Water Multilayers on TiO2 (101) Anatase Surface: Assessment of a DFTB-Based Method

Unraveling the atomic structure and dissociation of interfacial water on anatase TiO2 (101) under ambient conditions with solid-state NMR spectroscopy

Anatase TiO2 is a widely used component in photo- and electro-catalysts for water splitting, and the (101) facet of anatase TiO2 is the most commonly exposed surface. A detailed understanding of the behavior of H2O on this surface could provide fundamental insights into the catalytic mechanism. This, however, is challenging due to the complexity of the interfacial environments, the high mobility of interfacial H2O, and the interference from outer-layer H2O. Herein, we investigate the H2O/TiO2 interface using advanced solid-state NMR techniques. The atomic-level structures of surface O sites, OH groups, and adsorbed H2O have been revealed and the detailed interactions among them are identified on the (101) facet of anatase TiO2. By following the quantitative evolution of surface O and OH sites along with H2O loading, it is found that more than 40% of the adsorbed water spontaneously dissociated under ambient conditions on the TiO2 surface at a loading of 0.3 mmol H2O/g, due to the delicate interplay between water-surface and water-water interactions. Our study highlights the importance of understanding the atomic-level structures of H2O on the surface of TiO2 in catalytic reactions. Such knowledge can promote the design of more efficient catalytic systems for renewable energy production involving activation of water molecules.

DFT Calculation of Carbon-Doped TiO2 Nanocomposites

Titanium dioxide (TiO2) has been proven to be an excellent material for mitigating the continuous impact of elevated carbon dioxide concentrations. Carbon doping has emerged as a promising strategy to enhance the CO2 reduction performance of TiO2. In this study, we investigated the effects of carbon doping on TiO2 using density functional theory (DFT) calculations. Two carbon doping concentrations were considered (4% and 6%), denoted as TiO2-2C and TiO2-3C, respectively. The results showed that after carbon doping, the band gaps of TiO2-2C and TiO2-3C were reduced to 1.58 eV and 1.47 eV, respectively, which is lower than the band gap of pure TiO2 (2.13 eV). This indicates an effective improvement in the electronic structure of TiO2. Barrier energy calculations revealed that compared to pure TiO2 (0.65 eV), TiO2-2C (0.54 eV) and TiO2-3C (0.59 eV) exhibited lower energy barriers, facilitating the transition to *COOH intermediates. These findings provide valuable insights into the electronic structure changes induced by carbon doping in TiO2, which can contribute to the development of sustainable energy and environmental conservation measures to address global climate challenges.

Photoinduced Dynamics at the Water/TiO_{2}(101) Interface

We present a femtosecond time-resolved optical pump-soft x-ray probe photoemission study in which we follow the dynamics of charge transfer at the interface of water and anatase TiO_{2}(101). By combining our observation of transient oxygen O 1s core level peak shifts at submonolayer water coverages with Ehrenfest molecular dynamics simulations we find that ultrafast interfacial hole transfer from TiO_{2} to molecularly adsorbed water is completed within the 285 fs time resolution of the experiment. This is facilitated by the formation of a new hydrogen bond between an O_{2c} site at the surface and a physisorbed water molecule. The calculations fully corroborate our experimental observations and further suggest that this process is preceded by the efficient trapping of the hole at the surface of TiO_{2} by hydroxyl species (-OH), that form following the dissociative adsorption of water. At a water coverage exceeding a monolayer, interfacial charge transfer is suppressed. Our findings are directly applicable to a wide range of photocatalytic systems in which water plays a critical role.

Multi-scale modeling of folic acid-functionalized TiO2 nanoparticles for active targeting of tumor cells

Strategies based on the active targeting of tumor cells are emerging as smart and efficient nanomedical procedures. Folic acid (FA) is a vitamin and a well-established tumor targeting agent because of its strong affinity for the folate receptor (FR), which is an overexpressed protein on the cell membranes of the tumor cells. FA can be successfully anchored to several nanocarriers, including inorganic nanoparticles (NPs) based on transition metal oxides. Among them, TiO₂ is extremely interesting because of its excellent photoabsorption and photocatalytic properties, which can be exploited in photodynamic therapy. However, it is not yet clear in which respects direct anchoring of FA to the NP or the use of spacers, based on polyethylene glycol (PEG) chains, are different and whether one approach is better than the other. In this work, we combine Quantum Mechanics (QM) and classical Molecular Dynamics (MD) to design and optimize the FA functionalization on bare and PEGylated TiO₂ models and to study the dynamical behavior of the resulting nanoconjugates in a pure water environment and in physiological conditions. We observe that they are chemically stable, even under the effect of increasing temperature (up to 500 K). Using the results from long MD simulations (100 ns) and from free energy calculations, we determine how the density of FA molecules on the TiO₂ NP and the presence of PEG spacers impact on the actual exposure of the ligands, especially by affecting the extent of FA-FA intermolecular interactions, which are detrimental for the targeting ability of FA towards the folate receptor. This analysis provides a solid and rational basis for experimentalists to define the optimal FA density and the more appropriate mode of anchoring to the carrier, according to the final purpose of the nanoconjugate.

Quantum Anharmonic Calculations of Vibrational Spectra for Water Adsorbed on Titania Anatase(101) Surface: Dissociative versus Molecular Adsorption

The interaction of water molecules and hydroxyl groups with titanium dioxide (TiO₂) surfaces is ubiquitous and very important in anatase nanoparticle photocatalytic processes. Infrared spectroscopy, assisted by ab initio calculations of vibrational frequencies, can be a powerful tool to elucidate the mechanisms behind water adsorption. However, a straightforward comparison between measurements and calculations remains a challenging task because of the complexity of the physical phenomena occurring on nanoparticle surfaces. Consequently, severe computational approximations, such as harmonic vibrational ones, are usually employed. In the present work we partially address this complexity issue by overcoming some of the standard approximations used in theoretical simulations and employ the Divide and Conquer Semiclassical Initial Value Representation (DC-SCIVR) molecular dynamics. This method allows to perform simulations of vibrational spectra of large dimensional systems accounting not only for anharmonicities, but also for nuclear quantum effects. We apply this computational method to water and deuterated water adsorbed on the ideal TiO₂ anatase(101) surface, contemplating both the molecular and the dissociated adsorption processes. The results highlight not only the presence of an anharmonic shift of the frequencies in agreement with the experiments, but also complex quantum mechanical spectral signatures induced by the coupling of molecular vibrational modes with the surface ones, which are different in the hydrogenated case from the deuterated one. These couplings are further analyzed by exploiting the mode subdivision performed during the divide and conquer procedure.

A DFTB-Based Molecular Dynamics Investigation of an Explicitly Solvated Anatase Nanoparticle

We performed a self-consistent charge density functional tight-binding molecular dynamics (SCC DFTB-MD) simulation of an explicitly solvated anatase nanoparticle. From the 2 ps trajectory, we were able to calculate both dynamic and static properties, such as the energies of interaction and the formation of water layers at the surface, and compare them to the observed behaviour reported elsewhere. The high degree of agreement between our simulation and other sources, and the additional information gained from employing this methodology, highlights the oft-overlooked viability of DFTB-based methods for electronic structure calculations of large systems.

Comparative first principles-based molecular dynamics study of catalytic mechanism and reaction energetics of water oxidation reaction on 2D-surface

The study of the water-splitting process, which can proceed in 2e^- as well as 4e^- pathway, reveals that the process is entirely an uphill process, and the third step, that is, the oxooxo bond formation is the rate-determining step. The kinetic barrier of the oxygen evolution reaction (OER) on the 2D material catalysts in the presence of explicit solvents is scarcely studied. Here, we investigate the dynamics of the OER on the undoped graphene and the activation energy barrier of each step using first principles molecular dynamics simulations. Here we provide a detailed analysis of the kinetics of all the 4e^- transfer steps of OER on the graphene surface. We also compare the accuracy of one of the density functional theory (DFT) functionals and density functional based tight binding (DFTB) method in explaining the OER steps. The comparative study reveals that DFTB can be used for performing metadynamics simulations quipped with much less computational cost than DFT functionals. By both Perdew-Burke-Ernzerhof and DFTB methods, the third step is revealed to be the rate-determining step with an energy barrier of 21.19 ± 0.51 and 20.23 ± 0.20 kcal mol^-1 , respectively. DFTB gives an impression of being successful in predicting the energy barriers of OER in 4e- transfer pathway and comparable to the DFT method, and we would like to extend the use of DFTB for further studies with a sizable and complex system.

Reactive molecular dynamics simulations of hydration shells surrounding spherical TiO2 nanoparticles: implications for proton-transfer reactions

In many potential applications, nanoparticles are typically in an aqueous medium. This has strong influence on the stability, optical properties and reactivity, in particular for their functionalization. Therefore, the understanding of the chemistry at the interface between the solvent and the nanoparticle is of utmost importance. In this work, we present a comparative ReaxFF reactive molecular dynamics investigation on spherical TiO2 nanoparticles (NSs) of realistic size, with diameters from 2.2 to 4.4 nm, immersed in a large drop of bulk water. After force field validation for its use for a curved anatase TiO2 surface/water interface, we performed several simulations of the TiO2 nanoparticles of increasing size in a water drop. We found that water can be adsorbed jointly in a molecular and dissociative way on the surface. A Langmuir isotherm indicating an adsorption/desorption mechanism of water on the NS is observed. Regarding the dissociative adsorption, atomistic details reveal two different mechanisms, depending on the water concentration around the NS. At low coverage, the first mechanism involves direct dissociation of a single water molecule, whereas, at higher water coverage, the second mechanism is a proton transfer reaction involving two water molecules, also known as Grotthuss-like mechanism. Thermal annealing simulations show that several water molecules remain on the surface in agreement with the experimental reports. The capacity of adsorption is higher for the 2.2 and 3.0 nm NSs than for the 4.4 nm NS. Finally, a comparative investigation with flat surfaces indicates that NSs present a higher water adsorption capacity (undissociated and dissociated) than flat surfaces, which can be rationalized considering that NSs present many more low-coordinated Ti atoms available for water adsorption.

Dopamine-Decorated TiO2 Nanoparticles in Water: A QM/MM vs an MM Description

Nanoparticle functionalization is a modern strategy in nanotechnology to build up devices for several applications. Modeling fully decorated metal oxide nanoparticles of realistic size (few nanometers) in an aqueous environment is a challenging task. In this work, we present a case study relevant for solar-light exploitation and for biomedical applications, i.e., a dopamine-functionalized TiO₂ nanoparticle (1700 atoms) in bulk water, for which we have performed an extensive comparative investigation with both MM and QM/MM approaches of the structural properties and of the conformational dynamics. We have used a combined multiscale protocol for a more efficient exploration of the complex conformational space. On the basis of the results of this study and of some QM and experimental data, we have defined strengths and limitations of the existing force field parameters. Our findings will be useful for an improved modeling and simulation of many other similar hybrid bioinorganic nanosystems in an aqueous environment that are pivotal in a broad range of nanotechnological applications.

TETT-functionalized TiO2 nanoparticles for DOX loading: a quantum mechanical study at the atomic scale

In this work, we present a quantum mechanical investigation, based on the self-consistent charge density functional tight-binding (SCC-DFTB) method, of the functionalization with silane-type ligands (TETT) of a spherical TiO₂ nanoparticle of realistic size (2.2 nm containing 700 atoms) to create an efficient nanosystem for simultaneous photodynamic therapy and drug transport. We determine the mechanism of the TETT ligand anchoring and its stability under thermal treatment, through molecular dynamics simulations at 300 K. Then, we build a medium and a full coverage model (22 and 40 TETTs, respectively) and analyze the interaction among TETT ligands and between the ligands and the surface. Finally, on the fully covered nanoparticle, we succeed in localizing two minimum energy structures for an attached doxorubicin anticancer molecule (DOX) and provide the atomistic details for both the covalent and the non-covalent (electrostatic) types of interaction. A future development of this work will be the investigation of the loading capacity of this drug delivery system and of the pH effect of the surrounding aqueous environment.

Density-functional tight-binding: basic concepts and applications to molecules and clusters

The scope of this article is to present an overview of the Density Functional based Tight Binding (DFTB) method and its applications. The paper introduces the basics of DFTB and its standard formulation up to second order. It also addresses methodological developments such as third order expansion, inclusion of non-covalent interactions, schemes to solve the self-interaction error, implementation of long-range short-range separation, treatment of excited states via the time-dependent DFTB scheme, inclusion of DFTB in hybrid high-level/low level schemes (DFT/DFTB or DFTB/MM), fragment decomposition of large systems, large scale potential energy landscape exploration with molecular dynamics in ground or excited states, non-adiabatic dynamics. A number of applications are reviewed, focusing on -(i)- the variety of systems that have been studied such as small molecules, large molecules and biomolecules, bare orfunctionalized clusters, supported or embedded systems, and -(ii)- properties and processes, such as vibrational spectroscopy, collisions, fragmentation, thermodynamics or non-adiabatic dynamics. Finally outlines and perspectives are given.

On the stability of hydroxyl groups on substituted titania

The present study reports the stability of hydroxyl groups involving the surface coordinated oxygens of Pd,C,N-doped, and Pd/C and Pd/N-codoped anatase TiO₂, probed using DFT calculations.

Optimizing PEGylation of TiO2 Nanocrystals through a Combined Experimental and Computational Study

PEGylation of metal oxide nanoparticles is the common approach to improve their biocompatibility and in vivo circulation time. In this work, we present a combined experimental and theoretical study to determine the operating condition that guarantee very high grafting densities, which are desirable in any biomedical application. Moreover, we present an insightful conformational analysis spanning different coverage regimes and increasing polymer chain lengths. Based on 13C NMR measurements and molecular dynamics simulations, we show that classical and popular models of polymer conformation on surfaces fail in determining the mushroom-to-brush transition point and prove that it actually takes place only at rather high grafting density values.

Rational design of nanosystems for simultaneous drug delivery and photodynamic therapy by quantum mechanical modeling

Drug delivery systems are based on reversible interactions between carriers and drugs. Spacers are often introduced to tailor the type of interaction and to keep drugs intact. Here, we model a drug delivery system based on a functionalized curved TiO₂ nanoparticle of realistic size (700 atoms - 2.2 nm) by the neurotransmitter dopamine to carry the anticancer chemotherapeutic agent doxorubicin (DOX). The multiscale quantum chemical study aims at unraveling the nature and mechanism of the interactions between the components and the electronic properties of the composite system. We simulate the temperature effect through molecular dynamics runs of thermal annealing. Dopamine binds preferentially to low coordinated Ti sites on the nanoparticle through dissociated bidentate and chelate modes involving the diol groups. DOX is tethered by H-bonds, π-π stacking, dipole-dipole interactions and dispersion forces. Comparing different coverage densities of the spacer on the nanoparticle surface, we assess the best conditions for an effective drug transport and release: only at full coverage, DOX does not slip among the dopamine molecules to reach the nanoparticle surface, which is crucial to avoid the formation of stable coordinative bonds with under-coordinated Ti atoms. Finally, given the strong absorption properties and fluorescence of DOX and of the TiO₂ photocatalyst, we model the effect of light irradiation through excited state calculations to localize excitons and to follow the charge carrier's life path. This fundamental study on the nature and mechanism of drug/carrier interaction provides a solid ground for the rational design of new experimental protocols for a more efficient drug transport and release and its combination with photodynamic therapy.

Proton Transfers at a Dopamine-Functionalized TiO2 Interface

Despite the many successful syntheses and applications of dopamine-functionalized TiO₂ nanohybrids, there has not yet been an atomistic understanding of the interaction of this 1,2-dihydroxybenzene derivative ligand with the titanium dioxide surfaces. In this work, on the basis of a wide set of dispersion-corrected hybrid density functional theory (DFT) calculations and density functional tight binding (DFTB) molecular dynamics simulations, we present a detailed study of the adsorption modes, patterns of growth, and configurations of dopamine on the anatase (101) TiO₂ surface, with reference to the archetype of 1,2-dihydroxybenzene ligands, i.e., catechol. At low coverage, the isolated dopamine molecule prefers to bend toward the surface, coordinating the NH₂ group to a Ti_5c ion. At high coverage, the packed molecules succeed in bending toward the surface only in some monolayer configurations. When they do, we observe a proton transfer from the surface to the ethyl-amino group, forming terminal NH₃ ⁺ species, which highly interact with the O atoms of a neighboring dopamine molecule. This strong Coulombic interaction largely stabilizes the self-assembled monolayer. On the basis of these results, we predict that improving the probability of dopamine molecules being free to bend toward the surface through thermodynamic versus kinetic growth conditions will lead to a monolayer of fully protonated dopamine molecules.

Density-Functional Tight-Binding Study on the Effects of Interfacial Water in the Adhesion Force between Epoxy Resin and Alumina Surface

Adhesion is one of the most interesting subjects in interface phenomena from the viewpoint of wide-range applications as well as basic science. Interfacial water has significant effects on coatings, adhesives, and fiber-reinforced polymer composites, often causing adhesion loss. The way of thinking based on quantum mechanics is essential for a better understanding of physical and chemical properties of adhesive interfaces. In this work, the molecular mechanism of the adhesion interaction between epoxy resin and hydroxylated alumina surface in the presence of interfacial water molecules is investigated by using density-functional tight-binding calculations. Periodic slab model calculations demonstrate that hydrogen bond is an important factor at the adhesion interface. Effects of interfacial water molecules located between epoxy resin and hydroxylated alumina surface are assessed by using a dry model without interfacial water and wet models with water layers of 3, 6, and 9 Å thicknesses. Interesting first- and second-layer structures are observed in the distribution of interfacial water molecules in the tight space between the adhesive and adherend. Energy plots with respect to the displacement of epoxy resin from the alumina surface are nicely approximated by the Morse potential. The adhesion force and stress are theoretically obtained by differentiating the potential curve with respect to the displacement of epoxy resin. Computational results show that the adhesion force and stress are significantly weakened with an increase in the thickness of interfacial water layer. Thus, interfacial water molecules have a clue as to the role of water in the loss of adhesion.

Curved TiO2 Nanoparticles in Water: Short (Chemical) and Long (Physical) Range Interfacial Effects

In most technological applications, nanoparticles are immersed in a liquid environment. Understanding nanoparticles/liquid interfacial effects is extremely relevant. This work provides a clear and detailed picture of the type of chemistry and physics taking place at the prototypical TiO₂ nanoparticles/water interface, which is crucial in photocatalysis and photoelectrochemistry. We present a multistep and multiscale investigation based on hybrid density functional theory (DFT), density functional tight-binding, and quantum mechanics/molecular mechanics calculations. We consider increasing water partial pressure conditions from ultra-high vacuum up to the bulk water environment. We first investigate single water molecule adsorption modes on various types of undercoordinated sites present on a realistic curved nanoparticle (2-3 nm) and then, by decorating all the adsorption sites, we study a full water monolayer to identify the degree of water dissociation, the Brønsted-Lowry basicity/acidity of the nanoparticle in water, the interface effect on crystallinity, surface energy, and electronic properties, such as the band gap and work function. Furthermore, we increase the water coverage by adding water multilayers up to a thickness of 1 nm and perform molecular dynamics simulations, which evidence layer structuring and molecular orientation around the curved nanoparticle. Finally, we clarify whether these effects arise as a consequence of the tension at the water drop surface around the nanosphere by simulating a bulk water up to a distance of 3 nm from the oxide surface. We prove that the nanoparticle/water interfacial effects go rather long range since the dipole orientation of water molecules is observed up to a distance of 5 Å, whereas water structuring extends at least up to a distance of 8 Å from the surface.

Surface-Charge Dependent Orientation of Water at the Interface of a Gold Electrode: A Cluster Study

A gold/water interface has been investigated with the DFT-based self-consistent-charge density-functional tight-binding (SCC-DFTB) method using a cluster model. Born–Oppenheimer molecular-dynamics simulations for mono-, bi-, and trilayers of water on the surface of a Au55 cluster have been computed. We have demonstrated the applicability of this method to the study of the structural and dynamical properties of the gold/water-multilayer interface. The results of the simulations clearly show the charge-dependent orientation and the corresponding polarization of the water sphere around the gold cluster. However, it was also shown that this polarization is restricted almost only to the first solvation shell. This illustrates the rather short-range screening behavior of water. The present study builds the basis for further investigations of metal/electrolyte interfaces on a reliable atomistic level, avoiding the problems of spurious artifacts in models using periodic boundary conditions.