Preparation of ZnGa2O4 nanoflowers and their full-color luminescence properties

Gallate material, a luminescent matrix with excellent performance is normally prepared by vapor deposition or solid phase sintering method at high temperature. However, it has not been solved to prepare gallate-based fluorescent materials with full-color luminescent properties at low temperature. In this paper, ZnGa2O4 undoped or doped with Cr or Mn nanoflowers composed of nanosheet-level structure were prepared by hydrothermal method at low temperature. Under ultraviolet light irradiation, ZnGa2O4, ZnGa2O4:Mn2+ and ZnGa2O4:Cr3+ display three primary colors of blue, green and red luminescence through self-excitation, Mn2+ and Cr3+ excitation respectively. The solid fluorescence yields of blue, green, and red colors are 32.3, 36.5, and 40.7%, respectively. It is highly expected to be applied to color display, biological imaging, white light devices.


Sample characterization.
The phase structure of the sample was determined at room temperature by X-ray powder diffraction analyzer (Rigaku D/Max 2200PC, graphite monochromator filter, Cu Kα radiation, λ = 0.1542 nm) with the condition of tube voltage 40 kV, the tube current 20 mA, the scanning range 10°-80° (2θ) and the scanning speed 10 min −1 .The morphology and microstructure of the product were characterized by transmission electron microscope (JEM-100CXII, accelerating voltage 80 kV), high resolution transmission electron microscope (Philips Tecnai 20U-TWIN, accelerating voltage 200 kV) and scanning electron microscope (FE-SEM, S-4800, Hitachi, accelerating voltage 5 kV).X-ray photoelectron spectrometer (PHI-5300 ESCA spectrometer, Perkin Elmer, Al Kα as excitation light source) was used to analyze the surface properties of the samples.Before the spectrogram analysis, the electron binding energies of all elements were corrected with the C 1s peak at 284.6 eV as reference.Photoluminescence (PL) and fluorescence lifetime of samples were measured by Agilent Cary Eclipse Fluorescence Spectrometer.The UV-Vis absorption spectrum of the sample at room temperature was tested by Agilent Cary Series UV-VIS spectrometer, and BaSO 4 was used as baseline correction before the test.The infrared spectrum of the sample was tested by NICOLET FT-IR spectrometer, and the KBr was used as the background.

Results and discussion
Phase structure and morphology characteristics.ZnGa 2 O 4 is a bimetallic oxide composed of ZnO and Ga 2 O 3 , with Fd-3m space group symmetry, a = b = c = 8.335, and spinel structure with chemical formula AB 2 O 4 , in which Zn 2+ occupying tetrahedral center, Ga 3+ occupying octahedral center 21 , as shown in Fig. 1a.Usually, the charge imbalance caused by the introduction of impurity ions is unfavorable to the luminous intensity of luminophores, so higher energy is needed to eliminate the charge imbalance, such as calcination at high temperature for a long time 22 .The effective compensation factor φ when ions are substituted was calculated according to the formula φ = Z/r, in which Z is the charge number of ions and r is the effective radius of ions 23 .The greater the difference of φ, the more difficult it is to substitute ions.As shown in Table S1, for hexa-coordinate substitution of Mn 2+ , the effective compensation factor φ is 2.41, which is much lower than that of Ga 3+ 4.83, so it is difficult for Mn 2+ to replace Ga 3+ , while easy to replace Zn 2+ due to the small difference of effective compensation factor 11,17 .Furthermore, in the substitution reaction of ZnGa 2 O 4 , ions with similar effective radii are easy to be substituted with each other, that is, Mn 2+ replaces Zn 2+ to produce four coordinate substitutions, and Cr 3+ replaces Ga 3+ to produce six coordinate substitution 17,24 .
The SEM photos of the three nanomaterials (ZnGa 2 O 4 , ZnGa 2 O 4 :Mn 2+ and ZnGa 2 O 4 :Cr 3+ ) with different magnification is shown in Fig. 1b-g.They all present a flower-like nanostructure of about 5 μm, which is composed of multiple nanosheet-level substructures with a thickness of 6-10 nm interspersed together.The difference between the three samples is that the nanosheets composed of ZnGa 2 O 4 are thicker and the degree of curling of the nanosheets is smaller, while the nanosheets doped with Mn 2+ and Cr 3+ are thinner, and the nanosheets are freely curled to form spherical nanoflowers.As shown in Fig. S1.Zn, Ga, O and the corresponding doped elements Mn and Cr are uniformly distributed in the flower-like nanostructures confirmed by the element distribution surface scans.The flower-like structure of ZnGa 2 O 4 , ZnGa 2 O 4 :Mn 2+ and ZnGa 2 O 4 :Cr 3+ can also be seen from the TEM photos in Fig. 1h-j.Each flower is composed of the sub-structure of nano-flakes, and the size of nano-flowers is about 5 μm.From the contrast of the electron microscope photos, it can be clearly seen that the nano-flakes constituting the flower-like structure become thinner in turn, which is similar to that of Scanning electron microscope (SEM).The phenomenon may be due to the impurity ions adsorbed on the initial grain surface during hydrothermal process, which inhibiting the crystallization of the material to some extent, preventing the grain growth in some directions, and resulting in the formation of thinner nanosheets.High-resolution photos and corresponding selective electron diffraction photos of ZnGa 2 O 4 are shown in Fig. 1k.The lattice spacing 0.44 nm is corresponding to the (200) crystal plane of ZnGa 2 O 4 , and the selective electron diffraction photo clearly shows single crystal structure of a ZnGa 2 O 4 nanoflake.In the single crystal structure of the sheet, the diffraction points correspond to (200) and (111) crystal plane.The direction of the crystal zone axis is confirmed to [011] by calculating, therefore the exposed surface of the nanoplate is (110) plane.
As shown in Fig. 2a the diffraction peaks of ZnGa 2 O 4 , ZnGa 2 O 4 :Mn 2+ and ZnGa 2 O 4 :Cr 3+ all correspond to the diffraction peaks of standard card JCPDS38-1240 ZnGa 2 O 4 , which are cubic spinel structure.After doping Mn 2+ and Cr 3+ , the intensity of the diffraction peak decreases, and the corresponding FWHM (full width at half maximum) increases in sequence.The results of X-ray diffractometer (XRD) analysis also indicate that the secondary structure nanosheets that make up the nanoflowers become thinner in turn after doping Mn 2+ and Cr 3+ , which consistent precisely with the observation results of SEM and transmission electron microscopy (TEM).As shown in Fig. 2b-e the electron binding energies of 2p 1/2 orbitals and 2p 3/2 orbitals of Ga are located at 1143.6 eV and 1116.8 eV, respectively.And the electron binding energies of 2p 1/2 orbitals and 2p 3/2 orbitals of Zn are located at 1043.8 eV and 1021.1 eV, respectively.The characteristic peaks of electron binding energies located at 654.4 eV and 645.5 eV belong to 2p 1/2 orbitals and 2p 3/2 orbitals of Mn, respectively and manganese ions show +2 valences.The characteristic peaks of electron binding energy at 586.5 eV and 576.5 eV respectively belong to 2p 1/2 orbitals and 2p 3/2 orbitals of Cr and chromium ions show +3 valences.The intensity of these characteristic peaks is small due to the low contents of Mn and Cr.
Infrared and ultraviolet spectral characteristics.In order to understand the chemical composition of the sample, the Fourier transform infrared spectrum is conducted.The Fourier Transform infrared spectroscopy (FT-IR) of ZnGa 2 O 4 , ZnGa 2 O 4 :Mn 2+ and ZnGa 2 O 4 :Cr 3+ in Fig. S2 also indicate the samples were binary metal oxides consisting of Zn-O and Ga-O groups.As showed in Fig. S2, the broad absorption peak at the 3445 cm −1 wavelength belongs to the stretching vibration of O-H and N-H.The stretching vibration of N-H may come from the residual ethylenediamine in the sample, but there is no obvious stretching vibration peak of it near 1190 cm −1 .Therefore, the residual ethylenediamine may be very small and can be ignored after repeated cleaning of water and anhydrous ethanol.In the fingerprint region at low wavelength, the larger peaks of 585 cm −1 and 425 cm −1 are attributed to the vibration absorption of Zn-O and Ga-O, respectively.Through the infrared spectrum analysis, no other vibration peaks are observed except Zn-O and Ga-O in the sample, so it is determined that the sample is a binary metal oxide composed of Zn-O group and Ga-O group.The UV-vis absorption spectra of ZnGa 2 O 4 , ZnGa 2 O 4 :Mn 2+ and ZnGa 2 O 4 :Cr 3+ samples are shown in Fig. S3.It can be seen from the figure that the absorption regions of the three samples are basically the same, and there is only absorption in the region smaller than 350 nm.It is also confirmed that ZnGa 2 O 4 can only be excited at wavelengths less than 350 nm.However, in the 300-350 nm wavelength region, compared with the absorption peak of ZnGa 2 O 4 , the absorption of ZnGa 2 O 4 :Mn 2+ and ZnGa 2 O 4 :Cr 3+ increases slightly, which may be due to the absorption of a small amount of Mn and Cr itself, because the amount of Mn 2+ and Cr 3+ is very small, only 0.4% and 1%, so the absorption of these two elements is also very weak.
Luminescent properties.The excitation and emission spectra of ZnGa 2 O 4 , ZnGa 2 O 4 :Mn 2+ and ZnGa 2 O 4 :Cr 3+ are shown in Fig. 3a-c.For undoped ZnGa 2 O 4 , three peaks can be seen in the excitation spectrum, which are located at 226 nm 239 nm and 257 nm, respectively.These excitation peaks are caused by the charge transfer from O 2− to octahedral center Ga 3+ and the ultraviolet absorption of ZnGa 2 O 4 itself 25 .The emission spectrum obtained using 226 nm as the excitation wavelength is a broad peak with the highest peak of 456 nm in the range of 340-750 nm.This broad emission peak is in all probability caused by the self-excitation of Ga-O hexahedron in the spinel structure.The luminescent properties of fluorescent host materials are usually changed by introducing impurity ions 26 , that is also applicable to ZnGa 2 O 4 host materials.Mn 2+ -doped ZnGa 2 O 4 has green fluorescence emission, as shown in Fig. 3b.Except for the charge transfer from O 2-to Ga 3+ in the octahedral center and the ultraviolet absorption of ZnGa 2 O 4 itself, the absorption of Mn 2+ excites a red shift of 32 near 300 nm, which is consistent with the analysis of ultraviolet-visible absorption spectrum in Fig. S3.Due to the activation of Mn 2+ , the emission spectrum with the highest emission peak of 505 nm is located in the range of 470-600 nm with the excitation wavelength of 226 nm.
After enlarged locally as shown in Fig. S4, the five smaller excitation peaks located between 351-443 nm and centered at 351 nm, 379 nm, 410 nm, 422 nm and 443 nm respectively correspond to 5 A 1 -4 E, 6 A 1 -4 T 2 , 6 A 1 -4 A 1 , 4 E 1 and 6 A 1 -4 T of Mn 2+ . 27When excited at 226 nm, ZnGa 2 O 4 :Mn 2+ has green emission at 505 nm, which belongs to the 4 T 1 -6 A 1 d orbital electron forbidden transition of Mn 2+11,17 .This is the process of energy transfer from ZnGa 2 O 4 matrix to Mn 2+27 .The transition process of 4 T 1 -6 A 1 of Mn 2+ is accompanied by strong 3d shell lattice vibration coupling, and is affected by crystal field and symmetric sites.If Mn 2+ is in a weak crystal field, i.e. tetrahedron, the splitting of excitation energy will be weak, which will be accompanied by high energy emission, that is, green light and if Mn 2+ is in a strong crystal field, i.e., octahedron, it will emit yellow or red light 17,27 , which is consistent with our previous analysis of crystal structure.In our investigation, Mn 2+ replaces Zn 2+ with similar ionic radius in cubic ZnGa 2 O 4 matrix to generate tetrahedral coordination and emit green light, which is completely consistent with the test results of fluorescence spectrum.
The excitation and emission spectra of ZnGa 2 O 4 :Cr 3+ are shown in Fig. 3c.There is a wide excitation peak between 200-350 nm, including four intensity excitation peaks (226 nm, 240 nm, 257 nm, 266 nm), which related to the charge transfer transition of O 2− to the octahedral center Ga 3+ and the absorption transition with the belt, and the excitation spectrum of 300-350 nm is caused by the absorption of Cr 3+ .A red emission peak at 696 nm was obtained by excitation at 226 nm, which was attributed to the 2 E-4 A 2 characteristic transformation of Cr 3+28 .Meanwhile, a similar red emission peak at 696 nm was obtained by excitation at 416 nm and 572 nm.These two excitation peaks at 416 nm and 572 nm are caused by d-d electron-electron transitions of Cr 3+24,29 , corresponding to the 4 A 2 -4 T 1 and 4 A 2 -4 T 2 characteristic transitions of Cr 3+ , respectively 30 .
Undoped, Mn 2+ doped and Cr 3+ doped ZnGa 2 O 4 have blue, green and red emission properties under ultraviolet (UV) excitation, respectively.The excitation wavelengths used in the emission spectra of ZnGa 2 O 4 , ZnGa 2 O 4 :Mn 2+ and ZnGa 2 O 4 :Cr 3+ are all 226 nm, indicating that the hetero ions in ZnGa 2 O 4 :Mn 2+ and ZnGa 2 O 4 :Cr 3+ can effectively enter the lattice of ZnGa 2 O 4 under hydrothermal conditions to replace Zn 2+ and Ga 3+ to form tetrahedral and octahedral coordination, respectively.This is related to the addition of an appropriate amount of ethylenediamine during hydrothermal.Ethylenediamine aqueous solution is a strongly alkaline solution, which plays an effective role in promoting the crystallization of materials and the entry of hetero ions into the crystal lattice of the matrix under the condition of hydrothermal high temperature and high pressure.We also conduct experiment keeping other conditions remaining the same without ethylenediamine in the synthesis.The obtained ZnGa 2 O 4 doped with Mn 2+ and Cr 3+ does not show green and red emission properties after UV excitation, indicating that it is difficult for hydrothermal hetero ions to enter into the lattice of ZnGa 2 O 4 matrix under the condition of non-strong alkaline solvent.
The fluorescence attenuation curves of ZnGa 2 O 4 , ZnGa 2 O 4 :Mn 2+ and ZnGa 2 O 4 :Cr 3+ are fitted exponentially as shown in Fig. 3d.The attenuation curves of the three samples are all fitted by double exponents, which are in accordance with the formula.
where I is the fluorescence intensity when the time is t, I 1 and I 2 are fitting constants, and τ 1 and τ 2 are fluorescence lifetime.After fitting, each sample corresponds to two millisecond lifetimes, a shorter lifetime τ 1 and a relatively longer life τ 2 .The specific fitting parameters are shown in Table S2.The longer lifetimes of each sample of ZnGa 2 O 4 , ZnGa 2 O 4 :Mn 2+ and ZnGa 2 O 4 :Cr 3+ correspond to the self-excitation of Ga-O in the bulk phase of luminescent materials, the 4 T 1 -6 A 1 transition of Mn 2+ and the 2 E-4 A 2 transition of Cr 3+ , respectively.The short lifetime of the three samples is due to the fact that the surface effect of the materials has a great influence on the luminescence lifetime.These materials are all composed of ultra-thin 6-10 nm nanosheets with large surface area, and the increase of surface atoms leads to the appearance of more activated ions on the surface of the nanosheets.However, the impurities, unsaturated bonds, vacancies and other surface defects on the surface of the nanoparticles will quenched the activated ions and lead to radiation-free transition, thus shortening the life of the activated ions. 8xcited by 254 nm's handheld UV lamp, ZnGa color including white in the triangular area connected by the three points can be obtained by changing the ratio of the three luminescent materials.
In order to demonstrate whether there is white luminescence phenomenon when mixing the three samples, we conducted packaged white LED luminescence testing on their mixture.As shown in Fig. 5a, the encapsulated white LED device and the emitting color are shown.The encapsulated white LED prepared has a correlated colour temperature (CCT) of 26702K and a color rendering index (CRI) of 55.1.Within the constant voltage range of 20 mA to 120 mA and 3 V, the relationship between the luminous intensity of the encapsulated white LED and the input forward current is shown in Fig. 5b.From the color of the encapsulated white LED device and the CIE chromaticity diagram, we can see that the light emitted by this mixture is not pure white light, but a cyanish white-like light.According to the emission spectrum of the mixture, we speculate that the reason is  that the emission wavelength of red light is mostly in the invisible near-infrared region after 650 nm, and there is an empty window in the wavelength range of 600-650 nm, resulting in the emission color of encapsulated white LED being a cyan white light, rather than a pure white light like the results made by other phosphors 31,32 .

Conclusions
ZnGa 2 O 4 nanoflowers with single size composed of ZnGa 2 O 4 flake substructures with a thickness of 6-10 nm were synthesized by a simple hydrothermal method under the action of ethylenediamine template.The luminescence color of ZnGa 2 O 4 is controlled by doping Mn 2+ and Cr 3+ .Under UV excitation, undoped ZnGa 2 O 4 , ZnGa 2 O 4 doped with Mn 2+ and Cr 3+ have blue, green and red emission properties, respectively.The luminescence properties of the same matrix material with three primary colors are obtained.The blue, green and red fluorescence comes from the self-excited electron transfer of Ga-O itself and the 3d electron energy transfer of Mn 2+ and Cr 3+ .The three samples are mixed to do the encapsulated white LED luminescence test, which has the phenomenon of cyan white luminescence.These ZnGa 2 O 4 -based fluorescent nanomaterials are expected to be used in color display, biological imaging and white light devices.

Figure 5 .
Figure 5. (a) Encapsulated white LED devices and emitting colors.(b) The relationship between the luminous intensity of packaged white LED and the input forward current.