Defect-rich 1D TiO2 nanostructures show excellent photoelectrochemical water splitting property in the visible light region with a low onset potential of 0.3 V vs. RHE and a remarkably high conversion efficiency of 3.6%.

Uniform nanocrystalline films of tin oxide have been deposited on glass and silicon substrates by a solution spin-coating method followed by annealing under flowing oxygen at different temperatures. Complete conversion of the coated tin(IV) chloride solution to tin oxide at a post-oxygen-anneal temperature greater than 120 °C is confirmed by energy-dispersive X-ray analysis and depth-profiling X-ray photoemission measurements. Transmission electron microscopy and X-ray diffraction studies further reveal that the onset of SnO2 nanocrystallite formation in the amorphous film occurs at 350 °C. The resulting nanocrystallites that are accompanied by the observed granular structures and voids throughout the film as a result of crystallization and grain growth exhibit a narrow size distribution. The average nanocrystallite size is found to slowly increase from 7 nm at 350 °C to 10 nm at 500 °C (likely due to a strain-limited growth mechanism) and to grow exponentially above a second onset at 500 °C with a concomitant growth of the grains and a rapid increase in the roughness of the granular films. X-ray diffraction and Raman experiments further show a largely uniform depth distribution of nanocrystallites throughout the film, with a higher density near the surface. The facile physical control in the average size of the nanocrystallites by post-oxygen-anneal temperature in a desirable size regime (7−30 nm) promises a cost-effective, easily scalable fabrication method of SnO2 film for gas-sensing and nanoelectronic applications.

Shape and size of nanoparticles are fundamental structural properties that govern the development of novel surface-related applications. Traditionally external agents such as surfactants, reducing agents, or stabilizers have been used for enforcing preferential growth orientation, size, and shape to develop tailor-made nanoparticles. However, these external agents cover the pristine surface of the particle and invariably reduce the surface activity. Here, we introduce a surfactant-free, single-step electrochemical method to control the shape of air-stable FeNi alloy nanoparticles. Using glancing-incidence X-ray diffraction, we further demonstrate that the shape evolution of nanoparticles from concave cube to truncated sphere occurs concurrently with the phase transformation from bcc to fcc. This shape evolution can be achieved by fine-tuning a single parameter, the ratio of reactant concentrations (i.e., [Ni2+]/[Fe2+]). Addition of Ni2+ to the Fe2+ electrolyte changes the nucleation mechanism from progressive growth for pure Fe2+ electrolyte to instantaneous growth for mixed Fe2+/Ni2+ electrolyte, which leads to a remarkably narrow size distribution and very uniform dispersion on the Si substrate. Depth-profiling X-ray photoelectron spectroscopy and energy-dispersive X-ray analysis by both transmission electron microscopy and scanning electron microscopy for nanoparticles at different growth stages reveal alloy formation and preferential deposition of Fe during initial growth that results in a quasi-core–shell structure. We also observe the in-situ formation of a very thin Ni-doped FeOOH outer layer and NiFe2O4 intermediate layer on the skin of the nanoparticles, which passivates the surface and dramatically enhances the air stability. The present work provides a unique example of shape-controlled bimetallic nanostructures and offers insights into growth modification of a host metal structure by a guest metal.

Nanocrystalline tin (IV) oxide thin films doped with Eu(3+) ions are synthesized using a simple spin-coating method followed by postannealing in an O(2) flow at 700 °C. Transmission electron microscopy and x-ray photoelectron spectroscopy studies illustrate the incorporation of Eu(3+) ions in the films with a high atomic percentage of 2.7%-7.7%, which is found to be linearly dependent on the initial concentration of Eu(3+) in the precursor solution. Glancing incidence x-ray diffraction results show that the crystalline grain sizes decrease with increasing the Eu(3+) concentration and decreasing the postannealing temperature with the emergence of the Eu(2)Sn(2)O(7) phase at high Eu(3+) concentrations (≥5.3 at.%). Luminescence spectra of these doped samples show the characteristic narrow-band magnetic dipole emission at 593 nm and electric dipole emission at 614 nm of the Eu(3+) ions, arising from UV absorption at the SnO(2) band-edge followed by energy transfer to the emission centers. Manipulating the crystallite size, composition, and defect density of the samples greatly affects the absorption edge, energy transfer, and therefore the emission spectra. These modifications in the environment of the Eu(3+) ions allow the emission to be tuned from pure orange characteristic Eu(3+) emission to the broadband emission corresponding to the combination of strong characteristic Eu(3+) emission with the intense defect emissions.

Homogeneous, nanocrystalline films of tin(IV) oxide with controllable crystalline grains in the ultrasmall size range of 4À12 nm have been prepared by using a simple method of spin-coating followed by annealing in oxygen at different postannealing temperatures (T anneal ). These nanocrystalline films all exhibit a high optical transparency of 90À100% in the visible region with a band gap of 3.71 ( 0.05À3.87 ( 0.05 eV compared to 3.6 eV for bulk SnO 2 , indicating a high carrier density for all the TO films. The films obtained with T anneal g 350 °C, marking the onset of crystallization, are found to be conductive. The ac resistivity is measured as a function of temperature between 50 and 280 K for all the conductive films, and two distinct behaviors are observed between 50 and 90 K (LT) and 120À280 K (HT). The presence of two different media, i.e., the crystalline grains and the charge-depletion layer, can explain the observed resistivity behavior. The excellent fit of a parallel resistor model to the resistivity data for samples obtained with T anneal = 350À700 °C further validates the presence of the two media, revealing energy barrier heights of 48.0 ( 0.4À60.5 ( 0.4 meV for transport across the grain boundaries. The resistivity behavior in each medium is best described by the three-dimensional variable-range hopping (3D-VRH) model, given its excellent fit to the experimental data. On the basis of the resistivity results as analyzed within this model, we conclude that increasing T anneal leads to a reduction in the carrier density as defect density decreases. The 3D-VRH fits to the resistivity in the LT region further reveal that above the onset of exponential growth at T anneal = 500 °C, a remarkable improvement in the charge transport occurs likely due to the observed enhanced crystallinity. Postannealing at different temperatures, therefore, has a direct effect on the extent of crystallization in the amorphous matrix and the size of the resulting nanocrystallites, both of which affect the defect density and transport channels, and can therefore be used to provide fine control on the resistivity of the nanocrystalline SnO 2 film.