pH- and Facet-Dependent Surface Chemistry of TiO2 in Aqueous Environment from First Principles

Synthesis and Characterization of a Novel Sol-Gel-Derived Ni-Doped TiO2 Photocatalyst for Rapid Visible Light-Driven Mineralization of Paracetamol

The increasing presence of pharmaceutical contaminants, such as paracetamol, in water sources necessitates the development of efficient and sustainable treatment technologies. This study investigates the photocatalytic degradation and mineralization of paracetamol under visible light using nickel-doped titanium dioxide (Ni-TiO2) catalysts synthesized via the sol-gel method. The catalysts were characterized through Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), and surface area measurements. Ni doping enhanced the visible light absorption of TiO2, reducing its band gap from 3.11 eV (undoped) to 2.49 eV at 0.20 wt.% Ni loading, while Raman analysis confirmed Ni incorporation with anatase as the predominant phase. The Ni(0.1%)-TiO2 catalyst exhibited the highest photocatalytic activity, achieving 88% total organic carbon (TOC) removal of paracetamol (5 ppm) after 180 min under optimal conditions (catalyst dosage, 3 g L-1). Stability tests demonstrated 84% retained efficiency over five cycles, with a kinetic rate constant of 0.010 min-1. Hydroxyl radicals were identified as the main reactive species. The catalyst maintained high performance in tap water, achieving 78.8% TOC removal. These findings highlight the potential of Ni(0.1%)-TiO2 as a cost-effective, visible light-active photocatalyst for the removal of pharmaceutical pollutants, with promising scalability for industrial water treatment applications.

Ultraviolet irradiation penetration depth on TiO2

High-energy ultraviolet (UVC) irradiation of metal oxides (MOs, e.g., TiO2) results in photoinduced surface oxygen vacancies (PI-SOVs), which can change the charge carrier (e.g., electrons and holes) migration dynamics. Although PI-SOVs alter the electronic and chemical properties of MOs, there is no consensus on the penetration depth of the UVC irradiation, which induces PI-SOVs and is an important variable for the design and operation of MO-based systems. Here, we performed optical transmission and time-resolved atomic force microscopy measurements on back-illuminated TiO2 samples. Our experiments show that the effect of UVC irradiation on MOs can be observed hundreds of micrometers across the bulk, i.e., orders of magnitude larger than previously postulated values. We believe that our findings would be important both for the fundamental understanding of UVC irradiation/penetration and for device design/fabrication processes.

Advanced TiO2-Based Photocatalytic Systems for Water Splitting: Comprehensive Review from Fundamentals to Manufacturing

The global imperative for clean energy solutions has positioned photocatalytic water splitting as a promising pathway for sustainable hydrogen production. This review comprehensively analyzes recent advances in TiO2-based photocatalytic systems, focusing on materials engineering, water source effects, and scale-up strategies. We recognize the advancements in nanoscale architectural design, the engineered heterojunction of catalysts, and cocatalyst integration, which have significantly enhanced photocatalytic efficiency. Particular emphasis is placed on the crucial role of water chemistry in photocatalytic system performance, analyzing how different water sources-from wastewater to seawater-impact hydrogen evolution rates and system stability. Additionally, the review addresses key challenges in scaling up these systems, including the optimization of reactor design, light distribution, and mass transfer. Recent developments in artificial intelligence-driven materials discovery and process optimization are discussed, along with emerging opportunities in bio-hybrid systems and CO2 reduction coupling. Through critical analysis, we identify the fundamental challenges and propose strategic research directions for advancing TiO2-based photocatalytic technology toward practical implementation. This work will provide a comprehensive framework for exploring advanced TiO2-based composite materials and developing efficient and scalable photocatalytic systems for multifunctional simultaneous hydrogen production.

Protolytic Reactions at Electrified TiO2 P25 Interface: Quantitative and Thermodynamic Characterization

Protolytic reactions on the surface of a titania photocatalyst (TiO2 P25 containing chlorine impurities) were studied using potentiometric and calorimetric acid-base titration. The impurity was removed by either washing or heat treatment. The efficiency of purification was tested by chlorine (TOX) analysis and acid-base titration. Common intersection points of -0.023 and -0.021 mmol/g were obtained for the original and 400 °C heat-treated samples, which are in good agreement with the measured TOX value of 28 mmol/kg. The point of zero charge of the purified sample was determined to be 6.50. Titration data were fitted to simulate protolytic reactions during isothermal calorimetric titrations of purified titania. The evolved heat was measured, and data points were corrected with the heat of mixing and neutralization. The quantity of charged surface species formed in each step of titration was calculated using the parameters from the constant capacitance model fit. The partial molar enthalpy values of the exothermic and endothermic processes of surface protonation (ΔHpr, -17.47 to -16.10 kJ/mol) and deprotonation (ΔHdepr, 32.53 to 27.08 kJ/mol) depend slightly on the ionic strength of suspensions. The average standard enthalpy of one proton transfer reaction is -23.54 ± 1.75 kJ/mol, which is consistent with the literature.

Molecular picture of electric double layers with weakly adsorbed water

Water adsorption energy, Eads, is a key physical quantity in sustainable chemical technologies such as (photo)electrocatalytic water splitting, water desalination, and water harvesting. In many of these applications, the electrode surface is operated outside the point (potential) of zero charge, which attracts counter-ions to form the electric double layer and controls the surface properties. Here, by applying density functional theory-based finite-field molecular dynamics simulations, we have studied the effect of water adsorption energy Eads on surface acidity and the Helmholtz capacitance of BiVO4 as an example of metal oxide electrodes with weakly chemisorbed water. This allows us to establish the effect of Eads on the coordination number, the H-bond network, and the orientation of chemisorbed water by comparing an oxide series composed of BiVO4, TiO2, and SnO2. In particular, it is found that a positive correlation exists between the degree of asymmetry ΔCH in the Helmholtz capacitance and the strength of Eads. This correlation is verified and extended further to graphene-like systems with physisorbed water, where the electric double layers (EDLs) are controlled by electronic charge rather than proton charge as in the oxide series. Therefore, this work reveals a general relationship between water adsorption energy Eads and EDLs, which is relevant to both electrochemical reactivity and the electrowetting of aqueous interfaces.

Key Ingredients for the Screening of Single Atom Catalysts for the Hydrogen Evolution Reaction: The Case of Titanium Nitride

A computational screening of Single Atom Catalysts (SACs) bound to titanium nitride (TiN) is presented, for the Hydrogen Evolution Reaction (HER), based on density functional theory. The role of fundamental ingredients is explored to account for a reliable screening of SACs. Namely, the formation of H2-complexes besides the classical H* one impacts the predicted HER activity, in line with previous studies on other SACs. Also, the results indicate that one needs to adopt self-interaction-corrected functionals. Finally, predicting an active catalyst is of little help without an assessment of its stability. Thus, it is included in the theoretical framework the analysis of the stability of the SACs in working conditions of pH and voltage. Once unconventional intermediates and stability are considered in a self-interaction corrected scheme, the number of potential good catalysts for HER is strongly reduced since i) some potentially good catalysts are not stable against dissolution and ii) the formation of unconventional intermediates leads to thermodynamic barriers. This study highlights the importance of including ingredients for the prediction of new systems, such as the formation of unconventional intermediates, estimating the stability of SACs, and the adoption of self-interaction corrected functionals. Also, this study highlights some interesting candidates deserving of dedicated work.

How Doping Regulates As(III) Adsorption at TiO2 Surfaces: A DFT + U Study

The efficient adsorption and removal of As(III), which is highly toxic, remains difficult. TiO2 shows promise in this field, though the process needs improvement. Herein, how doping regulates As(OH)3 adsorption over TiO2 surfaces is comprehensively investigated by means of the DFT + D3 approach. Doping creates the bidentate mononuclear (Ce doping at the Ti5c site), tridentate (N, S doping at the O2c site), and other new adsorption structures. The extent of structural perturbation correlates with the atomic radius when doping the Ti site (Ce >> Fe, Mn, V >> B), while it correlates with the likelihood of forming more bonds when doping the O site (N > S > F). Doping the O2c, O3c rather than the Ti5c site is more effective in enhancing As(OH)3 adsorption and also causes more structural perturbation and diversity. Similar to the scenario of pristine surfaces, the bidentate binuclear complexes with two Ti-OAs bonds are often the most preferred, except for B doping at the Ti5c site, S doping at the O2c site, and B doping at the O3c site of rutile (110) and Ce, B doping at the Ti5c site, N, S doping at the O2c site, and N, S, B doping at the O3c site of anatase (101). Doping significantly regulates the As(OH)3 adsorption efficacy, and the adsorption energies reach -4.17, -4.13, and -4.67 eV for Mn doping at the Ti5c site and N doping at the O2c and O3c sites of rutile (110) and -1.99, -2.29, and -2.24 eV for Ce doping at the Ti5c site and N doping at the O2c and O3c sites of anatase (101), respectively. As(OH)3 adsorption and removal are crystal-dependent and become apparently more efficient for rutile vs. anatase, whether doped at the Ti5c, O2c, or O3c site. The auto-oxidation of As(III) occurs when the As centers interact directly with the TiO2 surface, and this occurs more frequently for rutile rather than anatase. The multidentate adsorption of As(OH)3 causes electron back-donation and As(V) re-reduction to As(IV). The regulatory effects of doping during As(III) adsorption and the critical roles played by crystal control are further unraveled at the molecular level. Significant insights are provided for As(III) pollution management via the adsorption and rational design of efficient scavengers.

Facet-Dependent Photocatalytic Behavior of Rutile TiO2 for the Degradation of Volatile Organic Compounds: In Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy and Density Functional Theory Investigations

In this study, a custom rutile titanium dioxide (TiO2) photocatalyst with a single exposed surface was utilized to investigate the facet-dependent photocatalytic mechanism of toluene. The degradation of toluene was dynamically monitored using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technology coupled with theoretical calculations. The findings demonstrated that the photocatalytic degradation rate on the TiO2 (001) surface was nearly double that observed on the TiO2 (110) surface. This remarkable enhancement can be attributed to the heightened stability in the adsorption of toluene molecules and the concurrent reduction in the energy requirement for the ring-opening process of benzoic acid on the TiO2 (001) surface. Moreover, the TiO2 (001) surface generated a greater number of reactive oxygen species (ROS), thereby promoting the separation of photogenerated charge carriers and concurrently diminishing their recombination rates, amplifying the efficiency of photocatalysis. This research provides an innovative perspective for a more comprehensive understanding of the photocatalytic degradation mechanism of TiO2 and presents promising prospects for significant applications in environmental purification and energy fields.

Modeling Single-Atom Catalysis

Electronic structure calculations represent an essential complement of experiments to characterize single-atom catalysts (SACs), consisting of isolated metal atoms stabilized on a support, but also to predict new catalysts. However, simulating SACs with quantum chemistry approaches is not as simple as often assumed. In this work, the essential factors that characterize a reliable simulation of SACs activity are examined. The Perspective focuses on the importance of precise atomistic characterization of the active site, since even small changes in the metal atom's surroundings can result in large changes in reactivity. The dynamical behavior and stability of SACs under working conditions, as well as the importance of adopting appropriate methods to solve the Schrödinger equation for a quantitative evaluation of reaction energies are addressed. The Perspective also focuses on the relevance of the model adopted. For electrocatalysis this must include the effects of the solvent, the presence of electrolytes, the pH, and the external potential. Finally, it is discussed how the similarities between SACs and coordination compounds may result in reaction intermediates that usually are not observed on metal electrodes. When these aspects are not adequately considered, the predictive power of electronic structure calculations is quite limited.

The pH dependent surface charging and points of zero charge. X. Update

Surfaces are often characterized by their points of zero charge (PZC) and isoelectric points (IEP). Different authors use these terms for different quantities, which may be equal to the actual PZC under certain conditions. Several popular methods lead to results which are inappropriately termed PZC. This present review is limited to zero-points obtained in the presence of inert electrolytes (halides, nitrates, and perchlorates of the 1st group metals). IEP are reported for all kinds of materials. PZC of metal oxides obtained as common intersection points of potentiometric curves for 3 or more ionic strengths (or by means of equivalent methods) are also reported, while the apparent PZC obtained by mass titration, pH-drift method, etc. are deliberately neglected. The results published in the recent publications and older results overlooked in the previous compilations by the same author are reported. The PZC/IEP are accompanied by information on the temperature and on the nature and concentration of supporting electrolyte (if available). The references to previous reviews by the same author allow to compare the newest results with the PZC/IEP of similar materials from the older literature.