Sensitized TiO2 Nanoparticles Research Articles

Improving the photocatalytic performance of conducting polymer polythiophene sensitized TiO2 nanoparticles under sunlight irradiation

Titanium dioxide (TiO2), polythiophene and polythiophene/TiO2 were prepared by sol–gel and solid-state reaction methods. Water-free iron(III) chloride (FeCl3) was used as an oxidant. The phase composition, morphology and the spectral properties of the products were characterized by XRD, TEM, UV–Vis and FT-IR techniques. The photocatalytic activity of the products was evaluated by the degradation of methyl orange under sunlight irradiation. TEM results showed that the polythiophene/TiO2 composite particles were well dispersed, rod-like shaped with 20 × 80 nm dimensions. UV–Vis analysis indicated that the absorption edge of polythiophene/TiO2 was 605 nm. Compared with the unmodified TiO2 and bare polythiophene, polythiophene/TiO2 exhibited largely enhanced activity for the photocatalytic degradation of methyl orange under sunlight irradiation. A degradation efficiency of methyl orange of 85.6% could be obtained within 120 min. The sensitization mechanism of polythiophene for the TiO2 photocatalyst is discussed briefly.

Effect of strong coupling on interfacial electron transfer dynamics in dye-sensitized TiO2 semiconductor nanoparticles

Dynamics of interfacial electron transfer (ET) in ruthenium polypyridyl complex [{bis-(2,2′-bpy)-(4-[2-(4′-methyl-[2,2′]bipyridinyl-4-yl)-vinyl]-benzene-1,2-diol)}ruthenium(II) hexafluorophosphate] (Ru-cat) and 5,10,15-tris phenyl-20-(3,4-dihydroxy benzene) porphyrin (TPP-cat)-sensitized TiO2 nanoparticles have been investigated using femtosecond transient absorption spectroscopic detection in the visible and near-infrared region. We have observed that both Ru-cat and TPP-cat are coupled strongly with the TiO2 nanoparticles through their pendant catechol moieties. We have observed a single exponential and pulse-width limited (<100 fs) electron injection from nonthermalized-excited states of Ru-complex. Here electron injection competes with the singlet-triplet manifold relaxation due to strong coupling of catecholate binding, which is a unique observation. Optical absorption spectra indicate that the catechol moiety interacts with TiO2 nanoparticles showing the characteristic pure catechol-TiO2 charge-transfer (CT) band in the visible region. Transient absorption studies on TPP-cat/TiO2 system exciting both the Soret band at 400 nm and the Q-band at 800 nm have been carried out to determine excitation wavelength-dependence on ET dynamics. The reaction channel for the electron-injection process has been found to be different for both the excitation wavelengths. Excitation at 800 nm, is found directly populate directly the excited CT state from where diffusion of electrons into the conduction band takes place. On the other hand, excitation at 400 nm light excites both the CT band of cat-TiO2 and also Soret band of TPP-cat.

Strongly Coupled Ruthenium−Polypyridyl Complexes for Efficient Electron Injection in Dye-Sensitized Semiconductor Nanoparticles

Dynamics of interfacial electron transfer (ET) in the ruthenium-polypyridyl complex [{bis(2,2'-bpy)-(4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol)} ruthenium(II) hexafluorophosphate] (Ru-cat)-sensitized TiO(2) nanoparticles has been investigated using femtosecond transient absorption spectroscopy detecting in the visible and near-infrared region. It has been observed that Ru-cat is coupled strongly with the TiO(2) nanoparticles through its pendant catechol moiety. Electron injection has been confirmed by direct detection of electrons in the conduction band, cation radical of the adsorbed dye, and a bleach of the dye in real time as monitored by transient absorption spectroscopy. A single-exponential and pulse width limited (<100 fs) electron injection has been observed, and the origin of it might have been from the nonthermalized excited states of the Ru-cat molecule. The result gave a strong indication that the electron injection competes with the thermalization of the photoexcited states due to large coupling elements for the forward ET reaction. Back-ET dynamics has been determined by monitoring the decay kinetics of the cation radical and injected electron and also from recovery kinetics of the bleach of the adsorbed dye. It has been fit with a multiexponential function, where approximately 30% of the injected electrons are recombined with a time constant of <2 ps, again indicating large coupling elements for the charge recombination reaction. However, our results have shown relatively long-lived charge separation in the Ru-cat/TiO(2) system as compared to other organic dye-sensitized TiO(2) nanoparticles with similar interactions.

Effect of Surface Modification on Back Electron Transfer Dynamics of Dibromo Fluorescein Sensitized TiO2 Nanoparticles

Electron injection and back electron transfer (BET) dynamics have been carried out for dibromo fluorescein (DBF) sensitized TiO2 nanoparticles capped (modified) with sodium dodecyl benzene sulfonate using transient absorption techniques in picosecond and microsecond time domain. BET dynamics have been compared with bare (unmodified) nanoparticles for the same DBF/TiO2 system. It has been observed that BET reaction is slow on the modified surface compared to a bare surface in earlier time domain (picosecond). This observation has been explained by the fact that on surface modification the energy levels of the semiconductor nanoparticles are pushed up in energy. As a result, the free energy of reaction (-deltaG zero) for BET reaction of a dye/SM-TiO2 system increases as compared to the dye/bare TiO2 system. High exoergic BET reaction in dye-sensitized TiO2 nanoparticles surfaces fall in the Marcus inverted regime, so with increasing free energy of reaction, BET rate decreases on the modified surface. However, a reversible trend in BET dynamics has been observed for the above systems in the longer time domain (microsecond). In microsecond time domain BET reaction is faster on the modified surface as compared to on the bare surface. Modification of this surface reduces the density of deep trap states. Recombination dynamics between deep-trapped electron and parent cation is slow due to low coupling strength of BET reaction. As the density of deep-trapped electrons is high in bare particles, BET reaction is slow in longer time domain.

Interfacial electron transfer in FeII(CN)6 4- -sensitized TiO2 nanoparticles: a study of direct charge injection by electroabsorption spectroscopy.

Electroabsorption (Stark) spectroscopy has been used to study the dye sensitized interfacial electron transfer in an Fe(II)(CN)(6)(4)(-) donor complex bound to a TiO(2) nanoparticle. The average charge-transfer distance determined from the Stark spectra is 5.3 A. This value is similar to the estimated distance between the Fe(II) center of the complex and the Ti(IV) surface site coordinated to the nitrogen end of a bridging CN ligand in (CN)(5)Fe(II)-CN-Ti(IV)(particle). This finding suggests that the electron injection is to either an individual titanium surface site or a small number of Ti centers localized around the point of ferrocyanide coordination to the particle and not into a conduction band orbital delocalized over the nanoparticle. The polarizability change, Tr(Deltaalpha), between the ground and the excited states of the Fe(II)(CN)(6)(4)(-)-TiO(2)(particle) system is approximately 3 time larger than normally observed in mixed-valence dinuclear metal complexes. It is proposed that the large polarizability of the excited state increases the dipole-moment changes measured by Stark spectroscopy.

Photoinduced electron transfer between dye IR-140 and TiO2colloids by femtosecond pump supercontinuum probing

We employ the femtosecond pump supercontinuum-probing technique to investigate the photoinduced electron transfer (ET) dynamics in dye IR-140 sensitized TiO2 nanoparticles (average diameter ~ 7 nm) in ethanol. After 807 nm excitation, transient absorption spectra and kinetics are recorded in the spectral region from 400 to 1000 nm, the fluorescence quenching correlated to the electron injection process is observed and the electron injection time from the excited singlet state of dye IR-140 into the conductor band of TiO2 nanoparticles is measured to be ~ 200 fs. The strong quenching of the emission yield and the shortening of the fluorescence lifetime were verified in the dye associated with the TiO2 surface in ethanol.

Interfacial Electron Transfer between Fe(II)(CN)64- and TiO2 Nanoparticles: Direct Electron Injection and Nonexponential Recombination

Photoinduced electron-transfer (ET) dynamics in Fe(II)(CN)64- sensitized TiO2 nanoparticles in D2O solution are studied by subpicosecond tunable laser spectroscopy in the mid-infrared and visible region. The dynamics of the injected electrons are monitored by the mid-IR absorption of electrons in the semiconductor, and the corresponding dynamics of the adsorbate are monitored by the vibrational spectra of the CN stretching mode region and electronic absorption in the visible. After 400 nm excitation, the forward electron injection time from Fe(II)(CN)64- to TiO2 occurs in <50 fs, indicating a direct photoinduced charge-transfer process. The back ET from TiO2 to Fe(III)(CN)63- in the <1 ns time scale is found to be a non-single-exponential process. The best three-exponential fit to the data yields back ET time constants of 3 ps (35%), 40 ps (30%), and >1 ns (35%). Combining with previous measurements in the nanosecond to microsecond time scale (Lu et al. J. Am. Chem. Soc. 1993, 115, 4927 and Vrachnou et al. J. Chem. Soc., Chem. Commun. 1987, 868), the total back ET process is found to be highly nonexponential with time constants ranging from 3 ps to 3 μs.

Direct Observation of Ultrafast Electron Injection from Coumarin 343 to TiO2 Nanoparticles by Femtosecond Infrared Spectroscopy

Transient infrared (IR) absorption of injected electrons in colloidal TiO2 nanoparticles in the 1900−2000 cm-1 region are measured by femtosecond IR spectroscopy. The direct detection of electrons in the nanoparticles with subpicosecond time resolution provides a new approach to study ultrafast interfacial electron transfer between semiconductor nanoparticles and molecular adsorbates. The dynamics of electron injection from sensitizers to nanoparticles and the subsequent back-transfer and relaxation dynamics of the injected electrons correspond to the rise and decay of the transient IR signal of injected electrons. Using this technique, the injection time for coumarin 343 sensitized TiO2 nanoparticles in D2O is determined to be 125 ± 25 fs. The subsequent decay dynamics of the injected electrons in nanoparticles are found to be different from conduction band electrons in a bulk TiO2 crystal.