Donald A. Tryk

Titanium dioxide photocatalysis

Abstract Scientific studies on photocatalysis started about two and a half decades ago. Titanium dioxide (TiO 2 ), which is one of the most basic materials in our daily life, has emerged as an excellent photocatalyst material for environmental purification. In this review, current progress in the area of TiO 2 photocatalysis, mainly photocatalytic air purification, sterilization and cancer therapy are discussed together with some fundamental aspects. A novel photoinduced superhydrophilic phenomenon involving TiO 2 and its applications are presented.

Recent topics in photoelectrochemistry: achievements and future prospects

The tremendous amount of research that has been carried out in the two closely related fields of semiconductor photoelectrochemistry and photocatalysis during the past three decades continues to provide fundamental insights and practical applications. The present review paper will attempt to describe some of the progress and resulting achievements in these two areas and to briefly discuss the future prospects. In order to provide a focal point, we will highlight work carried out in Japan over the last 5 years. However, we will try as much as possible to put this work into a global and historical context by tracing some of the key developments that have occurred outside this relatively narrow scope. We should note at the outset that we have made no attempt to cover the underlying theory or physics of photoelectrochemistry. Several excellent reviews have appeared during this same time period that cover fundamental and general aspects of photoelectrochemistry and photocatalysis.

Electrochemical selectivity for redox systems at oxygen-terminated diamond electrodes

Abstract Oxygen-terminated diamond electrodes were prepared by exposing as-grown hydrogen-terminated diamond thin films to oxygen plasma. The as-grown surfaces, which were highly hydrophobic, become hydrophilic after the oxygen plasma treatment. The apparent surface conductivity was not significantly changed after the oxygen plasma treatment. However, the electrochemical responses to several redox systems became remarkably different. For example, the cyclic voltammetric anodic–cathodic peak separations for the oxygen-terminated diamond electrodes became extremely large compared to those for the as-grown electrodes. This behavior was examined in comparison with as-grown diamond and glassy carbon electrodes.

TiO2 photocatalysts and diamond electrodes

Photocatalysis and electroanalysis are two seemingly disparate research areas, but they are linked by the fact that both involve the use of well-known materials, TiO2 and diamond, respectively, in new ways in the service of both environmental and medical sciences. In the present article, recent developments in the area of TiO2 photocatalysis and diamond electrochemistry are summarized, with emphasis on our findings at the University of Tokyo. In the photocatalysis section, we present the fundamental aspects of TiO2 photocatalysis and its practical applications, including air purification, self-cleaning surfaces and transparent superhydrophilic coatings. The diamond electrochemistry section deals with the electrochemical characterization and applications of diamond electrodes, which exhibit high sensitivity and excellent stability for electroanalysis, in contrast to conventional electrode materials. A particularly interesting environmental application of diamond electrodes has been developed; this involves the trace analysis of lead without the use of mercury.

TiO2-mediated photodegradation of liquid and solid organic compounds

The photocatalytic degradation of several liquid and solid organic compounds, including polymers, with molecular weights covering a wide range from 600 to 500,000 was studied on TiO2 thin films on glass under UV illumination. Nearly exact agreement was found between the weight losses of the solid compounds octadecane and stearic acid and the weights of CO2 produced during photocatalytic degradation. No other gas-phase degradation product was detected for these two compounds other than CO2, which means that potentially harmful products are not expected to pose a problem. For convenient comparison of degradation rates for various compounds and measurement methods, the values were converted to numbers of moles of carbon reacted per square centimeter per hour. Under appropriate conditions (50C, relative humidity 10% in air), octadecane was completely decomposed (<400 ng cm 2 ). In contrast, stearic acid did not decompose completely, even after more than 80 h of UV illumination. This may be due to the formation of a photocatalytically inert reaction product that blocks the TiO2 surface. The decomposition rates for all of the compounds examined spanned less than two orders of magnitude, suggesting that the photocatalytic reactions involved are rather versatile.

Anodic Voltammetry of Xanthine, Theophylline, Theobromine and Caffeine at Conductive Diamond Electrodes and Its Analytical Application

Boron-doped diamond (BDD) electrodes were used to examine the electrochemical oxidation of xanthine and its naturally occurring N-methyl derivatives, theophylline, theobromine and caffeine. Voltammetric studies showed that the mechanism of the overall reaction is similar to that of the oxidation of purine derivatives at the pyrolytic graphite electrode. The effects of pH, concentration and potential sweep rate on the voltammetric response were thoroughly investigated, and it was found that BDD exhibits excellent behavior, in terms of very well-defined, reproducible oxidation peaks, for xanthine, theophylline, theobromine and caffeine determination. The results enabled the measurement of the oxidation peak current to be used as the basis for a simple, accurate and rapid method for determining the investigated compounds, within a concentration range of 1 to 400 μM for theophylline, theobromine and caffeine, and of 1 to 100 μM for xanthine. Promising results were obtained for caffeine determination in real samples of commercially available products, without separation from the matrix.

Electroanalysis of dopamine and NADH at conductive diamond electrodes

Abstract Highly boron-doped diamond thin-film electrodes were examined for various possible applications in electroanalysis. Electrochemical oxidation of dopamine and NADH was investigated using cyclic voltammetry and chronoamperometry. Comparison experiments were performed using glassy carbon electrodes. Anodically treated diamond electrodes made it possible to determine dopamine selectively with high sensitivity in the presence of a large excess of ascorbic acid in acidic media. A detection limit of 50 nM was obtained using chronoamperometry. The treated electrodes were found to be stable for several months. Electrochemical oxidation of NADH was carried out at as-deposited diamond electrodes, with which very stable and reproducible cyclic voltammograms for NADH oxidation were obtained, unlike glassy carbon, at which a significant positive shift (∼200 mV) in the peak potential was observed within 1 h. The amperometric detection limit was found to be ∼10 nM. Interference of ascorbic acid was minimal using untreated electrodes when the concentration of ascorbic acid was comparable to the NADH concentration. Diamond microelectrodes small enough to consist of only one or two high quality microcrystals were fabricated in order to compare the electrochemical behavior with that of polycrystalline thin film electrodes, which contain large numbers of grain boundaries, at which non-diamond (sp 2 ) carbon can exist. This work demonstrates the potential of diamond electrodes for electroanalytical applications.

Electrochemical properties of Pt-modified nano-honeycomb diamond electrodes

The electrocatalytic behavior of boron-doped nanoporous honeycomb diamond films modified with Pt nanoparticles (10–150 nm) was examined with cyclic voltammetry (CV) and electrochemical impedance spectroscopy in acid solution. For two such films as well as an as-deposited film, the average number of exposed surface Pt atoms was estimated from CV to be 1.07(±0.05)×1015 cm−2 (real area, as estimated by SEM). These electrodes exhibited high electroactivity for hydrogen adsorption and oxidation of several alcohols. The current density (geometric basis) in the CV for methanol oxidation at a Pt-modified porous film with a pore diameter of 400 nm and a pore depth of 3 μm, was greatly enhanced, by a factor of 16, in comparison to the values obtained with a bulk Pt electrode. This enhancement is attributed to both the high surface area of the nano-honeycomb structure and the high electrocatalytic activity of Pt nanoparticles dispersed inside the pores. The electrocatalytic activities of the Pt-modified nano-honeycomb films were found to be dependent on the structural parameters of the honeycomb pores and the molecular sizes of the alcohols. Interestingly, ac impedance measurements have indicated a decrease in the penetration depths for a Pt-modified porous film with a pore diameter of 60 nm with increasing alcohol size, and an increase in the reaction resistances for ethanol oxidation with decreasing pore diameter.

Electroanalytical study of sulfa drugs at diamond electrodes and their determination by HPLC with amperometric detection

Abstract Conductive boron-doped diamond thin film electrodes were examined for the electroanalysis of three sulfa drugs, sulfadiazine, sulfamerazine and sulfamethazine. Cyclic voltammetry, flow injection analysis and liquid chromatography with electrochemical detection were used to study the oxidation reactions. At diamond electrodes, highly reproducible and well-defined cyclic voltammograms were obtained for all three drugs with a signal to background (S/B) ratio of a factor of ten greater than that obtained at two types of freshly polished glassy carbon (GC) electrodes. Diamond exhibited a highly reproducible amperometric response, with a peak variation of ∼5%, even at a concentration of 100 nM. A detection limit of 50 nM and a linear dynamic range of three orders of magnitude were obtained. No fouling of the electrode was observed within the experimental period of several hours. A rapid stabilization of the background current is achieved, within 10 min after the application of the operating potential under flow conditions, unlike the case of GC electrodes, which requires more than 30 min for reasonable stabilization. We have demonstrated the application of the diamond electrode for the simple and sensitive amperometric detection of the sulfa drugs in a standard mixture after their separation with reverse-phase HPLC to produce chromatograms with a flat baseline and high reproducibility, even at concentrations as low as 100 nM.

Band‐Edge Movements of Semiconducting Diamond in Aqueous Electrolyte Induced by Anodic Surface Treatment

Photoelectrochemical characterization of semiconducting diamond was carried out in a weakly UV-absorbing aqueous electrolyte using suprabandgap illumination in order to examine the influence of electrochemical oxidative surface treatment on the energetic positions of bandedges. Anodic treatment of diamond photoelectrodes resulted in a positive shift in both the photocurrent onset potential and the flatband potential obtained from Mott-Schottky plots, indicating the displacement of the bandedges. A corresponding increase in the photovoltage was also observed. The pH dependence of the flatband potential of anodically treated diamond indicates an acid-base equilibrium at the interface, suggesting the formation of oxygen-containing groups. The presence of the latter was confirmed using X-ray photoelectron spectroscopy. These results suggest that the changes in the potential drop in the Helmholtz layer are due to oxygen functional groups, and the loss of surface and subsurface hydrogen are mainly responsible for the observed shifts in the flatband potentials.