Yahya Alivov

Efficiency of dye sensitized solar cells based on TiO2 nanotubes filled with nanoparticles

From comparative studies of dye sensitized solar cell (DSSCs) fabricated using nanotubes (NTs) filled with nanoparticles (NPs) and empty NTs it is shown that filling of TiO2 NTs with NPs significantly increase efficiency of DSSCs. This increase depended on NT diameter and was found to be 17.4%, 55.6%, and 131% for solar cells with 40 nm, 80 nm, and 160 nm diameter NTs. The highest efficiency was observed for samples with 80 nm diameter NTs, which was 5.94% for ∼5 μm NT thickness DSSCs.

The anodization voltage influence on the properties of TiO2 nanotubes grown by electrochemical oxidation

A systematic study of titanium dioxide (TiO2) nanotubes (NTs) grown by electrochemical anodization in NH4F + glycerol electrolyte has been carried out in a broad range of anodization voltage of 5-350 V and acid concentration of 0.1-0.7 wt%. It is found that NTs can be grown in the voltage range from 10 to 240 V. The maximum NH4F acid concentration at which NTs can be formed decreases with the anodization voltage (V(a)). The maximum NH4F acid concentration is 0.7% for V(a)<60 V, and it decreases to 0.1% at V(a) = 240 V. Glancing angle x-ray diffractometer (GAXRD) measurements show that as-grown amorphous TiO2 transforms to the anatase phase when annealed at 400 degrees C, and further transforms to the rutile phase at annealing temperatures higher than 500 degrees C. The transition temperature from anatase to rutile phase depends on the anodization conditions. The electrical resistivity of the NT increases by eight orders of magnitude when V(a) increases from 10 to 240 V.

TiO2 nanotubes as a cold cathode for x-ray generation

Here we report on an x-ray source based on titanium dioxide (TiO2) nanotubes grown by electrochemical oxidation. From the analysis of current-voltage characteristics of TiO2 electron emitter field emission nature of the current was confirmed. The threshold voltage and field enhancement factors were derived to be ∼1.8 V/μm and ∼8363, respectively. The current density was ∼4.0 mA/cm2 at ∼2.4 V/μm. The stability tests showed that the current stayed stable within 6% for more than 720 h. TiO2 nanotubes were used as a cold cathode in x-ray tube and it was demonstrated that TiO2 nanotubes could be a good candidate for such applications.

A TiO2 nanostructure transformation: from ordered nanotubes to nanoparticles

Transformation of TiO2 nanotubes (NTs) to truncated tetragonal bipyramidal shape nanoparticles (NPs) was observed upon thermally annealing titanium dioxide (TiO2) ordered nanotube arrays in fluorine ambient, resulting from the reaction of fluorine ions (F(-)) from the electrolyte residues in long nanotubes grown by anodization in ethylene glycol+NH4F electrolyte. The size of the TiO2 nanoparticles formed depends on the fluorine concentration and can be controlled from 20 to 500 nm. The crystal and optical properties of the nanoparticle layers are superior compared with those of nanotube arrays which are also annealed but without undergoing a morphology transformation, as was shown by means of x-ray diffraction and photoluminescence spectroscopy measurements. The NT-NP transformation mechanism was studied by analyzing the initial stages of the NT-NP transformation. This was achieved by terminating the annealing process in F ambient after 1-5 min. It was found that amorphous NTs first contract, then break down, and finally merge and crystallize to form NPs.

Titanium nanotubes grown by titanium anodization

In this work we investigated the structural and electrical properties of titanium dioxide (TiO2) nanotubes (NTs) grown by electrochemical anodization of Ti metal sheets in NH4F+H2O+glycerol electrolyte at different anodization voltages (Va) and electrolyte composition. Our results revealed that TiO2 NTs can be grown in a wide range of anodization voltages from 10 to 240 V. The maximum NH4F acid concentration, at which NTs can be formed, decreases with the anodization voltage, which is 0.7% for Va<60 V, and decreases to 0.1% at Va=240 V. Addition of water to the electrolyte results in an increase in NT growth rate and modification of NT film morphology. Glancing angle x-ray diffraction experiments show that as-grown amorphous TiO2 transforms to anatase phase after annealing at 400 °C and further transforms to rutile phase at annealing temperatures above 500 °C. Samples grown in 30–120 V voltage range have higher crystal quality as seen from anatase (101) peak intensity and reduced linewidth. The electrical...

Effect of TiO2 nanotube parameters on field emission properties

The dependence of field emission properties of titanium dioxide (TiO(2)) nanotubes (NTs) has been studied as a function of NT diameter (D) and height (h), which varied in the ranges 18-500 nm and 500-12,000 nm, respectively. The studies showed a strong dependence of the field emission on these parameters. With an increase of NT diameter, the field enhancement factor increased monotonically from 120 to 3800; the current density also increased until D = 320 (current density ∼ 3.8 mA cm( - 2)), with subsequent decrease for larger diameters. The field emission properties initially improved with NT height until h = 5 µm, and later remained unchanged with further increases in h.

Origin of magnetism in undoped TiO2 nanotubes

Magnetic properties of undoped anatase, rutile, and amorphous titanium dioxide (TiO2) nanotubes grown by electrochemical anodization were studied by superconducting quantum interference device (SQUID) and electron spin resonance (ESR) methods in the temperature range 1.8-300 K. All anatase, rutile, and amorphous TiO2 nanotubes were found to exhibit paramagnetic behaviors in the entire temperature range when tested with magnetic center concentrations of 6×10(17), 3×10(16), and 3 × 10(15) cm(-3), respectively. The diameter of the TiO2 nanotubes varied from 40-160 nm and has no significant effect on the magnetic properties observed. SQUID data showed strong nonlinear M-H relationships for anatase at low temperatures, and Arrot plot analysis suggested ferromagnetism in the sample with a Curie temperature T(C) ~ 6 K. However, ESR studies showed no evidence for long-distance magnetic ordering. ESR studies revealed two magnetic centers with g1 = 1.928 and g2 = 2.028 that were common to all samples. The resonance peak at g1 = 1.922 was ascribed to Ti(3+) cations centers resulting from oxygen vacancies, while the peak at g2 = 2.028 was ascribed to surface absorbents. The amorphous sample ESR spectrum contained additional resonance peaks with corresponding g values at 2.228, 1.873, and 1.715 that possibly resulted from the disordered nature of these samples.

Imaging of nanoparticles with dual-energy computed tomography

A simulation study was performed to determine the feasibility and performance of imaging nanoparticles as contrast agents in dual-energy computed tomography. An analytical simulation model was used to model the relevant signal-to-noise ratio (SNR) in dual-energy imaging for the specific case of a three-material patient phantom consisting of water, calcium hydroxyapatite and contrast agent. Elemental gold and iodine were both considered as contrast agents. Simulations were performed for a range of monoenergetic (20-150 keV) and polyenergetic (20-150 kVp) beam spectra. A reference configuration was defined with beam energies of 80 and 140 kVp to match current clinical practice. The effect of adding a silver filter to the high-energy beam was also studied. A figure of merit (FOM), which normalized the dual-energy SNR to the square root of the patient integral dose, was calculated for all cases. The units of the FOM were keV(-1/2). A simple Rose model of detectability was used to estimate the minimum concentration of either elements needed to be detected (SNR > 5). For monoenergetic beams, the peak FOM of gold was 6.4 × 10(-6) keV(-1/2), while the peak FOM of iodine was 3.1 × 10(-6) keV(-1/2), a factor of approximately 2 greater for gold. For polyenergetic spectra, at the reference energies of 80 and 140 kVp, the FOM for gold and iodine was 1.65 × 10(-6) and 5.0 × 10(-7) keV(-1/2), respectively, a factor of approximately 3.3 greater. Also at these energies, the minimum detectable concentration of gold was estimated to be 58.5 mg mL(-1), while iodine was estimated to be 117.5 mg mL(-1). The results suggest that the imaging of a gold nanoparticle contrast agent is well suited to current conditions used in clinical imaging. The addition of a silver filter of 800 µm further increased the image quality of the gold signal by approximately 50% for the same absorbed dose to the patient.

Enhanced field emission from clustered TiO2 nanotube arrays

Field emission properties of clustered titanium dioxide (TiO2) nanotube arrays have been studied and compared with those of dense, highly aligned TiO2 nanotube arrays. It was found that clustered nanotube arrays showed significant increase of field emission current density (sevenfold) and field enhancement factor (fivefold) compared to regular shape, highly aligned forest-like TiO2 nanotube arrays, which can be explained by the reduction of electric field screening effects. Clustered TiO2 nanotubes arrays were achieved by electrochemical oxidation of titanium sheet in electrolyte consisting of diethylene or ethylene glycol solvents and ammonium fluorine, while the well aligned nanotube arrays were grown using glycerol.

Role of phonons in the optical properties of magnetron sputtered ZnO studied by resonance Raman and photoluminescence

The temperature dependence of phonons in ZnO has been studied using resonance Raman and photoluminescence (PL) emission measurements. Excitation with wavelength 363.8 nm (photon energy 3.409 eV) is used to establish incoming resonance near room temperature. Broad PL emission is seen at room temperature with peak position at 3.25 eV. This coincides with the overtone of the longitudinal optic (LO) band. Up to six LO phonon orders are observed. Temperature dependence of the LO phonon energy is described by a two-phonon decay mechanism with energies 100 and 496 cm−1. The temperature dependence of the PL shift is interpreted based on electron-phonon interactions. A two-phonon description is sufficient to describe the temperature shift in the band gap through occupation at average acoustic and optic phonon energies 125 and 500 cm−1, respectively. LO phonon sidebands (PSBs) are also observed at low temperature (23 to 100 K). The temperature shift in the PSB energies is interpreted based on the band gap shift com...