Diffusion and reaction pathways of water near fully hydrated TiO2 surfaces from ab initio molecular dynamics

Confinement Effects on Proton Transfer in TiO2 Nanopores from Machine Learning Potential Molecular Dynamics Simulations

Improved understanding of proton transfer in nanopores is critical for a wide range of emerging applications, yet experimentally probing mechanisms and energetics of this process remains a significant challenge. To help reveal details of this process, we developed and applied a machine learning potential derived from first-principles calculations to examine water reactivity and proton transfer in TiO2 slit-pores. We find that confinement of water within pores smaller than 0.5 nm imposes strong and complex effects on water reactivity and proton transfer. Although the proton transfer mechanism is similar to that at a TiO2 interface with bulk water, confinement reduces the activation energy of this process, leading to more frequent proton transfer events. This enhanced proton transfer stems from the contraction of oxygen-oxygen distances dictated by the interplay between confinement and hydrophilic interactions. Our simulations also highlight the importance of the surface topology, where faster proton transport is found in the direction where a unique arrangement of surface oxygens enables the formation of an ordered water chain. In a broader context, our study demonstrates that proton transfer in hydrophilic nanopores can be enhanced by controlling pore size, surface chemistry, and topology.

Adsorption of Glycine on TiO2 in Water from On-the-fly Free-Energy Calculations and In Situ Electrochemical Impedance Spectroscopy

We report here an experimental-computational study of hydrated TiO2 anatase nanoparticles interacting with glycine, where we obtain quantitative agreement of the measured adsorption free energies. Ab initio simulations are performed within the tight binding and density functional theory in combination with enhanced free-energy sampling techniques, which exploit the thermodynamic integration of the unbiased mean forces collected on-the-fly along the molecular dynamics trajectories. The experiments adopt a new and efficient setup for electrochemical impedance spectroscopy measurements based on portable screen-printed gold electrodes, which allows fast and in situ signal assessment. The measured adsorption free energy is -30 kJ/mol (both from experiment and calculation), with preferential interaction of the charged NH3+ group which strongly adsorbs on the TiO2 bridging oxygens. This highlights the importance of the terminal amino groups in the adsorption mechanism of amino acids on hydrated metal oxides. The excellent agreement between computation and experiment for this amino acid opens the doors to the exploration of the interaction free energies for other moderately complex bionano systems.

Water effect on the band edges of anatase TiO2 surfaces: A theoretical study on charge migration across surface heterojunctions and facet-dependent photoactivity

The charge migration mechanism across the surface heterojunction constructed on an anatase TiO2 nanocrystal is still under debate. To solve this longstanding question, we present a systematic study of the band edges (vs. standard hydrogen electrode, SHE) of aqueous TiO2 interfaces with anatase (101), (100) and (001) surfaces, using a combination of density functional theory-based molecular dynamics (DFTMD) and efficient computational SHE (cSHE) methods. Our calculations show that the conduction band minimum (CBM) of the (101) surface is lower than that of (001) and (100) surfaces, which is thermodynamically favorable for electrons migrating to the (101) surface through the surface heterojunction, while the hole preferentially accumulates on the (100) surface due to its highest valence band minimum (VBM). In addition, we qualitatively explore the facet-dependent photocatalytic activity of anatase TiO2. Due to the possession of both the beneficial atomic structure (with 100% undercoordinated Ti5c atoms at the surface) and electronic structure (more strongly oxidizing holes in the VBM and efficient electron-hole spatial separation separation), the (001) surface exhibits the most efficient photocatalytic performance for water oxidation. Furthermore, it is confirmed that the use of simplified theoretical models neglecting the detailed atomic structures of water at the aqueous interface is inadequate to predict the band alignment of semiconductors relative to water redox potentials, so that it may result in substantial errors in evaluating the photocatalytic performance of materials to be used for water splitting.

Thermal transport across TiO2-H2O interface involving water dissociation: Ab initio-assisted deep potential molecular dynamics

Water dissociation on TiO2 surfaces has been known for decades and holds great potential in various applications, many of which require a proper understanding of thermal transport across the TiO2-H2O interface. Molecular dynamics (MD) simulations play an important role in characterizing complex systems' interfacial thermal transport properties. Nevertheless, due to the imprecision of empirical force field potentials, the interfacial thermal transport mechanism involving water dissociation remains to be determined. To cope with this, a deep potential (DP) model is formulated through the utilization of ab initio datasets. This model successfully simulates interfacial thermal transport accompanied by water dissociation on the TiO2 surfaces. The trained DP achieves a total energy accuracy of ∼238.8 meV and a force accuracy of ∼197.05 meV/Å. The DPMD simulations show that water dissociation induces the formation of hydrogen bonding networks and molecular bridges. Structural modifications further affect interfacial thermal transport. The interfacial thermal conductance estimated by DP is ∼8.54 × 109 W/m2 K, smaller than ∼13.17 × 109 W/m2 K by empirical potentials. The vibrational density of states (VDOS) quantifies the differences between the DP model and empirical potentials. Notably, the VDOS disparity between the adsorbed hydrogen atoms and normal hydrogen atoms demonstrates the influence of water dissociation on heat transfer processes. This work aims to understand the effect of water dissociation on thermal transport at the TiO2-H2O interface. The findings will provide valuable guidance for the thermal management of photocatalytic devices.

Mechanistic insight on water dissociation on pristine low-index TiO2 surfaces from machine learning molecular dynamics simulations

Water adsorption and dissociation processes on pristine low-index TiO2 interfaces are important but poorly understood outside the well-studied anatase (101) and rutile (110). To understand these, we construct three sets of machine learning potentials that are simultaneously applicable to various TiO2 surfaces, based on three density-functional-theory approximations. Here we show the water dissociation free energies on seven pristine TiO2 surfaces, and predict that anatase (100), anatase (110), rutile (001), and rutile (011) favor water dissociation, anatase (101) and rutile (100) have mostly molecular adsorption, while the simulations of rutile (110) sensitively depend on the slab thickness and molecular adsorption is preferred with thick slabs. Moreover, using an automated algorithm, we reveal that these surfaces follow different types of atomistic mechanisms for proton transfer and water dissociation: one-step, two-step, or both. These mechanisms can be rationalized based on the arrangements of water molecules on the different surfaces. Our finding thus demonstrates that the different pristine TiO2 surfaces react with water in distinct ways, and cannot be represented using just the low-energy anatase (101) and rutile (110) surfaces.

Origin of the Hydrophobic Behaviour of Hydrophilic CeO2

The nature of the hydrophobicity found in rare-earth oxides is intriguing. The CeO2 (100) surface, despite its strongly hydrophilic nature, exhibits hydrophobic behaviour when immersed in water. In order to understand this puzzling and counter-intuitive effect we performed a detailed analysis of the confined water structure and dynamics. We report here an ab-initio molecular dynamics simulation (AIMD) study which demonstrates that the first adsorbed water layer, in immediate contact with the hydroxylated CeO2 surface, generates a hydrophobic interface with respect to the rest of the liquid water. The hydrophobicity is manifested in several ways: a considerable diffusion enhancement of the confined liquid water as compared with bulk water at the same thermodynamic condition, a weak adhesion energy and few H-bonds above the hydrophobic water layer, which may also sustain a water droplet. These findings introduce a new concept in water/rare-earth oxide interfaces: hydrophobicity mediated by specific water patterns on a hydrophilic surface.

Biomolecular Adsorprion at ZnS Nanomaterials: A Molecular Dynamics Simulation Study of the Adsorption Preferences, Effects of the Surface Curvature and Coating

The understanding of interactions between nanomaterials and biological molecules is of primary importance for biomedical applications of nanomaterials, as well as for the evaluation of their possible toxic effects. Here, we carried out extensive molecular dynamics simulations of the adsorption properties of about 30 small molecules representing biomolecular fragments at ZnS surfaces in aqueous media. We computed adsorption free energies and potentials of mean force of amino acid side chain analogs, lipids, and sugar fragments to ZnS (110) crystal surface and to a spherical ZnS nanoparticle. Furthermore, we investigated the effect of poly-methylmethacrylate (PMMA) coating on the adsorption preferences of biomolecules to ZnS. We found that only a few anionic molecules: aspartic and glutamic acids side chains, as well as the anionic form of cysteine show significant binding to pristine ZnS surface, while other molecules show weak or no binding. Spherical ZnS nanoparticles show stronger binding of these molecules due to binding at the edges between different surface facets. Coating of ZnS by PMMA changes binding preferences drastically: the molecules that adsorb to a pristine ZnS surface do not adsorb on PMMA-coated surfaces, while some others, particularly hydrophobic or aromatic amino-acids, show high binding affinity due to binding to the coating. We investigate further the hydration properties of the ZnS surface and relate them to the binding preferences of biomolecules.

Recent Advances in In Situ/Operando Surface/Interface Characterization Techniques for the Study of Artificial Photosynthesis

(Photo-)electrocatalytic artificial photosynthesis driven by electrical and/or solar energy that converts water (H2O) and carbon dioxide (CO2) into hydrogen (H2), carbohydrates and oxygen (O2), has proven to be a promising and effective route for producing clean alternatives to fossil fuels, as well as for storing intermittent renewable energy, and thus to solve the energy crisis and climate change issues that we are facing today. Basic (photo-)electrocatalysis consists of three main processes: (1) light absorption, (2) the separation and transport of photogenerated charge carriers, and (3) the transfer of photogenerated charge carriers at the interfaces. With further research, scientists have found that these three steps are significantly affected by surface and interface properties (e.g., defect, dangling bonds, adsorption/desorption, surface recombination, electric double layer (EDL), surface dipole). Therefore, the catalytic performance, which to a great extent is determined by the physicochemical properties of surfaces and interfaces between catalyst and reactant, can be changed dramatically under working conditions. Common approaches for investigating these phenomena include X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning probe microscopy (SPM), wide angle X-ray diffraction (WAXRD), auger electron spectroscopy (AES), transmission electron microscope (TEM), etc. Generally, these techniques can only be applied under ex situ conditions and cannot fully recover the changes of catalysts in real chemical reactions. How to identify and track alterations of the catalysts, and thus provide further insight into the complex mechanisms behind them, has become a major research topic in this field. The application of in situ/operando characterization techniques enables real-time monitoring and analysis of dynamic changes. Therefore, researchers can obtain physical and/or chemical information during the reaction (e.g., morphology, chemical bonding, valence state, photocurrent distribution, surface potential variation, surface reconstruction), or even by the combination of these techniques as a suite (e.g., atomic force microscopy-based infrared spectroscopy (AFM-IR), or near-ambient-pressure STM/XPS combined system (NAP STM-XPS)) to correlate the various properties simultaneously, so as to further reveal the reaction mechanisms. In this review, we briefly describe the working principles of in situ/operando surface/interface characterization technologies (i.e., SPM and X-ray spectroscopy) and discuss the recent progress in monitoring relevant surface/interface changes during water splitting and CO2 reduction reactions (CO2RR). We hope that this review will provide our readers with some ideas and guidance about how these in situ/operando characterization techniques can help us investigate the changes in catalyst surfaces/interfaces, and further promote the development of (photo-)electrocatalytic surface and interface engineering.

Water structure, dynamics and reactivity on a TiO2-nanoparticle surface: new insights from ab initio molecular dynamics

Water structure, dynamics and reactivity at the surface of a small TiO₂-nanoparticle fully immersed in water was investigated by an ab initio molecular dynamics simulation. Several modes of water binding were identified by assigning each atom to an atom type, representing a distinct chemical environment in the ab initio ensemble, and then computing radial distribution functions between the atom types. Surface reactivity was investigated by monitoring how populations of atom types change during the simulation. In order to acquire further insight, electron densities for a set of representative system snapshots were analyzed using an atoms-in-molecules approach. Our results reveal that water dissociation, where a water molecule splits at a bridging oxygen site to form a hydroxyl group and a protonated oxygen bridge, can occur by a mechanism involving transfer of a proton over several water molecules. The hydroxyl group and protonated oxygen bridge formed in the process persist (on a 10 ps time scale) and the hydroxyl group undergoes exchange using a mechanism similar to the one responsible for water dissociation. Rotational and translational dynamics of water molecules around the nanoparticle were analyzed in terms of reorientational time correlation functions and mean square displacement. While reorientation of water O-H vectors decreases quickly in the proximity of the nanoparticle surface, translational diffusion slows down more gradually. Our results give new insight into water structure, dynamics and reactivity on TiO₂-nanoparticle surfaces and suggest that water dissociation on curved TiO₂-nanoparticle surfaces can occur via more complex mechanisms than those previously identified for flat defect-free surfaces.

Self-Diffusion of Individual Adsorbed Water Molecules at Rutile (110) and Anatase (101) TiO2 Interfaces from Molecular Dynamics

The distribution of individual water molecules’ self-diffusivities in adsorbed layers at TiO2 surfaces anatase (101) and rutile (110) have been determined at 300 K for inner and outer adsorbed layers, via classical molecular-dynamics methods. The layered-water structure has been identified and classified in layers making use of local order parameters, which proved to be an equally valid method of “self-ordering” molecules in layers. Significant distinctness was observed between anatase and rutile in disturbing these molecular distributions, more specifically in the adsorbed outer layer. Anatase (101) presented significantly higher values of self-diffusivity, presumably due to its “corrugated” structure that allows more hydrogen bonding interaction with adsorbed molecules beyond the first hydration layer. On the contrary, rutile (110) has adsorbed water molecules more securely “trapped” in the region between Ob atoms, resulting in less mobile adsorbed layers.

A Contrastive Study of Self-Assembly and Physical Blending Mechanism of TiO2 Blended Polyethersulfone Membranes for Enhanced Humic Acid Removal and Alleviation of Membrane Fouling

In this study, membrane fabrication was achieved by two different methods: (i) self-assembly and (ii) physical blending of TiO₂ in PES membrane for humic acid filtration. The TiO₂ nanoparticles were self-assembled by using TBT as the precursor and pluronic F127 as triblock copolymers around the membrane pores. This was achieved by manipulating the hydrolysis and condensation reaction of TBT precursors during the non-solvent induced phase separation (NIPS) process. On the other hand, the TiO₂ was physically blended as a comparison to the previous method. The characteristic of the membrane was analysed to explore the possibility of enhancing the membrane antifouling mechanism and the membrane flux. The membrane morphology, pore size, porosity, and contact angle were characterised. Both methods proved to be able to enhance the antifouling properties and flux performance. The HA rejection increased up to 95% with membrane flux 55.40 kg m⁻² h⁻¹. The rejection rate was not significantly improved for either method. However, the antifouling characteristic for the self-assembly TiO₂/PES membrane was better than the physically blended membrane. This was found to be due to the high surface hydrophilicity of the MM membrane, which repelled the hydrophobic HA and consequently blocked the HA adsorption onto the surface.

Machine learning potentials for complex aqueous systems made simple

Understanding complex materials, in particular those with solid–liquid interfaces, such as water on surfaces or under confinement, is a key challenge for technological and scientific progress. Although established simulation approaches have been able to provide important atomistic insight, ab initio techniques struggle with the required time and length scales, while force field methods can often be limited in terms of their accuracy. Here we show how these limitations can be overcome in a simple and automated machine learning procedure to provide accurate models of interactions at the ab initio level, as illustrated for a variety of complex aqueous systems. These developments open up the prospect of the straightforward exploration of many technologically relevant systems by molecular simulations.

Simulation techniques based on accurate and efficient representations of potential energy surfaces are urgently needed for the understanding of complex systems such as solid–liquid interfaces. Here we present a machine learning framework that enables the efficient development and validation of models for complex aqueous systems. Instead of trying to deliver a globally optimal machine learning potential, we propose to develop models applicable to specific thermodynamic state points in a simple and user-friendly process. After an initial ab initio simulation, a machine learning potential is constructed with minimum human effort through a data-driven active learning protocol. Such models can afterward be applied in exhaustive simulations to provide reliable answers for the scientific question at hand or to systematically explore the thermal performance of ab initio methods. We showcase this methodology on a diverse set of aqueous systems comprising bulk water with different ions in solution, water on a titanium dioxide surface, and water confined in nanotubes and between molybdenum disulfide sheets. Highlighting the accuracy of our approach with respect to the underlying ab initio reference, the resulting models are evaluated in detail with an automated validation protocol that includes structural and dynamical properties and the precision of the force prediction of the models. Finally, we demonstrate the capabilities of our approach for the description of water on the rutile titanium dioxide (110) surface to analyze the structure and mobility of water on this surface. Such machine learning models provide a straightforward and uncomplicated but accurate extension of simulation time and length scales for complex systems.

Nanostructured versus flat compact electrode for triboelectric nanogenerators at high humidity

The triboelectric nanogenerator (TENG) is a promising technology for mechanical energy harvesting. TENG has proven to be an excellent option for power generation but typically TENGs output power drops significantly in humid environments. In this work, the effect of electrode's material on power output, considering smooth and nanostructured porous structures with various surface hydrophobicity, is investigated under various humidity conditions. A vertical contact-separation mode TENG is experimentally and numerically studied for four surface morphologies of Ti foil, TiO2 thin film, TiO2 nanoparticulated film, and TiO2 nanotubular electrodes. The results show that the TENG electrical output in the flat structures such as Ti foil and TiO2 thin film at 50% RH is reduced to 50% of its initial state, while in the nanoporous structures such as nanoparticle and nanotube arrays, this is observed at RH above 95%. The results show that the use of porous nanostructures in TENG due to their high surface-to-volume, and that the process of water adsorption on the pore leads to better performance than the flat surface in humid environments. Based on our study, employing nanoporous layers is vital for nanogenerators either for power generation or active sensor applications at high humidity conditions.

First principles characterisation of bio-nano interface

Nanomaterials possess a wide range of potential applications due to their novel properties and exceptionally high activity as a result of their large surface to volume ratios compared to bulk matter. The active surface may present both advantage and risk when the nanomaterials interact with living organisms. As the overall biological impact of nanomaterials is triggered and mediated by interactions at the bio-nano interface, an ability to predict those from the atomistic descriptors, especially before the material is produced, can present enormous advantage for the development of nanotechnology. Fast screening of nanomaterials and their variations for specific biological effects can be enabled using computational materials modelling. The challenge lies in the range of scales that needs to be crossed from the material-specific atomistic representation to the relevant length scales covering typical biomolecules (proteins and lipids). In this work, we present a systematic multiscale approach that allows one to evaluate crucial interactions at the bionano interface from the first principles without any prior information about the material and thus establish links between the details of the nanomaterials structure to protein-nanoparticle interactions. As an example, an advanced computational characterization of titanium dioxide nanoparticles (6 different surfaces of rutile and anatase polymorphs) has been performed. We computed characteristics of the titanium dioxide interface with water using density functional theory for electronic density, used these parameters to derive an atomistic force field, and calculated adsorption energies for essential biomolecules on the surface of titania nanoparticles via direct atomistic simulations and coarse-grained molecular dynamics. Hydration energies, as well as adsorption energies for a set of 40 blood proteins are reported.

Atomistic Molecular Dynamics Simulations of Lipids Near TiO2 Nanosurfaces

Understanding of interactions between inorganic nanomaterials and biomolecules, and particularly lipid bilayers, is crucial in many biotechnological and biomedical applications, as well as for the evaluation of possible toxic effects caused by nanoparticles. Here, we present a molecular dynamics study of adsorption of two important constituents of the cell membranes, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), lipids to a number of titanium dioxide planar surfaces, and a spherical nanoparticle under physiological conditions. By constructing the number density profiles of the lipid headgroup atoms, we have identified several possible binding modes and calculated their relative prevalence in the simulated systems. Our estimates of the adsorption strength, based on the total fraction of adsorbed lipids, show that POPE binds to the selected titanium dioxide surfaces stronger than DMPC, due to the ethanolamine group forming hydrogen bonds with the surface. Moreover, while POPE shows a clear preference toward anatase surfaces over rutile, DMPC has a particularly high affinity to rutile(101) and a lower affinity to other surfaces. Finally, we study how lipid concentration, addition of cholesterol, as well as titanium dioxide surface curvature may affect overall adsorption.

Encapsulation and Release of Doxorubicin from TiO2 Nanotubes: Experiment, Density Functional Theory Calculations, and Molecular Dynamics Simulation

Titanium dioxide (TiO₂) nanotubes are attractive materials for drug-delivery systems because of their biocompatibility, chemical stability, and simple preparation. In this study, we loaded TiO₂ nanotubes with anticancer drug doxorubicin (DOX) experimentally and in all-atom molecular dynamics (MD) simulations. The release of doxorubicin from the nanotubes was studied by high-performance liquid chromatography (HPLC) and confocal Raman spectroscopy, and drug-release profiles were evaluated under various conditions. The polyethylene glycol (PEG) coating and capping of the nanotubes led to a marked increase in the water contact angles from about 16 to 33° in keeping with reduced wettability. The capping retarded the release rate without decreasing the overall release amount. The MD simulations further show that the DOX molecule diffusion coefficients (D_i) are in the order of 10^-10 m²/s. The DOX molecules show a plethora of short- and long-range H-bonding interactions with TiO₂ nanotube walls and water. Calculated radial distribution functions (RDFs) and combined radial/angular distribution functions (CDFs) allowed gauging the strength of these hydrogen bonds. The strength does not fully correlate with the pKa values of DOX atoms which we assign to the confinement of DOX and water in the tubes. The lifetimes of hydrogen bonds between the DOX atoms and water molecules are shorter than that of the DOX^...TiO₂ interactions, and DOX^...DOX aggregation does not play an important role. These results suggest TiO₂ nanotubes as promising candidates for controllable drug-delivery systems for DOX or similar antiproliferative molecules.

pH Sensitivity Estimation in Potentiometric Metal Oxide pH Sensors Using the Principle of Invariance

A numerically solvable engineering model has been proposed that predicts the sensitivity of metal oxide- (MOX-) based potentiometric pH sensors. The proposed model takes into account the microstructure and crystalline structure of the MOX material. The predicted pH sensitivities are consistent with experimental results with the difference below 6% across three MOX (RuO2, TiO2, and Ta2O5) analysed. The model distinguishes the performance of different MOX phases by the appropriate choice of surface hydroxyl site densities and dielectric constants, making it possible to estimate the performance of MOX electrodes fabricated through different high-temperature and low-temperature annealing methods. It further addresses the problem, cited by theoreticians, of independently determining the C1 inner Helmholtz capacitance parameter while applying the triple-layer model to pH sensors. This is done by varying the C1 capacitance parameter until an invariant pH sensitivity across different electrolyte ionic strengths is obtained. This invariance point identifies the C1 capacitance. The corresponding pH sensitivity is the characteristic sensitivity of MOX. The model has been applied across different types of metal oxides, namely, expensive platinum group oxides (RuO2) and cheaper nonplatinum group MOX (TiO2 and Ta2O5). High temperature annealed, RuO2 produced a high pH sensitivity of 59.1082 mV/pH, while TiO2 and Ta2O5 produced sub-Nernstian sensitivities of 30.0011 and 34.6144 mV/pH, respectively. Low temperature annealed, TiO2 and Ta2O5 produced Nernstian sensitivities of 59.1050 and 59.1081 mV/pH, respectively, illustrating the potential of using cheaper nonplatinum group MOx as alternative sensor electrode materials. Separately, the usefulness of relatively less investigated, cheap, and readily available MOX, viz. Al2O3, as the electrode material was analysed. Low-temperature-annealed Al2O3 with a Nernstian sensitivity of 59.1050 mV/pH can be considered as a potential electrode material. The proposed engineering model can be used as a preliminary prediction mechanism for choosing potentially cheaper alternative sensor electrode materials.

Supercooled liquid-like dynamics in water near a fully hydrated titania surface: Decoupling of rotational and translational diffusion

We report an ab initio molecular dynamics (MD) simulation investigating the effect of a fully hydrated surface of TiO₂ on the water dynamics. It is found that the universal relation between the rotational and translational diffusion characteristics of bulk water is broken in the water layers near the surface with the rotational diffusion demonstrating progressive retardation relative to the translational diffusion when approaching the surface. This kind of rotation-translation decoupling has so far only been observed in the supercooled liquids approaching glass transition, and its observation in water at a normal liquid temperature is of conceptual interest. This finding is also of interest for the application-significant studies of the water interaction with fully hydrated nanoparticles. We note that this is the first observation of rotation-translation decoupling in an ab initio MD simulation of water.

Critical review of photocatalytic disinfection of bacteria: from noble metals- and carbon nanomaterials-TiO2 composites to challenges of water characteristics and strategic solutions

This critical review covers ways to improve TiO₂-based photocatalysts, how water characteristics may affect photocatalytic disinfection, and strategies to tackle the challenges arising from water characteristics. Photocatalysis has shown much promise in the disinfection of water/wastewater, because photocatalysis does not produce toxic by-products, and is driven by green solar energy. There are however several drawbacks that are curbing the prevalence of photocatalytic disinfection applications: one, the efficiency of photocatalysts may limit popular utilization; two, the water characteristics may present some challenges to the process. TiO₂-based photocatalysts may be readily improved if composited with noble metals or carbon nanomaterials. Noble metals give TiO₂-based composites a higher affinity for dissolved oxygen, and induce plasmonic and Schottky effects in the TiO₂; carbon nanomaterials with a tunable structure, on the other hand, give the composites an improved charge carrier separation performance. Other than photocatalyst materials, the characteristics of water/wastewater is another crucial factor in the photocatalysis process. Also examined in this review are the crucial impacts that water characteristics have on photocatalysts and their interaction with bacteria. Accordingly, strategies to address the challenge of water characteristics on photocatalytic disinfection are explored: one, to modify the semiconductor conduction band to generate long-lifetime reactive species; two, to improve the interaction between bacteria and photocatalysts.

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.

Prediction of Chronic Inflammation for Inhaled Particles: the Impact of Material Cycling and Quarantining in the Lung Epithelium

On a daily basis, people are exposed to a multitude of health-hazardous airborne particulate matter with notable deposition in the fragile alveolar region of the lungs. Hence, there is a great need for identification and prediction of material-associated diseases, currently hindered due to the lack of in-depth understanding of causal relationships, in particular between acute exposures and chronic symptoms. By applying advanced microscopies and omics to in vitro and in vivo systems, together with in silico molecular modeling, it is determined herein that the long-lasting response to a single exposure can originate from the interplay between the newly discovered nanomaterial quarantining and nanomaterial cycling between different lung cell types. This new insight finally allows prediction of the spectrum of lung inflammation associated with materials of interest using only in vitro measurements and in silico modeling, potentially relating outcomes to material properties for a large number of materials, and thus boosting safe-by-design-based material development. Because of its profound implications for animal-free predictive toxicology, this work paves the way to a more efficient and hazard-free introduction of numerous new advanced materials into our lives.

Direct Synthesis of Dimethyl Ether from Syngas on Bifunctional Hybrid Catalysts Based on Supported H3PW12O40 and Cu-ZnO(Al): Effect of Heteropolyacid Loading on Hybrid Structure and Catalytic Activity

The performance of bifunctional hybrid catalysts based on phosphotungstic acid (H3PW12O40, HPW) supported on TiO2 combined with Cu-ZnO(Al) catalyst in the direct synthesis of dimethyl ether (DME) from syngas has been investigated. We studied the effect of the HPW loading on TiO2 (from 1.4 to 2.7 monolayers) on the dispersion and acid characteristics of the HPW clusters. When the concentration of the heteropoliacid is slightly higher than the monolayer (1.4 monolayers) the acidity of the clusters is perturbed by the surface of titania, while for concentration higher than 1.7 monolayers results in the formation of three-dimensional HPW nanocrystals with acidity similar to the bulk heteropolyacid. Physical hybridization of supported heteropolyacids with the Cu-ZnO(Al) catalyst modifies both the acid characteristics of the supported heteropolyacids and the copper surface area of the Cu-ZnO(Al) catalyst. Hybridization gives rise to a decrease in the copper surface area and the disappearance of the strong acidic sites typical of HPW nanocrystals, showing all hybrids similar acid sites of weak or medium strength. The activity of the hybrids was tested for direct DME synthesis from syngas at 30 bar and 250 °C; only the hybrids with HPW loading higher than 1.4 monolayers showed activity for the direct synthesis of DME, showing that the sample loaded with 2.7 monolayers of heteropolyacid had higher activity than the reference hybrid representative of the most widely applied catalysts based on the combination of Cu-ZnO(Al) with HZSM-5. In spite of the high activity of the hybrids, they show a moderate loss in the DME production with TOS that denotes some kind of deactivation of the acidity function under reaction conditions.

Microstructural and Dynamical Heterogeneities in Ionic Liquids

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

Improved Sampling in Ab Initio Free Energy Calculations of Biomolecules at Solid–Liquid Interfaces: Tight-Binding Assessment of Charged Amino Acids on TiO2 Anatase (101)

Atomistic simulations can complement the scarce experimental data on free energies of molecules at bio-inorganic interfaces. In molecular simulations, adsorption free energy landscapes are efficiently explored with advanced sampling methods, but classical dynamics is unable to capture charge transfer and polarization at the solid–liquid interface. Ab initio simulations do not suffer from this flaw, but only at the expense of an overwhelming computational cost. Here, we introduce a protocol for adsorption free energy calculations that improves sampling on the timescales relevant to ab initio simulations. As a case study, we calculate adsorption free energies of the charged amino acids Lysine and Aspartate on the fully hydrated anatase (101) TiO2 surface using tight-binding forces. We find that the first-principle description of the system significantly contributes to the adsorption free energies, which is overlooked by calculations with previous methods.

Structure and reactivity of a water-covered anatase TiO2(001) surface

We systematically studied water adsorption and oxidation on the unreconstructed TiO₂(001) surface using first-principles calculations. Water first adsorbs on the surface in a dissociative state and then in a molecular state, as water coverage increases. The geometric properties of all adsorption structures suggest that the dissociative water molecules can induce stress release of the (001) surface at low coverage, reducing reactivity of the surface and thus leading to molecular adsorption of water on the surface at high coverage. The adsorption energy (or the surface energy) monotonously increases (or decreases) with the increase of the coverage, which further confirms that water, irrespective of its dissociative or molecular state, can improve the stability of the (001) surface and reduce its activity. We deeply investigated the mechanism of the oxygen evolution reaction (OER) on the water-covered (001) surface. A new water-assisted OER pathway is identified on the (001) surface, which includes the sequential transfer of protons from molecular water and surface hydroxyls, and O-O coupling processes. During the OER pathway, the O-O coupling step exhibits the largest thermodynamic energy and highest energy barrier, clarifying that it is the rate-determining step in the whole pathway. Our findings provide new insights into the strong dependence of water adsorption modes on coverage for the anatase TiO₂(001) surface and may explain the high oxidation activity of the TiO₂(001) surface in aqueous environments typical of TiO₂ photocatalysis.

Free energy of adhesion of lipid bilayers on titania surfaces

The adhesion strength between a flexible membrane and a solid substrate (formally the free energy of adhesion per unit area) is difficult to determine experimentally, yet is a key parameter in determining the extent of the wrapping of a particle by the membrane. Here, we present molecular dynamics simulations designed to estimate this quantity between dimyristoylphosphatidylcholine (DMPC) bilayers and a range of low-energy titanium dioxide cleavage planes for both anatase and rutile polymorphs. The average adhesion strength across the cleavage planes for rutile and anatase is relatively weak ∼-2.0 ± 0.4 mN m^-1. However, rutile has two surfaces (100 and 101) displaying relatively strong adhesion (-4 mN m^-1), while anatase has only one (110). This suggests a slightly greater tendency for bilayers to wrap rutile particles compared to anatase particles but both would wrap less than amorphous silica. We also estimate the adsorption free energies of isolated DMPC lipids and find that only the rutile 101 surface shows significant adsorption. In addition, we estimate the adhesion enthalpies and infer that the entropic contribution to the adhesion free energy drives adhesion on the rutile surfaces and opposes adhesion on the anatase surfaces.

Theoretical study of the adsorption characteristics and the environmental influence of ornidazole on the surface of photocatalyst TiO2

In this paper, density functional theory (DFT) was performed to study the adsorption properties of ornidazole on anatase TiO2(101) and (001) crystal facets under vacuum, neutral and acid-base conditions. We calculated the adsorption structure of ornidaozle on the anatase TiO2 surface, optimal adsorption sites, adsorption energy, density of states, electronic density and Milliken atomic charge under different conditions. The results show that when the N(3) atom on the imidazole ring is adsorbed on the Ti(5) atom, the largest adsorption energy and the most stable adsorption configuration could be achieved. According to the analysis of the adsorption configuration, we found that the stability of C(2)-N(3) bond showed a weakening trend. The adsorption wavelengths of the electronic transition between the valence band and conduction band of ornidazole on the TiO2 surface were in the visible light wavelengths range, showing that the TiO2 crystal plane can effectively make use of visible light under different conditions. We speculate the possibility of ornidazole degradation on the surface of TiO2 and found that the reactive site is the C-N bond on the imidazole ring. These discoveries explain the photocatalytic degradation of ornidazole by TiO2 and reveal the microscopic nature of catalytic degradation.

Water Adsorption on MO2 (M = Ti, Ru, and Ir) Surfaces. Importance of Octahedral Distortion and Cooperative Effects

Understanding metal oxide MO₂ (M = Ti, Ru, and Ir)-water interfaces is essential to assess the catalytic behavior of these materials. The present study analyzes the H₂O-MO₂ interactions at the most abundant (110) and (011) surfaces, at two different water coverages: isolated water molecules and full monolayer, by means of Perdew-Burke-Ernzerhof-D2 static calculations and ab initio molecular dynamics (AIMD) simulations. Results indicate that adsorption preferably occurs in its molecular form on (110)-TiO₂ and in its dissociative form on (110)-RuO₂ and (110)-IrO₂. The opposite trend is observed at the (011) facet. This different behavior is related to the kind of octahedral distortion observed in the bulk of these materials (tetragonal elongation for TiO₂ and tetragonal compression for RuO₂ and IrO₂) and to the different nature of the vacant sites created, axial on (110) and equatorial on (011). For the monolayer, additional effects such as cooperative H-bond interactions and cooperative adsorption come into play in determining the degree of deprotonation. For TiO₂, AIMD indicates that the water monolayer is fully undissociated at both (110) and (011) surfaces, whereas for RuO₂, water monolayer exhibits a 50% dissociation, the formation of H₃O₂ ^- motifs being essential. Finally, on (110)-IrO₂, the main monolayer configuration is the fully dissociated one, whereas on (011)-IrO₂, it exhibits a degree of dissociation that ranges between 50 and 75%. Overall, the present study shows that the degree of water dissociation results from a delicate balance between the H₂O-MO₂ intrinsic interaction and cooperative hydrogen bonding and adsorption effects.

Theoretical study of adsorption characteristics and the environmental influence for metronidazole on photocatalytic TiO2 anatase surfaces

The adsorption characteristics of metronidazole on anatase TiO₂(101) and (001) surfaces were studied by density functional theory (DFT). The adsorption structure of metronidazole on anatase TiO₂(101) and (001) surfaces has been optimized under vacuum, water, acidic, and alkaline conditions, respectively. The optimum adsorption site, adsorption energy, and electronic structure of the stable adsorption model were calculated. The adsorption characteristics of metronidazole on two different surfaces of TiO₂ were studied under acidic and alkaline conditions. Our calculated results found that the adsorption energy range is -0.95 ~ -3.11 eV on the TiO₂ (101) surface, and the adsorption energy range is -0.84 ~ -3.29 eV on the TiO₂ (001) surface. The adsorption wavelengths of electron transition between valence band and conduction band of metronidazole on the anatase TiO₂(101) surface is in the range of visible wavelength, indicating that the TiO₂(101) surface can effectively utilize visible light. However, the photocatalytic effect of the TiO₂(001) surface is greatly affected by the environment. The results reveal the adsorption characteristics and the environmental influence for metronidazole on photocatalytic anatase TiO₂ surfaces. Graphical abstract The adsorption characteristics of metronidazole on anatase TiO₂(101) and (001) crystal surfaces were studied by density functional theory (DFT).

Understanding CO oxidation on the Pt(111) surface based on a reaction route network

Kinetic analysis by the rate constant matrix contraction on the reaction route network of CO oxidation on the Pt(111) surface obtained by the artificial force induced reaction reveals the impact of entropic contributions arising from a variety of local minima and transition states.

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.

Modeling of solid–liquid interfaces using scaled charges: rutile (110) surfaces

The first application of the electronic continuum correction model with scaled charges to molecular dynamics simulations of solid–liquid interfaces.