Enhanced photocatalytic properties of the 3D flower-like Mg-Al layered double hydroxides decorated with Ag2CO3 under visible light illumination
Yanhui Ao , Dandan Wang, Peifang Wang * , Chao Wang, Jun Hou, Jin Qian
Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, Nanjing, 210098, China
►3D flower-like Ag2CO3/Mg-Al layered double hydroxide composite was prepared;
►The nanocomposites exhibited high photocatalytic activities on different organic pollutants;
► The mechanism of the enhanced activity were investigated.
Abstract:A facile anion-exchange precipitation method was employed to synthesize 3D flower-like Ag2CO3/Mg-Al layered double hydroxide composite photocatalyst. Results showed that Ag2CO3 nanoparticles dispersed uniformly on the petals of the flower-like Mg-Al LDH. The obtained nanocomposites exhibited high photocatalytic activities on different organic pollutants (cationic and anionic dyes, phenol) under visible light illumination. The high photocatalytic activity can be ascribed to the special structure which accomplishes the wide-distribution of Ag2CO3 nanoparticles on the surfaces of the 3D flower-like nanocomposites. Therefore, it can provide much more active sites for the degradation of organic pollutant. Then the photocatalytic mechanism was also verifified by reactive species trapping experiments in detail. The work would pave a facile way to prepare LDHs based hierarchical photocatalysts with high activity for the degradation of wide range organic pollutants under visible light irradiation.
Keywords: A. composites; A. layered compounds; A. semiconductors; B. chemical synthesis; D. catalytic properties;
Layered double hydroxides (LDHs) are a series of ionic lamellar compounds, which is constituted by positively charged brucited-like hydroxide layers with the presence of balancing anions located between the interlayers. The general formula for this layered structure is expressed as [M1-x 2+Mx 3+ (OH)2][A x/n] n-·mH2O. M2+ and M3+ represent divalent and trivalent metal ions, respectively; Anis an interlayer compensating anion that could be exchanged by inorganic or organic anions [1-4]. According to the special layered structure, flexible modification of chemical compositions, LDHs possess a significant potential in different fields, such as
catalysis, photocatalysis, adsorption, electrochemistry or drug delivery [5-9] . The common layered double hydroxides in nature are the magnesium aluminum hydroxyl carbonate compounds. And this type of the Mg-Al LDHs are always used as the adsorbent in waste water treatment [10-13].Compared to the TiO2 catalyst, Ag2CO3 has a narrow band gap. This advantage makes it a promising visible light responsive photocatalyst [14, 15] . However, Ag2CO3 is unstable. And the polyhedral morphology of Ag2CO3 generally has a big particle size (about 4 um) and could inversely hinder its photocatalytic performance [16, 17] . Therefore, it need to find a way to decrease the size of Ag2CO3 thus to increase its photocatalytic activity. Furthermore, it is important to find a good support for the uniform deposition of nanosized Ag2CO3 thus decrease the aggregation of the nanoparticles.
Thus, in this paper, we try to utilize the Mg-Al LDHs work as the substrate for the growth of nano-Ag2CO3 photocatalyst. Yu et al. had synthesized 3D hierarchical flflower-like Mg-Al layered double hydroxides by a simple solvothermal method  . So we refer to the literature and successfully synthesized 3D hierarchical flflower-like Ag2CO3/Mg-Al LDHs nanocomposites through a facile anion-exchange method. It is found that the small Ag2CO3 nanoparticles distributed evenly on the 3D flower-like Mg-Al LDHs. Meanwhile, the Ag2CO3/Mg-Al LDHs nanocomposites display excellent photocatalytic activities on both cationic/anionic dyes and phenol pollutant degradation.
2. Experimental details
Synthesis of 3D flower-like Mg-Al layered double hydroxide:
product was synthesized by the method reported in previous paper with a little modification . 8 mmol of Mg(NO3)2· 6H2O, 4 mmol of Al(NO3)3· 9H2O and 60 mmol of urea were added into 60 mL mixed dispersants of EG and H2O (V/V, 9/1). The mixed solution was well-mixed and transferred into the Teflon-lined stainless steel autoclaves with capacity of 100 mL. The autoclaves were sealed and maintained at 160℃ for 6 h, then cooled to room temperature naturally. Finally, the products were filtered and washed with ethanol and deionized water several times respectively, and dried at100℃ for 12 h
Preparation of Ag2CO3/Mg-Al LDHs nanocomposites:
To prepare Ag2CO3/Mg-Al LDHs nanocomposites, 1.2 g of the as-prepared Mg-Al LDHs were dispersed and stirred in the 300 mL of 0.5 M Na2CO3 solution for 24 h. The products were filtered and washed with deionized water two times, and dried at 60℃. Then 0.3 g of the intermediate product was added into 10 mL of 0.5 M Na2CO3 solution and maintained at 60℃in the oil bath pan until the solution completely evaporation. Finally, 0.3 g of the samples was fully-dispersed in 50 mL ultrapure water. 20 mL of 0.5 M AgNO3 was slowly added into the above solution which was under vigorous stirring at room temperature. The whole reaction was kept in the dark and stirring at room temperature for 1 h. The resulting product was centrifuged and washed with deionized water and dried at 60 ℃.
Photocatalytic activity experiments:
CEL-HXF300/CEL-HXUV300 Xe light source was used as the Vis-light illumination. The lamp with an optical fifilter (λ≥400 nm) was turned on above 30 min before the photocatalytic degradation experiments to make sure of the stability of the light intensity. The photocatalyst sample (50 mg) was dispersed in 100 mL solution with different initial concentration (MB 10, X-3B 25, phenol 10 mg/L) and stirred under a specific speed by a magnetic stirrer. The reactor was kept at room temperature by cooling with flowing water. Then, 1 ml of the reaction solution was withdrawn by a syringe every 10 min and analyzed.
Characterization:The X-ray diffraction (XRD) patterns of the samples were identified on a SiemensD5005 powder diffract meter in the range of 10°≤2θ≤80°. The UV/Vis diffuse reflectance spectra (DRS) of catalysts were characterized by a UV/Vis spectrometer (UV3600, SHIMADZU). The morphologies and microstructures were observed by field emission scanning electron micrograph (FE-SEM, Hitachi S-4800). TEM image was recorded on a Hitachi H-7650 transmission electron microscope.
3. Results and Discussion
3.1 Characterization of as-prepared products
The XRD patterns of the as-prepared products are shown in Fig. 1. The characteristic reflflections of the Mg-Al LDHs is similar to the literature . The characteristic plane appears at 2θ= 11.52⁰ correspond to (003), which illustrates that the Mg-Al LDHs is successfully synthesized with a basal d-spacing of 0.76 nm. The new peaks observed in the XRD patterns of the Ag2CO3/Mg-Al LDHs are readily indexed to the patterns of Ag2CO3 (JCPDS no. 26-0339)[17-20] . No other diffraction peaks are detected, indicating the high purity of Ag2CO3 crystals formation on the layers of the nanocomposites. Besides, the diffraction peaks of Mg-Al LDHs have no obvious change in the presence of the Ag2CO3; only the decrease in the intensity of diffraction peaks takes place. It verifies that Ag2CO3 decorates successfully onto the surface of the Mg-Al LDHs rather than intercalates between the interlayers [21, 22] . It also means that the growth of Ag2CO3 has no influence on the phase structure of the LDHs
Fig. 2(a) shows the SEM images of the prepared Mg-Al LDHs. As the literature reported that the Mg-Al LDHs material indeed composed of 3D flflower-like spheres and assembles by a series of well-organized plates clearly shown in the inset of the Fig. 2(a) . The morphology of the Ag2CO3/ Mg-Al-NO3 LDHs synthesized by a simple anion-exchange method is shown in Fig. 2(b). It is found that Ag2CO3 particles distribute evenly on the surfaces of the layers in a large scale. The delicate 3D flower-like morphology of the Mg-Al LDHs keeps entirely perfect after the formation of the Ag2CO3, which is consistent with the results of the XRD patterns. Moreover, the inset TEM image in the Fig. 2(b) demonstrates that the Ag2CO3 particles have a small diameter size about 10 nm and anchor closely on the nanoflakes. It further implies that the existence of Ag2CO3 would enhance the Mg-Al LDHs photocatalytic properties for providing more active sites.Fig. 3 shows the UV-Vis diffuse reflflectance spectra of the as-prepared products. The absorption of the Ag2CO3/ Mg-Al-NO3 LDHs nanocomposites is apparent higher than that of the Mg-Al LDHs in the range of 200 to 800 nm[20, 23] , which means the decorated Ag2CO3 nanoparticles play a key role in advancing the absorbance of the Ag2CO3/ Mg-Al-NO3 LDHs nanocomposites. Nevertheless, we chose Vis-light as the source in the photocatalysis based on the previous reports about Ag2CO3 [14, 24, 25] .
Scan and pay attention to us