Pulse laser ablation in water fragmentation of amorphous silica in water with optional NaCI spiking.

Amorphous SiO2 powders having two kinds of medium range order (MRO) were subjected to pulse laser ablation in water (PLAL) for X-ray diffraction, transmission electron microscopy and spectroscopy characterization. The powders fragmented down to 10-20 nm in size are mainly hydrogenated amorphous phase retaining Si-2nd O but not Si-2nd Si MRO. There are minor co-existing crystallites of beta-cristobalite, H2Si2O5 as well as additional alpha-tridymite and Na2SiO3 when water is spiked with NaCI for PLAL. Na-signature also caused lower vibration frequency of the hydrogenated silica network. The opal-like hydrogenated phase assemblage of amorphous phase, cristobalite and tridymite thus formed have a minimum band gap at ca. 5 eV for potential optoelectronic and catalytic applications in UV range and shed light on natural occurrence in dynamic settings such as meteorite impact and lightening strike on silicate-enrich crustal rocks.

(Zn,H)-codoped copper oxide nanoparticles via pulsed laser ablation on Cu-Zn alloy in water.

Nanosized (5 to 10 nm) amorphous and crystalline nanocondensates, i.e., metallic α-phase of Zn-Cu alloy in face-centered cubic structure and (Zn,H)-codoped cuprite (Cu2O) with high-pressure-favored close-packed sublattice, were formed by pulsed laser ablation on bulk Cu65Zn35 in water and characterized by X-ray/electron diffractions and optical spectroscopy. The as-fabricated hybrid nanocondensates are darkish and showed photoluminescence in the whole visible region. Further dwelling of such nanocondensates in water caused progressive formation of a rice-like assembly of (Zn,H)-codoped tenorite (CuO) nanoparticles with (001), (100), and {111} preferred orientations, (111) tilt boundary, yellowish color, and minimum bandgap narrowing down to ca. 2.7 eV for potential photocatalytic applications.

Size, composition and structural changes of TiO2 rutile nanoparticles by pulsed laser ablation in water.

The rutile-type TiO2 powders originally whitish and micrometer in size were successfully reduced in size with accompanied structure, composition and optical property change to become dark bluish by pulsed laser ablation in liquid under 532 nm excitation and 400 mJ per pulse at a specified water depth (5 or 15 mm) for up to 30 min. Transmission electron microscopic observations indicated the newly formed anatase nanoparticles are ca. 5 nm in size with spindle-like shape having well-developed {101} faces and -(001) facets for mutual coalescence into unity. The -(001)-specific coalescence twin of anatase was also observed. There are also amorphous nanoparticles in association with anatase, minor brookite and relic rutile with 2 x (110) superstructure. The resultant colloidal TiO2 nanoparticles are (Ti3+, Ti2+, H+) co-signified according to optical spectroscopic results and have a bimodal minimum band gaps 3.12 and 2.66 eV due to nanocrystals and amorphous phase, respectively, for potential optocatalytic applications in UV-visible range.

Formation of (H+, Al+, Al2+) co-doped bayerite and gamma-Al2O3 plates from spinel-type related nanocondensates in water.

The Al2O3 nanocondensates of spinel-type related structures, i.e., gamma- and theta-type with a significant internal compressive stress via pulsed laser ablation in water were subjected to prolonged dwelling in water to form columnar bayerite plates for further transformation as platy gamma-Al2O3. Transmission electron microscopic observations indicated the gamma-Al2O3 follows the crystallographic relationship (100)b//(011)gamma; [001]b//[111]gamma with relic bayerite (denoted as b). The gamma-Al2O3 also shows {111} twin/faults and rock salt-type domains due to dehydroxylation of bayerite which involves {1111} shuffling and disordering of the Al ions in the octahedral and tetrahedral sites. The combined evidences of X-ray photoelectron spectroscopy, vibrational spectroscopy and UV-visible absorbance indicated that the H+, Al+ and Al2+ co-doped bayerite and gamma-Al2O3 composite plates have a minimum band gap as low as approximately 5 eV for potential catalytic and electro-optical applications in water environment.

Nonstoichiometric Titanium Oxides via Pulsed Laser Ablation in Water.

Titanium oxide compounds TiO,Ti2O3, and TiO2 with a considerable extent of nonstoichiometry were fabricated by pulsed laser ablation in water and characterized by X-ray/electron diffraction, X-ray photoelectron spectroscopy and electron energy loss spectroscopy. The titanium oxides were found to occur as nanoparticle aggregates with a predominant 3+ charge and amorphous microtubes when fabricated under an average power density of ca. 1 × 108W/cm2 and 1011W/cm2, respectively followed by dwelling in water. The crystalline colloidal particles have a relatively high content of Ti2+ and hence a lower minimum band gap of 3.4 eV in comparison with 5.2 eV for the amorphous state. The protonation on both crystalline and amorphous phase caused defects, mainly titanium rather than oxygen vacancies and charge and/or volume-compensating defects. The hydrophilic nature and presumably varied extent of undercoordination at the free surface of the amorphous lamellae accounts for their rolling as tubes at water/air and water/glass interfaces. The nonstoichiometric titania thus fabricated have potential optoelectronic and catalytic applications in UV-visible range and shed light on the Ti charge and phase behavior of titania-water binary in natural shock occurrence.

Tapered ZnO Whiskers: {hkil}-Specific Mosaic Twinning VLS Growth from a Partially Molten Bottom Source.

Zn particulates overlaid with wurtzite (W)-type ZnO condensates having nearly orthogonal {101̄1} and {112̄1} facets were found to self-catalyze unusual tapered W-ZnO whiskers upon isothermal atmospheric annealing, i.e., thermal oxidation, at 600 °C. Analytical electron microscopic observations indicated that such whiskers formed tapered slabs having mosaic {101̄1} and {21̄1̄1} twinned domains. The tapered whiskers can be rationalized by an alternative vapor-liquid-solid growth, i.e., {hkil}-specific coalescence twinning growth from the ZnO condensates taking advantage of a partially molten bottom source of Zn and the adsorption of atoms at the whisker tips and ledges under the influence of capillarity effect. The tapered whiskers having strong photoluminescence at 391 nm and with a considerable flexibility could have potential applications.

Water-Driven Assembly of Laser Ablation-Induced Au Condensates as Mesomorphic Nano- and Micro-Tubes.

Reddish Au condensates, predominant atom clusters and minor amount of multiply twinned particles and fcc nanoparticles with internal compressive stress, were produced by pulsed laser ablation on gold target in de-ionized water under a very high power density. Such condensates were self-assembled as lamellae and then nano- to micro-diameter tubes with multiple walls when aged at room temperature in water for up to 40 days. The nano- and micro-tubes have a lamellar- and relaxed fcc-type wall, respectively, both following partial epitaxial relationship with the co-existing multiply twinned nanoparticles. The entangled tubes, being mesomorphic with a large extent of bifurcation, flexibility, opaqueness, and surface-enhanced Raman scattering, may have potential encapsulated and catalytic/label applications in biomedical systems. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11671-009-9359-x) contains supplementary material, which is available to authorized users.

Mesomorphic lamella rolling of au in vacuum.

Lamellar nanocondensates in partial epitaxy with larger-sized multiply twinned particles (MTPs) or alternatively in the form of multiple-walled tubes (MWTs) having nothing to do with MTP were produced by the very energetic pulse laser ablation of Au target in vacuum under specified power density and pulses. Transmission electron microscopic observations revealed (111)-motif diffraction and low-angle scattering. They correspond to layer interspacing (0.241-0.192 nm) and the nearest neighbor distance (ca. 0.74-0.55 nm) of atom clusters within the layer, respectively, for the lamella, which shows interspacing contraction with decreasing particle size under the influence of surface stress and rolls up upon electron irradiation. The uncapped MWT has nearly concentric amorphous layers interspaced by 0.458-0.335 nm depending on dislocation distribution and becomes spherical onions for surface-area reduction upon electron dosage. Analogous to graphene-derived tubular materials, the lamella-derived MWT of Au could have pentagon-hexagon pair at its zig-zag junction and useful optoelectronic properties worthy of exploration.

Laser ablation condensation, coalescence, and phase change of dense gamma-Al(2)O(3) particles.

Dense gamma-Al(2)O(3)condensates, with residual stress up to 3 GPa and ranging from nanometer to an unexpected micrometer size, were formed by pulsed laser ablation on Al target under oxygen background gas for a very rapid heating and cooling effect. Analytical electron microscopic observations indicated such nanoparticles tended to coalesce over {111} facets to form multiple twin and tilt boundary. The micrometer-size particles changed, upon electron irradiation, into metastable orthorhombic delta form full of twin variants and faults parallel to {100}.

Condensation and relaxation/transformation of dense t-ZrO2 nanoparticles.

Dense tetragonal (t)-ZrO(2) nanocondensates were synthesized under very rapid heating and cooling by pulsed Nd-YAG (YAG--yttrium aluminum garnet) laser ablation with oxygen background gas and characterized by electron diffraction. The t-ZrO(2) nanoparticles with a residual stress up to about 5 GPa tended to form deformation twins/faults upon local electron dosage. By contrast, the t-ZrO(2) nanoparticles formed at an-order-of-magnitude higher power have a residual stress above 6 GPa and tended to transform into a metastable cubic (c) phase vulnerable to amorphization. The relaxation/transformation of the self-constrained t-ZrO(2) nanoparticles can be rationalized by a lowering of internal energy under the influence of resolved shear stress and local electron heating.

Polymorphic transformation of dense ZnO nanoparticles: implications for chair/boat-type Peierls distortions of AB semiconductor.

Peierls distortion path was proved experimentally for dense ZnO nanoparticles prepared by static compression. Electron irradiation caused rock salt (R) to wurtzite (W) transition, following preferential (111)(R)//(0111)(W); [011](R)//[1213](W) and then transformation strain induced (111)(R)//(1011)(W); [011](R)//[0111](W). The two relationships can be rationalized by specified extent of chair- and boat-type Peierls distortions accompanied with band gap opening and intermediate {111}R slip for energetically favorable {111}R/(0111)W match.

Laser ablation condensation of alpha-PbO(2)-type TiO(2).

A high-pressure phase of TiO(2) with an alpha-PbO(2)-type structure has been synthesized via very energetic Nd-YAG laser pulse irradiation of oxygen-purged Ti target. The nanometer-size alpha-PbO(2)-type particles were (11;0), (010), and (001) faceted but the larger ones were spherical. The combined effects of rapid heating and cooling, the nanophase effect, and dense surfaces account for the formation of coherently strained alpha-PbO(2) particles. The refined cell volume indicated a considerable residual stress to stabilize the dense structure to ambient condition.