The highly active saddle-like Ag3PO4 photocatalyst

Catalysis Communications 85 (2016) 22–25

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Short communication

The highly active saddle-like Ag3PO4 photocatalyst under visible light irradiation Uyi Sulaeman a,⁎, Febiyanto Febiyanto a, Shu Yin b, Tsugio Sato b a b

Department of Chemistry, Jenderal Soedirman University, Purwokerto, 53123, Indonesia Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, 980-8577, Japan

a r t i c l e

i n f o

Article history: Received 1 January 2016 Received in revised form 26 March 2016 Accepted 2 July 2016 Available online 4 July 2016 Keywords: Facet Morphology Photocatalyst Rhodamine B Saddle-like Ag3PO4 Tetrahedron

a b s t r a c t Saddle-like Ag3PO4 particles of tetrahedron structure were successfully synthesized using a co-precipitation method by mixing H3PO4 ethanol solution and AgNO3 ethanol aqueous solution, where the percentage of ethanol in AgNO3 ethanol aqueous solution was varied at 0, 50, 80, 90 and 100% (v/v). The photocatalytic performance of the synthesized samples was evaluated by photodegradation of Rhodamine B (RhB) under blue light irradiation (λ = 455 nm). The results showed that the morphology of the Ag3PO4 particles greatly changed depending on the ethanol content in the reaction solution. Excellent photocatalytic activity was observed at 80% (v/v) of ethanol, where the Ag3PO4 showed saddle-like morphology derived from the tetrahedron structure. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Today, the morphology of silver phosphate has been receiving much attention for its ability to improve photocatalytic activity under visible light irradiation. Researchers have successfully controlled the morphology of Ag3PO4 to provide high photocatalytic activity for dye pollutant degradation [1–3] and antibacterial activities [4,5]. Wang et al. [1] synthesized spherical, polyhedral and irregularly shaped Ag3PO4 by co-precipitation method using various reactants under different temperatures. The polyhedral Ag3PO4 showed the highest photocatalytic activity because the polyhedral sample absorbed more visible light compared to the spherical and irregularly shaped samples. Wu et al. [4] synthesized three different morphologies of Ag3PO4, such as rhombic dodecahedron particles of 500 nm in diameter, spherical particles of 100 nm and small particles of 20 nm using the solvent of water, ethylene glycol and dimethyl sulfoxide, respectively. The highest activity could be found in small particles of 20 nm. The unique morphologies of Ag3PO4, which improve photocatalytic activity, were also synthesized [6–8]. Xu and Zhang [6] designed the truncated tetragonal bipyramid hollow microboxes of Ag3PO4. This morphology exhibited much higher photocatalytic activity than other morphologies of Ag3PO4, such as spherical and rhombic dodecahedral under visible light irradiation. A unique morphology of the flower-like Ag3PO4, which exhibited high photocatalytic activity under visible ⁎ Corresponding author. E-mail address: [email protected] (U. Sulaeman).

http://dx.doi.org/10.1016/j.catcom.2016.07.001 1566-7367/© 2016 Elsevier B.V. All rights reserved.

light irradiation was successfully synthesized using a facile aqueous solution route in the presence of polyethylene glycol [7]. The unique of concave trisoctahedral Ag3PO4 microcrystals consisting of {221} and {332} facets exhibited high photocatalytic activity [8]. The cubic morphology of Ag3PO4 microcrystals which enhanced the photocatalytic activity under visible light irradiation, were easily synthesized [9–11]. The ammonia played a crucial role in the formation of cubic Ag3PO4 microcrystal [9]. The cubic Ag3PO4 designed using ammonia, exhibited a higher photocatalytic activity, compared to irregularly shaped Ag3PO4. The cubic Ag3PO4 particles of around 120 nm in diameter, were synthesized in the presence of PVP [10]. This cubic Ag3PO4 showed superior photocatalytic activity for the photodegradation of methylene blue (MB) under visible light irradiation due to its larger specific surface area and longer life time of electron–hole pairs. The cubic– type structure could be well-controlled by volume ratio of water/ethylene glycol [11]. With this method, the uniform morphology of Ag3PO4 microcrystals could be designed and exhibited higher photocatalytic activities under visible light irradiation. The decomposition rate of RhB using these cubic Ag3PO4 microcrystals was three times higher than that of irregularly shaped Ag3PO4 microparticles. The most interesting morphology of Ag3PO4 is the tetrahedron which is highly reactive under visible light irradiation [12–16]. Hu et al. [12] reported that the tetrahedral Ag3PO4 structure synthesized by directly reacting commercial Ag foil with H2O2 and NaH2PO4 in an aqueous solution at room temperature, showed higher photocatalytic activity than Ag3PO4 cubes, irregular Ag3PO4 and N-doped TiO2. The novel type of regular tetrahedron nanocrystal exposing {111} facets was

U. Sulaeman et al. / Catalysis Communications 85 (2016) 22–25

designed by oxidizing Ag with H2O2 in the presence of PO3− ion [13]. 4 Dong et al. [14] fabricated the Ag3PO4 microcrystals with different morphologies, such as tetrahedra with round and sharp corners, short tetrapods, polyhedra, and dendritic long tetrapods via simple and green routes. Among these morphologies, the tetrahedral Ag3PO4 with round edges synthesized using KH2PO4 as a PO3− source showed the 4 highest activity and excellent stability. Zheng et al. [15] synthesized the single–crystalline tetrahedral Ag3PO4 microcrystal exposing {111} facet using a facile wet chemical method. This morphology structure showed higher photocatalytic activity compared to those with the {110} and {100} facets. Martin et al. [16] synthesized the tetrahedral Ag3PO4 with {111} facets by a novel kinetic control method using starting materials of AgNO3 and H3PO4 with the ethanol solution. This tetrahedral crystal showed higher activity for water photo-oxidation than rhombic dodecahedron {110} and cubic {100} structures. The excellent photocatalytic performance was attributed to a synergistic effect of the high surface energy and small hole mass which enhanced the charge carrier mobility and active surface reaction sites. Based on the above mentioned information, the design of tetrahedral Ag3PO4 structure to provide an excellent photocatalyst is very challenging. In the current result, the Ag3PO4 sample with the best photocatalytic reactivity, enabled by its saddle-like morphology, derived from the tetrahedron, could be easily prepared using ethanol aqueous solution. It is very important to provide a simple preparation method to be adopted for practical application. This finding could contribute to the improvement of photocatalytic reaction under visible light irradiation. 2. Experiment The five samples of Ag3PO4 were prepared as follows. At first, 0.85 g AgNO3 was dissolved in 50 mL ethanol aqueous solution with an ethanol volume percentage of 0, 50, 80, 90 or 100%. Then, the H3PO4 ethanol solution was made by dissolving 0.98 g H3PO4 in 50 mL of ethanol, and added to the AgNO3 ethanol aqueous solution drop by drop. The precipitates were separated by 14,000 rpm centrifugation, washed with water three times, and dried in a vacuum over night at 60 °C. The products were designated as E0, E50, E80, E90 and E100, respectively. The crystal structures of Ag3PO4 were characterized using X-ray diffraction (XRD, Bruker AXS D2 Phaser) using graphite-monochromatized CuKα radiation. The absorption spectra of powder samples were analyzed using a UV–Vis NIR spectrometer (JASCO V-670; JASCO Corporation, Tokyo, Japan), giving the output of absorbance in the UV and visible ranges of 200–800 nm with step size of 0.2 nm. The BET specific surface areas (SBET) of samples were determined by nitrogen adsorption (NOVA 4200e). The morphologies were observed by a scanning electron microscope (SEM, Hitachi S-4800). To investigate the binding energy, the X-ray photoelectron spectrometer (XPS, Perkin Elmer PHI 5600) was used. To evaluate the photocatalytic activities, 100 mg of catalyst was mixed with 100 mL of 10 mg/L Rhodamine B solution, and the solution was stirred at room temperature under dark condition for 20 min. After that, the solution was irradiated by a blue LED lamp (OptiLED, SP-E27BL, 2.5 W, λ = 455 nm) which is adjusted at 10 cm above the surface of solution. 4 mL of sample solution was withdrawn every 10 min and centrifuged at 14,000 rpm to separate the sample powder, and the concentration of RhB was measured using a spectrophotometer (JASCO V-670; JASCO Corporation, Tokyo, Japan), giving the output of absorbance in the UV and visible ranges of 300–700 nm with step of 0.2 nm [17]. 3. Results and discussion The tetrahedral Ag3PO4 was successfully synthesized by the co-precipitation method in ethanol aqueous solutions. Fig. 1(a) shows the XRD profile of samples synthesized using different percentage of ethanol.

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Fig. 1. XRD profiles (a) and diffuse reflectance spectra (b) of the Ag3PO4 synthesized by the co-precipitation method in ethanol aqueous solutions of E0, E80 and E100.

The body-centered cubic structure (JCPDS no.06-0505) was observed in all of the samples, similar to other results [18,19]. No impurities were observed on the samples, indicating that the samples were single phase Ag3PO4. However, a slightly different {111}/{100} intensity ratio of 1.12, 1.01 and 0.72 were observed in E0, E80, and E100, respectively, indicating that the samples had different facets. The absorption spectra of E0, E80 and E100 are shown in Fig. 1(b). The absorption spectra of E0 and E80 are similar, whereas significant broad absorption above 500 nm could be found in the sample of E100, indicating that the pure ethanol solution influences the properties of Ag3PO4. A high number of defects or deformations of morphology may generate the broad absorption of the sample in the visible region. It could be considered that the defect site of crystal affected the absorption in visible region [20]. The band gap energies were calculated based on the previous reports [17], and listed in Table 1. Fig. 2 shows that the morphology of Ag3PO4 could be controlled by increasing the volume percentage of ethanol. The morphology of the E0 consisted of triangular and irregularly shaped particles. The SEM image of the triangular particles confirmed that the microsized Ag3PO4 showed a tetrahedron feature. The tetrahedra are formed by the reaction of H3PO4 ethanol solution and AgNO3 aqueous solution. The side edge length of tetrahedron ranges from 1 to 3 μm and that of the irregular shape ranges from 0.5 to 2 μm. The tetrahedron particles appeared in the reaction of H3PO4 ethanol solution with AgNO3 ethanol aqueous solution (50% of ethanol). In this step, some of the tetrahedron particles were changed into a unique saddle–like shape of Ag3PO4, which has a round shape on one side edge of the tetrahedron feature. With a further increase of ethanol content in AgNO3 ethanol aqueous Table 1 The BET specific surface areas, band gap energies and rate constants of Ag3PO4 synthesized in variation of ethanol aqueous solutions. Sample

S.S.A. (m2/g)

Band gap energy (eV)

Rate constant (min−1)

E0 E50 E80 E90 E100

8.82 10.30 7.51 12.50 10.74

2.40 2.42 2.41 2.38 2.32

0.0216 0.0352 0.0637 0.0358 0.0300

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Fig. 2. FE-SEM images of the products synthesized in ethanol aqueous solutions of (a) E0, (b) E50, (c) E80, (d) E90 and (e) E100, and (f) higher magnification of (c).

solution up to 80%, the amount of saddle-like Ag3PO4 increased. However, the saddle–like Ag3PO4 of tetrahedron, dramatically decreased by increasing ethanol up to 90% and 100%, mostly irregularly shaped particles were formed. It is well known that the {111} facet of tetrahedron Ag3PO4 showed high reactivity. Therefore, it is very useful to control and modify this facet to enhance the reactivity of Ag3PO4. Martin et al. [16] reported that the exposed {111} facets could be controlled by manipulating the concentration of H3PO4 in ethanol, and excess amounts of H3PO4 can control the rate of Ag3PO4 nucleation and initial growth. With this method, the directional growth is halted at a specific concentration of H3PO4. However, in our experiment, since the concentrations of H3PO4 and AgNO3 were constant, the interaction between Ag+ and PO34 − was only affected by the strength of solution polarity which can be controlled by mixing of water and ethanol. Therefore, different morphologies resulted. Fig. 3 shows the photocatalytic activities of five samples which were synthesized using various ethanol aqueous solutions. All samples showed excellent photocatalytic activities. The highest photocatalytic activity could be found in E80, i.e., three times higher than E0. It is well known that the Ag3PO4 tetrahedral structure has high activity [16] due to the high surface energy of the {111} facet on the Ag3PO4. In this experiment, based on XRD results (see Fig. 1(a)), the highest ratio of {111}/{100} was observed in E0. However, E0 exhibited a lower photocatalytic activity compared to E80. This might be due to the lower specific surface area of large tetrahedron particles. In addition, the unique saddle-like morphology of the tetrahedra in E80 might

contribute to enhance the surface energy of Ag3PO4 leading to increased photocatalytic activity. With further increasing ethanol content (E90, E100), the photocatalytic activity decreased due to lower {111} facet and irregular shape. To evaluate the photocatalytic activity, the XPS spectra of Ag3PO4 were analyzed using XPS [Fig. 4]. The Ag4d binding energies (BE) of E0, E80 and E100, before and after Ar+ sputtering were investigated. Before Ar+ sputtering, BE peaks at 4.89, 4.84, 4.86 eV were observed for E0, E80 and E100, indicating no significant difference in BE for these three samples. However, after Ar+ sputtering, BE peaks of 5.25, 4.82 and 5.21 eV were observed for E0, E80 and E100, respectively, and the shoulder peaks appeared on a higher BE in all the samples. The BE peaks for E0 and E100 significantly shifted to higher BE, whereas the peaks for E80 shifted to a lower BE, indicating that the variation of synthesis methods affected the surface state of Ag3PO4. Table 2 shows the atomic ratios of Ag/P, O/P and C/P. The carbon was observed, mainly in the sample of E0. This might be due to the impurities of carbonates which were formed by the adsorption of the atmospheric CO2 on the surface [21]. It was very difficult to determine the composition of Ag3PO4 in the presence of the impurities. However, it could be concluded that by increasing the ethanol content in Na2HPO4–water–ethanol solution, the ratio of Ag/P was increased, indicating that the ethanol enriched the Ag+. The enriched Ag+ might enhance the photocatalytic activity. Hu et al. [12] explained that the {111} facet has high photocatalytic activity because the {111} facets contain only Ag atoms. It is different with the {100} plane which is composed mainly of P and Ag atoms. The enriched Ag+ cations in Ag3PO4

Fig. 3. Photocatalytic activities of the products synthesized in various ethanol aqueous solutions.


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sample exhibited a negative effect on photocatalytic activity. The increased ratio of Ag/P of E100 without increasing O/P ratio indicates the creation of an oxygen vacancy. Many reports showed that the oxygen vacancy could generate photocatalytic activity under visible light irradiation [22,23]. However, in this case, due to many irregularly shaped Ag3PO4 particles in E100, the photocatalytic activity decreased, compared to E80. 4. Conclusion The different morphologies of Ag3PO4 photocatalysts could be prepared by co-precipitation reaction of H3PO4 ethanol solution and AgNO3 ethanol aqueous solution, where the ethanol content in the AgNO3 ethanol aqueous solution was changed as 0, 50, 80, 90 and 100% (V/V). The results showed that the morphology of Ag3PO4 strongly depended on ethanol content. The highest photocatalytic activity was observed at 80% (V/V) of ethanol. This highest photocatalytic activity may correspond with the increase in saddle-shaped microcrystal derived from tetrahedron Ag3PO4. With the increase of ethanol (90% and 100%), decreased photocatalytic activity was observed which may be due to increased irregularly shaped Ag3PO4. Acknowledgements This research was supported by the JASSO (Japan Student Services Organization) program of the Follow-up Research Fellowship and the Ministry of Research, Technology and Higher Education of the Republic of Indonesia. References Fig. 4. XPS spectra of Ag3PO4 (E0, E80 and E100) before and after Ar+ sputtering.

Table 2 Atomic ratios of Ag/P, O/P and C/P in Ag3PO4 (E0, E80 and E100) measured by XPS. Sample

Before Ar+ sputtering

After Ar+ sputtering

Ag/P

O/P

C/P

Ag/P

O/P

C/P

E0 E80 E100

2.30 2.38 2.42

3.81 3.90 3.83

1.98 0.85 0.96

2.53 2.80 3.01

2.95 3.09 3.06

1.05 0.02 0.04

{111} planes may be partly reduced by the photogenerated electrons (EθAg+/Ag = 0.8 V vs. saturated calomel electrode (SCE)), and Ag nanolayers could also be formed on the {111} surfaces. An Ag nanolayer can facilitate the separation of photoexcited electron–hole pairs and enhance their photocatalytic activities. The partially reduced Ag+ site may also produce an oxygen vacancy on the surface which may facilitate the increase of photocatalytic active sites. Therefore, the tetrahedral Ag3PO4 structures can have higher activity for dye oxidation. However, in this experiment, the increase of the Ag+ in the crystal of E80 and E100 was not followed by the increase of P, indicating that a phosphorus deficiency was formed. It was suggested that E80 possessed the highest phosphorus deficiency. The increase of Ag+ in the E100

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