Showing papers by "Nathan S. Lewis published in 1994"

Flux-Matching Conditions at TiO2 Photoelectrodes: Is Interfacial Electron Transfer to O2 Rate-Limiting in the TiO2-Catalyzed Photochemical Degradation of Organics?

Abstract: A flux-matching condition has been applied to determine whether O_2 reduction is rate-limiting under photocatalytic conditions for the degradation of CHCl_3 at rutile TiO_2 single-crystal electrodes. In this approach, the potential dependence of the photooxidation current density is compared to the potential dependence of the current density for O_2 reduction. The potential at which the oxidation and reduction fluxes are equal determines the operating potential and the steady-state flux that will flow through the crystal under no applied bias. If this flux-matching condition occurs when the cathodic flux equals the flux of photogenerated carriers, then the predicted quantum yield should approach unity; otherwise, recombination should be significant in the TiO_2. Our measurements indicate that significant recombination will occur for the oxidation of typical organic molecules in H_2O over a range of pH values. The data also indicate that Pt catalysis of O_2 reduction should be beneficial for the oxidation of organic molecules, as would the use of alternate electron acceptors such as Fe(CN)_6^(3-). The O_2 reduction data and rotating disk electrode data collected in this work allow a quantitative comparison to theoretical estimates of the electron transfer rate constant for O_2 reduction at TiO_2. We also present an elucidation of the previously published theoretical treatments of TiO_2 charge transfer rate constants in view of the new data collected herein.

Measurement of barrier heights of semiconductor/liquid junctions using a transconductance method: Evidence for inversion at n-Si/CH3OH-1,1′-dimethylferrocene+/0 junctions

Abstract: Transconductance measurements have been used to characterize the space-charge regions of various n-Si/liquid contacts. To perform these measurements, Si electrodes were photolithographically processed to introduce p^+-contact areas into the surface of an n-type Si electrode. The electrical conductance between these p^+ regions was then used to probe the minority carrier concentration in the near-surface region of the n-type Si. Unlike conventional differential capacitance or current-voltage measurements, these transconductance measurements can be performed under near-equilibrium conditions and can be performed in the presence of gaseous ambients or when the sample is in contact with ionically conducting electrolyte solutions. In contact with the electrolyte solutions, faradaic and solid-state conduction pathways were distinguished using ac impedance measurements. The impedance spectra provided clear evidence that contact with Me_2Fc (1,1'-dimethylferrocene)^(+/0) and Fc^(+/0) redox couples in CH_3OH(l)-1.0 M LiClO_4 formed an inversion layer in the n-Si, but that CH_3OH(l)-1.0 M LiClO_4-Me_(10)Fc^(+/0) solutions did not yield an inversion layer. These observations are consistent with prior current-voltage measurements on these junctions. The barrier heights of the n-Si/CH_3OH-Me_2Fc^(+/0) and n-Si/CH_3OH-Fc^(+/0) junctions were determined to be 1.01 and 1.02 V, respectively. These measurements provide new insight into the photoelectrochemical behavior of SI/CH_3OH contacts and provide an alternate method for characterizing the energetics of semiconductor/liquid contacts.

Experimental Measurement of Quasi-Fermi Levels at an Illuminated Semiconductor/Liquid Contact

Abstract: A novel electrode geometry and contacting procedure has allowed measurement of the quasi-Fermi levels, i.e., the apparent electrochemical potentials, of electrons and holes at an illuminated semiconductor / liquid contact. The key feature of our experiments is the use of a lithographically patterned, high purity (100-400 Ω-km n-type float zone material), low dopant density Si sample in contact with CH_3OH-dimethylferrocene^(+/o) solutions. The photogenerated carriers can be collected at the back side of the Si sample through a series of diffused n+ and p^+ points. The lifetime of photogenerated carriers approaches 2 ms in this sample, indicating that electronhole recombination is minimized in the bulk of the semiconductor. Furthermore, surface recombination is minimized by use of low saturation current density, ohmic-selective contacts at the back of the sample. The solid/liquid contact also has a low recombination rate. Therefore, the potentials measured at the diffused points yield values for the quasi-Fermi levels of electrons and holes under illumination of the semiconductor/ liquid contact. Transient photovoltage measurements have also been performed to confirm quantitatively that the quasi-Fermi levels are flat across the Si samples used in this work.

Irreversible electrocatalytic reduction of V(V) to V(IV) using phosphomolybdic acid

Abstract: Although VO_22^+(aq) reduction is kinetically slow at glassy carbon and Pt electrodes, phosphomolybdic acid is shown to catalyze the electrochemical reduction of VO_2^+(aq) to VO_2^+(aq) in 1.0 M H_2SO_4(aq). A second-order rate constant of 33 M^(-1) s^(-1) was observed for this process. ^(31)P NMR spectra demonstrated that PMo_(11)VO_(40)^(4-) and PMo_(18)V_2O_(40)^(50-) were the dominant P-containing species under electrocatalytic conditions. The incorporation of V^V into the polyoxoanion led to a shift in potential from E^o(VO^(2+)(aq)/VO^(2+)(aq)) = +0.80 V vs Ag/AgCl for free V^V/V^(IV) to Eo' = +0.55 V vs Ag/AgCl for V^(V)/V^(Iv) bound in the heteropolyoxometalate (PMo_(11)VO_(40_^(4-)). This shift in formal potential corresponded to an equilibrium constant of 1.7 X 10^4 M^(-1) for preferential binding of V^V over V^(IV) by the heteropolyoxoanion. This negative shift in redox potential, combined with the slow electrochemical kinetics of free VO_2^+(aq) reduction and with the facile reaction of bound V^(IV) with free V^V in 1.0 M H_2SO_4(aq), resulted in the irreversible electrocatalytic reduction of VO_2^+(aq) to VO^(2+)(aq).

Irreversible Electrocatalytic Reduction of V(V) to V(IV) Using Phosphomolybdic Acid.

Abstract: Although VO_22^+(aq) reduction is kinetically slow at glassy carbon and Pt electrodes, phosphomolybdic acid is shown to catalyze the electrochemical reduction of VO_2^+(aq) to VO_2^+(aq) in 1.0 M H_2SO_4(aq). A second-order rate constant of 33 M^(-1) s^(-1) was observed for this process. ^(31)P NMR spectra demonstrated that PMo_(11)VO_(40)^(4-) and PMo_(18)V_2O_(40)^(50-) were the dominant P-containing species under electrocatalytic conditions. The incorporation of V^V into the polyoxoanion led to a shift in potential from E^o(VO^(2+)(aq)/VO^(2+)(aq)) = +0.80 V vs Ag/AgCl for free V^V/V^(IV) to Eo' = +0.55 V vs Ag/AgCl for V^(V)/V^(Iv) bound in the heteropolyoxometalate (PMo_(11)VO_(40_^(4-)). This shift in formal potential corresponded to an equilibrium constant of 1.7 X 10^4 M^(-1) for preferential binding of V^V over V^(IV) by the heteropolyoxoanion. This negative shift in redox potential, combined with the slow electrochemical kinetics of free VO_2^+(aq) reduction and with the facile reaction of bound V^(IV) with free V^V in 1.0 M H_2SO_4(aq), resulted in the irreversible electrocatalytic reduction of VO_2^+(aq) to VO^(2+)(aq).