Polydopamine Induced in-Situ Formation of ... - ACS Publications

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 14735−14743

Polydopamine Induced in-Situ Formation of Metallic Nanoparticles in Confined Microchannels of Porous Membrane as Flexible Catalytic Reactor Zhen Zeng,† Mingfen Wen,† Boxuan Yu,† Gang Ye,*,†,‡ Xiaomei Huo,† Yuexiang Lu,†,‡ and Jing Chen*,†,‡ †

Collaborative Innovation Center of Advanced Nuclear Energy Technology, Institute of Nuclear and New Energy Technology, and Beijing Key Lab of Radioactive Waste Treatment, Tsinghua University, Beijing 100084, China

‡

S Supporting Information *

ABSTRACT: Oxidant-regulated polymerization of dopamine was exploited, for the first time, for effective surface engineering of the well-defined cylindrical pores of nuclear track-etched membranes (NTEMs) to develop novel catalytic membrane reactor. First, in the presence of a strong oxidant, controlled synthesis of polydopamine (PDA) with tunable particle size was achieved, allowing a homogeneous deposition to the confined pore channels of NTEMs. The PDA interfaces rich in catechol and amine groups provided enhanced hydrophilicity to promote mass transport across the membrane and abundant nucleation sites for formation and stabilization of metallic nanoparticles (NPs). In-situ reductive growth of multiple metallic NPs, including Pd, Ag, and Au, was then achieved inside the cylindrical pores of NTEMs. Using the functionalized membrane as a catalytic reactor, efficient reduction of 4-nitrophenol (4-NP) was demonstrated in a flow-through mode. Moreover, after dissolution removal of the NTEMs, self-sustained one-dimensional (1D) PDA/M (M = Pd, Ag, or Au) hybrid nanotubes (NTs), with determined aspect ratio and a length reaching up to 10 μm, were obtained for catalysis of 4-NP in a batch reaction mode. This study established a facile and versatile method, by rational tuning of the polymerization behavior of dopamine, for effective modification of confined microscale/nanoscale cavities with different surface characteristics. The integration of PDA chemistry with NTEMs would provide more opportunities for development of novel catalytic membrane reactors as well as for the tailored synthesis of functional 1D nanotubes for broadened applications. KEYWORDS: metal nanoparticles, porous membrane, polydopamine, membrane reactor, catalysis

1. INTRODUCTION

reduced catalytic activity. An achievable strategy to address this challenge is to immobilize the metallic NPs to the matrix of porous materials, so that the metallic NPs can be well-dispersed and stabilized in the confined cavities, preserving highly catalytic activity and accessibility to substrate molecules.2 In this respect, miscellaneous scaffolding materials with different

Metallic nanoparticles (NPs) have aroused tremendous research enthusiasm during the past decades due to their great potential for versatile catalytic applications in organic synthesis and electrochemical processes.1 The marvelous performance of these nanosized metallic particles is associated with their high surface area to volume ratio as well as unique physical and chemical properties. However, nanosized metallic catalysts with high surface energy are prone to the formation of agglomeration and/or aggregation, frequently leading to © 2018 American Chemical Society

Received: February 6, 2018 Accepted: April 13, 2018 Published: April 13, 2018 14735

DOI: 10.1021/acsami.8b02231 ACS Appl. Mater. Interfaces 2018, 10, 14735−14743

Research Article

ACS Applied Materials & Interfaces porous characteristics, including alumina,3,4 mesoporous silicas5−7 and carbons,8,9 polymer membranes,10,11 hydrogels,12,13 metal−organic frameworks (MOFs),14,15 covalent organic frameworks (COFs),16,17 etc., have been employed as candidates to accommodate the metallic NPs, and diverse heterogeneous catalytic systems with improved performance have been established. Porous membranes with well-defined pore channels represent a class of attractive supports to host metallic NPs for developing catalytic membrane reactors.18,19 The membrane geometry allows the flow-through catalytic reactions and affords rapid convective mass transport of substrate molecules to the immobilized catalyst NPs, which overcomes the diffusion limitation usually found in other supports with irregular porosity.20 Established methodologies for embedding metallic NPs to the pore channels of membranes include the direct loading of mature NPs, or impregnation of metal salts, followed by chemical reduction to form the corresponding metallic NPs.21 Because of the transport hindrance within pore channels, the former often results in uneven distribution of the metallic NPs. The latter requires the introduction of additional reducing agents. Although there is less diffusion resistance for the soluble metal salts to enter the pore channels, it still suffers from the leaching of the metallic NPs because of relatively weak interaction with the inert pore walls. An alternative synthetic pathway involves chemical modification of the porous supports to generate functional groups on the pore walls, which serve as stabilizing and capping agents for in-situ conversion of metal salts to NPs.22−24 For instance, Sehayek et al. modified nanoporous alumina membranes with (aminopropyl)trimethoxysilane to accommodate Au NPs via the coalescence of citrate-stabilized gold colloids, followed by template removal to produce porous, high-surface-area “nanoparticle nanotube” (NPNT).23,24 Dotzauer and co-workers proposed a different protocol to immobilize Au NPs in porous alumina and polymeric membranes via layer-by-layer adsorption of polyelectrolytes within the pore channels.20 Basically, effective embedding of metallic NPs into porous membranes with good dispersion relies on the use of specialized modification technique, which is dependent on the surface chemistry of the supports and properties of the metal salts. Since the pioneering report in Messersmith’s group,25 the mussel-inspired polydopamine (PDA) chemistry has opened a new avenue for surface engineering of inorganic/organic materials and biomolecules.26−29 By manipulating the selfpolymerization of dopamine in weak alkaline solutions, uniform PDA coating can be deposited to virtually any surface. Moreover, the large number of functional groups such as catechol hydroxyls and amines in PDA structure create a versatile platform for further surface functionalization.30−34 Though the formation mechanism and specific structure of PDA are still under discussion,35−37 an encouraging opportunity for surface modification of porous scaffolding materials has emerged.38,39 Taking advantage of PDA chemistry, Azzaroni’s group reported a facile method to modulate the inner surface of polymer nanopores for fabrication of nanofluidic diodes.40 They also showed the PDA platform created inside the pore walls could be further integrated with more complex molecules through different covalent chemistries and self-assembly processes. In addition, Choi and co-workers employed an interesting dipping-based surface modification with PDA inside polymeric three-dimensional inverse-opal

(IO) structure and demonstrated the application as a catalytic membrane via synthetic incorporation of Ag NPs.41 Here, we present, for the first time, an oxidant-regulated PDA surface chemistry for in-situ formation of metallic (Pd, Ag, or Au) NPs within the well-defined cylindrical microchannels of polymeric membranes prepared by track-etched technique.42,43 Nuclear track-etched membrane (NTEM) contains artificially made cylindrical pores with homogeneous distribution, which is an attractive polymeric membrane widely used for various purposes such as high-quality water production, sensor, cell culture, laboratory filtration.44,45 The precisely determined structure, in terms of pore size, shape, and density, imparts the membrane with tunable hydraulic permeability with required transport and retention characteristics. In this study, with the aid of oxidant additive, the self-polymerization of dopamine occurred in a more controllable manner,46 which guaranteed an ideal modification of the cylindrical pore channels spanning across the thickness of the membrane. Moreover, the optimized PDA coating offered simultaneous advantages of enhanced hydrophilicity to promote mass transport inside the pore channels and effective creation of nucleation sites for formation and stabilization of metallic NPs. In-situ reductive growth of Pd, Ag, and Au NPs inside the confined microchannels was then realized, and we demonstrated that the obtained composite membrane could be used as a flexible catalytic reactor for efficient reduction of 4-nitrophenol (4-NP) in a flow-through mode.47,48 Besides, after dissolution removal of the NTEM, self-sustained one-dimensional (1D) PDA/M (M = Pd, Ag, or Au) hybrid nanotubes (NTs) with large aspect ratio were obtained, which could be also used as efficient catalysts for the 4-NP reduction in a batch reaction mode.

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium periodate (99%), dopamine hydrochloride (DA, 98%), ethanol (99.9%, ACS/HPLC certified), and methanol (99.9%, ACS/HPLC certified) were supplied by J&K Scientific Co., Ltd. Tris(hydroxymethyl)aminomethane (Tris) (>99%), potassium tetrachloropalladate(II) (98%), 4-nitrophenol solution (5000 μg/mL in methanol), and sodium borohydride (99%) were purchased from Sigma-Aldrich. Sodium hydroxide (98%) was bought from Beijing Chemical Works. Silver nitrate (99.8%) was purchased from Shantou Xilong Chemical Factory Co., Ltd. Sodium acetate anhydrous (99%) was obtained from Tianjin Zhiyuan Chemical Reagent CO., Ltd. Deionized water was used for the preparation of all aqueous solutions. All reagents were used as received without further purification. Polycarbonate (PC) membranes were purchased from Bayer Company. 2.2. Preparation of NTEMs. Nuclear track-etched membranes (NTEMs) were prepared according to our previously reported method.49 First, PC membranes were irradiated by 84Kr ions on heavy ion accelerator at normal incidence. After exposure to ultraviolet (UV) light for 1 h on each side, the sensitized membranes were immersed in 6.5 mol/L NaOH aqueous solution at 50 °C for 10 min. After chemical etching, the obtained membranes were washed by using deionized water for three times. The final NTEMs were dried at room temperature under vacuum for 24 h. 2.3. Modification of NTEMs via PDA Deposition. PDA deposition to modify the NTEMs was performed in traditional Tris buffer solution (pH 8.5) and in sodium acetate buffer solution (pH 5.0) for a comparative study. The former was carried out according to previously established procedure.50 The latter was briefly described as follows: 200 mL sodium acetate buffer solution was prepared in advance by dissolving anhydrous sodium acetate (4.1 g, 50 mmol) in deionized water, followed by the addition of 0.2 mol/L acetic acid to adjust the pH value to be 5.0. A piece of NTEM (∼5 cm2) was then submerged in the sodium acetate buffer solution under magnetic 14736


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Scheme 1. Illustration of PDA-Mediated Preparation of Catalytic Membrane Reactor and 1D Hybrid PDA/M (M = Pd, Ag, or Au) NPs Catalysts

heating rate of 10 °C/min. The surface chemistry and composition analysis of the samples were examined with a model 250XI X-ray photoelectron spectroscopy (XPS) spectrometer equipped with a mono Al Kα X-ray source (1361 eV). Elemental analysis of C, H, and N was performed on an Elementar Vario EL III instrument.

stirring. Dopamine hydrochloride (0.40 g, 2.0 mmol) and sodium periodate (0.86 g, 20 mmol) were added to the buffer solution. The reaction lasted for 2 h under stirring (300 rpm) at room temperature. The modified membrane was alternately washed with ethanol and deionized water and was dried in a vacuum oven (80 °C). 2.4. In-Situ Growth of Metallic NPs. Metallic nanoparticles (NPs) including Pd, Ag, and Au were immobilized to the pore channels of the PDA modified NTEMs via in-situ reduction of the corresponding metal ions by the catechol units in PDA structure. Taking the immobilization of Pd NPs as an example, a dry PDA modified NTEM was first fixed in a filtration equipment. 5 mL of K2PdCl4 (10 mmol) was introduced at room temperature and spontaneously passed through the NTEM at a flow rate of ∼0.1 mL/ min. Deionized water was then used for washing the products for three times. Finally, the products were dried at room temperature under vacuum for 24 h. To obtain the one-dimensional (1D) PDA/M (M = Pd, Ag, or Au) hybrid nanotubes (NTs), dichloromethane was used to dissolve the membrane. The hybrid nanotubes were separated by centrifugation, followed by washing with ethanol repeatedly. 2.5. Catalytic Reduction Performance. The catalytic performance to reduce 4-nitrophenol (4-NP) was demonstrated in a flowthrough mode using the Pd NPs immobilized NTEM and also in a batch mode using the self-sustained 1D PDA/Pd hybrid NTs. In the former case, the functionalized membrane was used as a catalytic reactor fixed in a self-made filtration device. A 10 mL reaction solution containing the mixture of 4-NP (0.5 mmol) and NaBH4 (0.3 mol) was introduced to the device at room temperature. The reaction solution passed through the catalytic membrane assisted by a vacuum pump (vacuum degree ∼0.09 MPa) at a flow rate of ∼0.2 mL/min. In a batch catalytic reaction, 20 mg of PDA/Pd hybrid NTs was mixed with 10 mL of 4-NP aqueous solution (0.5 mmol), followed by the addition of 0.3 mol of NaBH4. The catalytic reaction was also performed at room temperature under magnetic stirring. UV−vis spectrometer was employed to record the spectra of the reagent solution and the catalyzed product. 2.6. Characterizations. Scanning electron microscopy (SEM) was conducted by using a Merlin scanning electron microscope. The samples were disposed with metal spraying before observation and were then observed at an accelerating voltage of 30 kV in vacuum conditions. Transmission electron microscopy (TEM) images were recorded by using a model H-7700 microscope with an accelerating voltage of 120 kV. UV−vis spectra between 200 and 800 nm were recorded at room temperature with a Cary6000i spectrometer using a 1 cm path length quartz cuvette. Thermogravimetric analysis (TGA) was carried out by using a TA Instruments SDT Q600 instrument. The temperature ranged from room temperature to 1000 °C at a

3. RESULTS AND DISCUSSION This study aims to develop a versatile approach by taking advantage of the mussel-inspired PDA chemistry to effectively modify the inner surfaces of nuclear track-etched membranes (NTEMs) for in-situ nucleation and growth of metallic nanoparticles (NPs) in their cylindrical pore channels. The experimental protocol is illustrated in Scheme 1. First, a piece of NTEM with well-defined cylindrical pores is prepared by irradiating polycarbonate (PC) membrane with a projectile of heavy ions followed by chemical etching. PDA is then deposited, via the oxidant-controlled polymerization of dopamine, to the inner pore walls whereby reducing the metal salts of Pd, Ag, and Au for in-situ immobilization of the corresponding metallic NPs. The functionalized NTEM embedded with metallic NPs can serve as a membrane reactor for catalysis reaction in a flow-through mode. Besides, selfsupporting 1D PDA/M (M = Pd, Ag, or Au) hybrid nanotubes with large aspect ratio can be also obtained as heterogeneous catalysts via dissolution removal of the porous membrane. Porous membranes prepared by track-etched technique contain unique cylindrical pores with equal diameter and uniform density. The preparation procedure usually involves three steps, namely, heavy ion irradiation, ultraviolet (UV) treatment, and chemical etching. During the etching process, key factors including etching time, temperature, and concentration of the etching solution are adjusted to balance the pore geometry and membrane strength. In this study, after 84Kr ions bombardment, the polycarbonate membrane, with a thickness of 10 μm, was exposed to 6.4 mol/L NaOH aqueous solution for 10 min at 50 °C. The surface morphology and section view of the pore channels of the obtained NTEM are shown in Figure 1. The pore density of NTEM is ∼1 × 108/cm2 with a diameter of ∼120 nm for each cylindrical pore. Effective surface modification of the inner pore channels in NTEMs plays a key role for in-situ immobilization of metallic NPs. As mentioned above, the bio-inspired PDA chemistry 14737


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that the self-polymerization of dopamine occurs quickly under basic conditions, leading to poor control over the deposition process.51,52 Therefore, in pH 8.5 Tris buffer solution, the rapid formation of large-sized PDA nanoparticles and especially the aggregates impeded their embedding to the pore channels of NTEMs. With regard to the modification of porous scaffolds with micro/nano-confined spaces, it requires a more rational implementation of the PDA deposition with controllable nucleation rate and appropriate particle size. Structural investigation has suggested that PDA is a complex and heterogeneous ensemble of oligomers consisting of uncyclized dopamine units, cyclized 5,6-dihydroxyindole (DHI) units, and other moieties, which are assembled through interactions including cross-linking, hydrogen bonding, and π−π stacking.53,54 It has been accepted that the polymerization of dopamine starts from autoxidation and cyclization, followed by intra-/intermolecular interactions to form nanoparticles or aggregates.55 Recently, great efforts have been made to locate effective methods, such as tuning the pH values and buffer salts,51,52,56 introducing oxidants and other additives,57,58 UV irradiation,59 atmospheric plasma treatment,60 etc.,61 for a better control over the polymerization process of dopamine among which, performing the reaction under slightly acidic condition enables the inhabitation of uncontrolled autoxidation process of dopamine. The introduction of appropriate oxidants can regulate the nucleation and growth process of PDA nanoparticles, resulting in a controlled deposition process with improved homogeneity.46 So, we attempted to perform an oxidant-controlled PDA deposition with the addition of sodium periodate in acidic solution (pH 5.0) to modify the inner pore channels of NTEM. The surface morphology shows a homogeneous coverage on the NTEM without blocking the pores (Figure 2d), and evidently smaller PDA NPs (∼20 nm) with uniform distribution were obtained (Figure 2e). Such small-sized PDA NPs find better opportunity to enter the cylindrical pores of NTEM. The SEM section view shows that a large number of PDA NPs were deposited to the pore channels (Figure 2f). Therefore, the addition of sodium periodate greatly improved the PDA deposition to inner surfaces of the porous NTEMs. The immobilized PDA NPs in the confined channels would provide a reactive environment for further functionalization. An additional benefit of PDA deposition on NTEMs was the enhancement of their hydrophilicity. According to water contact angle (WCA) measurements (Figure 3), pristine NTEM was not hydrophilic with a WCA of 91.1°. After PDA modification in pH 8.5 Tris buffer solution, the abundant catechol hydroxyl and amine groups in PDA endowed the membrane with hydrophilicity (WCA = 61.6°). In comparison, by introducing sodium periodate to control the PDA deposition process, a much smaller WCA of 39.3° was obtained for the

Figure 1. Surface morphologies (a, b) of NTEMs and section views of the pore channels (c, d). The light-color dots in panel b are Au NPs sprayed on the membranes in sample preparation process.

shows great promise for surface modification because of its intriguing properties such as nonsurface specific adhesion and postfunctionalization accessibility. But so far, most of the reported researches focused on the surface engineering of external surfaces of nanoparticles or bulk materials. In the present study, first, according to a classic recipe, we performed the modification of NTEMs in pH 8.5 Tris buffer solution exposed to air. It was found that dopamine polymerized rapidly in the basic solution (judged by the color change of the reaction solution), and the resultant PDA nanoparticles had a strong tendency to form aggregates, which were deposited to the surface of the NTEM. Figure 2a shows a full coverage of the

Figure 2. SEM images of PDA deposited NTEMs in pH 8.5 Tris buffer solution (a−c) and in pH 5.0 sodium acetate buffer solution with the addition of sodium periodate (d−f).

membrane surface by the PDA nanoparticles and aggregates which led to complete blocking of the cylindrical pores. The PDA nanoparticles obtained under this conditions had nonuniform diameters ranging from 50 to 110 nm (Figure 2b). Besides, the SEM profile (Figure 2c) of the pore array in the membrane shows that a limited number of PDA nanoparticles were introduced to the cylindrical channels. Previous researches on traditional PDA chemistry have revealed

Figure 3. Water contact angle measurements on a pristine tracketched membrane (a) and the PDA modified counterparts in pH 8.5 Tris buffer solution (b) and in pH 5.0 sodium acetate buffer solution with the addition of sodium periodate (c). 14738


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Figure 4. (a) SEM profile of Pd NPs immobilized NTEM, (b, c) SEM images of the self-supporting 1D PDA/Pd nanotubes after membrane dissolution, (d) TEM image of 1D PDA/Pd nanotubes, and (e) SEM image and the corresponding EDX elemental mapping of 1D PDA/Pd nanotubes.

modified membrane. This may be attributed to the degradation of quinone units in PDA by the strong oxidant to yield carboxyl groups.46 The enhancement of hydrophilicity would promote the mass transport, resulting in better performance of the porous membranes. Then, the PDA coatings on the pore walls of NTEM served as a reactive platform for in-situ immobilization of metallic NPs to fabricate membrane reactor. As discussed above, to embed metal catalysts to porous membranes, in-situ nucleation to form the metallic NPs is a better way than direct loading of grown NPs due to the transport resistance in confined pore channels. Previous studies have proved that the catechol units in PDA are able to release electrons when oxidized into quinone groups, which can trigger the reduction of a series of metal ions to form the corresponding NPs.30,41 Here, by simply passing K2PdCl4 aqueous solution through the modified NTEMs by filtration, in-situ reduction of the palladium ions occurred, forming welldispersed Pd NPs (diameter ∼30 nm) which were stabilized in the PDA modified pore channels (Figure 4a). Supportive evidence of the successful immobilization of Pd NPs was also provided by energy dispersive X-ray (EDX) analysis. Clear Pd signal was found in the EDX spectrum (Figure S1). It should be pointed out that the synthetic protocol also resulted in the generation of PDA and Pd NPs on the surfaces of the NTEMs, which would enhance the hydrophilicity and permeability, and promote the catalytic ability of the functionalized membranes. In addition, to demonstrate the effective modification of the pore channels of NTEMs for accommodating Pd NPs, we dissolved the membranes in dichloromethane. As expected, selfsustained 1D tubular PDA/Pd NPs hybrid nanostructures were obtained. The PDA/Pd hybrid nanotubes have uniform size and large aspect ratio (Figure 4b). The length reaches up to 10 μm, suggesting the good development of the hybrid nanotubes which spanned the complete thickness of the supported membrane. Figure 4c shows the open cavities of the hybrid nanotubes, confirming that the pore arrays of NTEM were not blocked during PDA deposition with the addition of the oxidant. The hollow structure of the PDA/Pd hybrid nanotubes was recorded by TEM (Figure 4d), showing that a great number of Pd NPs are homogeneously dispersed in the PDA

nanotubes. This is further proved by the EDX mapping of the corresponding elements (Figure 4e). X-ray photoelectron spectroscopy (XPS) was used to examine the surface chemistry of NTEM and the modified counterparts. According to the wide scan XPS spectra in Figure 5, the pristine NTEM shows C 1s and O 1s signals (black line).

Figure 5. Wide scan XPS spectra of NTEM (black), PDA modified NTEM (red), and the Pd immobilized counterpart (blue).

After PDA deposition, evident N 1s peak shows up at around 400 eV (red line), which is associated with the amine species in PDA structure. With the immobilization of Pd NPs, a strong Pd 3d signal is observed on the spectrum of the functionalized membrane (blue line). It is noted that the Pd 3d signal contains two spin−orbital doublets, corresponding to the two electronic states of palladium, i.e., Pd(0) and Pd(II).62 Overall, the XPS evidence indicates the successful PDA deposition and subsequent immobilization of Pd NPs to the NTEM. Thermogravimetric analysis (TGA) was used to study the thermal stability of the functionalized NTEMs and examine their composition. Figure 6 shows the TGA curves of the functionalized membranes as well as the derivative thermogravimetric analysis (DTG) plots. The pristine NTEM exhibits a typical thermal degradation behavior of polycarbonate (PC) with a decomposition temperature (Td) of ∼510 °C. Increased weight loss is observed for the PDA modified intermediate, which shows a slight shift of Td to relatively lower temperature (∼506 °C) because of the earlier decomposition of PDA than 14739


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Figure 6. TGA curves (a) and DTG plots (b) of NTEM (black), PDA modified NTEM (red), and the Pd immobilized counterpart (blue).

PC.63 By comparing the weight loss until 600 °C, it can be estimated that about 16.5 wt % of PDA was deposited to the porous membrane. With the in-situ immobilization of Pd NPs, the functionalized NTEM shows less weight loss compared to the PDA modified intermediate. The loading of the Pd NPs is estimated to be ∼6.3 wt %, and the Td of the functionalized NTEM shifts to 490 °C. The versatility of the PDA platform integrated with porous NTEM was further proved by effective immobilization of other metallic NPs. Following the established procedure, welldispersed Ag and Au NPs were successfully embedded into the pore arrays of NTEM (Figure 7a,d). The functionalization

Figure 8. (a) XPS spectra of NTEM (black), PDA modified NTEM (red), and the Ag immobilized counterpart (blue). (b) XPS spectra of NTEM (black), PDA modified NTEM (red), and the Au immobilized counterpart (blue).

controlling the concentration of the feed solutions as well as their flow rate (contact time) during the synthesis.20,23,41 To evaluate the capability of the functionalized NTEMs as catalytic reactors, the Pd NPs immobilized NTEMs were employed for catalysis of a model reaction for reduction of 4nitrophenol (4-NP) to 4-aminophenol (4-AP).64−67 Figure 9a shows the digital photos of pristine NTEM (left), PDA deposited NTEM (middle), and the functionalized counterpart after Pd NPs immobilization (right). The membrane has a functional area (dark color area) of 3.5 cm2, which can be easily adjusted by varying the size of the filtration device employed in the fabrication process. The catalytic reduction of 4-NP to 4-AP starts with the conversion of 4-NP to 4-nitrophenolate as an intermediate upon contact with NaBH4 (Figure 9b), which shows a maximum absorption at around 400 nm in the UV−vis spectra. The further conversion of the intermediate to 4-AP will result in an evident change in absorption spectra, allowing to monitor the reaction process by UV−vis spectroscopy.62 The catalytic reaction in a flow-through mode was recorded using a digital camera (Figure 9c). The Pd NPs immobilized NTEM was settled on a sintered filter with the unmodified side exposed upward to the reaction solution. The absorption spectrum of the yellow-colored reaction solution is shown in Figure 9d (black line). Control experiments showed that the native NTEM and the PDA deposited intermediate could not effectively catalyze the reduction of 4-NP (Figure S2). For the Pd NPs immobilized NTEM, under a flow rate of 0.2 mL/min, colorless filtrate was obtained in the container below. The absorption signal showed a great decrease at around 400 nm and a new peak emerged at 295 nm (Figure 9d, red line), suggesting an efficient catalytic reduction of 4-NP. It is worth mentioning that the apparent catalytic performance of membrane reactor reflected the synergetic contribution of the metal NPs embedded to the pore arrays and those deposited on the surface of the NTEMs.

Figure 7. (a) SEM profile of Ag NPs immobilized NTEM, (b) SEM images of the self-supporting 1D PDA/Ag nanotubes after membrane dissolution, (c) TEM image of 1D PDA/Ag nanotubes, (d) SEM profile of Au NPs immobilized NTEM, (e) SEM images of the selfsupporting 1D PDA/Au nanotubes, and (f) TEM image of 1D PDA/ Au nanotubes.

processes were also evidenced by XPS surveys, showing nitrogen species after PDA deposition and distinct Ag 3d and Au 4f signals with the immobilization of corresponding metallic NPs (Figure 8). After removal of the membrane, PDA supported 1D hybrid nanotubes with large aspect ratio were obtained (Figure 7b,e). The TEM images in Figure 7c,f confirm the hollow structures of the hybrid nanotubes. Interestingly, the Ag NPs generated in the hollow PDA nanotubes show a relatively smaller diameter (∼40 nm) and more homogeneous distribution than the Au NPs (∼100 nm). This can be basically attributed to the different reductive growth behaviors of the corresponding metal ions within the PDA modified pore channels. Besides, as a routine procedure, the particle size and the distribution of the metallic NPs can be also tuned by 14740


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Figure 9. (a) Digital photos of NTEM (left), PDA deposited NTEM (middle), and the functionalized counterpart after Pd NPs immobilization (right). (b) Reaction mechanism of Pd NPs catalyzed reduction of 4-NP to 4-AP in the presence of NaBH4. (c) Digital photo of setup for 4-NP reduction using Pd NPs functionalized NTEM as a catalytic membrane reactor in a flow-through mode. (d) UV−vis absorption spectra of reaction mixture of 4-NP and NaBH4 (black line) and the filtrate containing reduced product (red line).

membrane reactor could be maintained >80% after five cycles. Moreover, after dissolution removal of the NTEMs, selfsustained 1D hybrid nanotubes (aspect ratio ∼83) with the metallic NPs homogeneously dispersed in the PDA matrix were obtained, which also exhibited efficient catalytic ability in batch reaction mode. To conclude, the non-surface-specific PDA chemistry provides an enabling tool for engineering the inner surfaces of porous membranes to develop new generation of catalytic membrane reactors. The methodology established in this work would expand the opportunity for template synthesis of 1D tubular nanostructures with defined aspect ratios and cavities for broadened applications.

The reusability of the Pd NPs immobilized NTEM for catalysis was evaluated. The results in Figure S3 show a slight decrease in the catalytic efficiency after the first run, which may be attributed to the leaching of the metal NPs. Dynamic light scattering (DLS) measurement shows an average size of 380 nm with unimodal distribution for the solid particles in the filtrate (Figure S4). Since the particle size is much larger than the diameter of the cylindrical pores of NTEM, the particles should be the PDA aggregates exfoliated from the surface of the membrane. Concurrently, it led to the partial loss of the Pd NPs from the PDA deposited surface, which was responsible for the decrease of catalytic efficiency after the first run. However, in the following cycle test, the catalytic efficiency of the membrane reactor remained constant (>80%), and no detectable solid particles were found in the filtrates according to DLS measurements. This proves the structural stability of Pd NPs immobilized by PDA within the pore channels of the membranes. Besides, previous researches have revealed that the reducing agent NaBH4 can possibly reduce the o-quinone units in the PDA structure. And, this would regenerate catechol groups, which also work for the reduction of 4-NP and contribute to stabilizing metallic NPs within the pore arrays.41,68 On the other hand, the catalytic ability of the selfsupporting 1D PDA/Pd hybrid nanotubes was examined in a traditional batch experiment as illustrated in Scheme 1. Because of the large specific surface area and good dispersion in the reaction mixture, the hybrid nanotubes also exhibited favorable catalytic performance for the reduction of 4-NP (Figure S5).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02231. EDX spectra, UV−vis spectra, and reusability evaluation (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(G.Y.) E-mail: [email protected]. *(J.C.) E-mail: [email protected]. ORCID

Gang Ye: 0000-0002-7066-940X Yuexiang Lu: 0000-0003-2755-7733 Notes

4. CONCLUSION In this work, by rational control of the mussel-inspired polydopamine (PDA) chemistry, we developed a facile and adaptable approach to engineer the well-defined pore channels of nuclear track-etched membranes (NTEMs), which provided an ideal platform for immobilization and stabilization of metallic nanoparticles (NPs) for fabricating catalytic reactors. First, studies on the polymerization behavior of dopamine revealed that an oxidant-regulated polymerization reaction resulted in controllable nucleation rate of PDA with appropriate particle size (∼20 nm), facilitating the modification of cylindrical pores of NTEMs. On this basis, in-situ reductive growth of Pd, Ag, and Au NPs inside the pore channels was realized. The functionalized membrane could serve as a catalytic reactor for effective reduction of 4-nitrophenol (4NP) in a flow-through mode with the flux of ∼0.2 mL/min. The reusability test suggested that the catalysis efficiency of the

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study was supported by the Changjiang Scholars and Innovative Research Team in University (IRT13026), the National Science Fund for Distinguished Young Scholars (51425403), and the National Natural Science Foundation of China under Projects 51673109, 51473087, and U1430234.

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REFERENCES

(1) Xu, Y.; Zhang, B. Recent Advances in Porous Pt-Based Nanostructures: Synthesis and Electrochemical Applications. Chem. Soc. Rev. 2014, 43, 2439−2450. (2) Kidambi, S.; Bruening, M. L. Multilayered Polyelectrolyte Films Containing Palladium Nanoparticles: Synthesis, Characterization, and Application in Selective Hydrogenation. Chem. Mater. 2005, 17, 301− 307.

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