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46040-AC10
Spectroscopy of Photovoltaic and Photoconductive Doped-Oxide Electrodes Related to Photocatalysis and Solar Energy Conversion

Daniel R. Gamelin, University of Washington

We have, with ACS-PRF support, been studying the mechanisms of sensitized photoinduced charge separation in transition-metal-doped oxides by photocurrent, absorption, and magnetic circular dichroism (MCD) spectroscopies. Our experiments allowed us to directly observe the electronic transitions responsible for initiating charge separation and we found a strong correlation between photocurrent activity and charge transfer transitions between the transition metal dopant and semiconductor. Similar to Co²⁺: ZnO,¹ we demonstrated that sub-bandgap transitions can give rise to photocurrent by charge transfer and excitonic state mixing. In the literature it has been suggested that d-d transitions are responsible for sub-bandgap photocurrent.^2,3 Our results indicate that ligand field transitions are not directly photoactive but may enhance photoelectrochemical responses by sensitization of charge transfer transitions.

Synthesis and construction of photovoltaic cells. Films of Co²⁺:ZnO, Ni²⁺:ZnO, Cr³⁺:TiO₂, Co²⁺:TiO₂, and Ni²⁺:InTaO₄ were fabricated using a variety of techniques including spin coating of nanoparticle colloidal suspensions,^4,5 sol solutions, and products of solid state synthesis⁶ on transparent conducting glass. Photovoltaic cells were constructed from the oxide photoanodes, an I^-/ I₃^- electrolyte, and platinum electrode. Many of the cells responded strongly to light, with current changes on the order of micro‐amps upon illumination with a 150 W halogen lamp.

Spectroscopy and photocurrent activity of transition metal doped oxides. Electronic transitions of Co²⁺:ZnO, Ni²⁺:ZnO, Cr³⁺:TiO₂ and Co²⁺:TiO₂ have been well studied by our group through absorption and MCD spectroscopies.^1,4,5,7 Building on our fundamental understanding of these materials and using ligand field and charge transfer calculations, we were able to give detailed electronic assignments to our spectra (Figure 1). As for Ni²⁺:InTaO₄, a sol-gel synthesis of Ni²⁺:InTaO₄ was successfully developed, as confirmed by XRD, and electronic absorption and MCD spectra of the material were collected for the first time (Figure 2). We observed a bandgap of 3.8 eV in InTaO₄, which narrowed to 3.2 eV upon nickel doping and the rise of a charge transfer transition. Pairing photocurrent action spectroscopy with the above experiments, we consistently observed internal quantum efficiencies (IQEs) corresponding to bandgap, metal-to-ligand conduction band charge transfer (ML_CBCT), ligand valence band-to-metal charge transfer (L_VBMCT) and some ligand field transitions (Figures 1 and 2). As a result, we successfully identified the electronic transitions giving rise to photocurrent.

Photoinduced charge separation processes. Several general trends were observed in our photocurrent experiments. Direct bandgap excitation always gave the highest efficiencies, followed by charge transfer, and certain spin-allowed ligand field transitions. The d‐d transitions that were inactive appear to be much lower in energy than the charge transfer transitions. A list of photoactive transitions in the metal oxides studied is given in Table 1. These trends along with a comparison of IQE values and transition energies, suggest that photoinduced charge separation in sub-bandgap regions are facilitated by the degree of mixing between ML_CBCT or L_VBMCT and excitonic states. An example of the energy level diagram leading to charge separation for Co²⁺:TiO₂ is given in Figure 3. These results agree quite well with the proposed configuration interaction model in Figure 4, where escape of the photogenerated carrier is governed by mixing between charge transfer and excitonic states. If ΔE is small, the large effective Bohr radius will allow extensive mixing of the two states and carrier migration is possible (Figure 4a). On the other hand, if ΔE is large, the effective Bohr radius will be small, limiting the amount of mixing of the two states. As a result, carrier migration to the electrical contact will be inefficient (Figure 4b). As for the ligand field regions, IQEs were generally extremely small and the above method did not apply. This suggests that they are not directly photoactive, but may improve photoresponses by sensitization of charge transfer transitions.

Further studies. Experiments in synthesizing Ni²⁺:InTaO₄ will continue for electrochemical studies and determination of band potentials. Photoconductivity experiments are in progress. As new oxide materials are being prepared in our lab, these results will be used to understand the role of doping in carrier generation, where the charges can be used for driving uphill redox reactions, such as water splitting.

References

1.       Liu, W. K., Mackay Salley, G., Gamelin, D. R. J. Phys. Chem. B. 2005, 109, 11486-11495.

2.       Kato, H., Kudo, A. J. Phys. Chem. B. 2002, 106, 5029-5034.

3.       Zou, Z., Ye, J., Kazuhlro, S., Arakawa, H. Nature. 2001, 414, 625-627.

4.       Schwartz, D., Norberg, N. S., Nguyen, Q. P., Parker, J. M., Gamelin, D. R. J. Am. Chem. Soc. 2003, 125, 13205-13218.

5.       Bryan, J. D., Santangelo, S. A., Keveren, S. C., Gamelin, D. R. J. Am. Chem. Soc. 2005, 127, 15568-15574.

6.       Zou, Z., Ye, J., Kazuhlro, S., Arakawa, H. J. Mater. Res. 2002, 17, 1419-1424.

7.       Bryan, J. D., Heald, S. M.,Chambers, S. A., Gamelin, D. R. J. Am. Chem. Soc. 2004, 126, 11640-11647.

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