The effect of thickness on the composition of passive films on a Ti-50Zr at% alloy

Anodic films were grown potentiodynamically in different electrolytes (pH = 1-14) on a Ti-50Zr at% cast alloy, obtained by fusion in a voltaic arc under argon atmosphere. The thickness of the films was varied by changing formation potential from the open circuit potential up to about 9 V; growth was followed by 30 min stabilization at the forming potential. Films having different thicknesses were characterized by photocurrent spectroscopy (PCS) and electrochemical impedance spectroscopy (EIS). Moreover, film composition was analyzed by X-ray photoelectron spectroscopy (XPS).Regardless of the anodizing conditions, passive films on the Ti-50Zr at% alloy consist of a single layer mixed oxide phase containing both TiO₂ and ZrO₂ groups. However, an enrichment of Ti within the passive film, increasing with the film thickness, is detected both by PCS and XPS. This leads to concentration profiles of Ti⁴⁺ and Zr⁴⁺ ions along the thickness, and to different electronic properties of very thin films (more insulating) with respect to thicker films (more semiconducting), as revealed by the photocurrent-potential curves.     

新一代光辐射测量仪器

介绍了国内新一代光辐射测量仪器的原理和目前进展情况。     

Photo-electrochemical investigation of anodic oxide films on cast Ti-Mo alloys. I. Anodic behaviour and effect of alloy composition

The anodic behaviour of cast Ti-Mo alloys, having different Mo contents (6-20 wt.%), was investigated in acidic and neutral aerated aqueous solutions. All sample showed a valve-metal behaviour, owing to formation and thickening of barrier-type anodic oxides displaying interference colours. Growth kinetics of passive films is influenced by both anodizing electrolyte and composition of the starting alloy. This last parameter was found to change also the solid-state properties of the films, explored by photoelectrochemical and impedance spectroscopy experiments. Thicker films (U_f = 8 V/MSE) grown on alloys richer in Mo showed more resistive character and a photocurrent sign inversion under negative bias, that revealed an insulating character, whereas corresponding films grown on alloys with lower Mo content, as well as thinner films, behaved as n-type semiconductors. Results are discussed in terms of formation of a mixed Ti-Mo oxide phase.     

Transport mechanism analysis of non-equilibrium charge carrier in heterojunctions with GaS-CdTe:Mn thin films

The current-voltage characteristics in the temperature range 170 ÷ 305 K at the direct polarization of GaS-CdTe:Mn heterojunction were studied. The tunneling process of carriers determines the electrical current through the junction. The ratio of the diffusion coefficient and surface recombination velocity in the CdTe:Mn film from the interface of junction was determined from the spectral characteristics of short circuit current and absorption. The non-equilibrium carriers generated in the CdTe:Mn film at the interface of heterojunction at the temperature of 293 K recombine through two recombination levels. At low temperatures (T < 170 K) the concentration of carriers decreases and only one recombination level appears with the short recombination lifetime.     

Significant improvement of photocurrent in dye-sensitized solar cells by incorporation thiophene into electrolyte as an inexpensive and efficient additive

For the first time, thiophene as an efficient additive is applied in electrolyte of dye-sensitized solar cells (DSSCs). In the cell based on 2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid (TPA) sensitizer, addition of 1 M thiophene to the electrolyte (instead of 4-tert-butylpyridine (TBP)) increases significantly photocurrent density (J_sc) from 4.62 to 7.80 mA cm⁻². Consequently, overall conversion efficiency (η) enhances from 2.17% to 3.18% with a 46% improvement. For DSSC based on 2-cyano-3-(2′-(5′,10′,15′,20′-tetraphenylporphyrinato zinc(II))yl)acrylic acid (Zn-1) with modified electrolyte, J_sc increases from 4.18 to 8.23 mA cm⁻² and η improves from 1.46% to 1.96% that shows an enhancement of 34% in η. Intensity modulated photocurrent spectroscopy (IMPS) indicates the faster electron transfer in the DSSC based on thiophene additive compared to the standard device. Also, the higher electron diffusion coefficient (D_n) of the DSSC with modified electrolyte shows the thiophene additive facilitates the electron transfer. Based on cyclic voltammetry (CV) and UV–vis data, thiophene forms a molecular complex with iodine molecule in the electrolyte solution that has a remarkable effect on the redox couple oxidation–reduction reactions which improves J_sc. A significant improvement in J_sc with a small decrease in V_oc and thereby an increase in η are observed from addition thiophene.     

Investigation of Semiconductor Quantum Dots for Waveguide Electroabsorption Modulator

In this work, we investigated the use of 10-layer InAs quantum dot (QD) as active region of an electroabsorption modulator (EAM). The QD-EAM is a p-i-n ridge waveguide structure with intrinsic layer thickness of 0.4 μm, width of 10 μm, and length of 1.0 mm. Photocurrent measurement reveals a Stark shift of ~5 meV (~7 nm) at reverse bias of 3 V (75 kV/cm) and broadening of the resonance peak due to field ionization of electrons and holes was observed for E-field larger than 25 kV/cm. Investigation at wavelength range of 1,300–1320 nm reveals that the largest absorption change occurs at 1317 nm. Optical transmission measurement at this wavelength shows insertion loss of ~8 dB, and extinction ratio of ~5 dB at reverse bias of 5 V. Consequently, methods to improve the performance of the QD-EAM are proposed. We believe that QDs are promising for EAM and the performance of QD-EAM will improve with increasing research efforts.     

Composite PbО2-TiO2 materials deposited from colloidal electrolyte: Electrosynthesis, and physicochemical properties

Electrodeposition of PbO₂ from nitrate solutions in the presence of TiO₂ nanoparticles leads to composite PbO₂-TiO₂ films. The content of the dispersed oxide which is finally occluded into the composite PbO₂ film depends on electrodeposition conditions such as pH, the value of the electrodeposition constant current or potential, the amount of added TiO₂ and on temperature. It also depends strongly on the presence of anionic additives such as sodium dodecyl sulfate (SDS) whose adsorption decreases the positive charge on the surface of the TiO₂ particles.The photo-electrocatalytic activity of the prepared materials has been tested in the oxidation of oxalic acid and benzyl alcohol. Electrodes showed a photoresponse to illumination at λ > 320 nm confirming literature reports on synergistic effects of illumination in electro-oxidation processes at PbO₂-based anodes. We also established that the life service of these electrodes increases by a factor of about 3 with respect to traditional PbO₂ anodes. On the other hand, the more striking achievement in the present work with PbO₂-TiO₂ electrodes is the enhancement of electroactivity in the dark for oxalic acid, benzyl alcohol as well as for O₂ evolution.     

Delta doping and positioning effects of type II GaSb quantum dots in GaAs solar cell

GaSb quantum dot (QD) solar cell structures were grown by molecular beam epitaxy on GaAs substrates. We investigate the reduction in open-circuit voltage and study the influence of the location of QD layers and their delta doping within the solar cell. Devices with 5 layers of delta-doped QDs placed in the intrinsic, n- and p-regions of a GaAs solar cell are experimentally investigated, and the deduced values of J_sc, V_oc, fill factor, efficiency (η) are compared. A trade-off is needed to minimize the V_oc degradation while maximizing the short circuit current density (J_sc) enhancement due to sub-bandgap absorption. The voltage recovery is attributed to the removal of the QDs from the high-field region which reduces SRH recombination. The devices with p- or n-doped QDs placed in the flat band potential (p- or n-region) show a recovery in J_sc and V_oc compared to devices with delta-doped QDs placed in the depletion region. However, there is less photocurrent arising from the absorption of sub-band gap photons. Furthermore, the long wavelength photoresponse of the n-doped QDs placed in the n-region shows a slight improvement compared to the control cell. The approach of placing QDs in the n-region of the solar cell instead of the depletion region is a possible route towards increasing the conversion efficiency of QD solar cells.     

Optimal Ag concentration for H2 production via Ag:TiO2 nanocomposite thin film photoanode

TiO₂ thin films containing different concentrations of Ag nanoparticles have been synthesized by sol-gel method. According to UV–visible spectra, presence of an intense surface plasmon resonance peak at 490 nm of wavelength indicated formation of silver nanoparticles in the TiO₂ films. Based on atomic force microscopy (AFM) analysis, the surface roughness and the effective surface ratio increased by increasing the Ag mol%. Moreover, scanning electron microscopy (SEM) images showed formation of Ag nanoparticles on the surface for the samples containing high Ag concentration. X-ray diffraction (XRD) patterns revealed that the size of Ag nanocrystals increased by increasing the Ag content in the films while the nanocrystalline size of TiO₂ reduced in the presence of silver nanoparticles. Based on x-ray photoelectron spectroscopy (XPS) data, a stoichiometric chemical composition was detected for TiO₂ while, Ag presented in a combination a metal/oxide states on the surface. Studying photoresponse of the samples showed that the highest photocurrent was obtained for the sample containing 1 mol% Ag. By measuring the photovoltage versus time, it was found that addition of silver nanoparticles to the TiO₂ layer resulted in reduction of the transient time of the photogenerated carriers in the samples. Impedance spectroscopy determined a slight decrease in charge transfer resistance by addition of Ag to the films. Moreover, measuring the amount of hydrogen produced during water splitting reactions verified that the highest quantum yield of 9.6% was obtained for the sample with 1 mol% Ag.     

Preparation and characterization of Co-doped TiO2 materials for solar light induced current and photocatalytic applications

Different amounts of Co-doped TiO₂ powders and thin films were prepared by following a conventional co-precipitation and sol–gel dip coating technique, respectively. The synthesized powders and thin films were subjected to thermal treatments from 400 to 800 °C and were thoroughly investigated by means of X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive analysis with X-rays, FT-infrared, FT-Raman, diffuse reflectance spectroscopy, ultraviolet–visible spectroscopy, BET surface area, zeta potential, flat band potential measurements, band-gap energy, etc. The photocatalytic ability of the powders was evaluated by methylene blue (MB) degradation studies. The thin films were characterized by photocurrent and ultraviolet–visible (UV–Vis) spectroscopy techniques. The characterization results suggest that the Co-doped TiO₂ powders synthesized in this study consist mainly anatase phase, and possess reasonably high specific surface area, low band gap energy and flat band potentials amenable to water oxidation in photoelectrochemical (PEC) cells. The photocatalytic degradation of MB over Co-doped TiO₂ powders followed the Langmuir–Hinshelwood first order reaction rate relationship. The 0.1 wt.% Co-doped TiO₂ composition provided the higher photocurrent, n-type semi-conducting behavior and higher photocatalytic activity among various Co-doped TiO₂ compositions and pure TiO₂ investigated.