Yttrium silicates Y2SiO5 upconversion nanomaterials with different doping concentrations of praseodymium ion Pr are prepared by using a sol-gel method. X-ray diffractometer, SEM, Fourier transform infrared spectrometer, and fluorescence spectrometer have been employed to test the crystal structure and upconversion luminescence performances. The results indicate that samples calcined higher than 950°C present fine crystal structures, of which Si-O-Si band at 757–1048 cm−1 splits into three fine peaks. The crystal size of the samples calcined at 950°C and 1000°C is 29.1 nm and 66.7 nm, respectively. The luminescence intensities of the samples are increasing at first and then decreasing, with the increasing of the doping concentrations of 0.47%, 0.77%, 0.96%, 2.95%, and 4.93%. Nanomaterial sample doped 0.96% Pr emits the highest upconversion luminescence intensity of 6.43 × 106 cps and shows the best photodegradation performance for nitrobenzene wastewater. It demonstrates that too much of Pr doping concentration would result in quenching of the fluorescence. Nevertheless, as the degradation time expands, sample doped 0.96% Pr shows much faster increasing of photodegradation rate than samples of other doping concentrations and reaches to a high photodegradation rate of 97.14% in 6 hours for 10 mg/L nitrobenzene wastewater.
Upconversion nanomaterials are known for its efficient emission of ultraviolet fluorescence under the exciting visible light [
In the past decades, high-quality rare earth-doped upconversion nanomaterials have been successfully synthesized with the rapid development of nanotechnology and are becoming more prominent in biological and environmental sciences [
To get the highest upconversion luminescence efficiency, another critical factor is doping with other ions to occupy or replace the ions in the host material [
Besides the host materials and doping ions, the synthetic methods are also critical for the high quality upconversion nanomaterials to obtain high luminescence efficiency. The synthesis methods are usually phase-based processes. So far, three kinds of methods are commonly used to synthesize upconversion nanomaterials, including thermal decomposition [
Many researches focused on the optical properties, luminescence properties, and fluorescence quenching mechanisms of Pr doped Y2SiO5 crystals [
Praseodymium ion Pr(III), doped Y2SiO5 upconversion nanomaterials were prepared by using a sol-gel method. First, 0.1 mol/L praseodymium nitrate solution was added into the mixture (1 : 1, vol) of HNO3 and H2O dissolved 0.663 g Y2O3. Heating was followed until the solution becomes a viscous mixture. A number of crystals were seed out after cooling down. The crystal was collected and dissolved in ethanol. Tetraethyl orthosilicate (TEOS) was added and mixed with the ethanol solution of the crystals. The obtained mixture was put into a water bath of 70°C until a gel was formed. The gel was dried in an oven of 104°C and then grinded into powder. At last, the powder was calcined at a temperature oR 900°C, 950°C, and 1000°C for 3 h in a muffle furnace to get the final product of Pr(III) doped Y2SiO5 upconversion nanomaterials. The Pr (III) doping concentrations, 0.47%, 0.77%, 0.96%, 2.95%, and 4.93%, which were confirmed by using inductively coupled plasma (ICP) spectrometer, were adjusted by changing the adding volume of the 0.1 mol/L praseodymium nitrate solution at the first stage.
X-ray diffractometer (D8 Advance, Bruker Corporation, German) was used to characterize the crystal form of the samples. A scanning electron microscopy (Hitachi S4800, SEM, Japan) was employed to characterize the particle size of the samples. Fourier transform infrared spectrometer (FTIR, MB154S, ABB BOMEN Corporation, Canada) was employed to check the crystal and groups of samples. The upconversion luminescence of the nanomaterials was tested by using a fluorescence spectrometer (FL3-TCSPC, Horiba Jobin Yvon Corporation, France). The exciting parameters were selected as 425 nm of the excitation wavelength, 370 nm of the optical filters, and 2 nm of the slit.
Nitrobenzene wastewater, 10 mg/L, which was from a TNT factory, was used as a target pollutant to test the photodegradation performances of the prepared Pr(III) doped Y2SiO5 upconversion nanomaterials. Filament lamp, 52 W, was used as the exciting light source for the upconversion nanomaterials. The treatment time was lasting for 1 h to 6.5 h. The degradation rate of nitrobenzene was tested by comparing the ultraviolet absorption values to the original value of the wastewater. The ultraviolet absorption values were tested by using an ultraviolet-visible spectrophotometer. The relationship of the nitrobenzene concentrations (
The heat treatment temperature is an important parameter for the crystal structure and crystal size of many kinds of nanomaterials, as well as upconversion nanomaterials. Figure
XRD patterns of Pr(III) doped samples calcined at different temperatures: (a) 900°C, (b) 950°C, (c) 1000°C.
Without obvious diffraction peak appears on the XRD pattern of sample calcined at 900°C, except for a bread curve at the 2-theta angle about 30°. It means that Y2SiO5 under the heat treatment temperature of 900°C is amorphous. Meanwhile, a higher heat treatment temperature of 950°C or 1000°C leads to good crystal forms of the Pr(III) doped Y2SiO5 upconversion nanomaterials, of which diffraction peak positions show high consistency with that of X1-Y2SiO5 phase [
It indicates that the crystal structure transition temperature of Pr (III) doped X1 pattern Y2SiO5 is about 950°C, heat treatment temperature higher than 950°C is not good for the forming of small crystal size for the materials.
The particle size of the samples characterized by a scanning electron microscopy also shows high dependence on the heat treatment temperature, as shown in Figure
SEM images of Pr(III) doped samples calcined at different temperature (a) 900°C; (b) 950°C; (c) 1000°C.
It has been proved that the crystal structures and bond groups of phosphor materials show high dependence on the heat treatment or thermal annealing temperature [
FTIR spectra of Pr(III) doped samples calcined at different temperatures: (a) 900°C, (b) 950°C, (c) 1000°C.
High heat treatment temperature is believed to be beneficial to the forming of fine crystal structures of the nanomaterials. Therefore, as a result, when the upconversion nanomaterials are calcined at the temperature of 950°C and 1000°C, the absorption band at 757–1048 cm−1 splits into three fine peaks at 849.7 cm−1, 933.8 cm−1, and 1008.3 cm−1, which result from the bending vibration absorption of Si-O bond, the symmetric vibration absorption of Si-O-Si bond, and the asymmetric vibration absorption of Si-O-Si bond, respectively, as curves (b) and (c) shown in Figure
The absorption bands at 1260–1620 cm−1 are attributed to the bending vibration of O-H bond or H2O, which is adsorbed on the internal holes surface of the nanomaterials or bonded with the nanomaterials in other forms. The bonded H2O in the materials is difficult to desorb at 900°C. A higher heat treatment temperature is usually beneficial to the desorption of the bonded or adsorbed H2O in the materials, and lead to the disappearing of the absorption bands at 1260–1620 cm−1 at the temperature of 950°C or 1000°C, as curves (b) and (c) shown in Figure
Since the ionic radius of Pr(III) and Y(III), 113 pm and 104 pm, respectively, is very close [
Emission spectra of Y2SiO5 doped different concentrations: of Pr(III), (a) 0.0%, (b) 0.47%, (c) 0.77%, (d) 0.96%, (e) 2.95% and (f) 4.93%.
The blank sample, the sample without doped Pr(III), shows relative low luminescence intensity. On the other hand, those samples doped Pr(III) emit obvious luminescence spectra at the wavelength of 360 nm. With the increasing of the dope concentration of Pr(III), the emission luminescence intensities are also enhanced obviously to a maximum of
As a typical environmental priority control pollutant, nitrobenzene wastewater usually comes from the factories manufacturing medicines, pesticides, plastics, explosives. It is difficult to degrade by normal methods because of its particular molecular structures [
The photodegradation performances of Pr(III) doped Y2SiO5 nanomaterials (a) 0.47%, (b) 0.96% and (c) 4.93%.
Sample doped 0.96% Pr(III) shows much higher photodegradation rate at all the time than the samples of the other two doping concentrations, 0.47% and 4.93%. It can be concluded that higher luminescence emission sample, such as Y2SiO5 nanomaterials doped 0.96% Pr(III) as shown in Figure
First-order fitting of the ln (
The photodegradation reaction kinetics data of nanomaterials (a) 0.47%, (b) 0.96% and (c) 4.93%.
The best photodegradation belongs to the upconversion nanomaterials doped 0.96% of Pr(III), and too much of doping concentrations of Pr(III) would not show high photodegradation of upconversion nanomaterials, which confirms high consistency with the test results indicated in Figure
Praseodymium ion, Pr(III), doped Y2SiO5 upconversion nanomaterials are prepared by using a sol-gel method. The doping concentrations of Pr(III) play important roles on the upconversion of the nanomaterials. The emission luminescence intensity of the nanomaterial reaches a maximum of
The work was supported by the Foundation of Jiangsu Environmental Protection Department, China (no. 2012015).