Chouhaid Nasr

Photoelectrochemical behavior of coupled SnO2|CdSe nanocrystalline semiconductor films

A photoelectrochemical cell with a coupled SnO2|CdSe nanocrystalline semiconductor electrode has been prepared by sequential deposition of SnO2 and CdSe films onto an optically transparent electrode (OTE), and its photoelectrochemical behavior has been studied. The results show that the coupling of CdSe with SnO2 leads to an improvement in the performance of OTE|SnO2|CdSe over OTE|CdSe cells in terms of increased incident photon-to-current conversion efficiency, increased stability and smaller reversal of current. The favorable positioning of the energy bands of SnO2 and CdSe is responsible for the above observations. Various photoelectrochemical parameters of the OTE|SnO2|CdSe cell obtained for an incident light power of 0.31 mW cm−2 at 470nm, are as follows: Isc ≈ 25–30 μA cm−2, Voc ≈ 0.5–0.6 V, ƒƒ = 0.47 and a power conversion efficiency of about 2.25%.

Photocatalytic reduction of azo dyes Naphthol Blue Black and Disperse Blue 79

Photocatalytic reduction of two textile azo dyes, Naphthol Blue Black (NBB) and Disperse Blue 79 (DB79) has been carried out in colloidal WO3 and TiO2 suspensions. Under bandgap excitation of the semiconductor colloids these dyes undergo irreversible reduction as they react with the trapped electrons. The quantum efficiency for the photocatalytic reduction of these dyes were 5.4% and 4.8% for NBB and DB79 respectively. The kinetics and mechanism of the interfacial charge transfer in these colloidal suspension has been elucidated with transient absorption spectroscopy. The reaction between the dye and trapped electrons is diffusion limited and occurs with rate constants of 1.1×108 M−1s−1 and 4.0×107 M−1s−1 for NBB and DB79 respectively.

Redox characteristics of Schiff base manganese and cobalt complexes related to water-oxidizing complex of photosynthesis

In an effort to obtain synthetic analogues of water-oxidizing complex (WOC) of photosystem II (PS II) of plant photosynthesis, a Schiff base manganese and a cobalt complex, employing Niten, a SALEN type ligand, have been prepared. Cyclic and square wave voltammetric measurements have been performed to assess their redox characteristics. Both complexes undergo several reduction processes in cathodic negative potential region at more or less similar potentials. In view of these reductions being independent of the nature of the metal, they are thought to be ligand-localized. Although similar in negative region, a marked difference in the behavior of the complexes is observed in anodic region. While the cobalt complex is electrochemically inactive in the positive potentials up to +1.0 V vs. Ag/AgCl, the manganese complex displays two oxidation waves at +0.25 and +0.5 V vs. Ag/AgCl. The presence of oxidation wave in manganese complex at +0.5 V vs. Ag/AgCl or +0.7 V vs. NHE suggests that this complex can catalyze the oxidation of water and can, thus, simulate the WOC of PS II.

Exciton diffusion length in microcrystalline chlorophyll a

The measurements of photocurrent as a function of depletion layer or Schottky barrier width have been performed to determine the exciton diffusion length (L) in microcrystalline chlorophyll a (Chl a). This method of determining L is particularly advantageous in that it does not require a prior knowledge of the absorption coefficient of the material as is usually the case with other, e.g., constant surface photovoltage or constant photocurrent techniques. A value of 180 A has been obtained for L in Chl a. While this value is comparable to those for many organic semiconductors, it is much inferior to that for the minority carriers in inorganic photoconductors. The smaller diffusion length may, in part, explain the feeble power conversion efficiency of Chl a and other organic photovoltaic cells.

Effect of temperature on capacitance of Al/microcrystalline chlorophyll a/Ag sandwich cells

The capacitance measurements of Al/microcrystalline Chl a/Ag sandwich cells have been carried out as a function of temperature. The results show that while the capacitance at 0.10 Hz is voltage dependent at 20 °C, it is totally voltage invariant at −150 °C. This is explained in terms of thermal mobilization of the trapped charges which are responsible for the depletion layer. The temperature supplies the necessary energy and brings the response time of the trapped charges within the modulation frequency of the applied voltage and makes capacitance voltage dependent. Further, depending upon the frequency, f, of the applied voltage, the temperature needed to mobilize the carriers to the same extent is also different. This is due to the fact that when the frequency is high, the trapped carriers are not able to respond as well to the variations in voltage compared to when the frequency is low. As a result, a comparatively high temperature is needed to mobilize the carriers to the same degree at high frequency...

Excited states and reduced and oxidized forms of a textile diazo dye, naphthol blue black. Spectral characterization using laser flash photolysis and pulse radiolysis studies

Abstract The excited singlet and triplet states of a diazo textile dye (Naphthol Blue Black) have been investigated by pico and nanosecond flash photolysis. The excited singlet state shows a difference absorption maximum at 500 nm and has a lifetime of 25–30 ps in ethanol. The triplet excited state generated by triplet-triplet energy transfer shows an absorption maxima at 710–730 nm. The oxidized form as well as the reduced form of the diazo dye have been generated pulse radiolytically by reacting with azide and e aq radicals, respectively. Preliminary results of the photoelectrochemical irreversible reduction of Naphthol Blue Black in colloidal TiO 2 are also reported.

Depletion layer characteristics of Al/microcrystalline chlorophyll a/Ag sandwich cells

Abstract The capacitance measurements of Al/microcrystalline chlorophyll a /Ag sandwich cells have been carried out at various frequencies in the dark and under illumination. The voltage-dependent capacitance, C , and the linear Schottky plots obtained at only low frequencies, ∼ 0.1 Hz, and under illumination suggest that the depletion layer mainly consists of trapped charges that are not able to follow the variations in applied voltage at high frequencies ∼ 100 Hz. From the linear Schottky plots, the depletion parameters, e.g., built-in potential, width at zero bias, and space charge density have been determined in the dark and under illumination (11 μW cm −2 ), which are, respectively, 620 mV, 300 A and 2 × 10 23 m −3 , and 700 mV, 205 A and 4.78 × 10 23 m −3 . The ageing of the cells in ambient air has been found to have dramatic effect on the capacitance characteristics of the cells. The diffusion of oxygen and presence of water vapour are most probably responsible for an inhomogeneous space charge developing within the depletion layer and the decrease of space charge density with time, respectively.

Photosensitization of nanocrystalline SnO2 films with a tris(2,2'-bipyridine) ruthenium(II)-fullerene dyad

Abstract A photoelectrochemical study of a tris(2,2′‐bipyridine)ruthenium(II)‐C60 donor–acceptor dyad adsorbed on nanocrystalline semiconductor SnO2 electrodes has been carried out. The results show that the incident photon‐to‐current conversion efficiency of dyad‐based photoelectrochemical cells is ∼10%.

CdSe-SnO2 coupled semiconductor thin films: Electrochemical and photoelectrochemical studies

The coupled OTE/SnO2/CdSe electrodes have been prepared by sequential deposition of SnO2 and CdSe colloidal films onto an optically transparent electrode (OTE), and their electrochemical and photoelectrochemical properties have been studied. It is observed that coupling SnO2 film with CdSe particles has extended its photoresponse into the visible. The incident photon-to-photocurrent conversion efficiency (IPCE) for CdSe film which is doubled when coupled with SnO2 film, shows a better charge separation in the coupled film. The cyclic voltammetric results of [Fe(CN)6]3−/4− with OTE, OTE/CdSe and OTE/SnO2/CdSe electrodes suggest that the coupling of CdSe with SnO2 creates an energy barrier that hinders electron flow from SnO2 to CdSe and into the electrolyte. Photocurrent stability of the cell employing OTE/SnO2/CdSe electrode has been improved by increasing the thickness of SnO2 particulate film. The photoelectrochemical performance of the cell has been evaluated and a net conversion efficiency of 2.25% has been obtained

Capacitance measurements of photovoltaic cells based on mixed monolayers of chlorophyll a and sulfoquinovosyldiacylglycerol

Abstract The capacitance measurements of Al/Chlorophyll a/Ag and Al/Chlorophyll a-Sulfoquinovosyldiacylglycerol/Ag sandwich cells have been carried out at different frequencies in dark and under illumination. The results show that while a voltage-dependent capacitance and a linear 1 C 2 versus V a plot is obtained for chlorophyll a (Chl a) at low frequencies, the addition of sulfoquinovosyldiacylglycerol (SQDG) in proportions ∼ 0.25 results in totally voltage-invariant capacitance. The capacitance characteristic of Chi a-SQDG (0.25) cells resemble more those for an insulator than those for a Schottky barrier. These results are explained in terms of negatively charged SQDG playing a role of traps which immobilize the holes. Alternatively, the interaction of negative polar head group of SQDG with Mg 2+ of Chl a can also be invoked as a possible reason for the observed results. The bound Chl a- SQDG species possibly does not form a Schottky barrier or forms a weak barrier with either Al or Ag electrodes, resulting in insulator-like capacitance characteristics of Al/Chl a-SQDG(0.25)/Ag cells.