A comparative study of ZnAl2O4 nanoparticles synthesized from different aluminum salts for use as fluorescence materials

Three ZnAl2O4 samples were prepared via a modified polyacrylamide gel method using a citric acid solution with different aluminum salt starting materials, including AlCl3∙6H2O, Al2(SO4)3∙18H2O, and Al(NO3)3∙9H2O under identical conditions. The influence of different aluminum salts on the morphologies, phase purity, and optical and fluorescence properties of the as-prepared ZnAl2O4 nanoparticles were studied. The experimental results demonstrate that the phase purity, particle size, morphology, and optical and fluorescence properties of ZnAl2O4 nanoparticles can be manipulated by the use of different aluminum salts as starting materials. The energy bandgap (Eg) values of ZnAl2O4 nanoparticles increase with a decrease in particle size. The fluorescence spectra show that a major blue emission band around 400 nm and two weaker side bands located at 410 and 445 nm are observed when the excitation wavelength is 325 nm. The ZnAl2O4 nanoparticles prepared from Al(NO3)3∙9H2O exhibit the largest emission intensity among the three ZnAl2O4 samples, followed in turn by the ZnAl2O4 nanoparticles prepared from Al2(SO4)3∙18H2O and AlCl3∙6H2O. These differences are attributed to combinational changes in Eg and the defect types of the ZnAl2O4 nanoparticles.

Scientific RepoRts | 5:12849 | DOi: 10.1038/srep12849 smaller particles have a relatively larger specific surface area, and therefore have a larger amount of dangling and unsaturated bonds on the particle surface. This in turn affects the defect levels and fluorescence properties of the powder 16 . However, the main disadvantage of preparing spinel ZnAl 2 O 4 by the traditional synthesis routes, such as the co-precipitation approach, the solid-state reaction method, and others, is the large particle size of the product.
The polyacrylamide gel route is a very good sol-gel method for the preparation of superfine nanoparticles 17 . Appropriate selection of a chelating agent, monomer systems, initiator, pH value, and sintering temperature can significantly improve the quality of the prepared nanoparticles 17 . In addition, different aluminum salts, i.e. different anionic species in the precursor solutions, can greatly influence the morphology, phase purity, and optical and fluorescence properties of the ZnAl 2 O 4 . However, most previously reported studies have only used a single aluminum salt as a starting material and have not investigated the influences of different aluminum salts on the morphology, structure, and optical and fluorescence properties of the obtained ZnAl 2 O 4 .
In this study, three different aluminum salts are used as starting materials to synthesize three ZnAl 2 O 4 gels via a polyacrylamide gel route, specifically aqueous solutions of citric acid with Al 2 (SO 4 ) 3 •18H 2 O, AlCl 3 •6H 2 O, or Al(NO 3 ) 3 •9H 2 O were used under identical conditions. In order to obtain superfine nanoparticles, N,N'-methylene-bisacrylamide was used as a cross-linking agent, and glucose was used to prevent gel collapse. After sintering these xerogels, three ZnAl 2 O 4 nanostructure samples were obtained. Their phase purity, morphologies, and optical and fluorescence properties were then characterized and compared. The objective of the present work is to investigate the influence of different aluminum salt starting materials on the resulting ZnAl 2 O 4 nanostructures and on their optical and fluorescence properties.

Results
The obtained ZnAl 2 O 4 xerogels decomposed into products after being sintered at 600 °C for 5 h in air. Figure 1 shows the XRD patterns of ZnAl 2 O 4 nanoparticles prepared from (S1) Al 2 (SO 4 ) 3  The mean grain size of samples S1, S2, and S3 were quantitatively evaluated based on the line broadening of the (220), (311), (511), and (440) peaks using the Scherrer formula, to be 13, 16, and 24 nm, respectively. XRD results indicate that the choice of the aluminum salts also has an influence on the phase purity of the final product. A possible reason for the formation of impurity phases when using citric acid as a chelating agent is that citric acid has a relatively weak coordinating capacity toward the metal ion of Al(NO 3 ) 3 •9H 2 O, and hence the formed metal complexonate is not expected to be highly stable. Fourier transform infrared (FT-IR) spectra of the ZnAl 2 O 4 nanoparticles prepared from (S1) Al 2 (SO 4 ) 3 23,24 , respectively. Figure 3 shows the TG/DTA curves of the ZnAl 2 O 4 xerogels obtained from (S1) Al 2 (SO 4 ) 3 •18H 2 O, (S2) AlCl 3 •6H 2 O, and (S3) Al(NO 3 ) 3 •9H 2 O. There are four weight loss stages observed for each sample. The first weight loss stage is seen at a low temperature range (before 200 °C) and corresponds to the evaporation of surface water in the ZnAl 2 O 4 xerogel precursors 25,26 . The second weight loss stage (around 200-250 °C) is due to the evaporation of structural water 25,26 . The third weight loss stage (between 250-400 °C) is due to the decomposition of small molecular organic compounds. The largest and final weight loss stage (around 400-620 °C) is due to decomposition of complexes, glucose, and the polyacrylamide side-chain, as well as combustion of the polyacrylamide backbone and other residues 27,28 . The total weight loss measured for the ZnAl 2 O 4 xerogel precursors were 97.31% for (S1) Al 2 (SO 4 ) 3 •18H 2 O, 95.201% for (S2) AlCl 3 •6H 2 O, and 98.234% for (S3) Al(NO 3 ) 3 •9H 2 O. In Fig. 3 (S1), the main endothermic peak appeared at around 535 °C and corresponds to the thermal decomposition of the complexes, polyacrylamide backbone, and other residues originating from Al 2 (SO 4 ) 3  nanoparticles prepared from Al 2 (SO 4 ) 3 •18H 2 O, a higher heat treatment temperature is usually needed to improve the phase purity.
To confirm whether the formation of ZnAl 2 O 4 nanoparticles prepared from Al 2 (SO 4 ) 3 •18H 2 O needed a higher heat treatment temperature, FT-IR measurements were carried out using a Bruker IFS 66 v/S spectrometer. The FT-IR spectra of the ZnAl 2 O 4 xerogel prepared form Al 2 (SO 4 ) 3 •18H 2 O and sintered at different temperatures are presented in Fig. 4. Here is can be seen that the S= O asymmetric stretching vibration (1186 cm −1 ) and S-O symmetric stretching vibration (1117 cm −1 ) peak intensities decrease with the increase of sintering temperature. This result indicates that the SO 4 2 − anion coordinates to Zn and Al cations and forms a bridged bidentate structure 23,29,30 .
In the case of the sample obtained by sintering the xerogel at 900 °C, all of the organic peaks disappear except for the H− O− H peak (1633 cm −1 ). These results indicate that the effects of aluminum salts and sintering temperature on the phase purity of ZnAl 2 O 4 cannot be neglected. Based on the subtle information gathered from the XRD and FT-IR results, the relevant reactions can be described as follows: The results indicate that the reaction (1) cannot occur at 600 °C, and that a higher sintering temperature is needed for the formation of pure ZnAl 2 O 4 nanoparticles.
In order to investigate the effects of different aluminum salts on the formation of ZnAl 2 O 4 , including the particle size and surface morphology, SEM and TEM images were collected of the ZnAl 2 O 4 samples prepared using different aluminum salts and sintered at 700 °C, and are shown in Fig. 5 and Fig. 6. The SEM images of the ZnAl 2 O 4 samples reveal that the particles are almost spherical in shape and have a narrow particle size distribution (Fig. 5(S1-S3)). When AlCl 3 •6H 2 O is added into the ZnAl 2 O 4 precursor, large particles form, however, if Al 2 (SO 4 ) 3 •18H 2 O is added into the precursor, small particles with little adhesion are observed, as shown in Fig. 5(S1-S2). Figure 6 shows (S1-S3) TEM image, (SH1-SH3) HRTEM image, and (SP1-SP3) the particle size distribution of ZnAl 2 O 4 nanoparticles prepared from (S1) Al 2 (SO 4 ) 3 •18H 2 O, (S2) AlCl 3 •6H 2 O, and Al(NO 3 ) 3 •9H 2 O. The ZnAl 2 O 4 nanoparticles are spherical in shape with a narrow particle size distribution, as shown in Fig. 6(S1-S3). The corresponding particle size distribution patterns are given in the Fig. 6 (SP1-SP3). The average particle sizes of samples S1, S2, and S3 are around 12, 18, and 24 nm, respectively. TEM results show that the particle size variation tendencies for samples S1, S2, and S3 are consistent with those calculated from XRD patterns (Fig. 1). In addition, the BET surface area of the sample decreases with the increase of particle size as shown in Table 1. Compared with sample S2, the average particle size and BET surface area of sample S1 is significantly reduced, an effect which may be due to the SO 4 2 − anion forming a bridged bidentate structure 31 . Fig. 6(S2) inset presents the SAED pattern taken from a portion of the ZnAl 2 O 4 nanoparticles shown in Fig. 6(S2). The associated electron diffraction pattern is consistent with that of pure ZnAl 2 O 4 crystals of a spinel structure, indexed as shown in Fig. 1(a)    using the Tauc relation 33 . This is shown in Fig. 7(b), where h is the Planck constant, α is the Kubelka-Munk (K-M) absorption coefficient, and ν is the frequency. The linear portions of the plots are extrapolated to the hν axis to give the values of Eg. The calculated Eg values of samples S1, S2, and S3 are 3.98 ± 0.01, 3.92 ± 0.01 and 3.22 ± 0.01 eV, respectively. Generally, the Eg values of nano-sized semiconductors increase with a decrease in particle size. In this case, the observed phenomenon is consistent with what has been previously reported.   (Fig. 8b). The three emission peaks at 400, 410, and 445 nm can be ascribed to intra band gap defects such as oxygen vacancies 34 . It has been noted that there is an obvious increase in the fluorescence intensity of the peak located at 400 nm when the Eg value decreases (see Fig. 8(a) inset).

Discussion
In order to understand the fluorescence mechanism of the prepared ZnAl 2 O 4 samples, it is necessary to propose a schematic band diagram to illustrate the process of excitation and emission for the system. Figure 9 shows a schematic band diagram for the fluorescence mechanism of ZnAl 2 O 4 samples obtained from (S1) Al 2 (SO 4 ) 3 •18H 2 O, (S2) AlCl 3 •6H 2 O, and (S3) Al(NO 3 ) 3 •9H 2 O. It is known that Al 3+ 2p orbitals and s orbitals located at the upper part of the Al 3+ 2p orbitals make up the major conduction band (CB) edge of ZnAl 2 O 4 , and the hybridization band composed of O 2− 2p and Zn 2+ 3d orbitals makes up the upper valence band (VB) 35 . When the Eg value > λ exc , (3.82 eV vs. 325 nm) it can be seen that one electron transition occurs from VB onto the intra band gap defects (IBGD) energy level ( Fig. 9(S1) and (S2)). After that, the electron will be driven by continued transition from IBGD to CB. Then, the electron on the CB drops down to the low energy level through loss of energy by vibration relaxation (VR). Finally, the electron on the low energy level undergoes a radiative recombination with a hole in the valence band, accompanied by three blue-light emissions. For sample S1, the impurity (SO 4 2− ) plays a crucial role to promote the electron transition from IL (impurity level) to CB and improve the fluorescence properties. When Eg < λ exc , one electron transition occurs from the VB to the high energy level ( Fig. 9(S3)). Then, the electron on the high energy level drops down to the CB by internal conversion. At the same time, the electron on the CB by VR drops down to the low energy level with an accompanying loss of energy. Finally, the electron on the low energy level undergoes a radiative recombination with a hole in the valence band, accompanied by a series of blue-light emissions. Figure 10 shows the Commission International De I'Eclairage (CIE) diagram of a ZnAl 2 O 4 phosphor under 325 nm laser excitations. The CIE color coordinates (x, y) of the ZnAl 2 O 4 phosphor was calculated using the fluorescence spectra. A typical CIE color coordinate of a ZnAl 2 O 4 phosphor was found to be x, y equals 0.1729, 0.0048 respectively under 325 nm laser excitations.
According to equation (7)     The fluorescence experiments revealed that the as-prepared ZnAl 2 O 4 phosphor exhibits interesting abilities for application in blue light-emitting materials. Interestingly, similar preparation methods may be employed for the synthesis of other metal oxides nanoparticles, including fluorescence materials, multiferroic materials, oxide thermoelectric materials, photocatalytic materials, solid oxide fuel cell materials, and high-temperature superconducting materials.  3 •9H 2 O are labeled sample S1, S2 and S3, respectively) were dissolved in the deionized water to obtain a final solution of 0.015 mol/L with the total cations. Starting compositions of samples S1, S2 and S3 are given in Table 2. After the solution was transparent, a stoichiometric amount of chelating agent (citric acid) was added into the solution in the mole ratio 1.5:1 with respect to the total cations (Zn 2+ and Al 3+ ) to complex the cations. After that, 20 g glucose was dissolved into the solution. Finally, the acrylamide and N, N'-methylene-bisacrylamide monomers were added into the solution. The resultant solution was heated to 90 °C on a hot plate to initiate the polymerization reaction, and a few minutes later a polyacrylamide gel was formed. The gel was dried at 120 °C for 24 h in a thermostat drier. The obtained xerogel precursor was ground into powder and some powder was sintered at 600, 700, 800, and 900 °C for 5 h in air to prepare the objective products.

Materials
Sample characterization. The ZnAl 2 O 4 xerogel precursor sintered at 600 and 700 °C were analyzed by X-ray diffractometer (DX-2700) with Cu Kα radiation. Fourier transform infrared (FTIR) spectra in the range 400-2000 cm −1 were recorded using a Bruker IFS 66 v/S spectrometer. Thermogravimetric (TG) and differential thermal analysis (DTA) analyses were performed in a SDT Q600 (TA instruments, Inc. USA) simultaneous thermal analyzer at a heating rate of 10 °C/min. The surface morphology of the synthesized ZnAl 2 O 4 sample was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The surface area of the samples were characterized by a 3H-2000BET-M instrument. The absorption spectra of the samples were examined on a Shimadzu UV-2500 UV-Visible spectrophotometer. The fluorescence spectra were collected at room temperature in a confocal Raman system using a He-Cd laser (325 nm, RGB laser system, NovaPro 30 mW, Germany).