Biomedical Engineering Reference
In-Depth Information
crystalline impurities except CNXL were detected by XRD, which indicates that pure
selenium can be obtained via CNXL-template synthesis under hydrothermal conditions.
The morphologies and structures of the CNXL-supported selenium nanoparticles were
examined by FESEM and TEM. FESEM images in Figure 12.11 show CNXL-supported
selenium nanoparticles prepared by hydrothermal treatment at different temperatures for
16 h. The images revealed that the selenium nanoparticles were uniformly bound to the
CNXL surface. These high-resolution FESEM images clearly indicate that the composite
material consists of many CNXLs (150-200 nm) coated with selenium nanocrystals. The
more highly magnified FESEM images (Figure 12.11b, d, and f) provide more details
on the size of the selenium nanoparticles as a function of preparation temperature. We
observed that particle diameters gradually increased from 10 to 15 to 20 nm when the
reaction temperature was 120, 140, and 160
◦
C, respectively. High resolution TEM
(HRTEM) images, along with a selected area electron diffraction (SAED) pattern, were
used to verify the structure of selenium nanoparticles at different temperatures as seen
in Figure 12.12. The diffraction rings in the SAED pattern (inset) could be indexed
as (100), (101), (110), (102), (111), (201), (112), and (202) reflections, indicating the
formation of the hexagonal selenium crystal phase, in agreement with the XRD results.
Selenium particle sizes are distributed about an average at each reaction temperature
(10, 15, and 20 nm at 120, 140, and 160
◦
C, respectively). This agrees well with the
SEM results. The energy dispersive spectroscopy (EDS) measurements obtained for each
sample (spectra not shown) indicated selenium signals in addition to strong copper and
carbon signals from the TEM copper grid and carbon coating. All of the above results
indicated that elemental selenium nanoparticles could be synthesized by using CNXL as
a reducing agent under mild hydrothermal treatment and the isolated elemental selenium
particles are in the range of 10-20 nm in diameter.
Although a complex reduction mechanism drives the formation of elemental selenium,
we believe that the presence of CNXL is critical for the reduction of H
2
SeO
3
at the
interface. Based on previous studies involving water-mediated reduction of metal ions
on cellulose, CNXL would provide the proper local chemical environment to reduce
Se(IV) ions (62). Reaction temperature is also a key factor in the formation of selenium
nanoparticles. The optimal reaction temperature range for detecting elemental selenium
peaks in the XRD patterns lied within 120-180
◦
C. At 100
◦
C, selenium particles were
too small to detect and at 180
◦
C, hydrolysis of cellulose by acid precluded product
collection by centrifugation.
The photocatalytic degradation of methylene blue (MB) on the prepared Se/CNXL
was studied at 25
◦
C and pH 8.0. The red shift in
λ
max
value of the MB in the solution
has been observed after added Se/CNXL materials. The dye shows the
λ
max
at 660 nm
in water, which is shifted to 665 nm in reaction solution. It is expected that the dye
molecules interact with the hydroxyl groups of CNXL surface (63). The progress of
the dye degradation was monitored by the decrease in absorbance of the peak due to
MB at 660 nm. Figure 12.13 shows the absorption spectra of successive degradation
of MB. Control experiment (photolysis) indicated that MB was not degraded (only 15%
in 1.5 h) when irradiated with UV in the absence of catalyst and about 40% remained
without decomposition after 30 h (Figure 12.13b).
It was also found that the degradation was very slow with the commercial Se particles
(30.2% degraded in 1.5 h). The uses of Se/CNXL led to the fast and complete degradation
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