(413e) Using Supercritical Fluids for CdS and CdTeS Nanoparticle Decorated TiO2 Nanostructures

TiO2 nanowires were sensitized with CdS and CdTeS quantum dots which were grown using an in situ colloidal method using both conventional organic solvents and supercritical carbon dioxide (scCO2). No pretreatment of the TiO2 nanowires was required prior to nanoparticle generation. The resulting nanostructure assembly and composition was confirmed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The electronic structure of TiO2 nanowires was preserved as indicated by Raman spectroscopy. The sensitization of the TiO2 nanowires by the QD's was confirmed by photocurrent measurements.

TiO2 has been a well-studied semiconductor photoactive material with the major challenge in using TiO2 being its large band-gap energy preventing efficient visible light absorption (3.0-3.2 eV). Narrow band-gap inorganic semiconducting materials including CdS, PbS, Bi2S3, CdSe and InP have been considered as promising sensitizers to enhance utilization of sunlight for energy production in photovoltaic devices. In particular, cadmium calchogenide semiconducting nanocrystals (CdX; X=S, Se and Te) belonging to II-VI semiconductors are very attractive as sensitizers for TiO2 due to their size-tunable optical properties. CdS/TiO2 and CdS quantum dots decorated TiO2 nanobelts and nanotubes have been prepared in the past and have shown enhanced optical properties.[1, 2] In comparison, semiconducting CdTeS sensitized TiO2 has not yet been well explored. CdTeS quantum dots are potentially superior to CdS for several applications including solar cells because of their additional near-IR activity. One-dimensional TiO2 nanostructures (nanotubes, nanowires) as supports of light harvesting semiconducting nanocrystals have been shown to be more effective than TiO2 [24] Wang et al used sonoelectrochemical synthesis to incorporate CdS nanoparticles on TiO2 nanotubes [7]. All these methods add an additional step in obtaining the QD decorated nanoparticles, which leads to an expensive and difficult process to efficiently attach hydrophobic QD's to the hydrophilic TiO2 surface. Herein, we present a simple method to prepare TiO2 nanowires sensitized with CdS/CdTeS without the need of a complex pretreatment. Sonication alone of TiO2 nanowires in TOA prior to in situ growing of CdS QD's in this solvent allowed obtaining CdS QD's decorated TiO2 nanowires. This method allowed the assembly of strongly anchored CdS and CdTeS QD's onto TiO2 nanowires.

The method followed for CdS QD's preparation has been presented elsewhere [9]. In a typical experiment, a 25 mL sample of TOA and a magnetic stirring bar were placed in a 100 mL three-necked flask under nitrogen equipped with a reflux condenser, a thermometer, and a thermocouple for automatic temperature control. When the temperature was stable at 235 °C, a solution of 0.5 g of Cd[S2CN(C2H5)2]2 in 9 mL of TOP was rapidly injected into the flask. 30 minutes after the addition of the cadmium diethyldithiocarbamate solution, the heater was removed to cool the reaction mixture. When the temperature dropped to approximately 75 °C, a large excess of methanol/ ethyl alcohol was added followed by separation of the quantum dots through centrifugation. The QDs were washed with methanol, and then dispersed into toluene. The mixture of QDs in toluene was further filtered under vacuum to remove any insoluble material. The solids obtained were thoroughly washed with toluene under sonication for 30 minutes, filtered to remove the nanoparticles weakly bonded to the TiO2 nanowires and dried under vacuum overnight at 40°C.

The morphologies of CdS and CdTeS nanoparticles anchored to the TiO₂ nanowire surface were clearly observed in the TEM images after sonication of the nanocomposites in toluene. CdTeS showed a high PL peak in the near IR region (650-800 nm) as a result of Te inclusion, in good agreement with previous studies [10, 11] (Figure 1).

Raman spectra for TiO2 nanowires and the assemblies showed the characteristic peaks of TiO2 that are similar to that of titanate. The similarity in the peak intensities shows a preserved structure of the TiO2 nanowires meaning that the CdS or CdTeS nanoparticles attachment to TiO2 is non-destructive as suggested by X. Li et al in SWCNT--CdS nanoparticle hybrids. Two signals corresponding to the longitudinal optical phonon mode (1

LO) and its overtone (2-LO) of the CdS nanoparticles at ca. 300cm^-1 and 600cm^-1 were found [18,19]. The signal found at ca. 300 cm^-1 in CdTeS/TiO2 spectrum is attributed to CdS-like LO phonons in CdTeS alloys by Fischer et al [20]. The signals from the nanoparticles are weak because the most abundant compound in the composites is TiO2. To further characterize the photosensitization of the TiO2 nanowires, photovoltaic devices were fabricated using commercial TiO2, TiO2 nanowires and the assemblies. Efficiency of solar devices depends on many factors (active layer, surface cleanliness, adsorption time, thickness of the coatings) that were not optimized. Figure 3 shows an increase in efficiency of 217% for CdS/TiO2 nanowires and 264% for CdTeS/TiO2 nanowires indicating the superior performance of our materials over commercial TiO2 and bare TiO2 nanowires.

A simple method was developed for preparing CdS/CdTeS anchored TiO2 nanowires without a pretreatment step or ligand exchange. The nanomaterias obtained showed an enhancement in its photocurrent properties compared to commercial TiO2 and bare TiO2 nanowires. Even if the solar device is not fully optimized and better results can be anticipated by controlling the parameters that affect the cell performance, the enhancement in efficiency render such materials interesting for applications like photovoltaics.

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