Showing papers by "Nathan S. Lewis published in 1980"

Photoreduction at illuminated p-type semiconducting silicon photoelectrodes. Evidence for Fermi level pinning

Abstract: Studies of p- and n-type Si electrodes are reported which show that semiconducting Si electrode surfaces do not allow efficient H/sub 2/ evolution in the dark (n type) or upon illumination with band gap or greater energy light (p type). The key experiment is that N,N'-dimethyl-4,4'-bipyridinium (PQ/sup 2 +/) is reversibly reduced at n-type Si in aqueous media at a pH where H/sub 2/ should be evolved at nearly the same potential, but no H/sub 2/ evolution current is observable. The PQ/sup 2+/+/.system may be useful as an electron-transfer mediator, since PQ/sup +/.can be used to effect generation of H/sub 2/ from H/sub 2/O using a heterogeneous catalyst. The PQ/sup +/.can be produced in an uphill sense by illumination of p-type Si in aqueous solutions. Studies of p-type Si in nonaqueous solvents show that PQ/sup 2 +/, PQ/sup +/., Ru(bpy)/sub 3//sup 2 +/, Ru(bpy)/sub 3//sup +/, and Ru(bpy)/sub 3//sup 0/ are all reducible upon illumination of the p-type Si. Interestingly, each species can be photoreduced at a potential approx. 500 mV more positive than at a reversible electrode in the dark. This result reveals that a p-type Si-based photoelectrochemical cell based on PQ/sup 2+/+/., PQ/sup +/l/sup 0//, Ru(bpy)/sub 3//sup 2+/+/,more » Ru(bpy)/sub 3//sup +/0/, or Ru(bpy)/sub 3//sup 0/-/ would all yield a common output photovoltage, despite the fact that the formal potentials for these couples vary by more than the band gap (1.1 V) of the photocathode. These data support the notion that p-type Si exhibits Fermi level pinning under the conditions employed.Surface chemistry is shown to be able to effect changes in interface kinetics for electrodes exhibiting Fermi level pinning.« less

Synthesis and characterization of a photosensitive interface for hydrogen generation: Chemically modified p-type semiconducting silicon photocathodes.

Abstract: p-Si photocathodes functionalized first with an N,N′-dialkyl-4,4′-bipyridinium redox reagent, (PQ2+/+-)surf, and then with a Pt precursor, PtCl62-, give significant efficiency (up to 5%) for photoelectrochemical H2 generation with 632.8-nm light. Naked p-Si photocathodes give nearly zero efficiency, owing to poor H2 evolution kinetics that are improved by the (PQ2+/+-)surf/Pt modification. The mechanism of H2 evolution from p-Si/(PQ2+/+-)surf/Pt is first photoexcitation of electrons to the conduction band of Si followed by (PQ2+)surf → (PQ+-)surf reduction. The dispersion of Pt then catalyzes H2O reduction to give H2 and regeneration of (PQ2+)surf. The overall energy conversion efficiency rivals the best direct optical to chemical conversion systems reported to date.

Heterogeneous electron transfer at designed semiconductor/liquid interfaces. Rate of reduction of surface-confined ferricenium centers by solution reagents

Abstract: Reduction of surface-confined ferricenium by solution reductants iodide, diindenyliron, (eta/sup 5/-C/sub 5/H/sub 5/)/sub 4/Fe/sub 4/(CO)/sub 4/, 1,1'-dimethylferrocene, ferrocene, and phenylferrocene has been studied in EtOH-0.1 M (n-Bu/sub 4/N)ClO/sub 4/ and also in H/sub 2/O-NaClO/sub 4/ for iodide. The surface-confined ferricenium can be generated on n-type Si by illumination of the electrode at some potential more positive than approx. -0.2 V vs. SCE. Linear sweep voltammetry was used to directly measure the time dependence of the surface ferricenium concentration in the dark and in the presence of the various reducing agents. The rate constant for the reaction with iodide was measured in both solvents, and the value in EtOH is somewhat lower than would be estimated from homogeneous solution reaction of ferricenium with iodide under the same conditions. Values of reaction rate constants for reaction with all other reductants indicate that the reduction rate is mass transport, not charge transfer, limited under the conditions employed. The relative ordering of the fast reductants has been determined to be diindenyliron > (eta/sup 5/-C/sub 5/H/sub 5/)/sub 4/Fe/sub 4/(CO)/sub 4/ approx. 1,1'-dimethylferrocene > ferrocene approx. phenylferrocene. Most of the derivatized surfaces have been prepared from (1,1'-ferrocenediyl)dichlorosilane, but preliminary results with polyvinylferrocene modified and (1,1'-ferrocenediyl)dimethylsilane derivatizedmore » surfaces are similar. Very high coverage surfaces from (1,1'-ferrocenediyl)dichlorosilane show some evidence for selective reduction of the more accessible ferricenium centers when a fast reductant is used. Steady-state photoanodic current at a given concentration of reductant generally accords well with the measured k/sub et/ values, and for the iodide experiments the steady-state photocurrent is directly proportional to surface coverage of electroactive ferrocene.« less

CHEMICALLY DERIVATIZED SEMICONDUCTOR PHOTOELECTRODES - A TECHNIQUE FOR THE STABILIZATION OF n-TYPE SEMICONDUCTORS.

Abstract: Pretreated Au, Pt, n-type Si, and n-type Ge can be derivatized with trichlorosilylferrocene, (1,1'-ferrocenediyl)dichlorosilane, and 1,1'-bis(triethoxysilyl)ferrocene to yield electroactive, surface-attached, oligomeric ferrocene material. Derivatized, n-type semiconductors exhibit photoeffects expected for such an electrode material; irradiated derivatized n-type Si can be used to effect the oxidation of solution reductants by mediated electron transfer, unique proof for which comes from the semiconductor electrode that responds to two stimuli, light and potential. The sustained, mediated oxidation of Fe(CN)/sub 6//sup 4 -/ in aqueous solution in an uphill sense by irradiation of derivatized n-type Si is possible whereas a naked n-type Si undergoes decomposition to SiO/sub x/ at a rate too fast to allow sustained energy conversion. This establishes the principle of manipulating interfacial charge-transfer kinetics for practical applications.