Micelles template for the synthesis of hollow nickel phosphate nanospheres

Materials Letters 251 (2019) 34–36

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Micelles template for the synthesis of hollow nickel phosphate nanospheres Bishnu Prasad Bastakoti ⇑, Samira Munkaila, Sudhina Guragain Department of Chemistry, North Carolina A&T State University, New Science Building, 1601 East Market Street, Greensboro, NC 27411, United States

a r t i c l e

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Article history: Received 9 April 2019 Received in revised form 7 May 2019 Accepted 9 May 2019 Available online 10 May 2019 Keywords: Hollow nanosphere Block copolymer Micelle template Nickel phosphate

a b s t r a c t Hollow nanospheres of amorphous nickel phosphate (NiP) was synthesized using micelles of the block copolymer, PS-PVP-PEO as template and structure directing agent. Phosphate ion is a bridge between PVP and Ni2+ ion. The removal of polymer template form the hollow spheres of NiP with the amorphous shell. The obtained hollow spheres of NiP have been successfully applied as a catalyst for the reduction of p-nitrophenol. Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction There is significant progress in the synthesis of transition metal phosphate nanoparticles for electrode materials and catalyst due to their unique redox properties [1]. Vanadium phosphate is a commercialized catalyst for the oxidation of butane to maleic anhydride; an important intermediate for the production of unsaturated resins and food additives [2]. Iron phosphates (FeP), cobalt phosphates (CoP) and nickel phosphates (NiP) showed high activity in a variety of oxidation-reduction reactions [3–6]. In this sense, the facile route for the synthesis of transition metal phosphates nanoparticles is highly desirable. Chen et al. synthesized colloidal spheres of transition metal phosphate using sodium dodecyl sulfate (SDS) as a capping agent. The nucleation and growth of colloidal nanoparticles were controlled by slow hydrolysis of urea and the stabilized by SDS [7]. The mesoporous ferric phosphate materials were synthesized by using poly(amidoamine) dendrimer as a single molecular template [8]. The mesoporous FeP synthesized via self-assembly of pluronics type block copolymer (P123) enhanced the lithium ion intercalation/deintercalation kinetics [9]. Among the other morphologies, the hollow nanospheres have attracted considerable interest due to well-defined morphology, low density, and high surface area [10–13]. The hollow structure of silica, carbon, and metal oxides are widely explored [14–16]. The sol-gel reaction between silica precursors and polymer ⇑ Corresponding author. E-mail address: [email protected] (B.P. Bastakoti). https://doi.org/10.1016/j.matlet.2019.05.034 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.

micelles allowed to synthesize the well-controlled SiO2 hollow nanospheres [17]. Despite huge scientific interest, the synthesis of the hollow structure of transition-metal phosphates has confronted many obstacles. The quick reaction between positively charged metal ions (Mn+) with negatively charged phosphate anions (PO34 ) often leads to irregular or bulk structures. Ethylenediamine tetra(methylene phosphonic acid) molecules are used as coupling agent to synthesize the micron-sized nickel phosphate [18]. However, there have been no simple ways for successful preparation of sub-50 nm hollow nanospheres of transition metal phosphates. Here, we report the micelles template method to synthesize the sub-50 nm hollow nanospheres of nickel phosphate (Fig. 1). The advantage of this method is that the chemically distinct three blocks of triblock copolymer contribute to the fabrication of hollow nanospheres [19]. The molecularly dissolved polymer chains undergo self-assembly to form polymeric micelles after addition of phosphoric acid. Phosphoric acid has a dual function as a source of phosphate and acid to protonate the PVP block of micelles. The positively charged micelles interacted with positively charged metal ions through phosphate ions bridge. After removing the polymer template, the hollow nanospheres of NiP was obtained. 2. Experimental 2.1. Method 50 mg PS13k-PVP9k-PEO16.5k with polydispersity index 1.15 (purchased from polymer source) triblock copolymer was dis-

B.P. Bastakoti et al. / Materials Letters 251 (2019) 34–36

Fig. 1. Schematic diagram of synthesis of NiP hollow nanospheres via direct micelles template.

solved in 10 mL of THF by mild sonication for 20 min followed by continuous stirring for 4 h. 0.12 mL of phosphoric acid was added into the polymer solution. 150 mg nickel nitrate (dissolved in 1 mL of ethanol) was added to the micelles solution and stirred for 1 hr. The precipitation was collected by centrifugation and dried at 50 °C. The composite was calcined at 550 °C to remove the polymer. 2.2. Characterization Hydrodynamic diameter and zeta-potential were measured using Otsuka ELS Z f-potential and particle analyzer. The morphology of the polymeric micelles, composites and hollow nanospheres were observed by field-emission scanning electron microscopy (SEM; Hitachi SU-8000) in the inverse mode and Transmission electron microscopy (TEM; JEOL JEM-1210). The crystalline phases and crystallinity were measured by X-ray powder diffraction (XRD: SHIMADZU XRD-7000) analysis. Thermogravimetric analysis (TGA) was carried out using a SEIKO-6300 TG/DTA instruments at a heating rate of 10 °C
min 1. The reduction of p-nitrophenol was monitored spectrophotometrically. 3. Results and discussion The triblock copolymer was molecularly dissolved in THF. No micelles were detected in DLS measurement. The addition of phosphoric acid induces the micellization. The formation of micelles was observed by Tyndall scattering as shown in Fig. 2a. The hydrodynamic diameter (Dh) was found to be 80 nm. For comparison, the polymer solution was prepared in THF/H2O solvent, and the Dh was found to be 75 nm which is less than that obtained after addition of phosphoric acid. The slight increase in the diameter in the presence of phosphoric acid is attributed to the protonation on nitrogen in pyridine ring. The protonation induces positive charge which increases the repulsion among the PVP blocks, and the micelles show expanded confirmation. However, the change in confirmation was not significant. The significant increase in hydrodynamic diameter of PS-PVP-PEO micelles in acidic condition (in the presence of dilute HCl) was reported [20]. The pKa of PVP is 4.98

Fig. 2. (a) Tyndal effect showing the micelles formation after addition of phosphoric acid into polymer solution in THF, (b) photograph of solution NiP/Polymer nanocomposites and (c) SEM image of PO34 /PS-PVP-PEO nanoaggregates.

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[21]. In the acidic condition (pH less than 4.98) the PVP gets protonated, and the protonation changes the confirmation of PVP from shrunken to extended because of the electrostatic repulsion between the protonated PVP. The protonated PVP has a positive charge. In the present study, the phosphate ions behave as counter ions and electrostatically binds with positively charged PVP. The zeta potential measured was 5 mV for micelles/phosphate composites. Fig. 2c shows the SEM image of composites micelles of PO34 / PS-PVP-PEO. The SEM images were recorded using the inverse mode of SEM. Spherical micelles of about 65 ± 5 nm diameter were clearly seen. The slight discrepancy of size between DLS measurement and SEM images is attributed to the drying of micelles during sample preparation prior to SEM measurement. The spherical nano-objects are an ideal template for hollow nanospheres. We used the micelles of PO34 /PS-PVP-PEO as a template for the synthesis of nickel phosphate nanospheres. The phosphate ions from phosphoric acid behave as a bridge between PVP and Ni2+ ions. After addition of Ni2+ ions, NiP was deposited at PVP units around the PS core of spherical micelles. The SEM images of NiP/PS-PVP-PEO nanocomposite particles prior to removal of the core is shown in Fig. 3a. After calcination, the template was removed leaving the hollow voids with a rigid shell of NiP. The complete removal of the polymer was confirmed by using FTIR spectra (Fig. S1). The main peaks from the polymer disappeared after calcination. The broad peak between 978 cm 1 and 1100 cm 1 showed the characteristic presence of the phosphate ions. The IR bands at 2885 cm 1 was assigned to the symmetric and asymmetric C–H stretching vibration bands of aliphatic polymer back bone [22]. The disappearing of characteristics vibration band of PVP ring at 1463 cm 1 and 1587 cm 1 further supports the removal of polymer template after calcination [23]. The thermogravimetric curve for NiP/PS-PVP-PEO nanocomposites was shown in Fig. 4. The initial part of the TG curves slightly declines. This implies the desorption of absorbed water molecules. The large weight loss at 500 °C is burning of the polymer. The much higher weight loss after 600 °C is possibly attributed to the decomposition of the NiP. The thermal stability of pure polymer was also shown

Fig. 3. SEM image of NiP/PS-PVP-PEO nanocomposites, (b, c) TEM images of NiP nanospheres. No lattice fringes were observed in the high-resolution TEM image. (d) SAED patterns are showing no distinct rings.

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study was relatively higher than the concentration of p-NP. The reaction can be regarded as zero order with respect to the borohydride concentration. Therefore, pseudo-first order kinetics with respect to the p-nitrophenolate. 4. Conclusion Hollow nanospheres of nickel phosphate are synthesized using block copolymer micelles as a template. The phosphate ions linked the positively charged micelles and metal ions. The Hollow nanospheres of nickel phosphate with amorphous shell shows the promising application in reduction of p-nitrophenol. It is believed that this synthetic method could be extended to other nanoarchitectures for different applications. Declaration of Competing Interest None. Fig. 4. Thermogravimetric analysis of (a) pure polymer and (b) NiP/PS-PVP-PEO nanocomposites.

for the comparison (Fig. 4b). It is obvious that the thermal stability of polymer was enhanced in the composites form. The fabrication of hollow nanospheres NiP with diameter sub 50 nm size is generally difficult because after the initial mixing of Ni ions and phosphate ions the growth of NiP is rapid to induce large crystal [18,24]. However, the use of polymeric micelles overcome this issue of crystal overgrowth, allowing the formation of hollow nanospheres with uniform void space. The use of triblock copolymer for the synthesis of hollow nanoparticles solves the problem encountered in another polymeric system to some extent. The PS core has a high glass transition temperature, and rigid structure in aqueous solution is a template to form void. PVP is a reaction site for PO34 ions and Ni2+ ions. The easy solubility of PEO in different solvents provides stability to the nanoaggregates by steric hindrance. Fig. 3(b,c) shows the TEM images of hollow NiP after removing the polymer template. The external diameter of the hollow nanospheres is around 45–50 nm with void diameter 32– 35 nm. The shell thickness is 6–8 nm. The void space and shell thickness could be easily tuned by changing molecular weight of polymer or solution properties [25,26]. From high-resolution TEM image, the shell exhibited an amorphous-like structure. The selected area diffraction pattern was shown in Fig. 3d. The diffuse rings in the selected area electron diffraction (SAED) patterns further confirm the amorphous characteristics. Crystalline phases of hollow NiP nanospheres were not seen in the wide angle XRD (Fig. S2). The elemental distribution in the hollow nanospheres was detected by EDX mapping in TEM mode. It was found that Ni, P, and O were uniformly distributed, as shown in Fig. S3. The amorphous NiP nano/microparticles have been already used in several applications such as supercapacitors [27] and methanol oxidation reaction [28]. Here, we have used NiP as a catalyst for the conversion of nitrophenol (NP) to aminophenol (AP). p-nitro phenol is an industrial intermediate in manufacturing drugs and anticorrosion. Noble metals (Au, Ag), are generally considered as an efficient catalyst for the hydrogenation of pnitrophenol [29]. It is very costly to use metal nanocatalyst. In order to evaluate the catalytic behavior of NiP hollow nanospheres, the reduction of p-NP to p-AP by NaBH4 used as a model reaction. The kinetics plot was shown in Fig. S4. As expected, a linear relation between ln(Ao/A) and time was observed giving the rate constant of 0.14 min 1. The concentration of NaBH4 for the present

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