Synthesis of Green and Red-Emitting Polymethyl Methacrylate Composites Grafted from ZnAl2O4:Mn-Bonded GO via Surface-Initiated Atom Transfer Radical Polymerization

A novel dual green and red-emitting photoluminescent polymer composite ZnAl2O4:Mn-bonded GO/polymethyl methacrylate (PMMA) was synthesized in a single-step reaction by surface-initiated atom transfer radical polymerization (SI-ATRP). The polymer chain was surface-initiated from the ZnAl2O4:Mn/GO, and the final products have a homogenous photoluminescent property from ZnAl2O4:Mn and better mechanical properties strengthened by graphene oxide (GO). The morphologies of ZnAl2O4:Mn/GO and the polymer composites were verified by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray diffraction analysis (XRD) revealed the two valence states of Mn (Mn2+, Mn4+) existing in the ZnAl2O4 host lattice, while Fourier-transform infrared spectroscopy (FTIR) spectra proved the transference of the active group, C-Br, from the initiator to the monomer during the polymerization. Gel permeation chromatography (GPC) shows the narrow dispersity of polymer composites fabricated through SI-ATRP. The SEM and FTIR results show the successful ‘graft’ of the polymer chains from the surface of ZnAl2O4:Mn/GO. The dual green and red-emitting polymer composites were synthesized, confirmed by the photoluminescence (PL) and photoluminescence excitation (PLE) results.


Introduction
In light of the current growth of the world's population, by 2050, the global population will have risen to 9 billion, 70% of which will be living in urban areas [1,2]. To solve the food shortage problem, plant factories have attracted increasing attention. A plant factory is "an indoor vertical farming system for efficient quality food production" whose particular emphasis is on plant factory artificial lights (PFAL) [2]. The indoor system has mainly two types of light sources: solely artificial light and artificial light plus sunlight. Most artificial lights used in plant factories are commercial light emitting diode (LED) lights which are produced by coating yellow phosphors (YAG: Ce 3+ , (Y, Gd) 3 (Al, Ga) 5 O 12 :Ce 3+ ) on blue LED chips, causing problems of a highly correlated colour temperature and low colour rendering index [3][4][5]. This kind of white light emitting diode (WLED) lacks red light, which is crucial for the growth of plants. To solve this limitation, phosphors containing red light-emitting elements are essential. Most recent red-emitting phosphors are fabricated by adding rare-earth ions like Eu 2+ to nitride due to their stable photoluminescent properties [6][7][8]. However, the fabrication of Eu-doped materials needs a high synthesis temperature (≥ 1800 • C), high pressure, and usually a nitrogen atmosphere [9]. To overcome these problems, non-rare-earth phosphors doped by transition metal elements (Mn, Co) have attracted rising attention. The selected phosphor in this project was a non-rare-earth manganese-doped zinc aluminate oxide (ZnAl 2 O 4 :Mn). Manganese is a promising replacement material that has been paid increasing attention to nowadays; it can emit green and red of transferring stress from the polymer to the filters. The GPMMA filters can improve the mechanical properties of pure PMMA significantly [29].
This project describes a novel kind of photoluminescent polymer composite by surfaceinitial atom transfer radical polymerization (SI-ATRP) on the graphene oxide surface, which contains green and red dual light-emitted phosphors. The fabricated polymer composites have narrow dispersity and photoluminescent properties, giving them a wide application in plant factories and greenhouses.

Fabrication of Photoluminescent ZnAl2O4:Mn/GO
A total of 10 g of Zn(COOCH3)2·2H2O and 22 g of AlCl3·6H2O (with the molar ratio of 1:2) were dispersed in 30 mL of deionized water by magnetic stirring. In the meantime, 0.54 g of MnCl2·4H2O was dissolved in 10 mL of DI water. Then, these two solutions were mixed by stirring for 2 h to make a homogeneous solution. Then, 30 mL of ammonium hydroxide was added to the mixed solution to change the pH value and form a white precipitate. The precipitate was washed with ethyl alcohol and DI water by centrifuging until the solution became neutral. The collected precipitate was preheated to 90 ℃ for 24 h before being sintered in air at different temperatures (1100 ℃, 1200 ℃, 1300 ℃, 1400 ℃, 1500 ℃).
A total of 2 mL of GO solution (8 mg) and 2 g of ZnAl2O4:Mn were dispersed in a mixed solution of 20 mL of CH3COOH and 7.5 mL of butanol. The mixture was stirred at 60 ℃ for 6 h. Then the solution was transferred into a 50 mL Teflon-sealed autoclave and

Fabrication of Photoluminescent ZnAl 2 O 4 :Mn/GO
A total of 10 g of Zn(COOCH 3 ) 2 ·2H 2 O and 22 g of AlCl 3 ·6H 2 O (with the molar ratio of 1:2) were dispersed in 30 mL of deionized water by magnetic stirring. In the meantime, 0.54 g of MnCl 2 ·4H 2 O was dissolved in 10 mL of DI water. Then, these two solutions were mixed by stirring for 2 h to make a homogeneous solution. Then, 30 mL of ammonium hydroxide was added to the mixed solution to change the pH value and form a white precipitate. The precipitate was washed with ethyl alcohol and DI water by centrifuging until the solution became neutral. The collected precipitate was preheated to 90 • C for 24 h before being sintered in air at different temperatures (1100 • C, 1200 • C, 1300 • C, 1400 • C, 1500 • C).
A total of 2 mL of GO solution (8 mg) and 2 g of ZnAl 2 O 4 :Mn were dispersed in a mixed solution of 20 mL of CH 3 COOH and 7.5 mL of butanol. The mixture was stirred at 60 • C for 6 h. Then the solution was transferred into a 50 mL Teflon-sealed autoclave and then heated to 180 • C for 24 h. After the product was collected by suction filtration using a 0.2 µm filter membrane and washed with ethanol and DI water, the product was dried in a vacuum oven at 80 • C. A total of 250 mg of ZnAl 2 O 4 :Mn/GO was dispersed in 15 mL of N, N-dimethylformamide (DMF) using an ultrasonic mixer. The mixture was transferred into a 50-mL two-neck spherical reaction bottle, which was pre-purged with nitrogen. Next, the whole reactor was purged with a vacuum and nitrogen for 30 min, and the purging cycle was repeated three times before beginning the reaction. Then, the reactor was put in an ice-water bath. After that, 4 mL of tetrahydrofuran (TEA) and 5 mL of α-bromoisobutyryl bromide were dropped into the reactor using an injection syringe. The reaction was kept at 0 • C for 3 h, and then the reaction was kept at room temperature for 48 h. After that, the product was collected by a vacuum filter through a 0.22 µm millipore polycarbonate membrane and washed with chloroform and acetone. Then, the product was dried under a vacuum at 70 • C overnight.

Polymerization of PMMA Composites and Pristine PMMA
Before polymerization, CuBr requires pretreatment. CuBr was dissolved in enough CH 3 COOH and stirred at room temperature overnight for purification. Then, the purified CuBr was washed with methyl alcohol, followed by drying in a vacuum oven at 40 • C for 12 h. It was then kept in a vacuum desiccator for further usage.
A total of 0.736 g of ZnAl 2 O 4 :Mn/GO-Br, 0.088 g of CuBr, 0.152 g of CuBr 2 , and 0.036 g of L-ascorbic acid were dispersed in a mixed solution containing 10 mL of DMF and 4 mL of CH 3 COOH, and then 20 mL of MMA was added into the system. The The polymerization of pristine PMMA was similar to the process of polymer composites. A total of 0.088 g of CuBr, 0.152 g of CuBr 2 , and 0.036 g of L-Ascorbic acid were dispersed in a mixed solution containing 10 mL of DMF and 4 mL of CH 3 COOH, and then 20 mL of MMA was added into the system. The whole system was purged by a vacuumnitrogen cycle three times before the reaction. Then, 40 µL of extra α-bromoisobutyryl bromide and 142.4 µL of PMDETA were dropped into the reaction bottom using an injection syringe. After adding PMDETA, the whole system was heated to 90 • C and kept in an oil bath for 48 h.
All the products were collected using a vacuum filter and purified through a dissolve filter process three times to remove extra ions. The purified composites were then dried at 80 • C for 48 h. The polymer sheet was produced by hot pressing at 145 • C. Then, the products were pressed into round pieces with a diameter of 50 mm and thickness of 1 mm at 150 • C with 30 KN pressure for further testing.

Characterization
The structure and morphologies of products obtained from each step were tested using a scanning electron microscope (SEM, TESCAN VEGA3) and transmission electron microscope (TEM, Talos F200S). The photoluminescent properties were tested by a Hitachi F-7100-3D using a Xe lamp as the energy source. The Fourier-transform infrared Polymers 2022, 14, 3689 5 of 17 spectroscopy (FTIR) spectrometers were recorded by Perkin Elmer with a wavenumber ranging from 500-4000 cm −1 . The crystallization structures were analyzed through X-ray diffraction (XRD) (Rigaku Smartlab, Tokyo, Japan) scanning from 15 • to 85 • . The molecular weight and distribution of the polymer chain were detected by a Shimadzu Rid-20A (Kyoto, Japan), and the polymer was dissolved in tetrahydrofuran (THF) to make a solution of 20 mg/mL. Thermogravimetry-different thermal gravity (TG-DTG) was used to characterize the thermal properties of the polymer composites. Vickers hardness indentation was recorded by an Alicona Infinite Focus.

Structure and Morphology
Structure and morphology are crucial for the research of composites, and SEM micrographs were recorded and compared to analyze the morphological changes during each experimental step. Figure 2 shows the morphological changes of ZnAl 2 O 4 : Mn during the annealing process at different temperatures from 1000 • C to 1500 • C. Obviously, with the increase in annealing temperature, the particle size of ZnAl 2 O 4 :Mn becomes larger, and the layered structure of phosphors becomes clearer. The tendency is consistent with the result in the literature [30].
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Characterization
The structure and morphologies of products obtained from each step were tested ing a scanning electron microscope (SEM, TESCAN VEGA3) and transmission electr microscope (TEM, Talos F200S). The photoluminescent properties were tested by a Hi chi F-7100-3D using a Xe lamp as the energy source. The Fourier-transform infrared sp troscopy (FTIR) spectrometers were recorded by Perkin Elmer with a wavenumber ran ing from 500-4000 cm −1 . The crystallization structures were analyzed through X-ray d fraction (XRD) (Rigaku Smartlab, Tokyo, Japan) scanning from 15° to 85°. The molecu weight and distribution of the polymer chain were detected by a Shimadzu Rid-2 (Kyoto, Japan), and the polymer was dissolved in tetrahydrofuran (THF) to make a so tion of 20 mg/mL. Thermogravimetry-different thermal gravity (TG-DTG) was used characterize the thermal properties of the polymer composites. Vickers hardness inden tion was recorded by an Alicona Infinite Focus.

Structure and Morphology
Structure and morphology are crucial for the research of composites, and SEM m crographs were recorded and compared to analyze the morphological changes duri each experimental step. Figure 2 shows the morphological changes of ZnAl2O4: Mn duri the annealing process at different temperatures from 1000 ℃ to 1500 ℃. Obviously, w the increase in annealing temperature, the particle size of ZnAl2O4:Mn becomes larg and the layered structure of phosphors becomes clearer. The tendency is consistent w the result in the literature [30].   In this experiment, the colour change of each step is quite noticeable. Firstly, Mndoped ZnAl 2 O 4 was dark pink because of the oxidation of Mn during the annealing process, and then the outcome turned grey after the hydrothermal reaction because of the existence of GO. When -Br decorated ZnAl 2 O 4 : Mn/GO was collected, the product's colour became a little bit light brown, which represents the colour of -Br. Figure 3 shows the morphologies of collected products after each experimental step. From Figure 3a,b, it can be seen that after being decorated by GO, the structure of photoluminescent ZnAl 2 O 4 : Mn changed  Figure 3d shows the surface morphologies of polymer composite ZnAl 2 O 4 : Mn/GO/PMMA. In order to confirm the successful decoration of GO on the surface of ZnAl 2 O 4 : Mn, the structural and elemental information needs to be further studied by XRD and EDS.
In this experiment, the colour change of each step is quite noticeable. Firstly, M doped ZnAl2O4 was dark pink because of the oxidation of Mn during the annealing p cess, and then the outcome turned grey after the hydrothermal reaction because of t existence of GO. When -Br decorated ZnAl2O4: Mn/GO was collected, the product's colo became a little bit light brown, which represents the colour of -Br. Figure 3 shows t morphologies of collected products after each experimental step. From Figure 3a,b, it c be seen that after being decorated by GO, the structure of photoluminescent ZnAl2O4: M changed to smaller layered pieces. The morphological changes are due to the interacti between ZnAl2O4: Mn and GO. Figure 3b,c show some transparent wrinkled structures the surface of ZnAl2O4: Mn pieces, which may come from GO. Figure 3d shows the surfa morphologies of polymer composite ZnAl2O4: Mn/GO/PMMA. In order to confirm t successful decoration of GO on the surface of ZnAl2O4: Mn, the structural and elemen information needs to be further studied by XRD and EDS. TEM micrographs are shown in Figure 4. Figure 4a shows the micrographs ZnAl2O4 annealed at 1400 ℃. The high-resolution TEM and SEAD results of Figure show the hexagonal crystal structure of ZnAl2O4, which corresponds to the XRD resu Figure 4c shows the wrinkled graphene oxide thin layer, and combined with Figure 4d is clear that the phosphor Mn-doped ZnAl2O4 is successfully located on the surface of G TEM micrographs are shown in Figure 4. Figure 4a shows the micrographs of ZnAl 2 O 4 annealed at 1400 • C. The high-resolution TEM and SEAD results of Figure 4b show the hexagonal crystal structure of ZnAl 2 O 4, which corresponds to the XRD results. Figure 4c shows the wrinkled graphene oxide thin layer, and combined with Figure 4d, it is clear that the phosphor Mn-doped ZnAl 2 O 4 is successfully located on the surface of GO. Figure 5a shows XRD patterns of phosphors annealed at different temperatures from 1000 • C to 1500 • C. The XRD spectra compare PDF#29-0880, PDF #46-1212, and PDF#30-8020 with the pdf cards of Mn ZnAl 2 O 4 , Al 2 O 3 , and MnO 2 . It shows that at 1000 • C, there are mainly peaks from Mn ZnAl 2 O 4 at 32 • , 37 • , 46 • , 55 • , 60 • , 75 • , and 79 • . Furthermore, when the temperature rises to 1200 • C, peaks representing Al 2 O 3 at 35 • , 39 • , 43 • , and 57 • begin to be observed. Moreover, at 1300 • C, the peak intensity of Al 2 O 3 becomes more vital. This proves that both ZnAl 2 O 4 and Al 2 O 3 exist in the final products when temperatures are higher than 1200 • C. Moreover, with the increase in temperature, the peak quality of Al 2 O 3 is higher.  Figure 5a shows XRD patterns of phosphors annealed at different temperatures fro 1000 ℃ to 1500 ℃. The XRD spectra compare PDF#29-0880, PDF #46-1212, and PDF#3 8020 with the pdf cards of Mn ZnAl2O4, Al2O3, and MnO2. It shows that at 1000 ℃, the are mainly peaks from Mn ZnAl2O4 at 32°, 37°, 46°, 55°, 60°, 75°, and 79°. Furthermor when the temperature rises to 1200 ℃, peaks representing Al2O3 at 35°, 39°, 43°, and 5 begin to be observed. Moreover, at 1300 ℃, the peak intensity of Al2O3 becomes mo vital. This proves that both ZnAl2O4 and Al2O3 exist in the final products when temper tures are higher than 1200 ℃. Moreover, with the increase in temperature, the peak qu ity of Al2O3 is higher.
Then, the XRD patterns of phosphors decorated by GO and Br are shown in Figu 5b below. XRD spectra of GO and reduced GO are referenced from L. Stobinski's wo [31], in which the main peak of graphene oxide is around 10°(2 ), while the promine peak of reduced graphene oxide is around 26°. Therefore, the peak around 25° in Figu 5b should be from the reduced graphene oxide, which proves the reduction of graphe oxide during the transfer reaction of the active initiating part from -bromoisobutyr bromide to GO. The peak of rGO also proves the existence of GO. Compared with PDF#2 0880 and PDF#30-0820 shown in Figure 5a, for the pdf cards of Mn ZnAl2O4 and MnO most of the peaks are from them, which means that Mn was doped in the ZnAl2O4 stru ture, and the rest of Mn was oxidized to MnO2. Figure 5c records the EDS ZnAl2O4:Mn/GO. The elemental dispersity of the wrinkled structure on the surface ZnAl2O4:Mn shows a 57.24% C atom on the tested surface. Figure 5d shows the mappin EDS result of ZnAl2O4:Mn/GO-Br; the result shows the successful location of -Br on t surface of phosphors/GO. Considering the XRD result, it can be confirmed that the tran parent structure is GO. Therefore, the SEM and XRD results prove the successful decor tion of GO on the surface of phosphors. Then, the XRD patterns of phosphors decorated by GO and Br are shown in Figure 5b below. XRD spectra of GO and reduced GO are referenced from L. Stobinski's work [31], in which the main peak of graphene oxide is around 10 • (2θ), while the prominent peak of reduced graphene oxide is around 26 • . Therefore, the peak around 25 • in Figure 5b should be from the reduced graphene oxide, which proves the reduction of graphene oxide during the transfer reaction of the active initiating part from α-bromoisobutyryl bromide to GO. The peak of rGO also proves the existence of GO. Compared with PDF#29-0880 and PDF#30-0820 shown in Figure 5a, for the pdf cards of Mn ZnAl 2 O 4 and MnO 2 , most of the peaks are from them, which means that Mn was doped in the ZnAl 2 O 4 structure, and the rest of Mn was oxidized to MnO 2 . Figure 5c records the EDS of ZnAl 2 O 4 :Mn/GO. The elemental dispersity of the wrinkled structure on the surface of ZnAl 2 O 4 :Mn shows a 57.24% C atom on the tested surface. Figure 5d shows the mapping EDS result of ZnAl 2 O 4 :Mn/GO-Br; the result shows the successful location of -Br on the surface of phosphors/GO. Considering the XRD result, it can be confirmed that the transparent structure is GO. Therefore, the SEM and XRD results prove the successful decoration of GO on the surface of phosphors.
The initiation of ATRP begins after the formation of carbon radicals from the initiator, and then the carbon radical reacts with the −C = C− bond of the monomers. The oxygen functional groups on the GO surface will make the initiating reaction of carbon radicals easier because the electron-drawing functional groups are beneficial to the formation of the C-Br active group, which is the former of carbon radicals [32]. The ATRP initiator EBiB was reacted with GO first, and then the active group was transferred from EBiB to the surface of GO to begin the process of the above-mentioned initiating reaction and halide atom transfer process. In this reaction, CuBr was used as the catalyst, CuBr 2 was used to keep the concentration balance of Cu(I) and Cu(II) in the reaction system, and α-bromoisobutyryl bromide was the initiator, which was covalently connected to the surface of GO, while PMDETA was a ligand. The covalent connection can be observed from the FTIR spectra in Figure 6 below. the C-Br active group, which is the former of carbon radicals [32]. The ATRP initiator EBiB was reacted with GO first, and then the active group was transferred from EBiB to the surface of GO to begin the process of the above-mentioned initiating reaction and halide atom transfer process. In this reaction, CuBr was used as the catalyst, CuBr2 was used to keep the concentration balance of Cu(I) and Cu(II) in the reaction system, and -bromoisobutyryl bromide was the initiator, which was covalently connected to the surface of GO, while PMDETA was a ligand. The covalent connection can be observed from the FTIR spectra in Figure 6 below.
(a) (b)  Figure 6. The FTIR results prove that the active group of ATRP -Br is located on the surface of GO. The FTIR pattern of ZnAl2O4: Mn/GO-Br displays a peak at 651 cm −1 , representing C-Br, which reveals the existence of a connection between GO and the active group, -Br. The peaks at 1577 cm −1 and 1480 cm −1 correspond to the vibration of O-C=O and C-H of GO. Compared with PMMA polymerized through ATRP, the weak peak at 651 cm −1 reveals the formation of the active group, C-Br, during the polymerization from ZnAl2O4: Mn/GO-Br to the polymer chain. Figure 6b is the FTIR of ZnAl2O4: Mn/GO-Br, which is clearer to reveal the structure of the product; the peak at 3689 cm −1 refers to the -OH functional groups on GO. Peaks at 1577 cm −1 and 1480 cm −1 refer to C=O cm −1 and C=C cm −1 of GO. In conclusion, the FTIR result consists of the SI-ATRP mechanism.

Thermal Stability
The thermal stabilities of ZnAl2O4:Mn/GO-Br, PMMA, and ZnAl2O4: Mn/GO/PMMA were tested by TG-DTG. From the TG results in Figure 7, PMMA began to lose its weight at 122 ℃, and it was decomposed entirely at 434 ℃. In comparison, the polymer composite began to lose weight at 150 ℃, which is 28 ℃ improved compared with PMMA, until  Figure 6b is the FTIR of ZnAl 2 O 4 : Mn/GO-Br, which is clearer to reveal the structure of the product; the peak at 3689 cm −1 refers to the -OH functional groups on GO. Peaks at 1577 cm −1 and 1480 cm −1 refer to C=O cm −1 and C=C cm −1 of GO. In conclusion, the FTIR result consists of the SI-ATRP mechanism.

Thermal Stability
The thermal stabilities of ZnAl 2 O 4 :Mn/GO-Br, PMMA, and ZnAl 2 O 4 : Mn/GO/PMMA were tested by TG-DTG. From the TG results in Figure 7, PMMA began to lose its weight at 122 • C, and it was decomposed entirely at 434 • C. In comparison, the polymer composite began to lose weight at 150 • C, which is 28 • C improved compared with PMMA, until 330 • C, and the composite lost 54% of its weight. The polymer composite ZnAl 2 O 4 : Mn/GO/PMMA was removed at 443 • C, which is 9 • C higher than for PMMA. Moreover, for the initiator, ZnAl 2 O 4 : Mn/GO-Br, the weight loss began at 127 • C, and it was decomposed completely at 443 • C. After the completion, it could be found that the thermal stability of polymer composite was a little bit higher than for pure PMMA but not evidently due to the limited addition of GO. The additive does not sacrifice the thermal stability of PMMA.

Molecular Weight
GPC recorded the molecular weight of PMMA and ZnAl 2 O 4 : Mn/GO/PMMA with different concentrations of phosphor, and the tested polymer and polymer composites were dissolved in THF. Figure 8a shows the PMMA trace of GPC, and Figure 8b,c show the polymer traces of PMMA composites with different concentrations of phosphor. Figure 8b,c have two peaks, where the left peak should belong to polymer chains with a larger molecular weight, and the right peak should refer to the initiator with a small polymer molecular weight of around 500. 330 ℃ , and the composite lost 54% of its weight. The polymer composite ZnAl2O4: Mn/GO/PMMA was removed at 443 ℃, which is 9 ℃ higher than for PMMA. Moreover, for the initiator, ZnAl2O4: Mn/GO-Br, the weight loss began at 127 ℃, and it was decomposed completely at 443 ℃. After the completion, it could be found that the thermal stability of polymer composite was a little bit higher than for pure PMMA but not evidently due to the limited addition of GO. The additive does not sacrifice the thermal stability of PMMA.

Molecular Weight
GPC recorded the molecular weight of PMMA and ZnAl2O4: Mn/GO/PMMA with different concentrations of phosphor, and the tested polymer and polymer composites were dissolved in THF. Figure 8a shows the PMMA trace of GPC, and Figure 8b,c show the polymer traces of PMMA composites with different concentrations of phosphor. Figure 8b,c have two peaks, where the left peak should belong to polymer chains with a larger molecular weight, and the right peak should refer to the initiator with a small polymer molecular weight of around 500.
The results are shown in Table 1 below. The number-average molecular weight (Mn) of PMMA is 56,720, the mass average molar mass (Mw) is 102,060, the molecular ion peak (MP) is 86,040, and the average molecular weight (Mz) is 178,120. Therefore, the dispersity can be calculated by Mw/Mn = 1.80. The dispersity of the ATRP PMMA is 1.80. The dispersity may be caused by the long polymerizing time of 48 h. In order to obtain a narrow dispersity, less reaction time is needed to control the active group number. Another possibility is the proportion between the initiator and monomers-the larger the proportion, The results are shown in Table 1 below. The number-average molecular weight (M n ) of PMMA is 56,720, the mass average molar mass (M w ) is 102,060, the molecular ion peak (M P ) is 86,040, and the average molecular weight (M z ) is 178,120. Therefore, the dispersity can be calculated by M w /M n = 1.80. The dispersity of the ATRP PMMA is 1.80. The dispersity may be caused by the long polymerizing time of 48 h. In order to obtain a narrow dispersity, less reaction time is needed to control the active group number. Another possibility is the proportion between the initiator and monomers-the larger the proportion, the wider the molecular weight dispersity due to a large number of active groups. The GPC results of polymer composites show that the M w is 377,940, which is 3.70 times that of pure PMMA with a narrow dispersity of 1.13. The difference in the initiator quantity causes this result. During the fabrication of PMMA, the initiator is only α-bromoisobutyryl bromide. The proportion of initiator and monomer is definite. However, when the initiator is transferred to the surface of GO, the proportion is not 100%, which leads to fewer initiators during the polymerization of polymer composites. Therefore, the proportion between initiator and monomer is less than for pure PMMA. Moreover, the dispersity is narrower than for pure PMMA due to the same reason. The narrow dispersity of polymer composites caused the homogeneous properties of polymer composites. causes this result. During the fabrication of PMMA, the initiator is only -bromoisobutyryl bromide. The proportion of initiator and monomer is definite. However, when the initiator is transferred to the surface of GO, the proportion is not 100%, which leads to fewer initiators during the polymerization of polymer composites. Therefore, the proportion between initiator and monomer is less than for pure PMMA. Moreover, the dispersity is narrower than for pure PMMA due to the same reason. The narrow dispersity of polymer composites caused the homogeneous properties of polymer composites.    Figure 9 shows the PL spectra of ZnAl 2 O 4 :Mn annealed in air at different temperatures from 1300 • C to 1500 • C. Earlier research reported that Mn 2+ behaves as an activator in the tetrahedral site of AB 2 O 4 structural compounds [30]. The same results are shown in Figure 9 below. With the increase in exciting resource wavelength, a stronger green light was excited, with the exciting peaks at around 510 nm showing the process of electron-hole pair recombination from the 4 T 1 to 6 A 1 state of Mn 2+ ion located in the tetrahedral site of Zn 2+ within the spinel structure of ZnAl 2 O 4 with a low crystal field strength.

Photoluminescent Properties
hole pair recombination from the 4 T1 to 6 A1 state of Mn 2+ ion located in the tetrahed of Zn 2+ within the spinel structure of ZnAl2O4 with a low crystal field strength.
As shown in Figure 9d, when the sintering temperature ranged from 1100 ℃ ℃, the excited green light became weak, and a stronger red light was excited. At a temperature, Mn 2+ was doped into a rich-Al2O3 structure in ZnAl2O4 compounds, was proved by XRD. With the quality increase in the Al2O3 structure, Mn 2+ could a stronger red light. Furthermore, with the increase in annealing temperature, oxida Mn 2+ to Mn 4+ occurred, as mentioned in XRD. Mn 4+ can replace Al 3+ in ZnAl2O4 comp at high temperatures due to its similar radius and activate the red luminescence emission centre [10]. As shown in Figure 9d, when the sintering temperature ranged from 1100 • C to 1500 • C, the excited green light became weak, and a stronger red light was excited. At a higher temperature, Mn 2+ was doped into a rich-Al 2 O 3 structure in ZnAl 2 O 4 compounds, which was proved by XRD. With the quality increase in the Al 2 O 3 structure, Mn 2+ could activate stronger red light. Furthermore, with the increase in annealing temperature, oxidation of Mn 2+ to Mn 4+ occurred, as mentioned in XRD. Mn 4+ can replace Al 3+ in ZnAl 2 O 4 compounds at high temperatures due to its similar radius and activate the red luminescence as the emission centre [10].
When the concentration of Mn rises from 3% to 5%, as Figure 10 shows, the emission is reduced, which is caused by the quenching of photoluminescence. In the structure, the Mn ion is located in the Zn ion tetrahedral coordination position in the ZnAl 2 O 4 spinel structure. However, in host crystals, over-adding Mn ions will lead to inhomogeneous distribution from the emission centre [11].
When the concentration of Mn rises from 3% to 5%, as Figure 10 is reduced, which is caused by the quenching of photoluminescence. Mn ion is located in the Zn ion tetrahedral coordination position in structure. However, in host crystals, over-adding Mn ions will lead distribution from the emission centre [11]. To analyze the red and green light-emittance of the photolumine and 679 nm were selected as the monitoring wavelength to test the P of ZnAl2O4: Mn annealed at 1400 ℃ was recorded by monitoring t band. The result shows three prominent peaks at 387 nm, 427 nm, an consistent with the electron transition of Mn 2+ in the host lattice 4 A1(G)/ 4 E(G) and 4 T2(G), respectively [30]. Additionally, from PLE m the 320 peak refers to the electron transition of Mn 2+ from 6 A1 to 4 E(D) nm refers to the electron transition of Mn 4+ . Figure 11a shows the most substantial peak at 427 nm. When 427 an excitation source, strong peaks at around 519 nm were excited, sh hole pair recombination process from the 4 T1 to 6 A1 state of the Mn 2 was selected as the exciting source, a peak at 679 nm was recorded, re bination of Mn 4+ .The results are consistent with literature [10,11,30]. To analyze the red and green light-emittance of the photoluminescent agents, 510 nm and 679 nm were selected as the monitoring wavelength to test the PLE spectra. The PLE of ZnAl 2 O 4 : Mn annealed at 1400 • C was recorded by monitoring the 510 nm emission band. The result shows three prominent peaks at 387 nm, 427 nm, and 455 nm, which are consistent with the electron transition of Mn 2+ in the host lattice from 6 A 1 to 4 T 2 (D), 4 A 1 (G)/ 4 E(G) and 4 T 2 (G), respectively [30]. Additionally, from PLE monitored by 679 nm, the 320 peak refers to the electron transition of Mn 2+ from 6 A 1 to 4 E(D), and the peak at 467 nm refers to the electron transition of Mn 4+ . Figure 11a shows the most substantial peak at 427 nm. When 427 nm was selected as an excitation source, strong peaks at around 519 nm were excited, showing the electron-hole pair recombination process from the 4 T 1 to 6 A 1 state of the Mn 2+ ion. When 320 nm was selected as the exciting source, a peak at 679 nm was recorded, referring to the recombination of Mn 4+ .The results are consistent with literature [10,11,30]. Figure 12 shows the PL spectra of the phosphor, ZnAl 2 O 4 :Mn/GO, which was modified by initiating the active group and the polymer composite, ZnAl 2 O 4 :Mn/GO/PMMA, with different concentrations of phosphors which were fabricated through SI-ATRP. Figure 12a shows the PL spectra of the modified phosphor excited by different wavelengths. From the picture, it can be observed that due to the reduction of GO during the initiating active group transfer reaction, the photoluminescent quenching from graphene oxide was avoided. If there are too many oxidation groups on the surface of GO, the excited electrons will be transferred to the surface GO instead of going through the luminescent process. Figure 12b shows the PL spectra of polymer composites with different concentrations of phosphors and pure PMMA. There is an apparent green light and red light excited by the source with a wavelength of 420 nm from ZnAl 2 O 4 :Mn/GO compared with PMMA. Additionally, with the increase in phosphor concentration, the excited green light and red light become more obvious. Figure 12c shows the PL spectrum of ZnAl 2 O 4 : Mn/GO/PMMA excited by 380 nm, and in this spectrum, both green light and red light were excited. The results show that the photoluminescent property of polymer composites is related to the concentration of phosphors. the 320 peak refers to the electron transition of Mn from A1 to E(D), and the peak at 467 nm refers to the electron transition of Mn 4+ . Figure 11a shows the most substantial peak at 427 nm. When 427 nm was selected as an excitation source, strong peaks at around 519 nm were excited, showing the electronhole pair recombination process from the 4 T1 to 6 A1 state of the Mn 2+ ion. When 320 nm was selected as the exciting source, a peak at 679 nm was recorded, referring to the recombination of Mn 4+ .The results are consistent with literature [10,11,30].  Figure 12 shows the PL spectra of the phosphor, ZnAl2O4:Mn/GO, which was modified by initiating the active group and the polymer composite, ZnAl2O4:Mn/GO/PMMA, with different concentrations of phosphors which were fabricated through SI-ATRP. Figure 12a shows the PL spectra of the modified phosphor excited by different wavelengths. From the picture, it can be observed that due to the reduction of GO during the initiating active group transfer reaction, the photoluminescent quenching from graphene oxide was avoided. If there are too many oxidation groups on the surface of GO, the excited electrons will be transferred to the surface GO instead of going through the luminescent process. Figure 12b shows the PL spectra of polymer composites with different concentrations of phosphors and pure PMMA. There is an apparent green light and red light excited by the source with a wavelength of 420 nm from ZnAl2O4:Mn/GO compared with PMMA. Additionally, with the increase in phosphor concentration, the excited green light and red light become more obvious. Figure 12c shows the PL spectrum of ZnAl2O4: Mn/GO/PMMA excited by 380 nm, and in this spectrum, both green light and red light were excited. The results show that the photoluminescent property of polymer composites is related to the concentration of phosphors.

Hardness Testing
The hardness of the pure polymerized PMMA and the photoluminescent polymer composites were tested through the Vickers hardness method. Figure 13 shows two examples of the indentation of PMMA and PMMA composites after the Vickers hardness experiment. Each species was tested for 20 points. The average Vickers hardness of PMMA is 51.42 HV, while the average Vickers hardness of polymer composites is 49.62 HV. From the figure, it could be found that with the addition of ZnAl 2 O 4 :Mn/GO, the hardness reduces slightly. The addition of ZnAl 2 O 4 :Mn/GO does not reduce the hardness much compared with pure PMMA, so the addition of phosphors does not sacrifice the hardness of the material.
source with a wavelength of 420 nm from ZnAl2O4:Mn/GO compared with PMMA. Additionally, with the increase in phosphor concentration, the excited green light and red light become more obvious. Figure 12c shows the PL spectrum of ZnAl2O4: Mn/GO/PMMA excited by 380 nm, and in this spectrum, both green light and red light were excited. The results show that the photoluminescent property of polymer composites is related to the concentration of phosphors.

Hardness Testing
The hardness of the pure polymerized PMMA and the photoluminescent polymer composites were tested through the Vickers hardness method. Figure 13 shows two examples of the indentation of PMMA and PMMA composites after the Vickers hardness experiment. Each species was tested for 20 points. The average Vickers hardness of PMMA is 51.42 HV, while the average Vickers hardness of polymer composites is 49.62 HV. From the figure, it could be found that with the addition of ZnAl2O4:Mn/GO, the hardness reduces slightly. The addition of ZnAl2O4:Mn/GO does not reduce the hardness much compared with pure PMMA, so the addition of phosphors does not sacrifice the hardness of the material.

Hardness Testing
The hardness of the pure polymerized PMMA and the photoluminescent polymer composites were tested through the Vickers hardness method. Figure 13 shows two examples of the indentation of PMMA and PMMA composites after the Vickers hardness experiment. Each species was tested for 20 points. The average Vickers hardness of PMMA is 51.42 HV, while the average Vickers hardness of polymer composites is 49.62 HV. From the figure, it could be found that with the addition of ZnAl2O4:Mn/GO, the hardness reduces slightly. The addition of ZnAl2O4:Mn/GO does not reduce the hardness much compared with pure PMMA, so the addition of phosphors does not sacrifice the hardness of the material.

Conclusions
In this research, homogeneous ZnAl2O4:3% Mn/GO/PMMA polymer composites with narrow dispersity (1.13) were successfully synthesized through SI-ATRP. The polymer composites have green and red dual light-emitting properties when excited by 400 nm, which can be used for LED lampshades. Compared with widely used physical methods, this research fabricated controllable molecular weight and homogeneous polymer composites with narrow dispersity. Mn-doped ZnAl2O4 could be excited as green and red

Conclusions
In this research, homogeneous ZnAl 2 O 4 :3% Mn/GO/PMMA polymer composites with narrow dispersity (1.13) were successfully synthesized through SI-ATRP. The polymer composites have green and red dual light-emitting properties when excited by 400 nm, which can be used for LED lampshades. Compared with widely used physical methods, this research fabricated controllable molecular weight and homogeneous polymer composites with narrow dispersity. Mn-doped ZnAl 2 O 4 could be excited as green and red dual light with one wavelength and could be synthesized under relatively simple conditions. In addition, ZnAl 2 O 4 :Mn is environmentally friendly and economical, which is a promising replacement for photoluminescent material of rare-earth metal ion-doped materials. GO is used as the bridge between phosphors and polymer chains because the polymerization is initiated by the Br on the GO surface. The existence of GO could also improve the mechanical properties of polymer composites. PMMA is a widely used polymer material with good optical properties. After polymerization with phosphor, the narrow-dispersed polymer composites can be machined directly on the surface without coating. This will give the polymer composite materials commercial potential in plant factories and greenhouses.