Green Synthesis of 8‐Hydroxyquinoline Barium as Visible‐Light‐Excited Luminescent Material Using Mechanochemical Activation Method

Abstract Using high‐energy UV‐light to excite 8‐hydroxyquinoline barium (BaQ2) is a short slab for this emerging area of organic luminescent materials. However, using visible light to excite BaQ2 has not been reported. To solve this problem, this study proposes the mechanochemical activation method to synthesize luminescent material of visible‐light‐excited BaQ2. This research applies infrared spectroscopy, X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, and fluorescence spectrometry to analyze the structure and luminescence properties. XRD shows that BaQ2 has a high crystallinity, small crystalline size, and high purity. According to the Scherrer equation, the mean particle size is 56 nm. The results of fluorescence spectrometry show that the excitation spectrum of the product is red‐shifted, and the maximum excitation wavelength is 408 nm. According to these results, the product has a high fluorescence and can be excited under visible light. This research explains the high efficiency of the mechanochemical‐activation method by thermodynamic and dynamic principles. This research also exemplifies luminescence theory of BaQ2 and at the microlevel explains the theory of visible‐light‐excited theory and the principle of luminescent intensity enhancement from the point of crystallography.


FTIR Spectra Analysis
The Fourier transform infrared (FTIR) spectra of BaQ 2 , which was synthesized by mechanochemical activation method (BaQ 2 -1), the FTIR spectra of BaQ 2 , which was synthesized by liquid-phase method (BaQ 2 -2), and the FTIR spectra of the 8-hydroxyquinoline (HQ) are shown in Figure 1. Peaks in 3600-3300 cm −1 are ascribed to the fundamental stretching of OH. The OH of BaQ 2 -1 may come from water in diluent KBr. Bands around 3039 cm −1 are due to CC of 8-hydroxyquinoline ring. The absorption peaks of CC of 8-hydroxyquinoline ring are observed at 1560, 1589, 1496, and 1460 cm −1 . The peak in 1382 cm −1 is assigned to CN. Because the hydroxyquinoline ring has aromaticity, it can trigger conjugated effect of π electron. And the conjugated effect makes the CN stronger, so the peak moves to higher numerical value. The characteristic absorption peaks of BaQ 2 are observed at 659, 549, and 484 cm −1 . They are ascribed to BaO and BaN.

X-Ray Diffraction Characterization
The X-ray diffraction patterns of BaQ 2 -1 and BaQ 2 -2 are shown in Figure 2. Comparing BaQ 2 -1 with JCPDS standard card 24-1879 of HQ and BaQ 2 -1 with JCPDS standard card 26-0155 of Ba(OH) 2 . They show that BaQ 2 was synthesized successfully. Comparing BaQ 2 -1 with BaQ 2 -2, it shows that peaks of BaQ 2 -1 are higher and more speculate than BaQ 2 -2 and BaQ 2 -1 has less impurity peaks, less full width at half maxima and less peak area then BaQ 2 -2. It means that BaQ 2 -1 has higher crystallinity, smaller particle size, and higher purity than BaQ 2 -2. The results show that using mechanochemical activation method can get BaQ 2 , which has high crystallinity, small particle size, and high purity. According to Scherrer equation, the mean particle size is 56 nm. The lattice parameters of BaQ 2 -1 cubic structure calculated using JADE 8.0 program are a = b = c = 5.34 Å. These results are consistent with synthesis mechanism of mechanochemical activation method below.

Photoelectron Spectroscopy Analysis
The X-ray photoelectron spectroscopy (XPS) of survey scan of BaQ 2 -1 is shown in Figure 3a. Consulting NIST X-ray photoelectron spectroscopy database and referring the survey scan of BaQ 2 -1, we can know that BaQ 2 -1 has barium, nitrogen, carbon, and oxygen. The Auger line of Ba is on 898.0 eV and it does not shift. It shows that the valence of Ba of BaQ 2 -1 has not changed. The survey scan of BaQ 2 -1 conforms to BaQ 2 .
Global Challenges 2019, 3,1900052     The XPS spectra detail scan of BaQ 2 -1 is shown in Figure 3b-e. During fitting limit energy interval of spin coupling and spin splitting, number of pear splitting and ratio of pear areas, full width at half maxima and ratio of Gaussian-lineshape and Lorentzian-lineshape. XPS spectra of Ba 3d of BaQ 2 -1 decomposes Ba 3d into four peaks (Figure 3b). The binding energy (BE) of these four peaks are 803.59, 802.89, 788.29, and 787.59 eV, respectively. The peaks in 802.89 eV (3d 3/2) and 787.59 eV (3d 5/2) can assign to OBaO. And the peaks in 803.59 eV (3d 3/2) and 788.29 eV (3d 5/2) can assign to OBaN. XPS spectra of C 1s of BaQ 2 -1 decomposes C 1s into four peaks (Figure 3c). The BE of these four peaks are 284.62, 285.22, 286.32, and 289.12 eV. And the peaks can respectively assign to CC, CO, CO, and OCO. XPS spectra of N 1s of BaQ 2 -1 decomposes N 1s into two peaks (Figure 3d). The BE of these two peaks are 398.11 and 399.11 eV. And the peaks can respectively assign to NBa and NH. XPS spectra of O 1s of BaQ 2 -1 decomposes O 1s into three peaks (Figure 3e). The BE of these three peaks are 530.99, 532.29, and 533.39 eV, respectively. And the peaks can respectively assign to BaOC/CO, OBa/CO, and OCO. The results showed that BaQ 2 was synthesized successfully. The molecular formula and structural model of BaQ 2 -1 were shown in Figure 3f,g according to these results.

Scanning Electron Microscope (SEM) Characterization and Energy Dispersive Spectroscopy (EDS) Analyzation
The scanning electron microscope image of BaQ 2 -1 is shown in Figure 4. It shows that the morphology of BaQ 2 -1 is regular and the surface of BaQ 2 -1 is uniform and smooth. SEM shows that the morphology of single BaQ 2 -1 crystal is short clavate, in which the average diameter is about 90 nm. Some BaQ 2 -1 crystals get together as coralline. The scanning electron microscope image of BaQ 2 -2 is shown in Figure 5. SEM shows that the morphology of single BaQ 2 -2 is in irregular shape and BaQ 2 -2 assembles more loosely. The diameter of clavate crystal of BaQ 2 -2 is from 400 to 900 nm. Comparing SEM images between BaQ 2 -1 and BaQ 2 -2. Results show that using the mechanochemical activation method can get BaQ 2 , which has better crystal forms, more uniform and smooth shapes and less grain size. The results are consistent with synthesis mechanism of theoretical studies of mechanochemical activation method below. It can be seen from the EDS image ( Figure 4e) of BaQ 2 -1 that the C, N, O, and Ba elements exist in it, and the weight percentage and atomic percentage of each element are basically consistent with the chemical formula of BaQ 2 .

Fluorescence Analysis
The excitation spectra and emission spectra of BaQ 2 -1 and BaQ 2 -2 are shown in Figure 6. The excitation spectra of them is obtained by scanning the full wavelength. The excitation spectrum of BaQ 2 -1 appears in three excitation peaks at 289, 372, and 408 nm, respectively. The maximum excitation wavelength of BaQ 2 -1 is 408 nm. The excitation spectrum of BaQ 2 -2 appears in two excitation peaks at 287 and 369 nm, respectively. The maximum excitation wavelength of BaQ 2 -2 is 369 nm.
Comparing the excitation spectrum of BaQ 2 -1 and the excitation spectrum of BaQ 2 -2. It shows that from 280 to 370 nm excitation peaks of BaQ 2 -1 are higher than excitation peaks of BaQ 2 -2 and have better peak shape and less full width at half maxima. It shows that BaQ 2 -1 has better selectivity than Global Challenges 2019, 3,1900052   BaQ 2 -2. The excitation spectrum of BaQ 2 -2 begins to decline after 370 nm and decline very fast after 400 nm. The excitation spectrum of BaQ 2 -1 has a strong peak at 408 nm. It shows that BaQ 2 -1 can be excited by visible light.
The emission spectrums show that the emission spectrum of BaQ 2 -1 at 408 nm has higher luminous intensity than 370 nm. And the maximum emission wavelength of BaQ 2 -1 is 475 nm, which belongs to blue laser. Comparing the emission spectrum of BaQ 2 -1 and BaQ 2 -2. The luminous intensity at 370 nm of BaQ 2 -1 is 1.5 times that of BaQ 2 -2. The luminous intensity at maximum emission wavelength of BaQ 2 -1 is 1.7 times that of BaQ 2 -2. It shows that BaQ 2 -1 has higher luminous efficiency than BaQ 2 -2. The results show that using the mechanochemical activation method can get visible-light-excited BaQ 2 at 408 nm visible light and has higher luminous efficiency. These results are consistent with the theoretical studies of the luminescence mechanism of BaQ 2 below.

Synthesis Mechanism of Mechanochemical Activation Method
The essence of mechanochemical activation method is using mechanical force to activate reactants before the solid-phase reaction. Mechanochemical activation method can decrease the radiuses of solid particles of reactants, increase the contact areas between reactants, and make reactants contact uniformly on a molecular-level by mechanical force. Mechanochemical activation method can also activate reactants to reduce the thermal energy of the solid-phase reaction needed. So that reactants can react at low-heating temperature and the reaction time can be short.

Thermodynamic Principles of Mechanochemical Activation Method
The equation of definition of the Gibbs free energy change (ΔG) of thermodynamic functions in a chemical reaction can be written as Equation (1) According to Equation (1), ΔS of the solid-phase reaction is small enough to ignore. So, ΔG of the pure solid-phase reaction is only relevant for ΔH. If ΔH is less than zero, then ΔG would be less than zero, too. That means that once the pure solid-phase reaction takes place, ΔG would always be less than zero. It causes that there is no chemical equilibrium in the pure solid-phase reaction and it goes on to the end as soon as the reaction takes place. The rate of production of pure solid-phase reaction is 100%. After mechanical activation, the chemical reactivity of 8-hydroxyquinoline and barium hydroxide (Ba(OH) 2 ) is enhanced. They cause that the reaction of 8-hydroxyquinoline and barium hydroxide can take place in the low heat and the rate of production of mechanochemical activation method is high.

Dynamic Principles of Mechanochemical Activation Method
The solid-phase reaction consists of several simple physical and chemical processes. This research divided the reaction of HQ and Ba(OH) 2 into two processes. One is chemical reaction process of HQ and Ba(OH) 2 on contact surface. The other is the diffusion process of Ba(OH) 2 through the product layer. The microcosmic dynamic model of the reaction of HQ and Ba(OH) 2 is built and shown in Figure 7.
Ba(OH) 2 is the diffusive phase, which is wrapped on the surface of HQ (Figure 7). HQ reacts with Ba(OH) 2     the surface of HQ to the center. R 0 is the radius of HQ at the beginning, and x is the thickness of the product layer. According to Equation (2), it shows the reaction rate on the surface = x Kt (2) where K is the reaction rate constant. That is that x is only concerned with the kind of reactant and reaction time. Pick a short arc (dy) on the contact surface and enlarge it. The arc can regard as a short straight line because the arc is enough short. The dynamic model of Ba(OH) 2 diffusing on dy is built (Figure 8).
HQ is the diffusion medium and x is the diffusion path. According to the Fick's first law, the equation on steady state diffusion can be written as where J i is the diffusion velocity of Ba(OH) 2 ; D is diffusion coefficient; Δc i is the concentration difference of Ba(OH) 2 ; Δx is the diffusion path of Ba(OH) 2 . According to Equation (3), the main factors affecting the rate of diffusion on steady state diffusion are physical property and material concentration. The smaller the particle radius is, the more regular particle shape will be. The larger the dispersion of particles is, the smaller the diffusional resistance will be. And finally, the greater the diffusion coefficient is, the faster the diffusion rate will be. This study grinds HQ and Ba(OH) 2 by mechanical force. It can effectively reduce particle radius and mix particles evenly. It causes that the diffusion rate is increased and the diffusion time is decreased. The diffusion coefficient of solid in a solid is much smaller than other media. That is, the rate of diffusion is relatively slower. According to the theory of rate-determining step, the rate of reaction between HQ and Ba(OH) 2 is determined by the rate of diffusion. That is the mechanochemical activation method increases the rate of diffusion and then increased the reaction rate of the whole reaction. The conclusion of appeal can also be confirmed by the theory of solid-state reaction rate (α). α means that the ratio of the reactive volume to the original volume. α can be written as Equation (4) And Equation (4) can transform as Equation (5) α ) ( Find the limit at the same time on both sides of Equation (5) as Equation (6) lim lim 1 1 According to Equation (6) the smaller R 0 is, the larger α will be. That is the smaller the radius of 8-hydroxyquinoline particles is, the faster the reaction rate will be.
Based on appeal analysis, mechanochemical activation method can effectively increase the rate of reaction between HQ and Ba(OH) 2 .

Luminescence Mechanism of BaQ 2
BaQ 2 belongs to metal ion perturbation ligand luminescence. 8-hydroxyquinoline ring absorbs energy and induces π-π + transition. And then the de-excitation of the exciton go by luminescing and vibrational relaxation. The addition of barium ions can increase molecular rigidity, reduce vibrational relaxation and increase luminous efficiency.

Excitation Spectrum Redshift of BaQ 2
Lattice parameter expansion of BaQ 2 . The crystal is composed of crystallite and grain boundary, and the interplanar spacing comes from the contribution of crystallite and grain boundary. For general large-sized crystals, the specific surface area increases and the surface atoms increase with the decrease of grain size. The coordination number of surface atoms is lower than internal atoms. It will cause the increase of dangling bond, increase of surface energy, decrease of atomic radius, and lattice parameter contraction. But there is no independent surface in the near nanometer crystal and the grain boundary energy is lower than the surface energy so the degree of lattice parameter contraction is much smaller. On the other hand, as the grain size decreases, because of the surface effect, the atomic spacing of the surface atoms is larger than the internal atoms. According to Equations (7) and (8) where U is van der Waals force; U e is coordinate covalent bonds energy. Third, the ordering of grain boundary is high, it will also lead to lattice parameter expansion.

Shrink of Bandgap and Decline of Zero-Phonon Transition Energy of BaQ 2
Some basic conclusions about electronic states in crystal materials can be deduced directly or inferred from the Global Challenges 2019, 3,1900052  where P is a constant; a is periodic potential field; k is wave vector; α is related to E e . The value of αa is not arbitrary according to the Equation (9), and the range is limited by the in Equation (11) So simultaneous Equations (9) and (10) can be deduced that the relative volume deformation of periodic potential field is related to the specified energy levels (E) where (αa) n+1 and ( ) α ′ a are constant value; v is unit cell volume; E gn is the nth bandgap. Get Equations (13) and (14) by the integral of Equations (15) and (16) where E g is bandgap. The Equations (15) and (16) show the relation between E g and relative volume deformation. That is the lattice parameter contraction will cause the increase of E g and the lattice parameter expansion will cause the decrease of E g . The Equations (15) and (16) can be further extended to the relation between specified energy levels (include the top of valence band and the bottom of conduction band) and relative volume deformation Or It means that the lattice parameter contraction will cause the increase of E and the lattice parameter expansion will cause the decrease of E.
The energy of transition of luminescence center from the ground state to the excited state is called charge-transfer energy (E CT ). According to Equation (19), E CT consists of zero-phonon energy (E zp ) and vibrational energy (E vib ) where E zp is the energy of transition of one electron from the top of valence band to the bottom of conduction band. So as the decrease of E g , the energy between valence band and conduction band would decrease and result in the decrease of E zp .
Similarly, E CT would decrease. Finally, with the decrease of E CT , less energy needed for excite and result in red-shifting of the excitation spectra. This research uses mechanochemical activation method to synthesize BaQ 2 , which grain size is less than 100 nm. Base on the above inference the excitation spectra of BaQ 2 redshift and the results are consistent with fluorescence spectrum of BaQ 2 .

New Excitation Peak of BaQ 2 on Visible Region
On the one hand with the decrease of grain size, 8-hydroxyquinoline barium's specific surface area, interface area, and the number of triple junction increase. And it leads to number of crystal defects, vacancy and vacancy cluster increase. The new crystal defects, vacancy or vacancy cluster increase would cause the new light absorption on visible region. On the other hand, because of the quantum confinement effect, the structure of boundary surface is unordered and the mean free path of an electron is short. It causes that the constraint of electron by the vacancy is weak and the probability of exciton formation is high. It results in high concentration of exciton. Finally, the high concentration of exciton would form a new and lower exciton level (Figure 9). It can decline the energy of electron transition and form a new excitation peak on visible region.

The De-Excitation Process of BaQ 2 Exciton
The de-excitation process of BaQ 2 exciton consists of radiative decay process and radiationless decay process (Figure 10). And radiationless decay process consists of Förster resonance energy transfer (FRET) and Dexter excitation transfer.
Global Challenges 2019, 3,1900052  According to Equations (20) and (21) where k F(D→A) is the rate constant of FRET; K is orientation factor; n is refractive indices of media; ω is radiative lifetime of energy donor; r is the distance between donor and acceptor; J is overlap integral of spectra where k D(D→A) is the rate constant of Dexter excitation; r is the distance between donor and acceptor; P and L are constant.
With the decrease of grain size of 8-hydroxyquinoline barium crystal, the lattice parameter is expanded and the bond length is elongated. It causes the distance between donor and acceptor longer and k ET would decline rapidly. Finally, radiationless decay process would reduce and result in the increase of fluorescence efficiency and the enhancing of luminous intensity. The results are consistent with the fluorescence spectrum of 8-hydroxyquinoline barium.

Conclusion
This study proposes a new method for the synthesis 8-hydroxyquinoline bivalent metal complex. We successfully synthesized BaQ 2 that can be excited by visible light with a maximum excitation wavelength of 408 nm. We have explained the efficiency of the mechanochemical activation method proposed by this study, by thermodynamic principles, and dynamic principles.
Our study is also explained by theoretical deduction that the maximum excitation wavelength of BaQ 2 can be red shifted by decreasing the grain size. We expanded photoluminescence theory as it applies to visible-light-excited BaQ 2 and further improved photoluminescence theory of 8-hydroxyquinoline bivalent metal complex. These findings can provide a new perspective on the potential for using luminescent materials for energy conservation. The study of the special luminescent properties of nanomaterials is still in its infancy. Improving the accuracy of detection equipment and the diversity of detection methods can help to improve and verify theories.

Experimental Section
Synthesis of BaQ 2 -1 and BaQ 2 -2: 1.4053 g (0.0097 mol) of 8-hydroxyquinoline (AR), 1.4869 g (0.0047 mol) of barium hydroxide (AR) were placed in the Planetary ball mill of Pulverisette 7 of FRITSCH company in Germany and grind for 1 h. The product powder was put into a 100 mL beaker. The beaker was put into a vacuum drying oven. The temperature of the vacuum drying oven was raised to 90 °C and kept at 1 h. Then the temperature was raised to 100 °C and kept at 0.5 h. Then it was cooled to room temperature. Finally, 1.9982 g BaQ 2 -1 was obtained at yield 99.60%. BaQ 2 -2 was obtained by liquid-phase method. [22] Characterization of BaQ 2 -1 and BaQ 2 -2: The infrared radiation spectra analysis (IR) of BaQ 2 -1 and BaQ 2 -2 was recorded using KBr pellets in the range of 4000-400 cm −1 on a Mattson Alpha-Centauri spectrometer. XRD of BaQ 2 -1 and BaQ 2 -2 was recorded on a Rigaku D-MAX 2550 radiation (λ = 0.15417 nm) with 2θ ranging from 5° to 70°. XPS with Al Kα radiation (hv = 1486.6 eV) as the photo source was used to investigate the surface properties of BaQ 2 -1. SEM and EDS were done on a JEOL JSM-6700F SEM microscope Japan. The photoluminescence properties of BaQ 2 -1 and BaQ 2 -2 were recorded using Hitachi F-7000 fluorescence spectrometer with 150 W monochromatic xenon lamp as excitation source.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.