29-09-2014, 12:32 PM
EXCIMER LASER INDUCED PHOTO-THERMAL CHANGES
OF SOL-GEL TiO2 THIN FILMS
EXCIMER LASER.pdf (Size: 222.58 KB / Downloads: 11)
Introduction
TiO2 is an n-type compound semiconductor that is used in sensor technique, as solar cell
element, antibacterial and self-cleaning coatings, functional medium in photocatalysis in both
nanoparticle and thin film form [1-6]. High chemical inertness, high mechanical stability, low cost
and nontoxicity make TiO2 in the last two decades a desirable photocatalytical material for air and
water decontamination [7-12]. Spray pyrolysis, sol-gel method and solvothermal method are usually
applied for preparation of nanostructured titania materials [13,14,15]. Sol-gel method appears to be
very precise, cost efficient and thus industrially very effective technique [16]. It is established that
titania sol-gel nanoparticles with high degree of crystallinity, high specific surface area and porosity
exhibit a pronounced photocatalytic activity toward various chemical processes [11,17]. However,
due to serious problems with long term stability of nanometer size particles they must be very often
subjected to regeneration that is known to be very expensive and complicate procedure. [18] Unlike
the nanosized particles, the sol-gel solid state thin films on suitable substrates have a very good
storage stability. The low specific surface area seems to be a serious disadvantage of sol-gel films
deposited on flat substrates. However, the evolution of high specific surface could be achieved
applying physical processing tools as for example laser absorption [19, 20]. It should be reminded
that UV light is necessary for initiation of the photocatalytic activity of titania. In order to reach high
photocatalytic efficiency titania has to be spectrally sensitized to lower energies. That spectral
sensitization has been achieved by selective organic dye adsorption or doping of titania with
compound semiconductors as CdS, CuO etc [7, 21]. The latter are known to have a narrower band
gap as compared to that of titania. Besides, it has been established that doping with Ag
+
, Pt
+
, Fe
2+
,
Ga
3+ Nb
5+
, Ta
5+
is accompanied by an introduction of effective traps for photo generated charge
carriers thus preventing the recombination process [4, 12, 22-24]. However, toxic complexes
between some chemical dopants and organic pollutants could be formed during UV irradiation [25].
In the present paper pulsed laser irradiation below the ablation threshold is used for
monitoring the spectral sensitivity and surface roughness design of sol-gel titania thin films for
photocatalitycal purposes. There are two main reasons to apply laser source in that research field.
First of all, the laser processing is a dry, fast and versatile modification technique. Besides, the fast
melting and cooling processes accompanying the absorption of laser irradiation result in the
evolution of catalytically most active
. Conclusions
The results obtained in the present study unambiguously show that excimer laser processing
at constant energy density varying the pulse number could be successfully used for tailoring the
structure and properties of sol-gel titania films that are of great importance for their further
application as photocatalyst. The established photo-thermal changes follow the same trend as those
observed earlier in laser modified samples at constant pulse number on increasing the pulse energy
density thus confirming the versatility of the laser modification as a processing tool. The evolution
of high surface roughness that is accompanied by bubble, pore and groove formation upon pulsed
UV light absorption in sol-gel titania films is demonstrated. Experimental evidence for the
occurrence of light stimulated spectral sensitization resulted in infrared induced spectral sensitivity
is presented. Therefore, a promising way is revealed increasing the efficiency of the photocatalytic
process on irradiation with low light energies.
In summary, the present results could be regarded as tracing of a new approach for
designing sol-gel titania films with high surface roughness degree and extended, infrared spectral
sensitivity for the purpose of water decontamination and purification.