Silver Orthophosphate Immobilized on Flaky Layered Double Hydroxides as the Visible-Light-Driven Photocatalysts

Flaky layered double hydroxide (FLDH) was prepared by the reconstruction of its oxide in alkali solution. The composites with FLDH/Ag3PO4 mass ratios at 1.6 : 1 and 3 : 1 were fabricated by the coprecipitation method. The powders were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscope, and UV-vis diffuse reflectance spectroscopy. The results indicated that the well-distributed Ag3PO4 in a fine crystallite size was formed on the surface of FLDH. The photocatalytic activities of the Ag3PO4 immobilized on FLDH were significantly enhanced for the degradation of acid red G under visible light irradiation compared to bare Ag3PO4. The composite with the FLDH/Ag3PO4 mass ratio of 3 : 1 showed a higher photocatalytic efficiency.


Introduction
The treatment of environmental pollutants in waste water by active semiconductor photocatalysts has recently attracted considerable attention for its ability to completely oxidize organic contaminants to carbon dioxide, water, and mineral acids [1][2][3][4][5].Among the semiconductor photocatalysts, TiO 2 has been extensively studied because of its outstanding photocatalytic activity, long-term stability, low cost, and nontoxicity [6,7].However, TiO 2 can only be activated with UV light with a wavelength of less than 385 nm due to its wide band gap (∼3.2 eV), which limits its utilization of the solar energy.Therefore, a growing interest is also focused on the development of the new photocatalysts that can operate effectively under visible light irradiation.The doping of TiO 2 with foreign elements such as metals and nonmetals to improve the energy band structure of TiO 2 has been extensively investigated [8], and nitrogen doping TiO 2 has been demonstrated as a visible light photocatalyst [9].However, the absorption edge of N-doped TiO 2 was just extended to the wavelength below 450 nm with a lower absorption constant.The non-TiO 2 -based photocatalysts with a larger absorption coefficient, such as Ta 3 N 5 [10], Ga 1−x Zn x N 1−x O x [11,12], BiVO 4 [13,14], Bi 2 WO 6 [15,16], and Ag 2 CrO 4 [17] have also been studied during the past decade.Up to now, the development of novel visible-light-responsive photocatalysts with expanded spectral response range and high photocatalysis quantum efficiency is still necessary.
Recently, Yi et al. reported the new use of Ag 3 PO 4 semiconductor as a visible light photocatalyst, which can oxidize water as well as decompose organic contaminants in aqueous solution [18].It was found that it exhibited extremely high photocatalytic efficiency for organic dye decomposition under visible light irradiation and the decomposition of methylene blue over Ag 3 PO 4 was dozens of times quicker than that over monoclinic BiVO 4 and commercial TiO 2−x N x .However, the effect about reducing the crystallite size of Ag 3 PO 4 on photocatalysis is still unknown.
Layered double hydroxide (LDH), also called hydrotalcite-like compound or anionic clay, is a family of lamellar ionic solids that in terms of layer charge are the counterpart of cationic clay minerals [19].The LDH has a unique property known as "memory effect," which refers to that LDH is decomposed into mixed metal oxide (MMO) when LDH calcined at 300-600 • C. The calcined LDH is able to recover the original layered structure easily when exposed to the air or put into anion aqueous solution [4,20].In our previous work, we have prepared flaky layered double International Journal of Photoenergy hydroxides (FLDH) composed of cross-linked nanoflakes by the reconstruction of their oxides in alkali solution, and the ZnO immobilized on FLDH generated highly dispersed ZnO nanoparticles (∼9.5 nm in diameter) with an enhanced photocatalytic efficiency [21].Nevertheless, the band gap of this composite (∼3.18 eV) is too wide to absorb sunlight efficiently.In this work, we synthesized the Ag 3 PO 4 /FLDH composites, and the much finer Ag 3 PO 4 was obtained in the presence of FLDH and the resulted composites exhibited a higher visible light photocatalytic efficiency compared to bare Ag 3 PO 4 .

Experimental
2.1.Synthesis of Materials.The Mg-Al-CO 3 LDH was prepared by coprecipitation by a high supersaturation method [22].A typical preparation process was described as follows.Solution A was prepared by dissolving AlCl 3 •6H 2 O (62.5 mmoL) and MgCl 2 •6H 2 O (125 mmoL) in 87.5 mL of deionized water and solution B prepared via dissolving NaOH (437 mmoL) and Na 2 CO 3 (208 mmoL) in 125 mL of deionized water.Solution A was added to solution B in a three-neck round-bottom reaction flask and then stirred at 50 • C, subsequently heated to 90 • C for 4 h.LDH was obtained after collection of the precipitates by filtration, washing several times with distilled water, and drying at 90 • C overnight.
FLDH was prepared by the reconstruction of its oxide in alkali solution [21].Firstly, LDH was calcined in air at 500 • C for 4 h, with a heating rate of 4 • C/min, to prepare the MMO.Secondly, 3.0 g MMO was dispersed into 200 mL 1.0 M Na 2 CO 3 solution and stirred for 5 min; the suspension was then transferred into a temperature-humidity chamber and stirred for 24 h at 6 • C. FLDH was obtained after collection of the precipitates by filtration, washing several times with distilled water, and drying at 90 • C overnight.
The Ag 3 PO 4 /FLDH composites were prepared as follows.3.0 g FLDH mixed with 4.8 g Na 2 HPO 4 was placed into 150 mL of the distilled water under room temperature and stirred for 5 min.A proper amount of AgNO 3 was dissolved in 50 mL of the distilled water and added to the suspension above.After stirring vigorously for 20 min at room temperature, the suspension was filtered, washed several times with distilled water, and dried at 60 • C overnight.By varying the amount of AgNO 3 , the composites with Ag 3 PO 4 /FLDH mass ratios at 1 : 1.6 and 1 : 3 (labeled Ag 3 PO 4 /FLDH-1.6 and Ag 3 PO 4 /FLDH-3, resp.) were prepared.Bare Ag 3 PO 4 sample without FLDH was also prepared by a similar method.

Characterization. The X-ray diffraction (XRD) patterns
of the powder phase compositions were identified by X-ray diffractometer (Model D/Max-2550, Rigaku Co., Japan) using Cu K α irradiation (λ = 1.5406Å) at 40 kV and 100 mA.The size and morphology of the samples were determined by field emission scanning electron microscopy (FE-SEM) (Model S-4800, Hitachi, Japan) and transmission electron microscope (TEM) (Model JEM-2100F, JEOL, Japan).The Brunauer-Emmett-Teller (BET) specific surface area measurement and the Barrett-Joyner-Halenda (BJH) pore volume were performed using a nitrogen adsorption apparatus (Model Autosorb-1MP, Quantachrome Instruments Co., USA).The UV-vis diffuse reflectance spectra of the powder samples were collected in the form of a dry-pressed disk at room temperature with a spectrophotometer (Model Lambda 950, Perkin-Elmer Co., USA).

Photocatalytic Tests.
Photocatalytic activities of the resultant bare Ag 3 PO 4 and Ag 3 PO 4 /FLDH composites were evaluated by the photocatalytic decomposition of acid red G (ARG) under visible light irradiation.A 500 W Xe lamp was used as the light source, and a ZJB 420 filter glass was used to cut off light of wavelength <420 nm.In view of the fact that Ag 3 PO 4 was slightly soluble in aqueous solution [23], the amount of catalysts in every experiment was normalized by the mass of Ag 3 PO 4 really present in each sample and made sure that the amount of Ag 3 PO 4 in ARG solution equal to 1 g/L.The experiments were performed as follows.Amount of photocatalyst was added into 100 mL ARG (50 mg/L).The aqueous suspension was stirred at room temperature and irradiated with visible light.About 3 mL of reaction suspension was sucked at a defined time interval, and the solid material was separated by centrifugation.The removal rates of ARG were measured on a UV-vis spectrophotometer (Model Lambda 35, Perkin-Elmer Co., USA) at the wavelength of 530 nm.

Results and Discussion
3.1.Characterization.Figure 1 shows the XRD patterns of the original LDH, MMO, and FLDH.The typical X-ray pattern of the original LDH (Figure 1(a)) exhibits the typical reflections of Mg-Al-CO 3 -LDH with a series of narrow, symmetric, and sharp peaks, indicating a high degree of crystallinity [24].Calcination of the LDH at 500 • C resulted in the formation of a mixed metal oxide phase with an MgOlike structure (Figure 1(b)).After regeneration by soaking the MMO into a Na 2 CO 3 solution, the calcined LDH have successfully recovered the original layered structure according to the characteristic reflections corresponding to the original LDH (Figure 1(c)), though a loss of some degree of crystallinity as reported elsewhere [25].
The XRD patterns of bare Ag 3 PO 4 , Ag 3 PO 4 /FLDH-1.6,and Ag 3 PO 4 /FLDH-3 are shown in Figure 2.For bare Ag 3 PO 4 (Figure 2(a)), all of the diffraction peaks are well indexed as the body-centered cubic Ag 3 PO 4 (JCPDS No. 06-0505).For the Ag 3 PO 4 /FLDH composites (Figures 2(b) and 2(c)), most of the diffraction peaks could be attributed to the body-centered cubic Ag 3 PO 4 .Meanwhile, small diffraction peaks of LDH have been detected just as that in Figure 1(c), confirming that the FLDH preserve its original structure after being coated with Ag 3 PO 4 .
The FE-SEM images in Figure 3 show the morphologies of LDH and FLDH. Figure 3(a) shows the plate-like morphology typical of LDH [26] with diameters in the range 50-100 nm.After regeneration, as shown in Figure 3  The sizes of the flaky sheets range from 50 to 200 nm, and the thickness is ∼10 nm.It means that these sheets have a large aspect ratio (defined as platelet diameter/thickness) which may lead to an efficient adsorption capability.
The TEM images in Figure 4 show the sizes and morphologies of Ag 3 PO 4 and Ag 3 PO 4 /FLDH-3.The TEM image of bare Ag 3 PO 4 shows agglomerated particles with diameters in the range 200-1000 nm (Figure 4(a)).The average diameter of the well-distributed Ag 3 PO 4 in the Ag 3 PO 4 /FLDH-3 composite (∼200 nm) is smaller than that of the bare Ag 3 PO 4 (Figure 4(a)).From these results, it can be concluded that FLDH plays an important role in hindering the crystal growth and agglomeration of Ag 3 PO 4 .Figure 4(c) shows the energy dispersive spectroscopy (EDS) pattern of Ag 3 PO 4 / FLDH-3.Except for the Cu and C peaks coming from the copper grid for the TEM analysis, the Mg and Al peaks are  1.It can be seen that the original LDH has a BET surface area 110.4 m 2 /g with a relative lower pore volume 0.336 cm 3 /g.After regeneration, the BET specific surface area and pore volume of FLDH are much larger than that of LDH, this may be because the newly formed FLDH has a coarse surface due to the irregular interconnected nanosheets and the remaining slit-like mesopores as shown in Figure 3(b) [21].This higher surface area and the special structure are considered to be a suitable support for Ag 3 PO 4 dispersion.The surface area of Ag 3 PO 4 /FLDH-1.6,and Ag 3 PO 4 /FLDH-3 are slightly lower than that of FLDH and decrease with increasing the Ag 3 PO 4 /FLDH mass ratios.This could be attributed to that the presence of Ag 3 PO 4 can clog the pore of FLDH, and this effect is more and more significant with the amount of Ag 3 PO 4 , which is further confirmed by change in the pore volume.It can also be observed that the surface area and pore volume of bare Ag 3 PO 4 are very low, which is expected to be responsible for the subsequent poor adsorption capacity of ARG.
The UV-vis diffuse reflectance spectra of the bare Ag 3 PO 4 , Ag 3 PO 4 /FLDH-1.6 and Ag 3 PO 4 /FLDH-3 are shown in Figure 5.It can be clearly seen that the bare Ag 3 PO 4 can absorb solar energy with a wavelength shorter than ∼530 nm as reported by Yi et al. [18].As for Ag 3 PO 4 /FLDH composites, the absorption edge of each sample shifts to a slightly shorter wavelength compared with bare Ag 3 PO 4 (∼525 nm for Ag 3 PO 4 /FLDH-1.6 and ∼520 nm for Ag 3 PO 4 /FLDH-3), though the absorption intensity slightly decreases.The decrease of absorption intensity is dependent on the content of Ag 3 PO 4 in these composites.

Degradation of ARG Solution.
The photocatalytic activity of Ag 3 PO 4 may be explained by that Ag 3 PO 4 has a large dispersion of conduction band due to the form of the delocalized π * antibonding states in the conduction band and the inductive effect of PO 4 3− , which helps the separation of electron-hole pairs [27], and a strong oxidation of photoexcited holes in the valence band could be responsible for the dye degradation [18].The photocatalytic activities in decomposing ARG are plotted in Figure 6, where C 0 and C t are the concentration of aqueous ARG in the starting aqueous (50 mg/L) and at time t, respectively.All of the samples showed efficient photocatalytic activities under visible light irradiation, indicating the potential of Ag 3 PO 4 as a photofunctional material for waste water cleaning.It can be clearly seen that both of the Ag 3 PO 4 /FLDH photocatalysts exhibited higher photocatalytic activities for the ARG degradation reaction than bare Ag 3 PO 4 , especially Ag 3 PO 4 /FLDH-3, the ARG dye can be completely degraded within 20 min under visible light irradiation.
The enhanced photocatalytic properties can be attributed to two main factors.First is the much smaller size and good dispersion of Ag 3 PO 4 immobilized on the FLDH.It is well known that the light-generated charge carriers in small-sized semiconductor grains can efficiently transfer to the surface, which results in the decrease of the opportunities for recombination [28].Meanwhile, for bare Ag 3 PO 4 , the by-products, black metallic Ag particles, would appear because of the photocorrosion during the photocatalytic process and attach themselves onto the surface of the Ag 3 PO 4 catalyst, which would inevitably prevent visible light absorption and decrease its photocatalytic activity [23].For the Ag 3 PO 4 /FLDH composites, the photocorrosion still existed, but the Ag particles resulted from the decomposition of much finer Ag 3 PO 4 in the composite were also much smaller compared to that from the bare Ag 3 PO 4 .The finer Ag particles scattered less light and improved the light harvesting in the photocatalytic process.Moreover, a large part of Ag particles would deposit onto the surface of FLDH not coated by Ag 3 PO 4 in the composites, which also further reduce the negative influence.Second is a higher adsorption capability for the composites compared with bare Ag 3 PO 4 .The photocatalytic degradation of a pollutant must proceed at the surface of the catalyst after the reactant has been adsorbed on the surface, so the adsorption is considered to be the prestep for the consequent photocatalytic reaction [29].According to the blank tests carried out like the way of photocatalytic degradation of ARG but without light irradiation, which are shown in Figure 7, it can be clearly seen that the adsorption of ARG on bare Ag 3 PO 4 in the dark was negligible after 60 min, however, the percentages of ARG removal with Ag 3 PO 4 /FLDH-1.6 and Ag 3 PO 4 /FLDH-3 were ∼35% and 55%, respectively.These results indicate that the high adsorption capability is very crucial to the improvement of photocatalytic activation for the Ag 3 PO 4 /FLDH composites.For Ag 3 PO 4 /FLDH-3, the excellent efficiency may be explained based on the smaller particle size, better dispersion of Ag 3 PO 4 on the FLDH, and higher adsorption capability with the amount of FLDH content.

Conclusions
In summary, we synthesized the Ag 3 PO 4 /FLDH composites with fine Ag 3 PO 4 crystalline grains through a wet chemical method.The Ag 3 PO 4 /FLDH composites, the Ag 3 PO 4 / FLDH-3 in particular, exhibited much higher catalytic efficiency than bare Ag 3 PO 4 for the degradation of ARG under visible light irradiation.The enhanced photocatalytic properties can be attributed to the combination of the smallersized and well-distributed Ag 3 PO 4 immobilized on the FLDH and the strong adsorption of the dye on the FLDH.
(b), FLDH is made up of flaky sheets coalesced irregularly with each other.

Table 1 :
Corresponding specific surface area and pore volume for the samples.toFLDH, and the presence of Ag and P elements indicates the formation of Ag 3 PO 4 .This result confirms that Ag 3 PO 4 /FLDH composites were successfully prepared in this study.The BET specific surface area and pore volume of the samples are summarized in Table corresponding