Materials Letters 92 (2013) 57–60

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Materials Letters

0167-57

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Highly photocatalytic activity of novel nano-sized Ag3PO4 for RhodamineB degradation under visible light irradiation

Tuan A. Vu n, Canh D. Dao, Thuy T.T. Hoang, Kien T. Nguyen, Giang H. Le,Phuong T. Dang, Hoa T.K. Tran, Tuyen V. Nguyen

Institute of Chemistry—VAST, 18 Hoang Quoc Viet Street, Cau Giay District, Hanoi City, Viet Nam

a r t i c l e i n f o

Article history:

Received 3 April 2012

Accepted 7 October 2012Available online 17 October 2012

Keywords:

Nano-sized Ag3PO4

Visible light

Rhodamine B degradation

7X/$ - see front matter & 2012 Elsevier B.V.

x.doi.org/10.1016/j.matlet.2012.10.023

esponding author. Tel.: þ84 915212318.

ail address: vuanhtuan.vast@gmail.com (T.A.

a b s t r a c t

Ag3PO4 nanoparticles have been successfully synthesized by precipitation of AgNO3 and H3PO4 in the

presence of oleylamine. The products were characterized by XRD, UV–vis and FE-SEM. It showed that

the particle sizes of products ranged from 5 to 10 nm depending on the synthesis conditions. UV–vis

result revealed a strong light absorbance in the visible light region at 470–475 nm. Photocatalytic

activities of TiO2 and Ag3PO4 nanoparticles were tested in photocatalytic degradation of Rhodamine B

(RhB) under visible light irradiation. In comparison to nano-TiO2, nano-sized Ag3PO4 samples exhibited

much higher photocatalytic activity in the RhB degradation. After 3 h of reaction, the conversion

reached the value of 96%.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Among various semiconductors, TiO2 has received great atten-tion owing to its high photocatalytic activity, low cost, non-toxicity and environment friendly advantages. However, the wideapplication of TiO2 is limited in the condition of solar irradiationdue to its wide band gap [1,2]. Therefore, considering energyconservation and environmental pollution issue, it is necessary todevelop highly efficient and visible light-driven photocatalysts.Visible light-induced engineering includes modification of TiO2

and development of new non-TiO2 based environmental cleaningmaterials. Several new non-TiO2 materials including BiOBr [3,4],Bi2WO6 [5,6], BiVO4 [7–9], In1�xNixTaO4 [10], ZnS–CuInS2–AgInS2

[11], Y2InSbO7 and Y2GdSbO7 [12], La2O3-modified silver vana-dates [13] and Ag–AgI/Fe3O4–SiO2 [14] have currently beendeveloped and exploited.

Recently, a breakthrough was made by Yi et al. [15], whoreported the use of silver orthophosphate (Ag3PO4) as an activevisible light photocatalyst for the oxidation of water andphotodecomposition of organic compounds. These authorsdemonstrated that the photocatalytic activity of Ag3PO4 is farexceeding those of current known visible light photocatalysts, e.g.BiVO4 or N-doped TiO2. However, the crystallite size of theAg3PO4 remained relatively large (0.5–2 mm) which mightactually limit the photocatalytic performance. To enhance photo-catalytic activity of this new material, it is thus desirable to

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Vu).

synthesize nanosized Ag3PO4 particles in order to improve lightabsorption efficiency. In this work, we report the synthesis andcharacterization of Ag3PO4 nanoparticles. The obtained sampleswere tested for photocatalytic degradation of RhB under visiblelight irradiation.

2. Experimental

Synthesis: For the synthesis of Ag3PO4 nanoparticles, a certainamount of silver nitrate and oleylamine (OM) were dissolved intoluene. To this mixture, H3PO4 dissolved in ethanol was thengradually added under stirring. The solution changed from color-less to yellow, indicating the formation of Ag3PO4 nanoparticles.After 30 min of stirring, excess ethanol was added to precipitateAg3PO4 nanoparticles. The obtained nanoparticles were thenre-dispersed in toluene and reprecipitated in ethanol. This processwas repeated three times to remove all unreacted reagents.

For Ag3PO4 nanoparticles synthesized in the presence of water,the procedure was similar to that mentioned above, except that20 g of water was added into H3PO4.

Removing OM surfactant from Ag3PO4 nanoparticles surface:10 mmol of as-synthesized Ag3PO4 nanoparticles was dispersedin 100 ml toluene. To this solution, a mixture containing 1 mmolNH4OH, 25 ml ethanol and 25 ml toluene was gradually addedunder stirring. After 1 h of additional stirring, the mixture wascentrifuged to obtain a precipitate of Ag3PO4 nanoparticles whichwas then redispersed in 100 ml of ethanol. To this mixture wasgradually added a solution of 50 ml ethanol containing 1 mmol

T.A. Vu et al. / Materials Letters 92 (2013) 57–6058

NH4OH. After 1 h of stirring, the Ag3PO4 nanoparticles obtainedthrough centrifugation were dried in an oven.

Characterization: The powder X-ray diffraction (XRD) patternsof the samples were recorded on a Shimadzu XRD-6100 analyzerwith Cu Ka radiation (l¼1.5417 A). The diffuse reflectance UV–visspectra of the catalysts were measured using a ShimadzuUV-2200 A spectrophotometer at 250– 650 nm and a ShimadzuUV–vis spectrophotometer at 295 K. The FT-IR spectra of theAg3PO4 were measured by the KBr pellet method (BIO-RADFTS-3000). Field emission scanning electron microscopy (FE-SEM) was carried out using HITACHI S-4800 instrument operatingat 10 kV and magnification of 80,000–100,000. Transmissionelectron microscopy (TEM) using JEOL 1010 instrument operatingat 80 kV with a magnification of 25,000–100,000.

Photocatalytic activity testing: Photocatalytic activities of thesamples were evaluated by the photocatalytic degradation ofRhodamine B (RhB). A mixture of RhB aqueous solution(15 ppm, 100 ml) and the given photocatalyst (30 mg) wasmagnetically stirred in absence of light for 40 min to determineamount of adsorbed RhB. The mixture was then stirred undervisible light irradiation using four fluorescent lamps (15 W each).At given time intervals, 10 ml of the suspension was collected andcentrifuged to remove photocatalyst particles. UV–vis adsorption

Fig. 1. XRD patterns of Ag3PO4 nanoparticles synthesized

spectrum of the centrifuged solution was recorded using a Cary300 Bio UV–visible spectrophotometer to determine the conver-sion of the reaction.

3. Results and discussion

Catalyst characterizationX-ray diffraction (XRD): Fig. 1 presents X-ray diffraction

patterns of Ag3PO4 nanoparticles synthesized in the absence(A) and presence of water (B). The XRD pattern of the nano-sized Ag3PO4 products exhibits well-defined peaks indicative ofhighly crystalline Ag3PO4 with body-centered cubic structure asobserved on micro-sized Ag3PO4 [15,16].

UV–vis diffuse reflectance spectroscopy: The UV–vis diffusereflectance spectra of Ag3PO4 nanoparticles synthesized in theabsence and presence of water are illustrated in Fig. 2. It isobserved that the absorption bands of Ag3PO4 nanoparticles shiftto longer wavelength (maximum at 475 nm) as compared to TiO2

nano particles (maximum at 345 nm), indicating they are highlyvisible light sensitized semiconductors. For a crystalline semi-conductor, it shows that the optical absorption near the bandedge follows the equation a(hv)¼A(hv�Eg)1/2, where a, n, A, and

in the absence (A) and in the presence of water (B).

Fig. 2. UV–vis diffuse reflectance spectra of TiO2 nanoparticles and Ag3PO4 nanoparticles.

Fig. 3. FE-SEM and TEM images of Ag3PO4 nanoparticles synthesized in the

absence of water (A) and in the presence of water (B).

T.A. Vu et al. / Materials Letters 92 (2013) 57–60 59

Eg are absorption coefficient, light frequency, the proportionalityconstant and the band gap energy, respectively [17]. The band gapenergy is estimated on the plot with (Ahn)2

¼ f(hn) by the interceptof the tangent to the plot with abscissa, and the obtained valueis 2.51.

Field emission scanning electron microscopy (FE-SEM) and Trans-

mission electron microscopy (TEM): FE-SEM and TEM images ofAg3PO4 nanoparticles synthesized in the absence and presence ofwater are presented in Fig. 3. From TEM images (Fig. 3A and B), itcan be observed that the particle sizes of Ag3PO4 synthesized inthe absence of water ranged in the nanometer scale from 10 nmto 12 nm and particle size distribution is more uniform. In thecase of Ag3PO4 synthesized in the presence of water, particle sizeranged from 5 nm to 10 nm (large portion) and 12 nm to 15 nm(small portion), and particle size distribution is less uniform andtended to form aggregates.

Photocatalytic activity testingIt is well known that Rhodamine B (RhB) is one of the most

important representatives of xanthene dyes and could yieldpotentially carcinogenic aromatic amines [18] when it underwentnatural reductive anaerobic degradation. Therefore, in this workwe choose RhB as a probe to test photocatalytic activity of nano-Ag3PO4 samples synthesized in absence and in presence of waterunder visible light irradiation. For comparison, photocatalyticactivity of nano-TiO2 also was investigated. Concentrations ofRhB were determined by using UV–vis spectroscopy (Fig. 4B).

Fig. 4A plotted the RhB concentration ratio (C/Co) as a functionof irradiation time over nano-TiO2, nano-sized Ag3PO4 synthe-sized in the absence and presence of water. Since RhB adsorptionis involved to decrease the overall RhB concentration duringreaction process, we need to investigate separately the RhBadsorption of the samples (in dark for 40 min under stirringcondition). As observed in Fig. 4A, after 40 min of adsorption onnano-TiO2, nano-Ag3PO4 synthesized in absence and presence ofwater in the dark, the RhB concentration was reduced by8.5%, 7.4% and 2.7%, respectively. Comparing photocatalytic activ-ities of three samples, nano Ag3PO4 synthesized in presence ofwater shows the highest conversion. Thus, after 1 h of visiblelight irradiation, RhB conversion over nano-TiO2, nano-Ag3PO4

Fig. 4. Variation of RhB concentrations versus irradiation time using nano-TiO2,

nano-Ag3PO4 synthesized in the absence and presence of water (A) and UV–vis

absorption spectra of RhB solutions at different times of reaction using Ag3PO4

nanoparticles (B).

T.A. Vu et al. / Materials Letters 92 (2013) 57–6060

synthesized in the absence and presence of water was 21%, 55%and 75%, respectively. The higher RhB conversion of nano-sizedAg3PO4 synthesized in the presence of water may be due to thehigher portion of small size particles (5 nm) in this sample ascompared to that of Ag3PO4 synthesized in absence of water,enhancing visible light absorption efficiency and consequentlyincreasing RhB conversion. Thus, the size of Ag3PO4 might begoverned by the relative rates of two reactions: (i) Oleylamine(OM) with H3PO4 and (ii) Ag ions with H3PO4, both of whichdepend on water content in the synthesis mixture. The formertends to induce small nano-Ag3PO4 while the latter favors growthof nano-Ag3PO4. In our case, it is possible that the reactionbetween Oleylamine (OM) with H3PO4 is favored by adding water,inducing smaller nano-Ag3PO4 particles. After 3 h of reaction, RhBconversion over nano-TiO2 reached the value of 60%, while RhBconversion over nano-Ag3PO4 samples reached to the value of96%. This clearly indicated the higher photocatalytic performanceof nano Ag3PO4 as compared to that of nano-TiO2. Note that inthis work, we used low light power (four fluorescent lamps of

15 W each) whereas, much higher light power (250–500 W) wasoften used in the published papers [13,14].

4. Conclusion

Ag3PO4 nanoparticles have been successfully synthesized byprecipitation of AgNO3 and H3PO4 in the presence of oleylamine.Synthesis of Ag3PO4 nanoparticles in the presence of water also wassuccessful. The products were characterized by XRD, UV–vis andFE-SEM. It showed that the crystal sizes of products were 5–10 nmdepending on the synthesis conditions. UV–vis result showed a stronglight absorbance in the visible light region at 470–475 nm, indicatinghighly visible light sensitized semiconductors. Photocatalytic activ-ities of TiO2 and Ag3PO4 nano particles were tested in photocatalyticdegradation of Rhodamine B (RhB) under visible light irradiation. Incomparison to nano-TiO2 and nano-sized Ag3PO4 samples exhibitedmuch higher photocatalytic activity in the RhB degradation. After 3 hof reaction, the conversion over nano-sized Ag3PO4 reached the valueof 96%. The obtained results show high photocatalytic performance ofnano-sized Ag3PO4 as novel non-TiO2 based photocatalyst.

Acknowledgments

The authors thanks the National Foundation for Science andTechnology Development of Vietnam—NAFOSTED (Grant no104.03-2010.12) for financial support.

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