A surface science perspective on TiO2 photocatalysis

A critical review of light-driven interfacial charge-transfer reactions of transition-metal compounds anchored to mesoporous, nanocrystalline TiO2 (anatase) thin films is described. The review highlights molecular insights into metal-to-ligand charge transfer (MLCT) excited states, mechanisms of interfacial charge separation, inter- and intra-molecular electron transfer, and interfacial charge-recombination processes that have been garnered through various spectroscopic and electrochemical techniques. The relevance of these processes to optimization of solar-energy-conversion efficiencies is discussed (483 references).

Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2 semiconductor surfaces

Micrometre- and nanometre-sized particles play important roles in many applications, including catalysis, optics, biosensing and data storage. Organic particles are usually prepared through polymerization of suitable monomers or precipitation methods. In the case of inorganic materials, particle fabrication tends to involve the reduction of a metal salt, or the controlled mixing of salt solutions supplying a metal cation and an elemental anion (for example, S2-, Se2-, O2-), respectively; in some instances, these methods even afford direct control over the shape of the particles produced. Another class of materials are metal-organic coordination polymers, which are based on metal ions coordinated by polydentate organic ligands and explored for potential use in catalysis, gas storage, nonlinear optics and molecular recognition and separations. In a subset of these materials, the use of organometallic complexes as ligands (so-called metalloligands) provides an additional level of tailorability, but these materials have so far not yet been fashioned into nano- or microparticles. Here we show that simple addition of an initiation solvent to a precursor solution of metal ions and metalloligands results in the spontaneous and fully reversible formation of a new class of metal-metalloligand particles. We observe initial formation of particles with diameters of a few hundred nanometres, which then coalesce and anneal into uniform and smooth microparticles. The ease with which these particles can be fabricated, and the ability to tailor their chemical and physical properties through the choice of metal and organic ligand used, should facilitate investigations of their scope for practical applications.

/pdf/chemically-tailorable-colloidal-particles-from-infinite-1bsqlk61o7.pdf

Chemically tailorable colloidal particles from infinite coordination polymers

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.

Ultrafast electron transfer at the moleculesemiconductor nanoparticle interface

The thermodynamics and kinetics of binding to nanocrystalline TiO_2 were investigated for five ruthenium complexes that differed structurally in the number of possible anchoring carboxy groups (one, two, four, or six) attached to coordinated bipyridyl ligands and in the number of auxiliary ligands (bipyridine, CN-, or SCN-). Diffuse reflectance infrared spectroscopic data indicated that the dyes predominantly bound to TiO_2 in a bridging mode in which the oxygen atoms of an attached carboxy group were bound to separate titanium atoms on the TiO_2 surface. Furthermore, in the dry state, complexes with only one monocarboxy or dicarboxy ligand used essentially all of their available carboxy groups to bind to the surface. However, complexes having two or three dicarboxy ligands used on average two carboxylato groups in binding to TiO_2. The structural differences between the complexes were manifested chemically in that the five dyes yielded similar maximum coverages (>100 nmol cm^(-2)) on nanocrystalline TiO_2 electrodes, but exhibited different binding constants (10^3−10^5 M^(-1)) and different adsorption and desorption kinetics (3−11) × 10^3 M^(-1) h^(-1) and 1−100 h, respectively). The binding constant for the monocarboxy dye was significantly lower than the binding constants for dyes with dicarboxy ligands, correlating primarily with an increase in the desorption rate of the monocarboxy complex. The adsorption rate constants were similar for all of the dyes, suggesting that formation of the first bond to TiO_2 was rate limiting. Binding of the dyes from an ethanolic solution that contained pyridine and pyridinium as an acidic proton activity buffer yielded lower coverages than binding from a nonbuffered ethanol solution, even though the binding constants were up to 100 times greater under buffered conditions. The lower equilibrium dye coverage in buffered ethanol did not correlate with changes in the protonation state of the dyes but rather indicated competition for, and/or deactivation of, TiO_2 active sites in buffered ethanol. The more weakly bound monocarboxy dye displayed the lowest short-circuit current density and open-circuit voltage under simulated solar illumination in a photoelectrochemical cell containing 0.50 M LiI, 0.040 M I_2, 0.020 M pyridine, and 0.020 M pyridinium triflate in acetonitrile. Additionally, even at constant coverage, the integrated quantum yield for photocurrent flow was lowest for TiO_2 sensitized with the monocarboxy dye. The potential required to drive 0.1 mA cm^(-2) of cathodic current density in the dark on dye-sensitized TiO_2 photoelectrodes was least negative for the monocarboxy dye, indicating more facile electron transfer between reduced TiO_2 and the solution redox couple. Hence, in this series of ruthenium carboxy-bipyridyl dyes, the most weakly bound species (i.e., the monocarboxy dye) yielded inferior photoelectrode properties, whereas differences between the dyes that contained at least one dicarboxy ligand resulted primarily from differences in the light absorption and energetic properties of the metal complexes. These observations suggest an important role for the linkage to the TiO_2 surface in achieving temporal stability as well as in tuning both the steady-state quantum yield and the magnitude of the predominant back-reaction rate in dye-sensitized TiO_2-based photoelectrochemical solar cells.

Anchoring Group and Auxiliary Ligand Effects on the Binding of Ruthenium Complexes to Nanocrystalline TiO2 Photoelectrodes

We have shown that Ru^(II)(bpy)_2(bpy-4-(xylyl)_x-≡-phenyl-COOH)(PF_6)_2 (abbreviated Rux, where x = 0, 1 or 2 xylyl groups; bpy = 2,2‘-bipyridine) dyes can act as sensitizers for nanocrystalline TiO_2 in functional photoelectrochemical cells under simulated solar illumination, albeit with low efficiencies. Both the short-circuit photocurrent density and the open-circuit voltage decreased as x was increased. Electron injection (10^6−10^8 s^(-1)) was slightly faster for the x = 0 dye, but both recombination (10^(-15)−10^(-13) cm^3 s^(-1)) and regeneration (10^4−10^6 s^(-1) for 10 mM I^-) were slightly faster for the x = 2 dye. We suggest that the lack of distance dependence is due to the flexible one-carboxyl attachment to the surface resulting in the Ru−TiO_2 electron-tunneling distance being very similar for x = 0, 1, and 2. For all of the Rux sensitizers, a relatively small potential was needed for generation of current in the dark, indicating that the reaction between electrons in TiO_2 and the I_3^-/I^- electrolyte solution is as favorable for the Rux sensitizers as for unmodified TiO_2 electrodes.

Elizabeth I. Mayo

Papers

Anchoring Group and Auxiliary Ligand Effects on the Binding of Ruthenium Complexes to Nanocrystalline TiO2 Photoelectrodes

Effects of bridging ligands on the current-potential behavior and interfacial kinetics of ruthenium-sensitized nanocrystalline TiO2 photoelectrodes