chitosan nanocomposites with enhanced visible-light photocatalytic performance using different molecular weight chitosan

Powder Technology 292 (2016) 186–194

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Morphology-controlled fabrication of Ag3PO4/chitosan nanocomposites with enhanced visible-light photocatalytic performance using different molecular weight chitosan Qihua Cao a, Ling Xiao a,⁎, Jin Li a, Chunhua Cao b, Sheng Li a, Jian Wang a a b

School of Resource and Environmental Sciences, Wuhan University, Wuhan 430072, PR China Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, College of Chemical and Environmental Engineering, Jianghan University, Wuhan 430056, PR China

a r t i c l e

i n f o

Article history: Received 27 October 2015 Received in revised form 14 January 2016 Accepted 2 February 2016 Available online 3 February 2016 Keywords: Chitosan Silver orthophosphate Nanocomposite Morphology-control Photocatalysis

a b s t r a c t A series of Ag3PO4 and chitosan (CS) nanocomposites (Ag3PO4/CS) were prepared through in situ growth of Ag3PO4 with different molecular weight chitosan as adsorbent and soft-template. The structure and properties of as-prepared Ag3PO4/CS nanocomposites were characterized by XRD, FT-IR, FESEM, TEM, XPS, BET and UV– vis/DRS. The visible-light photocatalytic decolorization behavior of Ag3PO4/CS nanocomposites was investigated using methyl orange as a model pollutant. It was found that the morphology and photocatalytic decolorization activity of the nanocomposites significantly depended on the molecular weight of chitosan. The decolorization of methyl orange was ascribed to the synergistic effect of photocatalysis and adsorption. When Ag3PO4/CS nanocomposites were prepared in the presence of high molecular weight chitosan, Ag3PO4 particles with cubic or rhombic dodecahedron shape and about 200 to 550 nm in diameter were wrapped in chitosan. Meanwhile, when Ag3PO4/CS nanocomposites were fabricated using low molecular weight chitosan as soft-template, 150– 400 nm flower-like spheres were obtained which were aggregated by a large number of nanoscale Ag3PO4 crystalline grains and connected by chitosan matrix. The special morphology of Ag3PO4/CS nanocomposites greatly increased the active surface area and the photocatalytic performance. The results in this study suggested that semiconductor photocatalysts with novel morphology and enhanced photocatalytic ability could be fabricated by using different molecular weight chitosan as soft-template. © 2016 Elsevier B.V. All rights reserved.

1. 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 [2–4]. 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 [5–7]. 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]. ⁎ Corresponding author at: The Luojia Mountain of Wuchang, Wuhan City, Hubei Province 430072, PR China. E-mail address: [email protected] (L. Xiao).

http://dx.doi.org/10.1016/j.powtec.2016.02.003 0032-5910/© 2016 Elsevier B.V. All rights reserved.

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 Ag3 PO 4 nanoparticles and demonstrated that their photocatalytic activities were superior to micron-sized Ag 3PO4 particles. Ma et al. [17] prepared Ag3PO4 / bentonite composites with fine Ag3PO 4 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-2amine and N-acetyl-D-glucose-2-amine, has excellent properties for the adsorption of metal ions and some organic pollutants owing to the

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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 [18,20–22]. 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 [20–23]. 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.

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The preparation procedure of Ag3PO4/CS nanocomposites was as follows: 0.1 g chitosan was dissolved in 100 mL 0.1% (V/V) acetic acid aqueous solution, and pH value was adjusted to 6.0 with 2 mol L− 1 NaOH solution. 6 mL 0.3 mol L−1 AgNO3 solution was dropped into chitosan colloidal solution under continuous stirring. The mixed solution was kept stirred for 2 h to facilitate chelating complexes between chitosan and Ag+ ions. Then, 6 mL 0.1 mol L−1 Na2HPO4 aqueous solution was dropped into the above mixture and the yellow precipitate formed subsequently. Vigorous stirring was continued for 1 h at pH 7.0. All experiments were carried out at room temperature. Finally, the yellow products were collected by centrifugation, washed with deionized water for several times, and then lyophilized in vacuum. According to the molecular weight of CS used in the synthesis, the obtained samples were labeled as Ag3PO4/CS0, Ag3PO4/CS4, Ag3PO4/CS6, and Ag3PO4/CS8, respectively. For comparison, Ag3PO4 was also prepared by a simple precipitation procedure in the absence of chitosan. 2.2. Characterization Crystal patterns of the samples were obtained on an X'Pert Pro X-ray diffraction spectrometer (PANalytical) with a Cu Kα target at 40 kV and 40 mA at a scan rate of 4°/min from 10° to 80° (2θ). FT-IR transmission spectra were recorded on a NICOLET 5700 Fourier transform infrared spectrometer (Thermo Fisher Scientific, USA) in the range of 4000– 400 cm−1. The size and surface morphology of samples were observed by using field emission scanning electron microscopy (FESEM) (Zeiss, German) and transmission electron microscopy (TEM) (Model JEM2100, JEOL, Japan). The X-ray photoelectron spectroscopy (XPS) measurements were conducted on an ESCALAB 250Xi spectrometer (Thermo Scientific, USA) by using monochromatic Al Kα X-ray excitation source. The ultraviolet–visible diffuse reflection absorptive spectra (UV–vis/DRS) were obtained at room temperature with UV-2550 UV–visible spectrophotometer (Shimadzu, Japan). The specific surface area was measured at 77 K through N2 sorption analysis using the Brunauer–Emmett–Teller (BET) method (BELSORP-max, Japan). 2.3. Evaluation of photocatalytic activity

2. Experimental 2.1. 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 taken out, the chitosan was precipitated with 10 mol L−1 NaOH solution and washed with deionized water until neutral, then lyophilized. The samples were labeled as CS0, CS4, CS6, and CS8, respectively, according to the degradation times of 0, 4, 6, and 8 h. The molecular weight of chitosan was determined by the capillary viscometry method using Ubbelohde viscometer at 25 °C. All of the molecular weight data of samples were displayed in Table 1. Table 1 Parameters of Ag3PO4and Ag3PO4/CS nanocomposites. Catalyst

CS MW (×103)

Surface area (m2 g−1)

Pore volume (cm3 g−1)

Particle size of Ag3PO4a (nm)

Ag3PO4 Ag3PO4/CS0 Ag3PO4/CS4 Ag3PO4/CS6 Ag3PO4/CS8

– 538 31.6 15.4 9.9

0.66 2.96 3.01 10.96 12.78

0.0011 0.0074 0.0403 0.0552 0.0793

200–700 200–550 12–32 11–30 8–30

a

The particle sizes of Ag3PO4 were estimated from TEM images.

The evaluation of photocatalytic activity was performed by the photocatalytic decolorization of MO solutions under visible light irradiation. The initial pH of MO solution was adjusted to 5.0 using the mixture of 0.01 mol L− 1 phosphoric acid and 0.01 mol L− 1 sodium dihydrogen phosphate. Typically, 10 mg of the photocatalyst was dispersed in 50 mL of 10 mg L−1 MO dye solution with constant magnetic stirring, and then the suspension was irradiated with a 300 W Xe arc lamp (15 A) equipped with an ultraviolet cutoff filter to provide visible light with λ N 420 nm. The irradiation source was positioned 10 cm away from the reactor to trigger the photocatalytic reaction. A cold water circulation device was used to eliminate the concomitant heat during irradiation. At certain time intervals, about 3 mL of the suspension was collected and separated by centrifugation. The concentration variations of MO dye solution were recorded by the light absorption of the clear liquor at 464 nm (λmax for MO solution). In order to investigate the adsorption behavior of photocatalysts, adsorption experiments were carried out in dark under other identical conditions. Decolorization rate (D) was calculated by the following equation (Eq. (1)): Dð%Þ ¼ ðC 0 −C t Þ=C 0  100:

ð1Þ

When initial concentration of dye is lower, the photocatalytic reaction kinetics is found to be expressed by the Langmuir–Hinshelwood model as follows [26,27]: ln ðC t =C 0 Þ ¼ kapp t

ð2Þ

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where D (%) was the decolorization rate of MO, Ct was the MO concentration (mg L−1) at time t minutes, C0 was the initial concentration of MO (mg L−1), kapp was the apparent pseudo-first-order reaction rate constant (min−1), and t was the reaction time (min). 3. Results and discussion 3.1. 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 dispersed in Ag3PO4/CS0 samples were irregular cubic or rhombic dodecahedron in shape with a diameter range from 200 nm to 550 nm, and their surface was rough due to the coating of chitosan layer (Fig. 1b, B). However, as shown in the FESEM images (Fig. 1C, D, E), Ag3PO4/CS4, Ag3PO4/CS6 and Ag3PO4/CS8 were flowerlike spheres with rough surface and the diameter ranges from 150 nm to 400 nm, completely different from the morphology of Ag3PO4/CS0. The TEM images in Fig. 1c, d, e showed that the structures of these flower-like spheres were different according to the molecular weight of chitosan. In Ag3PO4/CS4, the flower-like spheres were the aggregates of a large number of Ag3PO4 nanoparticles, and these Ag3PO4 nanoparticles were connected compactly by chitosan matrix (Fig. 1c). In Ag3PO4/CS6, the flower-like spheres were also the aggregates of a large number of Ag3PO4 nanoparticles, but, these Ag3PO4 nanoparticles were connected loosely by chitosan matrix (Fig. 1d). Meanwhile, the flower-like spheres in Ag3PO4/CS8 were the aggregates of small chitosan sphere which was covered by fine Ag3PO4 crystalline (Fig. 1e). The obvious contrast (dark/bright) between the boundary and the center of the small spheres implied that Ag3PO4 nanoparticles uniformly distributed on the surface of chitosan sphere. The particle sizes of Ag3PO4 nanoparticles in Ag3PO4/CS4, Ag3PO4/CS6 and Ag3PO4/CS8 were estimated from TEM images, and the results are listed in Table 1. The Ag3PO4 particle sizes decreased with the decrease of chitosan molecular weight involved in the synthesis process. The specific surface area and pore structure of samples were investigated by measuring N2 adsorption–desorption isotherms. Compared with that of pure Ag3PO4, the specific surface area, and pore volume of Ag3PO4/CS nanocomposites were much larger and increased as the molecular weight of chitosan decreased (Table 1). The pore volume of pure Ag3PO4 was 0.0011 cm3 g−1, thereby pure Ag3PO4 had a low BET specific surface area of 0.66 m2 g−1. Owing to the special aggregate structure (as shown in Fig. 1), Ag3PO4/CS8 nanocomposites had porous structure, with the pore volume increased to 0.0793 cm3 g−1. Thus a relatively high specific surface area of 12.78 m2 g−1 was measured. Low molecular weight chitosan exhibited outstanding properties for controlling the dimension and shape evolution of Ag3PO4 nanocrystallines. The special structure and high specific surface area of Ag3PO4/CS8 would be expected to effectively improve their adsorption and photocatalytic activity. The FTIR spectra of pure Ag3PO4 and Ag3PO4/CS nanocomposites were shown in Fig. 2. In FTIR spectra of pure Ag3PO4, the bands around 3100 cm−1 and 1656 cm− 1 could be assigned to the stretching and bending vibration of OH of water molecules which were connected onto Ag3PO4 [14]. Peaks at 1012 cm−1 and 560 cm−1 were assigned to P–O groups, which comprised the phosphate non-bridging oxygen portion of PO34 − tetrahedra in a chain structure [14,29]. Absorption bands at 3340 cm− 1 and 2877 cm− 1 in chitosan FTIR spectra were assigned to the amino (–NH2), hydroxyl (–OH) groups, and –CH2, – CH3 aliphatic groups, the strong absorption peaks centered at 1651 cm−1 and 1602 cm−1 corresponding to the amide I and amide II

vibrations respectively, and the band at 1421 cm−1 was attributed to the bending vibration of the –OH group. The characteristic adsorption in band at 560 cm−1 corresponded to the P–O bond vibration of PO3− 4 FTIR spectra of Ag3PO4/CS nanocomposites, indicating that Ag3PO4 was in situ formed in Ag3PO4/CS nanocomposites. Compared with the spectra of chitosan and Ag3PO4, Ag3PO4/CS0 showed mainly the characteristic absorption peaks of chitosan, Ag3PO4/CS4, Ag3PO4/CS6 and Ag3PO4/CS8 displayed mainly the prominent peaks of Ag3PO4. The band strength at 560 cm−1 increased as the molecular weight of chitosan decreased, proving that the content of Ag3PO4 in Ag3PO4/CS nanocomposites increased as the molecular weight of chitosan decreased. Meanwhile, the bands of O–H and –NH2 at 1602 cm− 1 and 1421 cm−1 shifted to 1588 cm−1 and 1411 cm−1 respectively, which demonstrated the chelation of Ag+ ion by the hydroxyl and the amino groups of chitosan. The as-synthesized Ag3PO4/CS nanocomposites were also identified by XRD analysis. For comparison, the XRD of pure Ag3PO4 and chitosan was also included. The XRD patterns of the samples were shown in Fig. 3. Chitosan material showed a typical peak at 2θ = 20.20°(inset in Fig. 3). In the XRD pattern of pure Ag3PO4 and Ag3PO4/CS nanocomposites, there were ten clear peaks with 2θ values at 20.93°, 29.72°, 33.35°, 36.63°, 47.84°, 52.73°, 55.07°, 57.32°, 61.68°, and 71.95°, corresponding to the crystal planes of (110), (200), (210), (211), (310), (222), (320), (321), (400), and (421) of body-centered cubic phase Ag3PO4 (JCPDS card No. 06-0505), respectively. These results indicated that Ag3PO4 particles were in situ formed in Ag3PO4/CS nanocomposites. No other diffraction peaks arising from possible impurities such as Ag (JCPDS card No. 04-0783) and AgO were detected, suggesting that the composites were composed of Ag3PO4 and chitosan. Characteristic peak of chitosan near 20° was not observed obviously in the XRD pattern of Ag3PO4/CS nanocomposites due to the relatively low content or weak intensity of chitosan. Moreover, the intensity of diffraction peaks of Ag3PO4 in Ag3PO4/CS nanocomposites was lower than that of pure Ag3PO4 and decreased as the molecular weight of chitosan decreased. The average crystallite sizes of Ag3PO4 nanoparticles in Ag3PO4/CS4, Ag3PO4/CS6, and Ag3PO4/CS8 nanocomposites calculated with the Scherrer equation based on the three diffraction peaks at 29.72°, 33.35°, and 36.63° were 25, 20 and 18 nm respectively. The data of Ag3PO4 and Ag3PO4/CS0 were not given out because the Scherrer equation was only valid with particles in the range of 10–100 nm. The average Ag3PO4 crystallite sizes in Ag3PO4/CS4, Ag3PO4/CS6, and Ag3PO4/CS8 nanocomposites were in agreement with results obtained from the TEM and decreased with the decrease of chitosan molecular weight involved in the synthesis process. XPS was employed to investigate the atomic compositions and the atomic binding energies on the surface of materials with about 4 nm of deep width. The XPS spectra of survey, Ag (3d), N (1s), and O (1s) of pure Ag3PO4, CS and Ag3PO4/CS nanocomposites were shown in Fig. 4a–d, respectively. As shown in Fig. 4a, all of the as-synthesized Ag3PO4/CS nanocomposites contained the elements Ag, P, O, N, and C, suggesting that Ag3PO4/CS nanocomposites had been successfully fabricated. The peaks at 373.9 eV and 367.9 eV in Fig. 4b could be assigned to the binding energies of Ag(3d5/2) and Ag(3d3/2), respectively, which were attributed to Ag+ ions in Ag3PO4 [19,30]. Compared with pure Ag3PO4, the binding energies of Ag(3d5/2) and Ag(3d3/2) in Ag3PO4/ CS nanocomposites had a slight shift, indicating that Ag+ was chelated with chitosan and the electron density at Ag+ was effected by other atoms. As seen in Fig. 4c, the O (1s) peak at 530.6 eV in XPS spectra of Ag3PO4/CS nanocomposites was from P–O of Ag3PO4 [31], and the peak at 532.7 eV was composed of HO from Ag3PO4 and chitosan [27, 32]. Although the total content of Ag3PO4 was maintained the same in all Ag3PO4/CS nanocomposites during preparation, the weight of organism in all prepared Ag3PO4/CS nanocomposites was in the range of 44– 48% (thermogravimetric analysis data not show), and the chitosan contents on the surface of Ag3PO4/CS nanocomposites calculated from the peak intensity ratio at 532.7 eV were 70.4%, 60.5%, 57.6% and 48.5%,

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Fig. 1. TEM (a–e) and FESEM (A–E) images of Ag3PO4 and Ag3PO4/CS nanocomposites. a, A: Ag3PO4; b, B: Ag3PO4/CS0; c, C: Ag3PO4/CS4; d, D: Ag3PO4/CS6; e, E: Ag3PO4/CS8.

corresponding to Ag3PO4/CS0, Ag3PO4/CS4, Ag3PO4/CS6 and Ag3PO4/ CS8 respectively, which indicated that chitosan content on the surface of Ag3PO4/CS nanocomposites was decreased with the chitosan

molecular weight decreasing. The broad peak of N (1s) in Ag3PO4/CS nanocomposites (Fig. 4c) was composed of two peaks at 399.5 eV and 401.5 eV. The peak with the binding energy of 399.5 eV was assigned

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3.2. Photocatalytic performance

Fig. 2. FT-IR spectra of pure Ag3PO4, Ag3PO4/CS nanocomposites, and CS.

to free N in chitosan (Fig. 4d), and the peak at 401.5 eV was attributed to N binding to Ag in chitosan. Dinh et al. [16] has observed the similar peak in the N1s XPS spectrum of oleylamine-capped Ag3PO4 nanocomposites, which was attributed to the interaction between Ag3PO4 and oleylamine through hydrogen-bonding interactions or proton exchange on the surface. The peak intensity at 401.5 eV increased with the decrease of the chitosan molecular weight, indicating that the chitosan content on the surface of Ag3PO4/CS nanocomposites decreased and the amount of Ag3PO4 nanoparticles on the surface of materials increased as the molecular weight of chitosan decreased. These results of N1s and O1s XPS spectra were in agreement with results observed from FTIR and TEM. The main ingredients on the surface of nanocomposites tended to be Ag3PO4 nanoparticles as the molecular weight of chitosan decreased. The light-absorbance property of pure Ag3PO4, and Ag3PO4/CS nanocomposites was investigated and the UV–vis/DRS spectra were displayed in Fig. 5. Four as-synthesized Ag3PO4/CS nanocomposites showed similar UV–vis/DRS spectra. The absorption edge of pure Ag3PO4 was at around 530 nm, the same as that reported by Yi et al. [1]. The absorption edges of Ag3PO4/CS nanocomposites extended largely to the whole visible light region, though the absorption intensity in visible light region (above 480 nm) decreased slightly, similar to the other case of Ag3PO4 based composites [13–15,17]. Generally, the red shift of the photoabsorption edges could enhance the photocatalytic efficiency of Ag3PO4 in the visible region [8,33].

Fig. 3. XRD patterns of pure Ag3PO4, CS and Ag3PO4/CS nanocomposites.

MO was a stable negatively charged organic dye and widely used as a target dye with no sensitization effect. The electrostatic interaction between positively charged chitosan molecule and the anionic MO dye was believed to be responsible for the high adsorption capacity. Therefore, the photocatalytic performance of the photocatalysts was explored using MO dyes as a model dye. Fig. 6a showed the time-dependent absorbance spectra of the MO solution in the presence of Ag3PO4/CS8 under visible light irradiation. The characteristic absorption peak of MO at 464 nm decreased with the irradiation time increasing and almost disappeared within 35 min. Fig. 6b showed the decolorization rates of MO in the presence of different photocatalysts under visible light irradiation. The kapp values were obtained from the slope of plot of − ln(Ct / C0) versus t (Fig. 6c) and were shown in Fig. 6d. Using Ag3PO4, Ag3PO4/CS0, Ag3PO4/CS4, Ag3PO4/CS6, or Ag3PO4/CS8 as catalysts, the decolorization rates of MO were about 88.2%, 90.5%, 94.5%, 97.7%, and 99.2% respectively after 35 min of irradiation, while the kapp values were 0.061, 0.067, 0.081, 0.103, 0.133 min−1 respectively. Compared with that of pure Ag3PO4, the decolorization rate and kapp value of Ag3PO4/CS nanocomposites were larger and increased as the molecular weight of chitosan decreased, and Ag3PO4/CS8 showed highest decolorization rate for MO. The heterogeneous photocatalytic reactions mainly occur on the interfacial layers of materials, and the efficiency of photocatalytic reaction was influenced by three factors [8,33]: 1) the affinity adsorption properties between the reactants and photocatalyst surface; 2) the efficiency of electron–hole pairs generated by excited photocatalyst; and 3) the separation and transfer of photo-generated electron–hole pairs. To further understand the adsorption and photocatalytic activity of Ag3PO4/CS nanocomposites, the solutions with MO concentration of 10 mg L−1 at pH 5.0 containing 200 mg L−1 Ag3PO4/CS8 photocatalyst were treated in dark or under visible light irradiation respectively. The experimental results were shown in Fig. 7. MO was hardly decolorized under visible light irradiation in the absence of the catalyst (curve a in Fig. 7), indicating that the photolysis factor of MO was weak and could be negligible in the evaluation of photocatalytic efficiency. The decolorization rate of MO was only 48.9% in the presence of Ag3PO4/CS8 nanocomposites in dark within 50 min (curve b in Fig. 7), which was ascribed to the adsorption behavior of the catalyst. After a 30 min visible light irradiation in the presence of Ag3PO4/CS8 nanocomposites, the decolorization rate of MO was up to 97.9%, revealing that the decolorization of MO was mainly attributed to the synergistic effect of photocatalysis and absorption. Because pure chitosan did not show photocatalysis activity and pure Ag3PO4 adsorbed only 2.1% MO (data not shown), the photocatalytic decolorization performances of the Ag3PO4/CS nanocomposites could be attributed to the photocatalysis activity of Ag3PO4 nanoparticles and absorption ability of chitosan. The highest decolorization rate of Ag3PO4/ CS8 nanocomposites was due to their special structure, the highest content of Ag3PO4 nanoparticles on the surface of Ag3PO4/CS nanocomposites and the highest specific surface area, which was demonstrated by the analyses results of XPS, TEM and specific surface area. The stability and reusability of composites were also evaluated by taking Ag3PO4/CS8 as an example. After each run, the photocatalysts were separated by centrifugation and washed with deionized water for several times. The recyclable results for pure Ag3PO4 and Ag3PO4/ CS8 nanocomposites were shown in Fig. 8a. After 4 cycles, the decolorization rates of pure Ag3PO4 for MO were largely decreased from 85.5% to 37.3%. Meanwhile, the photocatalytic efficiency of Ag3PO4/CS8 nanocomposites had only a slight loss, and the decolorization rates were only changed from 96.8% to 91.0%. Ag3PO4/CS8 exhibited much better photocatalytic stability than pure Ag3PO4. The XPS scans over the Ag3d peak of pure Ag3PO4 and Ag3PO4/CS8 nanocomposites before and after four cycling runs were shown in Fig. 8b. After four runs, the Ag3d peak could be decomposed into two

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Fig. 4. XPS spectra of Ag3PO4, CS, and Ag3PO4/CS nanocomposites. a: survey spectra; b: Ag(3d); c: O(1s); d: N(1s).

group peaks [5,6]: the group at ~373.9 eV and ~367.9 eV was attributed to Ag+ ions, and the small group at ~ 374.6 eV and ~ 368.6 eV was for typical values for Ag0. About 61.5% and 10.3% of Ag+ ions in pure Ag3PO4 and Ag3PO4/CS8 nanocomposites were reduced into metallic silver respectively. According to the reports [5,10,13,14], a big loss of Ag+ ions from the Ag3PO4 photocatalyst sharply reduced its photocatalytic activity. Thus, the XPS analysis further confirmed that Ag3PO4/CS8 nanocomposites had less photocorrosion and showed better stability and reusability than pure Ag3PO4, suggesting that chitosan could partially inhibit the photocorrosion of Ag3PO4. In addition, the strong interaction between Ag3PO4 and chitosan could efficiently prevent Ag3PO4 from dissolution during the photocatalytic process, and further enhanced the stability of Ag3PO4/CS8 nanocomposites. 3.3. Synthesis mechanism for Ag3PO4/CS nanocomposites It was commonly believed that there existed two mechanisms in the interaction between Ag+ ions and chitosan molecule: one was the chelation of Ag+ ion by the hydroxyl groups and/or the amino group existed in the chitosan molecule [25], and the other was the reduction of Ag+ ion to Ag with the hydroxyl groups or ether bonds [34]. According to the literature, the presence of the 1760 cm−1 signal in FTIR spectra was due to the reduction of the silver ions coupled to the oxidation of the hydroxyl groups in chitosan molecule [34]. In this study, the 1760 cm− 1 absorption peak was not observed in the FTIR spectra of Ag3PO4/CS nanocomposites. The characterization results from XPS, FTIR and XRD showed that the Ag+ ions were first chelated by the hydroxyl and the amino group that existed in the chitosan molecule, then Ag3PO4 was in situ formed, and no oxide-reduction process occurred. The fabrication of Ag3PO4/CS comprised two steps: the first is the adsorption between chitosan and Ag+ ions, the second is the formation of Ag3PO4/CS nanocomposites. The occurrence of molecular-weight-induced morphology transition depended on the conformation differences resulting from inter- and/or

intra-molecular interactions among high and low molecular weight chitosan [20]. For chitosan with same deacetylation degree in solution with the same pH and ionic strength, high molecular weight chitosan had more intra-molecular hydrogen bonding and/or more even charge distribution than low molecular weight ones. In turn, low molecular weight chitosan had greater tendency to become extended than high molecular weight ones [21,35]. Chen et al. demonstrated that the conformation of chitosan (MW ≥ 223 × 103) was in a random coil, and the conformation of chitosan (MW ≤ 148 × 103) was in a rod shape [20]. Additionally, due to existence of both hydrophilic and hydrophobic sites, chitosan could form both intra- and inter-molecular reversible junctions in solution, and then form either finite size clusters (aggregates) or infinite clusters (physical gels) [36]. The aggregation was accompanied with the formation of hydrophobic domains, and the size of chitosan chains was one of the key factors affecting the size of

Fig. 5. UV-DRS absorption spectra of pure Ag3PO4, and Ag3PO4/CS nanocomposites.

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Fig. 6. Photocatalytic decolorization of MO solution. a: Absorption spectra of MO solution using Ag3PO4/CS8 as catalyst; b: photocatalytic decolorization rate of different catalysts under visible light; c: plots of −ln(Ct / C0) versus irradiation time; d: apparent rate constant for photocatalytic decolorization of MO solution with different catalysts.

hydrophobic domains [36]. The foreign metal ions were only allowed to combine with –NH2 and/or –OH beyond hydrophobic domains of the chitosan chains, therefore, the size of hydrophobic domains and the stiffness of chain would jointly determine the ultimate morphology of chitosan-based composites. In present study, for Ag3PO4/CS0 nanocomposites, high molecular weight chitosan molecules entangled with each other via the intra- and inter-molecular hydrogen bonds to form an extremely compact network structure. Therefore, only few Ag+ ions were allowed to enter the structure and combine with amino and/or hydroxyl groups to form strict Ag+–CS complexes, while most Ag+ ions complexed with amino or hydroxyl groups out of the network structure. When PO34 − anions added, Ag3PO4 nanoparticles were in situ formed and submerged in chitosan layer. In contrast, chitosan molecules of low molecular weight (CS4, CS6) existed in wormlike in dilute solution owing to the less flexibility and more extended conformation, which were in favor of the uniform adsorption of Ag+ ions on chitosan chains. When the chitosan chains were short enough, such as CS8, the stiff chain aggregated together to form submicron-sized clusters of hydrophobic domains, and the foreign Ag+ ions were mainly combined with –NH2 and –OH groups located at the ends of chains beyond hydrophobic domains. Therefore, flower-like nanosphere morphology of Ag3PO4/CS8 formed. The possible synthesis mechanism of Ag3PO4/CS nanocomposites with different molecular weight chitosan was illustrated in Fig. 9. The morphology controlled Ag3PO4/CS nanocomposites could be fabricated by in situ growth of Ag3PO4 with different molecular weight chitosan as adsorbent and soft-template.

200 to 550 nm in cubic or rhombic dodecahedron shape were wrapped by chitosan layer on the surface. The adsorption and visible-light-driven photocatalytic activity of Ag3PO4/CS composites remarkably depended on the molecular weight of chitosan. It could be supposed that some novel morphology semiconductor photocatalysts such as other silver salt and copper compound with high photocatalytic performance could be fabricated by using different molecular weight chitosan as soft-template in the future.

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.

4. 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

Fig. 7. Photocatalytic decolorization curves of MO under different treatment conditions.

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Fig. 8. (a) Recyclability of Ag3PO4/CS8 nanocomposites and Ag3PO4; (b) XPS scans over Ag3d peaks before and after four cycling runs.

Fig. 9. Scheme illustration of the fabrication of Ag3PO4/CS nanocomposites.

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