John C. Flake

Electrochemical Reduction of CO2 to CH3OH at Copper Oxide Surfaces

The direct reduction of CO2 to CH3OH is known to occur at several types of electrocatalysts including oxidized Cu electrodes. In this work, we examine the yield behavior of an electrodeposited cuprous oxide thin film and explore relationships between surface chemistry and reaction behavior relative to air-oxidized and anodized Cu electrodes. CH3OH yields (43 μmol cm-2 h-1) and Faradaic efficiencies (38%) observed at cuprous oxide electrodes were remarkably higher than air-oxidized or anodized Cu electrodes suggesting Cu(I) species may play a critical role in selectivity to CH3OH. Experimental results also show CH3OH yields are dynamic and the copper oxides are reduced to metallic Cu in a simultaneous process. Yield behavior is discussed in comparison with photoelectrochemical and hydrogenation reactions where the improved stability of Cu(I) species may allow continuous CH3OH generation.

Composite Silicon Nanowire Anodes for Secondary Lithium-Ion Cells

A composite anode with active materials including 15% (w/w) silicon nanowires and graphite is demonstrated for use in secondary lithium-ion cells. The electrochemical behavior of the composite anode including voltammetry and charge/discharge capacities over the first 15 cycles is shown and compared with an equivalent graphite anode. Electrolessly etched nanowires swell in diameter upon lithiation and undergo reversible cycling without pulverization or agglomeration. Experimental results reveal high initial capacities (approximately 811 mA hg ―1 ) near theoretical predictions and a reversible capacity of 512 mA hg ―1 after 10 cycles with a capacity fade of approximately 1.4%/cycle. Capacity loss mechanisms are considered in comparison with other silicon-containing anodes.

Low temperature preparation of crystalline ZrO2 coatings for improved elevated-temperature performances of Li-ion battery cathodes

Ultrathin crystalline ZrO(2) nanofilms have been facilely deposited on LiMn(2)O(4) particles at 120 °C using atomic layer deposition. The ZrO(2) coating shows high crystallinity, conformality and homogeneity, which contribute to considerably improved electrochemical performance of LiMn(2)O(4) at elevated temperature in lithium-ion batteries.

Electrochemical Etching of Silicon in Nonaqueous Electrolytes Containing Hydrogen Fluoride or Fluoroborate

The electrochemical behavior and surface chemistry of anodic silicon etching in nonaqueous electrolytes was studied. Etching of single‐crystal p‐type and n‐type (100) silicon was carried out in acetonitrile and propylene carbonate with hydrofluoric acid (HF) or tetrafluoroborate providing fluoride to complex the oxidized silicon. Electrolytes containing HF resulted in tetravalent dissolution, and photocurrent quadrupling was observed. Electrolytes containing also resulted in tetravalent dissolution; however, calculated quantum efficiencies were lower depending upon the electrolyte. Current‐voltage behavior indicates the presence of surface states which affect both the onset potential for oxidation and the current multiplication. In situ multiple internal reflection Fourier transform infrared analysis confirms that silicon surfaces etched in electrolytes containing HF remain hydride‐terminated throughout etching; however, silicon etched in based electrolytes loses the initial hydride termination at the onset of etching.

Electrochemical and thermal grafting of alkyl grignard reagents onto (100) silicon surfaces.

Passivation of (100) silicon surfaces using alkyl Grignard reagents is explored via electrochemical and thermal grafting methods. The electrochemical behavior of silicon in methyl or ethyl Grignard reagents in tetrahydrofuran is investigated using cyclic voltammetry. Surface morphology and chemistry are investigated using atomic force microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS). Results show that electrochemical pathways provide an efficient and more uniform passivation method relative to thermal methods, and XPS results demonstrate that electrografted terminations are effective at limiting native oxide formation for more than 55 days in ambient conditions. A two-electron per silicon mechanism is proposed for electrografting a single (1:1) alkyl group per (100) silicon atom. The mechanism includes oxidation of two Grignard species and subsequent hydrogen abstraction and alkylation reaction resulting in a covalent attachment of alkyl groups with silicon.

Pulse Electrodeposition of Cu–ZnO and Mn–Cu–ZnO Nanowires

Cu-ZnO and Mn-Cu-ZnO nanowires are attractive catalysts for alcohol synthesis from CO hydrogenation reactions. Nanowire alloys are pulse electrodeposited into track etched polycarbonate membranes using aqueous electrolytes including Mn(NO 3 ) 2 , Cu(NO 3 ) 2 , Zn(NO 3 ) 2 , and NH 4 NO 3 . Pulse waveforms with a cathodic current density of 50.7 mA cm -2 for 50 ms (on-time), with varying off-times (400, 500, and 600 ms), are used to fabricate nanowire arrays (400 nm diameter, 25 μm long, and pore density of 1.5 × 10 8 pores cm -2 ). Pulse waveforms allow significantly higher copper concentrations and better control of zinc and manganese concentrations within nanowires. X-ray diffraction results show preferential growth in the (111) direction and crystallite size increases with an increase in off-time. Waveforms with longer off-times (500 and 600 ms) resulted in nanowires with relatively higher copper concentrations due to improved copper transport in nanopores. The nanowire surface has no manganese; however, the core shows manganese, which increases with the decrease in off-time. The effect of deposition conditions and electrolyte composition on nanowire properties are explained and discussed. .

Effects of Particle Concentration on Chemical Mechanical Planarization

Effects of particle concentration on removal rates in chemical mechanical planarization (CMP) were investigated. Experimental data shows the removal rate scales with the cubic root of weight percent solids and is proportional to mean separation distance between particles in the slurry. Results show similar removal rate scaling with solid concentration for both silicon dioxide and copper films. This work also identifies a critical solid concentration needed to initiate removal for oxide or copper surfaces. The effects of particles on copper surface oxidation and roughness are further characterized by voltammetry and atomic force microscopy. Results show copper films become progressively smoother with additional solids in the slurry.

Alternatives to Hydrogen Fluoride for Photoelectrochemical Etching of Silicon

Photoelectrochemical etching of silicon in nonaqueous solutions without the use of free-fluoride or HF has been demonstrated. Silicon was electrochemically oxidized and dissolved using stable, fluoride containing salts in acetonitrile solutions. The current-voltage behavior of silicon in acetonitrile solutions containing tetrabutylammonium tetrafluoroborate or tetrabutylammonium hexafluorophosphate were similar. The voltammetry of silicon in solutions containing tetrabutylammonium trifluoromethylsulfonyl, potassium hexafluoroarsenate, or sodium hexafluoroantimonate showed large hysteresis indicating the presence of silicon oxide on the surface. Although the dissolution rate of silicon dioxide was negligible in the absence of free-fluoride or HF, the oxidation and dissolution of silicon could be maintained, even when thin oxides were formed. Photocurrent oscillations associated with the buildup and removal of oxides were observed when traces of water were present.

Wafer-Scale Profile Evolution of Electrochemically Deposited Copper Films

Wafer-scale profile evolution of electrochemically deposited copper films is experimentally studied and compared with theoretical predictions. The effects of ohmic potential drop, electrochemical current-voltage behavior at the metal/electrolyte interface, and local concentration variations are considered. Results show good agreement with experimental data over a wide range of total currents and times.

Electrochemical Patterning of Organic Monolayers on Silicon

Organic monolayers may be grafted onto silicon surfaces using an in situ electrochemical patterning method. In this technique, dielectric templates such as polystyrene spheres (e = 2.5) or poly(dimethylsiloxane) stamps (e = 2.3-2.8) are placed in close proximity to, or in direct contact with, silicon electrodes while a potential is applied to drive electrografting reactions. In this work, the authors describe methyl monolayer patterns created in anodic processes and phenylacetylene monolayer patterns created in cathodic processes. Both anodic and cathodic processes show similar chronoamperometric behavior, suggesting silicon passivation associated with the formation of monolayers. Atomic force microscopy shows the sizes, geometries, and thickness of patterned films. Comparison of experimental results with electric field simulations also shows that solution resistance controls the feature sizes, resulting from electrografting with proximal dielectric templates. Similarly, electrochemical impedance spectroscopy shows that the films are densely packed with relatively low levels of defects. The versatile technique is further demonstrated as a monolayer resist for patterned electrodeposition of copper on silicon.