Production of NiMn2O4 hollow spheres and CoFe2O4 bowl-like structures by using block copolymer stabilized polystyrene spheres as a hard template

The aim of this study is to highlight the use of polystyrene (PS) latexes stabilized with block copolymers as a hard template in the production of metal oxide hollow spheres. PS latexes produced by dispersion polymerization by stabilizing with tertiary amine methacrylate-based diblock copolymer were used as a hard template in the preparation of nickel manganese oxide (NiMn2O4) hollow spheres and cobalt iron oxide (CoFe2O4) bowl-like structures. Thanks to the diblock copolymer stabilizer with tertiary amine functional groups on the PS surface, precursor salts of CoFe2O4 and NiMn2O4 were first homogeneously deposited on the surface of PS latexes with a controlled precipitation technique. Then, metal oxide hollow spheres and bowl-like structures were produced by calcination. XRD results showed that CoFe2O4 and NiMn2O4 structures were successfully obtained after calcination. The thermogravimetric analysis results showed that the CoFe2O4 and NiMn2O4 contents of the hybrid PS spheres were in the range of 26.0–28.6 wt%. SEM images showed that the inorganic-polymer spheres fused with each other after calcination to form larger magnetic CoFe2O4 bowl-like structures. SEM images also indicated successful production of highly rough NiMn2O4 hollow spheres with nanosheets on the surface.


5]
. With a pioneering approach proposed by us before, it is the use of block copolymer stabilizers that provide the basis for the interaction of inorganic species with PS spheres, allowing the accumulation of inorganic species on the PS surface.In our previous studies, we reported successful production of double layer nickel oxide and manganese oxide hollow spheres with a very rough surface with nanosheets on the surface [18], nickel oxide [19] and nickel iron oxide [20] hollow spheres.
In the present study, nickel manganese oxide (NiMn 2 O 4 ) hollow spheres and cobalt iron oxide (CoFe 2 O 4 ) bowl-like structures were produced by using block copolymer stabilized PS latexes as a hard template.The PS spheres with various sizewere produced by using poly [2-(diisopropylamino)ethyl methacrylate]-block-poly [2-(dimethylamino)ethyl methacrylate] (PDPA-b-PDMA), poly [2-(diethylamino)ethyl methacrylate]-block-poly [2-(dimethylamino)ethyl methacrylate] (PDEA-b-PDMA) and poly [2-(dimethylamino)ethyl methacrylate]-block-poly[2-N-mopholinoethyl methacrylate] (PDMA-b-PMEMA) diblock copolymers as stabilizer via dispersion polymerization.Thanks to the functionality provided by the tertiary amine methacrylate containing block copolymer fringes on the PS surface, it has been homogeneously coated (or deposited) with metal oxide precursor salts (metal hydroxides) in the presence of urea with the controlled precipitation technique.Finally, both polymeric compounds were removed and metal oxides were converted to CoFe 2 O 4 and NiMn 2 O 4 structures by calcination process.As stated above, PS spherical latexes stabilized with block copolymer, which can be produced in a simpler and more functional way than polymeric spheres to be used for this purpose, are candidates to be a new model in the production of different types of metal oxide hollow spheres.

Experimental section 2.1. Materials
2-(Diisopropylamino)ethyl methacrylate (DPA, SI-AL), 2-(dimethylamino)ethyl methacrylate (DMA, SI-AL), 2-(diethylamino)ethyl methacrylate (DEA, Aldrich) and 2-N-mofolinoethyl methacrylate (MEMA, Polysciences Inc.) monomers were first passed through the basic alumina column (SI-AL).Then, 2,2-diphenyl-1-picrylhydrazyl (DPPH) and granular calcium hydride were added and stored at -18 °C in a freezer.The monomers were distilled under vacuum before use.1-Methoxy-1-trimethylsiloxy-2-methyl-1-propene (MTS), which is used as the initiator of the group transfer polymerization (GTP), was distilled under vacuum at room temperature.Tetra-n-butyl ammonium bibenzoate (TBABB) as a catalyst was synthesized in accordance with the literature [21].THF was first dried with the addition of finely chopped solid sodium pieces by stirring 3 days at room temperature.It was then refluxed under dry nitrogen in the presence of solid potassium and used as a solvent in the polymerization reaction.PDPA-b-PDMA, PDEA-b-PDMA and PDMA-b-PMEMA diblock copolymers were synthesized by using group transfer polymerization technique as described before [22].n-Pentane (Merck) was used to remove homopolymer contaminants from the diblock copolymers before proton NMR spectroscopy measurements.

Instrumentation
Molecular weight distributions (M w /M n ) and number average molecular weights (M n ) of the polymers were measured using gel permeation chromatography (GPC) having the following parameters: An Agilent Iso Pump 1200 Series and a refractive index detector, connected to PLgel Mixed-D and Mixed-E (5 and 3 μm, respectively, 300 mm × 7.5 mm, Polymer Laboratories, Amherst, MA) columns and eluted with HPLC-grade tetrahydrofuran that was stabilized with BHT (0.5 g L −1 ) and TEA (0.02%) at a flow rate of 1.0 mL min −1 .PMMA standards (ex.Polymer Labs, M n : 1100-220000 g mol −1 ) were used for calibration.The comonomer ratio of PDPA-b-PDMA, PDEA-b-PDMA and PDMA-b-PMEMA diblock copolymers were determined from proton NMR spectra (deuteron solvents) by comparing related peaks of both blocks.The hydrodynamic diameters (R h ) and polydispersity index values (PDI or  2 /G 2 ) of PS spheres were determined by dynamic light scattering (DLS).DLS studies were performed using the ALV/CGS-3 compact goniometer system (Malvern, Inc, UK).This goniometer system is equipped with a 22 mW He-Ne laser at λ 0 632.8 nm, a photodiode detector operating with high quantum efficiency and an ALV/LSE-5003 multitau digital correlator electronic system.All measurements were made with 90° constant angle scattering of polymer dispersions.The data were evaluated by second order cumulative analysis.The solution temperature was kept constant at ±1 ºC sensitivity with a temperature controlled water bath.The wt% ratio of metal oxide or composites of metal oxide structures were determined with thermogravimetric analysis (TGA) device (Seiko SII Extar 6000 TG/DTA).Measurements were performed at different heating rates (1-10 ºC min -1 ) and under a flowing dry air atmosphere of 2 mL min -1 .Protherm furnaces PAF 110/10 muffle furnace was used in the calcination process.The morphologies of PS latexes and metal oxide hollow spheres prepared were examined by light microscope (Leica DM750), and scanning electron microscope (SEM, Zeiss Evo LS10).Powder diffraction patterns of metal oxide hollow spheres were determined by X-ray diffraction (powder-XRD, PANalytical Empyrean) analysis using Cu Kα-radiation (λ¼ 1.54 Å) with 2θ angle in the range of 1-90 at room temperature.The powder diffraction patterns were examined in the HighScore Plus software and the peak determinations were made, and the phase content of the sample was illuminated with the reference phases found by searching from the ICDD PDF4+ library.

Production of PS microspheres by dispersion polymerization
In this study, PDPA-b-PDMA, PDEA-b-PDMA [22] and PDMA-b-PMEMA [23] diblock copolymers, which we previously produced and characterized by GTP method, were used as stabilizers (Figure 1).The detail of the production of PDPA-b-PDMA diblock copolymer with GTP, which is used as a stabilizer in the synthesis of PS spheres, is given in the supporting information.
Synthesis of PS microspheres (latexes) was performed using PDPA 0.17 -b-PDMA 0.83 diblock copolymer (1.0 g, 25500 g mol -1 , M w /M n : 1.08) stabilizer, AIBN initiator (50 mg), styrene monomer (5.0 mL), 1-butanol or H 2 O/methanol (50.0 mL) under nitrogen atmosphere in oil bath at 60 °C at 1000 rpm stirring speed (Figure 2).The reaction was continued  overnight.The solution was centrifuged twice at 10000 rpm for 10 min to precipitate PS microspheres.PS spheres were dried overnight in vacuum.Experimental conditions of other produced PS spheres are given in Table 1.The spheres were characterized by DLS after centrifugation.

Preparation of metal oxide structures
Schematic representation of the production of NiMn 2 O 4 hollow spheres and CoFe 2 O 4 bowl-like structures using PS spheres stabilized with block copolymer are given in Figure 2. The deposition of metal hydroxides on the surface using the controlled precipitation technique in the presence of urea and then the production of both CoFe 2 O 4 [9,[24][25][26][27] and NiMn 2 O 4 [28,29] structures by calcination process have been studied previously.The production of metal oxides was carried out very similar to these previous studies.First, PS spheres (0.2 g) were dispersed in 180.0 mL of water (pH 7.0).The amounts of Fe(NO 3 ) 3 and Co(NO 3 ) 2 solutions given in Table 2 were then added and stirred for 24 h.Finally, urea was dissolved in water (20.0 mL) and added to the reaction medium and stirred at 80 °C for 24 h.Similar process was carried out using Ni(NO 3 ) 2 and MnCl 2 solutions, and all details are given in Table 2.The products were centrifuged three times at 5000 rpm and washed three times with distilled water.It was seen that the centrifuged solution part was completely clear, that is, all metal oxide precursor salt deposited on the PS surface in a controlled precipitation.It was observed that the PS spheres coated with both metal oxide precursor salts turned from white to brown tones over time (see Figure 3).The resulting material was dried in an oven at 100 °C overnight.The productions of all metal oxide structures are given in Table 2.
Finally, PS-inorganic hybrid spheres containing Co/Fe and Ni/Mn were calcinated at 700 ºC for 1 h and at 600 ºC for 2 h in air atmosphere, respectively.In these calcination processes in the furnace, both types of hybrid spheres were heated up to 300 ºC and kept at this temperature for 1 h, removing most of the polymeric structure.Then it was heated from 300 ºC to calcination temperatures (600 or 700 ºC) with a heating rate of 1 ºC/min and the calcination process was terminated by keeping at this temperature.The difference in the colors of the hybrid spheres before the calcination and the metal oxide structures formed after the calcination is given in Figure 3.The wt% ratios of metal oxides in PS-inorganic hybrid spheres were determined with by thermogravimetric analysis (TGA).TGA measurements were carried out in a dry air atmosphere (2 mL min -1 ) and the heating program applied in the measurements is given in Figure 4.It was aimed to remove polymeric parts by keeping at approximately 300 ºC for 60 min in TGA measurements of hybrid spheres as in the calcination process performed in the furnace.Since deviation was observed in TGA chromatograms due to intense combustion, it was planned to be kept at these temperatures for 60 min and to completely burn the polymeric part in a controlled manner.However, unexpected fluctuations were observed in TGA chromatograms, which we thought to be due to the polymer not being completely removed.The contents and morphology of the produced metal oxide structures were determined by XRD, light microscope and SEM.

Production of PS microspheres
The tertiary amine methacrylate based PDPA-b-PDMA, PDEA-b-PDMA and PDMA-b-PMEMA diblock copolymers each served as a good dispersing agent in the production of PS spheres and enabled the production of monodisperse PS spheres (Table 1).In PS latex stabilization using this block copolymer, the less soluble PDPA, PDEA or PMEMA block are adsorbed on the latex surface, while the more soluble PDMA blocks are responsible for the stabilization of latexes.The polymer chains belonging to the PDMA block are located in the form of fringes in the shell of PS spheres, just like spherical brushed polymers.Polymeric spheres stabilized with block copolymers have been preferred by polymer scientists to produce more monodisperse or environmentally sensitive spherical particles [30][31][32][33][34].The resulting spherical particles exhibit changes in swelling-shrinkage behavior or surface properties with external stimuli such as temperature and pH [30][31][32][33][34].In other words, interactions are established between block copolymers and polymeric spheres used as stabilizers in emulsion and dispersion polymerization techniques.These interactions are more stronger in emulsion polymerization, but some of these block copolymers remain on the surfaces of latex after their synthesis via dispersion polymerization as well [30,34,35].
As a result of DLS studies, hydrodynamic radius (R h ) values and polydispersity index values ( 2 /G 2 ) of PS spheres stabilized with different block copolymers are given in Table 1.According to these results, it can be said that PS spheres are produced as monodisperse with a diameter of 1-2 µm.There are many studies showing that PS spheres can be produced in planned diameters by changing many factors such as heterogeneous polymerization technique, stabilizer type, stabilizer amount, mixing speed and solvent type [30][31][32][33][34][35][36].When the previous studies are examined, if the amount of stabilizer increases or the mixing speed increases, the diameter becomes smaller.Comonomer ratios in the block copolymer have a significant effect on the diameter [23].The type of solvent and solvent mixtures are also very effective on diameter change [36].Since the comonomer ratios and molecular weights of the stabilizers used in the production of each PS sphere are different, it will be very difficult to compare with each other.However, it is well known that the ratios, lengths, hydrophilic/ hydrophobic nature of the blocks in the stabilizer structure are decisive in ensuring that the PS diameters are at the desired size.

Preparation of metal oxide structures
In the previous section, it was mentioned that block copolymers used as stabilizers in dispersion (or emulsion) polymerization adhere to the surface [30,34,35].The polymer fringes with this DMA unit provided a suitable environment for the absorption of metal ions.However, in this way, the metal oxide precursor salt can be deposited or adsorbed homogeneously on the structure used as a hard template.Incidentally, it is also known that PS spheres are frequently used as a hard template in the coating of polymeric cores with an inorganic layer [1,2].PS spheres are preferred because the phenyl ring is modifiable, easy to prepare, and is a low cost polymer that is easily available commercially.In addition, poly(methyl methacrylate) and formaldehyde resin are other common polymers used for this purpose [1,2].
Together with our previous studies [18][19][20], it will be very useful for the reader to compare diblock copolymer stabilized PS spheres which is used as a hard template with other polymeric rigid templates in terms of functionality and production technique in this pioneering work.Inorganic species must have a surface charge (or functionality) in order to adsorb to the surface of PS spheres.This can only be achieved with stabilizing agent [30][31][32][33][34], anionic radical (ammonium persulfate and potassium persulfate) and cationic radical [2,2'-azobis(2-methylpropionamidine) dihydrochloride] initiators used in the emulsifier-free polymerization method [11,12], various modifications made on the phenyl ring in PS spheres [2,11,13], and spherical polymeric brush polymers, which are surface initiated polymerization products [14][15][16][17].In addition, the fact that PS spheres have a charged surface is important in that it allows the coating of PS spheres with inorganic species with the layer-by-layer coating (LbL) technique [37].Polymeric spheres stabilized with block copolymers contain polymer fringes around them, just like brush polymers, can be produced quite simply compared to brush polymers which are surfaceinitiated polymerization products that require special monomers or various modifications [14][15][16][17].It is worth to mention that these polymeric spherical brushes are frequently used in the production of inorganic nanoparticles (NP) rather than the production of hollow spheres [14][15][16][17].On the other hand, it is quite common to use anionic PS spheres formed by the sulfonation of the polystyrene surface with the H 2 SO 4 treatment, but it can still be said that the PS spheres stabilized with the block copolymers used in this study are more functional [2,11,13].It can be assumed that block copolymers on the surface of PS spheres provide adsorption of inorganic species to the surface and provide a completely homogeneous coating by preventing separation from the surface by forming a steric barrier during nucleation-growth [14][15][16][17].
In this study, it was thought that the mechanism of action of the diblock copolymer used to stabilize PS spheres was to establish interaction with metal ions, increase the concentration of metal ions on the PS surface with adsorption, form nuclei on the surface of the inorganic species in the basic medium, and the growth of the crystals of metal oxide precursor salts on the PS surface thanks to the polymer fringes.The fact that tertiary amine methacrylate based polymers and many other polymers have already been discussed in many studies as metal ion adsorbents [38,39].However, it should be kept in mind that many different types of block copolymers can be designed and used for this purpose [38,39].It is also important that the polymer to be selected for this purpose has to have high metal ion adsorption capacity, low cost and easy availability.
Using the controlled precipitation technique as in this study, many metal oxide hollow spheres and bowl-like structures such as CuO, ZnO, SnO  [6,40] can be produced for different applications.To summarize briefly, urea added to the reaction mixture slowly decomposes to NH 3 at 80 ºC, that is, the hydroxide ion concentration in the mixture increases and metal hydroxides begin to precipitate in the PDMA fringes on the PS surface.In other words, Fe(OH) 3 /Co(OH) 2 and Mn(OH) 2 /Ni(OH) 2 crystals are grown in a controlled manner on the PS surface.It is then converted to CoFe 2 O 4 and NiMn 2 O 4 by thermal decomposition (calcination) and polymeric compounds are removed at this time [9,[24][25][26][27][28].
According to the results of thermogravimetric analysis (TGA), it was observed that 26.0-28.6 wt% residue remains at 650 ºC (Table 2).Considering that PS spheres did not leave any residue at the same temperature, almost all of the structures formed after the calcination of polymer-inorganic hybrid structures belonged to CoFe 2 O 4 and NiMn 2 O 4 residues (Figure 4).Of course, by adding higher proportions of precursor metal salts, these residue amounts can be further increased, and this change causing an increase in the shell thickness contributes to the hollow spheres remaining unbreakable.
It can be easily understood by comparing the light microscope images of PS spheres and inorganic-PS hybrid spheres that the surfaces of all PS spheres are successfully homogeneously coated with Fe(OH) 3 /Co(OH) 2 and Ni(OH) 2/ Mn(OH) 2 (Figure 6).Again, these images showed that no other precipitate structures were formed except for the PS surface (Figure 6).By taking SEM images of metal oxide structures, both their homogeneity and more detailed morphological structures were revealed.SEM images of the metal oxide structures taken after calcination also indicated that the PS spheres were coated homogeneously (Figure 7).From the SEM images of CoFe1 and CoFe2, it was seen that the inorganic-PS hybrid spheres fused with each other after calcination process to form larger structures (Figures 7a and 7b).Magnetic CoFe 2 O 4 hollow bowl-like structures were obtained in the outer layer of this structure, and hollow spheres were obtained in the inner layers.The formation of these bowl-like structures resulted from the mechanical abrasion of the hollow spheres in the outer layer.The diameters of the bowl-like structures formed on the surface were, as expected, approximately 1.85 µm for CoFe1 and approximately 0.80 µm for CoFe2, in relation to the diameters of the PS spheres used.In another study where polymeric spheres were used as a rigid template and similar metal oxide type was also similar, structures with similar morphological properties were obtained, which revealed the effect of temperature [42].Although the first structure planned to be produced is metal oxide hollow spheres, the obtained bowl-like structures can be related to the nature of the metal oxide as well as the calcination temperature and time.It is possible to produce metal oxide hollow spheres by experimenting with different calcination temperature and time [42,43].SEM images of NiMn1 and NiMn2 samples showed that NiMn 2 O 4 hollow spheres with a diameter of approximately 2.50 µm were successfully produced (Figures 7c  and 7d).The reason why both have similar diameters is, of course, because they are produced using the same PS template.SEM images showed that the shell thicknesses of NiMn1 and NiMn2 hollow spheres were approximately 285 nm and 318 nm, respectively (Figure 7).The difference between NiMn1 and NiMn2 is the amount of urea and a change in the color of the resulting inorganic-PS hybrid sphere was observed (Figure 3).There were nanosheets on the surface of both NiMn 2 O 4 hollow spheres.It has been emphasized in previous studies that the presence of nanosheets has an effect on increasing the surface area of the structure [44].
The cobalt iron oxide (CoFe 2 O 4 ) is an important type of metal oxide that has applications in various fields such as sensor [45], photocatalysts [8,9], electrocatalyst [46], cancer therapy [47], batteries [25,27], magnetic optical behavior [48] and supercapacitors [49].It is n-type semiconductor, highly stable, small optical band gaps (approximately 2.6 eV) making them active under visible light treatment [50].The nickel manganese oxide (NiMn 2 O 4 ) has been widely studied and applied in many fields such as sensor [51], negative temperature coefficient thermistors [52], photocatalysts [53], electrocatalyst [54], supercapacitors [28,51], and batteries [55] owing to its various advantages, such as low cost, resource abundance, good stability, environmental friendliness, convenience in use and excellent electrochemical performance [56].The effectiveness of the materials also depends on their morphology, size and composition of the materials.In this respect, it is undoubted that reproducible and uniform metal oxide structures with varying diameters depending on the choice of polymeric sphere used as template can be used in many similar applications with the positive effect of high surface area.

Conclusion
The PS spheres produced in different diameters by dispersion polymerization using different diblock copolymers as stabilizers were used in the production of NiMn 2 O 4 hollow spheres and CoFe 2 O 4 bowl-like structures.The dimensions of these structures formed according to the diameters of the spherical spheres also changed as expected.This study reports successful usage of spherical PS latexes stabilized with tertiary amine methacrylate based diblock copolymer as a template, which offers a new approach in terms of the use of hard templates.The surfaces of PS spheres are surrounded by a hydrated PDMA block of steric stabilizer, tertiary amine methacrylate based diblock copolymers.These stabilizers give them the ability to adsorb inorganic species at a higher capacity and stabilize the formed seeds on the surface, allowing the homogeneous metal oxide precursor salt to accumulate on the surface.Moreover, it should be noted that these PS spheres can be produced with a wide variety of block copolymers.Such hard templates have important advantages such as having more functional groups than surface modified PS spheres and being prepared with a simpler technique compared to spherical PS brushes.It is quite possible that the spheres produced in this study and other inorganic hollow spheres we continue to produce will be used in various catalysis studies in the future.

Figure 1 .
Figure 1.Chemical structures of the diblock copolymer stabilizers.

Figure 2 .
Figure 2. Schematic representation of the production of NiMn 2 O 4 hollow spheres and CoFe 2 O 4 bowl-like structures using PS spheres stabilized with block copolymer.

Figure 3 .
Figure 3. Digital pictures of PS-inorganic hybrid spheres (before calcination) and metal oxide structures (after calcination) (a) and the behavior of metal oxide species in a magnetic field (b).

Table 1 .
Experimental conditions in the synthesis of the PS latexes with various size and DLS measurements (styrene 5.0 mL, 1000 rpm and at 60 ºC).
a Mole% content determined by proton NMR spectroscopy.b GPC results (THF eluent, PMMA standards).

Table 2 .
Experimental conditions in the synthesis of the inorganic-PS hybrid spheres (in 200 mL water, at 80 ºC).