Controlled synthesis and wastewater treatment of Ag₂O/TiO₂ modified chitosan-based photocatalytic film

Abstract

A novel Ag₂O/TiO₂-modified chitosan-based photocatalytic film with high adsorption and photocatalytic activity was synthesized under simulated solar irradiation, based upon the coupling of the synergistic catalytic technique of nano Ag₂O/TiO₂ and membrane separation. XRD, XPS, FESEM, and TEM characterization results illustrated that the Ag₂O nanoparticles with a small size of 3–5 nm deposited on the surface of TiO₂. Subsequently, Ag₂O–TiO₂ nanoheterojunctions, as whole graft-structures, were coated on a chitosan-based polypropylene film. The as-synthesized photocatalytic film with a wide visible-light adsorption band and lower band gap of 2.4 eV enhanced the degradation ability for both ampicillin and methyl orange owing to the synergistic effect of AgO₂/TiO₂ nanoheterojunctions. Moreover, the photocatalytic film displayed excellent recycling properties for the degradation of methyl orange by being reused for 5 times without losing its photocatalytic activity.

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1. Introduction

Persistent organic pollutants (POPs) are causing concerned among the scientific community due to their persistence in the environment.¹ The photocatalytic process of nano TiO₂ has shown tremendous potential as a high-efficiency, low-cost, high physical-chemical stable and environmentally friendly technology to degrade POPs in wastewater.^2,3 Under ultraviolet (UV) light irradiation, electrons are emitted from TiO₂ inducing electron–hole pair formation, and subsequent charge separation induces the production of highly reactive oxygen species that can completely mineralize POPs. However, TiO₂ only responds to UV light with a wavelength below 387 nm.⁴ This drawback limits the possibility of employing sunlight as a light source because UV light occupies fewer proportions (4%) of the sun's radiation energy than that of visible light (43%). In addition, high recombination rates, about 10 ns between electron and hole pairs, reduce the quantum yield of a single TiO₂ semiconductor.⁵

In order to improve the photocatalytic activity and visible light utilization of TiO₂, numerous studies, pertaining to the surface modification of TiO₂ nanoparticles with depositing noble metal NPs (Au, Ag, Pt, Pd, etc.),^6–10 and coupling with semiconductors, such as CdS,¹¹ Ag₂O,¹² V₂O₅,¹³ WO₃ ¹⁴ and Bi₂O₃,¹⁵ have been carried out and obtained excellent achievements. Compared with other metallics, Ag and Ag-based oxides are the most suitable for practical applications because of their relatively low-priced and high efficiency.¹⁶ Ag₂O nanoparticles, a brown powder with a narrow band gap of 1.3 eV, are efficient electron absorbing agents under UV light irradiation and sensitizers to turn UV light response into the visible region.^17–19 Besides, Ag₂O has been used in various fields, as a mild oxidant, water cleaning agent, and catalyst.²⁰ The advantages of Ag₂O impel a novel strategy of combining Ag₂O and TiO₂ for superior photocatalysts in the visible light region.

Another main drawback of using nanosized TiO₂ is its complicated subsequent separation from water.²¹ To handle this problem, various materials, such as glass,²² stainless steel mesh,²³ fiber,²⁴ silica gel²⁵ and polymeric materials,²⁶ have been explored as a TiO₂ support for the photodegradation of contaminants in wastewater. Coupling nano-TiO₂ with organic polymers to fabricate membrane/globular composites is an efficient way for immobilization. Chitosan (CS) is the second most abundant natural polymer obtained from the cell walls of fungi and the shells of crust. CS has drawn a great attention for its excellent properties, such as blood compatibility, microbial degradation and environmental security.²⁷ Using chitosan as a carrier for TiO₂, chitosan-supported TiO₂ (CS–TiO₂) adsorbent was prepared and exhibited multifunctional performance for increasing the adsorption capacity of heavy metals and enhancing the adsorption–photocatalytic process of organic pollutants.²⁸ As a result, two forms of CS–TiO₂ composites were prepared: membrane and beads. Although globular resins can effectively degrade contaminants, the photocatalytic efficiency of CS–TiO₂ beads is usually lower than that of membrane systems due to the limited contact area with contaminants. It is highly desirable to develop membrane CS–TiO₂ composites with a better photocatalytic ability.

Based on the potential synergistic effects of Ag₂O and TiO₂, a new multilayer photocatalytic-membrane was fabricated, consisting of a Ag₂O/TiO₂ layer stacked on a chitosan sub-layer immobilized onto a polypropylene fiber substrate. FESEM, EDX, XRD, HRTEM, XPS, and UV-vis spectroscopy were used to characterize the films. The photocatalytic performance of Ag₂O/TiO₂-modified chitosan-based photocatalytic film (ATCPF) was analyzed by the degradation of ampicillin (AMP) and methyl orange (MO) with visible light irradiation. Chitosan-supported TiO₂ photocatalytic film (CTPF) was also prepared and its photocatalytic performance was evaluated with two model pollutants as reference. The reuse property of the obtained ATCPF was tested for a better understanding of its potential capability for practical applications. The present study developed an efficient photocatalyst towards organic pollutants and provided novel design ideas for effective visible-light-mediated nanocomposites photocatalysts.

2. Experimental section

2.1 Chemicals

Hydrophilic polypropylene fiber was purchased from Hengyue Nonwovens Company. Chitosan (90% degree of deacetylation) was bought from Jinan Haidebei Marine Bioengineering Co. Ltd. Nanosized bicrystalline TiO₂ (P25, 50 m² g⁻¹, average particle size 30 nm) was purchased from Degussa. Sodium ampicillin (AMP) was bought from Sigma Aldrich (CAS: 69-52-3). Other chemicals were obtained from Beijing Chemical Plant, including ammonium hydroxide (NH₄OH), silver nitrate (AgNO₃), methyl orange (MO), acetic acid (CH₃COOH), epichlorohydrin (C₃H₅OCl) and sodium hydroxide (NaOH). All chemicals were of analytical grade, and used without further purification.

2.2 Sample preparation

2.2.1. Synthesis of chitosan/polypropylene film (CPF). (i) The polypropylene fiber was shaken in a 2.5% NaOH aqueous solution for 24 h to complete basification, followed by a drying process at room temperature. (ii) 0.1 g CS was dissolved into 10 mL 2.5% CH₃COOH liquid and dispersed with 100 W ultra-sound for 15 min. Then, 1 mL epichlorohydrin (cross-linking agent) was added and stirred constantly for 4 h at 25 °C. (iii) The emulsion was dripped on the polypropylene film to fully cover it. By naturally drying the wet chitosan/polypropylene fiber, CP with a smooth surface was obtained.

2.2.2. Synthesis of Ag₂O/TiO₂-modified chitosan-based photocatalytic film (ATCPF). (i) 0.1 g TiO₂ nanoparticles were dispersed in 20 mL of 10 mM AgNO₃ aqueous solution. (ii) NH₄OH was dripped into the above TiO₂–AgNO₃ mixture solution to adjust the pH between 10 and 11, followed by photodeposition under 365 nm UV lamp (GE, 20 W) and vigorously stirring for 2 h. (iii) After switching off the UV lamp, CPF was impregnated into the solution for reacting another 5 h with a stirring rate of 100 rpm in the dark. Finally, ATCPF was neutralized and rinsed with deionized water when semi-damp, and then vacuum-dried at 40 °C. TiO₂-modified chitosan-based photocatalytic film (TCPF), Ag₂O/TiO₂ nanocomposites (AT) were also prepared as control.

2.3 Characterization

The morphology of ATCPF was observed by a FE-SEM (Hitachi S-4700, Tokyo, Japan) and a HRTEM (JOEL JEM 3010, Tokyo, Japan). The crystalline phase contained in TiO₂ and Ag₂O/TiO₂ was observed by XRD patterns in a Rigaku D/Max 2400 diffraction meter (Tokyo, Japan). XPS was recorded with the ESCALAB 250 system (Thermo Fisher Scientific, USA). UV-vis diffuse reflectance spectra were conducted on a UV-vis spectrophotometer (UV-3150, Shimadzu, Japan) attached with integrating sphere. All samples (5 mL) were filtrated with a 0.45 μm filter to remove COD and nitrogen, subsequently quantified the major cationic contents (Ti⁴⁺ and Ag⁺ ions) by an Agilent 7700x ICP-MS (Agilent technologies, USA).

2.4 Photocatalytic activity

The photocatalytic performance of the samples was evaluated by degrading two organic compounds: MO and AMP Photochemical experiments were accomplished in a photocatalytic reaction chamber provided with one 150 W tungsten–halogen lamp that facilitated continuous emission of UV-visible light (λ = 360–2000 nm). Reactions were performed in a commercially available dish with a diameter of 90 mm. In a typical photocatalytic experiment, photocatalytic film with a diameter of 80 mm was dispersed into 40 mL MO (10 mg L⁻¹) solution or 40 mL AMP (20 mg L⁻¹) solution with constant stirring. Here, each piece of ATCPF or TCP was coated with 60 mg loading mass of catalysts (Ag₂O/TiO₂ or TiO₂).

The residual concentrations of MO were detected by UV-vis spectrophotometry (Varian, Cary100) at the wavelength of 464 nm. The degradation of AMP was detected by HPLC analysis with a C₁₈ (5 μm, 4.6 mm × 150 mm) column at the wavelength of 220 nm. The degradation rate of MO and AMP could be calculated by eqn (1):

where C₀ and C_t are the initial and final concentration (mg L⁻¹), respectively.

The antibacterial effect of the samples was analyzed by antibacterial tests and Disk Diffusion Assays using the method introduced by M. J. Umapathy.^6,29

3. Results and discussion

3.1 Characterization of the prepared composite photocatalysts

3.1.1. Morphological investigation with SEM, EDS and TEM. The FE-SEMs of these samples were shown in Fig. 1. Compared to pristine polypropylene fiber (Fig. 1a), both the surface and gap of blank fiber were covered by smooth films, indicating good viscosity stability and film-forming properties of the chitosan/acetic acid solution (Fig. 1b). TiO₂ nanoparticles were evenly coated on the surface of the CP after synthesizing TCP (Fig. 1c). When Ag₂O/TiO₂ had been deposited on CP (Fig. 1d), a rough surface coated with homogeneous granulation was developed. With the 500× magnification, obvious nanoclusters were revealed and illustrated in Fig. 1d (inset), from which the particle size of aggregated Ag₂O/TiO₂ was measured in the range of 50–500 nm. It was the result of nanoparticle aggregation at high TiO₂-NPs content during the preparation. With the even coverage of polypropylene fiber by chitosan, the chitosan/polypropylene fiber composites (CP) provided a platform with high surface area for adhesion of Ag₂O/TiO₂.

Fig. 1 SEM images and EDX spectrum of (a) blank polypropylene fiber; (b) CPF; (c) TCPF (d) ATCPF with different magnifications.

Surface elemental analysis containing EDX spectrum has been done and the results are shown in Fig. 1 respectively. TCP shows more distinct elemental peaks of Ti and ATCPF shows more distinct elemental peaks of Ag and Ti than that of CP. Different color areas shown in Fig. 1d indicate C-, N-, O-, Ti and Ag-enriched areas of ATCPF. It revealed that the obtained film was composited of the elements of C, N, O, Ti and Ag, and also showed the homogeneous distribution of the Ag ions on the surface. The peaks of C element mainly resulted from the polypropylene film, while the peak of N element resulted from chitosan. The results demonstrated high-density TiO₂ and Ag₂O had been immobilized on the surface of the heterozygous nanocomposites.

In order to further confirm the distribution of both TiO₂ and Ag, morphology details of the ATCPF were analyzed by HRTEM. Dispersed Ag₂O nanoparticles were tightly coupled onto the surface of TiO₂ with an average diameter of 3–5 nm (Fig. 2a). The resolved interplanar distances was 0.27 nm, corresponding to the (111) plane of Ag₂O (Fig. 2b). On the basis of the SEM and TEM observation, a schematic structure of the ATCP was depicted in Fig. 2c. In the alkaline AgNO₃ solution, Ag⁺ was firstly adsorbed to the surface of TiO₂ and then transformed into Ag₂O under UV light illumination. The subsequent addition of chitosan/polypropylene fiber provided an excellent-carrier for the nanocrystals loading. As a result, Ag₂O/TiO₂ was deposited on the chitosan-modified polypropylene fiber, making the white fiber color purple-black. Our earlier study of FT-IR³⁰ indicated that the hydrogen bonding interaction was a main driving force for the self-assembly between TiO₂ with surface hydroxyl group and chitosan with amino and hydroxyl. Similarly, Ag₂O-modified TiO₂ also could easily combine with chitosan/polypropylene fiber via hydrogen bonding interaction. On the basis of the formation mechanism, ATCPF with a graft-like structure had been synthesized, and shown appropriate for follow-up application.

Fig. 2 (a and b) HRTEM image and (c) schematic structure of the ATCPF.

3.1.2. Elemental compositions analysis. To prove the synthesis of ATCPF, the crystal structures and chemical status of these samples were analyzed. XRD patterns of TCP and ATCPF were shown in Fig. 3a. The XRD pattern of TCP displayed that TiO₂ has a bicrystalline structure, which can be assigned to anatase and rutile phases. After coating with Ag nanoparticles, ATCPF showed additional peaks when compared with TCPF. Two obvious peaks for Ag₂O were detected at 32.8° {111} (JCPDS no. 41-1104) and 46.14° {300} (JCPDS no. 42-0874), which demonstrates that the Ag₂O nanoparticles are coated on the surface of TCP but not inserted in the crystal lattice.

Fig. 3 (a) XRD pattern of TiO₂ and Ag₂O/TiO₂ scraped off from ATCPF (A: anatase, R: rutile). (b) XPS survey scan spectra of pure TiO₂ and Ag₂O/TiO₂; (c) XPS spectra for Ag 3d_5/2 and 3d_3/2; (d) XPS spectra for TiO₂ 2P_3/2 and 2P_1/2; (e) UV-vis absorption spectra of Ag₂O/TiO₂ and TiO₂; (f) the band-gap energies of ATCPF and TCPF; (g) the energy band structure of Ag₂O and TiO₂ before contact and the energy band structure of p-Ag₂O/n-TiO₂ heterojunction at equilibrium (E_vac: vacuum level; E_F: Fermi level; ϕ: work function; χ: electron affinity).

The elemental compositions and chemical status of TCPF and ATCPF were characterized by XPS (Fig. 3b–d). Compared with TCP, additional peaks of Ag were found in ATCPF in addition to the Ti, O, and C peaks (Fig. 3b). High-resolution scan over Ag 3d_5/2 in Ag₂O/TiO₂ nanoparticles demonstrated that the binding energy is 367.47 eV, which is consistent with values reported for Ag₂O.³⁰ Meanwhile, the signals of Ti 2p_3/2 and Ti 2p_1/2 at ∼458 eV and ∼464 eV corresponding to TiO₂ (Fig. 3d).³¹ It indicated that silver existed in only one valence state (Ag⁺) as Ag₂O in the obtained mixture. Therefore, the nanoparticles scraped from ATCPF confirmed that TiO₂ nanoparticles were successfully modified by Ag₂O based on XRD and XPS characterization.

3.1.3. Mechanism of photocatalytic activity. The UV-vis diffuse reflectance spectra for TCP and ATCPF are shown in Fig. 3e. A broad intense adsorption of TCPF is in the UV region below 400 nm; however, a strong and wide adsorption of ATCPF is in the visible-light region at 545 nm with maximum. The band-gap energy of TCPF is 3.1 eV and the band-gap energy of ATCPF presents a marked displacement to lower energy values at 2.4 eV approximately, which were estimated by the Kubelka–Munk method (Fig. 3f).³² The wide visible light adsorption and low band gap in ATCPF nanocluster are all owe to the formation of p–n heterojunctions as visible-light sensitization. Ag₂O is a p-type narrow band gap semiconductor, while TiO₂ is an n-type wide band gap semiconductor. As shown in Fig. 3g, p–n heterojunctions are synthesised at the interface and electron transfer occurred from TiO₂ to Ag₂O when their Fermi levels align. At the equilibrium of junction, the charge of n-type TiO₂ regions is positive, while the charge of p-type Ag₂O is negative. Therefore, an opposing electric field (ξ, contact potential) at the junction and an equilibrium potential difference V₀ are formed across the transition region. During the photocatalytic reaction process of p-Ag₂O/n-TiO₂ nanojunction, the photogenerated electrons move to the conduction band (E_cn) of the n-type TiO₂ and holes move to the valence band (E_vp) of the p-type Ag₂O. The charge separation (e⁻/h⁺) were effectively by trapping photoelectrons on Ag₂O nanoparticles and powerful oxidizing agent and superoxide radical (OH⁻, O²⁻, ˙OOH, ˙OH) were produced subsequently in a photocatalysis degradation reaction.

3.1.4. Water contact angle measurement. In order to understand the surface hydrophilic for photocatalysis degradation and self-cleaning application, water contact angle of samples was analyzed. As shown in Fig. 4, the water contact angle of the bare substrate is 68°; however, the contact angle of the ATCPF is only 38°. The result demonstrated that hydrophilicity of the film tends to increase after modifying. A plausible mechanism for this phenomenon have been suggested that both chitosan and TiO₂ are abundant hydrophilic group, for example, ˙OH. In water treatment, the good hydrophilicity of ATCPF is conducive to water molecule diffusion and mass transfer, and it is easy to combine and degrade pollutant.

Fig. 4 Contact angle characterization of (a) blank film and (b) ATCPF.

3.2 Photocatalytic activities

To probe advantages of ATCPF, adsorbing capacity and degradation experiments were carried out by adsorbing and decomposing typical compounds: AMP and MO. These two typical compounds were chosen as model organic pollutants because antibiotic and azo dyes both involve serious risks for the human being and aquatic environment.

3.2.1. Adsorption and degradation of ampicillin (AMP). As shown in Fig. 5a, in the dark, ATCPF was more efficient in reducing the concentration of AMP than TCPF, CPF and AT, which indicated that the surface adsorption properties of ATCPF had been obvious increased when TCPF was modified by Ag₂O. Chitosan had no apparent adsorption effect on AMP, which was probably due to the steric macromolecular hindrance between chitosan and AMP. As shown in Fig. 5b, ATCPF exhibited the highest photocatalytic activity of MO degradation under simulated solar irradiation, the following series of photoactivity were found: ATCPF > AT > TCPF > CPF ≌ polypropylene film. It was clear that the removal of AMP by blank polypropylene fiber, CPF and AT were all depressed, which meant that AMP cannot be degraded directly by blank film or chitosan. In the case of the TCPF and AT, the concentration of AMP continued to decline as the irradiation time was prolonged; however, only about 40% of AMP was degraded within 3 h in the presence of TCP or AT. In comparison, ATCPF exhibited a considerable degradation ability of AMP under simulated solar irradiation, and the degradation rate of AMP reached 100% in 3 h. Compared with CPF, the distinguished performance of ATCPF should be attributed to the graft structure of Ag₂O/TiO₂ heterostructures, which expose to contact AMP in the solution and induce photocatalytic oxidation reaction. Combining CPF with Ag₂O/TiO₂ by self-assembly to form ATCPF also prevented the interference of chitosan to AMP, which was in good agreement with the schematic structure (Fig. 2c). The degradation activity of ATCPF under simulated solar irradiation was much higher than the dark case, and the corresponding removal rate was 59.2% after 3 h. Although ATCPF plays an important role in reducing AMP without irradiation, the light treatment was considered necessary to improve the photocatalytic performance of ATCPF.

Fig. 5 (a) The adsorption of AMP in dark condition within the presence of ATCPF, TCPF, CPF, AT nanoparticles and blank polypropylene film; (b) the degradation of AMP (20 mg L⁻¹, 40 mL) during simulated solar irradiation in the presence of ATCPF, TCPF, CPF, Ag₂O/TiO₂ nanocomposites (AT) and blank polypropylene film.

3.2.2. Degradation of MO. To further evaluate the multifunctionality of ATCPF, the adsorption and decomposition of MO were examined, as shown in Fig. 6. In the dark condition, ATCPF still performed good decolorization of MO. It was found that blank polypropylene film and AT again had no effect on MO, whereas CPF possessed absorptivity and reached saturation about 34.6% in 15 min. This phenomenon was slightly different from the case of AMP, which was due to the interaction between the functional groups (hydroxyl and amine) of chitosan and MO.³³ After the surface of CPF was coated with TiO₂, the obtained TCPF exhibited a slightly stronger adsorption capacity of MO in the dark, and about 50.3% decolorization was attained, indicating that TiO₂ also had affinity for MO owing to the hydroxyl groups on the surface of TiO₂.³⁴ When exposed to visible light, photocatalytic activity of TCP was more efficient than that of CP. It could be attributed to both the adsorption of CPF and photocatalysis of AT under simulated solar irradiation. Similar to the case of ATCPF for degrading AMP, the photocatalysis degradation of MO by ATCPF occurred at a significantly higher rate than that by TCPF, thoroughly completed within 30 min under visible light irradiation. The above results further indicated that the as-prepared ATCPF can be used as a highly efficient photocatalyst that can use simulated sunlight.

Fig. 6 (a) The adsorption of MO (10 mg L⁻¹, 40 mL) in dark condition within the presence of ATCPF, TCPF, CPF, AT and blank film polypropylene. (b) The degradation of MO (10 mg L⁻¹, 40 mL) during simulated solar irradiation in the presence of ATCPF, TCPF, CPF, AT and blank polypropylene film.

3.2.3. Comparison of reaction rate constant of ATCPF and TCPF. The reaction rate constants (k) of ATCPF and TCPF were analyzed by re-plotting the degradation rate of AMP and MO according to the following equation:

As shown in Table 1, during the degradation process of AMP and MO under simulated solar irradiation, the values of k indicated that the rate constant of ATCPF was much faster than that of TCP, CPF and AT. The photocatalysis mechanisms of ATCPF under UV- and visible-light irradiation are different. In the UV range, both TiO₂ and Ag₂O are excited to produced electron–hole pairs, and then the generated electrons and holes can react with H₂O to produce powerful oxidizing agent and superoxide radical.³⁵ In the visible range, only Ag₂O can be excited to produce electron–hole pairs, which then act as visible-light active component to enhance the Ag₂O/TiO₂ photocatalytic activity to decompose organic pollutants. Therefore, these two different mechanisms under UV-visible light jointly promoted the photocatalytic process of ATCPF.

Table 1 The rate constant during photocatalytic degradation of AMP and MO

Pollutants systems (under simulated solar irradiation)	kATCPF (×10^−4 min^−1)	kTCPF (×10^−4 min^−1)	kCPF (×10^−4 min^−1)	kAT (×10^−4 min^−1)
AMP (C0 = 20 mg L^−1)	196	27	0.11	38
MO (C0 = 10 mg L^−1)	164	32	14	89

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3.2.4. Mineralization studies. The mineralization effects were analyzed by studying the TOC contents of reaction solution during photocatalytic reaction. After reaction with ATCPF for 15 min, the degradation rate of MO was 96.00% and the removal rate of TOC was 83.77% (Fig. 7a). It demonstrated that mineralization is a synchronously process with degradation reaction. The results further indicated that the as-prepared ATCPF can be used as a highly efficient mineralization photocatalyst that can mineralize dye efficiently with simulated sunlight. However, as shown in Fig. 7b, after reaction with TCPF for 15 min, the degradation rate of MO was 73.41%, while the TOC removal rate was only 24.75%. The results demonstrated that the mineralization process was significantly improved because of the excellent photocatalysis of Ag₂O.

Fig. 7 Concentration change and TOC analysis of (a) ATCPF, (b) TCPF during photocatalytic degradation of MO (C_{0 MO} = 10 mg L⁻¹; V = 40 mL; M = 0.075 g; N = 70 rpm; UV; t = 6 h).

3.2.5. Photocatalytic antibacterial characteristics. To evaluate the antibacterial activity of the Ag₂O/TiO₂ modified chitosan-based photocatalytic film, the bactericidal efficiency of ATCPF was compared to the activities of the other photocatalysts materials. As shown in Fig. 8a, a negligible antibacterial activity was observed in the absence of photocatalyst (curve 1). Because of the lower photocatalytic activity of AT and CPF under visible light, only 56% and 35% inactivation of E. coli was found even after 60 min of irradiation (curve 3 and 4). In contrast, obvious enhancements in visible-light bactericidal efficiency was observed on TCPF (curve 5) and ATCPF (curve 6). In particular, ∼99.8% of the E. coli was killed on the ATCPF upon irradiation for 60 min (curve 6). Additionally, by comparing the bactericidal results on ATCPF in the presence (curve 6) and absence (curve 2) of visible-light irradiation, it is obvious that, apart from the effect of the Ag⁺ ions, the mainly bactericidal effects is related to the photocatalytic process. As shown in Fig. 8b, the antibacterial activity of ATCPF was also assessed by the diameter of inhibition zone (DIZ) test with various concentrations of E. coil. From the results of DIZ test under visible light (Fig. 8c), it was observed that PF, CPF and TCPF did not show any antimicrobial activities by visible light; however, ATCPF showed a palpable DIZ value which indicating significant antibacterial activity against E. coli with a concentrations of approximately 10⁴ CFU mL⁻¹. It can be predicted that the excellent biocidal function of ATCPF is derived from the Ag species and the oxidative stress caused by photo-generated reactive radicals.^6,29

Fig. 8 (a) Kinetics of bactericidal efficiency against E. coli under visible-light irradiation: (1) E. coli suspension without any catalyst irradiated by visible light, (2) E. coli suspensions with 0.1 g of ATCPF in the dark, (3–6) E. coli suspensions with 0.1 g of (3) CPF, (4) AT, (5) CTPF and (6) ACTPF. (b) Comparative study of the zone of inhibition (mm) for ATCPF in E. coli bacterial strain under visible-light irradiation. (c) Photographs of antibacterial results on E. coli for ATCPF.

3.2.6. Multiple reuse ability. To investigate the stability of ATCPF on the photocatalytic activity under simulated solar irradiation, the same samples were reused for five times to remove MO, and results were shown in Fig. 9. For comparison, the repeated degradation experiments were also carried out in the dark condition. Under irradiation, it was found that MO can be fully degraded within 30 min in the first two cycles. Along with the increase of reusing times, the photocatalytic efficiency of ATCPF decreases slightly. This might be because of the presumed loss of Ag₂O/TiO₂ nanoparticles or because of some degradation intermediates that cover the surface of the as-prepared samples. Nevertheless, the degradation rate of MO still remains over 90% after reusing 5 times. However, in the case of dark application, the degradation efficiency of ATCPF severely diminishes with the increase of repeated use. Once reaching adsorption equilibrium, the concentration of MO would no longer decrease. Therefore, irradiation seems to be necessary to keep the high photocatalytic activity and stability of ATCPF. In addition, ATCP can be easily separated and reused directly without extra regeneration processes.

Fig. 9 (a) Repeated adsorption–photocatalytic degradation experiments carried out under simulated solar irradiation and in dark showing the photodegradation efficiency and recyclability of ATCPF composites in the degradation of MO (C₀ = 10 mg L⁻¹, V = 40 mL). (b and c) Ionic concentration and release amount of Ti⁴⁺ and Ag⁺ during reused for five times.

The possible release amount of Ag ion and Ti ion in the solution after ATCPF was reused different times were determined by ICP-MS, and the results are shown in Fig. 9b and c. Compared with control group, a negligible concentration of Ti⁴⁺ was detected during reused for five times, which demonstrated that TiO₂ nanoparticles were modified on the surface of CPF firmly. While about 1 mg L⁻¹ Ag⁺ was detected during former three times reusing. It can be predicted that the increase of released amount of Ag⁺ led to the decrease of photocatalytic activity.

The observed adsorption performance, photocatalytic performance and stable reusability are far better than most of the reported nanostructures which has been summarized in Table 2. Therefore, the prepared ATCP can be regarded as a highly active, easily separated and highly stable sunlight photocatalyst, which shows a great potential in the field of environmental remediation through its simplicity and low-cost.

Table 2 Performance of photocatalytic activity in literature

Catalyst used and amount	Conc. and volume of MO	Light source (W)	Time (min)	Degradation (%)	Adsorption (%)	Repeated photooxidation (%)	Reference
Ag2O/TiO2 photocatalytic film (10 mg)	10 mg L^−1, 40 mL	150 (vis)	15	96	64	90 (5^th)	Our work
Ag2O/TiO2 nanobelt (10 mg)	20 mg L^−1, 10 mL	350 (vis)	80	86	∼3	—	36
Ag2O/TiO2 nanobelts (20 mg)	20 mg L^−1, 20 mL	300 (vis)	25	80	—	76 (4^th)	37
S-Doped Ag2O/TiO2 nanobelt (20 mg)	20 mg L^−1, 20 mL	300 (vis)	150	80	—	80 (6^th)	38
Ag2O/TiO2 microsphere (20 mg)	14 mg L^−1, 50 mL	500 (vis)	60	99	—	—	39
Ag2O/TiO2 nanosheet (100 mg)	160 mg L^−1, 100 mL	160 (UV)	60	20	87.5	78 (5^th)	40
Ag2O nanocomposites (30 mg)	16 mg L^−1, 10 mL	500 (UV)	8	98	5	100 (8^th)	41
Ag2O/TiO2 microspheres (40 mg)	∼3 mg L^−1, 40 mL	40 (UV)	15	93	∼3	93 (5^th)	42

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4. Conclusions

Based on the synergistic catalytic technique of Ag₂O and TiO₂ semiconductors, the sunlight active Ag₂O/TiO₂-modified chitosan-based photocatalytic film was successfully prepared. FESEM, HRTEM, XRD and XPS were used for the characterization of ATCPF, illustrating that Ag₂O nanoparticles were firstly deposited on TiO₂ nano-cluster and then Ag₂O/TiO₂ was as a whole immobilized on chitosan-modified polypropylene fiber, a multilayer composite with graft-like structure was obtained. The hydrophilic ATCPF film displayed excellent absorption capacity by modified chitosan, TiO₂ and Ag₂O. Because of its visible-light absorption and low band energy, the ATCPF film showed excellent photocatalytic property towards the complete degradation of two different target organics: AMP and MO, within 180 min and 30 min, respectively. ATCPF can be reused directly for 5 times with its photocatalytic efficiency for MO remaining in excess of 90%, which ensured a good stability and recyclability of the composite. The present study developed an efficient sunlight activated photocatalyst, and it is of realistic significance in wastewater treatment as well as scientific significance in designing visible light active nano-photocatalysts.

Acknowledgements

The authors express their thanks for the supports from the National Natural Science Foundation of China (21525625), the National Basic Research Program (973 Program) of China (2014CB745100), the (863) High Technology Project (2013AA020302) and the Chinese Universities Scientific Fund (JD1417).

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