Nitrogen-doped TiO₂ nanotube arrays (N-TNAs) were successfully fabricated by a simple thermal annealing process in ambient N₂ gas at 450 °C for 3 h. TNAs with modified morphologies were prepared by a two-step anodization using an aqueous NH₄F/ethylene glycol solution. The N-doping concentration (0–9.47 at %) can be varied by controlling N₂ gas flow rates between 0 and 500 cc/min during the annealing process. Photocatalytic performance of as-prepared TNAs and N-TNAs was studied by monitoring the methylene blue degradation under visible light (λ ≥ 400 nm) illumination at 120 mW·cm⁻². N-TNAs exhibited appreciably enhanced photocatalytic activity as compared to TNAs. The reaction rate constant for N-TNAs (9.47 at % N) reached 0.26 h⁻¹, which was a 125% improvement over that of TNAs (0.115 h⁻¹). The significant enhanced photocatalytic activity of N-TNAs over TNAs is attributed to the synergistic effects of (1) a reduced band gap associated with the introduction of N-doping states to serve as carrier reservoir, and (2) a reduced electron‒hole recombination rate.

Large-area surface ripple structures of indium-tin-oxide films, composed of self-organized nanodots, were induced by femtosecond laser pulses, without scanning. The multi-periodic spacing (∼800 nm, ∼400 nm and ∼200 nm) was observed in the laser-induced ripple of ITO films. The local conductivity of ITO films is significantly higher, by approximately 30 times, than that of the as-deposited ITO films, due to the formation of these nanodots. Such a significant change can be ascribed to the formation of indium metal-like clusters, which appear as budges of ∼ 5 nm height, due to an effective volume increase after breaking the In-O to form In-In bonding.

The evolution of microstructure and composition of titanium dioxide ( <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Ti</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>TiO2 ) nanotube arrays fabricated with hydrofluoric acid (HF) and <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">F</mml:mi></mml:mrow></mml:math>NH4F electrolytes as a function of annealing temperature up to <mml:math display="inline"><mml:mrow><mml:mn>400</mml:mn><mml:mi>°</mml:mi><mml:mi mathvariant="normal">C</mml:mi></mml:mrow></mml:math>400°C was investigated and compared using X-ray diffraction, scanning electron microscopy, and X-ray absorption near-edge structure spectroscopy. Results showed that <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Ti</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>TiO2 nanotube arrays grown in HF electrolyte contained 90% amorphous <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Ti</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>TiO2 and 10% lower oxidation states of titanium from <mml:math display="inline"><mml:msup><mml:mi mathvariant="normal">Ti</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math>Ti2+ (TiO) and <mml:math display="inline"><mml:msup><mml:mi mathvariant="normal">Ti</mml:mi><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math>Ti3+ <mml:math display="inline"><mml:mrow><mml:mo>(</mml:mo><mml:msub><mml:mi mathvariant="normal">Ti</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow></mml:math>(Ti2O3) cations. After annealing at <mml:math display="inline"><mml:mrow><mml:mn>400</mml:mn><mml:mi>°</mml:mi><mml:mi mathvariant="normal">C</mml:mi></mml:mrow></mml:math>400°C , <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Ti</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>TiO2 nanotube arrays underwent charge transfer and phase transformation to 93% anatase phase, 6% amorphous <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Ti</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>TiO2 , and 1% suboxides. In contrast, as-grown <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Ti</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>TiO2 nanotube arrays using <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">F</mml:mi></mml:mrow></mml:math>NH4F electrolyte possessed less amorphous <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Ti</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>TiO2 (82%) but more suboxides (18%) due to lower oxygen ion formation from scanty <mml:math display="inline"><mml:mrow><mml:mn>3</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi>wt</mml:mi><mml:mspace width="0.3em"></mml:mspace><mml:mi mathvariant="normal">%</mml:mi></mml:mrow></mml:math>3wt% of <mml:math display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>H2O addition. Its onset temperature of phase transformation was found to be higher than <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Ti</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>TiO2 nanotube arrays prepared by HF solution. Moreover, when annealed to <mml:math display="inline"><mml:mrow><mml:mn>400</mml:mn><mml:mi>°</mml:mi><mml:mi mathvariant="normal">C</mml:mi></mml:mrow></mml:math>400°C , the crystallinity of <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Ti</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>TiO2 nanotube arrays increased only by 86% for the anatase phase. The lower anatase phase could be attributed to the formation of <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">Ti</mml:mi><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>(NH4)2TiF6 type compounds presumably formed by the reaction of <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Ti</mml:mi><mml:msubsup><mml:mi mathvariant="normal">F</mml:mi><mml:mrow><mml:mn>6</mml:mn></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo></mml:mrow></mml:msubsup></mml:mrow></mml:math>TiF62− and <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msubsup><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mo>+</mml:mo></mml:msubsup></mml:mrow></mml:math>NH4+ ions dissociated from <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">F</mml:mi></mml:mrow></mml:math>NH4F .

Two types of periodic nanostructures, self-organized nanodots and nanolines, were fabricated on the surfaces of indium-tin-oxide (ITO) films using femtosecond laser pulse irradiation. Multiple periodicities (approximately 800 nm and 400 nm) were clearly observed on the ITO films with nanodot and nanoline structures and were identified using two-dimensional Fourier transformation patterns. Both nanostructures show the anisotropic transmission characteristics in the visible range, which are strongly correlated with the geometry and the metallic content of the laser-induced nanostructures.

In the present study, the structural, morphological, compositional, nanomechanical, and surface wetting properties of Bi₂Se₃ thin films prepared using a stoichiometric Bi₂Se₃ target and a Se-rich Bi₂Se₅ target are investigated. The Bi₂Se₃ films were grown on InP(111) substrates by using pulsed laser deposition. X-ray diffraction results revealed that all the as-grown thin films exhibited were highly c-axis-oriented Bi₂Se₃ phase with slight shift in diffraction angles, presumably due to slight stoichiometry changes. The energy dispersive X-ray spectroscopy analyses indicated that the Se-rich target gives rise to a nearly stoichiometric Bi₂Se₃ films, while the stoichiometric target only resulted in Se-deficient and Bi-rich films. Atomic force microscopy images showed that the films’ surfaces mainly consist of triangular pyramids with step-and-terrace structures with average roughness, R_a, being ~2.41 nm and ~1.65 nm for films grown with Bi₂Se₃ and Bi₂Se₅ targets, respectively. The hardness (Young’s modulus) of the Bi₂Se₃ thin films grown from the Bi₂Se₃ and Bi₂Se₅ targets were 5.4 GPa (110.2 GPa) and 10.3 GPa (186.5 GPa), respectively. The contact angle measurements of water droplets gave the results that the contact angle (surface energy) of the Bi₂Se₃ films obtained from the Bi₂Se₃ and Bi₂Se₅ targets were 80° (21.4 mJ/m²) and 110° (11.9 mJ/m²), respectively.