Preface This ACS Book presents studies of photoinduced processes in nanomaterials that fall into the category of basic research contributing to solar energy conversion. The book focuses on photophysical and photochemical processes at surfaces of semiconductor nanostructures that are related to photovoltaic and photocatalytic applications with a broader focus on time-resolved spectroscopic monitoring of related processes in photoactive materials. Changes of the composition, quantum confinement, size, shape, surface functionalization, doping, and relative spatial arrangements of nanocrystals, as well as the formation of semiconductor-to-metal nano-interfaces, tune the timescales of the relevant basic processes and properties of the materials. The book reports short, up-to-date reviews, recent experimental data, and computational results that all contribute to an atomistic description of electronic dynamics and charge transfer induced by optical excitations and lattice vibrations. Basic processes — such as light absorption, formation and evolution of charge transfer excitations, hot carrier relaxation, and reaction dynamics — all are perturbed by the environmental factors, such as lattice vibrations and solvent polarization, and are treated with different implementations of the "open system" approach. The merging of experimental and computational approaches allows for a holistic and coordinated effort to interpret, guide, and optimize the efficiency of photocatalytic and photovoltaic processes. Chapters cover the tuning of optoelectronic properties of nanomaterials for energy application via the broad modification of morphological variables. Properties of nanostructures are tuned by varying their dimensionality from zero-dimensional quantum dots (Chapters 5 and 12) and one-dimensional nanorods (Chapter 10) to two-dimensional thin films (Chapters 2, 6, and 7) and mesoporous materials (Chapter 3). Additional tune-ability is achieved via sensitization by dyes (Chapters 1, 8, and 12), doping (Chapters 1, 5, 6, and 9), or deposition of metal clusters on semiconductor surfaces (Chapters 7 and 11). Although the main focus of the reported research is basic science, several chapters report noticeable advances in dye-sensitized solar cells (Chapters 1 and 8), semiconductor-based photovoltaics (Chapters 7, 9, and 10), sensing applications (Chapter 6), pharmaceutical applications (Chapter 12), and conversion of solar energy to chemical energy in a broad variety of photoelectrochemical cells for photocatalysis and water splitting (Chapters 3, 4, and 11). The contributions come from both an experimental community (Chapters 1, 3, 4, and 12) and a community of researchers dealing with computing electronic structure (Chapters 2, 5, and 6), nonadiabatic dynamics of excited states (Chapters 7, 8, 9, and 10), and modeling chemical reaction dynamics (Chapter 11).
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King, Kern, and Parkinson (Chapter 1, Sensitization of Single Crystal Substrates) provide a starting point and a “system of coordinates” for other chapters dealing with the sensitization of single crystal wide-gap single crystal semiconductor surfaces by dyes, polymers, quantum dots, and doping (Chapters 2, 3, 4, and 8). Giorgi and Yamashita (Chapter 2, Electronic and Optical Properties of Low-Dimensional TiO2: From Minority Surfaces to Nanocomposites) focus on the confinement effect for ultra-thin titania films and their functionalization by graphene, as explored by electronic structure computations that reveal the excitonic nature of their optical spectra. Mahoney, Rasalingam, and Koodali (Chapter 3, Dye-sensitized and Doped TiO2 Mesoporous Materials for Visible Light Induced Photocatalytic Hydrogen Evolution) provide an overview of the benefits of doped mesoporous materials for visible light-induced photocatalytic hydrogen production: Sensitization of wide-gap semiconductors such as titania with dyes, quantum dots and doping by transition metals extends the response to visible light, while mesoporous morphology provides high surface-to-volume ratio, which collectively contributes to the efficiency of photocatalytic processes. This complements other chapters dealing with the effects of the confinement, morphology, and sensitization of nanomaterials for solar energy applications (Chapters 1, 2, and 4) and opens a call for the computational modeling of hydrogen evolution (Chapter 11). Pillai et al. (Chapter 4, Single Site Metal Ions on the Surface of TiO2 Nanorods: A Platform for Theoretical and Experimental Investigation) summarize the synthetic protocols for controlled deposition of transition metals on the surface of titania nanostructures and outline possible ways to characterize those nanomaterials for solar energy utilization, experimentally and computationally. Proshchenko and Dahnovski (Chapter 5, Transition Metal Doped Semiconductor Quantum Dots: Tunable Emission) examine the modeling of transition metal doping in semiconductor quantum dots, covering the impact of crystal field splitting and many-electron effects onto the absorption and luminescence of such models. Inerbaev et al. (Chapter 6, Theoretical Modeling of Oxygen and Water Adsorptionon Indium Oxide (111) Surface) address the control of the optical properties of an indium oxide thin film by the surface adsorption of oxygen at different air humidity for sensor applications. Micha (Chapter 7, Density Matrix Treatment of Optical Properties in Photovoltaic Materials: Photoconductivity at a Semiconductor Surface) focuses on metal clusters deposited on silicon surfaces. An overview of the practical implementations of the methods to describe the light-tomatter interaction, which are based on density operator techniques, paves the way to predict and interpret key observables in such nanostructures: photovoltage spectra, photocurrent, and photoconductivity. This chapter is closely connected to three subsequent chapters (Chapters 8, 9, and x
10), which focus on perturbations by light, solvent, substrate, and lattice vibrations onto the evolution of excited states. 8. Bowman, Chan, and Jakubikova (Chapter 8, Investigating Interfacial Electron Transfer in Highly Efficient Porphyrin-Sensitized Solar Cells) explore the way in which electron transfer in dye-sensitized titania depends on the details of the anchoring group composition, the surface adsorption, and the binding pattern. This chapter provides an excellent companion to experimental chapters as well as an overview of one of the first practical methods for the atomistic modeling of electron transfer that is based on the survival probability concept. 9. Akimov and Prezhdo (Chapter 9, Nonradiative Relaxation of Charge Carriers in GaN-InN Alloys: Insights from Nonadiabatic Molecular Dynamics) provide an overview of hot carrier relaxation in indium-doped gallium nitride — a promising material for solar energy conversion. The authors review a commonly used method for atomistic modeling of relaxation based on Tully’s surface-hopping algorithm and on-the-fly coupling along molecular dynamics trajectory. Prezhdo and Akimov highlight the importance of accounting for quantum decoherence effects, which are responsible for the precision of the computed rates of carrier relaxation. 10. Kryjevski (Chapter 10, Toward First-principles Description of Carrier Relaxation in Nanoparticles) focuses on the description of carrier relaxation due to phonon emission in semiconductor nanostructures, which is an important process in photo-excited nanostructures and is potentially relevant to photovoltaics. The chapter discusses an alternative to the on-the-fly procedure for computing electron–phonon interaction. Here, the electron–phonon interactions are treated perturbatively using the Keldysh technique, or non-equilibrium Green’s function, including one- and two-phonon processes. Nanoparticle-specific electron–phonon couplings are given by derivatives of the electron–ion interaction along each normal mode of lattice vibrations. 11. Meng (Chapter 11, Optical, Electronic and Catalytic Properties of Metal Nanoclusters by ab initio Molecular Dynamics Simulation: A Mini Review) covers two classes of photoinduced processes related to metal nanoparticles: their ability to catalyze chemical reactions and their influence on excited state relaxation. These two classes of processes relate to the dynamics of inter-atomic distances for molecules adsorbed at metal cluster and to the dynamics of electronic excited states, respectively. In the first half of the chapter, Meng provides computational predictions of the efficiency of proton reduction and hydrogen evolution at the surface of metal and bimetal clusters, thus making connections to the experimental chapter on hydrogen generation (Chapter 3). In the second part of the chapter, Meng addresses the details of electronic excited state dynamics in metal clusters by combining the reduced density operator method, similar to the one described in Chapter 7 by Micha, and the on-the-fly procedure for nonadiabatic coupling, similar to the one used in Chapter 9 by Prezhdo and Akimov. The xi
reported methodology paves the way to compute such nonequilibrium observables as photoluminescence spectra. 12. Zenkevich and von Borczyskowski (Chapter 12, Surface Photochemistry of “Quantum Dot – Porphyrin” Nanoassemblies: Exciton Relaxation Pathways and Singlet Oxygen Generation Generation) provide experimental time-resolved data on energy transfer and electron transfer at the interface of semiconductor quantum dot and a sensitizing dye molecule. A range of time-resolved spectoscopical techniques allows for the coexistence of several pathways of photoexcitation to be identified: charge transfer, resonant energy transfer, and multi-electron Auger processes, thus providing new opportunities for the quantitative comparison of computational and experimental approaches. In summary, complementary reviews on a broad variety of physical and chemical processes in functional nanostructures are presented to recognize recent achievements and to help explore open problems in order to coordinate experimental and computational efforts toward the further research of materials for renewable energy.
Dmitri Kilin Department of Chemistry University of South Dakota Vermillion, South Dakota 57069 and Department of Chemistry and Biochemistry North Dakota State University Fargo, North Dakota 58102
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