dimensionallyorderedmacroporousWO3 modified Ag3PO4

CERAMICS

INTERNATIONAL

Available online at https://www.360docs.net/doc/8a690545.html,

Ceramics International 42(2016)1392–

1398

Three-dimensionally ordered macroporous WO 3modi ?ed Ag 3PO 4

with enhanced visible light photocatalytic performance

Hui Xu a ,n ,Haozhu Zhao a ,Yuanguo Xu a ,Zhigang Chen a ,Liying Huang a ,Yeping Li a ,

Yanhua Song b ,Qi Zhang c ,Huaming Li a ,n

Institute for Energy Research,School of the Environment,Jiangsu University,Zhenjiang 212013,PR China

School of Environmental and Chemical,Engineering,Jiangsu University of Science and Technology,Zhenjiang 212003,PR China

Hainan Provincial Key Lab of Fine Chemistry,Hainan University,Haikou,Hainan 570228,PR China

Received 12August 2015;received in revised form 12September 2015;accepted 12September 2015

Available online 21September 2015

Abstract

A series of three-dimensionally ordered macroporous WO 3(3DOM WO 3)hybridized Ag 3PO 4photocatalysts with different 3DOM WO 3contents were prepared.The composites were characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM),transmission electron microscopy (TEM),X-ray photoelectron spectroscopy (XPS),UV –visible absorption spectra (UV –vis)and Fourier transform infrared (FT-IR)spectra.Under visible light irradiation,the as-prepared 3DOM WO 3–Ag 3PO 4composites showed enhanced photocatalytic performance for methylene blue (MB)degradation.The sample with 3DOM WO 3–Ag 3PO 4(4wt%)exhibited the highest photocatalytic activity.The enhanced photocatalytic performance under visible light irradiation could be due to a synergistic effect between 3DOM WO 3and Ag 3PO 4.After the introduction of 3DOM WO 3,the separation ef ?ciency of the photogenerated electron-hole pairs could be increased.&2015Elsevier Ltd and Techna Group S.r.l.All rights reserved.

Keywords:DOM WO 3–Ag 3PO 4;Photocatalytic;Visible light irradiation

1.Introduction

Photocatalytic technology has been widely used in the ?eld of environmental pollutant treatment and energy conversion.The key of photocatalysis research is to design and develop materials with high performance and high stability.Therefore,the development of photocatalytic material with speci ?c structure and nano-size is very important [1].

In recent years,three-dimensionally ordered macroporous (3DOM)materials,with uniform pore size and well-de ?ned periodic structure,have attracted many researchers'attention [2–4].3DOM materials possess relatively large surface areas,high thermal stability and good photocatlytic performance,and the increasing light harvesting ef ?ciency by 3DOM structure materials has been con ?rmed by literatures [5–7].The unique

ordered structure can interact with the light to enhance the light conversion ef ?ciency and facilitate the transfer of reactant molecules [8].Hence,there are many 3DOM semiconductor photocatalysts which have been developed and synthesized,including BiVO 4[9],InVO 4[7,10].However,the 3DOM-based composite photocatalysts have been rarely reported till now.Ag 3PO 4shows a good photocatalytic performance for water splitting and organic pollutant degradation [11–14].However,its stability always has a problem which is dif ?cult to solve.To the best of our knowledge,there have been no reports on the fabrication and photocatalytic applications of three-dimensionally ordered macroporous WO 3-supported Ag 3PO 4for the degrada-tion of organic pollutants under visible-light illumination.

Herein,a variety of 3DOM WO 3–Ag 3PO 4composites were synthesized.The 3DOM WO 3–Ag 3PO 4composites were con ?rmed to possess high photocatalytic performance under visible light irradiation,which was attributed to ef ?cient separation of electrons and holes.Moreover,the possible

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Corresponding authors.Tel.:t8651188791800;fax:t8651188791708.E-mail addresses:xh@https://www.360docs.net/doc/8a690545.html, (H.Xu),lihm@https://www.360docs.net/doc/8a690545.html, (H.Li).

mechanism for the enhanced photocatalytic activity of 3DOM WO 3–Ag 3PO 4photocatalysts was also discussed.2.Experimental 2.1.Sample fabrication

2.1.1.Synthesis of 3DOM WO 3

The polymethyl methacrylate (PMMA)colloidal crystal microspheres were synthesized by the method reported pre-viously [15].The 3DOM WO 3material was synthesized using PMMA-templating strategy. 1.0g of (NH 4)6H 2W 12O 40was dissolved in 30mL H 2O and methanol solution.1.0g of the PMMA hard template was added in the precursor solution and soaked for 8h.After ?ltration and one drying procedure,the solid was calcined at a ramp of 11C/min from RT to 3001C and held for 3h to remove the PMMA https://www.360docs.net/doc/8a690545.html,ter,the calcination temperature was continuously raised up to 5001C and held at this temperature for 2h.Then,the 3DOM WO 3could be obtained.

2.1.2.Synthesis of Ag 3PO 4material

In a typical synthesis,Ag 3PO 4material was prepared by the conventional ion exchange/precipitation method.AgNO 3(0.34g)was completely dissolved in distilled water.Na 3PO 4á12H 2O (0.25g)solution was added to the above solution.Then the resulting solution was stirred for 1h.The ?nal products were collected by centrifugation,washed with distilled water and absolute ethanol several times and then dried for 6h.

2.1.

3.Synthesis of 3DOM WO 3–Ag 3PO 4photocatalysts

3DOM WO 3–Ag 3PO 4photocatalysts were prepared as follows:0.34g of AgNO 3and the calculated content of 3DOM WO 3were dispersed in 20mL of distilled water and sonicated for 30min.Then,Na 3PO 4á12H 2O (0.25g)solution was added drop by drop to the obtained mixture at 601C with magnetical stirring for 0.5h.Finally,the washed precipitates were dried at 601C for 6h.The 3DOM WO 3–Ag 3PO 4with different 3DOM WO 3mass ratios of 1wt%,2wt%,4wt%and 6wt%were prepared and were denoted as 3DOM WO 3–Ag 3PO 4(1wt%),3DOM WO 3–Ag 3PO 4(2wt%),3DOM WO 3–Ag 3PO 4(4wt%),3DOM WO 3–Ag 3PO 4(6wt%).The synthesis process of 3DOM WO 3–Ag 3PO 4composites is shown in Fig.1.

2.2.Characterization of photocatalysts

XRD patterns were recorded on an X-ray diffractometer (Bruker D8)using Cu Ka radiation (λ?1.5418?)and operating at the angle range 2θ?10–801.Fourier transform infrared (FT-IR)spectra were recorded on a Nicolet Avatar-370spectrometer at room temperature.Transmission electron microscopy (TEM)images were performed on a JEOL-JEM-2010with an acceleration voltage of 200kV.Field emission scanning electron microscopy (FE-SEM)images were acquired with a JEOL JSM-7001F.DRS measurements were carried out using a UV-2450UV –vis spectro-photometer equipped with an integrating sphere.The analysis ranged from 200to 800nm,and BaSO 4was used as a re ?ectance standard.X-ray photoelectron spectroscopy (XPS)analysis was performed on an ESCALab MKII X-ray photo-electron spectro-meter using the Mg K αradiation.2.3.Photocatalytic activity

The photocatalytic activity of the samples was evaluated by MB degradation under visible light irradiation.A 300W Xe-lamp with a 420nm cutoff ?lter was used as the light source.In each run,40mg of the photocatalyst was added to 80mL of MB solution (10mg L à1).Prior to irradiation,the suspension was magnetically stirred in the dark for 30min to achieve the absorption –desorption equilibrium.At certain time intervals,3mL aliquots were sampled and centrifuged to remove the photocatalysts.The concentration of MB was analyzed by recording the absorbance at the characteristic band of 664nm using a UV2450spectrophotometer.3.Results and discussion

3.1.Crystal phase composition

The crystalline structure and phase purity of the as-prepared samples were studied by XRD measurements.The XRD patterns of the as-prepared 3DOM WO 3and 3DOM WO 3–Ag 3PO 4composites were shown in Fig.2.As shown in Fig.2,the characteristic diffraction peaks of (001),(020),(200),(120),

(111),

Fig.1.The synthesis process of 3DOM WO 3–Ag 3PO 4composites.

10203040

5060

2θ (degree)

WO 3

WO 3-Ag 3PO 4 (6 wt%)WO 3-Ag 3PO 4 (4 wt%)WO 3-Ag 3PO 4 (2 wt%)Ag 3PO 4

WO 3-Ag 3PO 4 (1 wt %)I n t e n s i t y (a .u )

(110)

(420)

(140)

(221)

(121)(201)(220)

(111)(120)(020)(200)

(001)(332)

(421)(420)(330)(400)(321)

(320)(222)(310)

(220)

(211)

(210)(200)Fig.2.XRD patterns of the as-prepared composite samples.

H.Xu et al./Ceramics International 42(2016)1392–13981393

(201),(220),(121),(221),(140)and (420)could be attributed to orthorhombic phase WO 3(JCPDS no.20-1324).For 3DOM WO 3–Ag 3PO 4composites,it could be clearly seen that all the diffraction peaks of the samples were indexed to the cubic structure of Ag 3PO 4(JCPDS no.06-0505).However,no diffrac-tion peaks of 3DOM WO 3were observed when 3DOM WO 3contents changed from 1wt%to 6wt%.3.2.Morphology and surface area

Fig.3showed the SEM images of 3DOM WO 3and 3DOM WO 3–Ag 3PO 4composites.The 3DOM WO 3sample displayed hierarchically macroporous architecture (Fig.3A –C).As shown in Fig.3A –C,it could be found that the well-ordered structure appeared in as-synthesized 3DOM WO 3material.The average pore diameters of 3DOM WO 3were about 90–110nm.As shown in Fig.3D,the Ag 3PO 4nanoparticles with a diameter range of

80–100nm were dispersed into the macropores of 3DOM WO 3.The EDS pattern (Fig.3E)further con ?rmed that 3DOM WO 3–Ag 3PO 4was composed of Ag,P,W and O elements.Fig.4showed the typical TEM images of the 3DOM WO 3and 3DOM WO 3–Ag 3PO 4composites.Obviously,the 3DOM structure maintained quite well even after loading Ag 3PO 4nanoparticles.The speci ?c surface areas of the samples were summarized in Table 1.It was found that the Ag 3PO 4sample showed the smallest (2.9m 2g à1),and the 3DOM WO 3sample showed the largest (10.1m 2g à1).For the 3DOM WO 3–Ag 3PO 4composites,it could be found that the BET surface areas were in the range of 3.8–6.1m 2g à1.3.3.XPS analysis

XPS was used to investigate the surface composition and the chemical status of 3DOM WO 3–Ag 3PO 4composites.Fig.5

100

150

200

250

I n t a n s t s ( a .u )

Energy (keV)

(E)

Fig.3.SEM images of 3DOM WO 3photocatalysts (A –C),3DOM WO 3–Ag 3PO 4(D)and EDS spectrum (E)of 3DOM WO 3–Ag 3PO 4.

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presented the over view of XPS spectrum of 3DOM WO 3–Ag 3PO 4(4wt%)composite.It could be found that the composite contained W,O,C,Ag and P elements,corresponding to W 4f,O 1s,C 1s,Ag 3d and P 2p,respectively.Fig.5B showed the main C 1s peak at 284.8eV,which was due to the adventitious hydrocarbon from the XPS instrument itself [16].As shown in Fig.5C,there were two peaks attributed to W 4f7/2and W4f5/2with values of 36.0eV and 38.2eV,respectively [17],which indicated that W in 3DOM WO 3–Ag 3PO 4composites had a valence of t6.In the Fig.5D,the high-resolution XPS spectrum showed the binding energies of the Ag 3d5/2and Ag 3d3/2peaks at 368.3eV and 374.2eV,respectively,which were typical values for Ag tin Ag 3PO 4material [18].The peaks at 132.9eV and 530.8eV could be assigned to the binding energies of P 2p (Fig.5E)and O 1s (Fig.5F)[19],respectively.These observations could further con ?rm the compositions of Ag 3PO 4and WO 3.

3.4.Optical properties and IR

The optical behavior of the samples was shown in Fig.6.It was observed that all samples showed strong absorption in the UV light range.For 3DOM WO 3–Ag 3PO 4materials,their absorption edge as well as the absorption intensity increased with enhanced mass ratios,which might be due to the photon effect of 3DOM WO 3.

FT-IR spectra of 3DOM WO 3–Ag 3PO 4samples were shown in Fig.7.It could be found that the FT-IR spectra were almost the same for all 3DOM WO 3–Ag 3PO 4composites with different 3DOM WO 3contents.The broad peak at 3000–3500cm à1was due to the stretching vibration of O –H of the adsorbed water molecules.For the 3DOM WO 3,only two IR peaks were shown.The peak at 770cm à1and 844cm à1could be the stretching vibration of W –O –W [20].In the case of 3DOM WO 3–Ag 3PO 4composites,the broad band located in 1400cm à1was related to P ?O band.Two absorption bands at 1015cm à1and 550cm à1

were always recognized as the typical breathing vibration of PO 43à

[19].It should be noted that the position of these characteristic peaks was the same as those of pure Ag 3PO 4.It could be also found that the peak intensity of WO 3–Ag 3PO 4composites decreased with the WO 3content (770cm à1).

3.5.Photocatalytic performance and proposed mechanism

The photocatalytic activities of 3DOM WO 3–Ag 3PO 4compo-sites with different 3DOM WO 3content were investigated under visible light irradiation after adsorption –desorption equilibration,as shown in Fig.8.It was obvious that photocatalytic activity was enhanced gradually with increasing mass fraction of 3DOM WO 3,which was due to the visible light response from 3DOM WO 3and the improved charge separation.It could be seen that the photocatalytic degradation ef ?ciency of MB was about 68.5%and 8.8%,for pure Ag 3PO 4and bulk 3DOM WO 3,respectively.Photocatalytic activity of the 3DOM WO 3–Ag 3PO 4composites was higher than that of the pure component,which suggested that there was a synergistic effect between 3DOM WO 3and Ag 3PO 4.It could be found that the optimal 3DOM WO 3content in WO 3–Ag 3PO 4composites was 4%by the molar ratio.When the 3DOM WO 3content increased 4wt%,the 3DOM WO 3–Ag 3PO 4composite exhibited the highest photocatalytic activity,and the photocatalytic degradation ef ?ciency was 83.2%.It was also seen that 3DOM WO 3–Ag 3PO 4(4wt%)catalyst exhibited much higher photocatalytic activity than the bulk WO 3–Ag 3PO 4(4wt%)material,which indicated that 3DOM WO 3possessed some advantages in the process of photocatalytic https://www.360docs.net/doc/8a690545.html,pared with pure WO 3,3DOM WO 3had a large BET speci ?c surface area,the increasing light harvesting ef ?ciency and the photon effect,which led to the enhanced photocatalytic activity of 3DOM WO 3–Ag 3PO 4.

Fig.9showed transient photocurrent responses of pure Ag 3PO 4and 3DOM WO 3–Ag 3PO 4(4wt%).It could be found that the photocurrent intensity of the 3DOM WO 3–Ag 3PO 4(4wt%)was almost 1.5times as that of pure Ag 3PO 4,suggesting that compared with the pure Ag 3PO 4,WO 3–Ag 3PO 4(4wt%)had a more ef ?cient separation and longer

lifetimes.

Fig. 4.TEM images of 3DOM WO 3(A)and 3DOM WO 3–Ag 3PO 4composites (B).

Table 1

The BET surface areas of 3DOM WO 3and 3DOM WO 3–Ag 3PO 4composites.Photocatalysts S BET (m 2g à1)Pure Ag 3PO 4 2.93DOM WO 3

10.13DOM WO 3–Ag 3PO 4(1wt%) 3.83DOM WO 3–Ag 3PO 4(2wt%) 4.63DOM WO 3–Ag 3PO 4(4wt%) 5.33DOM WO 3–Ag 3PO 4

(6wt%)

6.1

H.Xu et al./Ceramics International 42(2016)1392–13981395

280285290C1s

Binding energy (eV)I n t e n s i t y (a .u )

284.8 eV

2530354045

Binding energy (eV)I n t e n s i t y (a .u )

W4f

36.0 eV

38.2 eV

365370375380

374.2 eV

I n t e n s i t y (a .u )

Binding energy (eV)Ag3d 368.3 eV

120

125130135140

Binding energy (eV)

I n t e n s i t y (a .u )

P2p

132.9 eV

525

530535540

Binding energy (eV)

I n t e n s i t y (a .u )

O1s

530.8 eV 1000800600400

200

I n t e n s i t y (a .u )

A g 3p 1A g 3s

A g 3p 3O 1s

W 4p 1

W 4p s

A g 3d

C 1s

Binding energy (eV)W 4d

Fig.5.XPS spectra of 3DOM WO 3–Ag 3PO 4(4wt%)composite.(A)X-ray photoelectron survey spectra,(B)C 1s,(C)W 4f,(D)Ag 3d,(E)P 2p and (F)O 1s.

200

300

400

500600700800

0.2

0.40.60.81.01.2

1.4Wavelength (nm)

A d s o r b a n c e (a .u .)

Fig.6.DRS of Ag 3PO 4and 3DOM WO 3–Ag 3PO 4composites.

Wavenumber (cm -1)

T r a n s m i t t a n c e (%)

Fig.7.FT-IR spectra of 3DOM WO 3,Ag 3PO 4and 3DOM WO 3–Ag 3PO 4composites.

03691215

0.0

0.2

0.4

0.6

0.8

1.0

C /C 0

Irradiation time (min)

Fig.8.The photocatalytic activity of the photocatalysts under visible light irradiation.

700800900100011001200

Time (Sec)

P h o t o c u r r e n t (u A /c m -2)

Fig.9.Transient photocurrent responses of the samples.

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Fig.10revealed that the photocatalytic activity of 3DOM WO 3–Ag 3PO 4(4wt%)did not exhibit any signi ?cant loss after 4recycles and the degradation ef ?ciency of MB still reached over 80.1%under visible light irradiation.However,degradation ef ?ciency of MB by pure Ag 3PO 4reduced half after 1recycle and only reached over 11.4%after 4recycles.This could indicate that stability of Ag 3PO 4was tremendously

improved after the introduction of 3DOM WO 3for the pollutants degradation.The stability of fresh and used 3DOM WO 3–Ag 3PO 4(4wt%)material was also investigated by XRD and FT-IR analyses.As shown in Fig.10,the XRD and FT-IR patterns of the used 3DOM WO 3–Ag 3PO 4had no obvious change.

To further study the mechanism,the trapping experiments of radicals were https://www.360docs.net/doc/8a690545.html,ually,t-BuOH played the part of hydroxyl radical scavenger [21],and EDTA-2Na used as holes scavenger [22].As shown in Fig.11,when t-BuOH was added into the reaction system,the photocatalytic ef ?ciency had almost no change.However,the activity of the composite decreased signi ?cantly by the addition of the EDTA-2Na,indicating that holes were the main oxidative species.Therefore,a possible photocatalytic mechanism of 3DOM WO 3–Ag 3PO 4composites could be proposed.Under visible light irradiation,the electrons and holes could be generated on the surface of 3DOM WO 3and Ag 3PO 4.After combining 3DOM WO 3and Ag 3PO 4,the photo-generated electrons would transfer from the CB of Ag 3PO 4to WO 3,and the holes moved from the VB of 3DOM WO 3to Ag 3PO 4[23],enabling separation of electrons and holes.Therefore,organic contaminants could be directly oxidized by the photo-generated holes.4.Conclusion

In summary,3DOM WO 3–Ag 3PO 4composites exhibited the signi ?cantly enhanced photocatalytic activity under visible light irradiation.The photocatalytic activity of 3DOM WO 3–Ag 3PO 4(4wt%)was almost 1.5times higher than that of the pure Ag 3PO 4.The remarkable increase of photocatalytic activity was mainly ascribed to the synergistic effect between 3DOM WO 3and Ag 3PO 4.The experiment result indicated that holes were considered as the main reactive species during the photodegradation reaction process.Acknowledgments

This work was supported by the National Natural Science Foundation of China (21476097,21407065,21406094),Natural

100

2213

Number of Run

C /C 0

3DOM WO 3-Ag 3PO4

pure Ag 3PO 4

11020304050607080

I n t e n s i t y (a .u .)

2(degree)

4000350030002500200015001000500

the used 3DOM WO 33

-Ag PO 4

the used Ag PO 4

Wavenumber (cm -1)

T r a n s m i t t a n c e (%)

pure Ag PO 4

Fig.10.The comparison of photocatalytic degradation cycles of MB by pure Ag 3PO 4and 3DOM WO 3–Ag 3PO 4(4wt%)under visible light irradiation(A),the XRD patterns (B)and FT-IR patterns (C)of used recycled 3DOM WO 3–Ag 3PO 4(4wt%).

03691215

0.2

0.4

0.6

0.8

1.0

Irradiation time (min)

C /C 0

Fig.11.Effects of a series of scavengers on the degradation ef ?ciency of MB by 3DOM WO 3–Ag 3PO 4(4wt%)photocatalyst.

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Science Foundation of Jiangsu Province(BK20131207and BK20140533),Postdoctoral Foundation of China(2014M551520 and2013M541619)and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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