Morphology-controlled fabrication of Ag3PO4/chitosan nanocomposites with enhanced visible-light photocatalytic performance using different molecular weight chitosan
Graphical abstract
Introduction
Silver orthophosphate (Ag3PO4) is a semiconductor with an indirect band gap of 2.36 eV as well as a direct transition of 2.43 eV, whose photocatalytic capability under visible-light irradiation with quantum efficiency up to 90% was first reported in early 2010 [1]. This novel photocatalyst exhibits extremely high efficiency for the decomposition of organic dyes and has received much attention in the area of wastewater treatment [2], [3], [4]. However, there are some notable problems that existed in the Ag3PO4 photocatalytic system: relatively large particle size limits the photocatalytic performance of Ag3PO4, slight solubility in aqueous solution and severe photocorrosion reduces its stability and hinders their practical applications [5], [6], [7]. Thus, the fabrication of Ag3PO4 photocatalytic system with high photocatalytic activity and enhanced stability is still a great challenge.
Because photocatalytic reactions occur on the surface of catalysts, the large specific surface area and crystal-facet-controlled surface atomic structure are expected to enhance photocatalytic reactivity [8]. Nanoscale size particles and some crystal-facet-controlled crystal forms of Ag3PO4 including cube [2], rhombic dodecahedron [2], necklace-like [3], and tetrapods [9], have been successfully fabricated during the past few years. In addition, various Ag3PO4-based composite photocatalysts, such as Ag3PO4/TiO2 [5], Ag3PO4/AgX [7], CdS@Ag3PO4 [10], Ag3PO4/Bi2WO6 [11], Ag3PO4/FLDH [12], gC3N4-Ag3PO4 [13], Ag3PO4/RGO [14], and Ag@Ag3PO4/RGO [15], have also been developed to improve the photocatalytic activity and stability of catalysts. Dinh and co-workers [16] synthesized 8–16 nm Ag3PO4 nanoparticles and demonstrated that their photocatalytic activities were superior to micron-sized Ag3PO4 particles. Ma et al. [17] prepared Ag3PO4/bentonite composites with fine Ag3PO4 crystalline growing in the bentonite inter-layers, and the composites exhibited higher visible light photocatalytic efficiency compared with native Ag3PO4 particles. Reduced graphite oxide sheets (RGOs) were used to fabricate Ag3PO4/RGO [14] composites, and the photocatalytic rate and stability of composites were greatly enhanced owing to the properties of RGOs, such as the large specific surface area, adsorption to pollutants and the ability to inhibit the photocorrosion of Ag3PO4.
Chitosan (CS), a linear random copolymer of β-1,4-d-glucose-2-amine and N-acetyl-d-glucose-2-amine, has excellent properties for the adsorption of metal ions and some organic pollutants owing to the presence of high content of amino (–NH2) and hydroxyl (–OH) groups in the polymer matrix, which encourages its potential applications as adsorbents and soft-templates for synthesizing semiconductor composite catalysts [18]. Chitosan has also been used as stabilizing agent for silver nanomaterial [19]. As well known, the conformation of chitosan in solution is manipulated by the molecular structure and the properties of solution. The molecular structure parameters of chitosan include molecular weight (MW) and degree of deacetylation, and the properties of solution include ionic strength, solvent, temperature and pH value of solution [18], [20], [21], [22]. A lot of researches have demonstrated that the conformation of chitosan in solution played a major role on the behavior of the chitosan in solution through the coordination bond and electrostatic interactions between the chitosan molecules and other substances [21]. The effects of molecular weight of chitosan on their antibacterial, antifungal and antineoplastic activities have been demonstrated [20], [21], [22], [23]. Different agglomeration states of chitosan in solution provided various templates for the controlled growth of nanoparticles in chitosan-based composites [24]. Honary and co-workers [25] reported that the size of Ag–CS nanoparticles and antibacterial properties could be modulated by varying the molecular weight of chitosan, confirming that molecular weight had a profound effect on the adsorption of chitosan molecules to Ag+ ions. Our group has previously prepared CdS/CS [26] and Cu2O/CS [27] nanocomposites by using chitosan as precursor. As the chelation of –NH2 and –OH groups with metal ions dispersed metal ions homogeneously, the chitosan chain hindered nanoparticles from agglomeration and growing larger. The as-prepared nanocomposites exhibited enhanced visible-light photocatalytic activity and higher adsorb ability for dye molecules. Nevertheless, the influence of molecular weight of chitosan on the morphology and behaviors of nanocomposites was not discussed.
In the present study, we developed a series of nanocomposites of Ag3PO4/CS by a facile liquid phase precipitation procedure with chitosan as adsorbent and soft template. The effect of chitosan molecular weight on their morphology and structure was investigated. The photocatalytic performance of samples under visible light irradiation was also evaluated with methyl orange (MO) as a target pollutant.
Section snippets
Sample preparation
Chitosan with 88% of deacetylation degree was purchased from Jinke Biochemical Co., Ltd. (No. K101210245, Zhejiang, China). The low molecular weight chitosan was obtained according to a previously reported method [28]: 5.0 g chitosan was dissolved in 150 mL 3% (V/V) acetic acid solution, and then 50 mL 6% (V/V) H2O2 aqueous solution was added dropwisely in 1 h under continuous stirring. The solution was kept in water bath at 50 °C. At a given time interval, a certain volume of chitosan solution was
Morphology and structure
The morphology and size of the photocatalysts were researched by using TEM and FESEM. The TEM images were shown in Fig. 1a–e, and the FESEM images were shown in Fig. 1A–E, respectively. As seen in Fig. 1a, A, the pure Ag3PO4 powders were quasi-spherical or ellipsoidal in shape and about 200–700 nm in diameter, the surface of the block was smooth. Meanwhile, the obtained composites presented fantastic changes in morphology with the decrease of chitosan molecular weight. The Ag3PO4 particles
Conclusions
In summary, Ag3PO4/CS composites with different structures and morphologies were prepared via in situ growth of Ag3PO4 process in the presence of different molecular weight chitosan. Using low molecular weight chitosan (MW = 9.9 × 103) as adsorbent and soft-template, flower-like sphere (150–400 nm) distributed with nanoscale Ag3PO4 crystalline grains on its surface was formed. In the presence of high molecular weight chitosan, the Ag3PO4 particles with diameter range from 200 to 550 nm in cubic or
Acknowledgments
The work was financially supported by the Natural Science Foundation of Hubei Province, China (No. 2014CFB728). Special thanks to Prof. Hong Yuan and his group from Huazhong Agricultural University for providing experimental facilities.
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2018, Solid State SciencesCitation Excerpt :The N1s and Cl2p were also identified in the sample (Fig. 7(e,f)), indicating that the products of RhB degradation contain nitrogen and chlorine adsorbed on to the surface of Ag3PO4. The peak with the BE of 399.7 eV at N1s was assigned to C-N configuration [32,33]. The BE of 197.5 eV and 199.5 eV were assigned to Cl 2p3/2 and 2p1/2 spin-orbit doublet [34], respectively.