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Showing papers by "Nathan S. Lewis published in 1992"


Journal ArticleDOI
TL;DR: In this paper, a stochastic charge transfer formalism for photoelectrolysis reactions was proposed to predict a light intensity threshold for sustaining the net electrolysis of water using semiconductor electrodes.
Abstract: Conventional photoelectrochemical and photovoltaic theory predicts a light intensity threshold for sustaining the net electrolysis of water using semiconductor electrodes, but a stochastic charge-transfer formalism for photoelectrolysis reactions does not predict such threshold behavior.

109 citations


Journal ArticleDOI
TL;DR: In this paper, a series of metalloporphyrin hydride complexes of the type K[M(Por)(L)(H)] (M - Ru, Os; Por - OEP, TMP; L = THF, *Im, PPh{sub 3}, pyridine) has been synthesized by stoichiometric protonation of the corresponding K{sub 2}[M (Por)], followed by addition of L. The rate constant was determined to be 88 {+-} 14 M{sup -1} s
Abstract: A series of metalloporphyrin hydride complexes of the type K[M(Por)(L)(H)] (M - Ru, Os; Por - OEP, TMP; L = THF, *Im, PPh{sub 3}, pyridine) has been synthesized by stoichiometric protonation of the corresponding K{sub 2}[M(Por)], followed by addition of L. The addition of excess acids to these hydrides resulted in the elimination of dihydrogen. The kinetics showed no evidence for a bimolecular mechanism for this process and suggest simple protonation of the metal-hydride bond followed by dihydrogen loss. One-electron oxidation of the metal hydrides also resulted in dihydrogen formation. The kinetics of the oxidatively induced hydrogen evolution step from K[Ru(OEP)(THF)(H)] were examined and indicate a biomolecular mechanism in which two metal hydrides reductively eliminate one dihydrogen molecule. The rate constant was determined to be 88 {+-} 14 M{sup -1} s{sup -1}. These reaction mechanisms are discussed in the context of designing bimetallic proton reduction catalysts. The metal hydride K[Ru(OEP)(THF)(H)], was also synthesized by heterolytic activation of H{sub 2}. This hydride is a good one-electron reductant (-1.15 V vs FeCp{sub 2}) and is capable of reducing, by hydride transfer, the NAD{sup +} analogue, 1-benzyl-N,N-diethyl-nicotinamide. This nicotinamide reduction by a hydride formed from heterolytic dihydrogen activation is suggested as the mechanismmore » for hydrogenase enzymes. 38 refs., 4 figs., 3 tabs.« less

71 citations


Journal ArticleDOI
TL;DR: A series of monometallic dihydrogen complexes of the type M(OEP)(L)(H{sub 2}) (M = Ru, Os; L = THF, *Im) was synthesized and characterized by {sup 1}H NMR as mentioned in this paper.
Abstract: A series of monometallic dihydrogen complexes of the type M(OEP)(L)(H{sub 2}) (M = Ru, Os; L = THF, *Im) was synthesized and characterized by {sup 1}H NMR. The H-H bond length was found to increase when Os was replaced by Ru or when *Im was replaced by THF. The bond distances (as determined by T{sub 1}) range from 0.92 to 1.18 {angstrom}. The first example of a bimetallic bridging dihydrogen complex, Ru{sub 2}(DPB)(*IM){sub 2}(H{sub 2}), was also prepared. The H{sub 2} ligand is simultaneously bound to both Ru-metal centers. High-field {sup 1}H NMR experiments (620 MHz) revealed a -7.37 Hz dipolar splitting of the H{sub 2} ligand for this complex. Analysis of this splitting suggests that the H{sub 2} ligand is bound with the H-H axis perpendicular to the Ru-Ru axis. These complexes were examined as possible catalysts for the oxidation of dihydrogen through prior heterolytic activation of H{sub 2}. Only Ru(OEP)(THF)(H{sub 2}) can be conveniently deprotonated. Ru(OEP)(THF)(H{sub 2}) is also implicated in the Ru-(OEP)((THF){sub 2}) catalyzed isotopic exchange between H{sub 2} and D{sub 2}O in THF solution. Each step for this mechanism has been elucidated. We have also achieved catalytic dihydrogen oxidation using [Ru(OEP)]{sub 2} adsorbed onto graphite. Twomore » mechanisms for this ruthenium porphyrin catalyzed dihydrogen oxidation are presented and compared. 54 refs., 12 figs., 5 tabs.« less

48 citations


Journal ArticleDOI
TL;DR: The surface chemistry of n-type Si electrodes that had been etched, exposed to electrolyte, and electrochemically cycled has been probed using high-resolution X-ray photoelectron spectroscopy (XPS) as mentioned in this paper.
Abstract: The surface chemistry of n-type Si electrodes that had been etched, exposed to electrolyte, and electrochemically cycled has been probed using high-resolution X-ray photoelectron spectroscopy (XPS). n-Si surfaces etched in hydrofluoric acid-ethanol solutions (in air or N 2 ambients) displayed spectra in the Si 2p region that were free of detectable substrate oxide signals (≤5×10 -11 mol cm -2 SiO 2 ; equivalent to ≤4% of a monolayer)

24 citations


ReportDOI
15 Sep 1992
TL;DR: In this paper, the first atomic resolution images of polymers were obtained by using partially insulated STM tips as ultramicroelectrodes for the generation of lithographic features.
Abstract: : Our efforts through the last contract period have involved the fundamental study of the STM imaging process, the investigation of the use of partially insulated STM tips as ultramicroelectrodes, and applications of STM for the generation of lithographic features. STM investigations of conducting polymers dispersed on graphite surfaces and doped with I2 revealed that, although the conducting backbone of the polymer could be imaged under a range of tunneling conditions, the non-conducting side groups of the polymer were not imaged. These images represent the first atomic resolution images of polymers, and studies of this studies of this type, involving the simultaneous imaging of nominally conducting and non-conducting groups, should prove essential in developing a theoretical understanding of the contrast mechanisms which dominate the imaging of individual adsorbed molecules with STM. Studies of ultramicroelectrodes, fabricated initially as partially insulated STM tips for use in solutions containing significant concentrations of electroactive species, have revealed the possibility of fabricating electrodes as small as one nanometer in diameter.

2 citations


Journal ArticleDOI
TL;DR: In this article, a quantitative study has been performed on the stability of GaAs surfaces to photoanodic corrosion in contact with the 1.0 M KOH(aq)-0.10 M K[sub 2]Se(aq) electrolyte.
Abstract: A quantitative study has been performed on the stability of GaAs surfaces to photoanodic corrosion in contact with the 1.0 M KOH(aq)-0.10 M K[sub 2]Se(aq)-0.01 M K[sub 2]Se[sub 2](aq) electrolyte. In this electrolyte, chemically etched n-type GaAs electrodes displayed significant photocorrosion in competition with faradaic charge transfer to Se[sup 2[minus]](aq). Chemisorption of group VIIIB metal ions, including complexes of Co(III), Os(III), and Ru(III), onto the etched GaAs surface yielded improved current-voltage (I-V) behavior of the GaAs photoanode and also resulted in a significant reduction in photocorrosion. This behavior implies that the chemisorbed metal ions act to increase the rate of hole transfer to the Se[sup 2[minus]] donor species, as opposed to the passivation of nonradiative surface recombination. Related experiments on n-GaAs photoanodes in contact with the 1.0 M KOH(aq)-0.3 M Te[sup 2[minus]](aq)-0.01 M Te[sub 2][sup 2[minus]](aq) electrolyte have also been performed. The I-V properties of n-GaAs, p-GaAs, and Sn-doped In[sub 2]O[sub 3] electrodes have been investigated in this electrolyte, and these measurements have been used to elucidate common mechanistic features of charge-transfer reactions for the Se[sup 2[minus]](aq) and Te[sup 2[minus]](aq) redox systems.

Journal ArticleDOI
TL;DR: A series of monometallic dihydrogen complexes of the type M(OEP)(L)(H{sub 2}) (M = Ru, Os; L = THF, *Im) was synthesized and characterized by {sup 1}H NMR.
Abstract: A series of monometallic dihydrogen complexes of the type M(OEP)(L)(H{sub 2}) (M = Ru, Os; L = THF, *Im) was synthesized and characterized by {sup 1}H NMR. The H-H bond length was found to increase when Os was replaced by Ru or when *Im was replaced by THF. The bond distances (as determined by T{sub 1}) range from 0.92 to 1.18 {angstrom}. The first example of a bimetallic bridging dihydrogen complex, Ru{sub 2}(DPB)(*IM){sub 2}(H{sub 2}), was also prepared. The H{sub 2} ligand is simultaneously bound to both Ru-metal centers. High-field {sup 1}H NMR experiments (620 MHz) revealed a -7.37 Hz dipolar splitting of the H{sub 2} ligand for this complex. Analysis of this splitting suggests that the H{sub 2} ligand is bound with the H-H axis perpendicular to the Ru-Ru axis. These complexes were examined as possible catalysts for the oxidation of dihydrogen through prior heterolytic activation of H{sub 2}. Only Ru(OEP)(THF)(H{sub 2}) can be conveniently deprotonated. Ru(OEP)(THF)(H{sub 2}) is also implicated in the Ru-(OEP)((THF){sub 2}) catalyzed isotopic exchange between H{sub 2} and D{sub 2}O in THF solution. Each step for this mechanism has been elucidated. We have also achieved catalytic dihydrogen oxidation using [Ru(OEP)]{sub 2} adsorbed onto graphite. Twomore » mechanisms for this ruthenium porphyrin catalyzed dihydrogen oxidation are presented and compared. 54 refs., 12 figs., 5 tabs.« less

ReportDOI
31 May 1992
TL;DR: In this article, the same experimental procedure is extended to the fabrication of Pt ultramicroelectrodes, and a new method is described for the characterization of the tip geometry and the electrochemical response of the Pt-Ir wires to Ru(NH3) 62+/3+ in water and to FeCp20/+ and Co(CpCOOCH3)20+ in acetonitrile.
Abstract: : Recent work has detailed the preparation of conical and hemispherical Pt-Ir ultramicroelectrodes using a two-step procedure involving an electrochemical etch and the sealing of the resulting sharp wire tip by translation through molten glass. The ultramicroelectrodes were characterized by scanning electron microscopy (SEM) and electrochemically. In this work, the same experimental procedure is extended to the fabrication of Pt ultramicroelectrodes. A new method is described for the characterization of the tip geometry, the electrochemical response of Pt ultramicroelectrodes to Ru(NH3) 62+/3+ in water and to FeCp20/+ and Co(CpCOOCH3)20+ in acetonitrile is reported, and aspects related to ultramicroelectrode reliability are addressed. SEM micrographs of freshly etched Pt and Pt-Ir wires are shown in Fig. 1, and the method for characterizing the tip geometry is outlined in Fig.2. Based on this method, freshly etched Pt and Pt-Ir wires have hemispherical radii at their apex of respectively 0.36 and 0.57 microns (95% confidence limits).