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Hypercroslinked Polynaphthalene Semiconductor with Excellent VisibleLight Photocatalytic Performance in Degradation of Organic Dyes Lei Zhang, Xinhua Huang, Jin-Song Hu, Jian Song, and Il Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00190 • Publication Date (Web): 09 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017
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Langmuir
Hypercroslinked Polynaphthalene Semiconductor with Excellent Visible-Light Photocatalytic Performance in Degradation of Organic Dyes Lei Zhang,† Xin-Hua Huang,*,†, Jin-Song Hu,† Jian Song,† Il Kim*,‡ †
School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan, Anhui 232001, P. R. China. ‡
BK21 PLUS Centre for Advanced Chemical Technology, Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea. ABSTRACT: Hypercrosslinked polynaphthalene nanoparticles (PNNs) capable of catalyzing the degradation of organic pollutants upon exposure to visible light have been developed. The nascent and metal-free PNNs with a porous structure, high specific surface area and narrow bandgap are chemically and thermally stable in the catalytic system, which make it promising to be a kind of excellent photocatalytic material compared to conventional photocatalysts. The photocatalytic activity of the as-obtained PNNs exhibit remarkable photocatalytic performance for the degradation of rhodamine B (RhB) and methyl blue (MB) under the irradiation of visible-light. The easy preparation, high catalytic activity and recyclability of the PNNs open new opportunities in the visible-light-promoted degradation of organic pollutants.
INTRODUCTION Conducting polymers (CPs) that can be produced cheaply and conveniently on a large scale have been widely sought as photocatalysts1 in pollutant degradation.2−4 CPs with narrow bandgaps are capable of absorbing ultraviolet and visible light,5−7 which induce ππ* electron transitions in the π*-orbitals of CPs (lowest unoccupied molecular orbitals, LUMO) and react with water and oxygen to form superoxide radicals (O2•−), thus resulting in the oxidation of pollutants.6 Meanwhile, the holes in the highest occupied molecular orbitals (HOMO) of CPs are also capable of directly oxidising pollutants.8However, there are still two main drawbacks impeding their potential application in pollutant treatment. First, most CPs have linear structures that are deformed easily, decreasing photocatalytic efficiency.[3,5] To achieve more stable conjugated polymers, non-linear conjugated CPs have been designed,4,9 such as graphitic carbon nitride (g-C3N4),10−13 polyheptazine14, planarized fluorene-type conjugated polymers15 covalent organic frameworks16 have been widely investigated for photocatalytic applications. Second, the photocatalytic activities of many CPs under visible light are not ideal because of very broad bandgaps between the HOMO and LUMO. To achieve narrow bandgaps, further chemical modification is required, however, these processes are usually tedious.17−19 Therefore, design and preparation of photocatalysts with narrow bandgaps and perfect conjugated structures under milder conditions is still an appealing challenge. Herein, we report a visible-light-responsive photocatalyst which was prepared by Friedel-Crafts alkylation through cross-linking of naphthalene
molecules with chloromethyl methyl ether (CMME) (Scheme 1).20 The resulting polynaphthalene (PN) possesses a rigid coplanar main chain, where the bridging methylene carbons provide an odd number of electrons to methine carbons, with an additional π-electron available for delocalisation,21 thus enhancing the conjugation and carrier mobility.22 It was obtained as dark-yellow powder in the form of porous nanospheres23 with narrow bandgaps and good visible-light-responsive photocatalytic activity. Furthermore, it is insoluble in most common solvents; thus, it is easily separated from the photocatalytic system by filtration for reuse. To the best of our knowledge, this type of visible-light-responsive photocatalyst has not been reported yet.
Scheme 1. Schematic of the synthesis of polynaphthalene particles.
RESULTS AND DISCUSSION The PN nanoparticles (PNNs) were prepared without template as shown in Scheme 1. The bridging agent, CMME, participated in the reaction, resulting in a novel ladder-like conjugated polymer24 which finally self-
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assembled into porous nanospheres. FT-IR (Figure S1 in the Supporting Information (SI)) and solid-state 13C NMR (Figure S2 in the SI) spectra clearly show the formation of methylene bridges. The size of the PNNs was fairly uniform, as confirmed by the scanning electron microscopy (SEM) image in Figure 1A and field emission SEM (FE-SEM) (Figure 1B). HR-TEM shows the nanoscale particles (~5 nm in diameter) aggregated into microscale particles while forming a porous structure (Figure 1B inset). Such porous PNNs were thermally stable over a wide range of temperatures, as demonstrated by thermo gravimetric analysis. The major decomposition was at ~500 °C and 60 wt% of PNNs remained stable even at 800 °C (see Figure S3 in the SI). Thus, the PNNs are highly thermally stable under the environment of the catalytic system. The nitrogen sorption isotherm and pore size distribution (Figure 1C) revealed that the Brunauer-Emmett-Teller (BET) surface area of the PNNs was 131.4 m2 g−1 and the pore volume was 0.16 cm3 g−1. The pore was the result of the strong π-π stacking of the naphthalene and quick formation of the 3D framework structure.8
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conduction band were at 1.05 V (oxidation) and −0.56 V (reduction) vs. the saturated calomel electrode, respectively, yielding an energy gap of 1.61 eV as a first approximation (Figure S4(a) in the SI). This result is consistent with the UV/vis DRS. It is surprising that the PNNs are capable of absorbing visible light; this implies possible photocatalytic activity under visible light. The evaluated HOMO and LUMO levels in the PN moiety of the PNNs structures were calculated by density functional theory, which decreased with increasing numbers of PN moiety (Figure S4(b) in the SI), although they were underestimated, and the trend in our calculation results supported our deduction of the visible-light experiments. Note that the well-known metal-free graphitic carbon nitride (g-C3N4) material is also a visible-light-response material (2.7 eV bandgap), and the energy position of conduction band and valence band is at −1.1 and 1.6 eV versus normal hydrogen electrode, respectively.26
Figure 2. (A) XPS spectra of PNNs broad scan 1300-200 eV and (B) the C 1s separated peaks.
The surface composition and chemical states of the carbon elements in PNNs was revealed by XPS (Figure 2A). In Figure 2B, the C 1s peak can be separated into three signals at 284.7, 284.9 and 287.1 eV, which should be ascribed to the binding energy of C in the C-C, C=C and CO2 adsorbed on the surface, respectively.
Figure 1. (A) SEM and (B) FE-SEM images of PNNs (inset is the TEM image). (C) Nitrogen adsorption-desorption isotherm and corresponding pore size distribution curve (inset) of the PNNs. (D) UV/vis DRS spectrum of PNNs at 2 298 K and plot of (αhν) vs. photo energy (inset) for the estimation of bandgap (Eg ) of PNNs.
The UV/vis diffuse reflectance spectra (DRS) of the PNNs (Figure 1D) showed a broad absorption range from the visible to the near-infrared region (
