New Hybrid Properties of TiO2 Nanoparticles Surface Modified With Catecholate Type Ligands
© to the authors 2009
Received: 26 March 2009
Accepted: 24 September 2009
Published: 13 October 2009
Surface modification of nanocrystalline TiO2 particles (45 Å) with bidentate benzene derivatives (catechol, pyrogallol, and gallic acid) was found to alter optical properties of nanoparticles. The formation of the inner-sphere charge–transfer complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites. The binding structures were investigated by using FTIR spectroscopy. The investigated ligands have the optimal geometry for chelating surface Ti atoms, resulting in ring coordination complexes (catecholate type of binuclear bidentate binding–bridging) thus restoring in six-coordinated octahedral geometry of surface Ti atoms. From the Benesi–Hildebrand plot, the stability constants at pH 2 of the order 103 M−1 have been determined.
The mechanism of semiconductor-assisted photocatalytic processes is based on the principle that colloidal semiconductor nanoparticles behave as miniature photoelectrochemical cells [1–3]. The overall photoexcitation process of semiconductor nanoparticles by ultra bandgap energies involve the photogeneration of electron–hole pairs within particle, followed by the competition between recombination, interfacial charge transfer to adsorbed compounds, and migration into midgap surface states .
Nanocrystalline TiO2 has attracted widespread attention as a photocatalyst in various practical applications [5–7] as well as the part of photoelectrochemical systems, such as Grätzel cells . Although nanoparticulate TiO2 is very effective from an energetic point of view, it is a relatively inefficient photocatalyst. Due to its large bandgap (E g = 3.2 eV), TiO2 absorbs less than 5% of the available solar light photons. The main energy loss is due to the process of radiative or nonradiative recombination of charges generated upon photoexcitation of TiO2, which is manifested as the relatively low efficiency of long-lived charge separation. Hence, the successful photochemical energy conversion and subsequent chemical reactions require both the extended separation of photogenerated charges and the response in the visible spectral region.
To achieve larger separation distances, preventing the hole–electron recombination before desired redox reaction occurs, reconstructed surface of TiO2 nanoparticle surface was employed for establishing a strong coupling with electron-accepting and/or electron-donating species [8, 9]. Consequently, the lifetime of charge separation and photocatalytic activity of TiO2 nanoparticles are increased. The origin of the unique photocatalytic activities of TiO2 nanoparticles comparing to the bulk is found in larger surface area and the existence of surface sites with distorted coordination. Owing to large curvature of TiO2 particles in the nanosize regime, the surface reconstructs in such manner that distorts the crystalline environment of surface Ti atoms forming coordinatively unsaturated Ti atoms. The changes in the pre-edge structure of Ti K-edge spectrum (XANES) of TiO2 nanoparticles (d < 20 nm) revealed the existence of square–pyramidal coordination of surface Ti atoms (pentacoordinate) . The results obtained from measurements of XAFS spectra of TiO2 nanoparticles proved the existence of shorter Ti–O bond lengths (1.71 Å) as compared to bulk anatase TiO2 (1.96 Å) . The bond length distortion is also related to large curvature of nanometer size particles. These surface Ti atoms are very reactive and act as traps for photogenerated charges . It was reported, on a whole class of electron-donating enediol ligands [9, 13], benzene derivatives , or mercapto-carboxylic acids  that binding to coordinatively unsaturated Ti atoms simultaneously adjusts their coordination to octahedral geometry at the surface of nanocrystallites and changes the electronic properties of TiO2. In such hybrid structures localized orbitals of surface-attached ligands, are electronically coupled with the delocalized electron levels from the conduction band of a TiO2 semiconductor . As a consequence, absorption of light by the charge–transfer (CT) complex yields to the excitation of electrons from the chelating ligand directly into the conduction band of TiO2 nanocrystallites. This results in a red shift of the semiconductor absorption compared to that of unmodified nanocrystallites and enables efficient harvesting of solar photons. Additionally, this type of electronic coupling yields to instantaneous separation of photogenerated charges into two phases, the holes localize on the donating organic modifier, and the electrons delocalize in the conduction band of TiO2. Moreover, the enediol ligands were found to act as conductive leads, allowing wiring of oligonucleotides and proteins resulting in enhanced charge separation and ensuing chemical transformations [17, 18].
In last two decades the surface modification of commercial TiO2(Degussa P25,d = 30 nm) with benzene derivatives (mainly catechol and salicylic acid) was studied [19–34]. Just few articles [9, 13–15, 35] investigated CT complex formation between enediol ligands and colloidal TiO2 nanoparticles (d = 45 Å), where binding of modifiers is stabilized by ligand-induced surface reconstruction of the nanoparticles.
Herein, we report surface modification of TiO2 nanoparticles with enediol ligands (catechol, pyrogallol, and gallic acid) that are able to adjust the coordination geometry of the surface Ti atoms inducing shift of the absorption onset toward the visible region of the spectrum, compared to unmodified nanocrystallites. Since these novel CT semiconducting materials exhibit optical properties that are distinct from their constituents, not absorbing in the visible region, Benesi–Hildebrand analysis for molecular complexes was employed to determine the stability constants from the absorption spectra. The stoichiometry of formed complexes was obtained using Job’s method of continuous variation. The binding structures were investigated by using FTIR spectroscopy.
All the chemicals were of the highest purity available and were used without further purification (Aldrich, Fluka). Milli-Q deionized water (resistivity 18.2 MΩ cm−1) was used as solvent. The colloidal TiO2 dispersions were prepared by the dropwise addition of titanium(IV) chloride to cooled water. The pH of the solution was between 0 and 1, depending on the TiCl4 concentration. Slow growth of the particles was achieved by using dialysis at 4 °C against water until the pH = 3.5 was reached . The concentration of TiO2 (0.2 M) was determined from the concentration of the peroxide complex obtained after dissolving the colloid in concentrated H2SO4. The mean particle diameter of titania used in this study was 45 Å.
Surface modification of TiO2 resulting in the formation of a CT complex was achieved by the addition of surface-active ligands up to concentrations required to cover all surface sites ([Tisurf] = [TiO2]12.5/D, where Tisurf is the molar concentration of surface Ti sites, [TiO2] is the molar concentration of TiO2 in molecular units, and D is the diameter of the particle in angstroms). As the consequence of enhanced particle–particle interaction, upon surface modification that eliminates the surface charge, precipitation, and/or “gelling” of the solution may occur. In order to avoid these problems pH of the solution was adjusted to 2 with HCl. For the determination of CT complex binding constants the absorption spectra were recorded at room temperature in cells with 1 cm optical path length using Thermo Scientific Evolution 600 UV/Vis spectrophotometer. The experiments determining the composition of complexes by Job’s method  were conducted by using 7.2 mM TiO2 and 2 mM modifier solutions. Nine mixtures of TiO2 and modifier were prepared—volumes of TiO2 solution used varied from 1 to 9 mL and those of modifiers’ solutions from 9 to 1 mL; and total volume was always 10 mL.
Infrared spectra were taken in attenuated total reflection (ATR) mode using a Nicolet 380 FTIR spectrometer equipped with a Smart OrbitTM ATR attachment containing a single-reflection diamond crystal. The angle of incidence was 45°. In order to avoid the precipitation of modified TiO2 and the excess of unbound modifier, in the preparation of samples for FTIR measurements the quantity of ligands added was 50% of all Tisurf sites ([TiO2] = 0.2 M,c LIGAND = 25 mM). The dispersions containing surface modified TiO2 nanoparticles were dried under argon at room temperature, and powders obtained were placed into the vacuum oven for 8 h to get to complete dryness. Before measuring FTIR spectra, powders were triturated in the agate mortar. Typically, 64 scans were performed for each spectrum with 4 cm−1 resolution. The spectrum of the dried TiO2 aqueous slurry (not containing modifier) was used as the background.
Results and Discussion
Optical Properties of Surface Modified Nanocrystalline TiO2
Consequently, the onset of absorption of these CT nanocrystallites is red shifted compared to unmodified TiO2(Fig. 1). The shift in the absorption edge in the modified semiconductor nanoparticles is attributed to the excitation of localized electrons from the surface modifier into the conduction band continuum states of the semiconductor particle . Similar position of the absorption threshold for surface-modified nanoparticles with catechol, pyrogallol, and gallic acid (bandgaps are presented in Table 1) is probably the consequence of very similar dipole moments of various surface bound Ti–ligand complexes formed .
Apart from the shift in the absorption edge, the optical properties of surface modified semiconductor nanoparticles, having a continuous rise of absorption toward higher energies, paralleled the absorption properties characteristic of the band structure in bare semiconductor nanoparticles. A similar red shift, but in localized CT complex resulting in a pronounced absorption maximum, was previously observed for Ti4+ and catechol , salicylic , or ascorbic acid .
It should be noted that all investigated enediol ligands are by themselves extremely susceptible to oxidation. Apparently, because of the bidentate binding to nanoparticles, enediol ligands gain stability and are not easily oxidized. Enediol-modified TiO2 colloids preserved their optical properties even after exposure to daylight for 2–3 months.
Due to the existence of undercoordinated surface defect sites and their lower efficiency of covalent bonding with solvent molecules in comparison with covalent bonding between atoms within the TiO2 lattice, the surface species posses energy level in the midgap region . Apart from red shift of the absorption onset of surface modified TiO2 nanoparticles, CT interaction between the molecule of modifier and surface Ti atoms also induces fine-tuning of the electrochemical potential of semiconductor nanocrystals indicating changes in oxidizing abilities.
Upon surface modification, the effective bandgap of catechol-, gallic acid-, and pyrogallol-modified TiO2 nanoparticles at pH = 2 were determined to be: 1.96, 1.92, and 1.89 eV, respectively. These results indicate similar electrochemical potentials of semiconducting nanocrystals modified with chosen class of enediol ligands indicating similar binding structures and effective electronic coupling. TiO2 nanoparticles modified with this type of bidentate benzene derivatives can be used for development of Type II dye-sensitized nanoporous titania solar cells where the dyes bind to the particle surface through enediol groups . Also, blends of surface modified TiO2 nanoparticles and organic semiconductors cast from co-solutions can be used for synthesis of hybrid solar cells combining the unique properties of inorganic semiconductor and film-forming properties of polymers .
Determination of Stability (Binding) Constants
where [L] is the concentration of ligand, A and A max the absorbances of a CT complex for a given concentration of ligand L, and saturation concentration corresponding to the full coverage of TiO2 surface, respectively.
It must be pointed out that very interesting phenomenon was observed when TiO2 solutions of the same molecular concentration (3.5 mM) freshly prepared and 4-years-old were used for modification (Inset of Fig. 2).A 425 versus pyrogallol concentration curve for freshly prepared TiO2 shows much higher saturation value of A 425 than was observed in the case of 4-years-old TiO2 colloidal solution (A 425 4-years aged = 0.65 A 425 fleshly prepared). It is known that crystallinity increases with aging of TiO2 nanoparticles inducing the decrease in concentration of coordinatively unsaturated surface defect sites. In order to eliminate the possibility that decrease in number of Tisurf sites is simply the consequence of agglomeration or Oswald ripening we performed the measurements of the particle size diameter using dynamic light scattering (DLS). In both cases (freshly prepared and 4-years-old TiO2 solutions) the effective diameter was found to be the same.
Binding Structure of Ligands at Nanoparticle Surface
The way ligands bind to TiO2 surface was investigated by using ATR–FTIR spectroscopy. Since the infrared spectrum of dried TiO2 has only the characteristic broadband in 3700–2000 cm−1 region , we were able to measure spectra of modified colloids in 1750–1000 cm−1 region where the characteristic bands of modifiers exist. Spectra of adsorbed ligands were obtained by subtracting the spectrum of bare TiO2 nanoparticles from the spectrum of surface modified TiO2 nanoparticles.
According to the molecular structure of gallic acid two types of binding to Tisurf may appear—catecholate (OH, OH) or carboxylate (COOH) type. Literature data are quite controversial whether or not benzoic acid, having only COOH functional group, adsorbs on TiO2 surface. Tunesi and Anderson [19, 22] claim that no adsorption was observed, while the others [14, 16, 31, 33, 51] confirmed the formation of bidentate chelate or bridging complexes, pointing out weak adsorption [14, 31]. According to FTIR results obtained we may conclude that in the case of gallic acid binding through carboxylate group does not exist. Possible reason for that is as follows: in the carboxylate type of binding formation of chelated four-membered ring is proposed  being energetically less favorable than the ring formed through catecholate group (Scheme 1).
Hence, in all three ligands investigated (catechol, pyrogallol, and gallic acid) catecholate type of binding is obvious, since carboxylate type of binding takes no part in the binding of gallic acid to TiO2 nanoparticles.
All investigated ligands (catechol, pyrogallol, and gallic acid) form inner-sphere CT complexes with TiO2 nanoparticles (d = 45 Å). Binding of the modifier molecules to undercoordinated surface Ti atoms (defect sites) results in a significant change in the onset of absorption and the effective bandgap. From the Benesi–Hildebrand plot, the stability constants at pH 2 of the order 103 M−1 have been determined. For chosen enediol modifiers binding was found to be through bidentate binuclear (bridging) complexes leading to restoration of six-coordinated octahedral geometry of surface Ti atoms. Stabilized charge separation, being important feature of these systems opens-up possibility for using modifier molecules as conductive leads that allow electronic linking of the nanoparticle into molecular circuits providing further extension of photoinduced electron transfer.
Financial support for this study was granted by the Ministry of Science and Technological Development of the Republic of Serbia (Project 142066).
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