Son Hoang

Enhancing visible light photo-oxidation of water with TiO2 nanowire arrays via cotreatment with H2 and NH3: synergistic effects between Ti3+ and N.

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.

Visible Light Driven Photoelectrochemical Water Oxidation on Nitrogen-Modified TiO2 Nanowires

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.

Chemical bath deposition of vertically aligned TiO2 nanoplatelet arrays for solar energy conversion applications

We report a facile, scalable, and low cost chemical bath deposition of vertically aligned TiO2 nanoplatelet arrays on various substrates including fluorine-doped tin oxide coated glass substrates and their applications for photoelectrochemical (PEC) water splitting and dye sensitized solar cells. The TiO2 arrays consisting of single crystal rutile nanoplatelets with heights (film thicknesses) of up to 1 μm, lengths of up to 130 nm, and widths of ∼5 nm were grown via controlling oxidation and hydrolysis of TiCl3 at low pH (0.71–0.85) and low TiCl3 concentration (8–40 mM). As a photoanode for water oxidation in a PEC water splitting cell, the TiO2 nanoplatelets show excellent charge separation characteristics with a saturated photocurrent in 1 M KOH electrolyte under AM 1.5 G illumination of ∼0.4 mA cm−2 reached at an exceptionally low bias of −0.6 V vs. Ag/AgCl (0.4 V vs. reversible hydrogen electrode). Dye sensitized solar cells assembled using N719 dye sensitized-TiO2 nanoplatelet arrays also show promising performance with photoconversion efficiencies of 1.28% for as-synthesized (no thermal post-treatment) and 3.7% for annealed TiO2 nanoplatelets.

Atomic layer deposition of photoactive CoO/SrTiO3 and CoO/TiO2 on Si(001) for visible light driven photoelectrochemical water oxidation

Cobalt oxide (CoO) films are grown epitaxially on Si(001) by atomic layer deposition (ALD) using a thin (1.6 nm) buffer layer of strontium titanate (STO) grown by molecular beam epitaxy. The ALD growth of CoO films is done at low temperature (170–180  °C), using cobalt bis(diisopropylacetamidinate) and water as co-reactants. Reflection high-energy electron diffraction, X-ray diffraction, and cross-sectional scanning transmission electron microscopy are performed to characterize the crystalline structure of the films. The CoO films are found to be crystalline as-deposited even at the low growth temperature with no evidence of Co diffusion into Si. The STO-buffered Si (001) is used as a template for ALD growth of relatively thicker epitaxial STO and TiO2 films. Epitaxial and polycrystalline CoO films are then grown by ALD on the STO and TiO2 layers, respectively, creating thin-film heterostructures for photoelectrochemical testing. Both types of heterostructures, CoO/STO/Si and CoO/TiO2/STO/Si, demonstrate water photooxidation activity under visible light illumination. In-situ X-ray photoelectron spectroscopy is used to measure the band alignment of the two heterojunctions, CoO/STO and CoO/TiO2. The experimental band alignment is compared to electronic structure calculations using density functional theory.

Improvement of Solar Energy Conversion with Nb‐Incorporated TiO2 Hierarchical Microspheres

Niobium-modified TiO2 hierarchical spherical micrometer-size particles, which consist of many nanowires, are synthesized by solvothermal synthesis and studied as photoelectrodes for water photo-oxidation and dye-sensitized solar cell (DSSC) applications. Incorporation of Nb leads to a rutile-to-anatase TiO2 phase transition in the TiO2 hierarchical spheres (HSs), with the anatase percentage increasing from 0% for the pristine TiO2 HSs to 47.6% for the 1.82 at.% Nb-incorporated TiO2 sample. Incorporation of Nb leads to significant improvements in water photo-oxidation with the photocurrents reaching 70.5 μA cm(-2) at 1.23 V versus the reversible hydrogen electrode, compared with 28.3 μA cm(-2) for the pristine TiO2 sample. The photoconversion efficiency of Nb:TiO2 HS-based DSSCs reaches 6.09±0.15% at 0.25 at.% Nb, significantly higher than that for the pristine TiO2 HS cells (3.99±0.02%). In addition, the incident-photon-to-current efficiency spectra for DSSCs show that employing TiO2 and Nb:TiO2 HSs provides better light harvesting, especially of long-wavelength photons, than anatase TiO2 nanoparticle-based DSSCs.