[Show abstract][Hide abstract] ABSTRACT: 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.
The Journal of Physical Chemistry C 07/2015; 119(33):19030-19041. DOI:10.1021/acs.jpcc.5b05896 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The plant pigment betanin is investigated as a dye-sensitizer on TiO 2 with regard to its potential to undergo two-electron oxidation following one-photon excitation. Electrochemical, spectroelectrochemical and transient absorption measurements provide evidence for two-electron proton-coupled photo-oxidation leading to a quinone methide interme-diate which rearranges to 2-decarboxy-2,3-dehydrobetanin. Time-resolved spectroscopy measurements of betanin on nanocrystalline TiO 2 and ZrO 2 films were performed on femtosecond and nanosecond time-scales and provide evidence for transient species with absorption bands in the blue and the red. The results shed light on previous reports of high quantum efficiencies for electron injection and point the way to improved solar conversion efficiency of organic dye-sensitized solar cells.
Physical Chemistry of Interfaces and Nanomaterials XIII, San Diego, CA; 08/2014
[Show abstract][Hide abstract] ABSTRACT: 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.
The Journal of Physical Chemistry C 07/2014; 118(30):16831-16841. DOI:10.1021/jp500273q · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
The Journal of Physical Chemistry C 06/2013; 117(26):13654–13662. DOI:10.1021/jp402264p · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report resonance Raman spectra of individual porphyrin nanotubular aggregates of meso-tetra(4-sulfonatophenyl)porphyrin (TSPP) deposited on glass. Using a novel internal/external standard method, we show that absolute Raman cross sections of low-frequency vibrational modes are greatly enhanced by J-band excitation. We report single-aggregate resonance Raman spectra obtained without surface enhancement. Variations in the relative intensities of low- and high-frequency Raman modes of different aggregates and images of the resonance light scattering in epi-illumination reveal variations in aggregate structure and allow the possible correlation between Raman intensity and coherence to be explored. Polarized Raman spectra of individual aggregates confirm that the J-band is a composite of two closely spaced vibronically coupled transitions polarized parallel and perpendicular to the long axis of the aggregate, in accordance with our structural model of a hierarchical helical nanotube. The evolution of the Raman spectrum of a single aggregate during laser heating reveals the role of water in the assembly of structural subunits. Our experimental results provide insight into the concept of aggregation-enhanced Raman scattering.
The Journal of Physical Chemistry C 05/2013; 117(20):10856–10865. DOI:10.1021/jp404109u · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: At high concentrations, the nitrate ion alters the dynamics of ruthenium "blue dimer"-catalyzed water oxidation by Ce4+ such that the oxidation rate is enhanced and a unique reaction intermediate accumulates. This intermediate is characterized by an anomalous EPR signal, altered optical spectra, and the appearance of a new oxygen isotope-sensitive band in its resonance Raman (RR) spectrum. The-se features can be generated either by using high levels of ceric ammonium nitrate as oxidant or by adding HNO3 to solutions of the "blue dimer" undergoing Ce4+-driven catalytic turnover in nitrate-free media. This intermediate does not form if Co(III) is substituted as the oxidant, suggesting that Ce(IV) nitrate complexes are the causative agents. Use of 18O-labeled and 15N-labeled materials has established that: (1) the new RR band is not an O-O stretching mode (for example, as might be associated with formation of a peroxo species), but involves the O atom coordinated to a Ru center; (2) the O2 product does not contain an O atom derived from NO3-, eliminating several plausible pathways involving direct O-atom transfer to oxidized dimer. Although these results are surprising, similar phenomena have been reported for water oxidation catalyzed by monomeric Ru complexes. The dramatic effects observed for the "blue dimer" make it an ideal candidate for further study.
Journal of the American Chemical Society 11/2012; 134(49). DOI:10.1021/ja3093532 · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present the first report of photoluminescence spectra and images of single TiO(2) (anatase) nanotubes. In previous work using ensembles of conventional TiO(2) 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 TiO(2) 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 TiO(2) nanotubes compared to that of conventional TiO(2) 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.
[Show abstract][Hide abstract] ABSTRACT: 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 back-scattered 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 under-coordinated 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 Journal of Physical Chemistry C 05/2012; 116(19). DOI:10.1021/jp301680d · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Establishing analytical models at the nanoscale to interpret the mechanical and structural properties of vertically aligned carbon nanotubes (VACNTs) is complicated due to the nonuniformity in quality of as-grown samples and the lack of an accurate procedure to evaluate structural properties of nanotubes in these samples. In this paper, we present a comparative study of empirical methodologies to investigate the correlation between indentation resistance of multi-wall carbon nanotube (MWCNT) turfs, Raman features and the morphological properties of the turf structure using adaptive neuro-fuzzy system and probabilistic neural networks. Both methodologies provide comprehensive and innovative approaches for phenomenological modeling of VACNTs morphologies, mechanical properties and Raman Spectra using intelligent-based systems.
[Show abstract][Hide abstract] ABSTRACT: Helical porphyrin nanotubes of tetrakis(4-sulfonatophenyl)porphyrin (TSPP) were examined in DCl/D(2)O solution using resonance Raman and resonance light scattering spectroscopy to probe the influence of hydrogen bonding on the excitonic states. Atomic force microscopy reveals similar morphology for aggregates deposited from DCl/D(2)O and from HCl/H(2)O solution. Deuteration results in subtle changes to the aggregate absorption spectrum but large changes in the relative intensities of Raman modes in the J-band excited resonance Raman spectra, revealing relatively more reorganization along lower-frequency vibrational modes in the protiated aggregate. Depolarization ratio dispersion and changes in the relative Raman intensities for excitation wavelengths spanning the J-band demonstrate interference from overlapping excitonic transitions. Distinctly different Raman excitation profiles for the protiated and deuterated aggregates reveal that isotopic substitution influences the excitonic structure of the J-band. The deuterated aggregate exhibits a nearly two-fold increase in intensity of resonance light scattering as a result of an increase in the coherence number, attributed to decreased exciton-phonon scattering. We propose that strongly coupled cyclic N-mers, roughly independent of isotopic substitution, largely decide the optical absorption spectrum, while water-mediated hydrogen bonding influences the further coherent coupling among them when they are assembled into nanotubes. The results show that, similar to natural light-harvesting complexes such as chlorosomes, hydrogen bonding can have a critical influence on exciton dynamics.
[Show abstract][Hide abstract] ABSTRACT: The tunable optical properties of self-assembled chromophores are exploited by photosynthetic organisms to optimize their ability to harvest a broad range of the solar spectrum. Similarly, the efficiency of solar photovoltaic and photoelectrochemical devices depends strongly on the coincidence of the absorption spectrum of the photoactive components with the spectrum of the sun. While the possibility of borrowing ideas about light-harvesting aggregates from nature in order to improve the efficiency of solar energy conversion is quite attractive, progress to date is hindered by incomplete understanding of aggregate internal structure and its relation to excitonic states. In this Perspective, we describe our recent work on the hierarchal structure of self-assembled porphyrin aggregates that are similar to light-harvesting complexes of photosynthetic bacteria. We address the question of whether aggregation can be beneficial to dye-sensitized solar energy conversion and present promising results for a solar cell based on an abundant plant pigment that displays signatures of aggregation when adsorbed on TiO2.
[Show abstract][Hide abstract] ABSTRACT: The photoluminescence (PL) of dense nanocrystalline (anatase) TiO(2) 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 TiO(2) films used as gas sensors. The broad visible PL from defect states of compact and undoped TiO(2) 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 TiO(2) thin films as CO gas sensors and the chemical nature of luminescent defects.
[Show abstract][Hide abstract] ABSTRACT: 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.
The Journal of Physical Chemistry C 06/2011; 115(28). DOI:10.1021/jp1122954 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
Journal of Photochemistry and Photobiology A Chemistry 06/2011; 221(1):90-97. DOI:10.1016/j.jphotochem.2011.04.030 · 2.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Several characterization methods have been developed to investigate the mechanical and structural properties of vertically aligned carbon nanotubes (VACNTs). Establishing analytical models at nanoscale to interpret these properties is complicated due to the nonuniformity and irregularity in quality of as-grown samples. In this paper, we propose a new methodology to investigate the correlation between indentation resistance of multi-wall carbon nanotube (MWCNT) turfs, Raman spectra and the geometrical properties of the turf structure using adaptive neuro-fuzzy phenomenological modeling. This methodology yields a novel approach for modeling at the nanoscale by evaluating the effect of structural morphologies on nanomaterial properties using Raman spectroscopy.
Journal of Materials Science and Technology -Shenyang- 04/2011; 27(4):301-308. DOI:10.1016/S1005-0302(11)60066-2 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Well-defined monolayer islands of protonated meso-tetrakis(4-sulfonatophenyl)porphine (H2[H4TSPP]) self-assembled on highly oriented pyrolytic graphite (HOPG) from 0.75 M HCl solutions are studied in ultrahigh vacuum using scanning tunneling microscopy (STM), orbital-mediated tunneling spectroscopy (OMTS), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). HOPG proved to be a superior substrate because it does not form a thin surface chloride in the presence of HCl, as does Au(111). Unlike meso-tetrakis(4-carboxyphenyl)porphine (Hx[HyTCPP]), the carboxylate analog, monolayers are stable on HOPG and can be studied at room temperature without the addition of a second stabilizing compound. Thus, the surface organizational energy for H2[H4TSPP] is significantly stronger than that of Hx[HyTCPP]. This increased interaction is also apparent in the fact that the H2[H4TSPP] monolayer has a much smaller unit cell. It is suggested that sulfonate groups may be more useful than carboxylates for creating self-assembled structures on HOPG. Protonation of the porphyrin nitrogens in the surface species is confirmed by XPS. High-resolution images of single molecule layers show a well-defined deformation of the porphyrin ring, as expected with complete protonation of the central nitrogen atoms. OMTS and UPS were used to identify the HOMO and LUMO of the H2[H4TSPP] monolayer species, and results are contrasted to those of nickel(II) tetraphenylporphyrin (NiTPP). H2[H4TSPP] previously has been shown to form well-defined nanorods on HOPG. I−V curves of single and stacked rods taken by STM are consistent with conduction in a band formed from the LUMO of H2[H4TSPP].
The Journal of Physical Chemistry C 02/2011; 115(10). DOI:10.1021/jp112086p · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The visible photoluminescence of nanocrystalline TiO2 is examined in the presence of surface binding agents and as a function of vacuum annealing in order to probe the molecular nature of surface defects. The photoluminesence (PL) of bulk crystals of anatase TiO2 from (101) and (001) planes is also reported in order to test the hypothesis that electron and hole traps are spatially isolated on different crystal planes. We find that a number of hole scavengers are capable of quenching the PL associated with trapped electrons, while the ability of oxygen to quench PL through electron scavenging varies with the nature of the sample. We conclude that hole scavengers exert their influence on the PL through reaction with valence band holes rather than with spatially isolated trapped holes. Scavenging of electrons by O2, on the other hand, depends on adsorption at oxygen vacancies and varies with TiO2 sample.
[Show abstract][Hide abstract] ABSTRACT: 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.
The Journal of Physical Chemistry C 10/2010; 114(39):16357-16366. DOI:10.1021/jp106514g · 4.77 Impact Factor