples, as shown in Fig. 3(c)–(f), the grain-like Ag3 PO4 particles with a diameter of 200–500 nm covered the surface of rod-like WO3 . With growing content of Ag3 PO4 in the composite samples, more and more Ag3 PO4 particles existed on the surface of WO3 and these particles were partly
3.1. Characterization of the photocatalysts The phase purity and crystal structure of as-synthesized WO3 , Ag3 PO4 and Ag3 PO4 /WO3 composites were confirmed by XRD characterization. As shown in Fig. 1, the characteristic diffraction peaks of (1 1 0), (2 0 0), (2 1 0), (2 1 1), (3 1 0), (2 2 2), (3 2 0), (3 2 1), (4 0 0), (4 2 0) and (4 2 1) can be perfectly indexed as the body-centered cubic structure of Ag3 PO4 (JCPDS No. 06-0505). Moreover, the diffraction peaks of (0 0 1), (0 2 0), (2 0 0), (1 2 0), (1 1 1), (2 0 1), (2 2 0), (1 2 1), (2 2 1), (0 0 2), (0 4 0), (1 4 0) and (4 2 0) can be attributed to the orthorhombic phase WO3 (JCPDS No. 201324). There were no additional diffraction peaks observed in the XRD patterns of Ag3 PO4 /WO3 composites except for the characteristic peaks of Ag3 PO4 and WO3 . It also could be observed in Fig. 1, with the increase of Ag3 PO4 content in Ag3 PO4 /WO3 composites, the intensities of Ag3 PO4 diffraction peaks, such as (1 1 0), (2 0 0),
Fig. 3. SEM micrographs of as-prepared Ag3 PO4 (a), WO3 (b) and Ag3 PO4 /WO3 composites (c–f).
J. Zhang et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 12–18
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gap of 2.7 eV [44,45]) and Ag3 PO4 (with an optical band gap of 2.45 eV [1]) were combined. Under visible light irradiation, the electron–hole pairs were generated on the surface of Ag3 PO4 and WO3 . Because the redox potentials of CB and VB of WO3 are more positive than those of Ag3 PO4 , the photogenerated electrons could move from the CB of Ag3 PO4 to that of WO3 , and the photogenerated holes could move from the VB of WO3 to that of Ag3 PO4 . The holes could directly oxidize the organic dyes adsorbed on Ag3 PO4 surface [10,25,26] and the electrons could be consumed through a multi-electron reaction with oxygen (O2 + 2H+ + 2e− → H2 O2 , E0 = +0.682 V vs. NHE) [23]. The produced H2 O2 reacts with additional electron to produce • OH (H2 O2 + e− → OH− + • OH) [43], which could accelerate the degradation of organic dyes. Moreover, the transfer of photogenerated electrons and holes in the Ag3 PO4 /WO3 composites could prevent the recombination of electron–hole pairs, which also led to the improvement of photocatalytic activity. It could be also noticed from Fig. 4(a) that among these Ag3 PO4 /WO3 composites, the catalyst AW6/4 exhibited the highest catalytic activity for RhB degradation. When AW6/4 was used as photocatalyst, the RhB degradation rate could reach up to 97% under visible light irradiation for 6 min. While only 72%, 83% and 87% of RhB could be degraded over the other catalysts AW2/8 , AW4/6 , and AW8/2 , respectively, under the same reaction conditions. These results indicated that more remarkable synergistic effect was exhibited when the mass ratio of Ag3 PO4 :WO3 was 6:4. Moreover, the catalyst AW6/4 also exhibited improved activity for the photodegradation of MO, as shown in Fig. 4(b). Notable higher degradation rate of MO could be obtained catalyzed by AW6/4 compared with pure Ag3 PO4 and WO3 catalysts. 3.3. Photocatalytic stability of the Ag3 PO4 /WO3 composites Fig. 4. Photocatalytic degradation of RhB (a) and MO (b) under visible light irradiation ( > 420 nm).
agglomerated. When the Ag3 PO4 :WO3 mass ratio was 8:2, most of the WO3 crystal surfaces were covered by Ag3 PO4 particles. 3.2. Visible light photocatalytic activity of the Ag3 PO4 /WO3 composites The as-synthesized WO3 , Ag3 PO4 , and Ag3 PO4 /WO3 composites were used as photocatalysts for the decomposition of organic dye RhB and MO under visible light irradiation. Fig. 4(a) shows the degradation curves of RhB catalyzed by Ag3 PO4 /WO3 composites with different mass ratio of Ag3 PO4 to WO3 . It could be found that the Ag3 PO4 /WO3 composites exhibited excellent activities for RhB degradation, which were notably higher than those of pure Ag3 PO4 , WO3 and N-TiO2 catalysts under the same reaction conditions. After 10 min of visible light irradiation, nearly 100% of RhB was decomposed in the presence of Ag3 PO4 /WO3 photocatalysts. However, only 54% and 15% of RhB could be removed within the same time using pure Ag3 PO4 and WO3 as catalyst, respectively. The significantly lower catalytic activity of WO3 was mainly attributed to the more positive CB potential of WO3 compared with that of the single-electron reduction of O2 to • O2 − [42,43], as well as its absorption edge (∼460 nm) which would lead to the less absorption of visible light. While for Ag3 PO4 catalyst, the slightly lower activity can be ascribed to the relatively lower catalyst dosage in this study compared with the literature [2]. The notably enhanced photocatalytic performances of Ag3 PO4 /WO3 composites are mainly attributed to the highly effective separation of photogenerated electron–hole pairs. As shown in Fig. 5, the semiconductors of WO3 (with an optical band
The stability is an important issue for Ag3 PO4 catalyst. So the recyclability of as-synthesized Ag3 PO4 and AW6/4 catalysts were investigated in this section. After each photodegradation cycle, the photocatalyst was collected by centrifugation and reused in the following run. Fig. 6(a) shows the degradation curves of RhB in five consecutive applications with Ag3 PO4 and AW6/4 catalysts. As shown in Fig. 6(a), the catalyst AW6/4 exhibited excellent recyclability for the visible light degradation of RhB and no obvious decrease in catalytic activity was found after five runs. At the fifth run, the degradation rate of RhB was still up to 97% over AW6/4 under visible light irradiation for 6 min, which was the same as in the first run. However, the stability of Ag3 PO4 was found to be lower than that of AW6/4 , and the catalytic activity was notably decreased after the second run due to the absence of sacrificial reagent (such as AgNO3 [1]). Only 25% degradation rate of RhB was obtained over Ag3 PO4 catalyst for the fifth run. Fig. 6(b) shows the degradation curves of MO in three consecutive applications with Ag3 PO4 and AW6/4 catalysts. It could be found that the catalytic activity of AW6/4 was maintained absolutely throughout the three runs of MO degradation, however, the activity of Ag3 PO4 obviously decreased after the first run. These results indicated that the stability of Ag3 PO4 /WO3 composite was obviously improved compared with pure Ag3 PO4 catalyst. To exposit the reasons for the difference of catalytic stability between Ag3 PO4 and AW6/4 , the two catalysts used five runs in the degradation of RhB were characterized by XRD, SEM and XPS. Fig. 7 shows the XRD patterns of the used Ag3 PO4 and AW6/4 catalysts. Compared with Fig. 1, it could be found that the characteristic diffraction peaks of Ag3 PO4 and WO3 were still maintained in the XRD patterns of used catalysts. No characteristic peaks of other materials were detected, except a weak diffraction peak at 38.1◦ corresponding to metallic Ag in the used Ag3 PO4 catalyst. The SEM characterization results of used catalysts are shown in Fig. 8, which
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Fig. 5. Schematic diagram for the enhanced activity and stability of Ag3 PO4 /WO3 photocatalysts.
Fig. 7. Powder XRD patterns of used Ag3 PO4 (a) and AW6/4 (b) catalysts.
Fig. 6. Recycling tests of Ag3 PO4 and AW6/4 for the degradation of RhB (a) and MO (b) under visible light irradiation ( > 420 nm).
indicated that both the catalysts Ag3 PO4 and AW6/4 showed no obvious change in morphology after five runs. The XPS characterizations of the used Ag3 PO4 and AW6/4 catalysts were carried out and the full XPS spectra are depicted in Fig. 9. The peaks of Ag3d, O1s, and P2p were detected in the XPS spectrum of used Ag3 PO4 catalyst. Besides these, an additional peak of W4f was presented in the XPS spectrum of used AW6/4 catalyst. Fig. 10 shows the Ag3d XPS spectra of used Ag3 PO4 and AW6/4 catalysts. The peaks near 368 and 374 eV were attributed to Ag3d5/2 and Ag3d3/2 , respectively, each of which could be fitted to two
separate peaks corresponding to Ag0 and Ag+ ions [27]. As shown in Fig. 10, the peaks at 374.6 eV (374.7 eV for AW6/4 ) and 368.5 eV (368.6 eV for AW6/4 ) could be attributed to Ag0 [46], and the peaks at 373.8 eV (374.2 eV for AW6/4 ) and 367.8 eV (368.2 eV for AW6/4 ) could be assigned to Ag+ ions [47]. The slight peak shift was mainly attributed to the interaction between WO3 and Ag3 PO4 (or Ag) [48]. The calculated mole content of Ag0 was 42.6% of total silver element on the surface of Ag3 PO4 catalyst after five runs. While, only 14.5% of Ag0 was found on the surface of AW6/4 catalyst after five runs. That means, much less metallic Ag was formed on the surface of Ag3 PO4 /WO3 catalyst during the photodegradation process than that on Ag3 PO4 catalyst, which was consistent with the results of XRD characterization of used catalysts (Fig. 7). As reported in the literature [15], the formation of metallic Ag on the Ag3 PO4 surface would lead to the decrease of catalytic activity. When the Ag3 PO4 /WO3 catalysts were irradiated by visible light, the electrons and holes were generated on the surface of photocatalyst. Because of the special band gap structure of Ag3 PO4 /WO3 composites, the photogenerated electrons could move from the CB of Ag3 PO4 to that of WO3 , instead of be captured by Ag+ ions in Ag3 PO4 to produce metallic Ag. Thus, Ag3 PO4 was protected to a large extent in the Ag3 PO4 /WO3 catalyst and the stability of such photocatalyst was notably improved compared with single Ag3 PO4 .
J. Zhang et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 12–18
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Fig. 8. SEM micrographs of used Ag3 PO4 (a) and AW6/4 (b) catalysts.
Fig. 10. High-resolution Ag3d XPS spectra of used Ag3 PO4 (a) and AW6/4 (b) catalysts.
4. Conclusions A series of Ag3 PO4 /WO3 composites were prepared with different Ag3 PO4 :WO3 mass ratios and used as photocatalysts for the visible light ( > 420 nm) degradation of RhB and MO. These Ag3 PO4 /WO3 photocatalysts exhibited more excellent activity than single Ag3 PO4 and WO3 catalyst. The enhanced catalytic activity over Ag3 PO4 /WO3 composites was mainly attributed to the highly effective electron–hole separation. Among all these composite catalysts, AW6/4 with the Ag3 PO4 :WO3 mass ratio of 6:4 showed the highest catalytic activity and the degradation rate of RhB and MO could reached up to 97% under visible light irradiation for 6 min and 35 min, respectively. Furthermore, because of the special pathway of electron transfer, Ag3 PO4 was effectively protected in Ag3 PO4 /WO3 composites and thus their stability was significantly improved. Acknowledgements
Fig. 9. Full XPS spectra of used Ag3 PO4 (a) and AW6/4 (b) catalysts.
This work was supported by the National High Technology Research and Development Program of China (Grant No. 2012AA063008), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20100031120029), and the National Natural Science Foundation of China (Grant No. 21203102).
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