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Ultraviolet_Visible (UV_Vis) spectroscopy

The optical properties of the nanoparticles were
recorded on UV-Vis absorption spectrophotometer. The prepared nanoparticles
were first dispersed in methanol before the UV-Vis characterization with a
concentration of 0.1% wt to form a monodispersed solution, after this, each
nanoparticle was sonicated for 30min. Figure 6(a) shown the electronic spectrum
of ZnO nanoparticles exhibited one band 24 . These absorption band appeared
at 378 nm (26455.03) cm–1 due to the electron transition from the valence band
to the conduction band (O2p ? Zn3d) 25.The Co-doped ZnO, figure 6(b) show the
presences three bands attributed to d-d transition in the region 390 and 514 nm
which were assigned to 4A2(F)? 2A1(G)
and 4A2? 4T1(P) respectively,
These minima are in agreement with the Co+2
d-d (tetrahedral symmetry) 26. For Ni-doped ZnO the absorption band is
shifted to a longer wavelength at 396 nm. These electronic transitions indicate
that Ni2+ ions are incorporated 
Zn2+ ions in the octahedral ZnO. Usually, divalent Ni ions in
an octahedral symmetry show three main bands at about 8300,14080, 24995 cm-1.Those
bands are commonly attributed due to the spin-allowed triplet-triplet
transitions 27:

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 3A2g(F) ? 3T2g(F), 3A2g(F)
? 3T1g(F) and 3A2g(F) ? 3T1g(P).

 The obtained
data detect that the single observed absorption band is most probably due to 3A2g(F)
? 3T1g(P), electronic transitions occur between split d
levels of the central atom giving rise to so-called d-d transition or ligand
field spectra. Where this electronic transition attributed to d–d
transition is localized at 26525 cm-1 28. The bandgap energies were
calculated by the equation of 29.

Eg = 1240 ? ? max                   (2)

Where

Eg: is the band gap energy

1,240: is the hc

? max: is the wavelength in nanometer

The calculated values are 3.27 eV (378 nm), 3.17 eV
(390nm) , and 3.13 eV (396 nm) for undoped ZnO , Co,and Ni doped ZnO
respectively. These values are obviously red-shifted , this shift may be due to
the sp-d exchange interaction between the band electron and the
localized d-electron of theCo+2 ion and Ni+2 ion.

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