Self-assembled, excitonically coupled aggregates of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) in acidified alcohols were studied by absorption, resonance light scattering (RLS), and resonance Raman (RR) spectroscopy along with scanning probe microscopy. In ethanol, the major absorption bands of aggregates in 0.5 mM HCl (sample E1) were narrower, with smaller total RLS intensity, compared to those of aggregates in 50 mM HCl (sample E2). RR scattering cross sections were smaller, and depolarization ratio dispersion greater, for E2 than E1. Two distinct types of aggregates at differing acidity in 1-propanol, analogous to the ethanolic samples, but only one type in methanol, were also identified. Atomic force microscopy (AFM) images for E1 showed small, single layered structures (∼5–20 nm diameter, ∼1.5–2 nm thickness). Variable morphologies for E2 included double-layered, elongated structures consistent with collapsed nanotubes (∼25 nm width, ∼3.5–4 nm thickness, ∼100–200 nm length) and single molecule thick sheets (∼50–200 nm in characteristic diameter). The former were also observed in ultrahigh vacuum scanning tunneling microscopy (UHV-STM) images. Aggregate morphology appears to depend on protonation and deprotonation kinetics of porphyrin monomers. Spectroscopic observations suggest that a larger subset of J-band excitonic transitions are significantly active in E2 than in E1. These variations in excitonic properties are attributed to the differing aggregate sizes, shapes, and possibly molecular packing. Formation of long nanotubes appears to require a solvent able to donate two hydrogen bonds, such as water. An open sheet structure is favored otherwise, as in ethanol.

Self-assembled, excitonically coupled aggregates of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) prepared in aqueous LiCl, NaCl, KCl, CsCl, and HCl were studied with solution phase and single-particle resonance Raman (RR) spectroscopy. Solution phase excitation profiles are sharply peaked at 488.0 nm excitation for samples in HCl, LiCl, and CsCl, which show narrow J-bands in the corresponding absorption spectra, but more gently peaked for those induced by NaCl or KCl, which show broader absorption J-bands. The former three samples also exhibit larger low to high frequency mode intensity ratios with excitation near the J-band peak and smaller depolarization ratios compared to the latter two samples. Polarized spectra of individual aggregates correlate with the solution phase results, exhibiting an increase in intensities involving either incident or scattered light polarized transversely to the aggregate long axes in conditions shown previously to induce bundling of nanotubes or greater disorder. These results, along with previous absorption, resonance light scattering (RLS), and imaging data, suggest that excitonic coupling across nanotubular components in bundled aggregates leads to spectral broadening. This is attributed to increased spectral density of allowed excitonic transitions, particularly those polarized transversely to the aggregate length. Disorder leads to deviation of excitonic transition polarizations from pure axial and transverse directions, resulting in greater transverse relative to axial polarization but smaller excitonic coherence as measured by RLS intensity. These results suggest that environment-induced morphological variations can affect the energies, polarizations, and spatial structure of excitons in dye aggregates.

Enhanced light-harvesting and photoconversion efficiency of nanocrystalline TiO2 sensitized with the plant pigment betanin is observed in the presence of spectral signatures which reveal self-assembly of betanin on the surface. Though aggregation of sensitizing chromophores is generally considered detrimental to dye-sensitized solar energy conversion, solar cells constructed with aggregated betanin show a 2.5-fold increase in power conversion efficiency compared to those using mostly monomeric betanin. Dye aggregation results in a broadened absorbance spectrum with extended light harvesting at blue and red wavelengths. Variation in solution conditions and soaking times for film sensitization enable control of the relative amounts of adsorbed monomer and aggregate. Self-consistent modeling of the absorption spectra of betanin-sensitized TiO2 films as a function of dye loading suggests the templated formation of a betanin dimer on the TiO2 surface and associated splitting of the excited state. We show that dye aggregation increases the light-harvesting efficiency as well as the incident photon-to-current conversion efficiency (IPCE). Measurement of the absorbed photon-to-current conversion efficiency (APCE) reveals that electron injection and collection of the putative betanin dimer is more efficient than that of the monomer. Not only is this the first report of a dye-sensitized solar cell performance increase upon dye aggregation, but it also constitutes a record power conversion efficiency for a natural dye-based solar cell of 3.0%.

The spatial distribution of intra band gap traps in micrometer-sized single crystals of anatase TiO2 was explored using single-particle photoluminescence (PL) spectroscopy and imaging. The PL from microcrystals with well-defined {001} and {101} facets was imaged for the same particle before and after annealing to explore the influence of fluorine, used as a capping agent in the synthesis of the microparticles, on luminescent traps. Unannealed particles reveal weaker photoluminescence and distinctly different spatial distribution of PL emission compared to annealed particles. The results show that the capped particles have fewer surface defects as a result of passivation of surface electron traps associated with undercoordinated titanium. PL images suggest that the remaining surface defects in the fluorine-capped particles concentrate at edges and corners of the microcrystal, and that anisotropic carrier transport takes place via hopping between adjacent Ti atoms. Annealed particles reveal greater surface defect density and more isotropic, diffusional carrier transport than the as-synthesized particles. The results are interpreted in terms of the influence of fluorine-capping groups on the spatial distribution and occupancies of traps and different pathways for carrier transport. The results have implications for applications of TiO2 nanosheets in solar energy conversion and photocatalysis.

We report spectroelectrochemical and transient absorption spectroscopic studies of electron injection from the plant pigment betanin (Bt) to nanocrystalline TiO2. Spectroelectrochemical experiments and density functional theory (DFT) calculations are used to interpret transient absorption data in terms of excited state absorption of Bt and ground state absorption of oxidation intermediates and products. Comparison of the amplitudes of transient signals of Bt on TiO2 and on ZrO2, for which no electron injection takes place, reveals the signature of two-electron injection from electronically excited Bt to TiO2. Transient signals observed for Bt on TiO2 (in contrast to ZrO2) on the nanosecond time scale reveal the spectral signatures of photo-oxidation products of Bt absorbing in the red and the blue. These are assigned to a one-electron oxidation product formed by recombination of injected electrons with the two-electron oxidation product. We conclude that whereas electron injection is a simultaneous two-electron process, recombination is a one-electron process. The formation of a semiquinone radical through recombination limits the efficiency and long-term stability of the Bt-based dye-sensitized solar cell. Strategies are suggested for enhancing photocurrents of dye-sensitized solar cells by harnessing the two-electron oxidation of organic dye sensitizers.

Self-assembled, excitonically coupled aggregates of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) in aqueous solutions of HCl and of the alkali chloride salts LiCl, NaCl, KCl, and CsCl were studied by absorption and resonance light scattering (RLS) spectroscopy. Aggregates deposited from these solutions were imaged by transmission electron microscopy and atomic force microscopy. In NaCl and KCl, the J band in the absorption spectrum was broadened, while in CsCl it was narrowed, as compared to aggregates in HCl. In LiCl and NaCl, the relative excitonic coherence as determined from RLS was increased as compared to aggregates in HCl, while in KCl and CsCl it was reduced. The discrepancy between measures of coherence based on exchange narrowing in the absorption spectrum and those on RLS intensity points to morphology-dependent composite J band structures. Delocalization of excitons between nanotubular subunits of bundled aggregates appears to increase excitonic coherence. Loosening of intermonomer forces, observed as bending in the images, and possibly the inclusion of alkali and/or chloride ions within composite structures, appear to limit excitonic coherence by increasing disorder. The dependence of morphologies on counterion species can be explained, in part, by the cations’ differing kosmotropic versus chaotropic ion-pairing properties and effects on water structure. The results may inform methods to tune the spatial and spectral properties of excitons in these systems.

The solvent-dependent energies of surface trap states of TiO2 nanoparticles are examined by spectroelectrochemical photoluminescence using two different particle morphologies. Trap-state photoluminescence of nanocrystalline TiO2 in aqueous environment under Fermi level control reveals the pH-dependent redox Fermi levels of the surface Ti3+/4+ couple associated with 5-fold coordinated titanium. In aqueous environment, this trap-state distribution is populated at lower energy in TiO2 nanosheets rich in exposed (001) texture, compared to commercial anatase TiO2 nanoparticles with exposed (101) surfaces. Lower-energy traps appear to be partially passivated in the case of nanosheets in acetonitrile environment. Self-modeling curve resolution of the photoluminescence under Fermi level control reveals three spectral components in aqueous and acetonitrile environments: the red and green photoluminescence we have previously associated with electron and hole traps, respectively, and a third intermediate (yellow) component that may result from a separate distribution of electron traps. An apparent overvoltage, which is larger for nanoparticles than for nanosheets, is found for occupation of surface electron traps in aqueous environment. In contrast, electron traps in acetonitrile are occupied at potentials consistent with their energetic position within the band gap as determined by the photoluminescence spectrum. Our results reveal the solvent-dependent redox potential of electron traps and lend insight into the effects of contacting solvent on performance of nano-TiO2 in applications such as dye-sensitized solar cells.

Trap state photoluminescence of nanocrystalline TiO2 electrodes is investigated as a function of applied bias and pH in aqueous electrolyte. Films composed of the anatase polymorph reveal an increase in a broad red emission at increasingly negative potentials, with an onset about 200 mV positive of the pH-dependent literature value of the conduction band potential, followed by conversion to the green emission characteristic of hole traps at more negative bias. Green photoluminescence is the only emission seen from mixed-phase (P25, anatase/rutile) films at any applied potential, while red-emitting electron traps in P25 appear to be quenched by electron transfer to rutile hole traps. The influence of surface treatment by TiCl4 is investigated for both anatase and P25 in order to shed light on the mechanism by which this treatment improves the performance of TiO2-based solar cells. Our results reveal the difference between trap state distributions of P25 and anatase nanoparticles and address the molecular basis for red and green emitting traps. The results establish the redox potentials of the traps as a function of pH and reveal the breadth of their energetic distribution.

We present the first report of photoluminescence spectra and images of single TiO2 (anatase) nanotubes. In previous work using ensembles of conventional TiO2 nanoparticles, we interpreted the broad photoluminescence (PL) spectrum to be a superposition of hole trap emission, peaking in the green, and broad red PL arising from electron traps. PL spectra of individual nanotubes in inert environment show a similar broad emission, with peaks at around 560–610 nm. The PL from single nanotubes differs from the more blue-shifted PL of ordered nanotube films. The intensity of PL is found to be larger for single nanotubes than for ordered arrays, as a result of competition from transport in the contiguous samples and from introduction of additional trap states when the nanotubes are dispersed. PL images of single nanotubes show the emission to be concentrated in the area of excitation, but the peaks in the red and green components of the PL are not spatially coincident. Remote PL, occurring away from the excitation point, is observed in the green (∼510 nm), showing the possible contribution of charge transport to the observed PL. While the PL from ensembles of TiO2 nanotubes is fairly insensitive to contacting media, exposure of single nanotubes to air and ethanol changes the shape and intensity of the PL spectrum. Our results point to a very different trap state distribution in TiO2 nanotubes compared to that of conventional TiO2 nanoparticles, which we attribute to differences in exposed crystal facets. In addition, separation of nanotubes introduces additional photoluminescent trap states and changes the character of the emission from excitonic in the array to trap-mediated in single nanotubes.Keywords: single nanoparticle spectroscopy; TiO2 nanotubes; transport; trap states

The defect photoluminescence from TiO2 nanoparticles in the anatase phase is reported for nanosheets which expose predominantly (001) surfaces and compared to that from conventional anatase nanoparticles which expose mostly (101) surfaces. Also reported is the weak defect photoluminescence of TiO2 nanotubes, which we find using electron backscattered diffraction to consist of walls which expose (110) and (100) facets. The nanotubes exhibit photoluminescence that is blue-shifted and much weaker than that from conventional TiO2 nanoparticles. Despite the preponderance of (001) surfaces in the nanosheet samples, they exhibit photoluminescence similar to that of conventional nanoparticles. We assign the broad visible photoluminescence of anatase nanoparticles to two overlapping distributions: hole trap emission associated with oxygen vacancies on (101) exposed surfaces, which peaks in the green, and a broader emission extending into the red which results from electron traps on undercoordinated titanium atoms, which are prevalent on (001) facets. The results of this study suggest how morphology of TiO2 nanoparticles could be optimized to control the distribution and activity of surface traps. Our results also shed light on the mechanism by which the TiCl4 surface treatment heals traps on anatase and mixed-phase TiO2 films and reveals distinct differences in the trap-state distributions of TiO2 nanoparticles and nanotubes. The molecular basis for electron and hole traps and their spatial separation on different facets is discussed.

The photoluminescence (PL) of dense nanocrystalline (anatase) TiO2 thin films is reported as a function of calcination temperature, thickness, and tungsten and nickel doping. The dependence of the optical absorption, Raman spectra, and PL spectra on heat treatment and dopants reveals the role of oxygen vacancies, crystallinity, and phase transformation in the performance of TiO2 films used as gas sensors. The broad visible PL from defect states of compact and undoped TiO2 films is found to be much brighter and less sensitive to the presence of oxygen than that of mesoporous films. The dense nanocrystalline grains and the nanoparticles comprising the mesoporous film are comparable in size, demonstrating the importance of film morphology and carrier transport in determining the intensity of defect photoluminescence. At higher calcination temperatures, the transformation to rutile results in the appearance of a dominant near-infrared peak. This characteristic change in the shape of the PL spectra demonstrates efficient capture of conduction band electrons by the emerging rutile phase. The W-doped samples show diminished PL with quenching on the red side of the emission spectrum occurring at lower concentration and eventual disappearance of the PL at higher W concentration. The results are discussed within the context of the performance of the TiO2 thin films as CO gas sensors and the chemical nature of luminescent defects.Keywords: dense nanocrystalline film; gas sensors; Ni-doping; photoluminescence; titanium dioxide; W-doping;

A mathematical model is presented that accounts for the influence of dye–semiconductor electronic coupling on the optical absorption spectrum of a molecule adsorbed on the surface of a semiconductor nanoparticle. A quantum mechanical variational approach which treats the coupling of molecule and semiconductor transition dipoles permits the shifted and broadened absorption spectrum of the dye to be accounted for with a single adjustable parameter for coupling strength. The coupling strength is determined by the individual optical spectra of the separated molecule and semiconductor systems and by the relative orientation and distance of their transition dipoles in the dye–semiconductor system. We consider the role of coupling of the dye to semiconductor transitions involving surface states versus the conduction band and show that the former can result in the frequently observed spectral broadening of adsorbed dyes. The theory is applied to model the experimental absorption spectrum of retinoic acid and carotenoic acid-sensitized TiO2 colloidal nanoparticles. The resulting fitted coupling strengths are shown to be in good agreement with an estimate based on experimental values of the dye and semiconductor transition moments.

An improved separation technique employing medium pressure liquid chromatography is used to purify betanin from beet root for use as a sensitizer in a TiO2-based dye-sensitized solar cell. The use of a blocking layer and treatment by TiCl4 were explored in order to optimize the performance of the solar cell, resulting in energy conversion efficiencies as high as 2.7%, the highest yet recorded for a DSSC containing a single unmodified natural dye sensitizer. The fluorescence spectrum of betanin in aqueous solution is reported as a function of added colloidal TiO2, demonstrating efficient electron injection. Quenching of betanin fluorescence by TiO2 permits the observation of its resonance Raman spectrum, reported here for the first time and discussed in light of recent theoretical work on the electronic structure of betanin. We report the results of stability tests under continuous illumination and suggest ways to extend the lifetime of these solar cells.Highlights► Betanin and indicaxanthin are extracted from beet root and separated using medium-pressure chromatography. ► A betanin-based dye-sensitized solar cell shows the highest energy conversion efficiency (2.7%) reported to date for an unmodified natural-dye sensitized solar cell. ► The resonance Raman spectrum of betanin is reported for the first time using colloidal TiO2 to quench its fluorescence, revealing efficient electron injection.

We report polarized resonance Raman data of tetrakis(4-sulfonato)phenyl porphyrin (TSPP) aggregates in solution and deposited on Au(111) at wavelengths resonant with the red-shifted (J-band) and blue-shifted (H-band) components of the split Soret (B) band. We also report scanning tunneling microscopy (STM) images which reveal that the aggregate on Au(111) is a nanotube with a 2 nm wall thickness which tends to flatten on the substrate. Relative Raman intensities and their dependence on polarization of the incident and scattered light are found to vary greatly for H- and J-band excitation, revealing a much greater degree of coherence for the J-band, in agreement with the resonance light scattering spectrum. The J-band transition is found to have transition moment components both parallel and perpendicular to the long axis of the nanotube, consistent with a helical nanotube structure. The intensity increase of the Q-band on aggregation and the weak intensity of the H-band in both the absorption and the resonance light scattering spectra are explained by vibronic B-Q coupling, which is permitted in the lowered site symmetry of the aggregate. The resonance Raman data presented here provide insight into the molecular basis for the hierarchal structure of the aggregate.

We report the room-temperature photoluminescence spectra of nanocrystalline TiO2 in the anatase and rutile phases and in mixed-phase samples obtained commercially (Degussa P25) and by thermal treatment of nanocrystalline anatase. The photoluminescence spectrum of anatase spans a broad range of visible wavelengths, while the much more intense rutile emission is found in the near-infrared. Photoluminescence spectra as a function of contacting fluid provide insight into the microscopic nature of the luminescence, the basis for its breadth, and the influence of solvent on inter- and intraparticle electron transfer. Anatase photoluminescence results from at least two spatially isolated trap-state distributions, one of which is absent or quenched in P25 and in the presence of hole scavengers. TiO2 nanocrystalline films containing a small amount of rutile show solvent-dependent relative intensities of the anatase and rutile photoluminescence that reveal carrier transport between the two phases. Photoluminescence spectroscopy is shown to be a useful approach for determining the energetic distribution of midband gap states.