The effect of compression on electron transport and recombination in plastic TiO2 photoanodes

The compression method was applied for the preparation of plastic TiO₂ porous films on a conductive indium–tin oxide (ITO)-coated polyethylene naphthalate (PEN) substrate at low temperature for the generation of high-efficiency plastic dye-sensitized solar cells (DSCs). The compression parameters, including pressure and time, were varied in order to determine their effect on the photovoltaic performance of the plastic DSCs. The results from electrochemical impedance spectroscopy (EIS) showed that charge transport resistance in the porous TiO₂ films (R_t) gradually decreased when the applied pressure was increased from 0 MPa to 150 MPa, which indicated a better connection between the TiO₂ nanoparticles and electron transport in the TiO₂ films. In addition, a longer press time led to an increased resistance of electron recombination (R_ct) and an increased charge-collection efficiency. After optimization of the compression parameters, the efficiency of energy conversion was increased by approximately 81.6%. In addition, the efficiency of energy conversion was increased by an additional 4.65% under AM1.5 illumination.     

Influence of nitrogen dopants on N-doped TiO2 electrodes and their applications in dye-sensitized solar cells

Three different types of nanocrystalline, N-doped TiO₂ electrodes were synthesized using several nitrogen dopants through wet methods. The obtained nanocrystalline, N-doped TiO₂ electrodes possessed different crystallite sizes, surface areas, and N-doping amounts. Characterizations were performed to reveal the nitrogen-doping processes for the wet methods using ammonia, urea, and triethylamine as the nitrogen dopants. Additionally, a high conversion efficiency of 8.32% was achieved by the dye-sensitized solar cells, based on the N-doped TiO₂ electrodes. For instance, in comparison with the commercial P25 (5.76%) and pure anatase TiO₂ electrodes (7.14%), significant improvements (44% and 17%, respectively) in the efficiencies were obtained. The findings also indicated that the ammonia nitrogen dopant was more efficient than other two nitrogen dopants. The electron transports, electron lifetimes, and charge recombination in the dye-sensitized N-doped TiO₂ solar cells also differed from those in the pure TiO₂-based dye-sensitized solar cells (DSCs). Specifically, an enhanced photocurrent of ca. 36% in N-doped DSCs resulted from the synergistic effects of the high dye uptake and the efficient electron transport. Moreover, the relationship between charge and voltage revealed that less charge was needed to get a high open-circuit voltage in the N-doping films.     

Highly active TiO2−x−yNxFy visible photocatalyst prepared under supercritical conditions in NH4F/EtOH fluid

A novel N and F co-doped TiO₂ (TiO_2−x−yN_xF_y) photocatalyst is prepared by treating the TiO₂ precursor in NH₄F/ethanol fluid under supercritical conditions. During photocatalytic degradation of methylene blue under visible light irradiation, the as-prepared TiO_2−x−yN_xF_y exhibits higher activity than the undoped TiO₂, N-doped TiO₂ (TiO_2−xN_x), and F-doped TiO₂ (TiO_2−yF_y). Based on the characterizations including XRD, Raman, FTIR, TEM, PLS, UV–vis DRS, N₂ adsorption–desorption isotherms, XPS and NH₃-TPD, the synergetic promotions of N- and F-dopants incorporated into the TiO₂ lattice are discussed based on the enhanced spectral response in visible region, oxygen vacancies, and surface acidic sites. Meanwhile, the supercritical treatment also promotes the activity owing to the increase in both the surface area and the crystallization degree of anatase, and the enhanced incorporation of N- and F-dopants into the TiO₂ lattice.     

Preparation and characterization of a novel activated carbon‐supported N‐doped visible light response photocatalyst (TiO2−xNy/AC)

A photocatalyst, TiO_2?xN_y/AC (activated carbon (AC) supported N‐doped TiO₂), highly active in both the Vis and UV range, was prepared by calcination of the TiO₂ precursor prepared by acid‐catalyzed hydrolysis in an ammonia atmosphere. The powders were characterized by diffuse reflectance spectroscopy, scanning electron microscopy, X‐ray diffraction, N₂ adsorption, Fourier transform infrared spectroscopy and phenol degradation. The doped N in the TiO₂ crystal lattice creates an electron‐occupied intra‐band gap allowing electron‐hole pair generation under Vis irradiation (500–560 nm). The TiO_2?xN_y/AC exhibited high levels of activity and the same activity trends for phenol degradation under both Vis and UV irradiation: TiO_2?xN_y/AC calcined at 500 °C for 4 h exhibited the highest activity. The band‐gap level newly formed by doped N can act as a center for the photo‐generated holes and is beneficial for the UV activity enhancement. The performance of the prepared TiO_2?xN_y/AC photocatalyst revealed its practical potential in the field of solar photocatalytic degradation of aqueous contaminants. Copyright © 2007 Society of Chemical Industry     

A novel diphenylphosphinic acid coadsorbent for dye-sensitized solar cell

Diphenylphosphinic acid (DPPA) was adopted as a novel coadsorbent in dye-sensitized solar cells (DSCs) based on nanocrystalline TiO₂ sensitized with N719 dye [(Bu₄N)₂[Ru(dcbpyH)₂(NCS)₂]], leading to a significant enhancement of the cell's performance. Different ratios of dye-to-coadsorbent caused varying results, including a 12.5% increase in overall conversion efficiency coupled with a 10.6% increase in short-circuit current with a ratio of 2:1. Electrochemical impedance spectroscopy (EIS) results indicated that the augment ascribes to inhibited interfacial charge recombination between the conduction band electrons and triiodide ions in the electrolyte. On the other hand, different ratios caused different shift directions of the TiO₂ conduction band, which was confirmed by charge transport resistance obtained also from EIS analysis. To be specific, when the ratio of N719-to-DPPA was 2:1 and 1:1, the conduction band of TiO₂ film was positively shifted, while it was negatively shifted when the ratio was 4:1.     

Enhanced power conversion efficiency in dye-sensitized solar cells with TiO2 aggregates/nanocrystallites mixed photoelectrodes

Photoelectrodes of mixed microsized TiO₂ aggregates and individually dispersed TiO₂ nanocrystallites with different ratios were fabricated and studied for improved power conversion efficiency in dye-sensitized solar cells (DSCs). TiO₂ aggregates/nanocrystallites composites possess several advantages for high performance of DSCs, including the light scattering by the microsized TiO₂ aggregates and the high surface area of nanocrystallites both in aggregates and individually dispersed. A high power conversion efficiency of 7.59% was achieved with mixed TiO₂ aggregates/nanocrystallites photoelectrode using conventional dye N3, without applying anti-reflection coating, back-scattering layer, or chemical treatment. The electron transport properties of DSCs with mixed photoelectrodes were investigated by electrochemical impedance spectra, and the results showed that such a photoelectrode with mixed aggregates and nanocrystallites possess better connectivity for efficient electron transport.     

TiO2 micro-flowers composed of nanotubes and their application to dye-sensitized solar cells

TiO₂ micro-flowers were made to bloom on Ti foil by the anodic oxidation of Ti-protruding dots with a cylindrical shape. Arrays of the Ti-protruding dots were prepared by photolithography, which consisted of coating the photoresists, attaching a patterned mask, illuminating with UV light, etching the Ti surface by reactive ion etching (RIE), and stripping the photoresist on the Ti foil. The procedure for the blooming of the TiO₂ micro-flowers was analyzed by field emission scanning electron microscopy (FESEM) as the anodizing time was increased. Photoelectrodes of dye-sensitized solar cells (DSCs) were fabricated using TiO₂ micro-flowers. Bare TiO₂ nanotube arrays were used for reference samples. The short-circuit current (J_sc) and the power conversion efficiency of the DSCs based on the TiO₂ micro-flowers were 4.340 mA/cm² and 1.517%, respectively. These values of DSCs based on TiO₂ micro-flowers were higher than those of bare samples. The TiO₂ micro-flowers had a larger surface area for dye adsorption compared to bare TiO₂ nanotube arrays, resulting in improved J_sc characteristics. The structure of the TiO₂ micro-flowers allowed it to adsorb dyes very effectively, also demonstrating the potential to achieve higher power conversion efficiency levels for DSCs compared to a bare TiO₂ nanotube array structure and the conventional TiO₂ nanoparticle structure.     

Gas phase hydrogenation of maleic anhydride to tetrahydrofuran by Cu/ZnO/TiO2 catalysts in the presence of n-butanol

Cu/ZnO/TiO₂ catalysts were prepared via the coprecipitation method. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectrometry, temperature programmed reduction, and N₂ adsorption. The catalytic activity of Cu/ZnO/TiO₂ catalyst in gas phase hydrogenation of maleic anhydride in the presence of n-butanol was studied at 235–280 °C and 1 MPa. The conversion of maleic anhydride was more than 95.7% and the selectivity of tetrahydrofuran was up to 92.7%. At the same time, n-butanol was converted to butyraldehyde and butyl butyrate via reactions, namely, dehydrogenation, disproportionation, and esterification. There were two kinds of CuO species present in the calcined Cu/ZnO/TiO₂ catalysts. At a lower copper content, the CuO species strongly interacted with ZnO and TiO₂; at a higher copper content, both the surface-anchored and bulk CuO species were present. The metallic copper (CuO) produced by the reduction of the surface-anchored CuO species favored the deep hydrogenation of maleic anhydride to tetrahydrofuran. The deep hydrogenation activity of Cu/ZnO/TiO₂ catalyst increased with the decrease of crystallite sizes of CuO and the increase of microstrain values. Compensations of reaction heat and H₂ in the coupling reaction of maleic anhydride hydrogenation and n-butanol dehydrogenation were distinct.     

Effects of electrolyte in dye-sensitized solar cells and evaluation by impedance spectroscopy

Effects of the electrolyte of DSCs on impedance spectra were evaluated by changing concentration of redox couple, viscosity, and additives to electrolyte. The relation with current-voltage characteristics (I-V characteristics) was investigated. In many cases, the impedance component attributed to charge transfer at TiO₂|electrolyte interface demonstrated strong relation with the I-V characteristics. The recombination of electrons in TiO₂ with I₃⁻ in electrolyte was a key factor in determining performance of DSCs. To evaluate the effect of I₃⁻, diffusion-limiting current in the electrolyte for various viscosities was evaluated by cyclic voltammetry. When the short circuit current (SCC) was almost equal to the diffusion-limiting current, strong influence of the diffusion coefficient on the impedance spectra was observed: impedance arcs were enlarged as the diffusion coefficient was decreased. On the other hand, when the diffusion-limiting current was larger than the SCC, photo-excitation and electron injection processes became dominating factors in the DSCs performance. The SCC was regulated by the charge recombination process at TiO₂|electrolyte interface, and thus the impedance component ω₃ was related to the performance in such condition.     

The improvement of electron transport rate of TiO2 dye-sensitized solar cells using mixed nanostructures with different phase compositions

Dye-sensitized solar cells (DSCCs) in the form of mixed nanostructures containing TiO₂ nanoparticles and nanowires with different weight ratios and phase compositions are reported. X-ray diffraction and field emission scanning electron microscopy analyses revealed that the synthesized TiO₂ nanoparticles had average crystallite size in the range 21–39 nm, whereas TiO₂ nanowires showed diameter in the range 20–50 nm. The indirect optical band gap energy of TiO₂ nanowires, anatase- and rutile-TiO₂ nanoparticles was calculated to be 3.35, 3.28 and 3.17 eV, respectively. The power conversion efficiency of the solar cells changed with nanowire to nanoparticle weight ratio, reaching a maximum at a specific value. An increase of 4.3% in cell efficiency was achieved by introducing 10 wt% nanowire into the as-synthesized TiO₂ nanoparticles (WP1 cell). Furthermore, an increase of 27.6% in cell efficiency was achieved by using crystalline anatase-TiO₂ nanoparticles rather than as-synthesized TiO₂ nanoparticles in WP1 solar cell. It was found that the power conversion efficiency and short circuit current of WP1 cell were decreased down to around 30.8% and 39.1%, respectively using rutile nanoparticles rather than anatase nanoparticles. The improvement of cell efficiency was related to rapid electron transport and less recombination of photogenerated electrons, as confirmed by electrochemical impedance spectroscopy.     

Molecular structures and vibrational frequencies for cis-[PdCl2(tmen)] and cis-[Pd(N3)2(tmen)]: A DFT study

Theoretical molecular structures of the complexes cis-[PdCl₂(tmen)] and cis-[Pd(N₃)₂(tmen)] (tmen = N,N,N′,N′-tetramethylethylenediamine) were investigated using B3LYP/DFT method. The calculated molecular parameters, bond distances and angles, revealed a square-planar geometry around the metallic center for both compounds with the azide being linear. The theoretical infrared spectra of C₁ symmetry (electronic state ¹A) of the compounds are in agreement with the experimental data.     

TiO2 photocatalyst loaded on hydrophobic Si3N4 support for efficient degradation of organics diluted in water

TiO₂ photocatalyst loaded on Si₃N₄ (TiO₂/Si₃N₄) was prepared by a conventional impregnation method and its photocatalytic performance for the degradation of organics (2-propanol) diluted in water was compared with that of TiO₂ photocatalysts (TiO₂/SiO₂, TiO₂/Al₂O₃, and TiO₂/SiC) loaded on various types of supports (SiO₂, Al₂O₃, and SiC). The formation of the well-crystallized anatase phase of TiO₂ was observed on the calcined TiO₂/Si₃N₄ photocatalyst, while a small anatase phase of TiO₂ was observed on the TiO₂/SiC photocatalyst and amorphous TiO₂ species was the main component on the TiO₂/SiO₂ and TiO₂/Al₂O₃ photocatalysts. The measurements of the water adsorption ability of photocatalysts indicated that the TiO₂/Si₃N₄ photocatalyst exhibited more hydrophobic surface properties in comparison to other support photocatalysts. Under UV-light irradiation, the TiO₂/Si₃N₄ photocatalyst decomposed 2-propanol diluted in water into acetone, CO₂, and H₂O, and finally, acetone was also decomposed into CO₂ and H₂O. The TiO₂/Si₃N₄ photocatalyst showed higher photocatalytic activity than TiO₂ photocatalyst loaded on other supports. The well-crystallized TiO₂ phase deposited on Si₃N₄ and the hydrophobic surface of Si₃N₄ support are important factors for the enhancement of photocatalytic activity for the degradation of organic compounds in liquid-phase reactions.     

Quasi solid state dye-sensitized solar cells with modified TiO2 photoelectrodes and triphenylamine-based dye

Quasi solid state dye-sensitized solar cells (DSSCs) have been fabricated with organic sol or TiCl₄ modified TiO₂ and porous TiO₂ photoanode and a triphenylamine-based dye (TPAR3) used as photosensitizer. Dark current measurements suggested that both modified TiO₂ photoelectrodes had significantly reduced the recombination rate of photoelectrons due to the reduced bare FTO surface in comparison to porous photoelectrode. The DSSC based on modified TiO₂ photoelectrodes showed improved photovoltaic parameters compared to the porous TiO₂ photoelectrode. The overall power conversion efficiency (PCE) is 3.27%, 4.73% and 6.8% for porous, TiCl₄ modified and sol modified TiO₂ photoelectrodes, respectively. The improved PCE with modified TiO₂ electrodes was attributed to the formation of a compact layer. This effectively improves adherence of TiO₂ to FTO surface, providing a larger TiO₂/FTO contact area and reducing the electron recombination by blocking the direct contact between redox electrolyte and the conductive FTO surface and enhances the electron collection efficiency.     

TiO2/CdS/CdSe quantum dots co-sensitized solar cell with the staggered-gap (type-II) heterojunctions for the enhanced photovoltaic performance

The effect of co-sensitization and ZnS passivation on the photovoltaic performance of CdS quantum dot sensitized solar cells (QDSSCs) were investigated. The deposition of CdS, CdSe quantum dots (QD) and ZnS passivation on TiO₂ photoanode was carried out by successive ionic layer adsorption and reaction (SILAR) method. CdS/CdSe co-sensitization developed two staggered type-II heterojunctions at TiO₂/CdS and CdS/CdSe interfaces and resulted a cascade energy band structure. This suitable band alignment facilitated the double charge transfer mechanism at each heterojunction and transported the electrons easily into the photoanode. The narrow bandgap sensitizers CdS and CdSe significantly improved the potential utilization of solar spectrum with more charge carrier generation. ZnS passivation on QD surface suppressed electrode/electrolyte interfacial charge recombination and facilitated more electron injection from QDs into TiO₂ photoanode. The EDAX elemental mapping results inferred that CdS, CdSe and ZnS have efficiently covered the TiO₂ surface. TiO₂/CdS and CdS/CdSe interfaces and the amorphous nature of ZnS could be verified with HRTEM images. Hence, the co-sensitization and surface passivation played a significant role to enhance the PCE of CdS QDSSC from 1.9% to 4.05%.     

Enhancement of the photoelectric performance of dye-sensitized solar cells using Ag-doped TiO<Subscript>2</Subscript> nanofibers in a TiO<Subscript>2</Subscript> film as electrode

For high solar conversion efficiency of dye-sensitized solar cells [DSSCs], TiO₂ nanofiber [TN] and Ag-doped TiO₂ nanofiber [ATN] have been extended to be included in TiO₂ films to increase the amount of dye loading for a higher short-circuit current. The ATN was used on affected DSSCs to increase the open circuit voltage. This process had enhanced the exit in dye molecules which were rapidly split into electrons, and the DSSCs with ATN stop the recombination of the electronic process. The conversion efficiency of TiO₂ photoelectrode-based DSSCs was 4.74%; it was increased to 6.13% after adding 5 wt.% ATN into TiO₂ films. The electron lifetime of DSSCs with ATN increased from 0.29 to 0.34 s and that electron recombination was reduced.     

Role of surface state on the electron flow in modified TiO2 film incorporating carbon powder for a dye-sensitized solar cell

An investigation of surface-related traps in nanostructured TiO₂ films modified by the incorporation of carbon powder was conducted by the potential-step chronoamperometric method. For the modification of the morphology and surface state of the nanoporous TiO₂ electrode, the incorporation of carbon into the white TiO₂ powder was accomplished. In the chronoamperometric data, all of the transients showed an initial fast phase (<1 s) followed by a slower phase which is related to the trap filling process. The trap-filling period of the carbon incorporated TiO₂ film becomes longer, as the applied negative potential increases, due to the widely distributed traps induced by the increased surface area. Furthermore, the film capacitance was derived as a function of the applied bias by integrating the current to time curves of the chronoamperometric data. The accumulated charge of the carbon incorporated TiO₂ film increases prominently in two regions. The dominant increase shown in the positive region (−0.7 to −0.9 V vs. Ag/AgCl at pH 13) of the flat band potential implies that the electron occupancy in the surface-related traps is increased. At a more negative potential (below −1.2 V vs. Ag/AgCl), electrons from the conduction band of the TiO₂ film substantially influence the total current, thereby inducing an exponential increase in the current. Therefore, it is found that most of the traps are located in the positive region of the flat band potential, since the Fermi level of the nanostructured TiO₂ film is positioned at −1.14 V vs. Ag/AgCl at pH 13. The trap sites in the sub-bandgap region of the TiO₂ film are important in the electron transport of photoinjected electrons from dye molecules and partially charge recombination with redox electrolyte in operating dye-sensitized solar cell. The influence of charge trap formed by increased surface states on the electron transport and electron transfer was investigated by photovoltage and photocurrent transient measurements.     

Photovoltaic efficiency on dye-sensitized solar cells (DSSC) assembled using Ga-incorporated TiO2 materials

This study examined the photoelectric conversion efficiency of DSSC (dye-sensitized solar cell) when nanometer sized Ga (0.25, 0.50, and 1.00 mol%)–TiO₂ prepared using a hydrothermal method was employed as a working electrode material. The particle sizes observed in the transmission electron microscopy images were <20 nm in all samples. However, with increasing Ga concentration, the size increased and the shapes transformed to a stick form. The absorption band was slightly blue-shifted upon the incorporation of gallium ions, but the intensity of the photoluminescence (PL) curves of the Ga-incorporated TiO₂ was significantly smaller, with the smallest case being the 0.50 mol% Ga–TiO_2, which was related to recombination between the excited electrons and holes. When Ga–TiO₂ was applied in DSSC, the energy conversion efficiency was enhanced considerably compared to that using pure TiO₂; it was approximately 4.57% with the N3 dye under 100 mW/cm² of simulated sunlight. These results are in agreement with an electrostatic force microscopy (EFM) study showing that the electrons were transferred rapidly to the surface of Ga–TiO₂ film, compared with that on a pure TiO₂ film.     

Influence of electrolyte co-additives on the performance of dye-sensitized solar cells

The presence of specific chemical additives in the redox electrolyte results in an efficient increase of the photovoltaic performance of dye-sensitized solar cells (DSCs). The most effective additives are 4-tert-butylpyridine (TBP), N-methylbenzimidazole (NMBI) and guanidinium thiocyanate (GuNCS) that are adsorbed onto the photoelectrode/electrolyte interface, thus shifting the semiconductor's conduction band edge and preventing recombination with triiodides. In a comparative work, we investigated in detail the action of TBP and NMBI additives in ionic liquid-based redox electrolytes with varying iodine concentrations, in order to extract the optimum additive/I₂ ratio for each system. Different optimum additive/I₂ ratios were determined for TBP and NMBI, despite the fact that both generally work in a similar way. Further addition of GuNCS in the optimized electrolytic media causes significant synergistic effects, the action of GuNCS being strongly influenced by the nature of the corresponding co-additive. Under the best operation conditions, power conversion efficiencies as high as 8% were obtained.     

Selective hydrogenation of o-chloronitrobenzene over palladium supported nanotubular titanium dioxide derived catalysts

Titania derived nanotubes were synthesized by treating P-25 Degussa TiO₂ with a concentrated (18 M) KOH solution. Ageing the material in KOH solution for 2 days resulted in formation of tubular titania and Raman analysis revealed that the material has a titanate structure. The synthesized material was used as a catalyst support for the hydrogenation of ortho-chloronitrobenzene (o-CNB) with Pd as the active phase. The vapour-phase hydrogenation of o-CNB was carried out in ethanol at 523 K and atmospheric pressure over a Pd/TiO₂ derived nanotube catalyst (Pd/TiO₂-M). Pd/TiO₂-M gave complete conversion (100%) of o-CNB with the selectivity to ortho-chloroaniline (o-CAN) of 86%. The stability of the Pd/TiO₂-M catalyst was tested over 5 h during which time the conversion slowly dropped to 80% (selectivity 93%) due to catalyst poisoning. TPR analysis revealed the existence of a strong palladium-support interaction and this was found to be crucial to the overall activity of the catalyst.     

Effect of Carbon Nanotubes and Carbon Nanotubes/Gold Nanoparticles Composite on the Photocatalytic Activity of TiO2 and TiO2‐SiO2

A carbon nanotube (CNT)/gold nanoparticle (NP) nanocomposite was synthesized by simultaneously reducing the Au ions and depositing Au NPs on the surface of a CNT. The functional groups were investigated with Fourier transform infrared spectra. From the Raman spectra, the D‐band and G‐band of the CNT were identified. The deposition of nanometer‐sized Au NPs on the CNT sites was observed by transmission electron microscopy. The photodegradation of methylene blue (MB) in aqueous solutions was studied using various photocatalysts, including TiO₂, TiO₂‐SiO₂, CNT/TiO₂, CNT/TiO₂‐SiO₂, Au/TiO₂, CNT‐Au/TiO₂, and CNT‐Au/TiO₂‐SiO₂ composites. CNT addition leads to a synergic effect, improving the photoactivity of the catalysts. A possible physically based mechanism was proposed involving the reduction of electron‐hole recombination and fast electron‐transfer possibility.