Direct Observation of Photodriven Intermolecular Hole Transfer across TiO2 Nanocrystallites: Lateral Self-Exchange Reactions and Catalyst Oxidation

Unveiling photoinduced electron transfer in cobalt(iii)-R-pyridine complexes anchored to anatase nanocrystals: photoluminescence and magnetic studies

In this study, we synthesized mixed ligand complexes of the cis-[Co(tn)2(Rpy)Br]Br2 type using a novel mechanochemical approach. Characterization involved spectral measurements and single crystal X-ray diffraction analysis, confirming the structure of the cis-[Co(tn)2(4-Mepy)Br]Br2 complex. The single crystal refinement data revealed a monoclinic crystal system with a distorted octahedral geometry. The choice of the sixth ligand influenced the emission and magnetic properties, showing a ferromagnetic character in the Co(iii)-complex environment. We investigated efficient electron transfer to the cobalt(iii) center using TiO2 nanoparticles under UV-light irradiation. The adsorption characteristics of cis-[Co(tn)2(Rpy)Br]Br2 in aqueous 2-propanol varied, leading to surface compound formation. Under UV irradiation, the anatase surface exhibited remarkable adsorption capabilities, facilitating efficient electron transfer to the Co(iii) center and resulting in a high photoefficiency for Co(ii) formation. Our study has put forward a model for interfacial electron transfer (IET), taking into account the overlap between the TiO2 conduction band and the acceptor level of the Co center, as well as the electronic coupling between the donor level of the Ti center and the acceptor level of the Co center. This model sheds light on the accumulation of electrons for reducing the adhered complex ion. The IET process was corroborated by the conversion of 2-propanol into acetone, as verified by 1H NMR technique. Overall, our findings provide novel insights into the role of the Rpy moiety in modifying the structure of the TiO2-cobalt(iii)-Rpy compound and propose a mechanism for IET reactions, thus advancing the field.

Metal Complexes for Dye-Sensitized Photoelectrochemical Cells (DSPECs)

Since Mallouk's earliest contribution, dye-sensitized photoelectrochemical cells (DSPECs) have emerged as a promising class of photoelectrochemical devices capable of storing solar light into chemical bonds. This review primarily focuses on metal complexes outlining stabilization strategies and applications. The ubiquity and safety of water have made its splitting an extensively studied reaction; here, we present some examples from the outset to recent advancements. Additionally, alternative oxidative pathways like HX splitting and organic reactions mediated by a redox shuttle are discussed.

Electron-hopping across dye-sensitized mesoporous NiO surfaces

To gain a deeper understanding of the underlying charge processes in dye sensitized photocathodes, lateral electron hopping across dye-sensitized NiO photocathodes was investigated. For dye-sensitized systems, hole hopping across photoanodes has been studied extensively in the literature but no expansive studies on electron hopping in sensitized photocathodes exist today. Therefore, an organic p-type dye (TIP) with donor-linker-acceptor design, showing high stability and electrochemical reversibility, was used to study the electron transfer dynamics (electron-hopping) between dyes with temperature dependent spectroelectrochemistry and computational simulations. Besides intermolecular electron-hopping across the surface with a rate constant in the order of 10⁵ s^-1, our results show a second electron hopping pathway between NiO surface states with a rate constant in the order of 10⁷ s^-1, which precedes the electron hopping between the dyes. Upon application of a potential step negative enough to reduce both the dye and NiO surface states, the majority of NiO surface states need to be reduced before intermolecular electron transfer can take place. The results indicate that, in contrast to sensitized photoanodes where intermolecular charge transfer is known to influence recombination kinetics, intermolecular charge transport processes in TIP dye sensitized NiO photocathodes is less relevant because the fast electron transport between NiO surface states likely dominates recombination kinetics.

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

Reconciliation of Differences in Apparent Diffusion Coefficients Measured for Self-Exchange Electron Transfer between Molecules Anchored to Mesoporous Titanium Dioxide Thin Films

Redox-active sites present at large concentrations as part of a solid support or dissolved as molecules in fluid solutions undergo reversible self-exchange electron-transfer reactions. These processes can be monitored using a variety of techniques. Chronoamperometry and cyclic voltammetry are common techniques used to interrogate this behavior for molecules bound to mesoporous thin films of wide-bandgap semiconductors and insulators. In order to use these techniques to obtain accurate values for apparent diffusion coefficients, which are proxies for rate constants for self-exchange electron transfer, it is imperative to take into consideration nonidealities in redox titrations, parasitic currents, and ohmic resistances. Using spectroelectrochemical measurements taken concurrently with measurements of chronoamperometry data, we show that the spectroscopic data is not confounded from effects of parasitic currents or electroinactive dyes. However, we show that the thickness of the thin film over the region that is optically probed by the measurements must be known. When each of these considerations is included in data analyses, calculated apparent diffusion coefficients are, within error, independent of the method used to obtain the data. These considerations help reconcile variations in apparent diffusion coefficients measured using different techniques that have been reported over the past several decades and allow correct analyses to be performed in the future, independent of the method used to obtain the data.

An Insulating Al2O3 Overlayer Prevents Lateral Hole Hopping Across Dye-Sensitized TiO2 Surfaces

Three chromophores of the general form [Ru(bpy')₂(4,4'-(PO₃H₂)₂-2,2'-bipyridine)]²⁺, where bpy' is 4,4'-(C(CH₃)₃)₂-2,2'-bipyridine (Ru(dtb)₂P); 4,4'-(CH₃O)₂-2,2'-bipyridine (Ru(OMe)₂P), and 2,2'-bipyridine (RuP) were anchored to mesoporous thin films of TiO₂ nanocrystallites at saturation surface coverages to investigate lateral self-exchange Ru^III/II intermolecular hole hopping in 0.1 M LiClO₄/CH₃CN electrolytes. Hole hopping was initiated by a potential step 500 mV positive of the E_1/2 (Ru^III/II) potential or by pulsed laser (532 nm, 8 ns fwhm) excitation and monitored by visible absorption chronoabsorptometry and time-resolved absorption anisotropy measurements, respectively. The hole hopping rate constant k_R extracted from the potential step data revealed self-exchange rate constants that followed the trend: TiO₂|Ru(OMe)₂P (k_et = 1.4 × 10⁶ s^-1) > TiO₂|RuP (7.1 × 10⁵ s^-1) > TiO₂|Ru(dtb)₂P (6.5 × 10⁴ s^-1). Analysis of the anisotropy data with Monte Carlo simulations provided hole hopping rate constants for TiO₂|RuP and TiO₂|Ru(dtb)₂P that were within experimental error the same as that measured with the potential step. The hole hopping rate constants were found to trend with the TiO₂(e^-)|Ru^III → TiO₂|Ru^II charge recombination rate constants. The atomic layer deposition of an ∼10 Å layer of Al₂O₃ on top of the dye-sensitized films was found to prevent hole hopping by both initiation methods even though the chromophore surface coverage exceeded the percolation threshold and excited-state injection was efficient. The dramatic hole hopping turnoff was attributed to a larger outer-sphere reorganization energy for self-exchange due to the restricted access of electrolyte to the redox active chromophores. The implications of these findings for solar energy conversion applications are discussed.

The Effect of Chloride Anions on Charge Transfer in Dye-Sensitized Photoanodes for Water Splitting

The photoelectrochemical behavior of dye-sensitized photoelectrochemical cells based on a TiO₂ layer sensitized with ruthenium components, including an absorber, ruthenium(II)bis(2,2'-bipyridine)([2,2'-bipyridine]-4,4'-diylbis(phosphonic acid)) dibromide (RuP), and a catalyst, ruthenium(II) tris(4-methylpyridine)(4-(4-(2,6-bis((l1-oxidanyl)carbonyl)pyridin-4-yl)phenyl) pyridine-2,6-dicarboxylic acid) (RuOEC), was investigated in the following water-based electrolyte configurations: KCl (pH ≈ 5), HCl (pH ≈ 3), ethylphoshonic acid (pH ≈ 3) with a different KCl concentration, and a standard phosphate buffer (pH ≈ 7). The rate of charge transfer on the photoanode's surface was found to increase in line with the increase in the concentration of chloride anions (Cl^-) in the low pH electrolyte. This effect is discussed in the context of pH influence, ionic strength, and specific interaction, studied by cyclic voltammetry (CV) in dark conditions and upon illumination of the photoanodes. The correlations between photocurrent decay traces and CV studies were also observed.

Factors influencing the photoelectrochemical device performance sensitized by ruthenium polypyridyl dyes

The subtle variation in the alkyl substituents of the ruthenium photosensitizers controls the photo-electrochemical performance of the dye-only DSPECs (do-DSPECs) and those with sacrificial agent (sa-DSPECs).

Multimolecular assemblies on high surface area metal oxides and their role in interfacial energy and electron transfer

High surface area metal oxides offer a unique substrate for the assembly of multiple molecular components at an interface.

Ultrafast electron and hole transfer dynamics of a solar cell dye containing hole acceptors on mesoporous TiO2 and Al2O3

The stability of dye cations against recombination with conduction band electrons in mesoporous TiO₂ electrodes is a key property for improving light harvesting in dye-sensitised solar cells. Using ultrafast transient broadband absorption spectroscopy, we monitor efficient intramolecular hole transfer in the solar cell dye E6 having two peripheral triarylamine acceptors. After photoexcitation, two hole transfer mechanisms are identified: a concerted mechanism for electron injection and hole transfer (2.4 ps) and a sequential mechanism with time constants of 3.9 ps and 30 ps. This way the dye retards unwanted recombination with a TiO₂ conduction band electron by quickly moving the hole further away from the surface. Contact of the E6/TiO₂ surface with the solvent acetonitrile has almost no influence on the electron injection and hole transfer kinetics. Fast hole transfer (2.8 ps) is also observed on a "non-injecting" Al₂O₃ surface generating a radical cation-radical anion species with a lifetime of 530 ps. The findings confirm the good intramolecular hole transfer properties of this dye on both thin films. In contrast, intramolecular hole transfer does not occur in the mid-polar organic solvent methyl acetate. This is confirmed by TDDFT calculations suggesting a polarity-induced reduction of the driving force for hole transfer. In methyl acetate, only the relaxation of the initially photoexcited core chromophore is observed including solvent relaxation processes of the electronically excited state S₁/ICT.

Quantifying ultrafast charge carrier injection from methylammonium lead iodide into the hole-transport material H101 and mesoporous TiO2 using Vis-NIR transient absorption

We characterise time scales and yields of hole transfer and electron injection at lead perovskite interfaces with the hole-transport material and mesoporous TiO₂.

How mobile are dye adsorbates and acetonitrile molecules on the surface of TiO2 nanoparticles? A quasi-elastic neutron scattering study

Motions of molecules adsorbed to surfaces may control the rate of charge transport within monolayers in systems such as dye sensitized solar cells. We used quasi-elastic neutron scattering (QENS) to evaluate the possible dynamics of two small dye moieties, isonicotinic acid (INA) and bis-isonicotinic acid (BINA), attached to TiO2 nanoparticles via carboxylate groups. The scattering data indicate that moieties are immobile and do not rotate around the anchoring groups on timescales between around 10 ps and a few ns (corresponding to the instrumental range). This gives an upper limit for the rate at which conformational fluctuations can assist charge transport between anchored molecules. Our observations suggest that if the conformation of larger dye molecules varies with time, it does so on longer timescales and/or in parts of the molecule which are not directly connected to the anchoring group. The QENS measurements also indicate that several layers of acetonitrile solvent molecules are immobilized at the interface with the TiO2 on the measurement time scale, in reasonable agreement with recent classical molecular dynamics results.

Dye monolayers used as the hole transporting medium in dye-sensitized solar cells

Dye-sensitized TiO2 can be used as the active layer of solar-cell devices without an additional hole-transporting material. In this architecture, holes are transported through the dye monolayer.

Towards multielectron photocatalysis: a porphyrin array for lateral hole transfer and capture on a metal oxide surface

“An artificial photosynthetic model system is reported consisting of a porphyrin monolayer on a SnO₂ surface that enables oxidizing equivalents to be efficiently delivered to a thermodynamic trap via lateral hole transfer.”

High-performance photoelectrochemical cells based on a binuclear ruthenium catalyst for visible-light-driven water oxidation

Two photoanodes based on a binuclear (2) and a mononuclear ruthenium (3) water oxidation catalysts were assembled in combination with a molecular photosensitizer (1) by using a co-adsorption method. The anodes were used in dye-sensitized photoelectrochemical cells (DS-PECs) for visible-light-driven water splitting. A DS-PEC device using TiO2 (1+2) as working electrode (WE) exhibits better performance than TiO2 (1+3) as WE in light-driven water splitting. Detailed photoelectrochemical studies on these DS-PEC devices are discussed.

Effects of electron trapping and protonation on the efficiency of water-splitting dye-sensitized solar cells

Water-splitting dye-sensitized photoelectrochemical (WS-DSPECs) cells employ molecular sensitizers to absorb light and transport holes across the TiO2 surface to colloidal or molecular water oxidation catalysts. As hole diffusion occurs along the surface, electrons are transported through the mesoporous TiO2 film. In this paper we report the effects of electron trapping and protonation in the TiO2 film on the dynamics of electron and hole transport in WS-DSPECs. When the sensitizer bis(2,2'-bipyridine)(4,4'-diphosphonato-2,2'-bipyridine)ruthenium(II) is adsorbed from aqueous acid instead of from ethanol, there is more rapid hole transfer between photo-oxidized sensitizer molecules that are adsorbed from strong acid. However, the photocurrent and open-circuit photovoltage are dramatically lower with sensitizers adsorbed from acid because intercalated protons charge-compensate electron traps in the TiO2 film. Kinetic modeling of the photocurrent shows that electron trapping is responsible for the rapid electrode polarization that is observed in all WS-DSPECs. Electrochemical impedance spectroscopy suggests that proton intercalation also plays an important role in the slow degradation of WS-DSPECs, which generate protons at the anode as water is oxidized to oxygen.

Photogenerated carriers transfer in dye-graphene-SnO2 composites for highly efficient visible-light photocatalysis

The visible-light-driven photocatalytic activities of graphene-semiconductor catalysts have recently been demonstrated, however, the transfer pathway of photogenerated carriers especially where the role of graphene still remains controversial. Here we report graphene-SnO2 aerosol nanocomposites that exhibit more superior dye adsorption capacity and photocatalytic efficiency compared with pure SnO2 quantum dots, P25 TiO2, and pure graphene aerosol under the visible light. This study examines the origin of the visible-light-driven photocatalysis, which for the first time links to the synergistic effect of the cophotosensitization of the dye and graphene to SnO2. We hope this concept and corresponding mechanism of cophotosensitization could provide an original understanding for the photocatalytic reaction process at the level of carrier transfer pathway as well as a brand new approach to design novel and versatile graphene-based composites for solar energy conversion.

Simultaneous measurement of absorbance and quantum yields for photocurrent generation at dye-sensitized single-crystal ZnO electrodes

It is often assumed that the photoresponse or incident photon-to-current conversion efficiency (IPCE) spectrum of a sensitized semiconductor electrode is directly correlated with the amount of sensitizing species present on the semiconductor surface. In reality, the various forms of adsorbed species, such as dye aggregates or dye molecules bound to different adsorption sites, such as terrace edges, can have significantly different electron injection yields and carrier recombination rates. To provide information about the amounts of the various adsorbed dye species and their effectiveness as sensitizers, we report the simultaneous acquisition of IPCE and attenuated total reflectance (ATR) UV-vis spectra for a thiacyanine dye bound to a single-crystal oxide semiconductor electrode surface. ZnO single crystals were fashioned into internal-reflection elements to act both as a waveguide for the internally reflected probe beam for UV-vis spectra and as the substrate for dye sensitization using dyes with distinct spectral signatures for monomers and aggregates. Strong agreement was observed between the quantum efficiency and ATR UV-vis spectra, suggesting that, under the conditions employed, both monomers and aggregates of the dye studied generate photocurrent with the same efficiency.

Stabilizing small molecules on metal oxide surfaces using atomic layer deposition

Device lifetimes and commercial viability of dye-sensitized solar cells (DSSCs) and dye-sensitized photoelectrosynthesis cells (DSPECs) are dependent on the stability of the surface bound molecular chromophores and catalysts. Maintaining the integrity of the solution-metal oxide interface is especially challenging in DSPECs for water oxidation where it is necessary to perform high numbers of turnovers, under irradiation in an aqueous environment. In this study, we describe the atomic layer deposition (ALD) of TiO2 on nanocrystalline TiO2 prefunctionalized with the dye molecule [Ru(bpy)2(4,4'-(PO3H2)bpy)](2+) (RuP) as a strategy to stabilize surface bound molecules. The resulting films are over an order of magnitude more photostable than untreated films and the desorption rate constant exponentially decreases with increased thickness of ALD TiO2 overlayers. However, the injection yield for TiO2-RuP with ALD TiO2 also decreases with increasing overlayer thickness. The combination of decreased injection yield and 95% quenched emission suggests that the ALD TiO2 overlayer acts as a competitive electron acceptor from RuP*, effectively nonproductively quenching the excited state. The ALD TiO2 also increases back electron transfer rates, relative to the untreated film, but is independent of overlayer thickness. The results for TiO2-RuP with an ALD TiO2 overlayer are compared with similar films having ALD Al2O3 overlayers.

Accumulation of multiple oxidative equivalents at a single site by cross-surface electron transfer on TiO2

The photodriven accumulation of two oxidative equivalents at a single site was investigated on TiO2 coloaded with a ruthenium polypyridyl chromophore [Ru(bpy)2((4,4'-(OH)2PO)2bpy)](2+) (Ru(II)P(2+), bpy = 2,2'-bipyridine, ((OH)2PO)2-bpy = 2,2'-bipyridine-4,4'-diyldiphosphonic acid) and a water oxidation catalyst [Ru(Mebimpy) ((4,4'-(OH)2PO-CH2)2bpy)(OH2)](2+) (Ru(II)OH2(2+), Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine, (4,4'-(OH)2PO-CH2)2bpy) = 4,4'-bis-methlylenephosphonato-2,2'-bipyridine). Electron injection from the metal-to-ligand charge transfer (MLCT) excited state of -Ru(II)P(2+) (-Ru(II)P(2+)*) to give -Ru(III)P(3+) and TiO2(e(-)) was followed by rapid (<20 ns) nearest-neighbor -Ru(II)OH2(2+) to -Ru(III)P(3+) electron transfer. On surfaces containing both -Ru(II)P(2+) and -Ru(III)OH2(3+) (or -Ru(III)OH(2+)), -Ru(II)OH2(2+) was formed by random migration of the injected electron inside the TiO2 nanoparticle and recombination with the preoxidized catalyst, followed by relatively slow (μs-ms) non-nearest neighbor cross-surface electron transfer from -Ru(II)OH2(2+) to -Ru(III)P(3+). Steady state illumination of coloaded TiO2 photoanodes in a dye sensitized photoelectrosynthesis cell (DSPEC) configuration resulted in the buildup of -Ru(III)P(3+), -Ru(III)OH(2+), and -Ru(IV)═O(2+), with -Ru(IV)═O(2+) formation favored at high chromophore to catalyst ratios.

Intermolecular Interactions in Dye-Sensitized Solar Cells: A Computational Modeling Perspective

We present a unified overview of our recent activity on the modeling of relevant intermolecular interactions occurring in dye-sensitized solar cells (DSCs). The DSC is an inherent complex system, whose efficiency is essentially determined by the interrelated phenomena occurring at the multiple molecular-semiconductor-electrolyte heterointerfaces. In this Perspective, we illustrate the basic methodology and selected applications of computational modeling of dye-dye and dye-coadsorbent intermolecular interactions taking place at the dye-sensitized interface. We show that the proposed methodology offers a realistic picture of aggregation phenomena among surface-adsorbed dyes and nicely describes semiconductor surfaces cosensitized by different dyes. The information acquired from this type of studies might constitute the basis for an integrated multiscale computational description of the device functioning, including all of the possible interdependencies among the device constituents, which may further boost the DSCs efficiency. We believe that this direction should be the target of future computational research in the DSC field.