Nanostructured titania films sensitized by quantum dot chalcogenides
© Kontos et al; licensee Springer. 2011
Received: 9 December 2010
Accepted: 29 March 2011
Published: 29 March 2011
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© Kontos et al; licensee Springer. 2011
Received: 9 December 2010
Accepted: 29 March 2011
Published: 29 March 2011
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.
In recent years, nanostructured materials and quantum dots (QDs) light harvesting assemblies have emerged as highly promising building blocks for the development of and third generation solar cells affording efficient conversion of solar energy to electricity. Among different technologies, dye sensitized solar cells (DSCs)  hold great promise as an alternative renewable energy system with the advantages of low cost, transparency and flexibility . DSCs make use of nanocrystalline semiconducting electrodes (the most common being TiO2) sensitized with molecular dyes (the most efficient being polypyridyl ruthenium(II) complexes) in order to harvest solar light. In contrast to conventional p-n type devices, charge separation in DSCs takes place at the photoelectrode/sensitizer interface via electron injection from the dye into the conduction band of the semiconductor, followed by diffusive electron transport through the interpenetrated mesoporous network of the TiO2 semiconductor to the charge collector, while dye regeneration occurs via a redox electrolyte. Even though such devices have reached high performance and stability standards , the prospect of developing inorganic hybrid heterojunctions with enhanced selectivity, efficiency and robustness offering cost reduction and simplification in the DSCs manufacturing is attracting a great deal of attention.
One of the most attractive approaches for the utilization of inorganic heterojunctions in DSCs is the exploitation of the exceptional electronic properties of chalcogenide such as CdS, CdSe, PbSe, PbS and CdTe nanocrystals as light harvesting antennas [4–6]. Based on the unique quantum confinement effects, QDs offer unique high extinction coefficients and band gap tunability from the visible to the infrared spectral range by size control. Moreover, they can form favourable QDs/TiO2 as well as QDs/dye/TiO2 heterojunctions for efficient charge extraction [7–11]. A major drawback underlying the relatively low light harvesting ability and the concomitant reduced photocurrents in quantum dot sensitized solar cell devices is the amount of QDs adsorbed on the TiO2 electrode. Two main approaches have been so far exploited for the sensitization by QDs: in situ growth of QDs on TiO2 by chemical bath deposition (CBD) [7, 12] and successive ionic layer adsorption and reaction (SILAR) [13, 14] or attachment of preformed colloidal QDs to the TiO2 mesoporous structure by means of bifunctional linker molecules or direct adsorption using a suitable solvent in the colloidal solution [8, 11]. Linker-assisted and direct QD adsorption onto TiO2 allows fine control of the QD size, exploiting colloidal synthesis. However these systems suffer from rather low QD loading and relatively weaker electronic coupling between QDs and TiO2. On the other hand, CBD permits enhanced electron transfer to the wide band gap TiO2 electrode and significantly higher loading at the cost of appreciable QD aggregation that finally deteriorates solar cell performance [5, 6]. On the contrary, direct growth of QDs by SILAR has recently emerged as a promising deposition route combining high QD loading together with low degree of aggregation and efficient electron transfer to TiO2[14, 15].
In this work, we report a comparative investigation on the direct growth of chalcogenide CdS and PbS nanocrystals spanning a wide spectral range for light absorption on mesoporous TiO2 films employing the SILAR method. Reflectance and transmittance together with micro-Raman measurements were exploited to identify the optical and structural properties as well as quantum size effects of the sulfide nanocrystals and their stability upon air and light exposure. The electron injection efficiency of the sensitized films was accessed by photoluminescence (PL) measurements and the variation of the QD emission signal upon grafting onto TiO2.
Mesoscopic TiO2 films of a thickness of 15 μm were prepared using a TiO2 paste made of Degussa P25 nanoparticles on glass substrates, followed by sintering at 450°C . Films present excellent adherence to the glass substrate. For the CdS SILAR deposition , the TiO2 films were pretreated with a quick soaking in 1 M NH4F aqueous solution. Then, they were dipped into 0.05 M Cd(NO3)2, ethanol solution, rinsed in pure ethanol to remove excess of the precursor and dried in air. The same process was followed for depositing S2-, by successive dipping the films in 0.05 M Na2S solution, rinsing in pure methanol and drying. Each individual step lasted for 1 min and a total of 9 SILAR cycles were employed. PbS deposition was likewise carried out by sequential immersing the TiO2 film initially in a 0.02-M Pb(NO3)2 methanol solution, and then to a 0.02-M Na2S methanol solution. The process starts and terminates with Pb2+ deposition accomplishing 5.5 SILAR cycles .
Diffuse reflectance (R) and transmittance (T) measurements were carried out employing a Hitachi 3010 spectrophotometer equipped with a 60-mm diameter integrating sphere. The absorbance (A) spectra were derived as A = 1 - R - T. Surface morphology was examined with a digital Instruments Nanoscope III atomic force microscope (AFM), operating in the tapping mode. Micro-Raman and PL measurements were performed at room temperature employing a vacuum cell equipped with an optical window. For Raman, a Renishaw inVia spectrometer was employed, using an Ar+ ion laser (λ = 514.5 nm) and a high power near infrared (NIR) diode laser (λ = 785 nm) as excitation sources for CdS and PbS QDs, correspondingly. The spectra were recorded by focusing the laser beam on the film surface and controlling the light power to give 0.01 to 0.2 mW/μm2 at about 1.5 μm diameter spot. For PL experiments in PbS, the above facility was used, while for CdS, excitation of the film was done by focusing the 476.5-nm line of an Ar+ laser at 20 mW on the sample surface with an 8-cm focal length cylindrical lens. The emitted radiation was analyzed through a SPEX double monochromator, followed by photomultiplier detection.
Figure 1b shows the corresponding evolution of the PbS/TiO2 absorbance spectra with the SILAR cycles. In that case, the absorption edge of the sensitized system extended well in the NIR spectral region, presenting a marked shift from 690 nm for the first SILAR cycle up to 840 nm for the last PbS coating. These wavelengths are much shorter than the absorption edge (approximately 3000 nm) of bulk PbS that possess a narrow band gap of only 0.41 eV. This distinct variation of the PbS/TiO2 absorbance reflects essentially the large exciton Bohr radius (approximately 18 nm) of PbS QDs, affording wide tunability through the pronounced quantization effects for PbS nanocrystals over an extended particle size . Even though the broad spectral absorption of PbS/TiO2 is expected to comprise appreciable contributions from the whole electronic spectrum of the underlying PbS nanocrystals, its strong dependence on the coating cycles verifies that direct growth of PbS QDs on TiO2 and their optical response can be efficiently tuned by the SILAR technique through a broad size/spectral range.
However, storage of the PbS/TiO2 films under ambient conditions produced rapid degradation of their optical response. Specifically, brief exposure of the PbS/TiO2 to air for 90 min resulted in the drastic decrease of the absorbance and the shift of the absorption edge to shorter wavelengths, indicative of the reduction of the PbS size, as shown by the dashed line in Figure 1b. This variation can be associated with the prominent tendency of lead sulfide towards surface oxidation at ambient conditions, which is especially detrimental for the larger PbS nanocrystals . Storage under vacuum conditions in evacuated cells was accordingly found to be necessary to retain the PbS/TiO2 spectral characteristics intact. Similar degradation effects were also observed for the CdS/TiO2 films upon air exposure, though much less severe than those on PbS/TiO2, indicating their higher resistance to air oxidation that can be largely prevented by storage under inert atmosphere.
QD nanoparticles can be hardly identified in SEM and AFM images of the films, due to the rough characteristics of the TiO2 nanostructured substrate film. However, a morphological evidence of the CdS QDs came from 1 × 1 μm AFM surface images (not shown) on nanoparticulate sol-gel anatase TiO2 (chosen as a reference substrate) and comparing it with the surface of the CdS/TiO2 film corresponding to the full set of the 9 SILAR cycles. Thus, significant enhancement of the surface roughness was observed (Rms = 15.9 nm for CdS/TiO2 vs. 6.6 nm for bare TiO2), due to the CdS QDs growth on the surface, in agreement with literature .
Raman measurements under NIR excitation (785 nm) were applied to identify the structural integrity of the lead sulfide nanocrystals through resonance excitation on the PbS/TiO2 films. A composite band comprising two bands at 202 and 260 cm-1 could be accordingly resolved on the sensitized PbS/TiO2, as shown in the inset of Figure 2. Lead sulfide crystallizes in rock salt structure precluding first-order Raman scattering from phonons near the centre of the Brillouin zone (k = 0). However, the formally 'forbidden' LO scattering at 200 to 215 cm-1 may become allowed under conditions of resonant or quasi-resonant Raman excitation via the Fröhlich interaction, while appreciable contributions may also arise at these frequencies from two-phonon scattering of longitudinal acoustic and transverse optical modes in PbS . A characteristic broad Raman band has been also reported at approximately 430 cm-1 due to 2 LO scattering in PbS , which, however, cannot be safely discriminated in the PbS/TiO2 spectra due to the additional contribution of the rutile TiO2 phonon at approximately 447 cm-1.
In the case of PbS/TiO2, the PL emission spectra could be detected simultaneously with the Raman signal at 785 nm excitation. A very weak and broad PL band could be thus traced at 955 nm after subtraction of the glass background, as shown in the inset of Figure 4. This emission band emerges at wavelengths just above the absorption edge of the PbS/TiO2 (approximately 840 nm), complying with the excitonic PL of an ensemble of PbS QDs with a broad size distribution around 3 nm . Moreover, the PL emission band could be resolved only for freshly sensitized films PbS/TiO2, while it degraded rapidly upon air exposure verifying the great sensitivity of the system to surface oxidation. The drastic reduction of excitonic emission evidenced for both CdS and PbS nanocrystals upon direct growth on TiO2 by SILAR, markedly weaker than the emission colloidal QDs adsorbed on TiO2[11, 23], verifies the great potential of this deposition technique to enhance electronic coupling and the concomitant charge transfer between QDs and the underlying TiO2 substrate.
CdS and PbS nanocrystals can be efficiently deposited as sensitizers on mesoporous TiO2 substrates via the SILAR method. Enhanced electronic coupling and interfacial electron transfer are confirmed upon direct growth of the chalcogenide nanocrystals on TiO2 through the marked quenching of their excitonic emission. The optical absorbance of CdS/TiO2 can be tuned over a narrow spectral window in the visible range, reflecting essentially the small exciton Bohr radius of CdS QDs that inhibits utilization of quantum size effects for light harvesting. On the other hand, PbS/TiO2 exhibits pronounced band gap tunability spanning the visible to the NIR range, due to the prominent quantum size effects of PbS QDs. However, PbS/TiO2 degrades severely upon air exposure requiring a protection layer for application in solar cell devices. In contrast, CdS/TiO2 is appreciably more stable under ambient conditions, though it degrades readily under visible light irradiation.
atomic force microscope
chemical bath deposition
dye sensitized solar cells
full width at half maximum
successive ionic layer adsorption and reaction.
This work is financially supported by the "Sensitizer Activated Nanostructured Solar Cells -SANS"/FP7-NMP-2009-SMALL3-246124 project. The authors thank Ivan Mora-Seró and Juan Bisquert for valuable suggestions.
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.