Synthesis and visible light photocatalytic properties of

Synthesis and visible light photocatalytic properties of iron oxide–silver orthophosphate composites Febiyanto, Irma Vania Eliani, Anung Riapanitra, and U. Sulaeman

Citation: AIP Conference Proceedings 1725, 020021 (2016); doi: 10.1063/1.4945475 View online: https://doi.org/10.1063/1.4945475 View Table of Contents: http://aip.scitation.org/toc/apc/1725/1 Published by the American Institute of Physics

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Synthesis and Visible light Photocatalytic Properties of Iron 2[LGH௅Silver Orthophosphate Composites Febiyanto, Irma Vania Eliani, Anung Riapanitra, U Sulaemana) Department of Chemistry, Jenderal Soedirman University, Purwokerto, 53123, Indonesia. a)

Corresponding author: [email protected]

Abstract. The iron oxide-silver orthophosphate composites were successfully synthesized by co-precipitation method using Fe(NO 3 ) 3 .9H 2 O, AgNO 3 , and Na 2 HPO 4 .12 H 2 O, followed by calcination at 500oC for 5 hours. The Fe/Ag mole ratios of iron oxide-silver orthophosphate composites were designed at 0, 0.1, 0.2, 0.3 and 0.4. The samples were characterized using X-ray Diffraction, Diffuse Reflectance Spectroscopy, Scanning Electron Microscopy and Specific Surface Area. The photocatalytic activities were evaluated using Rhodamine B degradation under visible light irradiation. The iron oxide-silver orthophosphate composite with the Fe/Ag mole ratio of 0.2 exhibited higher photocatalytic activity compared to the pure Ag 3 PO 4 under visible light irradiation. The enhanced photocatalytic activity could be attributed to the effective separation of hole (+) and electron pairs in the iron oxide-silver orthophosphate composite.

INTRODUCTION Recently, the silver phosphate synthesis methods have been remarkably developed to provide a highly reactive photocatalyst for organic pollutant degradation. Among the synthesis methods, the Ag 3 PO 4 composite is very challenging due to easily generating the high photocatalytic properties. The composites of Ag 3 PO 4 /TiO 2 [1], Ag 3 PO 4 /ZnO [2], Ag 3 PO 4 /Bi 2 O 3 [3] and Ag 3 PO 4 /Fe 2 O 3 [4] have been investigated and enhanced the photocatalytic activity. The composites of Ag 3 PO 4 /TiO 2 synthesized by sol-gel method significantly improved the photocatalytic activity due to the inWHU௅VHPLFRQGXFWRU KROH௅transfer between the valence band of Ag 3 PO 4 and TiO 2 [1]. The Ag 3 PO 4 /ZnO prepared by a facile ball milling method also increased the photocatalytic under visible light irradiation [2]. In this composite, the hydroxyl radicals produced in the VB of ZnO played a significant role in the photocatalytic activity under visible light irradiation, doping Ag 3 PO 4 in this composite improved the transfer of SKRWRH[FLWHG HOHFWURQ௅hole and enhanced the stability. Another composite, Ag 3 PO 4 /Bi 2 O 3 , synthesized by co௅ precipitation, followed by the calcination process, enhanced the photocatalytic activity under visible light irradiation due to the effective separation of hole and electron pairs [3]. Yan et al. [4] reported that the new composite of Ag 3 PO 4 /Fe 2 O 3 synthesized by solvothermal method showed a high activity under visible light irradiation. Among these composite materials, Ag 3 PO 4 /Fe 2 O 3 is very promising to be further developed for commercial product due to its low price. To support this issue, we have designed and improved the LURQ R[LGH௅silver orthophosphate composite photocatalyst XQGHU FR௅precipitation method followed by the calcination. This paper GLVFXVVHGWKHQHZ௅method of iron oxide-silver orthophosphate composite synthesis and photocatalytic mechanism for RhB decomposition. This composite showed the excellent photocatalytic activity under visible light irradiation.

EXPERIMENTAL The coPSRVLWH PDWHULDOV RI LURQ R[LGH௅silver orthophosphate were synthesized using starting material of Fe(NO 3 ) 3 .9H 2 O, AgNO 3 , and Na 2 HPO 4 .12 H 2 O. The Fe/Ag mole ratios of composites were designed at 0, 0.1, 0.2, 0.3 and 0.4. Typically, the five variations of solution (solution 1) were made by dissolving 0 g (control), 0.404 The 3rd International Conference on Advanced Materials Science and Technology (ICAMST 2015) AIP Conf. Proc. 1725, 020021-1–020021-6; doi: 10.1063/1.4945475 Published by AIP Publishing. 978-0-7354-1372-6/$30.00

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g, 0.808 g, 1.212 g, and 1.616 g Fe(NO 3 ) 3 .9H 2 O in 10 ml of deionized water, respectively. An amount of 1.699 gram of AgNO 3 was dissolved in 10 ml of deionized water (Solution 2) and separately, a 3.5814 g of Na 2 HPO 4 .12 H 2 O was dissolved in 20 of deionized water (Solution 3). The reaction was started by mixing the solution 1 with the solution 2 under magnetic stirring, then the solution 3 was added slowly (dropwise). The precipitates were filtered and washed with water three times. The variation of Fe/Ag samples were named as Ag 3 PO 4 )H௅$J)H௅$J )H௅$J  DQG )H௅Ag 0.4 respectively. The precipitates were dried in oven at 105°C for 7 h and followed by calcination at 500°C for 5 h in the furnace. The samples were characterized using X-ray Diffraction (XRD), Direct Reflectance Spectroscopy (DRS) and Scanning Electron Microscopy (SEM) and BET Specific Surface Area. The photocatalytic activities were evaluated using Rhodamine B (RhB) degradation rate [5]. An amount of 0.2 gram catalyst was mixed with 100 ml of 10 mg/L RhB solution and was stirred under dark condition for 20 minutes. Afterward, the solution was irradiated under blue light of LED lamp under the distance of 10 cm. The sampling (5 ml) was performed every 10 minutes up to 100 minutes irradiation. The sample was centrifuged 1 hour to separate the catalyst, and RhB concentration was measured using UV-Vis spectrophotometer. The stability of catalyst was examined by cycling the photocatalytic reaction up to 4 times. The photocatalytic mechanism of the highest activity was analyzed using WKHLVRSURSDQROS௅ benzoquinone and ammonium oxalate for trapping the hydroxyl (•OH) radical, super oxide radical (•O 2 -), and hole (h+), respectively [2]. The photocatalytic activities of hole/radical trapper treatment were evaluated after 80 minutes reaction and compared with the sample without hole/radical trapper.

RESULTS AND DISCUSSION Figure 1 showed the XRD patterns of Ag 3 PO 4 )H௅$J)H௅Ag 0.2, )H௅$JDQG)H௅Ag 0.4 samples. All peaks of Ag 3 PO 4 can be identified as a body center cubic structure (JCPDS no. 06-0505) [6], indicating that the Ag 3 PO 4 sample is pure and no impurity obsHUYHG ZKHUHDVWKH VDPSOHRI)H௅$J)H௅Ag 0.3DQG )H௅$J exhibit the mixture of Ag 3 PO 4 and iron oxides. The Į-Fe 2 O 3 DQGȖ-Fe 2 O 3 could be observed in the composites [7], suggesting that the composites of iron oxide and Ag 3 PO 4 could easily synthesized XVLQJ WKH FR௅SUHFLSLWDWLRQ method followed by the calcination. However the iron oxide in the sample of Fe-Ag 0.1 could not be clearly observed due to small concentration.

Ag3PO4

ɲ--Fe2O3

ɶ-Fe2O3

Impurities

Fe-Ag 0.4

Intensity (a.u)

Fe-Ag 0.3

Fe-Ag 0.2

Fe-Ag 0.1

Ag3PO4

15

25

35

45

55

65

75

2ș (degree) FIGURE 1. X-ray diffraction of Ag 3 PO 4 DQGLURQR[LGH௅VLOYHURUWKRSKRVSKDWHFRPSRVLWHV

Figure 2(a) showed the absorption spectra of the pure Ag 3 PO 4 )H௅$J)H௅$J)H௅$JDQG)H௅ Ag 0.4. The absorption of Ag 3 PO 4 exhibited a single absorption edge of 539 nm, whereas the composites exhibited

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two absorptions edge. It indicated that the composites were composed of silver orthophosphate DQGLURQ௅R[LGHĮFe 2 O 3 DQG Ȗ-Fe 2 O 3 ). The Į-Fe 2 O 3 could be identified as the broad DEVRUSWLRQ RI í QP ZKHUHDV WKH ȖFe 2 O 3 could be identified as the absorption edge at 420௅520 nm. The band gap energy of Ag 3 PO 4 in the composites could be calculated by the method of Tauc’s relation ĮKȞ  % KȞ-E g )Ȗ [8], where Eg the optical band gap, h the 3ODQFN¶VFRQVWDQWDQGȞ the frequency of incident photons, B is a constant called the band tailing parameterȖLVWKH index which can have different values (2, 3, ½ and 1/3) corresponding to indirect allowed, indirect forbidden, direct allowed and direct forbidden transition, respectively. Using this equation, the band gap energy of these composites can be obtained by SORWWLQJWKHĮKY 2 against the SKRWRQHQHUJ\KȞ7KH band gap energies can be estimated from the intercept of the straight line fitted from the liner part of the graphics to the axis of the abscissa (Fig. 2(b)). With this estimation, the band gap energy of 2.38, 2.40, 2.40, 2.35 and 2.42 eV could be identified in the sample of Ag 3 PO 4 , )H௅$J)H௅$J)H௅$JDQG)H௅Ag 0.4 respectively. The Ȗ-Fe 2 O 3 band gap energies of 2.61, 2.76, 2.75 and 2.73 were also observed in the sample of )H௅Ag 0.1, )H௅$J  )H௅$J  DQG )H௅Ag 0.4 respectively, whereas tKHEDQGJDSRIĮ-Fe 2 O 3 could not be calculated due to broad absorption. The obtained band JDSHQHUJ\RIȖ-Fe 2 O 3 is higher than that of other reports [9,10], it may be due to the effect of composite formation. It is possible WKDWWKHQHZ௅properties could be generated by interaction of the materials. 1.0

7.0 (b)

(a)

Ag3PO4

6.0

0.8

5.0

0.6 Fe-Ag 0.2

(ɲhʆ) 2

Absorbance

Fe-Ag 0.1

Fe-Ag 0.3

0.4

4.0 2.76 eV

Ag3PO4

3.0

Fe-Ag 0.4

2.0

2.38 eV

0.2

Iron ŽdžŝĚĞവŐ3PO4 Composite (Fe-Ag 0.2)

1.0 2.40 eV 0.0

0.0 200

300

400

500

600

700

800

Wavelenght (nm)

2.0

2.2

2.4

2.6

2.8 hʆ (eV)

3.0

3.2

3.4

FIGURE 2. Diffuse reflectance spectroscopy spectra of Ag 3 PO 4 DQGLURQR[LGH௅VLOYHURUWKRSKRVSKDWHFRPSRVLWHV (a), Tauc relation of Ag 3 PO 4 and Fe௅$J 0.2 composite (b).

The morphology of the catalysts was observed by scanning electron microscope. Fig. 3 showed the SEM image of Ag 3 PO 3 DQGFRPSRVLWHRILURQR[LGH௅Ag 3 PO 4 with mole ratio of 0.2 (the highest activity). The ௅—POHQJWKRI URG௅like Ag 3 PO 4 connecting with each other formed the porous material (Fig. 3(a)). The addition of iron oxide into Ag 3 PO 4 decreased the particle size (Fig(2(b)), indicating that the FR௅SUHFLSLWDWLRQPHWKRGVLJQLILFDQWO\affected the morphology of composites.

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FIGURE 3. SEM image of Ag 3 PO 4 (a) and composite of iron oxide-Ag 3 PO 4 with mole ratio of 0.2 (b) TABLE 1. The surface area of Fe-Ag 0; Fe-Ag 0.2 and Fe-Ag 0.4 composites Surface area Composites (m2/g)

Fe-Ag 0 Fe-Ag 0.2 Fe-Ag 0.4

6,07 6,70 9,20

The photocatalytic properties of catalyst were evaluated by monitoring the Rhodamine B oxidation under blue light irradiation, the result was shown in Fig. 4(a). The highest activity was observed in the sample of Fe௅$J, indicating that the addition of iron oxide into silver phosphate significantly enhanced the photocatalytic activity. The rate of Rhodamine B oxidation under visible light irradiation was also investigated using the SVHXGR௅first௅RUGHU kinetics, ln(C 0 /C)=kt, where k is the rate constant, C 0 is the initial concentration and C is the concentration of dye in the reaction time [4]. Fig. 4(b) showed the plots ln(C 0 /C) vs. irradiation time (t). The photocatalytic degradation of RhB followed WKHSVHXGR௅ILUVW௅RUGHUNLQHWLFVmodel. The rate constant of 0.018, 0.021, 0.039, and 0.006 min-1 were observed in Ag 3 PO 4 , FH௅$J)H௅$JDQG)H௅Ag 0.3, respectively7KHVDPSOHRI)H௅Ag 0.2 exhibited the highest rate constant. 1.2

4.5 Light off

(a)

Light on

1.0

ln (Co/C)

ln (Co/C)

0.8

0.6 Photolysis Ag3PO4

0.4

0

3.5

Fe-Ag 0.1

3.0

Fe-Ag 0.2

Fe-Ag 0.4

2.0

1.0

Fe-Ag 0.3

0.5

20

(b)

Fe-Ag 0.3

2.5

Fe-Ag 0.2

Fe-Ag 0.4

0.0

Ag3PO4

1.5

Fe-Ag 0.1 0.2

4.0

40

60

80

100

0.0 0

Time (min.)

20

40

60

80

Time (min.)

FIGURE 4. Photocatalytic activity D DQGSVHXGR௅ILUVW௅order kinetics (b) of Ag 3 PO 4 and LURQR[LGH௅silver orthophosphate composites.

Table 1 showed the surface area of Ag 3 PO 4 )H௅$JDQG)H௅Ag 0.4. It revealed that the composite formation XVLQJFR௅SUHFLSLWDWLRQmethod followed by calcination did not significantly enhance the surface area, indicating that

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the enhanced photocatalytic activity was not because of surface area. The composite formation may generate the unique material which significantly enhanced the photocatalytic properties. The photocatalytic stability of the highest activity composite was evaluated by cycling the degradation of RhB. The photocatalytic activity gradually decreased during the cycling degradation (Fig.5), indicating that the composite was not good stability. It may be due to the formation of metallic silver (Ag0) on the surface of catalyst. The silver ion could be reduced (Ag+1 ĺ Ag0) by the excited electrons during photocatalytic processes leading to the decrease in photocatalytic activity [11]. 1.2 1st

2nd

3rd

4th

1.0

ln (Co/C)

0.8 0.6 0.4 0.2 0.0 0

50

100

150

200

250

300

350

400

Time (min.) FIGURE 5. Cycling runs of RhB degradation in the presence of Fe௅Ag 0.2 composite under visible light irradiation

Photocatalytic ability (%)

100 80 60 40 20 0 WT

IPA

BQ

AO

(a)

FIGURE 6. The effect of trapping radical and h+ (WT=without hole/radical trapper, IPA= isopropanol, BQ=p-benzoquinone, AO=ammonium oxalate on the photocatalytic ability (a) and the proposed mechanism of highest photocatalytic activity under visible light irradiation (b)

The mechanism of photocatalytic reaction in the highest photocatalytc activiW\)H௅Ag 0.2) was evaluated by a radical and hole (h+) trapper. The isopropanol, p-benzoquinone and ammonium oxalate were added into the solution as the trapper of hydroxyl (•OH) radical, super oxide radical (•O 2 -) and hole (h+), respectively. The results were compared to the system without trapper. The photocatalytic ability of 95.0%, 96,5%, 68.1% and 11.3% were obtained in the sample of without trapper, isopropanol, p-benzoquinone and ammonium oxalate, respectively (Fig.6a). Based on these results, p-benzoquinone and ammonium oxalate exhibited a significant role to decrease the photocatalytic activity compared to the sample without trapper, indicating that the mechanism of photocatalytic reaction mainly depended on a superoxide radical (•O 2 -) and hole (h+). The hole (h+) was the highest effect on the photocatalytic mechanism due to high excitation of silver phosphate and iron oxide. Under visible light irradiation,

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the electron (e-) in the valence band (VB) of silver phosphate and iron oxide were excited to their conduction band (CB). Portion of the excited electron in the iron oxide conduction band react with oxygen to form superoxide radical and the rest of the excited electrons may be transferred into conduction band of silver phosphate. The high accumulated electron in the conduction band of silver phosphate may generate the hydrogen peroxide [2]. It is clear that the composite formation of iron oxide and silver phosphate could suppress the electron recombination, hence increased the photocatalytic activity. The high effect of ammonium oxalate, indicates that the hole created in the valence band may directly react with the RhB to form RhB radical (•RhB). This mechanism was also supported by high adsorpWLRQ RI 5K% RQ )H௅Ag 0.2 at the 20 minutes of dark adsorption. The high interaction of RhB with composite could enhance the reaction with hole of valence band. It is also possible to generate the hole transfer from Ag 3 PO 4 VB to iron oxide VB due to highly reactive of silver phosphate, generating more hole in the surface could increase the separation of electron and hole pairs. The mechanism photocatalytic activity could be illustrated in Fig. 6b.

SUMMARY TKHLURQR[LGH௅silver orthophosphate composites with variation of Fe/Ag mole ratio (0-0.4) were successfully synthesized by facile co-precipitation method followed by calcinations. 7KH LURQ R[LGH௅silver orthophosphate composite with the Fe/Ag mole ratio of 0.2 has higher photocatalytic activity than pure Ag 3 PO 4 under visible light irradiation. The enhanced photocatalytic activity could be attributed to the effective separation of hole (+) and eOHFWURQSDLUVLQWKHLURQR[LGH௅silver orthophosphate composite.

ACKNOWLEDGMENTS This research was financially supported by the Ministry of Research, Technology and Higher Education of the Republic of Indonesia.

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