Luminescence, breakdown and colouring of anodic oxide films on aluminium

Photoluminescence properties of anodic aluminum oxide films formed in a mixture of malonic acid and oxalic acid

In this work, porous anodic aluminum oxide (AAO) films were fabricated by anodization in an electrolyte mixture with various concentration ratios of malonic acid and oxalic acid at room temperature. The photoluminescence (PL) properties of the AAO films before and after annealing from 300 °C to 650 °C in air or vacuum conditions were investigated, showing a strong PL band in the range of 300 - 550 nm. We observed a weak PL in the AAO film formed in the malonic acid electrolyte, while the films fabricated using an electrolyte mixture showed strong PL emissions, exhibiting a maximum. The broad PL band was decomposed into three Gaussian sub-bands, where the first two sub-bands could be attributed to the luminescence center oxygen vacancies (F⁺ and F defect centers), while the latter transformed from malonic impurities and oxalic impurities. More interestingly, the redshift of the PL bands occurred with increasing oxalic acid concentration, and the PL wavelength and intensity could be modulated by varying the concentration ratios in the malonic acid and oxalic acid electrolyte mixture.

Designing Carbon-Enriched Alumina Films Possessing Visible Light Absorption

Aluminum anodization in an aqueous solution of formic acid and sodium vanadate leads to the formation of alumina/carbon composite films. This process was optimized by varying the concentrations of formic acid and sodium vanadate, the pH, and the processing time in constant-voltage (60–100 V) or constant-current mode. As estimated, in this electrolyte, the anodizing conditions played a critical role in forming thick, nanoporous anodic films with surprisingly high carbon content up to 17 at.%. The morphology and composition of these films were examined by scanning electron microscopy, ellipsometry, EDS mapping, and thermogravimetry coupled with mass spectrometry. For the analysis of incorporated carbon species, X-ray photoelectron and Auger spectroscopies were applied, indicating the presence of carbon in both the sp² and the sp³ states. For these films, the Tauc plots derived from the experimental diffuse reflectance spectra revealed an unprecedentedly low bandgap (E_g) of 1.78 eV compared with the characteristic E_g values of alumina films formed in solutions of other carboxylic acids under conventional anodization conditions and visible-light absorption.

Recent Progress in the Fabrication and Optical Properties of Nanoporous Anodic Alumina

The fabrication of a thick oxide layer onto an aluminum surface via anodization has been a subject of intense research activity for more than a century, largely due to protective and decorative applications. The capability to create well-defined pores via a cost-effective electrochemical oxidation technique onto the surface has made a major renaissance in the field, as the porous surfaces exhibit remarkably different properties compared to a bulk oxide layer. Amongst the various nanoporous structures being investigated, nanoporous anodic alumina (NAA) with well-organized and highly ordered hexagonal honeycomb-like pores has emerged as the most popular nanomaterial due to its wide range of applications, ranging from corrosion resistance to bacterial repelling surfaces. As compared to conventional nanostructure fabrication, the electrochemical anodization route of NAA with well-controlled pore parameters offers an economical route for fabricating nanoscale materials. The review comprehensively reflects the progress made in the fabrication route of NAA to obtain the material with desired pore properties, with a special emphasis on self-organization and pore growth kinetics. Detailed accounts of the various conditions that can play an important role in pore growth kinetics and pore parameters are presented. Further, recent developments in the field of controlling optical properties of NAA are discussed. A critical outlook on the future trends of the fabrication of NAA and its optical properties on the emerging nanomaterials, sensors, and devices are also outlined.

Synthesis, spectroscopic and luminescence properties of Ga-doped γ-Al2O3

Gallium-doped aluminum oxide (Al_1-xGa_x)₂O₃ with γ-Al₂O₃ (spinel) structure has been synthesized by the precursor method using the formate Al_1-xGa_x(OH)(HCOO)₂ as a precursor. The examination of Al_1-xGa_x(OH)(HCOO)₂ (x = 0.0, 0.1, 0.2, 0.3, 0.4) was carried out by X-ray powder diffraction, Infrared, Raman spectroscopy and differential-thermal methods. The solid solutions γ-(Al_1-xGa_x)₂O₃ with х≤0.2 have been synthesized by thermolysis of precursors in helium atmosphere at 700 °C; they exhibit white-blue emission under UV excitation, whose intensity lowers with increasing dopant concentration. As an independent method, the DFT calculations confirmed thermodynamically the stability field of γ-(Al_1-xGa_x)₂O₃ solid solutions and the NMR data on relative abundance of Al and Ga within the tetrahedral and octahedral sites in the metal sublattice. Furthermore, the structural and thermodynamic properties of carbon-containing impurities within these compounds were suggested theoretically as possible models of luminescence emission centers. The experimentally observed Ga-dependent quenching of luminescence was explained using the competition between C2p and Ga4p states within the band gap of γ-Al₂O₃.

Fabrication of Self-Ordered Alumina Films with Large Interpore Distance by Janus Anodization in Citric Acid

Self-organized porous anodic alumina (PAA) formed by electrochemical anodization have become a fundamental tool to develop various functional nanomaterials. However, it is still a great challenge to break the interpore distance (D_int) limit (500 nm) by using current anodization technologies of mild anodization (MA) and hard anodization (HA). Here, we reported a new anodization mode named “Janus anodization” (JA) to controllably fabricate self-ordered PAA with large D_int at high voltage of 350–400 V. JA naturally occurs as anodizing Al foils in citric acid solution, which possessing both the characteristics of MA and HA. The process can be divided into two stages: I, slow pore nucleation stage similar to MA; II, unequilibrium self-organization process similar to HA. The as-prepared films had the highest modulus (7.0 GPa) and hardness (127.2 GPa) values compared with the alumina obtained by MA and HA. The optical studies showed that the black films have low reflectance (<10 %) in the wavelength range of 250–1500 nm and photoluminescence property. D_int can be tuned between 645–884 nm by controlling citric acid concentration or anodization voltage. JA is a potential technology to efficiently and controllably fabricate microstructured or hybrid micro- and nanostructured materials with novel properties.

Identification of the C2∏-X2Σ+ band system of AlO in the ultraviolet galvanoluminescence obtained during aluminum anodization

The first galvanoluminescence spectrum in the ultraviolet region obtained during anodization of high purity aluminum samples annealed at temperature above 525°C is presented. An intense broad peak with the maximum at about 31,900 cm(-1) is assigned to the transitions (some of them heretofore unobserved) between vibrational levels of the C(2)∏→X(2)Σ(+) spectral system of AlO, partly overlapped with the A(2)Σ(+)→X(2)∏ system of OH.