Research

Semiconductor/liquid interfaces play an important role in the conversion of solar energy to fuel using photoelectrochemical (PEC) cells. I look at the effect of organic linkers on electron transfer across interfaces, the effect of surface dipoles on band-edge positions, and the tethering of fuel forming catalysts to semiconductor surfaces.

Silicon

Hydride terminated Si surfaces are ideal for PEC cells because they have very, very low trap state densities, and the hydride is small, allowing for facile charge transfer. That is, hydride terminated Si is ideal in the absence of air and water. Upon exposure to air and water, hydride terminated Si develops a thick, insulating Si oxide layer that passivates the interface toward electron transfer - in short, it shuts down the cell.

The Lewis group uses a two step halogenation/alkylation reaction to stabilize Si surfaces in the presence of air and water. The chemistry can produce a Si(111) surface, on which every atop Si atom has a covalently linked methyl group. These CH₃-Si(111) surfaces are stable for months in air. Additionally, photoanodes can be functionalized in this way, and show stability in water containing PEC cells.

We have learned quite a lot about stabilizing Si through research on CH₃-Si(111), however, there still issues that prevent the CH₃-Si(111) surface from becoming a fixed component in an efficient PEC cell. One large issue being that the CH₃-Si(111) has poor kinetics for the HER reaction.

Figure 1. CH₃-Si(111) surfaces have high coverage, good resistance, and low defect densities. CH₂CHCH₂-Si(111) surfaces allow for further functionalization, however, do not have the coverage of the CH₃-Si(111) surface. The mixed allyl/methyl monolayer gives the best of both.

Mixed monolayers

Si can also be modified with organic groups capable of secondary chemistry, for example, the allyl group. The CH₂CHCH₂-Si(111) surface has a lower total coverage of the Si atop sites. This leaves non-functionalized sites ill-defined and susceptible to oxidation and trap state formation. To circumvent this, we developed a method to form mixed organic monolayers of allyl and methyl on Si(111). Mixed allyl/methyl monolayers, MMs, can have total coverage of Si atop sites, resistance to oxidation in air, and trap state densities indistinguishable from CH₃-Si(111) surfaces while having ~35% of Si atop sites linked to an allyl group. In this way, we can allow for covalent linkage of molecular catalysts without compromising the stability and electronics of the Si surface.

Figure 2. Mixed CH₃/thienylBr-Si(111) allow for coupling of small molecules via Heck mechanism. Such coupling allows for complete conjugation from the Si semiconductor through to the coupled molecule. There is no penalty in surface electronic quality, and no residual Pd on the Si(111) surfaces. Bound porphyrins can be selectively metallated with Co, Cu, or Zn.

Heck coupling chemistry

The Heck reaction has been used to couple olefins to a Si(111) surface that was functionalized with a mixed monolayer comprised of methyl and thienly groups. The coupling method maintained a conjugated linkage between the surface and the olefinic surface functionality, to allow for facile charge transfer from the silicon through the suface monolayer to the small molecule acceptor. While a Si(111) surface terminated with only thienyl groups displayed a surface recombination velocity, S, of 670 ± 190 cm s^-1, the mixed CH₃/SC₄H₃-Si(111) surfaces with a coverage of θ_SC₄H₃ = 0.15 ± 0.02 displayed a substantially lower value of S = 27 ± 9 cm s^-1. The olefinic substrates 4-fluorostyrene, redox active vinylferrocene, and protoporphyrin-IV were then coupled to the Pd-containing functionalized Si surfaces. The final Heck coupled surface exhibited S = 70 cm s^-1, indicating that high quality surfaces could be produced by this multi-step synthetic approach for tethering small molecules to silicon photo electrodes.