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Summary:

We have (1) synthesized negatively charged CdS quantum dots (QDs), (2) immobilized the QDs onto ITO (indium-tin oxide) substrate and fabricated the CdS QDs semiconductive photoelectrodes by the layer-by-layer assembly technique, (3) fabricated photoelectrodes covered with multilayered QDs and different sensitizers or dye molecules, (4) set up the apparatus for photoelectrochemical measurements and (5) investigated the photoelectrochemical behaviors of these electrodes. UV absorption and fluorescence measurements confirmed that stable, uniform CdS QDs had been synthesized. The negatively charged CdS QDs can be readily immobilized onto conductive ITO substrate by alternately assembling polydiallyldimethylammonium and QDs. Such an approach allows precise control of the thickness of the QDs film. In order to study the effect of different sensitizers on the photoelectrochemical behavior of the as-prepared electrodes, we have tested a few different sensitizers of for their spectral and photochemical properties (See Figure 1 for the sensitizer structures).

Figure 1. Structures of Pt-containing photosensitizers.

The photoelectrochemical set-up is shown in Figure 2a. Figure 2b shows the photocurrent response at an electrode covered with multilayered QDs. A significant photocurrent was observed, indicating the QD-modified electrode is capable of converting light energy into electrical current. A closer analysis of the current polarity upon irradiation reveals that a photooxidation process takes place. We found that electrodes constructed with QDs that were synthesized with different CdS and citrate ratios produced different amounts of photocurrent. The influence of the number of layers on the photoelectrochemical performance was also investigated. Electrode assembled for ten layers shows the best photoelectrochemical performance.

Figure 2. The home-made fluorescent spectrophotometer (a) and the photovoltaic current response at the (PDDA/QDs)₅ film (b). DY 2300 is a bipotentiostat.

We also investigated the sensitizing effect of the sensitizer molecules shown in Figure 1. Interestingly, the sensitizer-coated multilayer QDs photoelectrode behaves differently from the sensitizer-free electrode. Figure 3 shows photoelectric current response at a PDDA/QD film coated with FcNNNPtCl. Upon irradiation, instead of oxidation current, a reduction current was observed. The other two sensitizers (CH₃phNNNPtCl and   (CH₃)₂NphNNNPtCl), despite their structural and photochemical differences, exhibit similar photoelectrochemical behavior to the FcNNNPtCl. The finding in the current switch may be of both fundamental and applied significance. Further investigation into the mechanism and possible application is underway.

Figure 3. Photovoltaic current responses at a PDDA/QD film (black line) and a PDDA/QD film coated with FcNNNPtCl (red line).

Impacts of Research on the PI and his Student:

This grant enabled the PI to apply his expertise (electrochemistry, surface chemistry and material synthesis) to energy-related research. Performing such research allowed the PI to interact with other researchers in an NSF-funded center (Center for Energy and Sustainability) and to initiate related research activities (e.g., fuel cell testing and development and catalysis synthesis and characterization). Angela Ramos, an undergraduate chemistry major partially supported by an NSF-REU (Research Experience for Undergraduates) had worked closely with the PI on this project in the summer of 2010 and in the academic year between 2009 and 2010 through directed undergraduate research. As the research activities have drastically expanded in the past year, she will be supported by this grant starting from September 2010 so that more effort can be made to ensure the progress. Conducting research in the PI's group has exposed Angela to various research fields and motivated her to pursue an advanced degree in chemistry.