We report a synergistic effect involving hydrogenation and nitridation cotreatment of TiO(2) nanowire (NW) arrays that improves the water photo-oxidation performance under visible light illumination. The visible light (>420 nm) photocurrent of the cotreated TiO(2) is 0.16 mA/cm(2) and accounts for 41% of the total photocurrent under simulated AM 1.5 G illumination. Electron paramagnetic resonance (EPR) spectroscopy reveals that the concentration of Ti(3+) species in the bulk of the TiO(2) following hydrogenation and nitridation cotreatment is significantly higher than that of the sample treated solely with ammonia. It is believed that the interaction between the N-dopant and Ti(3+) is the key to the extension of the active spectrum and the superior visible light water photo-oxidation activity of the hydrogenation and nitridation cotreated TiO(2) NW arrays.

We report hydrothermal synthesis of single crystalline TiO(2) nanowire arrays with unprecedented small feature sizes of ~5 nm and lengths up to 4.4 μm on fluorine-doped tin oxide substrates. A substantial amount of nitrogen (up to 1.08 atomic %) can be incorporated into the TiO(2) lattice via nitridation in NH(3) flow at a relatively low temperature (500 °C) because of the small cross-section of the nanowires. The low-energy threshold of the incident photon to current efficiency (IPCE) spectra of N-modified TiO(2) samples is at ~520 nm, corresponding to 2.4 eV. We also report a simple cobalt treatment for improving the photoelectrochemical (PEC) performance of our N-modified TiO(2) nanowire arrays. With the cobalt treatment, the IPCE of N-modified TiO(2) samples in the ultraviolet region is restored to equal or higher values than those of the unmodified TiO(2) samples, and it remains as high as ~18% at 450 nm. We propose that the cobalt treatment enhances PEC performance via two mechanisms: passivating surface states on the N-modified TiO(2) surface and acting as a water oxidation cocatalyst.

The electrocatalytic oxygen reduction reaction (ORR) on noble metal surfaces [Eq. (1), RHE = reversible hydrogen electrode] is one of the most widely studied reactions in electrochemistry. Its fundamental scientific and technological importance is based on the fact that the oxygen/water half-cell reaction is a strongly oxidizing and ubiquitous redox couple. Combined with an electron-supplying redox process, such as shown in Equation (2), a direct electrochemical conversion ofthe overall Gibbs energy of reaction into electrical potentials is achieved. This conversion is the scientific basis for electrochemical conversion in fuel cells [1] or metal-air batteries. [2,3] The ORR is also used in oxygen depolarization cathodes (ODC) in modern chlorine technologies, [4,5] in which it replaces the hydrogen evolution process to improve electrical efficiencies. The reverse ORR process, that is, the evolution of oxygen from water, is crucial for efficient water (photo)-electrolysis [6,7] into hydrogen or in metal electrodeposition processes in the semiconductor industry. [2] In polymer electrolyte membrane fuel cells (PEMFCs), the ORR electrode catalyst material of choice has been platinum for decades. The ORR on Pt, however, is irreversible, thus causing overpotentials and losses in fuel-cell efficiency. Much research has been dedicated to the identification of more efficient catalysts, that is, materials with reduced precious-metal content and improved ORR activity.[8] Pt-rich alloys, most prominently Pt-Co formulations, have shown promise, with state-of-art activity improvements of two to three times over pure Pt. [9,10] However, a material with an at least fourfold activity improvement, deemed crucial for automotive applications, has remained elusive to date.[11]Herein, we report on carbon-supported Pt-Cu-Co ternary alloy nanoparticle electrocatalysts with previously unachieved four-to fivefold ORR activity improvements. We demonstrate the catalytic activities on rotating disk electrodes (RDEs) as well as in real H 2 /O 2 fuel-cell devices. enriched core-shell particle structures. Since metal dissolution into the membrane electrolyte has very detrimental effects in fuel cells, we also developed a new procedure to form the active catalyst phase in situ inside a fuel-cell electrode layer without compromising the membrane conductance. Figure 1 schematically illustrates our novel three-step procedure for preparation of the active catalyst phase. In step 1, the alloy precursor is applied in the cathode of a fuelcell membrane-electrode assembly (MEA). During step 2, a cyclic voltammetric treatment selectively dissolves the lessnoble metal atoms (mostly Cu) from the alloy particle surface. The Cu atoms migrate into the nafion polyelectrolyte and get trapped at negatively charged sulfonic acid groups. In step 3, the MEA is chemically treated with an inorganic acid, which results in complete exchange of Cu ions inside the polyelectrolyte with protons. After step 3, the catalyst has been converted into its active phase and is re...

Nanostructured BiVO4 films were synthesized by coevaporation of bismuth and vanadium in an oxygen ambient, a process referred to as reactive ballistic deposition (RBD). The films were tested in various electrolyte solutions to assess their activity for photoelectrochemical water oxidation. Deposition parameters, including the V/Bi atomic flux ratio and the incident angle of deposition, were adjusted. Films deposited with excess vanadium (V/Bi = 2) and incident angles of deposition at 65° showed the highest initial photocurrents with IPCE values above 21% for light wavelengths of 340−460 nm (in 0.5 M Na2SO4 at 1.0 V vs Ag/AgCl). With continued illumination the excess vanadium in these films dissolved into the electrolyte and the photocurrents dropped by 60−75% before reaching steady state. The steady-state photocurrent and IPCE values (above 14% for 340−460 nm light) were higher than the initial values for films synthesized with stoichiometric amounts of vanadium and bismuth (V/Bi = 1) and incident angles of deposition at 65°. Stoichiometric BiVO4 films remained stable under illumination but their photocurrents were limited by surface reaction kinetics. The addition of cobalt as an electrocatalyst to the surface of these films increased their photocurrent by a factor of 3.

Thin films of R-Fe 2 O 3 doped with either Ti or Sn were prepared by coevaporating iron and titanium/tin in a reactive oxygen ambient, and their physical, chemical, and photoelectrochemical properties were studied. It was found that manipulating the deposition angle had a profound effect on the photoelectrochemical water oxidation performance of 4% Ti-doped R-Fe 2 O 3 films, and a maximum in photocurrent at 1.4 V vs RHE (Reversible Hydrogen Electrode) was achieved for films grown at 75°i ncidence. It was also found that the nanocolumnar morphology and superior porosity attained using glancing angles improved the relative conversion of visible-light (λ > 420 nm) photons compared to dense films deposited at normal incidence. Sn-doped films were also prepared for comparison using the same deposition conditions, and although they were substantially better than undoped films, their performance was somewhat below that of Ti-doped films. The Ti-doped films deposited using optimum conditions resulted in incident photon-to-current efficiencies (IPCE) reaching 31% at 360 nm and 1.4 V vs RHE. By comparison, Sn-doped films reached only 21% under the same conditions. The increased photoconversion efficiency brought about through Ti 4þ or Sn 4þ incorporation appears to be due to both the improvement of electron transport within the bulk of the film and the suppression of recombination at the film-electrolyte interface due to the stronger electric field near the surface.