Hng Bin,Xuejin Zhng,Hongki Zho,Ning Zhng*

a School of Material Science and Engineering, Jilin Jianzhu University, Changchun 130118, China

b Department of Chemistry, Northeast Normal University, Changchun 130024, China

Keywords:

Polymer brushes

Polyoxometalate

Ag nanoparticles

Grafting polymerization

ABSTRACT

This contribution reports the immobilization of polyoxometalate (POM) into poly(4-vinyl pyridine)(P4VP)brushes and the controlled reduction of silver ions,in-situ generating metal nanoparticles in the brushes.P4VP brushes were straightforwardly created by UV-assisted photopolymerization of 4-vinyl pyridine(4 VP)on silicon or glass substrates.Phosphotungstic acid(H3PW12O40),one of the most widely used Keggin-type POM was anchored onto these pyridine moieties through electrostatic interaction,leading to the P4VP/POM hybrid brushes.The immobilized POM was further reduced to heteropolyblue,which could be used to generate silver nanoparticles in a controlled fashion.AFM, UV-vis and IR characterization indicate that P4VP brushes not only provide an efficient platform in the controlled preparation of Ag nanoparticles, but also efficiently disperse and stabilize POM, thus preventing the aggregation of the generated Ag nanoparticles.

Metal particles with nanometer size played a key role in a broad range of application spanning sensors [1,2], optoelectronics [3,4],medicine[5],and catalysis[6,7].In the past decades,great efforts have been devoted to fabricating nanoparticle assemblies to fulfill their advanced applications as well as a central concept in nanoscience and technology [8-10].Among all the noble metal nanoparticles, silver (Ag) nanoparticles have gained significant attention [11,12]owing to their superiority in diverse area, e.g.,photosensitive components [13], catalysts [14,15], and surface enhanced Raman spectroscopy [16].However, the high surface energy makes Ag nanoparticles tend to agglomerate,which limits their applications as outlined above.Forming polymer-nanoparticle composites is an efficient strategy in ensuring a well-defined spatial distribution of nanoparticles.The immobilization of the nanoparticle in a series of carriers such as polymers [17,18], latex particles [19], microgels [20], also achieved great success in the preparation of defined Ag nanoparticles.

As an assembly of semi-fixed polymer chains,polymer brushes in extended conformations have exhibited surface phenomena that are different from polymers deposited onto a substrate from solution,such as wetting, phase segregation, adsorption, lubrication, and diffusioncontrol[21-28].Moreover,polymerbrushes,withcontrolled conformation and grafting density, are promising candidates in generating environment with confined volume at surfaces due to the cooperative movement of stretched polymer chains[29].In a recent work,we employed polymer brushes as a confined environment to induce the metal-organicframeworks(MOFs)growth,thus the sizeof the surface-attached MOFs can be tailored from nanometer to micrometer range simply by the variation of structural parameter of polymer brushesand thenucleationtime[30].Thus,polymer brushes represent a novel and efficient confined environment, which are promising in the synthesis of other nanomaterials.

Polyoxometalates (POMs), a class of well-defined early transition metal-oxygen clusters [31], have attracted considerable attention in the past due to their unique properties such as conductivity [32], magnetism [33]and catalysis [34,35].Another intriguing property is that POMs can be photochemically reduced to blue POMs which is can be act as efficient reducing agents for some metal ions [36,37].However, the potential use of POMs usually limited by their poor process ability due to the poor solubility and the aggregation problem.

In the present work,we utilized polymer brushes as a confined environment in the controlled synthesis of Ag nanoparticles.POM was firstly dispersed and incorporated in the polymer brushes through static interaction.After the photoreduction of POM through UV light irradiation, the formed POM blue triggered the confined and controlled synthesis of Ag nanoparticles among the polymer brushes.

Scheme 1.Schematic route for the preparation of poly(4-vinylpyridine)brushes on a silicon substrate.

Fig.1.IR spectra for P4VP brushes and PW12 loaded P4VP brushes.

Poly(4-vinylpyridine) (P4VP) was chosen as supports for the immobilization of POM.The preparation of P4VP brushes is outlined in Scheme 1.P4VP brushes were prepared by self-initiated photografting and photopolymerization (SIPGP) from a selfassembled monolayers (SAMs) of α,ω-aminopropyltrimethoxysilane(APTMS)under UV-light irradiation(λmax=350 nm).After the polymerization,the substrate was cleaned by ultrasound in several solvents with different polarities to ensure that only chemically grafted polymer remains on the substrate.The formation of P4VP brushes is evident that the strong bands at 1593 cm-1and 1407 cm-1are originated from the C=N and C=C stretching modes in P4VP(Fig.1).AFM measurements show that the average height of P4VP brushes is 250 nm after 40 h of UV irradiation,and the P4VP grafted substrate appears to be homogeneous and smooth(Fig.2A).

Fig.2.AFM scan and corresponding section view on the indicated line for P4VP brushes(A),PW12 incorporated P4VP brushes(B)and Ag nanoparticle loaded P4VP brushes (C).

For the successive immobilization, we took the most common POM, [PW12O40]3-(PW12), as an example.DMF was used as a solvent because it has a high solubility for both P4VP and PW12.After the reaction with H3PW12O40, the thickness of P4VP as determined by AFM increases to 324 nm which is due to the adsorption of PW12anions on the protonated pyridine groups along polymer chains by electrostatic interaction [38,39].Fig.2 B presents the surface morphology and section view for AFM image of the PW12-immobilized P4VP brushes,which shows a significant roughness increase.The successful immobilization of PW12in the P4VP layer can be further confirmed by the IR characterization.After the PW12incorporation, new bands at 1077 cm-1and 968 cm-1which correspond to the typical P-O and W=O stretching modes of PW12,appear in the IR spectrum(Fig.1).Moreover,C=N and C=C stretching modes from the P4VP all shifted to high wavenumbers upon the electrostatic interaction with PW12[40].The schematic preparation route for the immobilization and reduction of PW12, and the successive generation of Ag nanoparticles on the polymer brushes are in shown Scheme 2.

It is known that POM such as Keggin ions undergo stepwise multielectron redox processes without structural change and may be reduced photochemically with suitable reducing agents[41].To a aqueous solution containing propan-2-ol(10 wt%)was added the PW12loaded P4VP brushes on a silicon substrate,and the mixture was irradiated by UV light for 4 h.This leaded to the reduction of(PW12O40)3-ions, which could be observed as a blue color appearing on the surface.This indicated the formation of oneelectron-reduced PW12, [PW12O40]4-[41].To this solution, was added AgNO3aqueous solution under continuous shaking.The substrate surface gradually displayed a yellow color.The reaction with AgNO3leaded to the formation of Ag nanoparticles.

Scheme 2.Schematic route for the loading of PW12, reduction of the PW12 and the successive in situ formation of silver nanoparticles among the polymer brushes.

Fig.3.UV-vis absorption spectra of P4VP brushes,PW12 loaded P4VP brushes and Ag incorporated P4VP brushes.

Fig.4.EDX spectra of P4VP brushes(top),PW12 loaded P4VP brushes(middle)and Ag incorporated P4VP brushes (bottom).

After the formation of Ag nanoparticles, the layer thickness further increased to 350 nm (Fig.2C).AFM measurement also indicates a nearly homogenous distribution of immobilized Ag nanoparticles with an average diameter of 21 nm on the P4VP brushes (Fig.2).

UV-vis absorption spectroscopy was used to monitor the process from the loading of PW12and the formation of Ag nanoparticles.As shown in Fig.3, the strong absorption bands at 218 nm and 259 nm are typical for P4VP.After the immobilization of POMs,a single band centered at 275 nm appeared,indicating the successful introduction of PW12into the polymer layer.UV irradiation caused the reduction of PW12, giving blue PW12.The formation of Ag nanoparticles revealed an absorption band at 429 nm, characteristic for the surface plasmon absorption of the immobilized Ag nanoparticles.

To further prove the successful incorporation of PW12and presence of Ag nanoparticles on the P4VP brushes, energy dispersive X-ray (EDX) analysis of the samples were investigated.As shown in Fig.4, the appearance of P and W signals and the successive occurrence of Ag signals at the characteristic binding energies in the wide-scan spectra confirm the presence PW12and Ag nanoparticles on P4VP brushes.

In conclusion, we have successfully incorporated POM in the P4VP brushes through static interaction.After the photoreduction to blue POM, the brush-POM hybrid showed excellent photoreductable capabilities to reduce Ag+ions to Ag nanoparticles.In the present work, polymer brushes ensure a well dispersion of POM in the brush layer, thus enabling the controlled synthesis of Ag nanoparticles.

Acknowledgment

The financial support of this work by Department of Science and Technology of Jilin Province (Nos.20180101196JC and 20180101170JC) is greatly acknowledged.

Appendix A.Supplementary data

Supplementary material related to this article can be found,in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.01.030.