Search Results (33)

The rapid growth of industrial activities has increased environmental pollution, and solar-driven heterogeneous photocatalysis using TiO₂ has emerged as a promising solution. However, its wide band gap limits its efficiency, prompting research into various optimization strategies. One of these approaches is surface functionalization. Thus, this study investigates the adsorption of CuSO₄ on the anatase TiO₂ (101) surface using density functional theory calculations. The adsorption process induced a magnetic moment of 0.97 µ_B and a slight reduction in overall bandwidth. A preferential adsorption geometry pattern with an energy of −4.31 eV was identified. Charge transfer analysis revealed a net transfer from the TiO₂ surface to the CuSO₄ molecule, with increased net atomic charges for atoms involved in new chemical bond formation, indicating a chemisorption process. These electronic structure modifications are expected to influence the electronic and catalytic properties of the material. The findings provide insights into the CuSO₄ adsorption mechanism on an anatase TiO₂ (101) surface and its impact on the properties of the material, contributing to a deeper understanding of this system. Full article

The acidity of anatase titania before and after KOH doping was probed by pyridine adsorption in a pulse microreactor and modeled by DFT optimization of the geometry of CO and pyridine adsorption on a periodic slab of (101) and (100) surfaces using a GGA/PBE functional and verified by an example of a single-point calculation of the optimized geometry using an HSE-06 hybrid functional. The anatase (101) surface was slightly more acidic compared to the (100) surface. Both experimental and computational methods show that the acidity of anatase surfaces decreased after KOH doping and increased after the dissociative adsorption of water. Higher acidity of Ti metal centers was indicated by the shortening of the Ti-N, Ti-C, and C-O bond lengths, increasing the IR frequency of CO and pyridine ring vibrations and energy of adsorption. The DFT calculated energy of pyridine adsorption was analyzed in terms of binding energy and the energy of lattice distortion. The latter was used to construct Hammett plots for the adsorption of 4-substituted pyridines with electron-donating and -withdrawing substituents. The Hammett rho constant was obtained and used to characterize the acidity of various metal centers of −1.51 vs. −1.46 on pristine (101) and (100) surfaces, which were lowered to −1.07 and −1.19 values on KOH-doped (101) and (100) surfaces, respectively. The mechanism of lowering surface acidity via KOH doping proceeds through the stabilization of the atomic structure of Lewis acid centers. When an alkaline metal cation binds to several lattice oxygen atoms, the surface structure becomes more rigid. The ability of Ti atoms to move toward the adsorbate is restricted. Consequently, the lattice distortion energy and binding energy are decreased. In contrast, higher flexibility of the outermost layer of Ti atoms as a result of electron density redistribution, for example, in the presence of water on the surface, allows them to move farther outward, make shorter contacts with the adsorbate, and attain higher energies of binding and lattice distortion. Full article

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 Ti_5c site), tridentate (N, S doping at the O_2c 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 O_2c, O_3c rather than the Ti_5c 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-O_As bonds are often the most preferred, except for B doping at the Ti_5c site, S doping at the O_2c site, and B doping at the O_3c site of rutile (110) and Ce, B doping at the Ti_5c site, N, S doping at the O_2c site, and N, S, B doping at the O_3c 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 Ti_5c site and N doping at the O_2c and O_3c sites of rutile (110) and −1.99, −2.29, and −2.24 eV for Ce doping at the Ti_5c site and N doping at the O_2c and O_3c 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 Ti_5c, O_2c, or O_3c 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. Full article

In this study, titanium dioxide (TiO₂) was deposited onto a fluorine-doped tin oxide (FTO) substrate using the sol–gel spin coating method. Through the implementation of calcination treatment on the thin film, enhancements were observed in terms of structural, optical, and morphological properties. Various calcination temperatures were explored, with TiO₂ annealed at 600 °C identified as the optimal sample. Analysis of the X-ray diffraction spectroscopy (XRD) pattern revealed the prominent orientation plane of (101), indicating the presence of anatase TiO₂ with a tetragonal pattern at this temperature. Despite fluctuations in the optical spectrum, the highest transmittance of 80% was observed in the visible region within the wavelength range of 400 nm. The estimated band-gap value of 3.45 eV reaffirmed the characteristic of TiO₂. Surface analysis indicated the homogeneous growth of TiO₂, uniformly covering the FTO substrate. Cross-sectional examination revealed a thickness of 263 nm with dense and compact nature of TiO₂ thin film. No presence of defects or pores reflects a well-organized structure and high-quality formation. Significant electrical rectification properties were observed, indicating the successful formation of a p–n junction. In summary, calcination treatment was found to be crucial for enhancing the properties of the thin film, highlighting its significance in the development of solar cell applications. Full article

Excited state calculations are performed to predict the electronic structure and optical absorption characteristics of Cu-doped anatase Ti$O_2$ nanofilms, focusing on their (101) and (001) surface terminations. Using model structures that successfully represent the equilibrium positions of deposited Cu atoms on the Ti$O_2$ surface, a comprehensive analysis of the absorption spectra for each considered model is made. The proposed modeling reveals phenomena when photogenerated electrons from Ti$O_2$ tend to accumulate in the vicinity of the deposited Cu atoms exposed to photon energies surpassing the band gap of Ti$O_2$ (approximately 3.2 eV). The crucial transition states that are essential for the creation of potential photocatalytic materials are identified through detailed calculations of the excited states. These insights hold substantial promise for the strategic design of advanced photocatalytic materials. The obtained results provide a base for subsequent analyses, facilitating the determination of heightened surface reactivity, photostimulated water splitting, and antibacterial properties. Full article

Titanium dioxide nanotubes (TNT) have been extensively studied because of their unique properties, which make such systems ideal candidates for biomedical application, especially for the targeted release of drugs. However, knowledge about the properties of TiO₂ nanotubes with typical dimensions of the order of the nanometer is limited, especially concerning the adsorption of molecules that can be potentially loaded in actual devices. In this work, we investigate, by means of simulations based on hybrid density functional theory, the adsorption of Vitamin C molecules on different nanotubes through a comparative analysis of the properties of different structures. We consider two different anatase TiO₂ surfaces, the most stable (101) and the more reactive (001)A; we evaluate the role of the curvature, the thickness and of the diameter as well as of the rolling direction of the nanotube. Different orientations of the molecule with respect to the surface are studied in order to identify any trends in the adsorption mechanism. Our results show that there is no preferential functional group of the molecule interacting with the substrate, nor any definite spatial dependency, like a rolling orientation or the concavity of the nanotube. Instead, the adsorption is driven by geometrical factors only, i.e., the favorable matching of the position and the alignment of any functional groups with undercoordinated Ti atoms of the surface, through the interplay between chemical and hydrogen bonds. Differently from flat slabs, thicker nanotubes do not improve the stability of the adsorption, but rather develop weaker interactions, due to the enhanced curvature of the substrate layers. Full article

Electrochemical nitrate reduction to ammonia is an efficient strategy for nitrate removal and ammonia production in ambient conditions. TiO₂ is a promising electrocatalyst for such a reaction, but chemical doping is still needed to further improve the electrocatalytic properties of TiO₂. Here, we investigated the effect of Zr-doping on the nitrate reduction reaction processes on the (101) surface of anatase TiO₂ using first-principles calculations. Two models with different Zr-doping levels were built. The reaction pathways and the potential-determining steps were established based on a thorough investigation of the variation in Gibbs free energy of each possible elementary step. The results show that a high level of Zr doping was effective to lower the Gibbs free energy for nitrate adsorption; however, Zr doping may promote the competing hydrogen evolution reaction (HER) by reducing the adsorption Gibbs free energy of H. Moreover, Zr doping also increases the adsorption Gibbs free energies for the intermediate products NO₂ and NO, which may result in an earlier termination of the reaction, by releasing the intermediates as the final products without producing ammonia. Therefore, Zr doping may decrease the Faradaic efficiency and selectivity of TiO₂ for the reaction and should be treated with caution experimentally. Full article

Studying the interaction of inorganic systems with organic ones is a highly important avenue for finding new drugs and treatment methods. Tumor cells show an increased demand for amino acids due to their rapid proliferation; thus, targeting their metabolism is becoming a potential oncological therapeutic strategy. One of the inorganic materials that show antitumor properties is titanium dioxide, while its doping was found to enhance interactions with biological systems. Thus, in this study, we investigated the energy landscape of glutamine (L), an amino acid, on pristine and doped TiO₂ surfaces. We first locally optimized 2D-slab structures of pristine and Au/Ag/Cu-doped anatase (001 and 101 surfaces) and similarly optimized a single molecule of glutamine in vacuum. Next, we placed the pre-optimized glutamine molecule in various orientations and on a variety of locations onto the relaxed substrate surfaces (in vacuum) and performed ab initio relaxations of the molecule on the substrate slabs. We employed the DFT method with a GGA-PBE functional implemented in the Quantum Espresso code. Comparisons of the optimized conformations and electronic structures of the amino acid in vacuum and on the surfaces yield useful insights into various biological processes. Full article

The relationship between structure and reactivity plays a dominant role in water dissociation on the various TiO₂ crystallines. To observe the adsorption and dissociation behavior of H₂O, the reaction force field (ReaxFF) is used to investigate the dynamic behavior of H₂O on rutile (110) and anatase (101) surfaces in an aqueous environment. Simulation results show that there is a direct proton transfer between the adsorbed H₂O (H₂O_ad) and the bridging oxygen (O_br) on the rutile (110) surface. Compared with that on the rutile (110) surface, an indirect proton transfer occurs on the anatase (101) surface along the H-bond network from the second layer of water. This different mechanism of water dissociation is determined by the distance between the 5-fold coordinated Ti (Ti_5c) and O_br of the rutile and anatase TiO₂ surfaces, resulting in the direct or indirect proton transfer. Additionally, the hydrogen bond (H-bond) network plays a crucial role in the adsorption and dissociation of H₂O on the TiO₂ surface. To describe interfacial water structures between TiO₂ and bulk water, the double-layer model is proposed. The first layer is the dissociated H₂O on the rutile (110) and anatase (101) surfaces. The second layer forms an ordered water structure adsorbed to the surface O_br or terminal OH group through strong hydrogen bonding (H-bonding). Affected by the H-bond network, the H₂O dissociation on the rutile (110) surface is inhibited but that on the anatase (101) surface is promoted. Full article

TiO₂-supported catalysts have been widely used for a range of both liquid-phase and gas-phase hydrogenation reactions. However, little is known about the effect of their different crystalline surfaces on their activity during the hydrodeoxygenation process. In this work, Au supported on anatase TiO₂, mainly exposing 101 or 001 facets, was investigated for the hydrodeoxygenation (HDO) of guaiacol. At 300 °C, the strong interaction between the Au and TiO₂-101 surface resulted in the facile reduction of the TiO₂-101 surface with concomitant formation of oxygen vacancies, as shown by the H₂-TPR and H₂-TPD profiles. Meanwhile, the formation of Au^δ−, as determined by CO-DRIFT spectra and in situ XPS, was found to promote the demethylation of guaiacol producing methane. However, this strong interaction was absent on the Au/TiO₂-001 catalyst since TiO₂-001 was relatively difficult to be reduced compared with TiO₂-101. The Au on TiO₂-001 just served as the active site for the dissociation of hydrogen without the formation of Au^δ−. The hydrogen atoms spilled over to the surface of TiO₂-001 to form a small amount of oxygen vacancies, which resulted in lower activity than that over Au/TiO₂-101. The catalytic activity of the Au/TiO₂ catalyst for hydrodeoxygenation will be controlled by tuning the crystal plane of the TiO₂ support. Full article

Au/TiO₂ photocatalysts were studied, characterized, and compared for CO₂ photocatalytic gas-phase reduction. The impact of the nature of the TiO₂ support was studied. It was shown that the surface area/porosity/TiO₂ crystal phase/density of specific exposed facets and oxygen vacancies were the key factors determining CH₄ productivity under solar-light activation. A 0.84 wt.% Au/TiO₂ SG (Sol Gel) calcined at 400 °C exhibited the best performance, leading to a continuous mean CH₄ production rate of 50 μmol.h⁻¹.g⁻¹ over 5 h, associated with an electronic selectivity of 85%. This high activity was mainly attributed to the large surface area and accessible microporous volume, high density of exposed TiO₂ (101) anatase facets, and oxygen vacancies acting as reactive defects sites for CO₂ adsorption/activation/dissociation and charge carrier transport. Full article

Photocatalytic water oxidation over titanium dioxide (TiO₂) was overviewed by surveying briefly the history of water photo-oxidation, followed by profiling the research for the molecular mechanism of oxygen evolution reaction (OER) at the TiO₂ surface. As the experimental approach to investigate the reaction mechanism, ESR, NMR, and STM were described as well as FTIR spectroscopy. Detection of reactive oxygen species, which are the intermediate species in the OER, was also involved in discussing the mechanism. As the theoretical approach to the reaction mechanism, some research with density functional theory (DFT) for anatase (101) surface was illustrated. Since the OER activity of rutile TiO₂ is higher than that of anatase, and the rutile (011) surface has been assigned to the oxidation facet, we performed a DFT calculation for a (011) surface model molecule. The results were successfully discussed with the reported mechanism. The first oxidation step occurs at the bridging OH site, which faces a Ti_5C site. The water molecule which coordinates both sites is oxidized, and the resultant radical coordinates the Ti_5C site to form a trapped hole Ti-O•. In the second step, a coordinated water molecule is oxidized at the Ti-O• site to form a Ti-OOH structure. Full article

In this work, brookite TiO₂ nanocrystals with co-exposed {001} and {120} facets (pH0.5-T500-TiO₂ and pH11.5-T500-TiO₂), rutile TiO₂ nanorod with exposed {110} facets and anatase TiO₂ nanocrystals with exposed {101} facets (pH3.5-T500-TiO₂) and {101}/{010} facets (pH5.5-T500-TiO₂, pH7.5-T500-TiO₂ and pH9.5-T500-TiO₂) were successfully synthesized through a one-pot solvothermal method by using titanium (V) iso-propoxide (TTIP) colloidal solution as the precursor. The crystal structure, morphology, specific surface area, surface chemical states and photoelectric properties of the pHx-T500-TiO₂ (x = 0.5, 1.5, 3.5, 5.5, 7.5, 9.5, 11.5) were characterized by powder X-ray diffraction (XRD), field scanning transmission electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), nitrogen adsorption instrument, X-ray photoelectron spectroscopy (XPS), UV-Visible diffuse reflectance spectra and electrochemical impedance spectroscopy (EIS). The photocatalytic activity performance of the pHx-T500-TiO₂ samples was also investigated. The results showed that as-prepared pH3.5-T500-TiO₂ nanocrystal with exposed {101} facets exhibited the highest photocatalytic activity (95.75%) in the process of photocatalytic degradation of methyl orange (MO), which was 1.1, 1.2, 1.2, 1.3, 1.4, 1.6, 10.7, 15.1 and 27.8 fold higher than that of pH5.5-T500-TiO₂ (89.47%), P25-TiO₂ (81.16%), pH9.5-T500-TiO₂ (79.41%), pH7.5-T500-TiO₂ (73.53%), pH0.5-T500-TiO₂ (69.10%), CM-TiO₂ (61.09%), pH11.5-T500-TiO₂ (8.99%), pH1.5-T500-TiO₂ (6.33%) and the Blank (3.44%) sample, respectively. The highest photocatalytic activity of pH3.5-T500-TiO₂ could be attributed to the synergistic effects of its anatase phase structure, the smallest particle size, the largest specific surface area and exposed {101} facets. Full article

MoS₂/TiO₂-based nanostructures have attracted extensive attention due to their high performance in many fields, including photocatalysis. In this contribution, MoS₂ nanostructures were prepared via an in situ bottom-up approach at the surface of shape-controlled TiO₂ nanoparticles (TiO₂ nanosheets and bipyramids). Furthermore, a multi-technique approach by combining electron microscopy and spectroscopic methods was employed. More in detail, the morphology/structure and vibrational/optical properties of MoS₂ slabs on TiO₂ anatase bipyramidal nanoparticles, mainly exposing {101} facets, and on TiO₂ anatase nanosheets exposing both {001} and {101} facets, still covered by MoS₂, were compared. It was shown that unlike other widely used methods, the bottom-up approach enabled the atomic-level growth of well-defined MoS₂ slabs on TiO₂ nanostructures, thus aiming to achieve the most effective chemical interactions. In this regard, two kinds of synergistic heterojunctions, namely, crystal face heterojunctions between anatase TiO₂ coexposed {101} and {001} facets and semiconductor heterojunctions between MoS₂ and anatase TiO₂ nanostructures, were considered to play a role in enhancing the photocatalytic activity, together with a proper ratio of (101), (001) coexposed surfaces. Full article

Earth abundant transition metal oxides are low-cost promising catalysts for the oxygen evolution reaction (OER). Many transition metal oxides have shown higher OER activity than the noble metal oxides (RuO₂ and IrO₂). Many experimental and theoretical studies have been performed to understand the mechanism of OER. In this review article we have considered four earth abundant transition metal oxides, namely, titanium oxide (TiO₂), manganese oxide/hydroxide (MnO_x/MnOOH), cobalt oxide/hydroxide (CoO_x/CoOOH), and nickel oxide/hydroxide (NiO_x/NiOOH). The OER mechanism on three polymorphs of TiO₂: TiO₂ rutile (110), anatase (101), and brookite (210) are summarized. It is discussed that the surface peroxo O* intermediates formation required a smaller activation barrier compared to the dangling O* intermediates. Manganese-based oxide material CaMn₄O₅ is the active site of photosystem II where OER takes place in nature. The commonly known polymorphs of MnO₂; α-(tetragonal), β-(tetragonal), and δ-(triclinic) are discussed for their OER activity. The electrochemical activity of electrochemically synthesized induced layer δ-MnO₂ (EI-δ-MnO₂) materials is discussed in comparison to precious metal oxides (Ir/RuO_x). Hydrothermally synthesized α-MnO₂ shows higher activity than δ-MnO₂. The OER activity of different bulk oxide phases: (a) Mn₃O₄(001), (b) Mn₂O₃(110), and (c) MnO₂(110) are comparatively discussed. Different crystalline phases of CoOOH and NiOOH are discussed considering different surfaces for the catalytic activity. In some cases, the effects of doping with other metals (e.g., doping of Fe to NiOOH) are discussed. Full article