Macroporous polymer resin with conjugated side-chains: an efficient Ag nanoparticle support for preparing a photocatalyst

Lin Wang a, Xuejiao Chen a, Yandong Duan a, Qingzhi Luo *a and Desong Wang *ab
aSchool of Sciences, Hebei University of Science and Technology, Shijiazhuang 050018, People's Republic of China. E-mail: lqz2004-1@163.com; dswang06@126.com; Fax: +86311 81669970; Fax: +86311 81669962; Fax: +86335 8058006; Tel: +86311 81669970 Tel: +86311 81669962 Tel: +86335 8058006
bState key Lab of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, 066004, People's Republic of China

Received 5th March 2020 , Accepted 11th May 2020

First published on 12th May 2020


Silver nanoparticles (Ag NPs), widely used as photocatalysts, exhibit low photocatalytic activity due to their serious aggregation and difficulty in separation for recycling. To solve these problems, a highly efficient and stable photocatalysis system was designed by immobilizing Ag NPs on a macroporous polymer resin with conjugated side-chains (denoted as Ag@sulfamide-resin). A macroporous polymer resin with conjugated side-chains (named sulfamide-resin) was constructed by grafting 4′-(p-ethanediamine phenyl)-2,2′:6′,2′′-terpyridine (TE) to polystyrene-based macroporous resin via sulfamide bond formation. The photocatalytic activity of Ag@sulfamide-resin was evaluated via the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction. Ag@sulfamide-resin exhibits excellent photocatalytic activity and stability, and the catalytic activity of Ag@sulfamide-resin under light irradiation was found to be 3.97 and 15.1 times those of Ag@sulfamide-resin in the dark and Ag@macroporous-resin under light irradiation, respectively. The higher photocatalytic activity was attributed to the fact that sulfamide resin can absorb broad wavelength light and easily transfer photogenerated electrons to Ag NPs; the abundant “N” atoms in this resin can act as anchoring sites for Ag NPs to prevent their aggregation and loss; and Ag NPs display smaller sizes of 2–3 nm. The macroporous polymer resin with conjugated side-chains may be a prospective material as a support for designing an efficient photocatalyst.


1 Introduction

Historically, sunlight, especially visible light, has been intensively used as an abundant, inexpensive, nonpolluting, and clean energy source to drive chemical reactions.1,2 Ag nanoparticles (Ag NPs) have been recognized as an excellent photocatalyst for converting light energy to chemical energy because of their low cost, unique optical and electronic properties, high catalytic activity and selectivity.3 However, their high surface energy causes their aggregation, resulting in a remarkable loss of catalytic activity, and their small size leads to difficulty in separating them from mixtures, which limits their recycling.4 One effective method to solve these problems is to immobilize Ag NPs on a support. Unfortunately, there is no synergistic effect between Ag NPs and the support to further improve the catalytic performances of the metal catalysts.

Polystyrene-based macroporous polymer resin, widely applied in the fields of ion exchange, adsorption, metal ion chelation etc., possesses many nano, meso, and macroscale pores and a large specific surface area, so it is an excellent catalyst support. The benzene rings in macroporous resin are relatively active and react easily with organic functional molecules. Based on this property, the conjugated side-chains can be grafted onto the surface of the macroporous resin to form a conjugated porous polymer (CPP) which exhibits some of the performance of organic semiconductors.

CPP used as an Ag NP support is unique for its permanent porous structure and photoactive π-conjugation component.5,6 The extended π-conjugation component along the skeleton not only shifts the light absorption of the polymer resin toward longer wavelengths, but also serves as an “energy donor” through the collection of light energy, and the three-dimensional pores can confine “energy accepting molecules”, resulting in the creation of a “donor–acceptor system” for energy transduction.7,8 Thus, it is believed that CPP has the ability to accept and transport light energy throughout its delocalized framework to metal nanoparticles, so that the photocatalytic activity of the metal catalyst can be significantly improved.9 Besides this, the high surface area of a porous polymer endows it with a high adsorption capacity toward reaction molecules and the ordered porous channels are beneficial for the transportation and diffusion of reaction molecules.10 Based on the above-mentioned advantages, it would be interesting and important to construct an efficient photocatalyst using CPP and Ag NPs, which have a synergistic effect in photocatalytic activity.

Herein, we have designed and synthesized a novel CPP by grafting (p-ethanediamine phenyl)-2,2′:6′,2′′-terpyridine (TE) carrying multi-aromatic rings onto a polystyrene-based macroporous resin, denoted sulfamide-resin, as illustrated in Fig. 1. Sulfamide-resin can absorb a broad range of light and migrate the photogenerated charges to Ag NPs, facilitating the photocatalytic activity of Ag NPs loaded on sulfamide-resin (designated Ag@sulfamide-resin). Furthermore, Ag+ bonds with the amino group in sulfamide-resin to form a complex compound, followed by the in situ reduction of Ag+ by reducing agent vanillin gas. Then Ag NPs are formed and immobilized with amino groups, which prevents the aggregation and loss of Ag NPs. The photocatalytic activity of Ag@sulfamide-resin was evaluated by the reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), which is an important chemical intermediate for the synthesis of pharmaceuticals, polymers, corrosion inhibitors and dyes.11 Finally, the synergistic mechanism for the enhanced photocatalytic activity of Ag@sulfamide-resin was elucidated.


image file: d0cy00435a-f1.tif
Fig. 1 Schematic representation of the preparation process of Ag@sulfamide-resin.

2 Experimental section

2.1 Materials

Acetylpyridine, bromobenzaldehyde, ethanediamine, triethylamine, ethylenediaminetetraacetic acid disodium (EDTA-2Na), cuprous chloride (CuCl), sodium hydroxide (NaOH) and potassium hydroxide (KOH) were purchased from J & K Chemical Technology Co., Ltd., China. Macroporous resin, silver nitrate (AgNO3), 4-nitrophenol (4-NP), sodium borohydride (NaBH4) and vanillin were obtained from Energy Chemical Co., Ltd., China. Concentrated H2SO4 (98%), N,N-dimethylformamide (DMF), thionyl chloride (SOCl2), ammonium hydroxide (NH3·H2O), anhydrous methanol (CH3OH) and 2-methoxyethanol (HOCH2CH2OCH3) were purchased from Sigma Chemical Co., Ltd., China. All reagents were used without further purification and Milli-Q grade ultrapure water was used in all experiments.

2.2 Synthesis of sulfamide-resin

The preparation routes of 4′-(p-ethanediamine phenyl)-2,2′:6′,2′′-terpyridine (TE) and chlorosulfonated-resin are presented in Fig. S1 and S2. The sulfamide-resin was synthesized via the sulfamide reaction of chlorosulfonated-resin and TE. Chlorosulfonated-resin (0.76 g) was dispersed in 10 mL of anhydrous methanol under stirring. Then, an anhydrous methanol solution of TE (100 mL, 0.045 M) was added dropwise to the above suspension with triethylamine (0.85 g, 8.4 mmol) as an acid binding agent, which was refluxed at 90 °C for 5 d. After that, the mixture was filtered, washed with anhydrous ethanol several times to remove triethylamine hydrochloride and residual TE, then dried at 60 °C to obtain the final product sulfamide-resin (see step I in Fig. 2). The structure of sulfamide-resin was confirmed by solid 13C NMR (Fig. S3).
image file: d0cy00435a-f2.tif
Fig. 2 Synthetic routes of Ag@sulfamide-resin.

2.3 Preparation of Ag@sulfamide-resin

The preparation of Ag@sulfamide-resin was carried out in the following steps. Firstly, sulfamide-resin (1.0 g) was dispersed in 10 mL of H2O. Subsequently, an AgNO3 aqueous solution (10 mL, 4.63 mM) was dropped into the sulfamide-resin suspension and stirred overnight in the dark. The solid powders were collected by filtration and washed three times with distilled water. After that, the solid powders were placed into a container containing vanillin gas at 120 °C for 2 d and then washed with distilled water several times to remove the remaining AgNO3 and vanillin. The final product, designated as 0.5%Ag@sulfamide-resin (0.5% is the designed mass percent proportion of Ag in the composite), was obtained after drying at 60 °C overnight (see step II in Fig. 2). ICP-AES measurements indicate that the actual loading amount of Ag NPs in sulfamide-resin is 0.44 wt%. Pure macroporous resin loaded with the same amount of Ag NPs (Ag@macroporous-resin) was obtained when the designed loading amount of Ag was 1.50 wt%. Moreover, Ag@sulfamide-resin with different designed Ag amounts of 0.1, 0.5, 1, and 2 wt% (designated as x%Ag@sulfamide-resin) were prepared and 0.5%Ag@sulfamide-resin exhibited the best photocatalytic activity, so it was chosen as the sample for characterization.

2.4 Photocatalytic activity and stability test of Ag@sulfamide-resin

The reduction of 4-NP to 4-AP was chosen as a model reaction to investigate the photocatalytic activity of the prepared samples in the presence of excess NaBH4. 100 mL of 4-NP aqueous solution (1.25 mM) was mixed with 10 mg of Ag@sulfamide-resin under stirring for 30 min in the dark to achieve an adsorption–desorption equilibrium between the catalyst and the reactant. Then, 472 mg of NaBH4 was added to this suspension and the color of reaction mixture changed to a dark yellow. Subsequently, the suspension was irradiated by a xenon lamp (300 W). During the photocatalytic reduction reaction process, 1.0 mL of the suspension was extracted at 10 min intervals and filtered through a 0.22 μm membrane to obtain the 4-NP solution, which was then diluted by 25 times to test the concentration using a UV-vis spectrophotometer (TU-1901, Beijing Purkinje General Instrument Co., Ltd., China). For the recycling test, Ag@sulfamide-resin was collected by membrane filtration after the first reaction and then re-dispersed into a mixed solution (100 mL) of 4-NP (1.25 mM) and NaBH4 (472 mg) for the next cycle.

2.5 Active species trapping experiments

To determine the photocatalytic reduction mechanism, trapping experiments were carried out using dimethylsulfoxide (DMSO), potassium iodide (KI), benzoquinone (BQ), and isopropyl alcohol (ISPA) as representative scavengers (5 mM) of e, h+, O2− and ˙OH, respectively. These scavengers were added to the 4-NP solution and the other procedures are the same as for the photocatalytic reduction experiments.

2.6 Structure and property characterization

1H nuclear magnetic resonance spectra (1H NMR) and solid-state 13C nuclear magnetic resonance spectra (13C NMR) were recorded on an Avance 500 MHz spectrometer (Bruker Co., Germany) and an Avance III 400 MHz spectrometer (Bruker Co., Germany), respectively. Fourier-transform infrared spectra (FTIR) of the samples were conducted on a Prestige-21 spectrometer (Shimadzu Co., Japan) in the range of 400–4000 cm−1 using spectroscopic grade KBr as the reference sample. Thermalgravimetric analysis (TGA) was carried out using a TGA-Q50STA 449 F5 spectrometer (Netzsch, Germany) at a heating rate of 10 °C min−1 under a nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) was measured with a Thermo Fisher Scientific K-Alpha with Al Kα X-ray ( = 1486.6 eV) irradiation. The X-ray anode was run at 250 W, and the high voltage was kept at 15.0 kV with a detection angle of 54°. The Brunauer–Emmett–Teller (BET) surface area was measured with a Micromeritics TriStar II 3020 surface area and porosity system (Micromeritics, America) using nitrogen as an adsorption gas at 77.36 K. Morphological characterization was carried out using an S-4800 scanning electron microscope (SEM) (Hitachi, Japan) at 10 KV, a JEM-2100F transmission electron microscope (TEM) (JEOL, Ltd., Japan) at 200 KV and a high angle annular dark field (HAADF). In addition, selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX) were also recorded using this TEM. The TEM sample was prepared using a negative-staining method. A drop of dilute ethanol suspension of the sample was placed on a carbon-coated copper grid for 5 min. Next, one drop of uranyl acetate solution (0.5%) as a staining agent was put on the copper grid for 5 min. Finally, the excess liquid was removed with filter paper and the copper grid was dried at room temperature overnight. UV-vis diffuse reflectance spectra (UV-vis DRS) were investigated with a Shimadzu-2550 Scan UV-vis system (Shimadzu, Japan) equipped with an integrating sphere attachment and a measurement wavelength range from 200 to 800 nm with BaSO4 as a reference. Photoluminescence (PL) emission spectra were recorded by an F-4600 FL fluorescence spectrophotometer (Hitachi Co., Japan) using a xenon lamp as the excitation source and the excitation wavelength was 270 nm. The actual amount of Ag NPs in the sample was quantified with an ICAP 6300 inductively coupled plasma-atomic emission spectrometer (ICP-AES) (Thermo, US). Electrochemical impedance spectra (EIS) and photocurrent response measurements were tested on a CHI 660E electrochemical system (CH Instruments, China) in a conventional three-electrode configuration with Pt wire and saturated calomel electrode as the counter electrode and reference electrode, respectively, and using 0.2 M Na2SO4 solution as the electrolyte. The working electrode was prepared as follows: 0.050 g of sample and 0.010 g of polyethylene glycol (PEG) were suspended in 5 mL of ethylalcohol to produce a slurry which was then deposited on an FTO substrate using a doctor-blade method, and finally the substrate was sintered at 200 °C for 2 h to obtain the working electrode. The alternating current frequency for EIS measurement ranged from 100 kHz to 10 mHz.

3 Results and discussion

3.1 Characterization of Ag@sulfamide-resin

In the FTIR spectrum (Fig. 3a), the characteristic absorbance peaks at 3305 and 1577 cm−1 belong to the stretching vibration and bending vibration of N–H in TE, respectively.12 After grafting TE onto the macroporous resin, two peaks at 1172 and 1373 cm−1 assigned to the S[double bond, length as m-dash]O symmetric and asymmetric stretching in the –SO2Cl group disappeared, and new peaks at 1155 and 1331 cm−1 from the symmetric and asymmetric stretching of S[double bond, length as m-dash]O in the –SO2NH– group were formed.13,14 Meanwhile, peaks at 3318 and 990 cm−1 related to stretching vibrations of –N–H and –S–N– in –SO2NH– also appeared.15 The peak at 1600 cm−1 belonging to the stretching vibrations of C[double bond, length as m-dash]C and C[double bond, length as m-dash]N in the aromatic rings of sulfamide-resin is significantly higher in intensity than that of chlorosulfonated-resin because there are more conjugated aromatic rings within the sulfamide-resin. These results indicate that TE was successfully grafted onto the macroporous resin through sulfamide bonds. The grafting degree was determined by TGA. Fig. 3b shows that the weight loss values of chlorosulfonated-resin and sulfamide-resin at 500 °C were 44% and 72%, respectively. The grafting degree (G) was calculated from eqn (1):16
 
image file: d0cy00435a-t1.tif(1)
where Wf and W0 are the mass loss values of the sulfamide-resin and chlorosulfonated-resin, respectively. Thus, it was calculated that 63.6% of was grafted onto the macroporous resin.

image file: d0cy00435a-f3.tif
Fig. 3 (a) FTIR spectra of TE, chlorosulfonated-resin and sulfamide-resin; (b) TGA thermograms of chlorosulfonated-resin and sulfamide-resin; (c) high resolution XPS spectra of N 1s, Ag 3d and wide-scan XPS survey spectrum of Ag@sulfamide-resin; (d) nitrogen adsorption–desorption isotherms of macroporous resin, sulfamide-resin and Ag@sulfamide-resin.

As shown in Fig. 3c, in the high resolution spectrum of N 1s, TE exhibits two peaks at 400.6 and 398.95 eV, corresponding to C–N and C[double bond, length as m-dash]N–C bonds, respectively.17 Compared to TE, the sulfamide-resin shows a new peak at 399.63 eV arising from –SO2NH–, which provides supporting evidence that the sulfamide-resin is constructed through sulfamide bonds.18 The peak at 401.5 eV attributed to Ag⋯N can be observed in Ag@sulfamide-resin (inset in Fig. 3c), indicating that the “N” atoms in sulfamide-resin can immobilize Ag NPs, preventing the aggregation of Ag NPs and improving their stability.19 Moreover, the XPS survey spectrum of Ag@sulfamide-resin confirms the presence of Ag in this material besides C, O, and N. The Ag 3d spectrum of Ag@sulfamide-resin has two independent peaks, corresponding to the spin–orbit of Ag 3d3/2 and Ag 3d5/2, which can be deconvoluted into four peaks: the peaks at 368.3 and 374.3 eV can be attributed to metallic Ag NPs and the other two peaks at 367.68 and 373.68 eV are assigned to Ag⋯N.20 The splitting of the Ag 3d doublet into Ag0 and Ag⋯N peaks is approximately 6.0 eV, indicating Ag in a zero-valent state Ag0 in Ag@sulfamide-resin. The above results confirm the presence of the desired elements and chemical groups in sulfamide-resin, and indicate that Ag NPs were successfully immobilized onto the sulfamide-resin.

BET results (Fig. 3d) show that the macroporous resin, sulfamide-resin and Ag@sulfamide-resin exhibit a typical IV isotherm (according to IUPAC classification) with a hysteresis loop at a relatively high P/P0 (>0.9), indicating the existence of abundant macropores in these samples. The porous structure of a catalyst is beneficial for the diffusion and transformation of reaction molecules in the catalytic/photocatalytic reaction. The BET surface area (200.24 m2 g−1) and pore volume (0.85 cm3 g−1) of sulfamide-resin have decreased compared to those of macroporous resin (BET ∼596.85 m2 g−1, pore volume ∼1.60 cm3 g−1) due to the existence of TE within the pore channels of the resin and pore breakage during the chemical treatment process. Moreover, the BET surface area (∼171.80 m2 g−1) and pore volume (∼0.79 cm3 g−1) of Ag@sulfamide-resin decrease further because Ag NPs fill the inside of the pore channels. The crystalline properties of Ag@sulfamide-resin were characterized by X-ray diffraction (XRD) measurement. As can be seen from Fig. S4, the sulfamide-resin is amorphous, with no characteristic diffraction peak. The amorphous characteristic of Ag@sulfamide-resin was also observed because of the dominant amorphous phase of sulfamide-resin with a low content of Ag NPs (0.44 wt% determined by ICP-AES measurement).

Fig. 4a shows that the skeleton of macroporous resin is composed of interconnected microspheres, and the interstices between microspheres are mesoporous voids (red arrows). After grafting TE onto the microporous resin (Fig. 4b), the porosity obviously decreased because TE was successfully grafted onto the surface of the microporous resin. Fig. 4c shows that Ag@sulfamide-resin contains a similar morphology to sulfamide-resin, and the TEM image (Fig. 4d) reveals that spherical Ag NPs (yellow arrows) are uniformly dispersed on the sulfamide-resin and their diameter is 2–3 nm (yellow circles) (Fig. 4e). This result can be ascribed to the stable immobilization of Ag NPs within the macroporous resin by the amide groups. The HRTEM image (inset in Fig. 4e) shows a visible lattice fringe spacing of 0.23 nm, which corresponds to the (110) plane of the Ag NPs (JCPDS card No. 00-004-0783). The EDX spectrum (Fig. 4f) shows the presence of Ag, C, N, Cl, S, O elements in the Ag@sulfamide-resin, and the mass percentage of Ag NPs is 0.44 wt%, which is close to the designed loading value of 0.5 wt%. The Cu peaks originate from the Cu grid in the TEM sample preparation process.21


image file: d0cy00435a-f4.tif
Fig. 4 SEM images of the (a) macroporous resin, (b) sulfamide-resin and (c) Ag@sulfamide-resin; TEM images (d and e) of the Ag@sulfamide-resin; (f) EDX spectrum of the Ag@sulfamide-resin; (g) HAADF-STEM image and corresponding EDS mapping images for Ag, C, N, S, O, and Cl elements of the Ag@sulfamide-resin; (h) SAED image of the Ag@sulfamide-resin.

Furthermore, these elements of Ag@sulfamide-resin are confirmed by HAADF-STEM and EDS mapping images (Fig. 4g), which also reveal the uniform distribution of Ag NPs on the Ag@sulfamide-resin. Moreover, the SAED image (Fig. 4h) shows four distinct diffraction rings, reflecting the polycrystalline character of Ag@sulfamide-resin and the position of the rings from inner to outer correspond to lattice spacings (d-spacings) of 0.236, 0.204, 0.145 and 0.123 nm, respectively, which are consistent with the d-spacings of the (111), (200), (220) and (311) planes of face-centered cubic (fcc) structured Ag NPs.22

3.2 Optoelectrical properties of Ag@sulfamide-resin

DRS measurements were used to investigate the light absorption ability of the samples. As shown in Fig. 5a, the macroporous resin has two absorption peaks at 222 and 256 nm, assigned to the π–π* transition of benzene rings, and has no absorption peak in the visible region. In the case of TE, the absorption band from 200 to 450 nm can be attributed to the π–π* transition of benzene rings and pyridine rings.23 Sulfamide-resin exhibits an obvious red-shift compared to TE and the macroporous resin, because the aggregated aromatic rings of TE grafted onto the macroporous resin skeleton increase the π-conjugated degree of the system, which narrows the energy gap for the π–π* transition, resulting in long wavelength light absorption.24 Furthermore, the Ag@sulfamide-resin shows broad light absorbance in the range from 200 to 800 nm and a further red-shift compared to sulfamide-resin, due to the surface plasmon resonance (SPR) absorption of Ag NPs.25 Under visible light irradiation, weakly bound electrons of Ag NPs give rise to a plasmon state, and when the incident light frequency matches the plasmon oscillation frequency, surface plasmon absorption occurs, reflected in the broad light absorption.26 Moreover, there is strong light absorbance of Ag@sulfamide-resin in the UV-vis region because of the small diameter of Ag NPs (2–3 nm).27 These absorbance curves correspond to an obvious color change from the white of macroporous resin, yellow of TE, and dark yellow of sulfamide-resin to the dark red of the Ag@sulfamide-resin (inset in Fig. 5a). Fig. S5a shows the band gap energy (Eg) of the sulfamide-resin (2.6 eV), and its highest occupied molecular orbital (HOMO) potential (1.32 eV) (Fig. S5b) based on the VB-XPS results. According to the equation ELUMO = EHOMOEg, the lowest unoccupied molecular orbital (LUMO) potential of the sulfamide-resin is −1.28 eV. This result further demonstrates that Ag@sulfamide-resin absorbs more visible light and generates more charges in the photocatalytic reduction reaction, which favors the visible light photocatalytic activity of the materials.
image file: d0cy00435a-f5.tif
Fig. 5 (a) UV-vis DRS, photographs (inset) and (b) PL spectra of macroporous resin (a1), TE (a2), sulfamide-resin (a3) and Ag@sulfamide-resin (a4); (c) EIS and (d) photocurrent response of macroporous resin, sulfamide-resin and Ag@sulfamide-resin.

PL, EIS and photocurrent response measurements were carried out to investigate the electron transfer behavior of the samples. As shown in the PL spectra (Fig. 5b), when excited at 270 nm, the macroporous resin exhibits a weak PL peak at 535 nm, and the sulfamide-resin shows a significantly higher PL peak at 547 nm, which should arise from the extended π-conjugation component. Besides, an obvious red-shift of ∼54 nm for sulfamide-resin in comparison with TE is due to the formation of a J-type π–π interaction between the grafted pyridine rings.28 This ordered structure facilitates charge transportation along the conjugated side-chains and reduces the charge recombination inside the polymer. The fluorescence strength of Ag@sulfamide-resin obviously decreases compared to that of sulfamide-resin, suggesting that Ag NPs as electron acceptors reduce the recombination probability of photogenerated electron–hole pairs, resulting in a decrease in PL intensity.29 EIS measurement (Fig. 5c) exhibits Ag@sulfamide-resin with a much decreased arc radius compared to those of sulfamide-resin or macroporous resin, indicating that Ag@sulfamide-resin possesses the lowest charge transfer resistance among them, which promotes its photogenerated electron and hole transfer and separation.30 This result was also verified by the photocurrent responses of the samples for ten switch-on and switch-off cycles. As shown in Fig. 5d, there are no significant photocurrents for any of the samples under dark conditions, whereas after light irradiation, the Ag@sulfamide-resin has a much greater photocurrent intensity (∼2.32 × 10−7 μA cm2) than the sulfamide-resin (∼5.42 × 10−8 μA cm2) or macroporous resin (∼1.50 × 10−8 μA cm2).31 This result demonstrates that the Ag@sulfamide-resin increases the number of photogenerated charges available because Ag NPs as an electron trap center boost the separation efficiency of the electron–holes.

3.3 Photocatalytic activity of Ag@sulfamide-resin

The catalytic activity of the prepared samples was studied by the model reaction of 4-NP to 4-AP in the presence of excess NaBH4.The pale yellowish 4-NP aqueous solution exhibits maximum absorption at 317 nm. The addition of NaBH4 to 4-NP solution results in this peak red-shifting to 400 nm, and the color changes to dark yellow due to 4-nitrophenotate ions being formed.32 The reaction finishes with a visual color change from dark yellow to colorless. The reduction process was monitored by the diminishing peak of 4-NP at 400 nm and the growing peak of 4-AP at 300 nm (Fig. 6a and S6). When the reduction reaction was carried out with NaBH4 but without a catalyst, the absorption intensity of 4-NP showed no obvious change (Fig. S7), indicating that the reduction reaction hardly proceeds. This result can be ascribed to the kinetic barrier between BH4 and 4-NP. Hence, the Ag@sulfamide-resin is indispensable for catalyzing the reduction of 4-NP to 4-AP.
image file: d0cy00435a-f6.tif
Fig. 6 (a) Photos of 4-NP, 4-NP after the addition of NaBH4 solution and 4-AP; time-dependent UV-vis spectra of the catalytic reduction of 4-NP catalyzed by Ag@sulfamide-resin under light irradiation (b) and in the dark (c); time-dependent UV-vis spectra of catalytic reduction of 4-NP catalyzed by Ag@macroporous-resin under light (d) and in the dark (e); (f) apparent kinetic relation for the photocatalytic reduction of 4-NP; (g) recycling test of the photocatalytic reduction of 4-NP catalyzed by Ag@sulfamide-resin; (h) reactive species trapping experiments of Ag@sulfamide-resin.

Ag@sulfamide-resin with different designed amounts of Ag of 0.1, 0.5, 1, and 2 wt% was prepared to investigate the relationship between the amount of Ag and the photocatalytic activity. As shown in Fig. S8, it was found that the conversion efficiency of 4-NP initially increases and then decreases with an increase in the amount of Ag, and the highest conversion efficiency of 4-NP was obtained for 0.5%Ag@sulfamide-resin. This is because the number of active sites increases with the amount of Ag, facilitating the photocatalytic reduction reaction, whereas further increasing the amount of Ag can lead to the serious agglomeration of Ag NPs, which can reduce the number of active sites in the photocatalyst. Additionally, Ag agglomeration shields the light absorption for the sulfamide-resin, and decreases the production efficiency of photo-generated electrons, resulting in lower photocatalytic reduction activity.33 The BET data further support this result because 0.5%Ag@sulfamide-resin shows the highest surface area (171.80 m2 g−1) compared with 0.1%Ag@sulfamide-resin (159.96 m2 g−1), 1%Ag@sulfamide-resin (153.85 m2 g−1) or 2%Ag@sulfamide-resin (121.42 m2 g−1). Thus, E-mail: 0.5%Ag@sulfamide-resin was chosen as a representative sample to study the reduction activity of 4-NP.

To investigate the effect of light on the catalytic activity of Ag@sulfamide-resin, the catalytic activities of Ag@sulfamide-resin under light and in dark conditions were determined. Under light (Fig. 6b), the absorption peak at 400 nm decreased sharply over time and decreased to nearly zero after only 24 minutes, whereas, in dark conditions, the complete reduction of 4-NP needs 80 minutes (Fig. 6c). As shown in Fig. 6f, the good linear correlation of ln(c0/c) versus time indicates that the 4-NP reduction process follows pseudo-first-order kinetics. The apparent kinetic rate constant (k) of 4-NP reduction in the presence of Ag@sulfamide-resin under light (k = 0.151 min−1) is 3.97 times that (k = 0.038 min−1) in the dark, indicating that Ag@sulfamide-resin acts as both a photocatalyst and a catalyst in the photocatalysis reaction. The higher reduction rate of 4-NP in light occurred because the conjugated side-chains within Ag@sulfamide-resin possesses stronger light adsorption ability and the photogenerated electrons migrate from sulfamide-resin to the Ag NPs. To further verify the role of the conjugated side-chains, the 4-NP reduction catalyzed by Ag@macroporous-resin with the same Ag loading (0.44 wt%) was also investigated. It was found that whether in the dark or under light (Fig. 6(d–f)), Ag@macroporous-resin has a much lower and almost unchanged rate constant (k = 0.010 min−1), revealing that Ag@macroporous-resin without conjugated side-chains does not exhibit photocatalytic activity and the conjugated side-chains in Ag@sulfamide-resin indeed play a synergistic role in 4-NP reduction.

As shown in Fig. 6g, the reduction rate of 4-NP hardly changes with an increase in the number of catalytic recycles, demonstrating that Ag@sulfamide-resin possesses excellent photocatalytic stability. The SEM and TEM images (Fig. S9) and XRD patterns (Fig. S10) of Ag@sulfamide-resin after 8-cycle experiments hardly change compared to those of the original sample, further confirming the superior photocatalytic stability of Ag@sulfamide-resin. The photocatalytic stability of Ag@sulfamide-resin can be ascribed to the anchoring effect of Ag NPs within sulfamide-resin by amino groups, leading to negligible loss and agglomeration of Ag NPs.

To better understand the 4-NP reduction mechanism and identify the active species involved, the effects of DMSO (e scavenger), KI (h+ scavenger), BQ (O2− scavenger) and ISPA (˙OH scavenger) on the photocatalytic activity were investigated. Fig. 6h shows that the photocatalytic conversion (c/c0) of 4-NP to 4-AP was significantly reduced to 20% after 24 min when DMSO was added to the 4-NP solution. Whereas, after the addition of BQ, KI or ISPA to the 4-NP solution, the photocatalytic conversion showed no obvious change, suggesting that the photogenerated e is the main active species in the photocatalytic reduction process.

3.4 Photocatalytic reduction mechanism of Ag@sulfamide-resin

Based on the above results, the catalytic reduction mechanism of Ag@sulfamide-resin under light irradiation was discussed, as shown in Fig. 7. Under dark conditions, the catalytic conversion of 4-NP to 4-AP by Ag@sulfamide-resin includes three steps: (i) diffusion and adsorption of reactants (borohydride ion (BH4) and 4-NP) onto the Ag@sulfamide-resin surface; (ii) transfer of electrons donated by BH4 to the acceptor of 4-NP by the electron transport system of Ag@sulfamide-resin, which leads to the formation of 4-AP; (iii) desorption of products (4-AP) from the catalyst surface and diffusion into aqueous solution. 4-NP was absorbed on the Ag@sulfamide-resin surface through π–π stacking interactions between the benzene rings of 4-NP and the aromatic rings of sulfamide-resin; thus, 4-NP at a high concentration can take part in the reduction reaction. Unlike the dark conditions, due to the existence of conjugated side-chains, sulfamide-resin under light can efficiently absorb broad wavelength light energy to produce photogenerated holes and electrons. NaBH4, as a hole-capture agent, can easily react with the photogenerated holes, which is beneficial for the separation probability between holes and electrons, so more photogenerated electrons in the LUMO of sulfamide-resin can transfer to the surface of Ag NPs because of the lower LOMO potential (−1.28 eV) than the Fermi level of the Ag NPs. Therefore, under light, Ag NPs in the Ag@sulfamide-resin can carry surplus electrons from BH4 and the photo-generated electrons in the sulfamide-resin. These plentiful electrons accelerate the catalytic reduction of 4-NP to 4-AP on the Ag surface.34,35 Moreover, the porous structure of the sulfamide-resin promotes the transportation and diffusion of both 4-NP and 4-AP molecules, and the formed Ag NPs are stabilized by the amide groups in the sulfamide-resin, which also benefits the catalytic reduction of 4-NP under light.36
image file: d0cy00435a-f7.tif
Fig. 7 Schematic representation of the photocatalytic reduction mechanism of Ag@sulfamide-resin.

4 Conclusion

In summary, an efficient photocatalyst (designated as Ag@sulfamide-resin) was successfully constructed by immobilizing Ag NPs onto macroporous polymer resin with conjugated side-chains. The Ag NPs were uniformly dispersed on sulfamide-resin with a narrow diameter of 2–3 nm. The Ag@sulfamide-resin exhibits 3.97 and 15.1 times more photocatalytic activity than those of Ag@sulfamide-resin in the dark and Ag@macroporous-resin under light, respectively. The enhanced photocatalytic activity was attributed to broad wavelength light absorption ability, high photogenerated electron density in the Ag NPs, and the small size of Ag NPs of 2–3 nm. Additionally, Ag@sulfamide-resin has excellent photocatalytic stability and recyclability, which is due to the fact that Ag NPs are immobilized by amide groups in the sulfamide-resin. Overall, the macroporous polymer resin with conjugated side-chains could be a promising support for preparing efficient visible-light photocatalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work received financial support from the Natural Science Foundation of Hebei Province (NO. B2019208128).

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Footnote

Electronic supplementary information (ESI) available: Synthesis of TE and sulfamide-resin, the figure of (αhν)2vs. hν of sulfamide-resin, VB-XPS spectra of sulfamide-resin and UV-vis spectra of 4-AP, 4-NP before and after addition of NaBH4 solution. See DOI: 10.1039/d0cy00435a

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