A surface science perspective on TiO2 photocatalysis

The increasing need for new materials capable of solar fuel generation is central in the development of a green energy economy. In this contribution, we demonstrate that black TiO2 nanoparticles obtained through a one-step reduction/crystallization process exhibit a bandgap of only 1.85 eV, which matches well with visible light absorption. The electronic structure of black TiO2 nanoparticles is determined by the unique crystalline and defective core/disordered shell morphology. We introduce new insights that will be useful for the design of nanostructured photocatalysts for energy applications.

Effect of nature and location of defects on bandgap narrowing in black TiO2 nanoparticles.

Single-crystal one-dimensional (1D) semiconductor architectures are important in materials-based applications requiring a large surface area, morphological control, and superior charge transport. Titania has widespread utility in applications including photocatalysis, photochromism, photovoltaics, and gas sensors. While considerable efforts have focused on the preparation of 1D TiO2, no methods have been available to grow crystalline nanowire arrays directly onto transparent conducting oxide (TCO) substrates, greatly limiting the performance of TiO2 photoelectrochemical devices. Herein, we present a straightforward low temperature method to prepare single crystal rutile TiO2 nanowire arrays up to 5 microm long on TCO glass via a non-polar solvent/hydrophilic substrate interfacial reaction under mild hydrothermal conditions. The as-prepared densely packed nanowires grow vertically oriented from the TCO glass substrate along the (110) crystal plane with a preferred (001) orientation. In a dye sensitized solar cell, N719 dye, using TiO2 nanowire arrays 2-3 microm long we achieve an AM 1.5 photoconversion efficiency of 5.02%.

Vertically aligned single crystal TiO2 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis details and applications

Porous anodic aluminum oxide: anodization and templated synthesis of functional nanostructures.

Second-generation TiO2-xDx photocatalysts doped with either anions (N, C, and S mostly) or cations have recently been shown to have their absorption edge red-shifted to lower energies (longer wavelengths), thus enhancing photonic efficiencies of photoassisted surface redox reactions. Some of the studies have proposed that this red-shift is caused by a narrowing of the band gap of pristine TiO2 (e.g., anatase, Ebg = 3.2 eV; absorption edge ca. 387 nm), while others have suggested the appearance of intragap localized states of the dopants. By contrast, a recent study by Kuznetsov and Serpone (J. Phys. Chem. B, in press) has proposed that the commonality in all these doped titanias rests with formation of oxygen vacancies and the advent of color centers (e.g., F, F+, F++, and Ti3+) that absorb the visible light radiation. This article reexamines the various claims and argues that the red-shift of the absorption edge is in fact due to formation of the color centers, and that while band gap narrowing is not an...

Is the Band Gap of Pristine TiO2 Narrowed by Anion- and Cation-Doping of Titanium Dioxide in Second-Generation Photocatalysts?

Highly ordered TiO2 nanowire (TN) arrays were prepared in anodic alumina membranes (AAMs) by a sol-gel method. The TNs are single crystalline anatase phase with uniform diameters around 60 nm. At room temperature, photoluminescence (PL) measurements of the TN arrays show a visible broadband with three peaks, which are located at about 425, 465, and 525 nm that are attributed to self-trapped excitons, F, and F+ centers, respectively. A model is also presented to explain the PL intensity drop-down of the TN arrays embedded in AAMs: the blue PL band of AAMs arises from the F+ centers on the pore walls, and the TNs first form in the center area of the pores and then extend to the pore walls.

Preparation and photoluminescence of highly ordered TiO2 nanowire arrays

We report an investigation of normal-incidence GeSn-based p-i-n photodetectors (PDs) with a Ge0.94Sn0.06 active layer grown using sputter epitaxy on a Ge(100) substrate. A low dark current density of 0.24 A/cm2 was obtained at a reverse bias of 1 V. A high optical responsivity of the Ge0.94Sn0.06/Ge p-i-n PDs at zero bias was achieved, with an optical response wavelength extending to 1985 nm. The temperature-dependent optical-response measurement was performed, and a clear redshift absorption edge was observed. This work presents an approach for developing efficient and cost-effective GeSn-based infrared devices.

GeSn p-i-n photodetectors with GeSn layer grown by magnetron sputtering epitaxy

Magnetron sputtering was successfully used to grow single-crystalline Ge1-x-ySixSny ternary alloys on Si (100) substrates. The lattice constants of the alloys were calculated by X-ray diffraction and corrected Vegard’s law, respectively, showing that the corrected Vegard’s law is suitable for the Ge1-x-ySixSny lattice constants. Thermal stability investigations showed that the Ge0.85Si0.051Sn0.099 alloy was stable at 500 °C. The Ge1-x-ySixSny can maintain good crystalline quality under moderate annealing temperature, with no indication of phase segregation or Sn precipitation. Optical absorption measurements were carried out at room temperature to determine the band gap energies of the Ge1-x-ySixSny alloys. These results suggest that magnetron sputtering is an effective alternative method to grow Ge1-x-ySixSny alloys for fabrication of novel devices.

Single-crystalline Ge1-x-ySixSny alloys on Si (100) grown by magnetron sputtering

A high-quality Ge0.88Si0.08Sn0.04/Ge0.94Sn0.06 multiple quantum well (MQW) structure was grown on a Ge (001) substrate by sputtering epitaxy. The MQW structure was characterized by high-resolution x-ray diffraction and transmission electron microscopy. Surface-illuminated Ge0.88Si0.08Sn0.04/Ge0.94Sn0.06 MQW pin photodetectors were fabricated with cutoff wavelengths of up to 2140 nm. The analysis of transitions from spectral response was fitted well with the theoretical calculations. Results suggest that sputtering epitaxy is a promising method for preparing high-quality low-dimensional Sn-based group IV materials and that Ge1−x−ySiySnx/Ge1−xSnx MQWs have potential applications in the development of efficient Si-based photonic devices.

Suyuan Wang

Papers

Preparation and photoluminescence of highly ordered TiO2 nanowire arrays

GeSn p-i-n photodetectors with GeSn layer grown by magnetron sputtering epitaxy

Single-crystalline Ge1-x-ySixSny alloys on Si (100) grown by magnetron sputtering

Characterization of a Ge 1-x-y Si y Sn x /Ge 1-x Sn x multiple quantum well structure grown by sputtering epitaxy.

Computational simulations using a low density ratio-based kinetic theory of granular flow in subcritical water fluidized beds