Mesoporous Nanocrystalline TiO2 Research Articles

The role of electron interfacial transfer in mesoporous nano-TiO2photocatalysis: a combined study of in situ photoconductivity and numerical kinetic simulation

In this research, a combination of in situ photoconductivity (σ) and kinetic simulations was used to study the role of electron interfacial transfer (IT) in the gaseous photocatalysis of formic acid by mesoporous nanocrystalline TiO2. The effects of light intensity, initial formic acid concentrations, oxygen amounts, and temperature on the in situ σ and the photocatalytic courses were studied in detail. The temperature dependence of in situ σ clearly shows that the electron transfer is determined by the IT of electrons to O2 rather than by the transport. It was seen that the electron IT limits the photocatalysis by correlating with the recombination and the hole IT via the dynamic change in electron densities. The numerical simulation of in situ σ shows that the IT of electrons belongs to a thermally activated process that presents a thermal barrier of 0.5 eV. It is considered that this high thermal barrier limits the IT of electrons. It was also seen that the thermal activation of photocatalysis does not relate to that of the electron IT, although the overall photocatalysis is limited by the IT of electrons. Our finding shows that it is an effective way to increase the photocatalytic activity by reducing the thermal barrier of electron IT.

Evaluation of photoconductive and photoelectrochemical properties of mesoporous nanocrystalline TiO2 powders and films prepared in acidic and alkaline media

Nanocomposite TiO2 powders were prepared via hydrothermal route using titanium(IV) butoxide as starting material at different pH media. In particular, three alkaline media composed of ammonium carbonate, ammonium bicarbonate and urea were used as modulators of the TiO2 nanostructure. Besides, sulfuric acid was also used succeeding an acidic environment for the growth of TiO2 nanoparticles. The structural properties of powders made either in acidic or alkaline environment were examined with microscopy techniques (FE-SEM), X-ray diffraction patterns (XRD), porosimetry (BET) and FTIR spectroscopy. TiO2 films were fabricated by suitably modifying the synthesized powders in the form of easily handled pastes. The powders were evaluated to their photoconductivity properties while films were sensitized with N719 ruthenium dye and employed as photoanodes in quasi-solid state dye sensitized solar cells (DSSCs). The cells were characterized in terms of their electrical and electrochemical characteristics using impedance spectroscopy. The overall performance of the solar cells employing TiO2 films prepared in acidic or alkaline environment was higher than that recorded for commercially obtained TiO2 powder (Degussa-P25) with a maximum difference of 16% in the case of TiO2 made in acidic environment.

Fabrication of TiO2 Based Microspheres By Spray Drying Method and Their Application for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have received considerable attention since Grätzel et.al.reported a high conversion efficiency using nanocrystalline mesoporous TiO2 film. A typical DSSC consists of a transparent conductive oxide, semiconductor oxide, dye sensitizer, electrolyte and counter electrode. The working electrode is a nanoporous semiconductor oxide that is placed on conducting glass and is separated from the counter electrode by only a thin layer of electrolyte solution. The photoanode of DSSCs is typically constructed using a thick film (~20 μm) of TiO2 or, less often, ZnO or SnO2 nanoparticles. The TiO2 film has a large inherent absorptive surface area for light scattering. Recently, TiO2 microspheres are of great interest for a great deal of applications, especially in the solar cell field. Because of their unique microstructure and light-scattering effect, TiO2 microsphere-based solar cells often exhibit superior photovoltaic performance. Hence, exploring new suitable TiO2 microspheres for high-efficiency solar cells is essential. In this study, we reported the fabrication of TiO2microspheres by a simple spray drying process, and applied for Dye-sensitized Solar Cells. TiO2 Microspheres were fabricated by spray drying method. TiO2 paste was prepared as follows, TiO2 microspheres (0.2g) was mixed into ethanol (1.8g). The resulting TiO2 paste was printed by doctor blade method on fluorine-doped SnO2-coated (FTO) glass electrodes and sintered at 500 °C for 30 min. DSSCs were prepared by a conventional procedure. We are successfully fabricated TiO2 microspheres by spray drying method. According to SEM images of the TiO2 microspheres, particle size was ca. 3mm. We made microsphere TiO2 based DSSCs, and their energy conversion efficiency was 6.5% under AM 1.5 simulated full sunlight (100 mW cm−2) illumination.

The impact of the incorporation of dual salts into poly(1-vinylpyrrolidone-co-vinyl acetate) based quasi-solid polymer electrolyte on the electrochemical and photovoltaic performances of the dye-sensitized solar cells

The incorporation of two different type of iodide salts in the quasi-solid polymer electrolyte (QSPE) has been reported previously to have the ability to enhance the electrical performance of the QSPE and photovoltaic performance in the application of dye-sensitized solar cells (DSSCs). In this research, potassium iodide (KI) and tetrapropylammonium iodide (Pr4NI) salts were used as the mobile ions providers for the poly(1-vinylpyrrolidone-co-vinyl acetate) (P(VP-co-VAc)) based quasi-solid polymer electrolytes (QSPEs). Dual salt system utilizing these two different salts will show improvement in the performance of the QSPE since they have different size and other characteristics. The resultant QSPEs with the dual iodide salts system in this research show better ionic conductivity compared to the QSPE with a single type of salt. The QSPE samples were sandwiched between a mesoporous nanocrystalline TiO2 coated fluorine-doped tin oxide (FTO) glass which works as the photoelectrode and a platinum (Pt) coated FTO as the counter electrode in order to form a functional P(VP-co-VAc)-based QSPE DSSC. Cells with either KI or Pr4NI, with identical QSPE compositions, were able to reach efficiencies of only 2.11% and 1.71% respectively. Interestingly, the cell with the dual iodide salts system under certain composition could display power conversion efficiency (PCE) of 5.53% with maximum short circuit density (Jsc) of 14.07mAcm−2, open circuit voltage (Voc) of 0.60V and a fill factor of 65% under standard light intensity of 100mWcm−2 irradiation of AM 1.5 sunlight. The cells have also been studied under different illumination condition and it was found that the overall photocurrent density of the cells is not only limited by the charge diffusion inside the QSPE samples. Electrochemical impedance studies of these cells have been studied to understand more on the electrical properties of the DSSCs. Tafel measurements which were carried out with two symmetrical Pt electrode has also been studied and shows that the dual salt system has higher electrocatalytic activity.

High Coverage Formation of CdS Quantum Dots on TiO2 by the Photocatalytic Growth of Preformed Seeds

Photoelectrochemical experiments and density functional theory calculations indicated that visible-light irradiation of the CdS quantum dots (QDs)–TiO2 direct coupling system (CdS/TiO2) causes the electron injection from the valence band (VB) of CdS into the conduction band (CB) of TiO2 (path 2) in addition to the inter-CB electron transfer from CdS to TiO2 (path 1). Path 2 can be induced by the sub-bandgap excitation of CdS QDs to extend the spectral response of the CdS/TiO2 system. For path 2 as well as path 1 to effectively work, CdS QDs should be directly deposited on the TiO2 surface with high coverage. According to the guideline, a photocatalytic growth of the preformed seed (PCGS) technique has been developed. Transmission electron microscopy observation and X-ray photoelectron spectroscopy measurements of the CdS/TiO2 prepared by the PCGS technique indicated that the TiO2 surface is highly covered by CdS QDs. The technique was applied to mesoporous TiO2 nanocrystalline films (mp-TiO2) to yield CdS...

Size-Dependence of the Activity of Gold Nanoparticle-Loaded Titanium(IV) Oxide Plasmonic Photocatalyst for Water Oxidation.

Mesoporous TiO2 nanocrystalline film was formed on fluorine-doped tin oxide electrode (TiO2 /FTO) and gold nanoparticles (NPs) of different sizes were loaded onto the surface with the loading amount kept constant (Au/TiO2 /FTO). Visible-light irradiation (λ>430 nm) of the Au/TiO2 /FTO photoanode in a photoelectrochemical cell with the structure of photoanode|0.1 m NaClO4 aqueous solution|Ag/AgCl (reference electrode)|glassy carbon (cathode) leads to the oxidation of water to oxygen (O2 ). We show that the visible-light activity of the Au/TiO2 /FTO anode increases with a decrease in Au particle size (d) at 2.9≤d≤11.9 nm due to the enhancement of the charge separation and increasing photoelectrocatalytic activity.

Well-organized, mesoporous nanocrystalline TiO2 on alumina membranes with hierarchical architecture: Antifouling and photocatalytic activities

We report the development of improved antifouling and photocatalytic membranes based on an organized mesoporous TiO2 (om-TiO2) layer on a porous alumina (Al2O3) support. The om-TiO2 layer was templated by an amphiphilic graft copolymer, i.e., poly(vinyl chloride)-graft-poly(oxyethylene methacrylate) (PVC-g-POEM), and was 50–70nm in pore size with high porosity and good pore interconnectivity. The precoating of poly(vinyl pyrrolidone) (PVP) on a porous alumina support was the key to providing a flat platform and improving the adhesion properties of alumina with TiO2, which allowed for the formation of om-TiO2 membranes. Fouling of photocatalytic om-TiO2 on the alumina membrane was also investigated using a laboratory-scale photocatalytic dead-end filtration reactor using a model dye compound. The fouling rate was observed to be much slower for the om-TiO2 membrane compared to the bare alumina support under UV illumination during 6h of membrane operation. The PVP precoating on the alumina support improved the permeation quality of the om-TiO2 membrane. The improved removal of the organic dye compound was thought to result from the enhanced adsorption capability of organic compounds and subsequent photocatalytic reactions on the om-TiO2 layer. However, UV illumination performed for longer than 6h gradually increased the fouling rate due to the deposition of the organic dye compound on the membrane surface, and prohibited the accessibility of the UV light to the TiO2 layer.

Lithium insertion properties of mesoporous nanocrystalline TiO2 and TiO2–V2O5 microspheres prepared by non-hydrolytic sol–gel

Mesoporous nanocrystalline TiO2 and TiO2–V2O5 microspheres were prepared by non-hydrolytic sol–gel from TiCl4, VOCl3, and iPr2O at 110 °C without any solvent or additives. The samples were characterized by elemental analysis, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, nitrogen physisorption, and impedance measurements. At low vanadium loadings, only TiO2 anatase was detected, and V2O5 scherbinaite was also detected at high vanadium loadings. The texture of the samples depended on the V loading, but all the samples appeared built of primary nanoparticles (≈10–20 nm in size) that aggregate to form mesoporous micron-sized spheres. The lithium insertion properties of these materials were evaluated by galvanostatic measurements taken using coin-type cells, in view of their application as electrode for rechargeable Li-ion batteries. The mesoporous TiO2 microspheres showed good performances, with a specific reversible capacity of 145 and 128 mAh g−1 at C/2 and C, respectively (C = 335.6 mA g−1), good coulombic efficiency, and a moderate capacity fade (6 %) from the 2nd to the 20th cycle at C/20. Although the addition of V effectively increased the electronic conductivity of the powders, the specific reversible capacity and cycling performances of the TiO2–V2O5 samples were only minimally improved for a 5 at% V loading and were lower at higher V loading.

Atomic layer deposition assisted sacrificial template synthesis of mesoporous TiO2 electrode for high performance lithium ion battery anodes

TiO2 has been considered a promising anode material particularly for large-size Li ion battery (LIB) along with the superior reliability in safety issue. Unfortunately, low Li storage capacity no more than the half of theoretical capacity has been the major bottleneck for the practical utilization of TiO2 anodes. We report mesoporous anatase TiO2 nanocrystalline anode materials produced from atomic layer deposition (ALD) based sacrificial carbon template synthesis for the significant enhancement in reversible capacity approaching the theoretical value. ALD conformal deposition of the nanoscale thick TiO2 layer at sacrificial carbon black template surface and subsequent thermal calcination at relatively low temperature produce mesoporous nanocrystalline TiO2 electrode with anatase crystalline phase. The resultant anode material with large specific surface area (260m2g−1) effectively facilitates the Li intercalation/deintercalation at electrode surface as well as enhances electrolyte permeability. Consequently, remarkable specific capacity (280mAhg−1), high first coulomb efficiency (85%) and excellent cycle ability (80% @100 cycles at 1 C) are achieved (Table S1). This ALD based sacrificial carbon templating method is a fast, facile and controllable advanced technology that offers a robust viable route for various high performance nanostructured materials.

Lateral Intermolecular Self-Exchange Reactions for Hole and Energy Transport on Mesoporous Metal Oxide Thin Films.

Intermolecular self-exchange energy and electron-transfer reactions occur without a loss of free energy. This behavior can be exploited for energy-transport applications when molecules that undergo self-exchange transfer reactions are immobilized on a solid support. This Article focuses upon lateral self-exchange reactions and the relevant interfacial chemistry that occurs on the mesoporous nanocrystalline (anatase) TiO2 thin films that are commonly used in dye-sensitized solar cells. It has been known for some time that all of the dye molecules (termed sensitizers) within such thin films can be reversibly oxidized and reduced by lateral self-exchange electron transfer provided that the sensitizer surface coverage exceeds a percolation threshold. Under conditions where excited-state electron injection into TiO2 is unfavored, lateral intermolecular energy-transfer reactions are also known to occur. The self-exchange rate constants have been quantified by electrochemical, absorption, and/or time-resolved anisotropy techniques and understood within the framework of Marcus theory. Such analysis reveals that the reorganization energy and the electronic coupling are sensitive to the identity of the molecular compound. Time-resolved anisotropy measurements have shown that lateral charge and energy-transfer reactions across the TiO2 surface occur in kinetic competition with charge recombination and excited-state relaxation, respectively. The extent to which lateral self-exchange reactions might be exploited for solar energy conversion applications is discussed, as are critical fundamental issues that remain unresolved.

Surface charge-transfer complex formation of catechol on titanium(IV) oxide and the application to bio-sensing

Adsorption properties of 2-hydroxyphenol (catechol) on TiO2 particles has been studied at 298K. The adsorption proceeds from the aqueous solution with the Langmuir type behavior. Diffuse reflectance infrared spectra of the catechol-adsorbed TiO2 suggested that catechol is adsorbed on TiO2 solution via the chelation to the surface Ti ions. The adsorption induces a strong absorption in the whole visible region, of which intensity increases with an increase in the adsorption amount. Photoelectrochemical experiments and molecular orbital calculations indicate that the absorption stems from the charge-transfer (CT) transition from the HOMO of catechol to the conduction band of TiO2. Time courses for the adsorption of catechol on mesoporous TiO2 nanocrystalline film-coated glass was traced by measuring the change in the absorbance of the CT band, and analyzed on the basis of the Langmuir model. This study would present a new simple technique for sensing of important biomolecules bearing the catechol moiety.

Mesoporous nanocrystalline TiO2 supported metal (Cu, Co, Ni, Pd, Zn, and Sn) catalysts: Effect of metal-support interactions on steam reforming of methanol

Mesoporous TiO2 supported Cu, Co, Ni, Pd, Sn and Zn catalysts (M-TiO2) were synthesized using facile one-step synthesis method and were characterized using BET, XRD, TGA-DSC, TEM, SEM-EDX, ICP-OES, and H2-TPR studies. The catalysts were further tested for steam reforming of methanol (SRM) to investigate their comparative catalytic performance. Depending on the nature of the metal component, the catalysts exhibited surface area, pore sizes, and TiO2 crystallite sizes in the range of 99–309m2/g, 2.63–4.69nm and 6.8–17.2nm, respectively. N2-physorption, TGA-DSC and XRD analysis demonstrated that the presence of metal in the TiO2 matrix stabilized the mesoporous structure by hindering the crystal growth during heat treatment and thereby preventing the collapse of porous structure. Furthermore, the characterization of 5–20% Zn-TiO2 catalysts indicated that there exist an optimum Zn loading to obtain highest surface area which was found to be 15% in the present study giving a high surface area of ∼258m2/g. This was consistent with the SRM studies where the activity increased up to 15% and then decreased significantly with further increase in Zn loading to 20%. The results of the SRM studies coupled with extensive TPR analysis of different M-TiO2 catalysts suggest that the specific metal-support interactions play a crucial role in controlling its performance on H2 production. The SRM activity order for different metals incorporated in mesoporous TiO2 was observed to be: Pd>Ni>Zn>Co>>Cu>>Sn. The Zn-TiO2 catalyst showed the lowest CO selectivity among the different catalysts studied.

Electron Transport, Trapping and Recombination in Anodic TiO<sub>2</sub> Nanotube Arrays

Anodically synthesized TiO2 nanotube arrays (TNTAs) constitute an exciting ordered large bandgap semiconductor nanoarchitecture for use as scaffolds and active layers for solutionprocessable devices including but not limited to, optoelectronic sensors, photovoltaics, photodetectors, photocatalysts and photoelectrochemical cells. Charge transport, trapping and recombination are key attributes of the material architecture that significantly influence the properties and performance of the resulting optoelectronic devices, thus motivating this review article. Since nanocrystalline mesoporous TiO2 films (np-TiO2) are actively researched for the same applications, in many cases, TNTAs and np-TiO2 are direct competitors and it is therefore meaningful to compare the optoelectronic properties of the two architectures head-to-head. In addition, there exists a whole host of TNTA-specific applications such as bottom-up fabricated photonic crystals, bulk heterojunction organic solar cells and metallodielectric metamaterials that leverage the ordered channel architecture. Recent studies have established the order of magnitude superior recombination lifetimes in sensitized TNTAs as compared to sensitized np-TiO2 as well as the salutary effect of lower structural disorder in TNTAs resulting in trap-free electron diffusion coefficients approaching those of single crystals and two orders of magnitude larger than np-TiO2. Photoconductivity measurements using bandgap illumination in both single nanotubes and nanotube ensembles have resulted in similar values of the mobility-lifetime product (10-5-10-4 cm2V-1), which are four to six orders of magnitude higher than in nanoparticle electrodes. At the same time, TiO2 nanotubes have a larger trap density and a greater average trap-depth than nanoparticulate Ti2 films, pointing to the importance of synthesis modification to improve material quality and post-synthesis techniques for trap passivation. Keywords: Cyclic voltammetry, electrochemical impedance spectroscopy, IMPS/IMVS studies, open-circuit voltage decay, photoconductivity, photoluminescence, resistivity, TiO2 nanotubes arrays.

Enhanced dye sensitized solar cell performance with high surface area thin ZnO film and PEDOT:PSS

ZnO coated meso-porous nanocrystalline TiO2 films were prepared and the corresponding photoelectrodes were used in the fabrication of Dye-Sensitized Solar Cells (DSSCs) to investigate its influence along with platinum (Pt), poly(3,4-ethylenedioxythiophene: poly (4-styrene sulfonate)) [PEDOT:PSS] counter electrodes on incident-photon-to-current conversion efficiency (IPCE) and power conversion performance (η). DSSCs fabricated based on the ZnO coated photoeletrodes exhibit an IPCE of 71% with η-value of 7.51±0.06% [short-circuit current density (JSC)=17.93±0.07mA/cm2, open-circuit voltage (VOC)=0.66±0.01V, fill factor (FF)=0.62±0.01] under Air Mass 1.5 sunlight, higher as compared to the reference DSSCs fabricated under similar conditions for ZnO free photoelectrodes, which exhibited η-value of 6.11±0.02% (JSC=15.04±0.04mA/cm2, VOC=0.68±0.01V, FF=0.60±0.01) with platinum decorated counter electrodes. The higher power conversion efficiency of DSSCs fabricated based on TiO2/ZnO electrode could be probably more competitive electron collection process due to sharper ZnO nanosoar paths.

Ultrathin Conformal Coating and Zn Doping in Nanocrystalline Mesoporous TiO2 Micron-Sized Beads for Highly Efficient Dye Sensitized Solar Cells

Doping in TiO2 has significant influence on donor density and dielectric parameters of TiO2 along with the introduction of defect levels. These variations in TiO2 hence could be exploited for enhancement in light scattering, charge transport and charge injection efficiency at the same time if the photoanode structure is properly designed. Herein, we attempted to demonstrate that Zn doping in nanocrystalline mesoporous TiO2 microspheres can be harnessed for such synergistic enhancements to realize a desirable photoanode for dye sensitized solar cells. A trace amount of Zn doping (2 at.%) along with resulting oxygen vacancies caused 69% enhancement in charge transport in electrode film, while the formation of an ultrathin disordered oxide layer (∼2nm) on the surface of the nanocrystals drastically improved the charge injection efficiency by restraining recombination at the interface between the electrode and electrolyte. Furthermore, compositional variation upon Zn doping altered the dielectric parameters which resulted in ∼28% enhancement in light scattering efficiency over a broad spectrum of sunlight. An overall 26% improvement in energy conversion efficiency has been achieved by exploiting these structures in dye sensitized solar cell devices.

Electric Fields and Charge Screening in Dye Sensitized Mesoporous Nanocrystalline TiO2 Thin Films

The photophysical and electron transfer properties of mesoporous nanocrystalline (anatase) TiO2 thin films sensitized to visible light with [Ru(dtb)2(dcb)](PF6)2, where dtb is 4,4′-(tert-butyl)2-2,2′-bipyridine and dcb is 4,4′-(CO2H)2-2,2′-bipyridine, were quantified in acetonitrile solutions that contained 100 mM concentrations of Li+, Na+, Mg2+, or Ca2+ perchlorate salts. The presence of these salts resulted in a dramatic and cation dependent bathochromic (red) shift of the metal-to-ligand charge transfer (MLCT) absorption and photoluminescence (PL) spectra of Ru(dtb)2(dcb)/TiO2 relative to the value measured in neat or 100 mM TBAClO4, where TBA is tetrabutyl ammonium cation, acetonitrile solutions. The magnitude of the shifts followed the trend: Na+ < Li+ < Ca2+ < Mg2+. The PL intensity was also found to decrease in this same order and comparative actinometry studies showed that this was due to MLCT excited state electron transfer quenching by the TiO2 acceptor states. The RuIII/II redox chemistry was found to be non-Nernstian; the ideality factors were cation-dependent, suggestive of an underlying electric field effect. Electrochemical reduction of the TiO2 resulted in a black coloration and a blue shift of the fundamental (VB → CB) absorption, the normalized spectra were cation independent. Reduction of sensitized TiO2 also resulted in a blue shift of the MLCT absorption, the magnitude of which was used to determine the surface electric fields. Under conditions where about 20 electrons were present in each anatase nanocrystallite, the electric field strength reported by the Ru compound followed the trend Na+ < Li+ < Mg2+ < Ca2+, with Na+ being 1.1 MV/cm and Ca2+ 2.3 MV/cm. In pulsed laser experiments, the first-derivative absorption signature was observed transiently after excited state injection and iodide oxidation. These absorption amplitudes were time-dependent and decayed over time periods where the number of injected electrons was constant, with behavior attributed to screening of the surface electric field by cations present in the electrolyte. The monovalent cations screened charge much more rapidly than did the dications, kLi+,Na+ = 5.0 × 104 s–1 and kMg2+,Ca2+ = 5.0 × 102 s–1, presumably because the small number of injected electrons resulted in spatially isolated singly reduced Ti(III) sites that were more easily screened by the monocations.

Development of a new solid-state absorber material for dye-sensitized solar cell (DSSC)

In contrast to the conventional DSSC systems, where the dye molecules are used as light harvesting material, here a solid-state absorber was used as a sensitizer in conjunction with the dye. The materials like ZnO and Al2O3 : C, which will show optically stimulated luminescence (OSL) upon irradiation were used as extremely thin absorber layers. This novel architecture allows broader spectral absorption, an increase in photocurrent, and hence, an improved efficiency because of the mobility of the trapped electrons in the absorber material after irradiation, to the TiO2 conduction band. Nanocrystalline mesoporous TiO2 photoanodes were fabricated using these solid-state absorber materials and after irradiation, a few number of samples were co-sensitized with N719 dye. On comparing both the dye loaded photoanodes (ZnO/TiO2 and Al2O3 : C/TiO2), it can be concluded from the present studies that, the Al2O3 : C is superior to ZnO under photon irradiation. Al2O3 : C is more sensitive to photon irradiation than ZnO and hence there can be more trap centres produced in Al2O3 : C.

High temperature and water-based evaporation-induced self-assembly approach for facile and rapid synthesis of nanocrystalline mesoporous TiO2

Through a high-temperature and water-based EISA (HW-EISA) approach, mesoporous TiO2 with high surface areas, ultra-large mesopores/pore volumes and tuneable bi-crystallinity (anatase plus rutile) can be facilely prepared in a ternary templating system (peroxotitanic acid/P123/H2O).

Ex Situ CdSe Quantum Dot-Sensitized Solar Cells Employing Inorganic Ligand Exchange To Boost Efficiency

We present an effective way to boost the as-synthesized CdSe quantum dot-sensitized solar cells (QDSSCs) performance by introducing an inorganic ligand exchange strategy into this traditional system. Inorganic ligand exchange, to the best of our knowledge, is designed for the first time for CdSe-based QDSSCs, and it features low-cost, easy operation, and repeatable process. The route involves the direct deposition of the CdSe quantum dots (QDs), which were initially capped with trioctylphosphine (TOP) ligands onto mesoporous TiO2 nanocrystalline electrodes and followed by a post-treatment of the sensitized photoanode films with sulfur ions (S2–) solution. Here, changes in surface chemical status of CdSe QDs during the inorganic ligand exchange process and the influence of ligand exchange on the electron’s ultralfast transfer between nanoparticles were investigated through XPS and femtosecond transient grating techniques, respectively. With the inorganic ligand passivated CdSe QDs, the QDSSCs exhibited a power conversion efficiency of 3.17% (AM1.5G, 100 mW/cm2), 65% higher than that of the organic ligands capped QDSSCs.

Homoleptic “Star” Ru(II) Polypyridyl Complexes: Shielded Chromophores to Study Charge-Transfer at the Sensitizer-TiO2 Interface

Three homoleptic star-shaped ruthenium polypyridyl complexes, termed Star YZ1, Star YZ2, and Star YZ3, where the Ru(II) center is coordinated to three bipyridine ligands each carrying two oligo(phenylene ethynylene) (OPE) rigid linker units terminating with isophthalic ester (Ipa) groups for binding to metal-oxide surfaces were synthesized. In Star YZ3, each OPE linker was substituted with two n-butoxy (n-BuO) solubilizing groups. Star complex YZ4, which is homoleptic but lacks the octahedral symmetry, was synthesized as a reference compound. The Star complexes were synthesized using two approaches: in the first, Ru(4,4'-(Br)2-2,2'-bpy)3 was reacted in a Sonogashira cross coupling reaction with the ethynyl-OPE-Ipa linkers; in the second, the 2,2'-bpy-OPE-Ipa ligands were reacted with Ru(DMSO)4(PF6)2. The photophysical behavior of the Star complexes were studied in fluid solution and anchored to the surface of mesoporous nanocrystalline TiO2 thin films (Star/TiO2). To a first approximation the excited state behavior in CH3CN was unchanged when the compounds were anchored to a TiO2 thin film, indicating that the highly symmetrical (octahedral) and rigid molecular structure of the ligands shielded the chromophoric core from the TiO2 semiconductor. Inefficient excited state injection, φ(inj) < 0.05, was observed to occur on a nanosecond time scale with slow recombination. In addition, the presence of n-BuO groups on the linker unit gave a large increase in the extinction coefficient of YZ3, which allows for enhanced harvesting of sunlight. The results indicate that molecular design on the nanometer length scale can be utilized to control excited state relaxation pathways at semiconductor surfaces.