The surface science of titanium dioxide

The invasion of woody vegetation into deserts, grasslands and savannas is generally thought to lead to an increase in the amount of carbon stored in those ecosystems. For this reason, shrub and forest expansion (for example, into grasslands) is also suggested to be a substantial, if uncertain, component of the terrestrial carbon sink1,2,3,4,5,6,7,8,9,10,11,12,13,14. Here we investigate woody plant invasion along a precipitation gradient (200 to 1,100 mm yr-1) by comparing carbon and nitrogen budgets and soil δ13C profiles between six pairs of adjacent grasslands, in which one of each pair was invaded by woody species 30 to 100 years ago. We found a clear negative relationship between precipitation and changes in soil organic carbon and nitrogen content when grasslands were invaded by woody vegetation, with drier sites gaining, and wetter sites losing, soil organic carbon. Losses of soil organic carbon at the wetter sites were substantial enough to offset increases in plant biomass carbon, suggesting that current land-based assessments may overestimate carbon sinks. Assessments relying on carbon stored from woody plant invasions to balance emissions may therefore be incorrect.

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Ecosystem carbon loss with woody plant invasion of grasslands.

This review describes recent research into the properties of the chromophore-TiO2 interface that forms the basis for photoinduced charge separation in dye-sensitized semiconductor solar cells. It focuses particularly on an atomistic picture of the electron-injection dynamics. The interface offers an excellent case study, pertinent as well to a variety of other photovoltaic systems, photo- and electrochemistry, molecular electronics, analytical detection, photography, and quantum confinement devices. The differences between chemists' and physicists' models for describing molecules and bulk materials, respectively, create challenges for the characterization of interfaces that include both of these components. We give an overall picture of the interface by starting with a description of the properties of the chromophores and semiconductor separately, and then by discussing the coupled system, including the chromophore-semiconductor binding, electronic structure, and electron-injection dynamics. Explicit time-dependent modeling is particularly valuable for an understanding of the ultrafast electron injection because it shows a variety of individual injection events with well-defined dynamical features that cannot be made apparent by an average reaction-rate description.

Theoretical studies of photoinduced electron transfer in dye-sensitized TiO2.

Understanding the surface chemistry of TiO2 is key to the development and optimisation of many technologies, such as solar power, catalysis, gas sensing, medical implantation, and corrosion protection. In order to address this, considerable research effort has been directed at model single crystal surfaces of TiO2. Particular attention has been given to the rutile TiO2(110) surface because it is the most stable face of TiO2. In this critical review, we discuss the chemical reactivity of TiO2(110), focusing in detail on four molecules/classes of molecules. The selected molecules are water, oxygen, carboxylic acids, and alcohols-all of which have importance not only to industry but also in nature (173 references).

Chemical reactions on rutile TiO2(110)

Electron transfer across the molecule-semiconductor interface is a fundamental process that is relevant to many applications of nanoparticles, such as dye-sensitized solar cells and molecular electronics. This review summarizes recent progress in understanding electron transfer dynamics from molecular adsorbates to semiconductor nanoparticles. Photoexcitation of molecular adsorbates to their excited states is followed by electron injection into semiconductor nanoparticles. The products of electron injection (oxidized adsorbate and electrons in semiconductor) are monitored by their electronic and vibrational spectra, allowing direct measurement of injection rate. The dependence of injection rate on the properties of semiconductor nanoparticle, molecular adsorbate, intervening bridging and anchoring group, and interfacial environment are discussed and compared with Marcus theory of interfacial electron transfer.

Ultrafast electron transfer at the molecule-semiconductor nanoparticle interface.

Bi-isonicotinic acid ~2,28-bipyridine–4,48-dicarboxylic acid! is the ligand of several organometallic
dyes, used in photoelectrochemical applications. Therefore the atomic scale understanding of the
bonding of this molecule to rutile TiO2(110) should give insight into the crucial dye–surface
interaction. High resolution x-ray photoelectron spectroscopy ~XPS!, near edge x-ray absorption
fine structure ~NEXAFS!, and periodic intermediate neglect of differential overlap ~INDO!
calculations were carried out on submonolayer bi-isonicotinic acid rutile TiO2(110). Data from
multilayers is also presented to support the submonolayer results. For a multilayer, XPS shows that
the carboxyl groups remain in the ~pristine! protonated form, and NEXAFS show that the molecular
plane is tilted by 57° with respect to the surface normal. For the submonolayer, the molecule bonds
to the rutile TiO2(110) surface via both deprotonated carboxyl groups, with a tilt angle of 25°, and
additionally an azimuthal orientation of 44° with respect to the @001# crystallographic direction. The
adsorbant system was also investigated by quantum mechanical calculations using a periodic INDO
model. The most stable theoretical adsorption geometry involves a twist around the molecular axis,
such that the pyridine rings are tilted in opposite directions. Both oxygen atoms of each carboxyl
group are bonded to five-fold coordinated Ti atoms ~2M-bidentate!, in excellent agreement with the
experimental results.

Adsorption of Bi-Isonicotinic Acid on Rutile TiO2 (110)

A series of Ru-polypyridine dyes has been studied with electron spectroscopy using AlKα and synchrotron radiation. Both pure complexes and complexes adsorbed on nanostructured TiO2 (anatase) surfaces have been examined and special emphasis was given to the dye complex cis-bis(4,4′-dicarboxy-2,2′-bipyridine)-bis-(isothiocyanato)-ruthenium(II) [Ru(dcbpy)2(NCS)2]. The measurements provide information concerning the energy level matching between the dyes and the TiO2, which is of importance in photoinduced charge transfer reactions and in applications such as dye-sensitized solar cells. The measurements also support the general picture of bonding of carboxylated complexes to the surfaces via the carboxyl groups of a single bi-isonicotinic acid ligand, and that, for Ru(dcbpy)2(NCS)2, the NCS-ligand–TiO2 interaction is small. Corroborative support is provided via quantum chemical calculations on the ligand (bi-isonicotinic acid) adsorbed on a TiO2 anatase (101) surface.

XPS studies of Ru-polypyridine complexes for solar cell applications

Excited state properties of fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene have been investigated theoretically using multireference CASSCF and CASPT2 methods. Experimentally, chlorobenzene and bromobenzene are known to exhibit one fast dissociation channel, whereas iodobenzene exhibits two fast dissociation channels. The calculations indicate that the chlorobenzene, the bromobenzene, and the slower iodobenzene dissociation channels are due to intersystem crossings from a bound (π,π*) singlet excited state to a repulsive (n,σ*) triplet excited state. The faster iodobenzene dissociation channel is instead found to be caused by a direct dissociation of an antibonding (n,σ*) singlet excited state. The CASPT2 calculations predict that the onset of fluorobenzene photodissociation should occur around 196 nm, with a single time constant longer than 1 ns. CASSCF geometries and accurate MSCASPT2 calculated vertical excitation energies are presented for the ground state as well as the first excited singlet and triplet states of all the monohalobenzenes.

Theoretical Study of the Fast Photodissociation Channels of the Monohalobenzenes

The photochemistry of low lying excited states of six different fluorinated bromobenzenes has been investigated by means of femtosecond laser spectroscopy and high level ab initio CASSCF/CASPT2 quantum chemical calculations. The objective of the work was to investigate how and to what extent light substituents, position on the benzene ring and number, would influence the dissociation mechanism of bromobenzene. In general, the actual position of a fluorine atom affects the dissociation rate to a less extent than the number of fluorine atoms. A clear connection between a lowering of a repulsive πσ* relative to a bound ππ* state and the number of fluorine substituents exists, and the previously suggested model of coupling between dissociation rate and relative location of bound and repulsive state still holds for these molecules. A more elaborate examination of the electronic structure of the excited states in bromobenzenes than previously reported is presented.

Photochemistry of bromofluorobenzenes.

The C1s core photoelectron spectra of a series of alkene molecules, ethene, propene, 1‐butene, cis and trans 2‐butene, 2‐methyl‐propene, and 1‐pentene are discussed. The experimental spectra are assigned using intermediate neglect of differential overlap‐configuration interaction (INDO‐CI) calculations and comparative discussions. It is shown that hyperconjugation is a useful concept in the assignment of the transitions. INDO‐CI is shown to give a reasonable description of the low energy part of the spectra. The results are used in the discussion of molecular models for the interpretation of the electronic structure of polyacetylene.

S. Lunell

Papers

Adsorption of Bi-Isonicotinic Acid on Rutile TiO2 (110)

XPS studies of Ru-polypyridine complexes for solar cell applications

Theoretical Study of the Fast Photodissociation Channels of the Monohalobenzenes

Photochemistry of bromofluorobenzenes.

The C1s core shake‐up spectra of alkene molecules: An experimental and theoretical study