Photochemical Charge Transfer Research Articles

Mechanistic insights into CO2 conversion to CO using cyano manganese complexes.

Without the use of a photosensitizer, [Mn(bpy)(CO)3(CN)] (MnCN) can photochemically form [Mn(bpy)(CO)3]-, the active species for CO2 reduction. While cases of the axial X-ligand dissociating upon irradiation of fac-[M(N-N)(CO)3X] complexes (M = Mn or Re; N-N = bipyridine (bpy) ligand; X = halogen or pseudohalogen) are well documented, the axial cyanide ligand is retained when either [Mn(bpy)(CO)3(CN)] or [Mn(mesbpy)(CO)3(CN)], MnCN(mesbpy), are irradiated anaerobically. Infrared and UV-vis spectroscopies indicate the formation of [Mn(bpy)(CO)2(MeCN)(CN)] (s-MnCN) as the primary product during the irradiation of MnCN. An in-depth analysis of the photochemical mechanism for the formation of [Mn(bpy)(CO)3]- from MnCN is presented. MnCN(mesbpy) is too sterically hindered to undergo the same photochemical mechanism as MnCN. However, MnCN(mesbpy) is found to be electrocatalytically active for CO2 reduction to CO. Thus providing an interesting distinction between photochemical and electrochemical charge transfer.

(Invited) Surface Photovoltage Spectroscopy Observes Junctions and Carrier Separation in Gallium Nitride Nanowire Arrays for Overall Water-Splitting

Gallium nitride (GaN) nanowire arrays on silicon are able to drive the overall water-splitting reaction with up to 3.3% solar-to-hydrogen efficiency. Photochemical charge separation is key to the operation of these devices, but details are difficult to observe experimentally because of the number of components and interfaces. Here we use surface photovoltage spectroscopy (SPS) to study charge transfer in i- and n- and p-GaN nanowire arrays on n+-Si wafers in the presence and absence of Rh/Cr2O3 co-catalysts. The effect of the space charge layer and sub-bandgap defects on majority and minority carrier transport can be clearly observed and estimates of the built-in potential of the junctions can be made. Transient illumination of the p-GaN/n+-Si junction generates up to -1.4 V surface photovoltage by carrier separation along the GaN nanowire axis. This process is central to the overall water-splitting function of n+-Si/p-GaN/Rh/Cr2O3 nanowire array. These results improve our understanding of photochemical charge transfer and separation in group III-V semiconductor nanostructures for the conversion of solar energy into fuels. Figure 1

Surface photovoltage spectroscopy observes junctions and carrier separation in gallium nitride nanowire arrays for overall water-splitting.

Gallium nitride (GaN) nanowire arrays on silicon are able to drive the overall water-splitting reaction with up to 3.3% solar-to-hydrogen efficiency. Photochemical charge separation is key to the operation of these devices, but details are difficult to observe experimentally because of the number of components and interfaces. Here, we use surface photovoltage spectroscopy to study charge transfer in i-, n-, and p-GaN nanowire arrays on n+-Si wafers in the presence and absence of Rh/Cr2O3 co-catalysts. The effect of the space charge layer and sub-bandgap defects on majority and minority carrier transport can be clearly observed, and estimates of the built-in potential of the junctions can be made. Transient illumination of the p-GaN/n+-Si junction generates up to -1.4 V surface photovoltage by carrier separation along the GaN nanowire axis. This process is central to the overall water-splitting function of the n+-Si/p-GaN/Rh/Cr2O3 nanowire array. These results improve our understanding of photochemical charge transfer and separation in group III-V semiconductor nanostructures for the conversion of solar energy into fuels.

Interface Engineering of MoS2 -Modified Graphitic Carbon Nitride Nano-photocatalysts for an Efficient Hydrogen Evolution Reaction.

Understanding of photochemical charge transfer processes at nanoscale heterojunctions is essential in developing effective catalysts. Here, we utilize a controllable synthesis method and a combination of optical absorption, photoluminescence, and electrochemical impedance spectroscopic studies to investigate the effect of MoS2 nanosheet lateral dimension and edge length size on the photochemical behavior of MoS2 -modified graphitic carbon nitride (g-C3 N4 ) heterojunctions. These nano-heterostructures, which comprise interlayer junctions with variable area (i. e., MoS2 lateral size ranges from 18 nm to 52 nm), provide a size-tunable interfacial charge transfer through the MoS2 /g-C3 N4 contacts, while exposing a large fraction of surface MoS2 edge sites available for the hydrogen evolution reaction. Importantly, modification of g-C3 N4 with MoS2 layers of 39±5 nm lateral size (20 wt % loading) creates interfacial contacts with relatively large number of MoS2 edge sites and efficient electronic transport phenomena, yielding a high photocatalytic H2 -production activity of 1497 μmol h-1 gcat -1 and an apparent QY of 3.3 % at 410 nm light irradiation. This study thus offers a design strategy to improve light energy conversion efficiency of catalysts by engineering interfaces at the nanoscale in 2D-layered heterojunction materials.

Non-adiabatic molecular dynamics of molecules in the presence of strong light-matter interactions.

When the interaction between a molecular system and confined light modes in an optical or plasmonic cavity is strong enough to overcome the dissipative process, hybrid light-matter states (polaritons) become the fundamental excitations in the system. The mixing between the light and matter characters modifies the photophysical and photochemical properties. Notably, it was reported that these polaritons can be employed to control photochemical reactions, charge and energy transfer, and other processes. In addition, according to recent studies, vibrational strong coupling can be employed to resonantly enhance the thermally-activated chemical reactions. In this work, a theoretical model and an efficient numerical method for studying the dynamics of molecules strongly interacting with quantum light are developed based on nonadiabatic excited-state molecular dynamics. The methodology was employed to study the cis-trans photoisomerization of a realistic molecule in a cavity. Numerical simulations demonstrate that the photochemical reactions can be controlled by tuning the properties of the cavity. In the calculated example, the isomerization is suppressed when polaritonic states develop a local minimum on the lower polaritonic state. Moreover, the observed reduction of isomerization is tunable via the photon energy and light-molecule coupling strength. However, the fluctuation in the transition dipole screens the effect of light-matter, which makes it harder to tune the photochemical properties via the coupling strength. These insights suggest quantum control of photochemical reactions is possible by specially designed photonic or plasmonic cavities.

(Invited) Using Surface Photovoltage Spectroscopy to Observe Photovoltage Generation at the Interfaces of Cu2o, BiVO4, and Rh:SrTiO3 Particles

Surface photovoltage spectroscopy (SPS) allows contact-less measurement of the Galvani photopotential at the interfaces of particles under illumination. Here we use SPS to observe the light intensity and wavelength dependent photopotential in a Ru–SrTiO3:Rh/BiVO4 particle tandem overall water splitting photocatalyst and in particle layers made of Cu2O and BiVO4. For the tandem, a photovoltage develops above 2.20 eV, the effective band gap of the tandem, and reaches its maximal value of −2.45 V at 435 nm (2.44 mW cm–2), which corresponds to 96% of the theoretical limit of the photocatalyst film on the fluorine-doped tin-oxide-coated glass (FTO) substrate. Charge separation is 82% reversible with 18% of charge carriers being trapped in defect states. The unusually strong light intensity dependence of the photovoltage (1.16 V per decade) is attributed to depletion layer changes inside of the BiVO4 microcrystals. Isolated Cu2O particle layers on FTO show p-type behavior and BiVO4 particles show n-type behavior. In a FTO/Cu2O/BiVO4 particle stack, both particles act in tandem, producing up to -2.1 V photovoltage under 2.5 eV excitation, which corresponds to 90% of the ideal limit. The photovoltage spectra also reveal the effects of shading at high excitation energy and the effects of low absorber strength at low excitation energy. These results are relevant to the understanding of photochemical charge transfer at irregular particle surfaces and to the construction of devices for artificial photosynthesis and water splitting. Figure 1

(Invited) Quantum Confinement Controls Effective Band Gap, Photocatalytic H2 Evolution and Photovoltage in CdSe Nanocrystals

Here, we provide the first quantitative analysis of quantum-size-controlled photocatalytic H2 evolution at the semiconductor–solution interface. We find that the hydrogen evolution rate from suspended mercaptoethanol-ligated CdSe quantum dots in aqueous sodium sulfite solution depends on nanocrystal size. Photoelectrochemical measurements on CdSe nanocrystal films reveal that the observed reactivity is controlled by the free energy change of the system, as determined by the proton reduction potential and the quasi-Fermi energy of the dots. The corresponding free energy change can be fitted to the photocatalytic activity using a modified Butler–Volmer equation for reaction kinetics. In parallel, Surface photovoltage spectroscopy (SPS) was used to photochemical charge transfer at the CdSe quantum dot -indium doped tin oxide (ITO) interface. Negative photovoltages are observed under super band gap illumination from majority carrier (electron) injection into the ITO substrate. The photovoltage onset energies track the optical band gaps of the samples and are assigned as effective band gaps of the films. The photovoltage values (−125 to −750 mV) vary with quantum dot sizes and are modulated by the built-in potential of the CdSe–ITO Schottky type contacts. Deviations from the ideal Schottky model are attributed to Fermi level pinning in states approximately 1.1 V negative of the ITO conduction band edge. Positive photovoltage signals of +80 to +125 mV in films of >4.0 nm nanocrystals and in thin (70 nm) nanocrystal films are attributed to electron–hole (polaron) pairs that are polarized by a space charge layer at the CdSe–ITO boundary. The space charge layer is 70–150 nm wide, based on thickness-dependent photovoltage measurements. These findings establish a quantitative experimental basis for quantum-confinement-controlled proton reduction and electron transfer with semiconductor nanocrystals. Figure 1

Surface Photovoltage Spectroscopy Resolves Interfacial Charge Separation Efficiencies in ZnO Dye-Sensitized Solar Cells

Any optimization of dye-sensitized solar cells (DSSCs) must consider the energetics and charge transfer kinetics of the dye, substrate, and redox couple. Here, we use surface photovoltage spectroscopy to probe the energetics and photochemical charge transfer efficiency in fluorenyl-thiophene dye (OD-8)-sensitized ZnO films. Discrete photochemical charge transfer events at the dye–ZnO interface and at the dye– I–/I3– or [Co(2,2′-bipyridyl)3]3+/2+ interfaces can be observed as negative photovoltage under dye excitation at 1.7 eV (460 nm). Without a redox couple, charge separation at the ZnO/dye interface is only 4% effective, likely due to the short electron hole separation distance. In the presence of the redox couples, charge separation approaches 26–54% of the theoretical limit, emphasizing the importance of the dye regeneration reaction via the redox couple. On the basis of the open circuit voltage, charge separation in fully assembled DSSCs is 100% efficient with iodide, but only 61% efficient with the...

Wavelength dependent photochemical charge transfer at the Cu2O-BiVO4 particle interface - evidence for tandem excitation.

The understanding of the photochemical charge transfer properties of powdered semiconductors is of relevance to artificial photosynthesis and the production of solar fuels. Here we use surface photovoltage spectroscopy to probe photoelectrochemical charge transfer between bismuth vanadate (BiVO4) and cuprous oxide (Cu2O) particles as a function of wavelength and film thickness. Optimized conditions produce a -2.10 V photovoltage under 2.5 eV (0.1 mW cm-2) illumination, which suggests the possibility of a water splitting system based on a BiVO4-Cu2O direct contact particle tandem.

Photochemical charge separation at particle interfaces: the n-BiVO4-p-silicon system.

The charge transfer properties of interfaces are central to the function of photovoltaic and photoelectrochemical cells and photocatalysts. Here we employ surface photovoltage spectroscopy (SPS) to study photochemical charge transfer at a p-silicon/n-BiVO4 particle interface. Particle films of BiVO4 on an aluminum-doped p-silicon wafer were obtained by drop-coating particle suspensions followed by thermal annealing at 353 K. Photochemical charge separation of the films was probed as a function of layer thickness and illumination intensity, and in the presence of methanol as a sacrificial electron donor. Electron injection from the BiVO4 into the p-silicon is clearly observed to occur and to result in a maximum photovoltage of 150 mV for a 1650 nm thick film under 0.3 mW cm(-2) illumination at 3.5 eV. This establishes the BiVO4-p-Si interface as a tandem-like junction. Charge separation in the BiVO4 film is limited by light absorption and by slow electron transport to the Si interface, based on time-dependent SPS measurements. These problems need to be overcome in functional tandem devices for photoelectrochemical water oxidation.

Photochemical charge transfer observed in nanoscale hydrogen evolving photocatalysts using surface photovoltage spectroscopy

The application of inorganic nanostructures for solar water splitting is currently limited by our understanding of photochemical charge transfer on the nanoscale, where space charge layers are less effective for carrier separation.

Use of potential determining ions to control energetics and photochemical charge transfer of a nanoscale water splitting photocatalyst

Tetrabutylammonium (TBA) stabilized H[Ca2Nb3O10] nanosheets catalyze hydrogen evolution from aqueous methanol under illumination with UV light. Here we show that surface treatment with protons, potassium, and strontium potential-determining cations (PDIs) in aqueous solution modifies the electrostatic, energetic and photocatalytic properties of this nanomaterial. Attachment of cations to the nanocrystals was verified with elemental dispersive spectroscopy. Zeta potentials were measured as −40 mV (TBA+, pH = 4.8), −50 mV (K+, pH = 4.3), and −20 mV (Sr2+, pH = 4.4). Photoelectrochemical measurements in methanol containing 0.1 M tetraethylammonium chloride revealed anodic current photoonset potentials/Fermi energies ranging between −0.59 V (Sr2+) and −0.71 V (at pH = 7, vs. NHE). The photocatalytic proton reduction ability of the modified nanocrystals was assessed in aqueous methanol at pH = 1. Here, KxH1−x[Ca2Nb3O10] evolved hydrogen at 350 μmol H2 h−1, SrxH1−2x[Ca2Nb3O10] at 70 μmol H2 h−1, and H[Ca2Nb3O10] at 160 μmol H2 h−1. In addition, the photocatalytic activity was found to increase (20–160 μmol H2 h−1) with solution pH. These observed activity variations can be quantitatively understood using a linear free energy relationship between the proton reduction rate constant and the free energy of proton reduction. This shows that the photocatalytic activity of the nanocrystals depends on the electrochemical potentials/Fermi energies of the modified catalysts. The effect of the PDI charge on the nanomaterial energetics can be rationalized by considering the surface potential. The latter can be related to the particle surface charge and the concentration of counterions in solution using the Grahame equation. These results provide a quantitative basis for the understanding and manipulation of nanomaterial photocatalysts with PDIs.

(Invited) Photochemical Charge Transfer in Niobium Oxide Nanocrystal Films Studied With Surface Photovoltage Spectroscopy

not Available.

ChemInform Abstract: Kinetics of Fast Photochemical Charge Separation and Charge Recombination Reactions

AbstractReview: 187 refs.

ChemInform Abstract: Electron Transfer Processes in Imaging

Many photoimaging technologies involve electron-transfer processes in the critical image capture, or light-sensitive, step of the process. This chapter reviews the photochemical and photophysical basis for these processes. The chapter is divided into several major sections: an introduction to basic imaging concepts; a brief discussion of the chemistries and photochemistries involved in conventional silver halide based imaging processes and those involving other semiconductors as primary light absorbers; and sections describing less conventional processes that are based on organic photochemical charge transfer processes (electrophotography, color formation systems, and photopolymerization processes).

Bonded Exciplex Formation: Electronic and Stereoelectronic Effects

As recently proposed, the singlet-excited states of several cyanoaromatics react with pyridine via bonded-exciplex formation, a novel concept in photochemical charge transfer reactions. Presented here are electronic and steric effects on the quenching rate constants, which provide valuable support for the model. Additionally, excited-state quenching in poly(vinylpyridine) is strongly inhibited both relative to that in neat pyridine and also to conventional exciplex formation in polymers, consistent with a restrictive orientational requirement for the formation of bonded exciplexes. Examples of competing reactions to form both conventional and bonded exciplexes are presented, which illustrate the delicate balance between these two processes when their reaction energetics are similar. Experimental and computational evidence is provided for the formation of a bonded exciplex in the reaction of the singlet excited state of 2,6,9,10-tetracyanoanthracene (TCA) with an oxygen-substituted donor, dioxane, thus expanding the scope of bonded exciplexes.

On the non-adiabatic dynamics of solvation: A molecular hydrodynamic formulation

An extended hydrodynamic formulation is developed to describe non-adiabatic solvation dynamics for coupled electronic states. Starting from a hybrid quantum–classical kinetic equation, coupled equations for the solvent local density and momentum density are derived which feature the characteristic population/coherence coupling of the non-adiabatic quantum evolution. These equations have the same formal structure as the non-adiabatic quantum hydrodynamic equations described in [I. Burghardt, L.S. Cederbaum, J. Chem. Phys. 115 (2001) 10312]. In accordance with the molecular hydrodynamics approach, a generalized free energy functional is introduced, adapted to the hybrid quantum–classical setting. A microscopic Marcus-type functional for polar solvation is considered as a special case. The present formulation is particularly appropriate to describe ultrafast solvation dynamics coupled with charge transfer, for example in photochemical charge transfer processes. By the explicit consideration of quantum coherence, the details of the population transfer and its susceptibility to decoherence effects, become amenable to direct investigation.

Photochemical charge transfer and trapping at the interface between an organic adlayer and an oxide semiconductor.

Identification of charge transfer and trapping sites on semiconducting oxide surfaces is of fundamental importance in furthering the field of heterogeneous photocatalysts. Using scanning tunneling microscopy, electron energy loss spectroscopy, and photodesorption, we observed both electron trapping and hole transfer events on the (110) surface of TiO2 rutile. UV irradiation of a saturated monolayer of trimethyl acetate (TMA) on TiO2(110) at room temperature resulted in hole transfer to the carboxylate group, followed by (CH3)3C-COO bond cleavage and desorption of CO2 and isobutene/isobutane. Hole transfer to TMA proceeded in the absence of a gas-phase electron scavenger (which is typically O2) because the accompanying photogenerated electrons could be trapped at the surface as Ti3+ cations bound to bridging OH groups. The extent of electron trapping, gauged by electron spectroscopy, correlated directly with the yields of photodesorption fragments resulting from the hole transfer channel. Charge at the Ti3+ sites was titrated in the dark via a reaction between O2 and the Ti3+-OH groups.

Photokatalytische Aktivierung von molekularem Sauerstoff mittels Eisen(III)porphyrin-Komplexen

Photochemical charge transfer excitation of tetra-phenyl(porphyrinato)iron(III) complexes yields tetraphenyl) (porphyrinato)iron(II) which is able to coordinate molecular oxygen under formation of oxo-[tetraphenyl(porphyrinato)]- iron(IV). Based on this photochemical reaction pathway photocatalytic oxygenation of α-pinene and other alkenes can be initiated. Iron(III) complexes of tetramesitylporphyrin, tetrakis-(pentafluorophenyl)porphyrin, octa-β-bromo-tetrakis-(pentafluorophenyl)porphyrin, and octa-β-chloro-tetrakis-(pentafluorophenyl)porphyrin were investigated photochemically with the aim to improve the low photochemical efficiency of tetraphenyl(porphyrinato)iron(III). The influence of substituents on the porphyrin ligand on the photochemical behavior of the corresponding iron(III) complexes is measured mainly by means of temperature dependent UV/Vis spectroscopy. Both, the yield of oxygenation products formed photocatalytically with α-pinene and the product distribution (allylic alcohols versus epoxide) depend on the design of the porphyrin ligands coordinated with iron(III).

Photochemical charge transfer excitation of trans-4-stilbazole at a silver electrode

When adsorbed at the surface of a polycrystalline silver electrode, trans-4-stilbazole photoreacts, in the presence of dissolved oxygen, to form trans-4′-hydroxy-4-stilbazole. The structure of the photoproduct is confirmed by (1) a comparison of the surface Raman results with the normal Raman of trans-4′-hydroxy-4-stilbazole, and by (2) by ex-situ analysis of the irradiated electrode surface using high-resolution mass spectrometry. The surface photochemistry appears to be a one-photon process whose threshold wavelength lies in the middle of the visible range though neat trans-4-stilbazole absorbs only in the ultraviolet. The surface Raman intensity of the photoproduct increases synchronously with the Faradaic current of oxygen reduction. Attendant incorporation of oxygen exclusively at the 4′ position of end-on trans-4-stilbazole evidences a photo-initiated electron transfer from the electrode to oxygen reducing at the other end of the molecule. Scattering from stilbazole adsorbed at two different surface sites is evident in the surface Raman spectrum: charged (Ag +) sites and more neutral surface sites. We assign excitation profile maxima for stilbazole at Ag + sites and the photochemical initiation to a silver-to-stilbazole charge transfer at those sites. Ab-initio molecular orbital calculations on a model composed of a ground state Li 0- trans-4-stilbazole and the anion radical excited state Li +- trans-4-stilbazole − reveal a low-energy transition that supports assignment of the visible excitation to a silver-to-stilbazole electron transfer.