Figure 1
(a) General representation of a dye-sensitized TiO2film, (b) representation of the excitation of the chromophore and formation of a charge-separated state in a TiO2film sensitized by a ruthenium(II) dye, and (c) schematic representation of the working principles of a typical TiO2molecular device for solar energy conversion, featuring the excitation of the dye sensitizer, excited state electron injection into TiO2and some of the electron transfer reactions that may follow the charge-separation step.
Figure 2
Representation of an electron transfer reaction between a donor (D) and an acceptor (A). The nuclear configurations of the reactants and of the surrounding medium must partially reorganize before the electron transfer occurs.
Figure 3
Reaction Gibbs free energy surfaces as a function of the nuclear configuration for a nonadiabatic electron transfer reaction. The dashed arrow represents the reaction pathway.
Figure 4
(a) Parabolic dependence of -ΔG‡on the driving force -ΔG° for electron transfer and (b) schematic representation of free Gibbs energy surfaces along the nuclear configuration of a nonadiabatic electron transfer for four kinetic regimes: self-exchange reactions (ΔG° = 0), normal regime for weak exergonic reactions (0 < -ΔG° < λ), activationless reactions (ΔG‡= 0 and ΔG° = λ), and inverted electron transfer for highly exergonic reactions (λ< -ΔG° < 0).
Figure 5
Schematic energy surfaces of the reactants and products and the vibronic overlap between them, which is represented by the gray regions (adapted from reference 8383 Barbara, P. F.; Meyer, T. J.; Ratner, M. A.; J. Phys. Chem. 1996, 100, 13148.).
Figure 6
(a) Energy diagram for the oxidation or reduction of a solvated molecular species according to the Gerischer model and (b) the corresponding energy distributions (adapted from reference 8787 Memming, R; Semiconductor Electrochemistry, 2nd ed.; Wiley: Weinheim, Germany, 2015.).
Figure 7
Schematic Gerischer diagram illustrating the interfacial electron injection from an excited sensitizer into the acceptor states of TiO2 and subsequent back-electron transfer to the oxidized sensitizer (adapted from references 113113 Watson, D. F.; Meyer, G. J.; Annu. Rev. Phys. Chem. 2005, 56, 119.
114 Ardo, S.; Meyer, G. J.; Chem. Soc. Rev. 2009, 38, 115.-115115 Aghazada, S.; Nazeeruddin, M. K.; Inorganics 2018, 6, 34.).
Figure 2
Representation of an electron transfer reaction between a donor (D) and an acceptor (A). The nuclear configurations of the reactants and of the surrounding medium must partially reorganize before the electron transfer occurs.
Figure 3
Reaction Gibbs free energy surfaces as a function of the nuclear configuration for a nonadiabatic electron transfer reaction. The dashed arrow represents the reaction pathway.
Figure 4
(a) Parabolic dependence of -ΔG‡on the driving force -ΔG° for electron transfer and (b) schematic representation of free Gibbs energy surfaces along the nuclear configuration of a nonadiabatic electron transfer for four kinetic regimes: self-exchange reactions (ΔG° = 0), normal regime for weak exergonic reactions (0 < -ΔG° < λ), activationless reactions (ΔG‡= 0 and ΔG° = λ), and inverted electron transfer for highly exergonic reactions (λ< -ΔG° < 0).
Figure 5
Schematic energy surfaces of the reactants and products and the vibronic overlap between them, which is represented by the gray regions (adapted from reference 8383 Barbara, P. F.; Meyer, T. J.; Ratner, M. A.; J. Phys. Chem. 1996, 100, 13148.).
Figure 4
(a) Parabolic dependence of -ΔG‡on the driving force -ΔG° for electron transfer and (b) schematic representation of free Gibbs energy surfaces along the nuclear configuration of a nonadiabatic electron transfer for four kinetic regimes: self-exchange reactions (ΔG° = 0), normal regime for weak exergonic reactions (0 < -ΔG° < λ), activationless reactions (ΔG‡= 0 and ΔG° = λ), and inverted electron transfer for highly exergonic reactions (λ< -ΔG° < 0).
Figure 5
Schematic energy surfaces of the reactants and products and the vibronic overlap between them, which is represented by the gray regions (adapted from reference 8383 Barbara, P. F.; Meyer, T. J.; Ratner, M. A.; J. Phys. Chem. 1996, 100, 13148.).
Figure 6
(a) Energy diagram for the oxidation or reduction of a solvated molecular species according to the Gerischer model and (b) the corresponding energy distributions (adapted from reference 8787 Memming, R; Semiconductor Electrochemistry, 2nd ed.; Wiley: Weinheim, Germany, 2015.).
Figure 7
Schematic Gerischer diagram illustrating the interfacial electron injection from an excited sensitizer into the acceptor states of TiO2 and subsequent back-electron transfer to the oxidized sensitizer (adapted from references 113113 Watson, D. F.; Meyer, G. J.; Annu. Rev. Phys. Chem. 2005, 56, 119.
114 Ardo, S.; Meyer, G. J.; Chem. Soc. Rev. 2009, 38, 115.-115115 Aghazada, S.; Nazeeruddin, M. K.; Inorganics 2018, 6, 34.).
Figure 8
Schematic representation of (a) the multiple-trapping transport mechanism, the respective (b) nearest-neighbor continuous-time random walk and (c) random flight models, and (d) the hopping transport mechanism. The gray lines represent localized, trap states (adapted from references 138138 Katoh, R.; Furube, A.; Barzykin, A. V.; Arakawa, H.; Tachiya, M.; Coord. Chem. Rev. 2004, 248, 1195.
139 Bisquert, J.; J. Phys. Chem. C 2007, 111, 17163.-140140 Adhikari, P.; Kobbekaduwa, K.; Shi, Y.; Zhang, J.; Abass, N. A.; He, J.; Rao, A.; Gao, J.; Appl. Phys. Lett. 2018, 113, 183509.).
Figure 9
(a) Schematic representation of the arrangement of the components of a dye-sensitized solar cell. (b) Overview of the electron transfer and transport processes in a DSSC with an I−/ redox mediator: (1) photoexcitation (< 10−15s), (2) hot electron injection (10−13to 10−12s), (3) thermalization (10−13to 10−12s), (4) thermalized electron injection (10−12to 10−10s), (5) thermal relaxation to the ground state (10−9to 10−8s), (6) regeneration of the oxidized dye (10−9to 10−6s), (7) electron transport through the TiO2film (10−3to 10−2s), (8) recombination with acceptor species in the mediator (10−3to 100s), (9) back-electron transfer (10−6to 10−3s), and (10) lateral intermolecular self-exchange (10−8to 10−6s). The blue arrows represent the processes that promote energy conversion, while the processes that inhibit energy conversion are represented by orange arrows (adapted from reference 1919 Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H.; Chem. Rev. 2010, 110, 6595.).
Figure 10
Schematic representation of a DSPEC for water oxidation and hydrogen production. The following processes are illustrated: (1) sequential photon excitation (<10−15 s), (2) excited state electron injection (10−13 to 10−10 s), (3) sequential electron transfer events from the catalyst to the photosensitizer (catalyst activation) (10−12 to 10−6 s), (4) water oxidation (10−1 to 101 s), (5) electron transport through the TiO2 mesoporous film (10−3 to 10−2 s), (6) proton transfer to the cathode compartment, and (7) catalytic reduction of protons to H2. PS is the photosensitizer, cat is the catalyst, and PEM is a proton exchange membrane (adapted from reference 4141 Brennaman, M. K.; Dillon, R. J.; Alibabaei, L.; Gish, M. K.; Dares, C. J.; Ashford, D. L.; House, R. L.; Meyer, G. J.; Papanikolas, J. M.; Meyer, T. J.; J. Am. Chem. Soc. 2016, 138, 13085.).
Figure 11
Representation of electron transfer processes in DSPEC photoanodes where the photosensitizer and catalyst are coloaded (a) or covalently linked in a molecular assembly (b). The following processes are illustrated: (1) excited state electron injection (10−13 to 10−10 s), (2) lateral intermolecular self-exchange (10−6 to 10−3 s), (3) electron transfer from the catalyst to the photosensitizer (catalyst activation) (10−12 to 10−6 s), (4) back-electron transfer (10−7 to 10−3 s), and (5) recombination to the activated catalyst (10−7 to 10−3 s). The blue arrows represent the forward electron transfer processes, and the orange arrows represent processes that inhibit energy conversion.
Figure 12
Schematic representation of a tandem DSPEC for the reduction of CO2to CO. The following processes are illustrated at the photoanode: (1) sequential photon excitation (< 10−15s), (2) excited state electron injection (10−13to 10−10s), (3) sequential electron transfer events from the catalyst to the photosensitizer (catalyst activation) (10−12to 10−6s), (4) water oxidation (10−1to 101s), (5) electron transport through the TiO2mesoporous film (10−3to 10−2s), and (6) proton transfer to the cathode compartment. At the photocathode, the following processes are illustrated: (7) sequential photon excitation (< 10−15s), (8) excited state electron transfer (10−13to 10−10s), (9) sequential electron transfer events from the photosensitizer to the catalyst (catalyst activation) (10−12to 10−6s), (10) CO2reduction (10−1to 101s), and (11) electron transport through the NiO film (10−2to 1 s). PS is the photosensitizer, cat is the catalyst, and PEM is a proton exchange membrane (adapted from reference 4141 Brennaman, M. K.; Dillon, R. J.; Alibabaei, L.; Gish, M. K.; Dares, C. J.; Ashford, D. L.; House, R. L.; Meyer, G. J.; Papanikolas, J. M.; Meyer, T. J.; J. Am. Chem. Soc. 2016, 138, 13085.).
Figure 9
(a) Schematic representation of the arrangement of the components of a dye-sensitized solar cell. (b) Overview of the electron transfer and transport processes in a DSSC with an I−/ redox mediator: (1) photoexcitation (< 10−15s), (2) hot electron injection (10−13to 10−12s), (3) thermalization (10−13to 10−12s), (4) thermalized electron injection (10−12to 10−10s), (5) thermal relaxation to the ground state (10−9to 10−8s), (6) regeneration of the oxidized dye (10−9to 10−6s), (7) electron transport through the TiO2film (10−3to 10−2s), (8) recombination with acceptor species in the mediator (10−3to 100s), (9) back-electron transfer (10−6to 10−3s), and (10) lateral intermolecular self-exchange (10−8to 10−6s). The blue arrows represent the processes that promote energy conversion, while the processes that inhibit energy conversion are represented by orange arrows (adapted from reference 1919 Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H.; Chem. Rev. 2010, 110, 6595.).
Figure 10
Schematic representation of a DSPEC for water oxidation and hydrogen production. The following processes are illustrated: (1) sequential photon excitation (<10−15 s), (2) excited state electron injection (10−13 to 10−10 s), (3) sequential electron transfer events from the catalyst to the photosensitizer (catalyst activation) (10−12 to 10−6 s), (4) water oxidation (10−1 to 101 s), (5) electron transport through the TiO2 mesoporous film (10−3 to 10−2 s), (6) proton transfer to the cathode compartment, and (7) catalytic reduction of protons to H2. PS is the photosensitizer, cat is the catalyst, and PEM is a proton exchange membrane (adapted from reference 4141 Brennaman, M. K.; Dillon, R. J.; Alibabaei, L.; Gish, M. K.; Dares, C. J.; Ashford, D. L.; House, R. L.; Meyer, G. J.; Papanikolas, J. M.; Meyer, T. J.; J. Am. Chem. Soc. 2016, 138, 13085.).
Figure 11
Representation of electron transfer processes in DSPEC photoanodes where the photosensitizer and catalyst are coloaded (a) or covalently linked in a molecular assembly (b). The following processes are illustrated: (1) excited state electron injection (10−13 to 10−10 s), (2) lateral intermolecular self-exchange (10−6 to 10−3 s), (3) electron transfer from the catalyst to the photosensitizer (catalyst activation) (10−12 to 10−6 s), (4) back-electron transfer (10−7 to 10−3 s), and (5) recombination to the activated catalyst (10−7 to 10−3 s). The blue arrows represent the forward electron transfer processes, and the orange arrows represent processes that inhibit energy conversion.
Figure 12
Schematic representation of a tandem DSPEC for the reduction of CO2to CO. The following processes are illustrated at the photoanode: (1) sequential photon excitation (< 10−15s), (2) excited state electron injection (10−13to 10−10s), (3) sequential electron transfer events from the catalyst to the photosensitizer (catalyst activation) (10−12to 10−6s), (4) water oxidation (10−1to 101s), (5) electron transport through the TiO2mesoporous film (10−3to 10−2s), and (6) proton transfer to the cathode compartment. At the photocathode, the following processes are illustrated: (7) sequential photon excitation (< 10−15s), (8) excited state electron transfer (10−13to 10−10s), (9) sequential electron transfer events from the photosensitizer to the catalyst (catalyst activation) (10−12to 10−6s), (10) CO2reduction (10−1to 101s), and (11) electron transport through the NiO film (10−2to 1 s). PS is the photosensitizer, cat is the catalyst, and PEM is a proton exchange membrane (adapted from reference 4141 Brennaman, M. K.; Dillon, R. J.; Alibabaei, L.; Gish, M. K.; Dares, C. J.; Ashford, D. L.; House, R. L.; Meyer, G. J.; Papanikolas, J. M.; Meyer, T. J.; J. Am. Chem. Soc. 2016, 138, 13085.).
Figure 13
(a) Time evolution of the changes in the IR absorbance at a given probe wavelength, and (b) the TRIR transient spectra for a given time delay t after the pump flash. The orange dot is the ∆A determined for the time delay t at the probe wavenumber where the kinetic curve (a) was measured.
Figure 14
Representation of the contributions to a ∆A transient absorption spectrum at a given time delay. Each individual contribution (dashed lines) is time-dependent, and the overall spectral sum (solid line) varies as a function of time after excitation.
Figure 15
Schematic diagram of the components in the (a) nanosecond and (b) ultrafast pulsed laser spectroscopy setup.
Figure 16
Schematic diagram of an ultrafast pump-probe technique, in which the changes in absorbance as a function of time resulting from different concentrations of transient species are measured at different pump-probe delay times.
Figure 13
(a) Time evolution of the changes in the IR absorbance at a given probe wavelength, and (b) the TRIR transient spectra for a given time delay t after the pump flash. The orange dot is the ∆A determined for the time delay t at the probe wavenumber where the kinetic curve (a) was measured.
Figure 17
Representation of the sinusoidal voltage perturbation V(ω,t) and the corresponding alternating current response I(ω,t) in an EIS experiment. The phase shift j is the time delay between the input perturbation and the output response (adapted from reference 270270 von Hauff, E.; J. Phys. Chem. C 2019, 123, 11329.).
Figure 18
Representation of typical Nyquist impedance spectra of a DSSC. In the order of decreasing frequency, the regions correspond to (1) the electron transfer processes at the counterelectrode|electrolyte and uncovered TCO|electrolyte interfaces, (2) electron diffusion in the TiO2 film, (3) electron recombination with oxidized species in the electrolyte, and (4) diffusion of redox ions in the electrolyte (adapted from references 252252 Bisquert, J.; Fabregat-Santiago, F. In Dye-Sensitized Solar Cells, 1st ed.; Kalyanasundaram, K., ed.; EPFL Press: Lausanne, Switzerland, 2010. and 253253 Sarker, S.; Ahammad, A. J. S.; Seo, H. W.; Kim, D. M.; Int. J. Photoenergy 2014, 2014, 851705.).
Figure 19
Complete equivalent circuit model for a DSSC (adapted from reference 272272 Fabregat-Santiago, F.; Bisquert, J.; Palomares, E.; Otero, L.; Kuang, D.; Zakeeruddin, S. M.; Grätzel, M.; J. Phys. Chem. C 2007, 111, 6550.).
Figure 20
Schematic diagram showing the changes in the density of states of the trapped electrons as a function of energy during a transient photovoltage decay measurement.
Figure 21
(a) Representation of the time evolution of the oxidation of surface-bound molecules during a chronoabsorptometry experiment (adapted from reference 9191 Motley, T. C.; Brady, M. D.; Meyer, G. J.; J. Phys. Chem. C 2018, 122, 19385.). (b) Example of absorption changes during the application of a sufficiently positive electrical potential to a dye-sensitized film. The inset shows the normalized absorbance change plotted against the square root of time, fitted by the Anson equation (blue dashed line).