Oliver Diwald

Stability and Photoelectronic Properties of Layered Titanate Nanostructures

Layered titanate nanostructures offer promising photoelectronic properties that are subject to surface chemistry-induced morphology changes. For a systematic evaluation of the bulk and surface contributions to the photoactivity of these structures, we investigated their photoelectronic properties and in particular their dependence on the condition of the gas-solid interface. We comprehensively explored the stability of Na(2)Ti(3)O(7) nanowires and scrolled up H(2)Ti(3)O(7) nanotubes by means of transmission electron microscopy, Raman, and FT-IR spectroscopy and subjected both titanate sheet-based structures to controlled thermal activation treatment under high vacuum conditions. We found that throughout thermal annealing up to T = 870 K the structure and morphology of Na(2)Ti(3)O(7) nanowires are retained. Consistent with the significant photoluminescence emission that is attributed to radiative exciton annihilation in the bulk, UV-induced charge separation is strongly suppressed in these structures. H(2)Ti(3)O(7) nanotubes, however, undergo transformation into elongated anatase nanocrystals during annealing at temperatures T >OR= 670 K. Photoexcitation experiments in O(2) atmosphere reveal that these structures efficiently sustain the separation of photogenerated charges. Trends in the abundance of trapped holes and scavenged electrons were characterized quantitatively by tracking the concentration of paramagnetic O(-) and O(2)(-) species with electron paramagnetic resonance spectroscopy EPR, respectively. An incisive analysis of these results in comparison to those obtained on airborne anatase nanocrystals underlines the critical role of surface composition and structure on charge separation and, in consequence, on the chemical utilization of photogenerated charge carriers.

STM studies of defect production on the -(1×1) and -(1×2) surfaces induced by UV irradiation

Abstract The effect of UV radiation ( 1.6 eV ⩽hν⩽5.6 eV ) on the rutile surface was studied in ultrahigh vacuum (UHV) using the scanning tunneling microscope (STM). It was found that the TiO 2 (1 1 0) -(1×1) surface remains unaffected by UV irradiation. However, line defects along the 〈0 0 1〉 direction were formed on the TiO 2 (1 1 0) -(1×2) surface, the cross-section for the defect formation being 10 −23.5±0.2 cm 2 photon −1 ( hν⩾3.0 eV ). The defects are attributed to collective oxygen removal by UV light. It was also found that the defects were stable in UHV (up to 48 h), and their formation was not temperature dependent.

Wavelength selective excitation of surface oxygen anions on highly dispersed MgO

Monochromatic UV light in the spectral interval between 4.0 and 5.5 eV is used in order to selectively excite 3- and 4-coordinated oxygen anion sites on the surface of MgO nanoparticles exposed to O2 gas. As a result, two different paramagnetic O− surface species and also ozonide anions O3− are observed by electron paramagnetic resonance (EPR) spectroscopy. The relative abundance of each of the O− species exhibits a specific dependence on the energy of the exciting photons. EPR data together with the results of theoretical modeling suggest that both O− species are located at 3-coordinated sites having different local environments. At sufficiently high O2 pressures molecular oxygen does not only act as an electron trap, favoring the O− formation, but it also contributes to UV induced O3− formation with a maximum efficiency at 4.2 eV.

Chemical vapour deposition — a new approach to reactive surface defects of uniform geometry on high surface area magnesium oxide

Abstract Chemical vapour deposition (CVD) is particularly well suited for the preparation of high surface area magnesium oxide which exhibits a considerably reduced surface heterogeneity. This is shown in the present study for three types of surface defects: low coordinated anions and cations as well as anion vacancies. For each of them, only two to three different species with discrete geometries are observed. The characterization of these defects is based on specific EPR active molecular surface probes, namely, electron deficient oxygen anions O − , superoxide anions O 2 − and surface colour centres F S + , respectively. Some of the O 2 − and F S + centres exhibit a dipolar magnetic interaction with the proton of a closely spaced OH group. The electronic interaction is reflected by a change of the intramolecular force field of the respective OH group. Consequently, there is an IR spectroscopic access to the characterization of the defects in question. On the other hand, the surface O − species are absolutely insensitive to the hydroxylation state of the MgO surface.

Particle Networks from Powder Mixtures: Generation of TiO(2)-SnO(2) Heterojunctions via Surface Charge-Induced Heteroaggregation.

We explored the impact of interfacial property changes on aggregation behavior and photoinduced charge separation in mixed metal oxide nanoparticle ensembles. TiO2 and SnO2 nanoparticles were synthesized by metal organic chemical vapor synthesis and subsequently transformed into aqueous colloidal dispersions using formic acid for adjustment of the particles’ surface charge. Surface charge-induced heteroaggregation was found to yield blended nanoparticle systems of exceptionally high mixing quality and, after vacuum annealing, to extremely high concentrations of heterojunctions between TiO2 and SnO2 nanoparticles with dehydroxylated surfaces. For tracking charge transfer processes across heterojunctions, the photogeneration of trapped charge carriers was measured with electron paramagnetic resonance (EPR) spectroscopy. On blended nanoparticles systems with high concentrations of SnO2–TiO2 heterojunctions, we observed an enhanced cross section for interparticular charge separation. This results from an effective interfacial charge transfer across the interfaces and gives rise to substantially increased concentrations of electrons and hole centers. The here presented insights are key to the rational design of particle-based heterojunctions and mesoporous nanoparticle networks and help to engineer composite nanomaterials for photocatalysis and solar energy conversion.

Optical Properties of Nanocrystal Interfaces in Compressed MgO Nanopowders

The optical properties and charge trapping phenomena observed on oxide nanocrystal ensembles can be strongly influenced by the presence of nanocrystal interfaces. MgO powders represent a convenient system to study these effects due to the well-defined shape and controllable size distributions of MgO nanocrystals. The spectroscopic properties of nanocrystal interfaces are investigated by monitoring the dependence of absorption characteristics on the concentration of the interfaces in the nanopowders. The presence of interfaces is found to affect the absorption spectra of nanopowders more significantly than changing the size of the constituent nanocrystals and, thus, leading to the variation of the relative abundance of light-absorbing surface structures. We find a strong absorption band in the 4.0−5.5 eV energy range, which was previously attributed to surface features of individual nanocrystals, such as corners and edges. These findings are supported by complementary first-principles calculations. The possibility to directly address such interfaces by tuning the energy of excitation may provide new means for functionalization and chemical activation of nanostructures and can help improve performance and reliability for many nanopowder applications.

Site selective hydroxylation of the MgO surface

On the surface of MgO nanoparticles 1- and multiple (n = 3 and 4) coordinated hydroxyl groups are generated site-selectively and studied by transmission infrared spectroscopy. Their formation is based either on the stepwise dehydration of previously hydrated samples or on simple surface reactions involving molecular hydrogen on the one hand, and molecular oxygen, nitrous oxide or properly selected UV quanta on the other as reactants. 4-Coordinated OH groups may interact (a) with adjacent surface adsorbants such as hydrides (Mg2+H−) or superoxide anions (Mg2+⋯O−2) and (b) with surface defects like oxygen anions on steps (O2−) or paramagnetic surface colour centers. In each case the frequency of the respective IR-active OH stretching vibration is indicative of a specific local surface environment to which the OH probe is exposed. On the other hand, only perfectly free and isolated 3-coordinated OH groups have so far been observed. For sterical reasons they are exempt from any interaction with adjacent surface groups or surface defects.

UV induced local heating effects in TiO2 nanocrystals

When isolated TiO(2) nanocrystals are subjected to UV light at 77 K and pressures below 10(-6) mbar, trapping of photogenerated hole centers occurs on the surface of the nanocrystals and can be tracked by time-resolved electron paramagnetic resonance spectroscopy. Irrespective of the selected UV irradiance used, the maximum concentration of trapped charges was found to be constant for a given number of nanocrystals ( approximately 10(15)) and corresponds to one electron-hole pair per particle. On a time scale of seconds to minutes the dynamics for the trapping process depend on the number of photons with supra band gap energy. A local temperature rise of the TiO(2) nanocrystals was observed for irradiances above 1.55 mW cm(-2) (10(15) photons cm(-2) s(-1)). This is attributed to enhanced nonradiative recombination of photogenerated charge carriers via heat production and points to a substantial contribution of thermal chemistry in photocatalytic reaction cycles.

Solid−Solid Interface Formation in TiO2 Nanoparticle Networks

Aiming at a comparison of microstructure and paramagnetic properties of mesoporous TiO(2) nanoparticle networks, we subjected entirely different TiO(2-x) precursor structures to vacuum annealing. The transformation of an amorphous TiO(2-x) gel--obtained by sol-gel processing of an ethylene glycol-modified titanium precursor--into a network of interconnected anatase nanocrystals was explored by means of X-ray diffraction, nitrogen sorption, and electron microscopy. Crystalline junctions between the particles emerge from temperature treatment. This process of particle network formation is different from that related to the vapor phase grown anatase nanocrystals where particle-particle interface formation is induced by contact with water. It was found that, after annealing up to 873 K and controlled sample purification in oxygen atmosphere, both types of samples exhibit high concentrations of particle-particle interfaces and comparable properties in terms of surface area, porosity, and microstructure. With electron paramagnetic resonance (EPR) we observed on nonstoichiometric TiO(2-x) networks an identical type of subsurface defect which is related to the presence of solid-solid interfaces.

Size effects in MgO cube dissolution.

Stability parameters and dissolution behavior of engineered nanomaterials in aqueous systems are critical to assess their functionality and fate under environmental conditions. Using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, we investigated the stability of cubic MgO particles in water. MgO dissolution proceeding via water dissociation at the oxide surface, disintegration of Mg(2+)-O(2-) surface elements, and their subsequent solvation ultimately leads to precipitation of Mg(OH)2 nanosheets. At a pH ≥ 10, MgO nanocubes with a size distribution below 10 nm quantitatively dissolve within few minutes and convert into Mg(OH)2 nanosheets. This effect is different from MgO cubes originating from magnesium combustion in air. With a size distribution in the range 10 nm ≤ d ≤ 1000 nm they dissolve with a significantly smaller dissolution rate in water. On these particles water induced etching generates (110) faces which, above a certain face area, dissolve at a rate equal to that of (100) planes.1 The delayed solubility of microcrystalline MgO is attributed to surface hydroxide induced self-inhibition effects occurring at the (100) and (110) microplanes. The present work underlines the importance of morphology evolution and surface faceting of engineered nanomaterials particles during their dissolution.