This award on solar energy research is co-funded by the Divisions of Chemistry, Materials Research, and Mathematical Sciences of the Directorate for Mathematical and Physical Sciences. The team of three scientists from the University of Texas at Austin is focused on the design, development, fabrication, testing, and understanding, (through mathematical modeling) of performance characteristics of nanostructured, semiconducting photocatalysts for high efficiency photoelectrochemical (PEC) devices. These devices will be used for the direct conversion of solar photons into hydrogen fuel via water electrolysis. The key to the discovery of these new photocatalysts is finding new compositions and structures at the nanometer scale. A combination of computational mathematics and rapid synthesis and screening tools are used to reveal structure/function relationships. Although the modeling of PEC systems has been performed at the macroscale, the behavior changes significantly at the nanoscale and these systems have not been simulated previously. The research group prepares nanostructured PEC thin films of promising materials compositions as identified by the rapid screening techniques and refines these structures via mathematical modeling and photoelectrochemical experiments. The nanostructured films are grown using a technique referred to as reactive ballistic deposition (RBD). This technique enables the tuning of photocatalytic and optical properties for high visible-light activity. In addition to the researchers' efforts to develop alternative energy sources from water and sunlight, the group is creating streaming animated videos on energy for high school students in both English and Spanish. High school student researchers and faculty from four-year undergraduate institutions assist in the research efforts during the summer months, designing and constructing experimental apparatus and collaborating with scientists at the national laboratories and at other universities.
In our work, we have demonstrated multiple techniques to synthesis, screen, and characterize new photoactive materials for use in photoelectrochemical applications. We used scanning electrochemical microscopy (SECM) to quickly screen Pb-Cr bimetallic oxides and find high-performing candidates. A picoliter dispenser was used to create spot arrays with varied molar ratios of Pb and Cr as well as the single metal oxides. The highest photocurrent was observed in the spots with a molar ratio of approximately 2:1 Pb:Cr. These best candidates were then prepared on a larger scale and tested as photoanodes to confirm that they were in fact photoactive materials. Pb2CrO5 produced 0.22 mA/cm2 of photocurrent under simulated sunlight, far exceeding that of PbCrO4 (0.03 mA/cm2) and Pb5CrO8 (0.06 mA/cm2). In addition to this synthesis and characterization of new photoactive materials, we have also demonstrated a new direct method of characterization of semiconductor powders. The advantage of this technique is that it does not require the fabrication of an electrode, either thin-film or pellet, to evaluate a sample. Because of this, less stable semiconductors, such as hydrogen- and nitrogen-doped TiO2, which may change during the electrode fabrication, can be tested in the same state as they were prepared. "Standard" powders of TiO2, WO3, and BiVO4 were used to confirm that current-voltage and action spectrum measurements generated by the powder slurry technique matched reasonably well with known literature results. Accordingly, we found that hydrogen-doped TiO2, even with absorption across a significant portion of the visible region, did not actually exhibit any visible-region photocurrent response. Nitrogen-doped TiO2, on the other hand, did in fact produce photocurrent at wavelengths up to 520 nm.
