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Nanostructured titania films sensitized by quantum dot chalcogenides.

Kontos AG, Likodimos V, Vassalou E, Kapogianni I, Raptis YS, Raptis C, Falaras P - Nanoscale Res Lett (2011)

Bottom Line: The optical absorbance of CdS/TiO2 can be tuned over a narrow spectral range.On the other side PbS/TiO2 exhibits a remarkable band gap tunability extending from the visible to the near infrared range, due to the distinct quantum size effects of PbS quantum dots.Degradation effects are much less pronounced for CdS/TiO2 that is appreciably more stable, though it degrades readily upon visible light illumination.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Physical Chemistry, NCSR "Demokritos", Aghia Paraskevi Attikis, Athens 15310, Greece. akontos@chem.demokritos.gr.

ABSTRACT
The optical and structural properties of cadmium and lead sulfide nanocrystals deposited on mesoporous TiO2 substrates via the successive ionic layer adsorption and reaction method were comparatively investigated by reflectance, transmittance, micro-Raman and photoluminescence measurements. Enhanced interfacial electron transfer is evidenced upon direct growth of both CdS and PbS on TiO2 through the marked quenching of their excitonic emission. The optical absorbance of CdS/TiO2 can be tuned over a narrow spectral range. On the other side PbS/TiO2 exhibits a remarkable band gap tunability extending from the visible to the near infrared range, due to the distinct quantum size effects of PbS quantum dots. However, PbS/TiO2 suffers from severe degradation upon air exposure. Degradation effects are much less pronounced for CdS/TiO2 that is appreciably more stable, though it degrades readily upon visible light illumination.

No MeSH data available.


Related in: MedlinePlus

Evolution of the CdS/TiO2 Raman spectra upon simultaneous variation of the laser power and acquisition time (irradiation dose remains constant). The inset shows the variation of the intensity ratio I(LO)CdS/I(Eg)TiO2 determined from the integrated areas of the CdS LO mode and the Eg anatase TiO2 mode, with the spectral acquisition time.
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Figure 3: Evolution of the CdS/TiO2 Raman spectra upon simultaneous variation of the laser power and acquisition time (irradiation dose remains constant). The inset shows the variation of the intensity ratio I(LO)CdS/I(Eg)TiO2 determined from the integrated areas of the CdS LO mode and the Eg anatase TiO2 mode, with the spectral acquisition time.

Mentions: Degradation effects were also observed in the CdS Raman signal when acquired in ambient conditions, though considerably less pronounced than those of PbS/TiO2. Most importantly, an intriguing photodegradation effect on the CdS Raman intensity was evidenced by varying the laser irradiation time in ambient conditions. Figure 3 shows characteristic resonance Raman spectra of CdS/TiO2 acquired in air under variable laser power density and different acquisition times so that the total irradiation dose (product of laser power × acquisition time) remains constant. In that case, a marked increase of the CdS LO Raman intensity relative to that of the Eg anatase TiO2 mode occurred by decreasing the spectral acquisition time (inset of Figure 3). Ordinary local heating effects are excluded since the relative CdS LO intensity was found to increase with the laser power and no appreciable shift and broadening of the LO mode or variation of the I2LO/ILO intensity ratio were identified [20], indicating that the observed behavior is related to the duration of exposure of the CdS/TiO2 films to the laser beam. This variation was completely suppressed when Raman experiments were conducted in an isolated cell compartment under vacuum conditions, pointing to a photodegradation effect of the CdS nanocrystals under ambient conditions. A similar result was recently reported for CdSe QDs anchored to TiO2 following visible light irradiation under atmospheric conditions [23]. In that case, time resolved transient absorbance and emission measurements revealed that electrons injected from CdSe to TiO2 may be scavenged by surface adsorbed oxygen leaving behind reactive holes, which cause anodic corrosion of the CdSe QDs. An analogous mechanism can be accordingly proposed for the CdS/TiO2 system upon resonant laser irradiation at 514.5 nm, causing electron injection to TiO2 and the surface oxidation of CdS nanocrystals through the remaining valence band holes.


Nanostructured titania films sensitized by quantum dot chalcogenides.

Kontos AG, Likodimos V, Vassalou E, Kapogianni I, Raptis YS, Raptis C, Falaras P - Nanoscale Res Lett (2011)

Evolution of the CdS/TiO2 Raman spectra upon simultaneous variation of the laser power and acquisition time (irradiation dose remains constant). The inset shows the variation of the intensity ratio I(LO)CdS/I(Eg)TiO2 determined from the integrated areas of the CdS LO mode and the Eg anatase TiO2 mode, with the spectral acquisition time.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211329&req=5

Figure 3: Evolution of the CdS/TiO2 Raman spectra upon simultaneous variation of the laser power and acquisition time (irradiation dose remains constant). The inset shows the variation of the intensity ratio I(LO)CdS/I(Eg)TiO2 determined from the integrated areas of the CdS LO mode and the Eg anatase TiO2 mode, with the spectral acquisition time.
Mentions: Degradation effects were also observed in the CdS Raman signal when acquired in ambient conditions, though considerably less pronounced than those of PbS/TiO2. Most importantly, an intriguing photodegradation effect on the CdS Raman intensity was evidenced by varying the laser irradiation time in ambient conditions. Figure 3 shows characteristic resonance Raman spectra of CdS/TiO2 acquired in air under variable laser power density and different acquisition times so that the total irradiation dose (product of laser power × acquisition time) remains constant. In that case, a marked increase of the CdS LO Raman intensity relative to that of the Eg anatase TiO2 mode occurred by decreasing the spectral acquisition time (inset of Figure 3). Ordinary local heating effects are excluded since the relative CdS LO intensity was found to increase with the laser power and no appreciable shift and broadening of the LO mode or variation of the I2LO/ILO intensity ratio were identified [20], indicating that the observed behavior is related to the duration of exposure of the CdS/TiO2 films to the laser beam. This variation was completely suppressed when Raman experiments were conducted in an isolated cell compartment under vacuum conditions, pointing to a photodegradation effect of the CdS nanocrystals under ambient conditions. A similar result was recently reported for CdSe QDs anchored to TiO2 following visible light irradiation under atmospheric conditions [23]. In that case, time resolved transient absorbance and emission measurements revealed that electrons injected from CdSe to TiO2 may be scavenged by surface adsorbed oxygen leaving behind reactive holes, which cause anodic corrosion of the CdSe QDs. An analogous mechanism can be accordingly proposed for the CdS/TiO2 system upon resonant laser irradiation at 514.5 nm, causing electron injection to TiO2 and the surface oxidation of CdS nanocrystals through the remaining valence band holes.

Bottom Line: The optical absorbance of CdS/TiO2 can be tuned over a narrow spectral range.On the other side PbS/TiO2 exhibits a remarkable band gap tunability extending from the visible to the near infrared range, due to the distinct quantum size effects of PbS quantum dots.Degradation effects are much less pronounced for CdS/TiO2 that is appreciably more stable, though it degrades readily upon visible light illumination.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Physical Chemistry, NCSR "Demokritos", Aghia Paraskevi Attikis, Athens 15310, Greece. akontos@chem.demokritos.gr.

ABSTRACT
The optical and structural properties of cadmium and lead sulfide nanocrystals deposited on mesoporous TiO2 substrates via the successive ionic layer adsorption and reaction method were comparatively investigated by reflectance, transmittance, micro-Raman and photoluminescence measurements. Enhanced interfacial electron transfer is evidenced upon direct growth of both CdS and PbS on TiO2 through the marked quenching of their excitonic emission. The optical absorbance of CdS/TiO2 can be tuned over a narrow spectral range. On the other side PbS/TiO2 exhibits a remarkable band gap tunability extending from the visible to the near infrared range, due to the distinct quantum size effects of PbS quantum dots. However, PbS/TiO2 suffers from severe degradation upon air exposure. Degradation effects are much less pronounced for CdS/TiO2 that is appreciably more stable, though it degrades readily upon visible light illumination.

No MeSH data available.


Related in: MedlinePlus