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The synthesis of monodispersed M-CeO2/SiO2 nanoparticles and formation of UV absorption coatings with them

CeO2/polymer nanoparticles have drawn considerable attention for their excellent UV absorption properties. However, many challenges still exist in the successful incorporation of ceria into the polymer matrix for the easy agglomeration and photocatalytic activity of CeO2 nanoparticles. Herein, we address these issues by constructing three-layer structured nanoparticles (M-CeO2@SiO2) and incorporating them into a polymer matrix through a mini-emulsion polymerization process. During this process, small-sized nano-ceria became uniformly anchored on the surfaces of monodisperse silica particles first, and then the particles were coated with an MPS/SiO2 shield. The morphology and dispersion of the nanoparticles were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The performance of the hybrid films was characterized using UV-vis absorption spectroscopy (UV-vis) and water contact angle (WCA) measurements. Results showed that the M-CeO2@SiO2 nanoparticles exhibited a three-layer structure with a mean diameter of 360 nm, and they possess good compatibility with acrylic monomers. After the addition of M-CeO2@SiO2, hybrid films exhibited enhanced UV absorption capacity as expected, accompanied by an obvious improvement in hydrophobicity (the water contact angle increased from 84.2° to 98.2°). The results showed that the hybrid films containing M-CeO2@SiO2 particles possess better global performance as compared with those containing no particles.


Introduction
Fluorinated acrylics, which generally contain peruoroalkyl segments, are typical low surface energy materials with outstanding water/oil repellence characteristics, antifouling properties and satisfactory adhesion on different substrates. [1][2][3][4][5] These unique properties make them ideal candidates for outdoor coatings, which has attracted widespread attention from researchers. [6][7][8] The purpose of outdoor coatings is to protect the substrates from harsh environments, such as sunlight, rain, and air pollution. Therefore, it is important for uorine-containing acrylate copolymers to maintain greater protective abilities than simple physical isolation.
Among various harsh environmental factors, ultraviolet rays, which may cause the degradation of organic matrices, leading to the failure of protective coatings, is a major source of damage. Although the polymers themselves are durable in outdoor environments, it is necessary to incorporate some UV absorbers to improve the lifetime and improve the properties of these coatings. 9 Compared with organic UV absorbers that may release environmental pollutants, such as volatile organic compounds (VOC), inorganic nanoparticles such as TiO 2 , ZnO and CeO 2 seem to be better choices. [10][11][12][13][14] Among the numerous UV absorbers, ceria nanoparticles are ideal candidates due to their excellent UV absorption ability, visible light transparency and lower light refractive index. [15][16][17][18] Several studies have been published on the incorporation of CeO 2 nanoparticles into polymer latex to shield UV light. [19][20][21] Most of these studies achieved good dispersion of cerium oxide via physical blending or modication with ligands or hydrogen bonds, such as citric acid, polyethylene glycol and poly(acrylate acid). [22][23][24][25] For hybrid latex obtained using these methods, the interaction between ceria nanoparticles and the organic matrix are weak, making the long-term stability of the system open to question. To the best of our knowledge, although several approaches have been reported for the preparation of CeO 2 /polyacrylate hybrid latex, the use of ceria in uorinated polyacrylate coatings has been seldom reported.
Constructing core-shell structures is commonly used in nano-modication. In this way, the surfaces of nanoparticles are covered with a layer of other materials for protection or modication. [26][27][28][29][30] In this paper, we constructed a core-shell-shell structure to solve the above-mentioned problems. In this structure, silica has been used as the core and shell material due to its wellknown characteristics of easy dispersion and high stability. 31 Firstly, small-sized ceria particles were uniformly deposited on the surface of monodispersed silica nano-spheres. Then, a layer of SiO 2 /MPS (3-methacryloxypropyltrimethoxysilane) shell was introduced to isolate the ceria from the external environment, which provides the active site for the subsequent mini-emulsion reaction. The resulting nanoparticles were used to prepare a polyacrylate/M-CeO 2 @SiO 2 latex via a mini-emulsion polymerization process for outdoor functional coatings. In addition to the original advantages of the uorine-containing acrylic resin, the latex coating exhibits excellent UV absorption capabilities. Furthermore, due to the increase in surface roughness and the enrichment of the uorine component on the surface, the hydrophobicity of the coating is also improved.
Methyl methacrylate (MMA) and butyl acrylate (BA) were used as monomers which were puried using a neutral alumina column to eliminate inhibitors before use. Ammonium persulfate (APS) was obtained from Tianjin Bodi Chemical Co., Ltd, and was recrystallized before use. SiO 2 with a mean diameter of 310 nm was synthesized in our lab using the Störber method. 32 Deionized water and absolute ethanol were used in all processes.

Characterization techniques
The morphologies and sizes of all the prepared particles (SiO 2 , CeO 2 @SiO 2 , and M-CeO 2 @SiO 2 ) were characterized using eld emission scanning electron microscopy (FESEM, JSM-7500F, Japan) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). The TEM samples were prepared by dispersing the particles into ethanol followed by dropping the suspension onto a carbon-coated Cu grid. Particle sizes were measured directly, taking about 50 particles in the SEM pictures into account using soware named Nano measurer. An X-ray diffractometer (XRD, D/MAX-2500RB, Japan) was used to identify the crystal structure using Cu Ka radiation with a scan speed of 6 min À1 in the 2q range from 10 to 100 . Fourier transformed infrared (FT-IR) absorption spectra were recorded on a NEXUS-470 FTIR analyzer (Nicolet, USA) from 4000 cm À1 to 500 cm À1 at a resolution of 8 cm À1 . A UV-3600 ultraviolet transmittance analyzer (Shimadzu, Japan) was used to record the UV-vis absorption spectra of the latex lms in the wavelength range of 250-700 nm. The static contact angle of water was measured on the surface of lms using a DSA 20 (KRÜSS, Germany) at room temperature. Inorganic nanoparticle lms were prepared using the compression method with a powder compressing machine (769YP-15A, Tianjin Keqi High & New Technology Corporation), which pressed nano-powder into round akes under high pressure. Atom force microscopy (AFM) was used to investigate the surface topography of the latex lms. 4 ml water droplets were used and ve measurements were done for each example. The lms were prepared by dropping 1 ml of emulsion onto each slide and then drying at 60 C for 3 h. All optical photographs were obtained using a digital camera (Olympus E-PL1, Japan).

Synthesis of CeO 2 @SiO 2 nanoparticles
The deposition of CeO 2 onto SiO 2 is affected by electrostatic attraction in a weak alkaline environment. In a typical experiment, 1 g of the prepared SiO 2 and various amounts of Ce(NO 3 ) 3 $6H 2 O were added into 60 ml of ethanol in a threenecked ask followed by sonicating for 30 min to make it dispersed. Aerward, 3.23 g HMT and 20 g H 2 O were added into the ask with agitation. The solution was stirred for 2 h at a temperature of 75 C, and then 400 ml H 2 O 2 was added to oxidize Ce 2 O 3 to CeO 2 . The precipitates were centrifuged and washed with absolute ethanol three times before drying at 60 C overnight in an oven.

Synthesis of M-CeO 2 @SiO 2 nanoparticles
Subsequently, the CeO 2 @SiO 2 particles were covered with a mixture of MPS and TEOS, so as to separate CeO 2 from the atmosphere, to prevent the catalytic decomposition of CeO 2 . Specically, 0.2 g CeO 2 @SiO 2 was dispersed in a solution of 1.2 g water and 16 g ethanol under constant ultra-sonication for 30 min. Aer that, 0.3 g TEOS and 0.72 g ammonia were added in sequence. Aer 5 min, 0.2 g MPS (this amount is based on our previous work 33 ) dissolved in 10 ml ethanol was added into the ask. The reaction was carried out at room temperature and aged for 8 h. The resulting M-CeO 2 @SiO 2 nanoparticles were centrifuged and then washed three times with ethanol, aer which the M-CeO 2 @SiO 2 nanoparticles were obtained by drying overnight in an oven at 60 C.

Preparation of PFA/M-CeO 2 @SiO 2 latex and lm
The PFA/M-CeO 2 @SiO 2 latex was prepared using a typical miniemulsion polymerization procedure. As shown in Table 1, the oil phase was prepared by mixing all the monomers, the costabilizer (HD) and a certain amount of M-CeO 2 @SiO 2 (the amount of M-CeO 2 @SiO 2 varied from 0 g to 0.12 g, and the latex was denoted as FPA-0 to FPA-12 accordingly). The mixture was sonicated and stirred by a mechanical agitator for 30 min to disperse the nanoparticles evenly. Then the water phase was added into the oil phase and sonicated in an ice bath for 30 min. Aerward, a high-speed shearing machine was used at a speed of 8000 rpm for 30 min to generate a homogeneous miniemulsion. Finally, the resultant mini-emulsion was transferred into a 100 ml three-necked ask equipped with a thermometer, a reux condenser, a mechanical agitator and a nitrogen inlet. 10 ml of 1 wt% APS aqueous solution was used as an initiator and the reaction was kept at 75 C for 6 h.
Latex was spin-coated onto the slides and dried at 60 C for 3 h to form a lm for follow-up tests.

Preparation of M-CeO 2 @SiO 2 nanoparticles
In order to avoid the agglomeration of cerium oxide, improve its dispersion within the coatings, and allow it play the role in the UV absorption, a three layer structure was designed. In the structure, monodispersed SiO 2 particles were used as cores to ensure the even distribution of cerium oxide, and MPS/SiO 2 was used as the shell material to improve the compatibility with the polymer system and prevent the catalytic performance. The particle synthesis diagram is illustrated schematically in Fig. 1a.
SEM/TEM photos in Fig. 1b-f show an intuitive view of the three-layer structure. In Fig. 1b and e, uniform and monodispersed SiO 2 with a diameter of ca. 300-320 nm can be clearly observed. When CeO 2 was deposited ( Fig. 1c and f), particle sizes increased to 335-350 nm, and a distinctive layer of CeO 2 (the dark layer) can be clearly observed. As for M-CeO 2 @SiO 2 ( Fig. 1d and g), the particle sizes are further increased to 355-365 nm, with an MPS/SiO 2 layer (the semi-transparent layer) completely covering the surface of CeO 2 to separate CeO 2 from the environment. As is well known, silica particles prepared by the Stöber method usually have smooth surfaces because of the relatively complete hydrolysis and condensation of TEOS in water/ethanol solution. 34 They are negatively charged in neutral or alkaline dispersion. When Ce(NO 3 ) 3 $6H 2 O is added into SiO 2 dispersion, the positively charged Ce 3+ ions are attracted to the vicinity of SiO 2 through electrostatic interaction and become deposited on the SiO 2 surface in a weakly alkaline environment. Then the deposited Ce 2 O 3 is oxidized to CeO 2 by the addition of H 2 O 2 . For the purpose of optimizing the cerium loading, the Ce(NO 3 ) 3 $6H 2 O to SiO 2 mass ratio was varied as follows: XRD patterns are typically used to study the phase structure of various nanoparticles. As shown in Fig. 2a, a broad diffraction peak at 23 can be seen in the XRD curve of pure SiO 2 , which corresponds to the typical characteristics of amorphous silica. Aer coating with ceria, four distinct diffraction peaks (28.4 , 32.9 , 47.4 , and 56.2 ) can be observed, which correspond to the reections of the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of the CeO 2 crystal, respectively. This result proves the existence of CeO 2 in CeO 2 @SiO 2 and conrms it to have the cubic uorite structure. The strong and sharp peaks reveal that CeO 2 possesses an intact crystal structure. No new diffraction   peaks appear in M-CeO 2 @SiO 2 , but the relative intensities of the CeO 2 diffraction peaks decrease, owing to the coverage of MPS/SiO 2 . Fig. 2b shows the FT-IR curves of pure SiO 2 , CeO 2 @SiO 2 and M-CeO 2 @SiO 2 . The absorption at 1100 cm À1 is assigned to the stretching vibration of Si-O-Si, while the peaks at 802 and 470 cm À1 belong to the bending vibration of Si-O. The broad peak at around 3447 cm À1 is ascribed to the stretching vibration of -OH. The peaks at 1460 and 1390 cm À1 are attributed to the Ce-O bond. Compared with CeO 2 @SiO 2 and SiO 2 , the absorption of the carboxyl group (C]O) at 1716 cm À1 in the M-CeO 2 @SiO 2 curve demonstrates the successful graing of MPS on the surface of the CeO 2 @SiO 2 nanoparticles.
In summary, the monodispersed three-layer structure nanoparticles were synthesized as anticipated. Silica is the central core, cerium dioxide forms the middle layer and MPS/ SiO 2 is the outmost layer.

Performance of M-CeO 2 @SiO 2 nanoparticles
The MPS/SiO 2 shell shielded the catalytic properties of CeO 2 , and photo-degradation experiments were carried out to prove it. The results are shown in Fig. 3a. It can be clearly observed that the Rh B concentration decreased rapidly when CeO 2 is not coated. Aer 4 h of irradiation with a Xe lamp, about 79.44% and 98.2% of Rh B was degraded by pure CeO 2 and CeO 2 @SiO 2 , respectively. As a comparison, only 31.94% Rh B was degraded in M-CeO 2 @SiO 2 suspension, which is even slightly lower than that of the blank sample (35.65%). Throughout the process, the decomposition curve of the M-CeO 2 @SiO 2 suspension was almost parallel with that of the blank sample. The tiny gap between the M-CeO 2 @SiO 2 suspension and the blank sample arises from the fact that the opaque M-CeO 2 @SiO 2 suspension blocked part of the light. It is obvious that nanoparticles with the MPS/SiO 2 layer show no catalytic properties. As a result, the synthesized M-CeO 2 @SiO 2 can be safely added into the organic system, avoiding the potential decomposition of the polymer matrix.
The MPS/SiO 2 layer also improves the compatibility of nanoparticles with the polymer latex. It is well known that the compatibility of nanoparticles with the monomer mixture is of vital importance for the stability and global properties of the nal emulsion. 1 wt% of pure CeO 2 , CeO 2 @SiO 2 and M-CeO 2 @SiO 2 nanoparticles were dispersed separately into the monomer mixture which was composed of MMA/BA ¼ 50/ 50 wt%. As shown in Fig. 3b, aer a 12 h sedimentation test at room temperature, CeO 2 and CeO 2 @SiO 2 nanoparticles precipitated at the bottom of the monomer mixture; however, the M-CeO 2 @SiO 2 nanoparticles did not form sediment and exhibited a uniform opaque feature, presenting constant stability and compatibility. The contact angle of the nanoparticles reected this phenomenon to some extent. As is well known, acrylic monomers are oleophilic monomers, which are incompatible with water. To be dispersed uniformly within acrylic monomers, nanoparticles should also be hydrophobic, too. That means the water contact angle (WCA) should be larger than 90 . Fig. 3c presents the WCAs of the nanoparticles. Obviously, pure CeO 2 and CeO 2 @SiO 2 nanoparticles are hydrophilic, and the WCAs are 36.9 and 21.9 , respectively. Aer modication with MPS, however, the WCA dramatically increased to 112 . This may be attributed to the uniform dispersion of M-CeO 2 @SiO 2 nanoparticles amongst the acrylate monomers. The M-CeO 2 @SiO 2 nanoparticles are more likely to maintain their stability in the polymer system compared with pure CeO 2 and unmodied CeO 2 @SiO 2 nanoparticles.

Preparation of PF/M-CeO 2 @SiO 2 latex coatings
The above-synthesized nanoparticles were applied to the acrylate emulsion system via a mini-emulsion polymerization process to improve the performance of polyacrylate resin.
The morphology of the latex particles was observed using TEM to investigate the formation mechanism of the composite lms. Hybrid latex containing 1.0 wt% M-CeO 2 @SiO 2 nanoparticles was dropped onto a copper grid. As can be observed from Fig. 4a, the hybrid particles show a raspberry-like structure with several polymeric particles graed onto the surface of the nanoparticles. The amount of PF/M-CeO 2 @SiO 2 particles is small because of the low content of the M-CeO 2 @SiO 2 nanoparticles. This structure is formed because of the good compatibility between nanoparticles and organic monomers.  This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 4554-4560 | 4557 Fig. 4c illustrates the preparation of the PF/M-CeO 2 @SiO 2 latex using a mini-emulsion polymerization process. In the pre-emulsication process, the organic segments on the surface of the nanoparticles attract the surrounding acrylic monomers. Aer that, these monomers polymerize directly to form raspberry-like structures during the mini-emulsion. The uneven distribution of polymeric particles on the M-CeO 2 @SiO 2 surface is probably due to steric hindrance among polymeric particles. The raspberry-like structures would benet the even dispersion of M-CeO 2 @SiO 2 particles in the latex system and subsequent coatings (as shown in Fig. S2 †).
The chemical composition of the PF/M-CeO 2 @SiO 2 lm was characterized by FT-IR as shown in Fig. 5. The absorption peaks at 2956 cm À1 and 2874 cm À1 are the stretching vibrations peaks of C-H, while the peak at 1723 cm À1 is assigned to the stretching vibration peak of C]O. Absorption at 3345 cm À1 belongs to -OH. Furthermore, the characteristic peaks at 1236 cm À1 and 1143 cm À1 are the stretching vibration peaks of C-F. Compared with PFA0, few changes can be observed aer the incorporation of M-CeO 2 @SiO 2 nanoparticles due to the small amount of nanoparticles in the hybrid latex. Only a weak peak at 800 cm À1 , which corresponds to Si-O, indicates the existence of M-CeO 2 @SiO 2 nanoparticles within uorinated polyacrylate.

Performance of PF/M-CeO 2 @SiO 2 latex coatings
Due to the excellent UV absorption capacity of ceria, the hybrid lms containing M-CeO 2 @SiO 2 also show the ability of UV absorption. The UV-vis absorption capacity of hybrid latex lms with different M-CeO 2 @SiO 2 content was measured, and the results are shown in Fig. 6. Obviously, hybrid latex lms containing M-CeO 2 @SiO 2 nanoparticles show considerably stronger absorption of light below 600 nm than the pristine lm, especially in the wavelength range of 250-400 nm. Furthermore, the UV-vis absorption capacity of the lms increases gradually with the amount of nanoparticles present in the nal lm.
Surface hydrophobicity is an important property of uoropolymers. Thus, water contact angle measurements were taken to evaluate the wettability properties of PF/M-CeO 2 @SiO 2 lms, and the results are displayed in Fig. 7. It can be seen that the surface hydrophobicity of the hybrid lms signicantly improved with a small amount of M-CeO 2 @SiO 2 nanoparticles added. As shown in Fig. 7, the bare PFA0 lm exhibits an average WCA of 84.2 . When 0.2 wt% M-CeO 2 @SiO 2 nanoparticles were added, however, the WCA dramatically increased to 90.5 . When the content of M-CeO 2 @SiO 2 was increased further, the hydrophobicity of the hybrid lms gradually increased initially, and then decreased. The PFA10 lm, which contains about 1.0 wt% M-CeO 2 @SiO 2 , shows the highest water contact angle of 98.2 . All the hybrid lms containing M-CeO 2 @SiO 2 show a higher level of surface hydrophobicity than that of PFA0. As is well known, solid surface hydrophobicity is determined by the surface microstructure and chemical composition. The altered WCA data indicate that some changes have taken place on the surface aer the addition of M-CeO 2 @SiO 2 . To investigate the changes, AFM and EDS tests were performed to study the lm surface properties. Fig. 8 shows the AFM height and Rq (root mean square roughness) of the hybrid lms. It can be seen that the hybrid lms containing M-CeO 2 @SiO 2 nanoparticles displayed rougher surfaces than that of PFA0. As shown in Fig. 8a, bare PFA0 shows a rather smooth surface, with Rq ¼ 0.268, and revealed good compatibility of the uorinated segment with the polyacrylate system. Aer the addition of M-CeO 2 @SiO 2 , the   lm surface roughness increased. As the M-CeO 2 @SiO 2 nanoparticle content increased from 0 wt% to 1 wt%, Rq increased slowly from 0.268 to 2.89. However, as the nanoparticle content continued to increase to 1.2 wt%, there was a dramatic change in the surface roughness, with Rq increasing to 8.89. This sharp increase is likely ascribed to the partial agglomeration of nanoparticles, according to our group's previous work. 35 Although the M-CeO 2 @SiO 2 nanoparticles exhibit relatively good compatibility with the polymeric system, when too many nanoparticles are added during the polymerization process, partial agglomeration might happen, resulting in the sharp increase in surface roughness. The Wenzel and Cassie-Baxter models are two well-known classic wetting models that are commonly used to explain the relationship between wettability and surface roughness. According to those theories, the WCA of a hydrophobic surface can be improved by a rough surface, which is consistent with our experimental results. Table 2 presents the chemical composition at the air-lm interface of the PF/M-CeO 2 @SiO 2 hybrid lms. With the addition of M-CeO 2 @SiO 2 into PFA copolymers, a remarkable increase of uorine content on the lm surface can be observed. It can be seen from Table 2 that only 6.80 wt% uorine content can be detected from the latex lm containing no M-CeO 2 @SiO 2 nanoparticles. As the M-CeO 2 @SiO 2 content increases from 0 wt% to 1.2 wt%, the uorine content on the lm surface increases signicantly from 6.80 wt% to 9.45 wt% at rst, and then decreases slightly to 8.54 wt%. This result indicates that the addition of M-CeO 2 @SiO 2 helps the enrichment of uorine on the surface because of the unique structure of the hybrid particles. By constructing a raspberry-like structure, parts of the latex particles grow directly on the M-CeO 2 @SiO 2 surface. During the lm formation process, those latex particles concentrate on the surface along with the M-CeO 2 @SiO 2 nanoparticles. 33 Furthermore, the uorine-containing groups always migrate to the surface during the lm formation process due to their relatively low surface energy. By the addition of the nanoparticles, the surface roughness and the specic surface area of the lm layer increase, which helps to increase the content of the F element per unit area. A slight decrease of uorine content when more nanoparticles were added resulted from the partial agglomeration of nanoparticles, as mentioned before.

Conclusions
Ceria was uniformly incorporated into polymeric matrices and maintained high stability when a three-layer nanoparticle structure was constructed. The synthesized nanoparticles are about 360 nm in diameter. Monodispersed silica spheres with controlled sizes ensure the even distribution of ceria. An MPS/ SiO 2 layer protects the catalytic activity of ceria and improves the compatibility between the nanoparticles and the organic matrix. The synthesized M-CeO 2 @SiO 2 nanoparticles were incorporated into uorinated polyacrylate resin via a miniemulsion method. Aer the addition of M-CeO 2 @SiO 2 , the hydrophobicity of the hybrid coatings improved signicantly from 84.2 to a maximum of 98.2 , due to the enrichment of uorine and an increase in the surface roughness. Furthermore, the UV absorption capacity of the hybrid coating was improved, as expected, with the use of M-CeO 2 @SiO 2 . The hybrid coating is a promising candidate material for outdoor coatings.

Conflicts of interest
There are no conicts to declare.