PVDF–Poly(MMA-co

Jun 11, 2012 - This paper reports the fabrication of self-supporting three-dimensional ZnIn2S4/PVDF–poly(MMA-co-MAA) (ZIS/Polymer) composite mats wi...
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Self-Supporting Three-Dimensional ZnIn2S4/PVDF−Poly(MMA-coMAA) Composite Mats with Hierarchical Nanostructures for High Photocatalytic Activity Shengjie Peng,*,†,‡ Peining Zhu,§ Subodh G. Mhaisalkar,† and Seeram Ramakrishna*,§ †

School of Materials Science and Engineering, Nanyang Technological University, Singapore, 639798 NUS Nanoscience and Nanotechnology Initiative (NUSNNI)-NanoCore, National University of Singapore, 117576 § Department of Mechanical Engineering, National University of Singapore, Singapore, 117574 ‡

S Supporting Information *

ABSTRACT: This paper reports the fabrication of self-supporting threedimensional ZnIn2S4/PVDF−poly(MMA-co-MAA) (ZIS/Polymer) composite mats with hierarchical nanostructures by a simple combination of an electrospinning technique and a hydrothermal process and their high photocatalytic activity. The characterization results show that ZnIn2S4 (ZIS) nanosheets with a thickness of about 20 nm distribute uniformly on the surface of the nanofiber polymers to form mats. The coverage density of the ZIS nanosheet coating on the surface of the polymer mats could be controlled by simply adjusting the amount ratios of feeding reactants to polymers. Furthermore, the growth process of ZIS coating is investigated based on time-dependent experiments. The obtained ZIS/ Polymer heteroarchitectures show high photocatalytic property and stability to degrade methyl orange (MO) because of the formation of hierarchical nanostructures, which might improve the adsorption and catalysis of the dyes. Due to the self-supporting property of the mats, the ZIS/Polymer mats could be laid or hung conveniently anywhere under solar irradiation and recycled easily, which provides a solution to the separation problem for conventional catalysts that are small in size. The study may open a new way to build hierarchical device fabrics with optical and catalytic properties.

1. INTRODUCTION Global environmental problems associated with harmful pollutants have received much attention in past decades. As a “green” technique, efficient semiconductor photocatalysts can offer the potential complete mineralization of hazardous wastes, especially for the photodegradation of organic pollutants with solar energy.1,2 It has been proved that photocatalytic performance was closely related to the structures of the semiconductor materials.3 In particular, three-dimensional architectures with high specific morphologies and high orders have been included because of their important role in the systematic study of structure−property relationships and improved physical and chemical properties.4,5 Therefore, considerable efforts have been taken to synthesize hierarchical photocatalysts with specific morphologies, novel properties, and superior photocatalytic performances. However, photocatalytic nanosized semiconductor powder is difficult to recycle and is easily aggregated, which hinders its large-scale application. Although many researchers are making efforts to make it practical, satisfactory results have not been fully achieved. Therefore, it is still a big challenge to design efficient and practical photocatalysts. © 2012 American Chemical Society

As the only member of the AB2X4 family of semiconductors with a layered structure, ZnIn2S4 (ZIS) has been extensively studied because of its important potential uses in charge storage, electrochemical recording, thermoelectricity, and photocatalysis especially.6−8 In recent years, various ZIS nanostructures have been successfully fabricated by a variety of methods, including the hydro/solvothermal methods, microwave-solvothermal method, and so on, and showed enhanced visible light photocatalytic activity for methyl orange degradation.9−12 Compared to other methods, hydro/solvothermal methods have proven to be powerful approaches, owing to the environmentally friendly process, low cost, and controllable morphologies. In recent years, some powder can be loaded onto inorganic and polymer carriers, which have been demonstrated to be effective photocatalysts.13,14 Electrospinning, as a comparatively low-cost and applicable technique, is able to synthesize materials in the form of fabric with a certain strength and flexibility on a large scale.15 The electrospun nanofibers with both high porosity and large Received: March 22, 2012 Revised: June 4, 2012 Published: June 11, 2012 13849

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autoclave was sealed, maintained at 160 °C for 12 h, and cooled to room temperature naturally. Finally, a yellow mat with 50 wt % ZIS weight load (ZIS(50 wt %)/Polymer) was obtained and washed by water and ethanol, and then freeze-dried. A set of experiments were performed via adjusting hydrothermal reaction times and temperatures under a procedure similar to that for the ZIS(50 wt %)/Polymer composite mat. The experimental procedure is shown in Figure 1. The ZIS powder

surface area are promising materials for surface modification and functionalization in many applications.16 One of the attractive advantages for this method is their ready formation of self-supporting mats with favorable recycling characteristics. As one of the electrospun polymer mats, poly(vinylidene difluoride) (PVDF) has excellent weatherability and resistance to chemicals, and can endure high temperature treatment, due to the stable C−F bond in the main chain.17 In addition, the carboxyl in poly(methyl methacrylate-co-methacrylic acid) [P(MMA-co-MAA)] can coordinate with metal ions to make them bind closely with each other.18 Furthermore, the electrospun mats possess large surface areas and good loading abilities for inorganic semiconductor materials.19,20 All these advantages can make electrospun PVDF−P(MMA-co-MAA) nanofiber composite mats a suitable polymer carrier and enhance the photocatalytic performance. Motivated by the above concerns, in this paper we successfully fabricated three-dimensional ZnIn2S4/PVDF−P(MMA-co-MAA) (ZIS/Polymer) composites with hierarchical nanostructures on a large scale by a simple and effective route that combines the electrospinning technique with the hydrothermal process. By simply adjusting the amount ratios between the feeding reactants and the polymers, the coverage density of ZIS nanosheets on the surface of the polymer nanofibers could be tailored. Furthermore, the as-electrospun ZIS/Polymer products with hierarchical nanostructures exhibited a much higher photocatalytic performance to the degradation of methyl orange (MO) dye than the only ZIS powder prepared by the hydrothermal method. Owing to the simplicity, low-cost, large scale, and ready recyclability, the present fabrication strategy can be adopted to fabricate other systems combined with semiconductors and polymers, which have practical applications in devices.

Figure 1. (a) Schematic illustration of the synthetic route for making three-dimensional ZIS/Polymer mat. (b) Photos of ZnIn/Polymer mat (left) and ZIS(50 wt %)/Polymer mat (right).

2. EXPERIMENTAL SECTION 2.1. Fabrication of PVDF−P(MMA-co-MAA) Nanofiber Mats Incorporated with Zn(CH3COO)2 and InCl3 Precursors. The PVDF−P(MMA-co-MAA) nanofiber mats incorporated with Zn(CH3COO)2 and InCl3 precursors were fabricated by a simple electrospinning method according to the following route: First, 0.40 g of poly(vinyldine fluoride) (PVDF, M w = 275 000) and 0.10 g of poly(methyl methacrylate-co-methacrylic acid) (P(MMA-co-MAA), Mw = 15 000) were dissolved in 5 mL of N,N-dimethylformamide (DMF) solution by stirring for 1 h. Then 0.08 g of Zn(CH3COO)2 and 0.13 g of InCl3 were added to the above solution. After stirring for another 2 h, a solution with sufficient viscosity was obtained for subsequent electrospinning. The solution was electrospun at 25 kV, 10 cm working distance (the distance between the needle tip and the target), and 1.0 mL h−1 flow rate. The as-electrospun mat was collected on aluminum foil that was attached on the edge of the collecting wheel. Then the polymer mat was cut into required dimensions for the following hydrothermal experiments. 2.2. Fabrication of ZnIn2S4/PVDF-P(MMA-co-MAA) Composite Mats. The ZIS coatings on the polymer mats were obtained by a hydrothermal method. In a typical synthesis of ZnIn2S4/PVDF−P(MMA-co-MAA) (ZIS/Polymer) mats, first, 0.1 mM C2H5NS (TAA) and 35 mL of distilled water were dissolved in a Teflon-lined stainless steel autoclave with 50 mL capacity and stirred for 30 min. Then 0.1 g of the aboveobtained polymer mat with dimensions of 3.0 cm × 3.0 cm was loaded into the autoclave and was stirred for 1 h. After that, the

was fabricated by the same method without using the electrospun fiber mats for comparison. It is noted that ZIS(10 wt %)/Polymer, ZIS(30 wt %)/Polymer, ZIS(50 wt %)/Polymer, and ZIS(70 wt %)/Polymer composites are referenced with different ZnIn2S4 weight loads (10, 30, 50, and 70 wt %) in the composites, which were determined by the feeding reactants of Zn(CH3COO)2 and InCl3 with different molar amounts according to the stoichiometry of ZIS. The ZIS loads of the composite polymer mats were further determined by thermogravimetric (TG) analysis of the obtained composites (Figure S1 and Table S1 in the Supporting Information). It was found that the ZIS loads were 10.3, 30.9, 50.7, and 72 wt %, which was similar to their corresponding estimated ZnIn2S4 loads of 10, 30, 50, and 70 wt %. For simplicity, the eletrospun PVDF−P(MMA-co-MAA) mat incorporated with Zn(CH3COO)2 and InCl3 precursors is described as ZnIn/ Polymer. 2.3. Characterization. X-ray diffraction (XRD) measurements were carried out using a D/max 2500 XRD spectrometer (Rigaku) with a Cu Kα line of 0.1541 nm to characterize the crystal structure of the products. The thermal analysis was obtained with a thermogravimeter/differential thermal analyzer (TG/DTA 6300, SII Nanotechnology). The heating rate was 5 °C min−1 with a nitrogen flow (50 mL 3 min−1). Scanning electron microscopy (SEM; XL-30 ESEM FEG, Micro FEI Philips, 10 kV) was used to characterize the morphologies of the products. Energy dispersive X-ray spectroscopy (EDS) coupled with scanning electron microscopy (SEM) was used to 13850

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observed, suggesting that the obtained mat was the composite of ZIS and PVDF/P(MMA-co-MAA). The surface morphologies of the electrospun polymer incorporated with Zn(CH3COO)2 and InCl3 and the asobtained ZIS(50 wt %)/Polymer mat are presented in Figure 3.

analyze the composition of samples. X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB LKII instrument with a Mg KR-ADES (hν = 1253.6 eV) source at a residual gas pressure of below 1× 10−8 Pa. Infrared (IR) spectra of the composite films were performed on a Fourier transform infrared spectrometer (FTS-3500ARX). The surface area of the samples was detected by Brunauer−Emmett−Teller (BET) nitrogen adsorption−desorption measurement (BELSORPmini analyzer). Ultraviolet−visible diffuse reflectance spectra were recorded on a UV−visible spectrophotometer (Cary 100 Scan Spectrophotometer, Varian) equipped with a Labsphere diffuse reflectance accessory. 2.4. Photocatalytic Test. The photocatalytic degradation of methyl orange (MO) was carried out in an aqueous solution at ambient temperature under visible light irradiation of a 500 W Xe lamp with a 420 nm cutoff filter. Briefly, 0.1 g of ZnIn2S4 powder photocatalyst was added to 100 mL of methyl orange (MO) aqueous solution (10 ppm). The system was cooled by a fan and circulating water to maintain room temperature. Before illumination, the solution was slightly stirred for 60 min in the dark in order to reach adsorption−desorption equilibrium between the photocatalyst and MO. It should be noted that the amount of added electrospun mats were based on the ZnIn2S4 weight loads. Photocatalytic degradation was monitored by measuring the absorbance of solution using a Varian Cary 50 Scan UV−vis spectrophotometer. At given intervals of illumination, the samples of the reaction solution were taken out and analyzed. The degradation efficiency was expressed by (C/C0)%.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. XRD analysis was performed to investigate the purity of the obtained samples. Figure 2a reveals the XRD pattern of the ZIS/Polymer mat

Figure 3. Low magnification SEM images of (a) electrospun ZnIn/ Polymer and (b−d) ZIS(50 wt %)/Polymer mat. (e) High magnification SEM image of ZIS(50 wt %)/Polymer mat. (f) EDS of ZIS(50 wt %)/Polymer mat.

From Figure 3a, it can be clearly seen that the precursor ZnIn/ Polymer nanofibers oriented randomly because of the bending instability associated with the spinning jet (Figure 3a). The nanofibers with a smooth and uniform surface interconnected with each other to form a nonwoven mat, and the average diameter of the nanofibers in the mat was approximately 600 nm (inset in Figure 3a). After hydrothermal reaction for 12 h, the product still maintained the mat structure. The SEM image in Figure 3b indicates that there was little change of the polymer nanofibers during the growth of ZIS, except for the increase of the average diameter of nanofibers to 1 μm. Figure 3c is the magnified image of the ZIS(50 wt %)/Polymer product; it can be observed that the ZIS nanosheets were uniformly distributed across the surface of the polymer nanowires without aggregation, offering a high surface area. The SEM image of the nanofibers indicated by the arrow in Figure 3d confirms that the polymer nanofibers are coated by ZIS nanosheets, which is further demonstrated by their corresponding TEM and HRTEM images (Figure S2 in the Supporting Information). The high magnification SEM image of the surface in Figure 3e shows that the thickness of the nanosheets is about 20 nm and there are many pores among the nanosheets, which is expected to be advantageous to the pollutant dye adsorption and degradation. The EDS of the ZIS/ Polymer product indicates that C, O, F and Zn, In, S elements were ascribed to the polymer nanofibers and ZIS nanosheets,

Figure 2. XRD patterns of (a) ZnIn2S4 powder, (b) ZnIn/Polymer mat, and (c) final ZIS(50 wt %)/Polymer mat.

obtained by combining the electrospinning method and hydrothermal process. For comparison, the XRD patterns of the electrospun PVDF/P(MMA-co-MAA) mat incorporated with Zn(CH3COO)2 and InCl3 and the ZIS powder obtained by a hydrothermal method are shown in Figure 2b and 2c, respectively. From Figure 2c, the main diffraction peaks of the ZIS/Polymer centered at about 28.9 and 47.2° are attributed to the (102) and (112) planes of hexagonal ZnIn2S4 (JCPDS Card No. 72-0773). Besides the peaks from the ZIS and polymer, no characteristic peaks for impurity, such as ZnS and In2S3, were 13851

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Figure 4. XPS spectra of (a) O 1s, (b) Zn 2p, (c) In 3d, and (d) S 2p of ZIS(50 wt %)/Polymer composite.

In 3d5/2, and In 3d3/2).11 These chemical shifts of Zn and In ions may result from the interaction between ZIS and the PVDF/P(MMA-co-MAA) polymer, due to the interaction of carboxyl coordinated with Zn and In ions.24 Such an interaction can make the obtained ZIS nanosheets immobilized tightly on the surface of fluoropolymer nanofibers. Furthermore, the wavenumbers at 2980 and 1720 cm−1 after the introduction of ZnIn2S4 are blue shifts, compared with the peaks at ∼3023 and ∼1765 cm−1 in PVDF−P(MMA-co-MAA) sample corresponding to a hydroxyl and a carbonyl stretching of the carboxyl groups present in P(MMA-co-MAA) (Figure S4 in the Supporting Information). The blue shifts of the wavelength coincide with the XPS results in Figure 4. Due to the interaction between ZIS and PVDF−P(MMA-co-MAA) polymer, the ZIS can grow into nanosheets from ZIS nuclei and bind with the polymer nanofibers firmly under the hydrothermal condition.26,27 The effect of precursor concentration on the size and density of ZIS nanosheets of ZIS/Polymer composites was also investigated. Figure 5 displays the SEM images of the obtained composites with different amounts of ZnIn2S4 loading, which shows that the surface morphologies of the composite are different from that of the ZIS(50 wt %)/Polymer. After the hydrothermal reaction, the obtained ZIS(10 wt %)/Polymer composite still presents a one-dimensional structure, with a uniform diameter of 700 nm. It is found that small outgrowths with platelike structures can be seen on the surface of the prepared nanofibers. The magnified SEM image indicates that some nanofibers are coated by curved nanosheets (Figure 5b). When the amount of ZIS loading is increased to 30 wt %, the low magnification SEM image in Figure 5c shows that the number of the nanosheets per unit area increases significantly, and more polymer nanofibers are coated by ZIS nanosheets; as a result, almost the entire surface of each nanofiber has been decorated with the nanosheets. A part of some naked polymer nanofibers can also be observed. A detailed analysis of Figure

respectively. The atomic ratio of Zn:In:S is about 13.5%:29.0%:57.5%, which is close to the stoichiometry of ZIS. The EDS spectrum in Figure 3f confirms that the ZIS(50 wt %)/Polymer with the heteroarchitecture structure could be successfully obtained by the combination of electrospinning and the hydrothermal process. The chemical composition and purity of the ZIS(50 wt %)/Polymer was further investigated by XPS analysis, which is shown in Figure 4. The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing carbon 1s to 284.5 eV. Only C, O, F, Zn, S, and In peaks can be observed in the survey of the XPS spectrum (Figure S3 in the Supporting Information). The shape of a wide and asymmetric peak of the O 1s spectrum in Figure 4a shows that there can be more than one chemical state according to the binding energy. Using XPSPeak, version 4.1, the O 1s XPS spectrum can be curve-fitted into four peak components with binding energies, which includes In−O at 529.7 eV, Zn−O at 530.5 eV, carbonyl at 531.3 eV, and hydroxyl at 531.8 eV.21−23 The presence of Zn−O and In−O peaks could be ascribed to the influence of carboxyl coordinated with zinc and indium ions. The chemical interaction between ZIS and carboxyl can greatly improve the bonding strength of ZIS nanosheets with the polymer electrospun fiber. The Zn 2p5/2 and Zn 2p3/2 bonding energies are located at 1021 and 1044 eV, indicating that the valence state of Zn is +2. Also, the peaks located at 444 and 451.6 eV correspond to In 3d5/2 and In 3d3/2, indicating the +3 valence state of In. Furthermore, the S 2p1/2 peak split at 161.1 and 162.1 eV can be assigned to S coordinated to Zn and In in ZnIn2S4. These results showed that the chemical states of the sample were In3+, S2−, and Zn2+, and pure ZIS products coating electrospun polymer nanofibers can be obtained by the hydrothermal method. It is found that the observed binding energies of Zn 2p and In 3d have 0.9 and 0.7 eV shifts compared with those typical for Zn 2p (1021.9 and 1044.9 eV for Zn 2p5/2 and Zn 2p3/2) and In 3d (444.7 and 452.2 eV for 13852

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Figure 6. (a, c) Low magnification and (b, d) high magnification SEM images of ZIS(50 wt %)/Polymer products obtained for different times: (a, ) 0.5 h; (c, d) 3 h.

reaction proceeded, the nucleation of ZIS continued during the ZIS crystal growth, thereby resulting in the higher distribution of ZIS nanosheets. 3.2. Formation Mechanism. The formation of the ZIS/ Polymer composites with different amounts of ZnIn2S4 loadings can be explained on the basis of the present study. The main coating process commonly occurring in the reaction is schematically described in Figure 7, including coordination,

Figure 5. (a, c, e) Low magnification and (b, d, f) high magnification SEM images of (a, b) ZIS(10 wt %)/Polymer, (c, d) ZIS(30 wt %)/Polymer, and (e, f) ZIS(70 wt %)/Polymer composites.

5d shows that the ZIS nanosheets interconnect with each other to coat the surface of nanofibers, which form a type of core− shell nanofiber morphology, with a core of about 600 nm thickness and a shell of 600 nm in diameter. In contrast, the surfaces of polymer nanofibers are all coated by ZIS nanosheets, when the reactant content gradually increases to 70 wt %. Some more ZIS nanosheets are aggregated with each other to form microspheres, depositing on the polymer/ZIS composites. This microsphere morphology is similar to that of the ZIS powder obtained by the hydrothermal method (Figure S5 in the Supporting Information). A comparison between the SEM images of the ZIS/Polymer with different amounts of ZIS loading indicates that the amount and density of the ZIS nanosheets can be increased by increasing the precursor concentrations during the hydrothermal reaction and a fully ZIS coating of the polymer can be achieved when the amount of ZIS is 50 wt % of the polymer. To understand the formation mechanism of the ZIS/ Polymer composite, a temperature-dependent experiment was conducted based on the ZIS(50 wt %)/Polymer sample. Figure 6 shows the SEM images of the ZIS(50 wt %)/Polymer obtained under different times of 0.5 and 3 h. After 0.5 h hydrothermal reaction, the surface of polymer nanofibers was rough and the entire surface was homogeneously coated by many ZIS nanoparticles (Figure 6a). These nanoparticles interconnected with each other, and it was difficult to determine the individual nanoparticle size. Upon prolonging the reaction time to 3 h, it is interesting to find that these nanoparticles could grow to little nanosheets, which were directed out from the polymer surface (Figure 6c,d). As the

Figure 7. Schematic diagram of the growth process of ZIS/Polymer composite.

nucleation, and growth steps. Initially, Zn2+ and In3+ in the precursor electrospun solution can be absorbed by the PVDF− P(MMA-co-MAA) polymer, because the polymer consists of −COOH functional groups, which enable it to chelate strongly with inorganic cations and to form metal−carboxyl complexes that can serve as precursors for the preparation of inorganic nanomaterials. This coordination can lead to good combination of subsequent ZIS with the polymer.20 It is worth noting that the introduction of P(MMA-co-MAA) to the polymer plays a crucial role in the subsequent nucleation and growth. If the P(MMA-co-MAA) was not mixed with the PVDF polymer and the other reaction conditions were not changed, only some sparse ZIS nanosheets dispersed on the polymers and the ZIS/ Polymer nanoarchitecures were not obtained. That is, the PVDF−P(MMA-co-MAA) polymer could act as a structuredirecting molecule which is responsible for the formation of the three-dimensional ZIS/Polymer nanostructures in our systems. Then the reaction may precede heterogeneous ZIS nucleation on the polymer surface and subsequent homogeneous ZIS nanosheet growth on the ZnIn2S4 nanoparticles. 13853

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However, when the precursor concentration exceeded a certain concentration, some nanosheets aggregated to form quasisphere morphology, covering the ZIS/Polymer nanocomposite. The reason may be that some redundant Zn and In in the precursor could not coordinate with the −COOH of the polymer and aggregated with each other, after the coordination between the cations and −COOH achieved saturation. Therefore, the redundant Zn and In cations may escape from the polymer matrix under the hydrothermal condition due to the little weak interaction of cations with the polymer. Then some quasi-sphere ZIS structures formed by the reaction of these redundant Zn and In cations with TAA, following the self-assembled three-dimensional nanosheets coating the polymer matrix. 3.3. Optical Properties. The UV−vis absorption spectra and the corresponding band-gap energies are shown in Figure 8. It is observed that ZnIn/Polymer has no evident absorption

When the hydrothermal reaction started, the sulfide resource TAA dissolved in water solution released H2S through heating. Then H2S was ionized to produce S2−, being the sulfur source for next reaction. The sulfur ions penetrated the outer surface and inner section of the polymer nanofiber matrix.28,29 Meanwhile, under the hydrothermal condition, the Zn− carboxyl and In−carbonxyl bonds of complexes in the polymer ruptured and reacted with the diffusing S2− due to the drive of the combining force between Zn2+ and In3+ ions and S2−. As a result, ZIS nuclei were formed on the active growth sites provided by the polymer nanofiber matrix. Once the nuclei formed, they would aggregate to form ZIS nanoparticls at the expense of Zn−carboxyl and Zn−carboxyl complexes on the surface of thepolymer, which suppressed the precursor diffusion in the nearby environment and facilitated further homogeneous growth. The high porosity and large surface area of the ZIS nanoparticles coating the polymers provided many nucleation sites for the growth of ZIS nanostructures. It is known that agglomeration of nanoparticles to form nanostructures was a very commonly occurring phenomenon because the nanoparticles tended to decrease the exposed surface in order to lower the surface energy. In principle, when two crystals were in contact, likely they tended to rotate with each other to minimize the interface strain energy. Therefore, the same types of crystal plane tended to align with each other, forming a coherent interface to decreasing interface energy. According to the oriented attachment mechanism,30,31 the adjacent nanoparticles spontaneously self-organized so that they shared a common crystallographic orientation, followed by the joining of these particles at a planar interface. During the oriented attachment, bonding between the nanoparticles reduced the total energy by removing surface energy associated with unsatisfied bonds. In our case, ZIS nanoparticles fused to each other by facets to grow by sideways and led to the formation of interconnected nanosheets on the polymer nanofibers. The formation of nanosheet morphology was thought to have developed naturally as a result of an intrinsic lamellar structure of hexagonal ZIS phase under a certain hydrothermal condition, in which Zn and half the In atoms were tetrahedrally coordinated by sulfur atoms, whereas the other half were octahedrally coordinated.9 From a thermodynamics point of view, the surface energy of an individual nanosheet was quite high with two main exposed planes, and thus they tended to aggregate to the surface planes to decrease the surface energy by reducing exposed areas. During the process of ZIS nanoplate growth, cation−carboxyl complexes and TAA could release Zn, In, and sulfur donor gradually to decrease the formation rate of ZIS, which could also help to lead to the controllable growth of numerous ZIS nanosheets monodispersed on primary ZnIn2S4 nanoparticles. As a result, the nanosheets can aggregate to form a three-dimensional nanoarchitecture through a self-assembly process in the bulk solution to achieve a low surface energy. For the growth of ZIS nanosheets on the polymer, the precursor concentration is also determinative. It has been reported that reactant concentration will induce the nucleation and crystal rate.32 As the precursor concentration was low, only a few cation−carboxyl complexes formed in the precursor solution and the obtained ZIS nucleus grew so that a few ZIS nanosheets coating the polymers were obtained. With the increase of precursor concentration, more crystal seeds resulted in more nucleation centers, and as a result of their faster growth, more well-crystallized nanosheets film were formed.

Figure 8. UV−vis absorption spectra and (inset) corresponding (αhν)2 vs hν curves of ZnIn/Polymer, ZIS(10 wt %)/Polymer, ZIS(30 wt %)/Polymer, ZIS(50 wt %)/Polymer, and ZIS(70 wt %)/Polymer composites.

above 300 nm. After coating with ZIS nanosheets, the UV−vis absorption spectra of the ZIS(10 wt %)/Polymer with different amounts of ZIS composites show that the absorption edge red shifts to 510−560 nm. This is attributed to the ZIS nanosheets on the surface of the polymers, resulting in the improvement of the visible light absorption ability of the ZIS/Polymer.33 The inset shows the band-gap energies of the samples estimated from the plots of (αhν)2 vs hν, which was derived from its corresponding UV−vis absorbance spectra. In the absorption region, the absorption characteristics of the samples obey the model equation (αhν)2 = A(hν − Eg), where α is the optical absorption coefficient, h is the Planck constant, ν is the photon frequency, A is a constant, and Eg is the band-gap energy. The extrapolated value (the straight line to the X-axis) of hν at 0 indicates the absorption band-gap energy of the semiconductors.34 Based on the above-mentioned method, Eg values of the ZIS/Polymer composites are calculated to be in the range 2.44−2.61 eV, which are smaller than that of the ZnIn/Polymer (Eg = 3.25 eV). The smaller Eg value of ZIS(10 wt %)/Polymer is attributed to the fully ZIS coating on the polymer. The Eg values of ZIS/Polymer composites are close to that of the ZIS powder (Eg = 2.31 eV).11 Therefore, these bandgap energy values show that the obtained ZIS/Polymer composites have a good absorption intensity in the wavelength range, indicating a high photoresponse in the visible light region. 13854

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Figure 9. (a) Absorption spectra of a solution of MO in the presence of ZIS(50 wt %)/Polymer under exposure to UV light (inset: photographs of the resulting solution taken at regular time intervals). (b) Photocatalytic degradation of MO under visible light irradiation in the presence of ZnIn/ Polymer, ZIS(10 wt %)/Polymer, ZIS(30 wt %)/Polymer, ZIS(50 wt %)/Polymer, and ZIS(70 wt %)/Polymer composites, and ZIS powder under visible light irradiation (λ > 420 nm). (c) Lifetime of ZIS(50 wt %)/Polymer for MO degradation under visible light irradiation. (d) Proposed mechanism for the photocatalysis of ZIS/Polymer heteroarchitectures.

3.4. Photocatalytic Properties. It is well-known that nanostructured ZIS has been employed as an efficient photocatalyst for the photocatalytic degradation of organic dyes.11,12 The photodegradation efficiency is dependent on the morphological structures of ZIS. In our work, as the ZIS/ Polymer composites possess hierarchical structures and a wide absorption in the visible region, they can be potentially used as photocatalysts for the degradation of organic pollutants. The photocatalytic activities of the ZIS/Polymer composites were evaluated by the degradation of methyl orange (MO) dye, a typical pollutant in the textile industry under visible light irradiation. Before exposure of the sample to UV light, the suspension was magnetically stirred in the dark for 60 min to obtain a good dispersion to reach adsorption−desorption equilibrium (Figure S6 in the Supporting Information). UV−vis absorption spectroscopy was applied to record the adsorption behavior of the MO solution before and after adsorption by the ZIS(50 wt %)/Polymer composites (Figure 9a). Figure 9a shows the time-dependent absorption spectra and the photographs of MO solution in the presence of ZIS(50 wt %)/Polymer composite from 0 to 120 min. As can be seen in Figure 9a, in the presence of ZIS(50 wt %)/Polymer catalyst, the intensity of a maximum absorption band at 553 nm gradually decreased with an increase of UV exposure time and eventually disappeared almost completely after 120 min, and the color of the dye also correspondingly changed gradually from yellow to colorless (inset in Figure 9a). This indicates the excellent degradation of MO from aqueous solution by ZIS(50 wt %)/Polymer composite.

To compare the photocatalytic properties of the as-prepared ZIS/Polymer nanocomposites with different amounts of ZIS loading, ZIS(50 wt %)/Polymer and ZIS powder were employed as contrastive catalysts to understand the effect of hierarchical structure on the photocatalytic activity of ZIS/ Polymer composites. Figure 9b shows the variation in the characteristic absorption of MO at 553 nm while monitoring the photocatalytic degradation process of the polymer and the ZIS powder with the irradiation time. Very little MO photodegradation can be observed in the presence of ZnIn/ Polymer under UV light irradiation (less than 5% within 120 min). It was found that the MO degradation rate in ZIS/ Polymer nanocomposites increased from 30%, to 40%, and to 92%, when the ZIS content increased from 10%, to 30%, and to 50%. However, the MO degradation rate in the system decreased to 82% with the further increase of ZIS weight load to 70%. When the ZIS powder was used in the system, MO was photodegraded to 80% after irradiation for 120 min, which is smaller than the 92% of MO degraded by ZIS(50 wt %)/Polymer composites during the same irradiation time. These results suggested that the visible light photocatalytic activity of pure ZIS powder was enhanced by coating on the electrospun polymer mat and there was an optimum amount of ZIS nanosheets. The stability of a photocatalyst is a crucial factor for industrial application; however, traditional visible light active photocatalytic powder is either unstable or hard to recycle. In this experiment, ZIS(50 wt %)/Polymer was easily recycled after bleaching MO under visible light irradiation and was reused five times in the decomposition of MO to test the 13855

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in Figure 9d, and hypothetical major reactions that occur in our experiment can be summarized in reactions 1−6.

chemical stability. It was found that the ZIS(50 wt %)/ polymer composite exhibited a very good durability and there was no significant decrease in the activity even after seven cycles. The degradation rate of ZIS(50 wt %)/Polymer still remains up to 91.0%, indicating relatively high durability of the photodegradation of the porous ZIS even after five cycles. Moreover, XRD, EDS, and SEM were also performed to confirm the chemical stability of ZIS(50 wt %)/Polymer after the fifth run (Figures S7−S9 in the Supporting Information). Therefore, the photocatalyst is stable during the reaction and can be used repeatedly. It is known that the photocatalytic ability of a material is determined by many factors, such as surface area, structure, and active sites.35 The superior photocatalytic performance of the ZIS(50 wt %)/Polymer composites with hierarchical nanostructures might be attributed to the following factors. First, it is believed that the photocatalytic activity is related to the adsorption and desorption of molecules at the surface of the catalyst. In our experiment, the degradation of MO by using ZIS/Polymer photocatalyst may precede an adsorption− migration−photodegradation process. When ZIS/Polymer was immersed into the MO solution, MO molecules were absorbed onto the surface of polymer fibers due to the C−F bonds, then migrated to the ZIS nanosheet catalyst surface, and finally were degraded by ZIS catalyst under UV irradiation.25 When the ZIS amount of the ZIS/Polymer composite was 10%, the degradation rate was low, although the adsorption effect of fluoropolymer nanofibers was obvious. The degradation rate gradually increased with the increase in ZIS weight load. The degradation rate achieved a 92% maximum value when the ZIS loading was 50%. However, the degradation rate decreased if the ZnIn2S4 loading continually increased to 70 wt %. This may be due to the existence of the surplus ZIS microsphere powder (Figure 5e), which degraded the degradation rate. Second, the superior photocatalytic performance of the composite is attributed to the structure of ZnIn2S4 nanosheets hierarchically assembled on polymer nanofiber backbones. This structure would effectively prevent aggregation of ZIS nanosheets and thus maintain a large active surface area, which is different from that of the degradation ZIS powder. The results of the BET analysis revealed that the surface area of ZIS(50%)/ Polymer with the hierarchical nanostructure (59.2 m2 g−1) was greater than that of the ZIS powder (51.3 m2 g−1) (Table S1 in the Supporting Information). It indicated that the higher surface area could offer not only more opportunity for the diffusion and mass transportation of MO molecules, but also more active catalytic sites. Therefore, it is helpful for maintaining high active surface area and improving the photocatalytic activity of the material. Third, the many boundaries among ZIS nanosheets and polymers can function as a separation between the photogenerated electron−hole pairs, therefore forming more •OH radicals, and facilitate the degradation of salicylic acid to some degree.36 Compared with the interface provided by random collisions, a steadier and tighter interface formed by direct growth of ZIS is nanosheets on polymers, which will lead to easier charge transfer and more efficient separation of electron−hole pairs before recombination. Moreover, the lamellar structure favors preventing the composition of the electron cavity, which helps to improve the photocatalytic activity.37 Therefore, a photocatalytic degradation mechanism of the ZIS/Polymer heteroarchitectures was proposed. An illustration of interparticle electron transfer behavior is shown

ZnIn2S4 + hν → ecb− + h vb+

(1)

ecb− + O2 → •O2−

(2)

• •

O2− + H 2O → •HO2 + OH−

(3)

HO2 + H 2O → H 2O2 + •OH

(4)

H 2O2 → 2•OH •

OH + MO → degraded products

(5) (6)

When the ZIS/Polymer composite system is irradiated by visible-range light, electrons are injected from the valence band to the conduction band of the photoexcited ZIS nanosheets. The conduction band electrons (e−) in the excited state can react with dissolved oxygen to yield superoxide radical anions, O2•−, which on protonation generate the hydroperoxy, •HO2, radicals. The •OH radical can be formed from the trapped electron after formation of the •HO2 radical. The large amount of the generated hydroxyl radical OH•, a strong oxidizing agent to decompose the organic dye, can oxidize the MO dyes absorbed on the surface of polymer and ZIS nanosheets to produce degraded compounds. Besides the above advantages, compared with ZIS powder photocatalyst, ZIS/Polymer hierarchical photocatalyst can be readily separated from the water in a slurry system after photocatalytic reaction because of their self-supporting structures. More importantly, it was indicated that these heteroarchitecture nanofiber photocatalysts with high photocatalytic activities could be easily separated and recovered by sedimentation, which would greatly promote their practical application to eliminating organic pollutants from wastewater. Therefore, the synthesized ZIS/Polymer hierarchical photocatalyst can be used suitably for industrial applications. On the basis of the experiment results, it is indicated that the photocatalytic activity could be improved by using the ZIS/ Polymer photocatalyst, and it could be further improved by optimizing the size and exposed density of the ZIS nanosheets, in addition to the diameter size of the polymer matrix. Meanwhile, further study should be performed to investigate the detailed photocatalytic mechanism, and hence to improve the photocatalytic properties of the ZIS/Polymer structure. Considering the above factors, it is reasonable to consider that the ZIS/Polymer hierarchical nanostructures prepared by the combination of electrospinning and a hydrothermal process can exhibit photocatalytic activity superior to that of pure ZIS powder.

4. CONCLUSION In conclusion, ZnIn2S4/PVDF−P(MMA-co-MAA) composites with ZnIn2S4 nanosheets hierarchically assembled on polymer nanofiber backbones have been successfully fabricated by combining electrospinning and a hydrothermal process. The coverage density of the ZIS nanosheet coating on the polymer mats could be controlled by adjusting the amount ratios of the precursors to the polymers and the parameters of the hydrothermal reaction. Experimental results demonstrated that the ZIS(50 wt %)/Polymer with unique hierarchical nanostructures exhibited superior photocatalytic performance to that of pristine ZIS powder obtained by a hydrothermal 13856

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(12) Chen, Z. X.; Li, D. Z.; Zhang, W. J.; Shao, Y.; Chen, T. W.; Sun, M.; Fu, X. Z. J. Phys. Chem. C 2009, 113, 4433. (13) Sun, C. H.; Wang, N. X.; Zhou, S. Y.; Hu, X. J.; Zhou, S. Y.; Chen, P. Chem. Commun. 2008, 44, 3293. (14) Huang, Z. M.; Zhang, Y. Z.; Kotaki, M.; Ramakrishna, S. Compos. Sci. Technol. 2003, 63, 2223. (15) Wang, Z. R.; Wang, H.; Liu, B.; Qiu, W. Z.; Zhang, J.; Ran, S. H.; Huang, H. T.; Xu, J.; Han, H. W.; Chen, D.; Shen, G. Z. ACS Nano 2011, 5, 8412. (16) Greiner, A.; Wendorff, J. H. Angew. Chem., Int. Ed. 2007, 46, 5670. (17) Yee, W. A.; Kotaki, M.; Liu, Y.; Lu, X. H. Polymer 2007, 48, 512. (18) Adden, N.; Gamble, L. J.; Castner, D. G.; Hoffmann, A.; Gross, G.; Menzel, H. Langmuir 2006, 22, 8197. (19) Chang, Z. J. Chem. Commun. 2011, 47, 4427. (20) Fan, W. J.; Zhou, Z. F.; Xu, W. B.; Shi, Z. F.; Ren, F. M.; Ma, H. H.; Huang, S. W. Int. J. Hydrogen Energy 2010, 35, 6525. (21) Xu, L.; Song, H. W.; Dong, B.; Wang, Y.; Chen, J. S.; Bai, X. Inorg. Chem. 2010, 49, 10590. (22) Wang, C. H.; Shao, C. L.; Zhang, X. T.; Liu, Y. C. Inorg. Chem. 2009, 48, 7261. (23) Shi, N.; Li, X. H.; Fan, T. X.; Zhou, H.; Ding, J.; Zhang, D.; Zhu, H. X. Energy Environ. Sci. 2011, 4, 172. (24) Riha, S. C.; Johnson, D. C.; Prieto, A. L. J. Am. Chem. Soc. 2011, 133, 1383. (25) He, T. S.; Ma, H. H.; Zhou, Z. F.; Xu, W. B.; Ren, F. M.; Shi, Z. F.; Wang, J. Polym. Degrad. Stab. 2009, 94, 2251. (26) Cui, W. G.; Li, X. H.; Zhou, S. B.; Weng, J. Polym. Degrad. Stab. 2008, 93, 731. (27) Yu, G.; Li, X. H.; Cai, X. J.; Cui, W. G.; Zhou, S. B.; Weng, J. Acta Mater. 2008, 56, 5775. (28) Hu, J. Q.; Deng, B.; Zhang, W. X.; Tang, K. B.; Qian, Y. T. Inorg. Chem. 2001, 40, 3130. (29) Peng, S. J.; Zhu, P. N.; Thavasi, V.; Mhaisalkar, S. G.; Ramakrishna, S. Nanoscale 2011, 3, 2602. (30) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (31) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930. (32) Govender, K.; Boyle, D. S.; Kenway, P. B.; O’Brien, P. J. Mater. Chem. 2004, 14, 2575. (33) He, T.; Zhou, Z. F.; Xu, W.; Cao, Y.; Shi, Z. F.; Pan, W. P. Compos. Sci. Technol. 2010, 70, 1469. (34) Liu, M. L.; Chen, I. W.; Huang, F. Q.; Chen, L. D. Adv. Mater. 2009, 21, 3808. (35) Hernández-Alonso, M. D.; Fresno, F.; Suárez, S.; Coronado, J. M. Energy Environ. Sci. 2009, 2, 1231. (36) Zhang, H.; Lv, X. J.; Li, Y. M.; Wang, Y.; Li, J. H. ACS Nano 2010, 4, 380. (37) Xia, J. X.; Yin, S.; Li, H. M.; Xu, H.; Yan, Y. S.; Zhang, Q. Langmuir 2011, 27, 1200.

method. Furthermore, the ZIS(50 wt %)/Polymer mat with a high cycle degradation stability for MO dye can be easily recycled without deactivation, indicating that the ZIS(50 wt %)/Polymer mat did not photocorrode during the photocatalytic oxidation of the pollutant molecules, which was especially important for its application and as a promising candidate for practical photocatalysts. Since the electrospinning and hydrothermal methods are simple processes for the fabrication of inorganic nanofiber mats and nanomaterials with controlled shapes, the combination of the two methods in our work opened a new door for producing controlled functional three-dimensional hybrid architectures with ultrahigh-volume fractions of semiconductor nanomaterials, which could have promising applications in photovoltaics, photocatalysis, and sensors.

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ASSOCIATED CONTENT

S Supporting Information *

Thermal gravity analysis, BET of PVDF−P(MMA-co-MAA) and ZIS/Polymer composite mats with different amounts of ZnIn2S4, TEM and HRTEM images of ZIS nanosheets of ZIS(50 wt %)/Polymer, XPS survey spectrum of ZIS(50 wt %)/Polymer composite, SEM image of ZnIn2S4 powder, physical adsorption properties of samples in the dark, and XRD, SEM, and EDS of ZIS(50 wt %)/Polymer composite after five cycles of photocatalytic degradation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.P.); [email protected] (S.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Singapore NRF-CRP grant on “Nanonets for Harnessing Solar Energy and Storage”, and we also thank NUS and NTU for providing facilities to carry out the research.

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REFERENCES

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