Nanocrystalline TiO2 Film Electrode Research Articles

Tropical marine Chlorella sp. PP1 as a source of photosynthetic pigments for dye-sensitized solar cells

To understand the potential for cultivating tropical marine Chlorella sp. PP1 to produce photosynthetic pigments, particularly β-carotene (β-Car), we investigated the growth characteristics and pigment properties of Chlorella sp. PP1 under three different irradiances and properties of solar cells sensitized by β-Car. Chlorella sp. PP1 cells that were cultured under fluorescent lamp irradiance (12:12-h photoperiods) showed better cell development and density. However, photoinhibition of cell growth was observed in the cells under both continuous fluorescent lamp irradiance and direct natural sunlight exposure. Photosynthetic pigments of Chlorella sp. PP1 were identified as chlorophylls and carotenoids, and carotenoids were produced more dominantly under light stress conditions. The solar cells that only used β-Car adsorbed by a nanocrystalline TiO2 film electrode exhibited photovoltaic effects, indicating that β-Car was capable of performing as an antenna of the solar cells.

Some Features of Dye-sensitized Solar Cell Combining with Single-walled Carbon Nanotubes

A dye-sensitized solar cell (DSSC) was fabricated with a nanocrystalline TiO2 film electrode on FTO glass, N719 dye, electrolytes (or CsSnI3), and counter Pt electrode by incorporating it with single-walled carbon nanotubes (SWNTs). SWNTs were combined with TiO2 film, CsSnI3, Pt electrode, separately, and the SWNTcontaining cell was compared with a pristine cell in cell performance. We also examined the performance change by pressing TiO2 film, during cell fabrication, inside a high pressure chamber. Mostly, the change of conversion efficiency was compared for each cell, and an atomic force microscopy data were suggested to explain our results.

StSt/TiO2 compact layer/TiO2 triple-layered conducting substrates for large active area dye-sensitized solar cells

Efficient flexible dye-sensitized solar cells (DSSCs) using a stainless steel (StSt) substrate for preparing nanocrystalline TiO2 film electrodes were developed. Large active area (>1cm2) and SiOx/ITO free StSt DSSCs were fabricated for a systematic investigation. As cell area increased from 1cm2 (device A) to 4cm2 (device D) the power conversion efficiency dropped significantly, mostly due to a decrease in the fill factor (FF) and suppression of the short-circuit current density (Jsc). By integrating small-area cells onto StSt substrate to fabricate the large-area DSSC (device C), Rsheet-TCO (sheet resistance of the TCO) can be reduced significantly, yielding the improved performance of the DSSCs. Compared with other methods to prepare StSt-based photoelectrodes, such as sputtering, we provide a simple, low cost and suitable method for the large scale preparation of DSSCs.

Formation of efficient dye-sensitized solar cells by introducing an interfacial layer of hierarchically ordered macro-mesoporous TiO2 film

Hierarchically ordered macro-mesoporous TiO2 films (Ti-Ma-Me) were fabricated on fluorine-doped tin oxide (FTO) substrates through the confinement self-assembly method. The prepared Ti-Ma-Me possesses periodically ordered structure and a large specific surface area, which was applied as an interfacial layer between the nanocrystalline TiO2 film (P25-TiO2) and FTO electrode in the dye-sensitized solar cell (DSSC). The introduction of a Ti-Ma-Me interfacial layer increased the short- circuit current density ( J sc) from 7.49 to 10.65 mA/cm2 and the open-circuit voltage ( V oc) from 0.65 to 0.70 V as the result of its improved light harvesting efficiency by allowing for the high roughness factor and enhanced multiple internal reflection or scattering as well as reducing the back-transport reaction by blocking direct contact between the electrolyte and FTO electrode. Therefore, the photovoltaic conversion efficiency ( η ) was improved by 83% from 3.04% to 5.55%, as compared to a device using a bare P25 TiO2 photoanode.

Study on the Feasibility of Bacteriorhodopsin as Bio-Photosensitizer in Excitonic Solar Cell: A First Report

Bacteriorhodopsin (bR) is a membrane protein found in the archae Halobacterium salinarum. Here, we studied wild type bR and especially the triple mutant bR, 3Glu [E9Q/E194Q/E204Q], in combination with wide gap semiconductor TiO2 for their suitability as efficient light harvester in solar cell. Our differential scanning calorimetry data show thermal robustness of bR wild type and 3Glu mutant, which make them good candidates as photosensitizer in solar cells. Molecular modeling indicates that binding of bR to the exposed oxygen atoms of anatase TiO2 is favorable for electron transfer and directed by local, small distance interactions. A solar cell, based on bR wild type and bR triple mutant immobilized on nanocrystalline TiO2 film was successfully constructed. The photocurrent density-photo voltage (J-V) characteristics of bio-sensitized solar cell (BSSC), based on the wild type bR and 3Glu mutant adsorbed on nanocrystalline TiO2 film electrode were measured. The results show that the 3Glu mutant displays better photoelectric performance compared to the wild type bR, giving a short-circuit photocurrent density (J(sc)) of 0.09 mA/cm2 and the open-circuit photovoltage (V(oc)) 0.35 V, under an illumination intensity of 40 mW/cm2.

Photoenergy Conversion System Based on the Photosynthesis Dyes Conjugated Nanoparticle

The solar energy conversion system based on photosynthesis is promising as alternative energy invention technology of the future. In this review, the photoenergy conversion system based on the photosensitization of major photosynthesis dye pigment molecule, chlorophyll immobilized onto titanium dioxide or assembled into surfactant micellar system are introduced. One is the photoinduced hydrogen production system with the system containing an electron donor reagent, an electron carrier reagent, hydrogen producing catalyst and chlorophyll molecule assembled in surfactant micellar system. The other one is the photovoltaic conversion system based on the chlorophyll or chlorophyll derivative molecules immobilized onto nanocrystalline TiO2 film electrode.

The effect of potential on the electron-trapping process of surface states in nanocrystalline TiO2 film electrode

The effect of electrode potential (U) on the electron-trapping process of surface states in nanocrystalline TiO2 film electrode was investigated by current-potential (I-U) curves and transient photocurrent spectra. A simple model, the transfer property of photogenerated carriers in alkaline electrolyte varied with U, was built up. As a result, two kinds of surface states, deep surface state OL− (Sc) formed by nonprotonic lattice oxygen ion and shallow surface state Ti–O–̈O–̈O− (So) formed by chemisorbed O2, were formed on the hydroxylated surface of TiO2 particles, which could efficiently trap the photogenerated electrons. The variation of U could remarkably change the relative position between Fermi level of TiO2 (EFn) and surface state level (ES), consequently affecting the electron-trapping probability of surface states. As U>0.4V, the more negative EFn could significantly suppress the electron-trapping process of Sc and So, improving the intensity and stability of anodic photocurrent. When U was decreased, the EFn shifted to positive direction, which resulted in the facilitative trapping process of Sc and the decrease in anodic photocurrent. As U⩽−0.4V, the EFn was more positive than the energy level of So (ESo). Therefore, So could easily trap the photogenerated electrons, and the produced cathodic current resulted in the further decrease in anodic photocurrent.

Preparation of transparent TiO2 nanocrystalline film for UV sensor

The nanocrystalline TiO2 film electrodes were prepared by sol-gel method at different calcining temperatures, which had characteristics of different film thickness, uniform transparency, as well as high photoelectric and mechanical stability. Photoelectric measurements show that calcining temperature and film thickness could remarkably influence the photoelectric properties of the electrodes. The film calcined at 450°C is anatase phase with high crystallinity and strong photoelectric activity, and shows the largest photocurrent. When the temperature is lower than 450°C, the film has weaker crystallinity because of a large number of defects in the film, and this is not favorable for the transport of the photogenerated carriers. And at a temperature higher than 450°C, the photocurrent of the electrode is decreased due to anatase-rutile phase transition in the film. The increase in film thickness is favorable to the enhancement of ultraviolet light (UV) absorption amount, which would improve the photoelectric activity of the film. But, excessive thickness will increase the recombination rate of the electron-hole pairs, and result in a reduction in electrode’s photoelectric activity. In addition, the response sensitivity and stability of the photocurrent produced in the electrode are related to bias potential. At a potential of 0.4 V, the electrode shows a saturated photocurrent of 30.8 μA and a response time of ∼1 s, suggesting that the prepared TiO2 film electrode can be used for making UV sensors.

Molecular and electronic ground and excited structures of heteroleptic ruthenium polypyridyl dyes for nanocrystalline TiO2 solar cells

Heteroleptic ruthenium complexes of the type cis-[Ru(H2dcbpy)(L)(NCS)2], where H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine and L = 1,10-phenanthroline (phen) (1) or dipyrido[3,2-a∶2′,3′-c]-phenazine (dppz) (2), were synthesized and their photochemical properties were investigated. The complexes showed a broad and intense metal-to-ligand charge transfer (MLCT) transition band in the visible region. The complexes were anchored to nanocrystalline TiO2 film electrodes, and the photovoltaic properties of the resulting dye-sensitized solar cells were characterized and compared with the properties of cells prepared with cis-(NBu4)2[Ru(Hdcbpy)2(NCS)2] (N719). The efficiency of the 2-sensitized solar cell was 20% lower than that of the 1-sensitized solar cell, but neither was as efficient as the N719-sensitized solar cell. The electronic structures of the complexes were investigated by means of a time-dependent density functional theory method in an effort to better understand their effectiveness in TiO2-based photoelectrochemical cells. The calculation results indicated that the character of the MLCT transitions in the long wavelength region differed between 1 and2, although their energy levels are nearly the same in the protonated forms. It was suggested that the performance of the 2-sensitized solar cell could be improved by the introduction of electron-donating groups on the dppz ligand of 2.

A 4.2% efficient flexible dye-sensitized TiO2 solar cells using stainless steel substrate

An efficient flexible dye-sensitized solar cells (DSSCs) using stainless steel supporting substrate for fabricating nanocrystalline TiO2 film electrodes were developed, intending to improve the photoelectrochemical properties of plastic substrate-based DSSCs. The most important advantage of a stainless steel-based TiO2 film electrode over a plastic-based electrode lies in its high-temperature sinterability. Optimal photovoltaic properties were obtained with a cell where the TiO2 film was coated on both ITO- and SiOx-sputtered stainless steel (denoted as TiO2/ITO/SiOx/StSt). The photocurrent of the flexible cells with a TiO2/ITO/SiOx/StSt electrode increased significantly, leading to a much higher overall solar conversion efficiency η of 4.2% at 100 mW/cm2, based on short-circuit photocurrent density, open-circuit voltage and fill factor of 11.2 mA/cm2, 0.61 and 0.61 V, respectively, than those reported for cells with plastic substrates.

Near-IR Light-Sensitized Voltaic Conversion System Using Nanocrystalline TiO2 Film by Zn Chlorophyll Derivative Aggregate

A Zn chlorophyll-a derivative, Zn chlorin-e6 (ZnChl-e6), adsorbed onto a nanocrystalline TiO2 film (ZnChl-e6/TiO2) electrode was prepared, and the photovoltaic properties of the ZnChl-e6/TiO2 electrode were studied. The absorption peaks of ZnChl-e6/TiO2 observed at 420, 654, and 795 nm were attributed to the ZnChl-e6 molecules aggregating onto TiO2 film. The fluorescence attributed to the ZnChl-e6 monomer and aggregate was observed at 710 and 820 nm, respectively, and the fluorescence in both cases was quenched by TiO2 particles. The maximum of the incident photon-to-current conversion efficiency (IPCE) value in the photocurrent action spectrum was 800 nm, and the IPCE value was 7.0%. ZnChl-e6 molecules formed aggregates on a nanocrystalline TiO2 film electrode. From the photocurrent-photovoltage characteristics of the ZnChl-e6/TiO2 electrode irradiated with 100 mW cm(-2), the short-circuit photocurrent (I(SC)) was found to be 0.19 mA cm(-2) and the open-circuit photovoltage (V(OC)) was found to be 375 mV. The maximum power was estimated to be 28.7 microW cm(-2), and the fill factor (FF) was estimated to be 40.1%. A near-IR light induced photovoltaic conversion system using a ZnChl-e6 aggregate formed onto a nanocrystalline TiO2 film electrode was achieved.

Photo-operated Glucose-O<sub>2</sub> Biofuel Cell Based on the Visible-light Photosensitization of Chlorophyll Derivatives Adsorbed on Nanocrystalline TiO<sub>2</sub> Film

A novel type of photo-operated biofuel cell was based on the combination of NAD+ reduction with glucose and GDH, with photosensitization of zinc chlorin-e6 (ZnChl-e6) as a model of chlorophyll a on a nanocrystalline TiO2 film electrode as the anode, and electrochemical reduction of oxygen to the water on a platinum electrode as the cathode. The short-circuit photocurrent (ISC) and the open-circuit photovoltage (VOC) were 6.8 μA·cm−2 and 444 mV, respectively. The peaks in the photocurrent action spectrum of this cell were observed at 400 and 780 nm, and the IPCE values at 400 and 780 nm were estimated to be approximately 11.0 and 6.2%. This novel type of photo-operated glucose-O2 biofuel cell depends on the visible and near IR photosensitization of ZnChl-e6 molecules on nanocrystalline TiO2 film electrode.

Photovoltaic Cell Based on the Near-IR Sensitization of Zn Chlorin-e<sub>6</sub> Adsorbed on a Nanocrystalline TiO<sub>2</sub> Film Electrode

The photovoltaic cell using visible and near-infrared sensitization of nanocrystalline TiO2 films by chlorophyll derivative, zinc chlorin-e6 (ZnChl-e6), is developed. The short-circuit photocurrent (I SC) is 0.27 mA·cm−2, the open-circuit photovoltage (V OC) is 422 mV, and the fill factor (FF) of device using ZnChl-e6 adsorbed on nanocrystalline TiO2 film electrode is estimated to be 41.0%, respectively.

Effect of Phosphonate-Functionalized Surface Modification on Nanocrystalline TiO2Film Electrode

Since the pioneering works of Gratzel’s group for dyesensitized solar cells (DSSC), nanocrystalline semiconductor film electrodes have been intensively investigated. In DSSC, the interfacial charge transfer between the surface of the film and the solution electrolyte is very important. Especially, the effect of surface complexation on the rate of interfacial electron transfer reaction is known to be crucial. In the Gratzel’s DSSC, the light-absorbing ruthenium complexes were adsorbed on the surface of TiO2 through the carboxylate chelation. The strong electronic coupling between π orbital of the ligand and 3d orbital of the conduction band of TiO2 rendered the very rapid electron transfer from the metal-to-ligand charge transfer state to the conduction band feasible and the rate of recombination between the conduction band and Ru(III) was retarded by the high driving force in the Marcus inverted region and the spacer ligand. In the result, about 80% incident photon-to-current conversion efficiency in a certain wavelength region was achieved in those cells. Recent studies showed that a phosphonate group instead of a carboxylate group was also strongly adsorbed on the surface of nanocrystalline TiO2 film and an efficient electron transfer between them occurred. Using the metal bisphosphonate multiplayer technique, we have also demonstrated that the redox couple multiplayers of bisphosphonate derivatives were constructed on the surface of a nanocrystalline TiO2 film. Whereas a carboxylate compound was sensitive to pH of an electrolyte solution and was desorbed from the surface of TiO2 at the pH higher than 4, a phosphonate compound was less sensitive to the solution pH and remains on the surface until pH 9. To elucidate the surface sites and the surface complexation effect of nanocrystalline TiO2 (NT) film electrode, here we briefly report the effect of surface chelation of two different types of phosphonate-functionalized compounds on the interfacial electron transfer of NT film electrode for water oxidation: one is electron-donating and the other is electron-accepting. VP and DBPA were both adsorbed directly on the surface of NT electrode by a simple immersion in aqueous and ethanolic solutions, respectively. After the immersion for an hour, the electrodes were thoroughly washed with copious amounts of water and ethanol. The adsorption of DBPA on the surface of TiO2 film could be confirmed from CHstretching mode (at 2919 and 2851 cm−1) in the IR spectra of DBPA-treated TiO2 that is not shown here, although the amount of the adsorbed DBPA could not be estimated quantitatively. The cyclic voltammograms of NT electrodes prepared here are shown in Figure 1. For the bare NT electrode, the cathodic current appeared at potentials negative than flat band potential (Efb) and increased as the applied potential increased. Since there was no appreciable faraday component except solvent water, the current can be assigned to the capacitive current that can be due to the complex capacitance such as Helmholtz layer, double layer and space charge layer capacitance, if space charge layer exists, etc.

Dye-Sensitized Solar Cell Using a TiO2 Nanocrystalline Film Electrode Modified by an Aluminum Phthalocyanine and Myristic Acid Coadsorption Layer

A dye-sensitized solar cell using a nanocrystalline TiO2 film electrode modified by aluminum phthalocyanine, aluminum 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine hydroxide (AlTPPc), and a myristic acid adsorbed layer was developed. The UV−vis absorption spectrum of the TiO2 film electrode modified by an AlTPPc and myristic acid adsorbed layer shows that the aggregation of AlTPPc molecules was suppressed by myristic acid. For a cell using AlTPPc adsorbed on a nanocrystalline TiO2 film electrode, ISC, VOC, and FF were 0.026 mA cm-2, 186 mV, and 40.4%, respectively. The maximum power output was estimated to be 2.25 μW cm-2. In contrast, ISC, VOC, and FF values were 0.061 mA cm-2, 225 mV, and 40.0%, respectively, and the maximum power output was estimated to be 5.40 μW cm-2 using a TiO2 film electrode modified by an AlTPPc and myristic acid layer. The ISC and VOC values increased with the concentration ratio of myristic acid to AlTPPc until 0.1 and then decreased, and the IPCE value at 700 nm of the solar c...

Observation of Cathodic Photocurrents at Nanocrystalline TiO2 Film Electrodes, Caused by Enhanced Oxygen Reduction in Alkaline Solutions

Nanocrystalline TiO2 (rutile and anatase) film electrodes usually show anodic photocurrents in aqueous electrolytes. Detailed photoelectrochemical studies have revealed that they also show cathodic photocurrents under particular conditions, e.g., in alkaline solutions containing dissolved oxygen under illumination from the TiO2-film side at short wavelengths such as 300 nm. The cathodic photocurrents appear in a potential region about 0.5∼0.9 V more positive than the flat-band potential (Ufb) of single-crystal n-TiO2 electrodes. It is shown that the appearance of the cathodic photocurrents can be attributed to efficient electron transfer from the conduction band of TiO2 particles to chemically adsorbed oxygen molecules, the density of which is largely increased in alkaline solutions through a charge-transfer interaction between surface anionic groups such as Ti−O- as an electron donor and oxygen molecules as an electron acceptor. The dependences of the cathodic and anodic photocurrents on the illumination wavelength, the illumination direction, the electrode potential, and the crystal form of TiO2 particles are discussed in relation with the photocatalytic activity of particulate TiO2 films.

Sub-picosecond Injection of Electrons from Excited [Ru(2,2′-bipy-4,4′-dicarboxy)2(SCN)2] into TiO2 Using Transient Mid-Infrared Spectroscopy*

We have used femtosecond pump-probe spectroscopy to time resolve the injection of electrons into nanocrystalline TiO2 film electrodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4'-dicarboxy-2,2'-bipyridine)2(NCS)2] (N3). Pumping at one of the metal-to-ligand charge transfer adsorption peaks and probing the absorption of electrons injected into the TiO2 conduction band at 1.52 {micro}m and in the range of 4.1 to 7.0 {micro}m, we have directly observed the arrival of the injected electrons. Our measurements indicate an instrument-limited {approx}50-fs upper limit on the electron injection time under ambient conditions in air. We have compared the infrared transient absorption for noninjecting (blank) systems consisting of N3 in ethanol and N3 adsorbed to films of nanocrystalline Al2O3 and ZrO2, and found no indication of electron injection at probe wavelengths in the mid-IR (4.1 to 7.0 {micro}m). At 1.52 {micro}m interferences exist in the observed transient adsorption signal for the blanks.

High Photocurrent Quantum Yields in Short Wavelengths for Nanocrystalline Anatase-Type TiO2 Film Electrodes Compared with Those for Rutile-Type

Abstract Photoelectrochemical activity of nanocrystalline TiO2 film electrodes prepared by use of various TiO2 particles has been studied comparatively. It is confirmed that the photocurrent (or photoactivity) for anatase-type TiO2 is in general higher than that for the rutile-type. The photocurrent quantum yield (ηpc) for anatase-type TiO2 is nearly equal to that for rutile-type in long wavelengths but increases more and more with decreasing wavelength in contrast to the ηpc for rutile-type, and thus the ηpc for anatase-type becomes much higher than the ηpc for rutile-type in short wavelengths around 300 nm, indicating that the high photoactivity for anatase-type TiO2 is due to this high ηpc in short wavelengths. There are some discussions on the reasons for the high ηpc for anatase-type TiO2 in short wavelengths, including the possibility of contribution of hot electrons or holes in the TiO2 film.

Dynamics of Electron Injection in Nanocrystalline Titanium Dioxide Films Sensitized with [Ru(4,4‘-dicarboxy-2,2‘-bipyridine)2(NCS)2] by Infrared Transient Absorption

We have used femtosecond pump−probe spectroscopy to time resolve the injection of electrons into nanocrystalline TiO2 film electrodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4‘-dicarboxy-2,2‘-bipyridine)2(NCS)2] (N3). Pumping at one of the metal-to-ligand charge-transfer absorption peaks and probing the absorption by injected electrons in the TiO2 at 1.52 μm and in the range of 4.1−7.0 μm, we have directly observed the arrival of electrons injected into the TiO2 film. Our measurements indicate an instrument-limited ∼50 fs upper limit on the electron injection time. We have compared the infrared transient absorption for noninjecting systems consisting of N3 in ethanol and N3 adsorbed to films of nanocrystalline Al2O3 and ZrO2 and found no indication of electron injection at probe wavelengths in the mid-IR (4.1−7.0 μm).

Photoelectrical performance of nanocrystalline TiO2 film electrode sensitized by Ru(II)-bipyridine complexes

Two kinds of Ru(II)-bipyridine complexes,cis-di(thiocyanate)bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium(II) andcis-di(thiocyanate)bis(2, 2′-bipyridyl-3, 3′-dicarboxylate) ruthenium(II), were utilized as the sensitizers to the nanocrystalline TiO2 film electrodes. Study shows that the two dyes have quite different sensitization properties due to the strong steric effect of carboxyl groups. In addition, the pretreatment to nanocrystalline TiO2 film electrodes with TiCl4 was investigated, which is an effective way to improve the photoelectric conversion performances of sensitized TiO2 electrodes.