Wilaiwan Chanmanee

Formation and characterization of self-organized TiO2 nanotube arrays by pulse anodization.

This paper describes TiO2 nanotube arrays prepared by anodic oxidation of Ti substrates using pulse voltage waveforms. Voltages were pulsed between 20 and -4 V or between 20 and 0 V with varying durations from 2 to 16 s at the lower limit of the pulse waveform. Ammonium fluoride or sodium fluoride (and mixtures of both) was used as the electrolyte with or without added medium modifier (glycerol, ethylene glycol, or poly (ethylene glycol) (PEG 400)) in these experiments. The pulse waveform was optimized to electrochemically grow TiO2 nanotubes and chemically etch their walls during its cathodic current flow regime. The resultant TiO2 nanotube arrays showed a higher quality of nanotube array morphology and photoresponse than samples grown via the conventional continuous anodization method. Films grown with a 20 V/-4 V pulse sequence and pulse duration of 2 s at its negative voltage limit afforded a superior photoresponse compared to other pulse durations. Specifically, the negative voltage limit of the pulse (-4 V) and its duration promote the adsorption of NH4+ species that in turn inhibits chemical attack of the growing oxide nanoarchitecture by the electrolyte F- species. The longer the period of the pulse at the negative voltage limit, the thicker the nanotube walls and the shorter the nanotube length. At variance, with 0 V as the low voltage limit, the longer the pulse duration, the thinner the oxide nanotube wall, suggesting that chemical attack by fluoride ions is not counterbalanced by NH3/NH4+ species adsorption, unlike the interfacial situation prevailing at -4 V. Finally, the results from this study provide useful evidence in support of existing mechanistic models for anodic growth and self-assembly of oxide nanotube arrays on the parent metal surface.

Tailoring Copper Oxide Semiconductor Nanorod Arrays for Photoelectrochemical Reduction of Carbon Dioxide to Methanol

Solar photoelectrochemical reduction of carbon dioxide to methanol in aqueous media was driven on hybrid CuO/Cu2O semiconductor nanorod arrays for the first time. A two-step synthesis was designed and demonstrated for the preparation of these hybrid copper oxide one-dimensional nanostructures on copper substrates. The first step consisted in the growth of CuO nanorods by thermal oxidation of a copper foil at 400 °C. In the second step, controlled electrodeposition of p-type Cu2O crystallites on the CuO walls was performed. The resulting nanorod morphology with controllable wall thickness by adjusting the Cu2O electrodeposition time as well as their surface/bulk chemical composition were probed by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. Photoelectrosynthesis of methanol from carbon dioxide was demonstrated at -0.2 V vs SHE under simulated AM1.5 solar irradiation on optimized hybrid CuO/Cu2O nanorod electrodes and without assistance of any homogeneous catalyst (such as pyridine or imidazole) in the electrolyte. The hybrid composition, ensuring double pathway for photoelectron injection to CO2, along with high surface area were found to be crucial for efficient performance in methanol generation under solar illumination. Methanol formation, tracked by gas chromatography/mass spectrometry, indicated Faradaic efficiencies of ~95%.

Electrochemical grafting of poly(3,4-ethylenedioxythiophene) into a titanium dioxide nanotube host network.

This study focuses on electrodeposition for infiltrating in situ a conducting polymer such as poly(3,4-ethylenedioxythiophene) (PEDOT) into a host titanium dioxide (TiO(2)) nanotube array (NTA) framework. The TiO(2) NTA was electrosynthesized on titanium foil in turn by anodization in a fluoride-containing medium. The PEDOT layer was electrografted into the TiO(2) NTA framework using a two-step potentiostatic growth protocol in acetonitrile containing supporting electrolyte. The nanoscopic features of oligomer/polymer infiltration and deposition in the NTA interstitial voids were monitored by field-emission scanning electron microscopy. Systematic changes in the nanotube inner diameter and the wall thickness afforded insights into the evolution of the TiO(2)NTA/PEDOT hybrid assembly. This assembly was subsequently characterized by UV-visible diffuse reflectance, cyclic voltammetry, and photoelectrochemical measurements. These data serve as a prelude to further use of these hybrids in heterojunction solar cells.

Solar photothermochemical alkane reverse combustion.

Significance An efficient solar process for the one-step conversion of CO2 and H2O to C5+ liquid hydrocarbons and O2 would revolutionize how solar fuel replacements for gasoline, jet, and diesel solar fuels could be produced and could lead to a carbon-neutral fuel cycle. We demonstrate that this reaction is possible in a single-step process by operating the photocatalytic reaction at elevated temperatures and pressures. The process uses cheap and earth-abundant catalytic materials, and the unusual operating conditions expand the range of materials that can be developed as photocatalysts. Whereas the efficiency of the current system is not commercially viable, it is far from optimized and it opens a promising new path by which such solar processes may be realized. A one-step, gas-phase photothermocatalytic process for the synthesis of hydrocarbons, including liquid alkanes, aromatics, and oxygenates, with carbon numbers (Cn) up to C13, from CO2 and water is demonstrated in a flow photoreactor operating at elevated temperatures (180–200 °C) and pressures (1–6 bar) using a 5% cobalt on TiO2 catalyst and under UV irradiation. A parametric study of temperature, pressure, and partial pressure ratio revealed that temperatures in excess of 160 °C are needed to obtain the higher Cn products in quantity and that the product distribution shifts toward higher Cn products with increasing pressure. In the best run so far, over 13% by mass of the products were C5+ hydrocarbons and some of these, i.e., octane, are drop-in replacements for existing liquid hydrocarbons fuels. Dioxygen was detected in yields ranging between 64% and 150%. In principle, this tandem photochemical–thermochemical process, fitted with a photocatalyst better matched to the solar spectrum, could provide a cheap and direct method to produce liquid hydrocarbons from CO2 and water via a solar process which uses concentrated sunlight for both photochemical excitation to generate high-energy intermediates and heat to drive important thermochemical carbon-chain-forming reactions.

Photocatalytic Generation of Syngas Using Combustion‐Synthesized Silver Bismuth Tungstate

Silver bismuth tungstate (AgBiW(2)O(8)) nanoparticles were prepared for the first time by solution combustion synthesis by using the corresponding metal nitrates as the precursor and urea as the fuel. These nanoparticles were subsequently modified with Pt catalyst islands using a photocatalytic procedure and used for the photogeneration of syngas (CO+H(2)). Formic acid was used for this purpose for the in situ generation of CO(2) and its subsequent reduction to CO. In the absence of Pt modification, H(2) was not obtained in the gas products evolved. These results were compared with those obtained with acetic acid in place of formic acid. The combustion process was simulated by thermogravimetry and the synthesized powder was characterized using transmission electron microscopy, diffuse reflectance UV/Vis spectroscopy, X-ray diffraction, surface area measurements, and X-ray photoelectron spectroscopy. Tauc plots derived from the diffuse reflectance data yielded an optical band gap of 2.74 eV. The photocatalytic activity of these nanoparticles was superior to a sample prepared by solid-state synthesis. Mechanistic aspects are finally presented, as are structural models and electronic calculations, using density functional theory (DFT).

Self-organized TiO2 nanotube arrays by anodization of Ti substrate: Effect of anodization time, voltage and medium composition on oxide morphology and photoelectrochemical response

This study probes the relationship between the morphology of anodic titania (TiO 2 ) layers grown on Ti foil substrates and their subsequent photoelectrochemical response in 0.5 M Na 2 SO 4 supporting electrolyte. The effects of anodization variables (voltage and time) and anodization medium composition [water, glycerol, poly(ethylene glycol), ethylene glycol] along with fluoride ion concentration on the oxide layer of morphology and photoresponse are described. The degree of order of the self organized TiO 2 nanotube arrays and the extent to which these arrays organize themselves over the entire substrate surface are key variables dictating the corresponding quality of the resultant photoresponse.

Photocatalytically Generated Bimetallic ( Pt – Au ∕ C – TiO2 ) Electrocatalysts for Polymer Electrolyte Fuel Cell Applications

Heterogeneous photocatalysis was used to prepare bimetallic Pt―Au modified carbon―TiO 2 matrices for use in polymer electrolyte fuel cells. These new generation electrocatalysts were characterized by transmission electron microscopy, energy-dispersive X-ray analyses, X-ray diffraction, and X-ray photoelectron microscopy. The electrocatalytic activity of these materials for the oxygen reduction reaction (ORR) was assessed by rotating disk hydrodynamic voltammetry. Of the three variant scenarios that can be envisioned for photocatalytic deposition of the two metals, i.e., sequential deposition (with Pt first and Au second or Au first and Pt second) or simultaneous deposition of Pt and Au on the C―TiO 2 nanocomposite surface from a single bath, electrocatalyst samples with Pt decorating the initially deposited Au nanoclusters (designated as Pt/Au/C―TiO 2 ) performed the best in terms of ORR kinetic facility, even relative to the monometallic case of Pt supported on C―TiO 2 . The durability of these electrocatalysts (in terms of corrosion) was assessed via galvanostatic polarization tests; once again Pt/Au/C―TiO 2 fared best relative to the other two samples as well as the Pt/C―TiO 2 control case. For all the electrochemical analyses, the total metal loading in the electrocatalysts was kept constant at 20% (by mass) for meaningful comparison.

Solution combustion synthesis of BiVO4 nanoparticles: Effect of combustion precursors on the photocatalytic activity

Abstract This paper describes the solution combustion synthesis, solid-state characterization, photoelectrochemical behavior, and photocatalytic properties of bismuth vanadate (BiVO4). In particular, the influence of combustion precursor was addressed in this study. Bismuth nitrate pentahydrate was used as the bismuth precursor and either vanadium chloride or vanadium oxysulfate was used as the vanadium precursor. Urea, glycine, or citric acid was used as the fuel. Stoichiometric mixtures (1:1) of the fuels and oxidants (with the Bi:V mole ratio also maintained at 1:1) were subjected to solution combustion synthesis. The resultant samples were characterized by X-ray diffraction, highresolution transmission electron microscopy, diffuse reflectance spectrophotometry, thermal analyses, and laser Raman spectroscopy. Methyl orange was used as a probe to test the photocatalytic attributes of the combustionsynthesized (CS) samples, and benchmarked against a commercial bismuth vanadate sample. The CS samples were superior to the commercial benchmark sample, and samples derived from vanadium chloride were superior to vanadium oxysulfate counterparts. The photoelectrochemical properties of the various CS samples were also studied and these samples were shown to be useful both for environmental photocatalytic remediation and water photooxidation applications.

Cathodic Electrosynthesis of Niobium Oxide One-Dimensional Nanostructures with Tailored Dimensions

A cathodic electrosynthesis strategy was conceived and demonstrated for the preparation of Nb 2 O 5 nanorods/nanotubes supported on Nb foil. This method involved an interplay of several processes including the chemical attack of fluoride species on the Nb cathode, the electrogeneration of O 2 at a proximal Pt anode, its subsequent diffusion to and electrocatalytic reduction on the Nb cathode, and finally, the chemical reaction between the ionic Nb and oxygen precursor species at the interface to generate the Nb 2 O 5 nanoarchitectures. The oxide nanorods/nanotubes were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, energy dispersive X-ray analyses, and Raman spectroscopy, and their critical dimensions could be tailored via control of the growth variables.

Anodic Growth of Titania Nanotube Array on Titanium Substrate: A Study by Electrochemical Impedance Spectroscopy

In this collaborative study, the variables that affect the morphology of nanotubular arrays of titanium oxide (grown by anodization of titanium substrate) were analyzed by a fractional factorial (Taguchi) statistical experimental design. The growth voltage and water content of the medium were found to have the maximal impact on nanotube length as shown by an analysis of variance. Nanotubular array growth was monitored by electrochemical impedance spectroscopy (EIS) and the results were corroborated by other measurement probes (scanning electron microscopy, transient current-time profiles, voltammetry) as needed. Taken as a whole, the EIS data presented above support the validity of the Bojinov equivalent circuit model especially at growth times past a few minutes. Trends in the values of the equivalent circuit elements obtained from fits to the EIS data as a function of the growth variables are rationalized within the framework of a model proposed by Schmuki and co-workers for the growth and self-assembly of titania nanotubes on Ti substrates in fluoride-containing media.