Redox-active poly(ionic liquid)-engineered Ag nanoparticle-decorated ZnO nanoflower heterostructure: A reusable composite catalyst for photopolymerization into high-molecular-weight polymers
Graphical abstract
Introduction
Metal/metal oxide nanostructures have been extensively studied during the last two decades because of their wide range of applications in optoelectronics, biosensors, chemical sensors, water desalination, and hazardous waste remediation as well as majorly in catalysis/photocatalysis [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. TiO2 nanostructures has been the most efficient and well-studied photocatalyst compared to other metal oxide nanostructures [14], [15], [16], [17]. However, ZnO nanoparticles (NPs) are suitable alternative to TiO2 NPs due to their similar band gap energy (3.37 eV), a much higher electron mobility than exhibited by TiO2 and its lower cost and attracted extensive attention towards photocatalytic degradation of organic pollutants in water and air [4], [5], [10], [18], [19], [20], [21]. Beside these metal oxide photocatalysts, the photocatalysts, composed of noble metal (Au, Ag and Pt) NPs and semiconductor NPs (e.g., ZnO or TiO2) (deposited on the surface of semiconductor NPs) have recently been utilized as very promising candidates with much higher activities for many organic reactions including degradation of organic pollutants [22], [23], [24], [25], [26], [27], [28]. It is known that the metal-semiconductor interface efficiently separates the photogenerated charges and metal nanoparticles (MNPs) act as sensitizers for harvesting higher wavelength light due to their surface plasmon resonance (SPR) [26], [28], [29], [30]. It is also reported that the photocatalytic activities under both ultraviolet and visible light irradiation drastically enhance when the surface of ZnO/TiO2 NPs is decorated with MNPs, such as Au, Ag, Pt NPs etc. [22], [23], [29], [31], [32], [33] Thus the low photocatalytic efficiency of ZnO/TiO2 NPs due to recombination of electrons with holes can easily be overcome by making heteronanostructures through physical blending of these metal oxide NPs with MNPs [24], [33], [34]. Therefore, it would be interesting to check whether such type of semiconductor-metal heteronanostructure can be utilized as efficient composite photocatalyst for polymerization of vinyl monomers without any photoinitiator, which has not been explored much recently.
Regarding the synthesis of metal oxide-metal nanoparticles heteronanostructures, there exists several different wet chemical methods to prepare different-shaped structures such as AgNPs coated ZnO nanorods [35], radical-shaped Ag/ZnO microstructures [36], dendritic ZnO@Ag NPs [37] etc. It should be noted that these heteronanostructures are mostly prepared by two-step wet-chemical protocol. The first step is the preparation ZnO NPs of different shapes. The key to prepare the different-shaped ZnO NPs is the use of different capping agents including small molecules citrate salts [38], [39], citric acid [40], water-soluble diblock copolymers [41], and surfactants, [42], [43]. The MNPs are prepared separately and blended with different-shaped ZnO nanostructures to prepare such heteronanostructure. In this context, we have reported the synthesis of flower-shaped ZnO nanostructures hydrothermally using ascorbate-based salt [12]. Additionally, we have also shown that ascorbate ion is redox-active and can be able to generate AuNPs from gold salt [44]. Thus, there is possibility that ascorbate ion can be used to make ZnO@Ag heteronanostruture, but the anchoring of AgNPs with ZnO wouldn't be that good because of small molecular ascorbate binder. Furthermore, it is known that the polymer can act as better anchoring materials for stabilizing MNPs than small molecules [45]. Additionally, it is also known that the imidazolium group have affinity towards AgNPs [12], [44]. Therefore, the designing of a poly(ionic liquid) (PIL) containing cationic imidazolium and anionic ascorbate groups would beneficial for preparing shaped composite ZnO@Ag heteronanostructure as well as neat ZnO NPs. There are many reports including ours on the use of ascorbic acid or ascorbate ion-based ionic liquids for synthesis of MNPs [12], [44], [46], [47], [48]. But, there is no such report where a water soluble ascorbate-based PIL is used for the synthesis of ZnO or ZnO@Ag nanostructures.
As mentioned above, the metal oxide-metal nanoparticle heteronanostructure photocatalyst have been mostly used for water purification through the degradation of organic pollutants/dyes present in it [22], [28]. Beside these applications, photopolymerization of vinyl monomers with these composite photocatalysts is also one of the important area, though it is not studied at all. Also there exists only very few reports where TiO2, ZnO or other semiconductor NPs are utilized alone as photocatalyst for initiation of vinyl monomers [16], [19], [29], [49], [50], [51]. But, the detail study of polymerization kinetics, mechanism and the effect of shape of these semiconductor NPs on polymerization are absent.
In this work, we report the synthesis of spherical AgNPs decorated flower-shaped ZnO (ZnO@Ag) heteronanostructures by a simple two-step wet-chemical approach using a newly design redox-active PIL, poly(1-vinyl-3-butylimidazolium ascorbate) (P[VBuIm][As]) as capping-cum-shape directing as well as reducing agent. First step is the synthesis of flower-shaped ZnO NPs by hydrothermal method from zinc salt using P[VBuIm][As]. In the second step, AgNPs are then deposited on the surface of ZnO by the reduction of by surface adsorb ascrobate ion of the PIL. The ZnO@Ag heteronanostructures is then utilized as efficient composite photocatalysts for polymerization of different vinyl monomers. It is worthy to mention that the ZnO NPs are extensively used in photocatalytic degradation of organic pollutants/dyes [4], [5], [10], [12], [18], [19], [20]. Furthermore, the heteronanostructures consisting of either AuNPs or AgNPs and metal oxide such as TiO2, ZnO, Fe2O3 etc., are also utilized as composite photocatalysts with higher activities in degradation of organic pollutants and in other organic reactions [22], [23], [25], [26], [52]. However, the use of neat ZnO NPs as photoinitiator for the free radical polymerization vinyl monomers is very limited [16], [19], [29], [51]. In particular, there is no such report of use of this photocatalyst (ZnO@Ag) as a photoinitiator for the bulk and solution polymerization of vinyl monomers. In this case, ZnO@Ag heteronanostructure is utilized as an efficient photoinitiator to successfully initiate the polymerization of different monomers such as methyl methacrylate (MMA) and styrene in bulk and in solution under the irradiation of UV light (λ = 350 nm). This composite photocatalyst also produces poly(N-isopropylacrylamide) (PNIPAM) in aqueous medium. The kinetics of MMA polymerization using ZnO@Ag photocatalyst shows that the conversion increases with time. But, as such there in no increase of molecular weight with conversion. The obtained PMMA molecular weights are quite high, ranging from 186000 to 194300. ZnO NPs also produces PMMA from MMA upon UV irradiation but suffers from poor conversion compared to that obtained using ZnO@Ag heteronanostructures. This interesting part of this catalytic polymerization is that these NPs can easily be isolated by centrifugation and can be reused without any significant loss of its virgin photocatalytic activities.
Section snippets
Materials
Zinc acetate dihydrate (ZnAc2.2H2O), silver nitrate (AgNO3), sodium hydroxide (NaOH) were purchased from Merck (India) and were used without further purification. Ascorbic acid (AA, Aldrich) was used without further purification. 1-Vinylimidazole (Aldrich) was purified by vacuum distillation prior to use. 2,2-Azobisisobutyronitrile (AIBN, Aldrich) was recrystallized from acetone before use. Methyl methacrylate (MMA) and styrene were purchased from Aldrich and were purified by passing through a
Synthesis of flower-shaped ZnO NPs and ZnO@Ag composite heteronanostructures
It is generally observed that to fabricate metal oxide-metal composite nanostructures with heterointerface, a surfactant or a polymeric stabilizer is usually necessary. In this case, we chose to use a newly designed PIL with ascorbate anion and imidazolium cation (Scheme 1) to generate hierarchical ZnO@Ag composite heteronanostructures. It is well known that the ascorbic acid can be used as reducing agent for generations of MNPs, mainly in an alkali medium [47], [54], [55]. We also prepared
Conclusions
In summary, the composite heterostructure consisting of ZnO nanoflower decorated with spherical AgNPs (ZnO@Ag) have been fabricated by a two-step chemical method. Flower-shaped ZnO NPs was first synthesized wet-chemically using a redox-active PIL, P[VBuIm][As] as the shape-directing-cum-capping agent, followed by growing of spherical AgNPs on its surface by the reduction of added AgNO3 with the adsorbed ascrobate ion in the second step. SEM and HRTEM analyses confirmed that the surfaces of ZnO
Acknowledgements
M.D. thanks CSIR, New Delhi for providing fellowship. Y.B. thanks IACS for providing fellowship. This research was supported by the grants from SERB (No. EMR/2016/002321), India.
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