The surface science of titanium dioxide

The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited

Oxygen vacancies in crystal have important impacts on the electronic properties of ZnO. With ZnO2 as precursors, we introduce a high concentration of oxygen vacancies into ZnO successfully. The obtained ZnO exhibits a yellow color, and the absorption edge shifts to longer wavelength. Raman and XPS spectra reveal that the concentration of oxygen vacancies in the ZnO decreased when the samples are annealed at higher temperature in air. It is consistent with the theory calculation. The increasing of oxygen vacancies results in a narrowing bandgap and increases the visible light absorption of the ZnO. The narrowing bandgap can be confirmed by the enhancement of the photocurrent response when the ZnO was irradiated with visible light. The ZnO with oxygen vacancies are found to be efficient for photodecomposition of 2,4-dichlorophenol under visible light irradiation.

Oxygen vacancy induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO.

In the past few decades, there has been a wide research interest in titanium dioxide (TiO2) nanomaterials due to their applications in photocatalytic hydrogen generation and environmental pollution removal. Improving the optical absorption properties of TiO2 nanomaterials has been successfully demonstrated to enhance their photocatalytic activities, especially in the report of black TiO2 nanoparticles. The recent progress in the investigation of black TiO2 nanomaterials has been reviewed here, and special emphasis has been given on their fabrication methods along with their various chemical/physical properties and applications.

Black titanium dioxide (TiO2) nanomaterials

This review focuses primarily on homogeneous catalysts for the oxidation of water, but does include selected heterogeneous systems. It does not attempt to summarize all chemistry related to systems that are capable of oxidizing water. In particular, heterogeneous catalysts that have no direct relevance to understanding the WOC [water-oxidizing complex] are not discussed in detail. Neither are nonbiomimetic systems for artificial photosynthesis or water-splitting in its elements discussed. However, a list of some of the recent references in these areas can be found in section X (Nonbiomimetic Water Oxidation Catalysts). The authors will first give a brief summary of the current view of the photosynthetic WOC and its functionality, followed by analysis of the thermodynamic and kinetic constraints for water-oxidation that have to be overcome by any catalyst. Since manganese is the metal that performs this reaction in the WOC, manganese catalysts will be discussed first, followed by other transition metals, particularly ruthenium. In the final section the authors summarize the principles of reactivity learned from theory and existing models that will guide one toward synthesis of better catalysts in the future. This review does not attempt to summarize manganese chemistry relevant to nonfunctional (structural) models of the WOC, whichmore » has been reviewed recently. Water-oxidation catalysts are intrinsically important in their own right, independent of possible biological relevance. They have direct applications as catalysts in artificial photosynthetic systems for the splitting of water that could be used in future fuel cells for the generation of electricity. 133 refs.« less

Synthetic Water-Oxidation Catalysts for Artificial Photosynthetic Water Oxidation.

A new operational transconductance amplifier and capacitor based sinusoidal voltage controlled oscillator is presented. The transconductor uses two cross-coupled class-AB pseudo-differential pairs biased by a flipped voltage follower, and it exhibits a wide transconductance range with low power consumption and high linearity. The oscillator has been fabricated in a standard 0.8-/spl mu/m CMOS process. Experimental results show a frequency tuning range from 1 to 25 MHz. The amplitude is controlled by the transconductor nonlinear characteristic. The circuit is operated at 2-V supply voltage with only 1.58 mW of maximum quiescent power consumption.

A low-power low-voltage OTA-C sinusoidal oscillator with a large tuning range

A systematic comparison of flipped voltage followers circuits is presented. Two new versions of the flipped voltage follower are introduced. They are characterized by very low output impedance, high bandwidth speed, wide signal swing. All structures can be easily modified for class AB operation. Simulation results show that the newly introduced structures have optimal characteristics

Comparison of conventional and new flipped voltage structures with increased input/output signal swing and current sourcing/sinking capabilities

We report experimental data on the photocurrent-time dependence observed during the photoassisted oxidation of water at a H[sub 2] reduced (001) TiO[sub 2] rutile single crystal, as a function of the concentration of generated donor centers. The observed photocurrent transients are fitted with a kinetic model previously proposed, according to which the photocurrent time decay at high band-bending is controlled by the second-order rate constant, k[sub 2], for H[sub 2]O[sub 2] generation from the recombination of surface-bound OH[degree][sub s] radicals, photogenerated by hole trapping at Ti-coordinated OH[sup [minus]] ions of basic character. k[sub 2] is found to increase linearly with the concentration of donor centers, N[sub D] in the range (1.5-43) [times] 10[sup 17] cm[sup [minus]3], from 7.0 [times] 10[sup [minus]15] to 8.5 [times] 10[sup [minus]14] cm[sup 2] s[sup [minus]1]. The apparent increase of the catalytic activity of TiO[sub 2] surface in the photooxidation of H[sub 2]O to H[sub 2]O[sub 2] is attributed to the formation of Ti[sub 2]O[sub 3] corundum type lattice defects during TiO[sub 2] reduction process in H[sub 2]. The catalytic properties of these defects are thought to be associated with the short distance between Ti[sup 3+] pairs, which makes easy covalent bonding between adsorbed OH[degree][sub s]more » radicals. 14 refs., 6 figs., 1 tab.« less

Catalytic role of lattice defects in the photoassisted oxidation of water at (001) n-titanium(IV) oxide rutile

This paper presents a new CMOS low-voltage linear transconductor for very high frequency. It uses multiple-input floating-gate transistors in each inverter of the differential structure transconductor presented by Nauta [1992]. The proposed transconductor operates under a constant low-voltage supply as low as 1.2 V, and its transconductance and output resistance are independently tunable, as shown by experimental measurements. This architecture is suitable for use in high-frequency continuous-time filters with programmable center frequency and quality factor. Simulation results of a 10.7-MHz and Q=40 g/sub m/-C filter operating with a voltage supply of 1.4 V and with rail-to rail input swing are presented.

Floating-gate-based tunable CMOS low-voltage linear transconductor and its application to HF g/sub m/-C filter design

A CT complex /spl Delta//spl Sigma/ ADC with built-in LPF is presented. A modified feedback topology is used to improve robustness to interferers near f/sub g//2 or f/sub s/. Adding programmable gain control, the 0.18 /spl mu/m CMOS ADC achieves 89.5dB DR in a 1MHz BW, consuming 4.7mW from a 1.8V supply.

F. Munoz

Papers

A low-power low-voltage OTA-C sinusoidal oscillator with a large tuning range

Comparison of conventional and new flipped voltage structures with increased input/output signal swing and current sourcing/sinking capabilities

Catalytic role of lattice defects in the photoassisted oxidation of water at (001) n-titanium(IV) oxide rutile

Floating-gate-based tunable CMOS low-voltage linear transconductor and its application to HF g/sub m/-C filter design

A 4.7mW 89.5dB DR CT complex /spl Delta//spl Sigma/ ADC with built-in LPF