Interfacial Charge Transfer Complex Research Articles

Photocatalytic Reactions over TiO2-Based Interfacial Charge Transfer Complexes

The present review is related to the novel approach for improvement of the optical properties of wide bandgap metal oxides, in particular TiO2, based on the formation of the inorganic–organic hybrids that display absorption in the visible spectral range due to the formation of interfacial charge transfer (ICT) complexes. We outlined the property requirements of TiO2-based ICT complexes for efficient photo-induced catalytic reactions, emphasizing the simplicity of the synthetic procedure, the possibility of the fine-tuning of the optical properties supported by the density functional theory (DFT) calculations, and the formation of a covalent linkage between the inorganic and organic components of hybrids, i.e., the nature of the interface. In addition, this study provides a comprehensive insight into the potential applications of TiO2-based ICT complexes in photo-driven catalytic reactions (water splitting and degradation of organic molecules), including the identification of the reactive species that participate in photocatalytic reactions by the spin-trapping electron paramagnetic resonance (EPR) technique. Considering the practically limitless number of combinations between the inorganic and organic components capable of forming oxide-based ICT complexes and with the knowledge that this research area is unexplored, we are confident it is worth studying, and we emphasized some further perspectives.

Interfacial charge transfer complexes between ZnO and benzene derivatives: Characterization and photocatalytic hydrogen production

The interfacial charge transfer (ICT) complex formation is a simple procedure to bring optical absorption of wide-bandgap oxide materials in the visible spectral range, crucial for enhancing their use in photo-driven reactions. The optical absorption of the prepared ICT complexes between ZnO and five different colorless benzene derivatives is red-shifted compared to pristine ZnO nanopowder. The density functional theory (DFT) calculations provided realistic energy level alignment in hybrid systems. Also, the DFT-calculated infrared spectra support the binding structures derived based on experimental measurements of free and adsorbed ligands onto ZnO surfaces. The photocatalytic performance of prepared hybrids was evaluated using photocatalytic hydrogen generation in the water-splitting reaction. The ZnO nanopowders modified with catechol and caffeic acid have over 50% higher hydrogen production rate than pristine ZnO, displaying steady hydrogen production under long-run working conditions.

The photocatalytic ability of visible-light-responsive interfacial charge transfer complex between TiO2 and Tiron

The surface modification of commercial TiO2 powder with catecholate-type ligand Tiron (TIR) leads to the formation of the interfacial charge transfer complex (ICT) absorbing in the visible spectral range. The estimated band gap energy of the ICT complex (Eg = 2.2 eV) by density functional theory (DFT) calculations agrees with experimental measurements. The surface-modified TiO2 with TIR has enhanced sorption capacity towards Pb2+ ions compared to the pristine one due to the presence of free sulfonate groups. Our attempt to reduce Pb2+ ions to metallic form failed. The TiO2-based ICT complex with TIR can serve as an efficient sorbent to remove Pb2+ ions from the solution without the ability to recover them in metallic form in a photo-driven catalytic process. The photocatalytic ability of the ICT complex to induce oxidation reactions is significantly improved since the complete degradation of organic dye methyl orange occurs under exclusive excitations with visible light photons.

Fabrication of Visible Light Sensitive Electrospun TiO2 Nanofibers Using Squaric Acid for Photocatalytic Application

Degradation of organic pollutants using photocatalysts has gained utmost importance, due to the increasing environmental pollution. Despite various attempts to improve the photocatalytic efficiency of well-known photocatalysts such as titanium dioxide (TiO2), by making them visible light active, various issues need to be resolved. In this work, attempts have been made to improve the visible light absorption capacities of the electrospun TiO2 nanofibers by modification using squaric acid (SqA). An interfacial charge transfer complex is formed by the condensation reaction between the hydroxyl groups on the surface of the TiO2 nanofibers and the SqA ligand. Various characterizations confirmed that the modification using SqA had led to the formation of the interfacial charge transfer layer, without affecting the crystallinity or morphology of the TiO2 nanofibers. The modified TiO2 nanofibers showed sensitivity to visible light with red shift in the optical absorption. It exhibited an improved photocatalytic efficiency of 85% against the degradation of tetracycline, compared with 60% for unmodified TiO2 nanofibers. It also showed an increased rate of degradation of 0.21 mg/L/min, when compared with the 0.13 mg/L/min of unmodified TiO2 nanofibers.

New insights into cupric ion-mediated ligand-to-metal charge transfer between TiO2 with peroxydisulfate under visible light for bolstering benzophenone-3 degradation

Effective removal of hazardous and low biodegradable benzophenone-3 (BP-3) has been a challenging problem. In this study, a new Vis/TiO2/PDS/Cu2+ system was constructed to evaluate the catalytic performance of BP-3 during the co-induced ligand-to-metal charge transfer (LMCT) process by Cu2+, PDS and TiO2. Under visible light irradiation, trace Cu2+ efficiently triggering LMCT process from PDS to TiO2 via Cu–O bridging effect was firstly reported. In this LMCT-mediated photocatalysis process, Cu2+ was not only involved in the formation of visible-light responsive TiO2-Cu-PDS complexes but also inhibited the back electron transfer and promoted PDS activation by accelerating the electron transfer from the conductive band (CB) of TiO2 to PDS via Cu2+/Cu+ cycle. Compared with BP–3 removal in Cu2+/PDS system (18%) and Vis/TiO2 system (48%), that in Vis/TiO2/Cu2+ system (58%) and Vis/TiO2/PDS system (54%) exhibited a slight improvement. It was worth noting that BP-3 removal in Vis/Cu2+/TiO2/PDS system reached 91% within 30 min and the corresponding TOC removal was up to 76% in 120 min, which proved that the significantly enhancement of BP-3 degradation in Vis/Cu2+/TiO2/PDS system. Meanwhile, this system could maintain efficient catalytic performance in a wide pH range from 3 to 8. The formation of an interfacial charge transfer complex of Cu-PDS on TiO2, and the boosted generation of O2–, SO4− and HO radicals over the Vis/Cu2+/TiO2/PDS system were confirmed. Notably, visible light excited Cu-PDS complexes to distribute electrons to TiO2 (CB) via Cu–O bridge action, thereby inducing the enhanced visible-light activity of TiO2 and achieving superior catalytic performance for various pollutants in Vis/Cu2+/TiO2/PDS system. Furthermore, enhanced activation mechanisms of TiO2 and PDS by visible-light induced LMCT process were proposed in detail. Hydroxylation, demethylation and ring-opening dominated BP-3 degradation, and the weakened toxicity of BP-3 in Vis/TiO2/PDS/Cu2+ system was confirmed via the distinct increase of E.coli colonies numbers at the introduction of intermediates. This study offers new insights into LMCT activation approach and broadens the understanding of the ability of Cu2+ to be applied in photo-activated persulfate-based advanced oxidation processes for environmental remediation.

A low cost and green curcumin/ZnO nanocomposites: Preparation, characterization and photocatalytic aspects in removal of amaranth dye and hydrogen evolution generation

Curcumin is natural non-toxic yellow pigment with potent biological and chemical reactivity due to the existence of keto-enol chemical structure. Incorporation of curcumin on ZnO surface enhances the absorbability of visible light radiations and increases the redox power of the charge carriers, In this research, curcumin/ZnO nanoparticles are generated for efficient decomposition of anionic amaranth dye and production of hydrogen gas as renewable fuel resource. ZnO nanoparticles were synthesized through sol-gel concepts by controlled hydrolysis of zinc acetylacetonate in presence of pluronic template. Curcumin with various concentration [1–5] wt% are deposited on ZnO nanoparticles in ultrasonic bath of 200 W intensity. The solid specimens are well explored by XRD, adsorption isotherms of nitrogen at 77K, HRTEM, XPS, DRS and PL. Curcumin molecules are interacted chemically with ZnO nanoparticles through interfacial charge transfer complex that improves the visible light harvesting and enhances the efficiency of the charge carriers separation. In solar reactor, ZnO with 3 wt % curcumin decomposes 95% of amaranth dye and produce hydrogen gas with evolution rate 5.1 mmolg−1hr−1. Depend on step S-scheme charge migration route, the positive hole and negative electrons in the valence band and LUMO energy level of ZnO and curcumin, respectively exerts a strong oxidative and reductive efficiency for the photocatalytic processes. The trapping experiments reveal that superoxide, positive holes and hydroxyl radicals are generated for rapid decomposition of amaranth dye. The durability and high photocatalytic performance of this novel nanocomposites can be used in various industrial and environmental purposes.

Interfacial Charge Transfer Complexes in TiO2-Enediol Hybrids Synthesized by Sol-Gel.

Metal oxide-organic hybrid semiconductors exhibit specific properties depending not only on their composition but also on the synthesis procedure, and particularly on the functionalization method, determining the interaction between the two components. Surface adsorption is the most common way to prepare organic-modified metal oxides. Here a simple sol–gel route is described as an alternative, finely controlled strategy to synthesize titanium oxide-based materials containing organic molecules coordinated to the metal. The effect of the molecular structure of the ligands on the surface properties of the hybrids is studied using three enediols able to form charge transfer complexes: catechol, dopamine, and ascorbic acid. For each system, the process conditions driving the transition from the sol to chemical, physical, or particulate gels are explored. The structural, optical, and photoelectrochemical characterization of the amorphous hybrid materials shows analogies and differences related to the organic component. In particular, electron paramagnetic resonance (EPR) spectroscopy at room temperature reveals the presence of organic radical species with different evolution and stability, and photocurrent measurements prove the effective photosensitization of TiO2 in the visible range induced by interfacial ligand-to-metal charge transfer.

Interfacial charge transfer complex formation between silver nanoparticles and aromatic amino acids.

The optical properties of surface-modified silver nanoparticles (Ag NPs) with aromatic amino acids tryptophan (Trp) and histidine (His) were examined using the cluster model for density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Also, the redistribution of electronic charges upon chemisorption of ligand molecules onto silver's surfaces is determined. The obtained theoretical data, on one side, undoubtedly indicate the the formation of an interfacial charge transfer (ICT) complex between silver and this type of ligand, and, on the other side, partial oxidation of surface silver atoms accompanied by an increase of electron density in ligand molecules. The ICT complex formation, based on noble metal nanoparticles, has never been reported previously to the best of our knowledge. The experimental spectroscopic measurements support the theoretical data. A new absorption band in the visible spectral range appears upon surface modification of Ag NPs, and, when exposed to air, oxidation of surface-modified Ag NPs is significantly faster than the oxidation of the unmodified ones.

Interfacial charge transfer complex between TiO2 and non-aromatic ligand squaric acid

The attachment of squaric acid, a non-aromatic molecule, to the surface of TiO2 powder induced the optical absorption of the obtained hybrid material in the visible spectral range due to the interfacial charge transfer complex formation. The optical characterization of the hybrid is supported by the density functional theory calculations of the model cluster. The paramagnetic species generated upon excitation with ultraviolet or visible light, in both TiO2 powders, pristine and surface-modified, were identified conducting low-temperature solid-state and indirect electron paramagnetic resonance (EPR) spectroscopy experiments (spin trapping and spin scavenging). The solid-state EPR experiments indicated the promotion of electrons from the organic moiety to the titania conduction band under visible-light excitation of hybrid. Also, the spin scavenging experiments showed that the electrons generated in the hybrid upon the visible-light activation facilitate the reduction of the radical cations present in the dispersion, while these effects are not observed for pristine TiO2.

Tuning Properties of Cerium Dioxide Nanoparticles by Surface Modification with Catecholate-type of Ligands.

Cerium dioxide (CeO2) finds applications in areas such as corrosion protection, solar cells, or catalysis, finding increasing applications in biomedicine. This work reports on surface-modified CeO2 particles in order to tune their applicability in the biomedical field. Stable aqueous CeO2 sol, consisting of 3-4 nm in size crystallites, was synthesized using forced hydrolysis. The coordination of catecholate-type of ligands (catechol, caffeic acid, tiron, and dopamine) to the surface-Ce atoms is followed with the appearance of absorption in the visible spectral range as a consequence of interfacial charge-transfer complex formation. The spectroscopic observations are complemented with the density functional theory calculations using a cluster model. The synthesized samples were characterized by X-ray diffraction analysis, transmission electron microscopy, and nitrogen adsorption-desorption isotherms. The ζ-potential measurements indicated that the stability of CeO2 sol is preserved upon surface modification. The pristine CeO2 nanoparticles (NPs) are nontoxic against pre-osteoblast cells in the entire studied concentration range (up to 1.5 mM). Hybrid CeO2 NPs, capped with dopamine or caffeic acid, display toxic behavior for concentrations ≥0.17 and 1.5 mM, respectively. On the other hand, surface-modified CeO2 NPs with catechol and tiron promote the proliferation of pre-osteoblast cells.

Fermi level pinned molecular donor/acceptor junctions: reduction of induced carrier density by interfacial charge transfer complexes

Charge transfer complex (CPX) formation at a donor–acceptor interface reduces the amount of Fermi-level pinning induced interfacial charge transfer.

Efficiency of the interfacial charge transfer complex between TiO2 nanoparticles and caffeic acid against DNA damage in vitro: A combinatorial analysis

The genotoxic and antigenotoxic behavior of the interfacial charge transfer (ICT) complex between nano-sized TiO2 particles and caffeic acid (CA) was studied in in vitro experiments. The formation of the ICT complex is indicated by the appearance of absorption in visible-spectral range. The continual variations method indicated bridging coordination between the ligand, caffeic acid, and the surface Ti atoms, while the stability constant of the ICT complex was found to be 1.5?103 mol-1 L. An agreement between the experimental data and the theoretical results, based on the density functional theory, was found. The ICT complex and its components did not display genotoxicity in the broad concentration range 0.4?8.0 mg mL-1 TiO2 at a mole ratio c(TiO2)/c(CA) = 8. On the other hand, post-treatment of damaged DNA by the ICT complex induced antigenotoxic effect at lower concentrations, but at higher concentrations, 5.125?10.250 mg mL-1 ICT, the ICT complex did not show any beneficial effect on H2O2-induced DNA damaged cells. The experimental data were analyzed using the combinatorial method to determine the effect of component interaction on the genotoxic and antigenotoxic behavior of the ICT complex.

Computational study of interfacial charge transfer complexes of 2-anthroic acid adsorbed on a titania nanocluster for direct injection solar cells

Adsorption and light absorption properties of interfacial charge transfer complexes of 2-anthroic acid and titania, promising for direct-injection solar cells, are studied ab initio. The formation of interfacial charge transfer bands is observed. The intensity of visible absorption is relatively low, highlighting a key challenge facing direct injection cells. We show that the popular strategy of using a lower level of theory for geometry optimization followed by single point calculations of adsorption or optical properties introduces significant errors which have been underappreciated: by up to 3eV in adsorption energies, by up to 5 times in light absorption intensity.

Visible-light photocurrent response of TiO2–polyheptazine hybrids: evidence for interfacial charge-transfer absorption

We investigated photoelectrodes based on TiO(2)-polyheptazine hybrid materials. Since both TiO(2) and polyheptazine are extremely chemically stable, these materials are highly promising candidates for fabrication of photoanodes for water photooxidation. The properties of the hybrids were experimentally determined by a careful analysis of optical absorption spectra, luminescence properties and photoelectrochemical measurements, and corroborated by quantum chemical calculations. We provide for the first time clear experimental evidence for the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO(2) (acceptor), which is responsible for a significant red shift of absorption and photocurrent response of the hybrid as compared to both of the single components. The direct optical charge transfer from the HOMO of polyheptazine to the conduction band edge of TiO(2) gives rise to an absorption band centered at 2.3 eV (540 nm). The estimated potential of photogenerated holes (+1.7 V vs. NHE, pH 7) allows for photooxidation of water (+0.82 V vs. NHE, pH 7) as evidenced by visible light-driven (λ > 420 nm) evolution of dioxygen on hybrid electrodes modified with IrO(2) nanoparticles as a co-catalyst. The quantum-chemical simulations demonstrate that the TiO(2)-polyheptazine interface is a complex and flexible system energetically favorable for proton-transfer processes required for water oxidation. Apart from water splitting, this type of hybrid materials may also find further applications in a broader research area of solar energy conversion and photo-responsive devices.

Performances enhancement in OLEDs by inserting ultrathin trilayer in electron injection structure and using MoO 3 as hole buffer layer

In order to improve performance of Organic Light-emitting Diodes (OLEDs) in this paper, the electron injection is enhanced by inserting an ultrathin Alq 3–LiF–Al trilayer in between the organic layer and the LiF/Al cathode, which is explained by the formation of a thin n-doped Alq 3 layer that improves electron injection. At the same time, a buffer layer using MoO 3 was induced between m-MTDATA and N,N′-bis-[1-naphthy(-N,N′diphenyl-1,1′-biphenyl-4,4′-diamine)](NPB) to prompt hole injection because not only the Highest Occupied Molecular Orbital (HOMO) of MoO 3 is suitable to be used as buffer layer but also the interfacial doping effect of MoO 3 induces interfacial charge transfer complex formation between MoO 3 and NPB. At current density of 20 mA/cm 2, compared with control device, the current efficiency, power efficiency and luminance of device using ultrathin Alq 3–LiF–Al trilayer and MoO 3 were increased by 64%, 101% and 63% respectively, while driving voltage was reduced by 26%. After a series of single-carrier devices were made to analyze performance improvement of device using ultrathin Alq 3–LiF–Al trilayer and MoO 3, it is found that efficient carrier balance can improve device performance remarkably.

Effect of Insulating Oxide Overlayers on Electron Injection Dynamics in Dye-Sensitized Nanocrystalline Thin Films

Insulating metal oxide coatings has been investigated as a potential approach for improving the efficiency of dye-sensitized solar cells. Thin insulating overlayers were thought to improve the cell efficiency by retardation of the recombination kinetics of injected electrons in semiconductor thin films with the oxidized sensitizer molecules and the electrolytes. However, how they affect the electron injection dynamics is still unclear. In this work, we examined the effects of Al2O3 overlayers on the electron injection dynamics in Ru and Re bipyridyl complexes sensitized nanocrystalline TiO2 and SnO2 films. Electron injection dynamics was measured as a function of the number of overlayers by ultrafast infrared transient absorption spectroscopy. The uniformity of the coating was also probed using interfacial charge-transfer complexes. The injection rate was found to decrease when the number of Al2O3 overlayers increases, and the mechanisms by which the overlayers affect the injection dynamics were discussed.

Escape dynamics of photoexcited electrons at catechol:TiO2(110)

Ultrafast electron escape dynamics following excitation of the interfacial charge transfer complex of catechol prepared on the rutile ${\mathrm{TiO}}_{2}(110)$ surface was investigated with femtosecond two-photon photoemission (2PPE). Laser pulses were generated with two noncollinear optical parametric amplifiers operated simultaneously at a repetition rate of $150\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$ delivering a cross-correlation function with $35\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ full width at half maximum. Catechol was absorbed from the solution. The experimental data were not depending on the choice between three different solvents. Photoinduced interfacial charge transfer was instantaneous and thus the rise of the signal was controlled by the cross-correlation function. The energy distribution of the hot electrons generated at the surface was measured as 2PPE spectrum. The decay of the 2PPE signal was nonexponential with a first time constant below $10\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, a dip in the $50\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ to $100\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ range, and a tail lasting for picoseconds. It was attributed to the release of the electrons from the surface and their escape into the bulk of the semiconductor.

Study of Interfacial Charge-Transfer Complex on TiO2 Particles in Aqueous Suspension by Second-Harmonic Generation

We report the first direct observation of an interfacial charge-transfer complex using second-harmonic spectroscopy. The second-harmonic spectrum of catechol adsorbed on 0.4 micron-sized TiO2 (anatase) colloidal particles in aqueous suspension reveals a charge-transfer band centered at 2.72 eV (456 nm). In addition, the adsorption isotherm of catechol on the colloidal TiO2 suspension was obtained and gave an excellent fit to the Langmuir adsorption model. From this, we infer the free energy of the adsorption to be ΔG° = −6.8 kcal/mol.

Hole Reactions from d‐Energy Bands of Layer Type Group VI Transition Metal Dichalcogenides: New Perspectives for Electrochemical Solar Energy Conversion

Holes, photogenerated in d‐energy bands of covalent semiconducting layer‐type group VI ‐transition metal dichalcogenides react electrochemically differently from holes generated in semiconductors with valence bands based on p‐orbitals (e.g., , , , ). They do not constitute broken crystal bonds and do therefore not lead directly to an anodic photodecomposition of the electrode. The chemical character of these holes as missing d‐electrons, on the other hand, gives rise to very specific electrochemical surface reactions with electron donors such as I−, Br−, and even OH−, for example. The specific nature of the interfacial charge transfer complexes formed, their advantageous effect on the potential distribution in the electrode surface, and their favorable oxidation potential (in the case of the photoreaction with OH− ions), respectively, may be the clue to a promising new approach to several unaccomplished goals of photoelectrochemical research, among them the construction of stable regenerative electrochemical solar cells and the oxidation of water with visible light. Photoelectrochemical measurements performed with single crystals as well as experiments on solar energy conversion and photoelectrochemical reactions with water induced by visible and near infrared light are described as experimental evidence in support of this finding.