The synergistic effect of graphene oxide and silver vacancy in Ag3PO4-based photocatalysts for rhodamine B degradation under visible light

Accepted Manuscript Full Length Article The synergistic effect of graphene oxide and silver vacancy in Ag3PO4-based photocatalysts for rhodamine B degradation under visible light Ruidi Liu, Hui Li, Libing Duan, Hao Shen, Qian Zhang, Xiaoru Zhao PII: DOI: Reference:

S0169-4332(18)32070-1 https://doi.org/10.1016/j.apsusc.2018.07.173 APSUSC 39988

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

28 May 2018 23 July 2018 25 July 2018

Please cite this article as: R. Liu, H. Li, L. Duan, H. Shen, Q. Zhang, X. Zhao, The synergistic effect of graphene oxide and silver vacancy in Ag3PO4-based photocatalysts for rhodamine B degradation under visible light, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.07.173

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The synergistic effect of graphene oxide and silver vacancy in Ag3PO4-based photocatalysts for rhodamine B degradation under visible light Ruidi Liu, Hui Li, Libing Duan, Hao Shen, Qian Zhang, Xiaoru Zhao* MOE Key Laboratory of Material Physics and Chemistry under Extraordinary Conditions; Department of Applied Physics, School of Science, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China Tel. /Fax: +86 29 88431678. * Corresponding author: [email protected] (Xiaoru Zhao).

Abstract Spherical-like Ag3PO4 and graphene oxide (GO) wrapped Ag3PO4 (Ag3PO4@GO) composites with different GO contents were successfully synthesized via facile co-precipitation and ultrasonic processes. The morphologies, structures, and surface bonds of the samples were characterized by SEM, TEM, XRD, FTIR, and XPS spectroscopy. Furthermore, photocatalytic activities toward rhodamine B (RhB) degradation of these samples under visible light were evaluated. The experimental results indicated that the breaking of the weak Ag-O bond led to silver vacancies (VAg) in Ag3PO4 lattice, which acted as trapping centers of photo-generated holes. The enhanced photocatalytic performance of the Ag3PO4@GO samples could be explained by the synergistic effect of V Ag and GO. Ag3PO4 hybrid with 10 mg GO (Ag3PO4@10GO) was turned out to have the highest content of VAg, thus leading to the optimum improvement of photocatalytic activity. Keywords: Ag3PO4; graphene oxide; silver vacancy; photocatalytic activity; visible light. Introduction Nowadays, the poisonous, hazardous, and cancerogenic wastewater discharged from industrial factories has caused serious damage to the environment[1], which not only seriously threatens human being’s health, but also hinders the social sustainable 1

development. Therefore, efficient technologies which are able to control environmental pollution have attracted much attention and fascination[2]. During the past decades, many semiconductors (ZnO[3], TiO2[4], SnO2[5], ZnS[6], CdS[7] and MoS2[8] and so on) have been developed to purify waste water as efficient photocatalysts based on their unique bandgap structures. Among them, TiO2 has been extremely explored for its chemical stability, non-toxcity and low-cost[9, 10]. However, the poor absorption in visible regions and low efficiency of photo-produced electrons (e-) and holes (h+), seriously impede the photocatalytic performance[11]. Recently,

many

efforts

have

been

devoted

to

explore

high-efficient

visible-light-responsible photocatalysts, such as noble metal deposition, doping and forming composites. Among these strategies, coupling with two-dimensional materials, such as GO, has been considered to be an effective method, because these materials generally have excellent transparency and high chemical stability and can be integrated on a flexible surface.[5, 11-14] More recently, many researchers have reported the application of Ag3PO4 semiconductor (indirect band gap of about 2.36 eV and direct band gap of about 2.43 eV) as a new photocatalyst[15, 16], owing to its excellent photooxidation potentials for O2 evolution from water and organic dye decomposition under visible light[16, 17]. Unfortunately, the practical applications of Ag3PO4 are intensively limited by its low structural stability[18]. To further enhance the efficiency of the novel material, lots of efforts such as shape control[19-21], facet control[15] and hybridization[22-25] have been carried out to improve the efficiency and stability of photocatalysts. Graphene, a two-demensional, single layer carbon material, has attracted much attention because of its great specific area, excellent mobility of charge transfer and extremely high electrical conductivity. Graphene oxide (GO), an oxidation of graphene with hydroxyl and carboxyl groups modified[26], has been deemed to be a potential candidate for the organic wastewater treatment, CO2 conversion and hydrogen production[27]. In recent years, several well-defined hybridized composites in terms of GO and semiconductors have been developed, such as GO/TiO2[28], GO/ZnO[29] and GO/SnO2[30]. The hybridization of GO and Ag3PO4 has also been 2

applied to develop photocatalyst with enhanced photocatalytic performance and excellent stability by many researchers[22, 31, 32]. However, to the best of our knowledge, there are no previous reports on the synergistic effect of GO and the native defect of Ag3PO4. In this study, Ag3PO4 sphere-like nanomaterials were synthesized via a facile co-precipitation process. Ag3PO4@GO composites (GO wrapped Ag3PO4) with different GO contents were constructed via an ultrasound-precipitation process. Afterwards, the structures, morphologies and chemical properties of the composites were systematically investigated. The roles of GO in the visible-light-driven Ag3PO4@GO composites, especially the synergism of GO and the native defect of Ag3PO4 were analyzed and discussed. A possible mechanism for rhodamine B degradation in Ag3PO4@GO composites was proposed. Experimental Details 1. Synthesis of Ag3PO4. In the typical synthesis of Ag3PO4, firstly, 1.00 g of Polyvinyl Pyrrolidone (PVP) was dispersed uniformly in 100 mL deionized water (DI) with 10 min of ultrasonic treating. Subsequently, 1.20 g of silver nitrate (AgNO3) was put into the solution and dissolved with 30 min of magnetic stirring. After that, 0.84 g of Na2HPO4 was dissolved in 20 mL water and added into the above solution drop by drop with magnetic stirring for about 1 h. Then the obtained yellowish products were separated by centrifugation and washed with DI water for several times. After dried at 60 ℃ for 12 hrs, the Ag3PO4 microspheres were obtained finally. 2. Synthesis of Ag3PO4@GO core-shell structure. To begin with, different amounts of GO were dispersed in 100 ml DI water and then treated with ultrasonic for 4 hrs at 150 W. Separately, the 200 mg of as-prepared Ag3PO4 products was dispersed in 20 ml of DI water after ultrasonic for several minutes to form a suspension, then the suspensions were added into the different concentration of GO aqueous solutions drop by drop respectively. Then, the obtained mixed suspensions were kept stirring for 24 hrs and collected by centrifugation, then washed with DI water and then dried at 60 oC for 12 hrs. The final products were 3

noted as Ag3PO4@10GO, Ag3PO4@20GO and Ag3PO4@40GO, corresponding to the GO amount of 10 mg, 20 mg and 40 mg, respectively. 3. Photocatalytic activity. The

photocatalytic

performance

of

the

Ag3PO4

and

Ag3PO4@GO

(Ag3PO4@10GO, Ag3PO4@20GO and Ag3PO4@40GO) composites was explored by degradation of rhodamine B (RhB, 6 mg/L, 50 mL) with 50 mg photocatalysts under visible light irradiation at room temperature. A 500 W Xeon-lamp equipped with a 420 nm ultraviolet cut-off filter was chosen as the visible light source. Before light on, the

suspensions

were

magnetically

stirred

for

60

min

to

attain

the

adsorption-desorption equilibrium of rhodamine B. During illumination, an external water circulation was used to ensure the experiment temperature at about 25 oC. After given appropriate time intervals, the reaction samples were withdrawn and centrifuged to remove the solid photocatalysts. The photocatalytic degradation rate was determined by measuring the concentrations of the solution samples with an UV-vis spectrophotometer. 4. Characterization The phase and structure of the samples were determined by X-ray diffraction (XRD) on a PANalytical X’pert MPD PRO Diffractometer using Cu Kα radiation (λ=0.154 nm) in range from 5 ° to 8 °. The surface morphology details of the as-prepared samples were characterized by scanning electron microscope (SEM, JSM-7000F) and transmission electron microscope (TEM, Tecnai F30 G2). The different functional groups present on the surface were analyzed by Fourier transform infrared spectroscopy (FTIR, Bruker TENSOR27). X-ray photoelectron spectroscope (XPS) measurements were performed by Kratos AXIS Ultra DLD to obtain surface chemical states and binding energy of the samples. The optical properties were carried out by UV-vis diffuse reflectance spectroscopy (DRS) using a PE Lambda 950 spectrophotometer.

Results and Discussion 4

Fig.1. SEM images of Ag3PO4 (a), and Ag3PO4@GO hybrids (b-d): Ag3PO4@10GO, Ag3PO4@20GO and Ag3PO4@40GO, respectively; TEM images of Ag3PO4@10GO (e-f) and its corresponding element maps of Ag, O, P, C in red dashed box (g).

5

As shown in Fig.1, the morphologies of the as-prepared products were performed by SEM and TEM. The figure revealed that the pure Ag 3PO4 displayed spherical and spherical-like shapes with average 200~500 nm diameters in size. Some aggregations could be observed in the pure Ag3PO4, after decorated with GO, the dispersion of the particles slightly improved. Noted, the morphologies of Ag3PO4 particles in Ag3PO4@GO composites did not change obviously. However, when decorated with GO, the surface of the Ag3PO4 was well wrapped by the GO, which was further confirmed by the TEM images (Fig.1e and Fig.1f). To further illustrate the element distribution in the composite, Fig.1g gave the element maps of Ag, O, P, C of Ag3PO4@10GO composite corresponding to the red dashed box in Fig.1f, displaying the good dispersion of GO on the surface of Ag3PO4. The SEM and TEM images indicated that the intimate contact between Ag3PO4 and GO could possibly induce electron transport channel, which could benefit the charge separation.

Fig.2. XRD patterns of the as-prepared Ag3PO4 and Ag3PO4@GO composites.

XRD patterns were investigated to demonstrate the crystal structure of the Ag3PO4 and Ag3PO4@GO samples. As shown in Fig.2, all peaks of the obtained 6

samples could be legibly distinguished. The main peaks were noted at 2θ= 21.1 °, 29.9 °, 33.5 °, 36.8 °, 42.7 °, 48 °, 52.9 °, 55.2 °, 57.5 °, 61.8 °, 66.0 ° and 72.1 °, which could be related to the (110), (200), (210), (211), (220), (310), (222), (320), (321), (400), (330) and (421) planes of the body-centered cubic structure Ag3PO4 (PDF#06-0505), respectively. Specifically, with the increase of GO concentration, a slight diffraction peak was discovered in Ag3PO4@40GO located at 11.2 °, which was corresponding to the GO[1]. There was no characteristic peak of GO observed in the Ag3PO4@10GO and Ag3PO4@20GO, which was attributed to the relatively low diffraction.

Fig.3. The UV-vis diffuse reflectance absorption spectra of as-prepared Ag3PO4 and Ag3PO4@GO composites.

As reported, the optical property of the semiconductor is the crucial factor which determines its photocatalytic performance. The UV-vis diffuse reflectance spectra (DRS) of Ag3PO4 and Ag3PO4@GO composites were employed (Fig.3a) to investigate the absorption properties. An obvious enhanced absorbance in visible region was observed when GO was incorporated, and when the GO content increased, the absorbance curves held the same trend. The band gap energy (E g) could be figured out by the following formula[32]:  Here,

are parameters corresponding to absorption coefficient, the Plank

constant, the light frequency, a constant and band gap energy, respectively. Moreover, 7

for Ag3PO4, the parameter n=1 for a direct band gap and n=4 for an indirect band gap[33]. Consequently, the Eg of these samples was shown in Fig.3b as they could be elicited from the plots. It could be found that the band gap energies decreased with increasing GO contents, which indicated the enhanced absorption of visible light region.

Fig.4. FT-IR of Ag3PO4 and Ag3PO4@GO composites.

Fig. 4 showed FT-IR spectra of GO, Ag3PO4 and the obtained Ag3PO4@GO composites. In the pure Ag3PO4 spectrum, the obvious broad absorption bands at 3000-3500 cm-1 and 1650 cm-1 were attributed to the stretching vibration of –OH from the adsorbed H2O molecules on the surface[34, 35]. The absorption band centered at 540 cm-1 due to the O=P-O bending vibration of PO43-, while the bands located at 850 cm-1 and 1097 cm-1 were ascribed to the symmetric and asymmetric stretching vibration of P-O-P ring[36]. Additionally, the band which was observed at 1383 cm-1 is assigned to the synergistic effect of P=O stretching vibration and PO43symmetric stretching vibrations[36]. It was worth noting that when the pure Ag3PO4 was decorated with 40 mg GO (Ag3PO4@40GO), a notable difference compared with the pure Ag3PO4 sample was the presence of bands located at 1604 cm-1 and 1204 cm-1, which might be assigned to skeletal vibration of GO and C-O stretching vibration of epoxy groups[32], indicated the existence of GO and the fine combination of the Ag3PO4@GO composites[37]. 8

Fig.5. XPS spectra of GO, as-prepared Ag3PO4 and Ag3PO4@GO composites (a), and high resolution XPS spectra of C 1s(b), Ag 3d(c), O 1s(d) over Ag3PO4 and Ag3PO4@GO composites.

X-ray photoelectron spectroscopy (XPS) was used to further investigate the bonding configuration and understand the properties of the as-prepared Ag3PO4 and Ag3PO4@GO composites, as displayed in Fig.5. The full-scale XPS spectra of all as-prepared samples were presented in Fig.5a, peaks responding to Ag 3p1/2, Ag 3p3/2, O 1s, C 1s, P 2s and P 2p were observed in both Ag3PO4 and Ag3PO4@GO. The 9

high-resolution XPS spectra of C 1s were demonstrated in Fig.5b to understand the surface chemical composition. The peaks were deconvoluted into three sub-peaks which could be attributed to C-C/C=C (284.6 eV), C-O/C=O (286.0 eV) and O-C=O (288.5 eV), respectively. With different amounts of GO added, the energy of C-C/C=C and O-C=O peaks was similar to the as-prepared Ag3PO4 whereas the peaks shifted to the lower binding energy. Notably, it may be concluded that the increased intensities of the C-O/C=O peaks indicated enhanced GO content of the samples, which were corresponding with the adsorption of the photocatalysis results[38]. As many researchers pointed out, native defects and self-decomposition of the Ag3PO4 could mainly influence the properties and determine the catalytic efficiency[39-43]. Therefore, high-resolution Ag 3d peaks (Fig.5c) were illustrated to study the form of Ag elements. The Ag 3d5/2 and Ag3/2 peaks were located near 368 eV and 374 eV, which could be resolved to two separate peaks corresponding to Ag+ and Ag vacancy (VAg). Due to the weak Ag-O bonds, the nature of the chemical bonding in Ag3PO4 suggested that the removal of an Ag atom to form VAg did not cost much energy[40]. With the presence of the VAg, occupied states were induced and led to acceptors which could accept additional electrons. It could be calculated that in the as-prepared Ag3PO4, about 7.8 % VAg was detected. While decorated with GO, the ratios of VAg in Ag3PO4@10GO, Ag3PO4@20GO and Ag3PO4@40GO were 26.4 %, 23.9 % and 12.3 %, respectively. It might be concluded that GO could influence the VAg amount in the crystalline. The XPS of O 1s (Fig.5d) was also carried out to gain a better understanding of the crystal structure of the Ag3PO4. The peaks in all samples could be resolved into three peaks at about 530.3 eV, 531.7 eV and 533.0 eV, which were agreed well with the P=O, O-H and P-O-Ag bonds respectively[44], the ratios of the peak area were presented in Table.1. With the increasing of the GO amounts, ratios of the O-H peak were gone up due to the outstanding adsorption of the GO, while the ratios of P-O-Ag reached the lowest level in the Ag3PO4@10GO sample, which was well coordinated with the VAg result.

10

Table.1. The integral area ratios of XPS peak for O 1s in Ag3PO4 and Ag3PO4@GO composites Sample

Peak ratio (%) P=O (530.3 eV)

O-H (531.7 eV)

P-O-Ag (533.0 eV)

Ag3PO4

34.2

43.8

22.0

Ag3PO4@10GO

29.6

52.7

17.6

Ag3PO4@20GO

21.4

60.3

18.3

Ag3PO4@40GO

17.3

62.4

21.3

Fig.6. Photodegradation of rhodamine B by Ag3PO4 and Ag3PO4@GO composites under visible light irradiation (a), adsorption and degradation of rhodamine B with Ag3PO4 and Ag3PO4@GO composites (b).

The photocatalytic behavior of the obtained samples was evaluated by rhodamine B (RhB) degradation under visible light, as displayed in Fig.6a, where C was the corresponding concentration in real time while C0 was the initial concentration at the beginning. Noted, a blank experiment was carried out in case that rhodamine B may probably have a self-degradation under irradiation. It could be observed from the results that before light on, the adsorption of rhodamine B among these composites was significantly improved with the amounts of GO increasing, which was coordinated with the C 1s XPS discussions. Moreover, compared with the pure Ag3PO4, the photocatalytic activities of the Ag3PO4@GO were evidently 11

enhanced. To further understand the improvements of the whole irradiation, the adsorption and photocatalytic performance were shown in Fig.6b separately. The Ag3PO4@10GO sample showed the best photocatalytic performance which was consistent with the highest VAg amount, while Ag3PO4@20GO sample possessed the optimum removal of the rhodamine B. In order to investigate the process of the rhodamine B removal, the mechanism of the formation of V Ag was discussed as follows. Theoretically[39, 40], VAg, oxygen vacancy (VO), silver interstitial (Agi), oxygen interstitial (Oi) and interstitial hydrogen (Hi) are the primary native defects of Ag3PO4. Due to the weak formation of Ag-O bonds, it suggests that the removal of Ag atoms and formation of V Ag can be easily created other than other native defects. When the Ag atom is absented from the Ag3PO4 crystal, it is easy to create partially occupied states in the band gap, which are mostly derived from the d orbitals of the neighboring Ag atoms. As these occupied states could accept additional electrons, VAg could act as an acceptor and forms V-Ag when Fermi energy increases. In the model proposed by P. Reunchan et al[40], the distance of the two Ag atoms across the VAg is ~16 % shorter than that equilibrium distance of the perfect Ag3PO4 crystals. It is probably leads to a high VAg mobility owing to the strong attractive interaction between VAg and the neighboring Ag atoms. Moreover, the low migration of barrier possibly allows V-Ag to migrate along the path, which involves the displacement of a nearest Ag atom. Finally, the silver vacancies could not be the isolated defects, they could bind with other defects or impurities and possibly be trapped on the surface of the Ag3PO4 particles.

12

Fig.7. Schematics of the carrier separation and transfer in Ag3PO 4@GO composite.

From the above experimental results, the photocatalytic mechanism of as-prepared Ag3PO4 and carrier transfer of Ag3PO4@GO samples was illustrated schematically in Fig.7. In nature, as Ag3PO4 is a narrow band gap semiconductor, the electrons could be excited under visible light and transfer to conduction band (CB), while the holes are left on VB. The generated electrons on CB may directly flow to GO meanwhile the photo-induced holes are captured by the surface Ag vacancies in the lattice, which lead to enhanced charge transfer and separation efficiency. Once photo-generated carriers are trapped by GO and silver vacancies, the electrons will reduce dissolved oxygen (O2) in water to superoxide radical anions (•O2-), while holes oxidize absorbed water (H2O) and hydroxyl ion (OH-) to hydroxyl radical (• OH). The h+, •O2- and •OH exhibit excellent oxidative potentials, which could directly oxidize organic dye to CO2 and H2O. As demonstrated by the experimental results, the photocatalytic activities of the Ag3PO4@GO samples were significantly improved than that of as-prepared Ag3PO4. It could be speculated that the enhancement of the hybrid samples was attributed to the successful transfer of the electrons to GO and the holes to Ag vacancies. Moreover, GO could also strongly enhance the adsorption capacity and play the role of adsorption center which could be 13

observed in the adsorption-desorption equilibrium before light on. To conclude, a synergistic mechanism of V Ag and GO was proposed to explain the enhanced photocatalysis of the Ag3PO4@GO samples. Due to the electronic conductivity of GO and the effective caption of VAg, Ag3PO4@10GO could show the best photocatalytic performance. Meanwhile, owing to the super adsorption capacity of GO, Ag3PO4@20GO could exhibit optimum removal of rhodamine B.

Conclusions Spherical-like Ag3PO4 crystals, and Ag3PO4@GO composites with different GO contents, had been successfully synthesized via facile co-precipitation method and ultrasound-precipitation process. It was found that silver vacancies (V Ag) were easily formed in as-prepared Ag3PO4 due to the weak Ag-O bond in Ag3PO4 crystals. The defect states of Ag vacancies would act as trapping centers for photo-excited holes, which subsequently enhanced the separation of electron-hole pairs. Our experiments further confirmed that the amount of Ag vacancies in the Ag3PO4 crystals varied with the wrapped GO contents. Because GO could provide adsorption sites for organic dye molecules, it might act as an electron acceptor and probably affect the amount of V Ag in Ag3PO4 crystal. The Ag3PO4@10GO sample showed the largest VAg amount and highest degradation rate, just because of the synergistic effect of GO and V Ag. Acknowledgements The authors acknowledge the financial support of National Natural Science Foundation of China (Grant Nos. 51472205, 51302218), Fundamental Research Funds for the Central Universities of China (Grant No.3102016ZY033).

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(RGO)/ZnFe2O4/Ag3PO4

Nanocomposites

for

Enhanced

Photocatalytic

Performance

toward

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graphical abstract

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Highlights： 1. Spherical-like Ag3PO4 and different concentrations of Ag3PO4@GO composites have been synthesized via facile co-precipitation and ultrasound-precipitation process. 2. Due to the weak Ag-O bond in Ag3PO4 crystals, it is easy to form silver vacancies (VAg) in the lattice, which act as trapping centers of photo-generated holes. 3. The roles of different GO contents in the composites played on the visible-light-driven photocatalysis have been carried out. 4. The enhanced photocatalytic performances of the Ag3PO4@GO samples could be explained by the synergistic effect of V Ag and GO.

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