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2. Experimental
2.1. Materials
The main chemicals include (NH
4
)
2
SO
4
, phenol, NaClO, Sodium citrate, Tm
2
O
3
, HF, CH
3
COOH,
C
2
H
5
OH, tetrabutyl titanate(TBOT), Ca(NO
3
)
2
, HNO
3
and H
2
O
2
.
2.2. Preparation of CaF
2
(Tm
3+
) nanoparticles
CaF
2
(Tm
3+
) nanoparticles were prepared by co-precipitation method. In a typical experiment, a
saturated solution of Ca(NO
3
)
2
was prepared using deionized water as a solvent. Under heating
conditions, a prescribed amount of Tm
2
O
3
was dissolved in concentrated HNO
3
. Then, under the
condition of stirring, Ca(NO
3
)
2
solution and Tm(NO
3
)
3
respectively drops into the HF solution at a
certain rate (Ca
2+
: F
-
=1:2, Tm
3+:
F
-
=1:3;molar ratio),after the titration, the magnetic stirring was
continued for 1 h. After this, the solution was centrifuged and washed 6 times to remove the
remaining NO
3-
.After drying and grinding, the CaF
2
(Tm
3+
) nanoparticles was obtained.
2.3. Preparation of CaF
2
(Tm
3+
)/TiO
2
photocatalyst
CaF
2
(Tm
3+
)/TiO
2
photocatalyst was synthesized using sol-gel method. TBOT, ethanol and a small
amount of glacial acetic acid were mixed and vigorously stirred to form solution A. The precise
amount of CaF
2
(Tm
3+
) nanoparticles was dissolved in deionized water and an appropriate amount of
ethanol was added. After 15 min of ultrasonic separation, solution B was obtained. Under the
condition of stirring, solution B was added dropwise to solution A. After the titration, the stirring was
continued for 60 min. After standing for 24 h, the sol was dried at 80°C for 12 h and ground to
powder, then the powder was calcined at 450°C for 2 h. Then, the samples were ground to form
CaF
2
(Tm
3+
)/TiO
2
photocatalyst. The molar ratios of Tm
3+
to TiO
2
are 0, 5, 10, 15, 20 and 25% .
2.4. Characterization techniques
The crystal structures of all prepared samples were characterized by X-ray diffraction (XRD) using a
D/MAX-2500X ray diffractometer with Cu Kα radiation (Japan). Scanning electron microscopy
(SEM) images using a Quanta 200FEG field emission environment scanning electron microscopy
(American) were employed to obtain a variety of physical and chemical properties of prepared
samples, such as morphology, composition, crystal structure, etc.
2.5. Photocatalytic activity measurements.
The photocatalytic activity of the CaF
2
(Tm
3+
)/TiO
2
photocatalyst under UV light(intensity: 18W) was
evaluated by measuring degradation of NH
3
-N in aquaculture wastewater. According to the
experimental requirements, different amounts of (NH
4
)
2
SO
4
were added in the seawater after
pumping (sampled in the waters near Dalian Ocean University, ammonia concentration of 0.9725
mg/L, pH=7.98) to obtain simulated aquaculture wastewater. The photocatalytic degradation of NH
3
-
N was analysed by various experimental parameters namely doping ratio, dosage, initial
concentration of NH
3
-N, pH value,concentration of H
2
O
2
and illumination time. The content of NH
3
-
N was estimated by indophenol blue colorimetric method[7].
3. Results and discussion
3.1. The characterization of CaF
2
(Tm
3+
)/TiO
2
photocatalyst
The XRD patterns of materials are shown in Figure 1. TiO
2
has obvious peaks at 2θ of 25.4287°,
37.8204°, 47.9708° and 70.3778°. The CaF
2
(Tm
3+
)/TiO
2
with doping ratio of 15% has obvious peaks
when 2θ is 25.3284°, 28.0843°, 38.0536° and 55.2359°. The photocatalyst of 25% has obvious peaks
at 2θ of 25.3289°, 28.0858°, 37.9646° and 55.1043°. Scherrer formula was used to calculate the
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