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

Graphical abstract

Research Highlight:

►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;

1. Introduction

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 [10] .  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[10] . 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[10] . 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)[10] . 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] .