Steps on anatase TiO2(101)

Tailoring surface morphology on anatase TiO2 supported Au nanoclusters: implications for O2 activation

Strong interaction between the support surface and metal clusters activates the adsorbed molecules at the metal cluster-support interface. Using plane-wave DFT calculations, we precisely model the interface between anatase TiO2 and small Au nanoclusters. Our study focusses on the adsorption and activation of oxygen molecules on anatase TiO2, considering the influence of oxygen vacancies and steps on the surface. We find that the plane (101) and the stepped (103) surfaces do not support O2 activation, but the presence of oxygen vacancies results in strong adsorption and O-O bond length elongation. Modifying the TiO2 surface with supported small Au n nanoclusters (n = 3-5) also significantly enhances O2 adsorption and stretches the O-O bond. We observe that manipulating the cluster orientation through discrete rotations results in improved O2 adsorption and promotes charge transfer from the surface to the molecule. We propose that the orientation of the supported cluster may be manipulated by making the cluster adsorb at the step-edge of (103) TiO2. This results in activated O2 at the cluster-support interface, with a peroxide-range bond length and a low barrier for dissociation. Our modeling demonstrates a straightforward means of exploiting the interface morphology for O2 activation under low precious metal loading, which has important implications for electrocatalytic oxidation reactions and the rational design of supported catalysts.

Unveiling diverse coordination-defined electronic structures of reconstructed anatase TiO2(001)-(1 × 4) surface

Transition metal oxides (TMOs) exhibit fascinating physicochemical properties, which originate from the diverse coordination structures between the transition metal and oxygen atoms. Accurate determination of such structure-property relationships of TMOs requires to correlate structural and electronic properties by capturing the global parameters with high resolution in energy, real, and momentum spaces, but it is still challenging. Herein, we report the determination of characteristic electronic structures from diverse coordination environments on the prototypical anatase-TiO2(001) with (1 × 4) reconstruction, using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/atomic force microscopy, in combination with density functional theory calculation. We unveil that the shifted positions of O 2s and 2p levels and the gap-state Ti 3p levels can sensitively characterize the O and Ti coordination environments in the (1 × 4) reconstructed surface, which show distinguishable features from those in bulk. Our findings provide a paradigm to interrogate the intricate reconstruction-relevant properties in many other TMO surfaces.

Enhanced Sensing Performance of SnX Ti1-X O2 -TiX Sn1-X O2 Core-Shell Heterostructure via Increasing the Density of Unsaturated Sn and Ti Atoms

The strategy of combining different semiconductor materials is adjudged an effective approach to improve the sensing performances of semiconductor materials. However, the specific synergistic mechanism for the excellent gas-sensitive performances of composite materials has not been elucidated. Herein, a facile solvothermal method is employed to synthesize SnX Ti1-X O2 -TiX Sn1-X O2 core-shell heterostructures using SnCl4 •5H2 O and tetrabutyl titanate (TBOT) as raw materials. When the molar ratio of SnCl4 •5H2 O/TBOT is 1.8/3.0, the afforded composite exhibited the highest gas sensing performances compared with other composites prepared with other molar ratios. The enhanced sensing performance is attributed to the simultaneous incorporation of Sn and Ti ions into each other's lattice, leading to an increase in the density of unsaturated Sn and Ti atoms on the surface. Ultimately, more oxygen vacancies are formed by the unsaturated Sn and Ti atoms, which benefits electron capture and the redox reaction of adsorbed gases. Thus, the concept of increased unsaturated metal atoms and oxygen vacancy resulting from the doping of different metal ions into each other's lattice has deepened the understanding of gas sensing and the catalytic reaction mechanisms. The lattice synergy of different metals provides a pathway for the design of advanced gas-sensing materials and catalysts.

Effects of Photodynamic Therapy on Streptococcus mutans and Enamel Remineralization of Multifunctional TiO2-HAP Composite Nanomaterials

BACKGROUND

As photosensitizer and photocatalyst, titanium dioxide (TiO₂) can produce a photodynamic reaction for antibacterial treatment. This study aims to explore a Titanium dioxide/nano-hydroxyapatite (TiO₂-HAP) composite combined with the dental curing lamp (385-515 nm) in clinical which could inhibit the dental plaque biofilm formed by Streptococcus mutans (S. mutans) and promote the enamel surface remineralization simultaneously.

METHODS

X-ray Diffraction (XRD) and high resolution transmission electron microscope (HRTEM) were used to detect the characterization of TiO₂-HAP composite nanomaterials. Photodynamic properties of TiO₂-HAP were detected by Diffuse reflectance spectrum (DRS) and fluorescence spectroscopy. Bacterial growth was measured by reading the absorbance of bacterial cultures and confocal microscope was used to observe the biofilm removal ability of nanomaterials. The ability of TiO₂-HAP to promote enamel remineralization was measured by Scanning electron microscope (SEM).

RESULTS

The OD 600 of S. mutans was 0.76 in the control group and 0.13 in group of TiO₂-HAP with exposure to light-emitting diode (LED) (150 mW/cm²) for 5 min, suggesting its sustained antibacterial potency and inhibition of the metabolic activity of dental plaque microcosm biofilm. Also, the release of calcium and phosphorus ions in TiO₂-HAP can promote enamel mineralization simultaneously. After 15 days of remineralization, the Ca/P ratio of demineralized enamel surface increased from 1.28 to 1.67, which was similar to that of normal enamel.

CONCLUSIONS

The TiO₂-HAP exhibit a promising anti-bacterial activity and remineralization capacity which can prevent the occurrence of caries to the greatest extent and promote the biomimetic mineralization of dental tissues.

Synthesis of 2D anatase TiO2 with highly reactive facets by fluorine-free topochemical conversion of 1T-TiS2 nanosheets

Two-dimensional (2D) anatase titanium dioxide (TiO₂) is expected to exhibit different properties as compared to anatase nanocrystallites, due to its highly reactive exposed facets. However, access to 2D anatase TiO₂ is limited by the non-layered nature of the bulk crystal, which does not allow use of top-down chemical exfoliation. Large efforts have been dedicated to the growth of 2D anatase TiO₂ with high reactive facets by bottom-up approaches, which relies on the use of harmful chemical reagents. Here, we demonstrate a novel fluorine-free strategy based on topochemical conversion of 2D 1T-TiS₂ for the production of single crystalline 2D anatase TiO₂, exposing the {001} facet on the top and bottom and {100} at the sides of the nanosheet. The exposure of these faces, with no additional defects or doping, gives rise to a significant activity enhancement in the hydrogen evolution reaction, as compared to commercially available Degussa P25 TiO₂ nanoparticles. Because of the strong potential of TiO₂ in many energy-based applications, our topochemical approach offers a low cost, green and mass scalable route for production of highly crystalline anatase TiO₂ with well controlled and highly reactive exposed facets.

Ultrahigh Photocatalytic CO2 Reduction Efficiency and Selectivity Manipulation by Single-Tungsten-Atom Oxide at the Atomic Step of TiO2

The photocatalytic CO₂ reduction reaction is a sustainable route to the direct conversion of greenhouse gases into chemicals without additional energy consumption. Given the vast amount of greenhouse gas, numerous efforts have been devoted to developing inorganic photocatalysts, e.g., titanium dioxide (TiO₂ ), due to their stability, low cost, and environmentally friendly properties. However, a more efficient TiO₂ photocatalyst without noble metals is highly desirable for CO₂ reduction, and it is both difficult and urgent to produce selectively valuable compounds. Here, a novel "single-atom site at the atomic step" strategy is developed by anchoring a single tungsten (W) atom site with oxygen-coordination at the intrinsic steps of classic TiO₂ nanoparticles. The composition of active sites for CO₂ reduction can be controlled by tuning the additional W⁵⁺ to form W⁵⁺ -O-Ti³⁺ sites, resulting in both significant CO₂ reduction efficiency with 60.6 μmol g^{- 1} h^{- 1} and selectivity for methane (CH₄ ) over carbon monoxide (CO), which exceeds those of pristine TiO₂ by more than one order of magnitude. The mechanism relies on the accurate control of the single-atom sites at step with 22.8% coverage of surface sites and the subsequent excellent electron-hole separation along with the favorable adsorption-desorption of intermediates at the sites.

DFT Investigation of Substitutional and Interstitial Nitrogen-Doping Effects on a ZnO(100)-TiO2(101) Heterojunction

Density Functional Theory (DFT) calculations have been performed to investigate the structural and electronic properties of the ZnO(wurtzite)-ATiO2(anatase) heterojunction in the absence and presence of substitutional, interstitial nitrogen (N) doping and oxygen vacancies (OV). We report a detailed study of the interactions between the two nonpolar ZnO and TiO2 surfaces and on the role of N-doping and oxygen vacancies, which are decisive for improving the photocatalytic activity of the heterojunction. Our calculations show that substitutional N-doping is favored in the ATiO2 portion, whereas the interstitial one is favored in the ZnO region of the interface. Both substitutional and interstitial N-doped sites (i) induce gap states that act as deep electronic traps improving the charge separation and delaying electron-hole recombination, (ii) facilitate the OV formation causing a decrease in the formation energy (E FORM), and (iii) do not affect the band alignment when compared to the undoped analogue system. The presented results shed light on the N-doping effect on the electronic structure of the ZnO(100)-TiO2(101) heterojunction and how N-doping improves its photocatalytic properties.

Promotion of the Co3O4/TiO2 Interface on Catalytic Decomposition of Ammonium Perchlorate

Supports can widely affect or even dominate the catalytic activity and selectivity of nanoparticles because atomic geometry and electronic structures of active sites can be regulated, especially at the interface of nanoparticles and supports. However, the underlying mechanisms of most systems are still not fully understood yet. Herein, we construct the interface of Co₃O₄/TiO₂ to boost ammonium perchlorate (AP) catalytic decomposition. This catalyst shows enhanced catalytic performance. With the addition of 2 wt % Co₃O₄/TiO₂ catalysts, AP decomposition peak temperature decreases from 435.7 to 295.0 °C and activation energy decreases from 211.5 to 137.7 kJ mol^-1. By combining experimental and theoretical studies, we find that Co₃O₄ nanoparticles can be strongly anchored onto TiO₂ supports accompanied by charge transfer. Moreover, at the interfaces in the Co₃O₄/TiO₂ nanostructure, NH₃ adsorption can be enhanced through hydrogen bonds. Our research studies provide new insights into the promotion effects of the nanoparticle/support system on the AP decomposition process and inspire the design of efficient catalysts.

Li3[Al(PO4)2(H2O)1.5] and Na[AlP2O7]: from 2D layered polar to 3D centrosymmetric framework structures

Two new aluminophosphates with moderate NLO property or a short UV cut-off edge (∼190 nm) are reported.

Photocatalytic Air Purification Using Functional Polymeric Carbon Nitrides

The techniques for the production of the environment have received attention because of the increasing air pollution, which results in a negative impact on the living environment of mankind. Over the decades, burgeoning interest in polymeric carbon nitride (PCN) based photocatalysts for heterogeneous catalysis of air pollutants has been witnessed, which is improved by harvesting visible light, layered/defective structures, functional groups, suitable/adjustable band positions, and existing Lewis basic sites. PCN-based photocatalytic air purification can reduce the negative impacts of the emission of air pollutants and convert the undesirable and harmful materials into value-added or nontoxic, or low-toxic chemicals. However, based on previous reports, the systematic summary and analysis of PCN-based photocatalysts in the catalytic elimination of air pollutants have not been reported. The research progress of functional PCN-based composite materials as photocatalysts for the removal of air pollutants is reviewed here. The working mechanisms of each enhancement modification are elucidated and discussed on structures (nanostructure, molecular structue, and composite) regarding their effects on light-absorption/utilization, reactant adsorption, intermediate/product desorption, charge kinetics, and reactive oxygen species production. Perspectives related to further challenges and directions as well as design strategies of PCN-based photocatalysts in the heterogeneous catalysis of air pollutants are also provided.

Investigating the naturally occurring forms of TiO2 on electronic and optical properties using OLCAO-MGGA-TBO9: a hybrid DFT study

In this paper, a detailed study and analysis on the electronic and optical properties of anatase, rutile and brookite titanium dioxide (TiO2) which are the naturally occurring phases of TiO2 have been carried out. We have obtained these properties using the self-consistent orthogonalized linear combination of atomic orbitals with meta-generalized gradient approximation (MGGA) and Tran and Blaha (TBO9) as exchange–correlation under the framework of density functional theory. Obtained results on band gap value (E g), dielectric constant and refractive index as calculated by considering the optimal value of c (system-dependent parameter) have been analyzed statistically and are found to be much closer to the experimental values and are better than the other approaches published in the literature. It is seen that optical absorption for all the three phases of TiO2 occurs in UV region of EM spectrum. Using statistical analysis in correlation with other effective methods such as mBJ, GGA + U, GGA + Ud + Up, LSD + U, GW and HSE06 functional, it is found that MGGA-TB09 gives a better description of electronic structure and optical properties with less computation time. This work provides good understanding of electronic and optical properties of TiO2, stems a foundation for its possible applications in photo catalytic activities of dye sensitized solar cells.

Creating self-assembled arrays of mono-oxo (MoO3)1 species on TiO2(101) via deposition and decomposition of (MoO3)n oligomers

Hierarchically ordered oxides are of critical importance in material science and catalysis. Unfortunately, the design and synthesis of such systems remains a key challenge to realizing their potential. In this study, we demonstrate how the deposition of small oligomeric (MoO3)1-6 clusters-formed by the facile sublimation of MoO3 powders-leads to the self-assembly of locally ordered arrays of immobilized mono-oxo (MoO3)1 species on anatase TiO2(101). Using both high-resolution imaging and theoretical calculations, we reveal the dynamic behavior of the oligomers as they spontaneously decompose at room temperature, with the TiO2 surface acting as a template for the growth of this hierarchically structured oxide. Transient mobility of the oligomers on both bare and (MoO3)1-covered TiO2(101) areas is identified as key to the formation of a complete (MoO3)1 overlayer with a saturation coverage of one (MoO3)1 per two undercoordinated surface Ti sites. Simulations reveal a dynamic coupling of the reaction steps to the TiO2 lattice fluctuations, the absence of which kinetically prevents decomposition. Further experimental and theoretical characterizations demonstrate that (MoO3)1 within this material are thermally stable up to 500 K and remain chemically identical with a single empty gap state produced within the TiO2 band structure. Finally, we see that the constituent (MoO3)1 of this material show no proclivity for step and defect sites, suggesting they can reliably be grown on the (101) facet of TiO2 nanoparticles without compromising their chemistry.

Abundant hydroxyl groups decorated on nitrogen vacancy-embedded g-C3N4 with efficient photocatalytic hydrogen evolution performance

Engineering hydroxyl and N vacancies on g-C₃N₄ led to dual mitigation of the recombination rate of photogenerated carriers, which was achieved by enriched hydroxyl groups trapping the holes and stable N vacancies capturing the electrons.

Machine learning approach for elucidating and predicting the role of synthesis parameters on the shape and size of TiO2 nanoparticles

In the present work a series of design rules are developed in order to tune the morphology of TiO₂ nanoparticles through hydrothermal process. Through a careful experimental design, the influence of relevant process parameters on the synthesis outcome are studied, reaching to the develop predictive models by using Machine Learning methods. The models, after the validation and training, are able to predict with high accuracy the synthesis outcome in terms of nanoparticle size, polydispersity and aspect ratio. Furthermore, they are implemented by reverse engineering approach to do the inverse process, i.e. obtain the optimal synthesis parameters given a specific product characteristic. For the first time, it is presented a synthesis method that allows continuous and precise control of NPs morphology with the possibility to tune the aspect ratio over a large range from 1.4 (perfect truncated bipyramids) to 6 (elongated nanoparticles) and the length from 20 to 140 nm.

22% Efficiency Inverted Perovskite Photovoltaic Cell Using Cation-Doped Brookite TiO2 Top Buffer

Simultaneously achieving high efficiency and high durability in perovskite solar cells is a critical step toward the commercialization of this technology. Inverted perovskite photovoltaic (IP-PV) cells incorporating robust and low levelized-cost-of-energy (LCOE) buffer layers are supposed to be a promising solution to this target. However, insufficient inventory of materials for back-electrode buffers substantially limits the development of IP-PV. Herein, a composite consisting of 1D cation-doped TiO2 brookite nanorod (NR) embedded by 0D fullerene is investigated as a top modification buffer for IP-PV. The cathode buffer is constructed by introducing fullerene to fill the interstitial space of the TiO2 NR matrix. Meanwhile, cations of transition metal Co or Fe are doped into the TiO2 NR to further tune the electronic property. Such a top buffer exhibits multifold advantages, including improved film uniformity, enhanced electron extraction and transfer ability, better energy level matching with perovskite, and stronger moisture resistance. Correspondingly, the resultant IP-PV displays an efficiency exceeding 22% with a 22-fold prolonged working lifetime. The strategy not only provides an essential addition to the material inventory for top electron buffers by introducing the 0D:1D composite concept, but also opens a new avenue to optimize perovskite PVs with desirable properties.

Process-Induced Nanostructures on Anatase Single Crystals via Pulsed-Pressure MOCVD

The recent global pandemic of COVID-19 highlights the urgent need for practical applications of anti-microbial coatings on touch-surfaces. Nanostructured TiO₂ is a promising candidate for the passive reduction of transmission when applied to handles, push-plates and switches in hospitals. Here we report control of the nanostructure dimension of the mille-feuille crystal plates in anatase columnar crystals as a function of the coating thickness. This nanoplate thickness is key to achieving the large aspect ratio of surface area to migration path length. TiO₂ solid coatings were prepared by pulsed-pressure metalorganic chemical vapor deposition (pp-MOCVD) under the same deposition temperature and mass flux, with thickness ranging from 1.3-16 mm, by varying the number of precursor pulses. SEM and STEM were used to measure the mille-feuille plate width which is believed to be a key functional nano-dimension for photocatalytic activity. Competitive growth produces a larger columnar crystal diameter with thickness. The question is if the nano-dimension also increases with columnar crystal size. We report that the nano-dimension increases with the film thickness, ranging from 17-42 nm. The results of this study can be used to design a coating which has co-optimized thickness for durability and nano-dimension for enhanced photocatalytic properties.

Shape-controlling effects of hydrohalic and carboxylic acids in TiO2 nanoparticle synthesis

The ability to synthesize nanoparticles (NPs), here TiO₂, of different shapes in a controlled and reproducible way is of high significance for a wide range of fields including catalysis and materials design. Different NP shapes exhibit variations of emerging facets, and processes such as adsorption, diffusion, and catalytic activity are, in general, facet sensitive. Therefore, NP properties, e.g., the reactivity of NPs or the stability of assembled NPs, depend on their shape. We combine computational modeling based on density functional theory with experimental techniques such as transmission electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray powder diffraction to investigate the ability of various adsorbates, including hydrohalic and carboxylic acids, to influence NP shape. This approach allows us to identify mechanisms stabilizing specific surface facets and thus to predict NP shapes using computational model systems and to experimentally characterize the synthesized NPs in detail. Shape-controlled anatase TiO₂ NPs are synthesized here in agreement with the calculations in platelet and bi-pyramidal shapes by employing different precursors. The importance of the physical conditions and chemical environment during synthesis, e.g., via competitive adsorption or changes in the chemical potentials, is studied via ab initio thermodynamics, which allows us to set previous and new results in a broader context and to highlight potentials for additional synthesis routes and NP shapes.

A DFT+U revisit of reconstructed CeO2(100) surfaces: structures, thermostabilities and reactivities

Cerium dioxide (CeO2) shows wide catalytic applications by virtue of its excellent oxygen storage capacity. The CeO2(100) surface has aroused particular interest because of its intrinsic polarity; however, it suffers from structural reconstruction, which consequently hinders experimental and theoretical studies. In this work, we performed density functional theory calculations with on-site Coulomb interaction correction to investigate and further correlate the geometric and catalytic properties of reconstructed CeO2(100) surfaces. By introducing CeO2 units on a previous O-terminal model, the surface exposed CeO4 pyramids with gradual increase in coverage and eventually transformed into a Ce-terminal structure. The corresponding thermostabilities were evaluated by calculating the surface energy and oxygen vacancy formation energy. We also showed that the CO oxidation on the reconstructed CeO2(100) surfaces favored the Mars-van-Krevelen mechanism. The most stable CeO4-terminal type of reconstruction, covered with a half overlayer of CeO4 pyramids on the surface, was capable of directly producing CO2 without forming bent CO2 intermediates and carbonate byproducts. Moreover, coordinatively unsaturated Ce ions at the pyramid apex provided extra accommodation to the reacting CO, thus lowering the reaction barrier of the key CO coupling step relative to that of the O-terminal surface. We finally generalized a unified picture of the dynamic changes in the thermostability and catalytic activity along with the structural reconstruction of the CeO2(100) surface. The CeO4-terminal type of reconstruction was theoretically predicted to be highly efficient for catalyzing CO oxidation.

Screening Doping Strategies To Mitigate Electron Trapping at Anatase TiO2 Surfaces

Nanocrystalline anatase titanium dioxide is an efficient electron transport material for solar cells and photocatalysts. However, low-coordinated Ti cations at surfaces introduce low-lying Ti 3d states that can trap electrons, reducing charge mobility. Here, a number of dopants (V, Sb, Sn, Zr, and Hf) are examined to replace these low-coordinated Ti cations and reduce electron trapping in anatase crystals. V, Sb, and Sn dopants act as electron traps, while Zr and Hf dopants are found to prevent electron trapping. We also show that alkali metal dopants can be used to fill surface traps by donating electrons into the 3d states of low-coordinated Ti ions. These results provide practical guidance on the optimization of charge mobility in nanocrystalline TiO₂ by doping.

Roles of N-Vacancies over Porous g-C3N4 Microtubes during Photocatalytic NO x Removal

The development of catalysts that effectively activate target pollutants and promote their complete conversion is an admirable objective in the environmental photocatalysis field. In this work, graphitic carbon nitride (g-C₃N₄) microtubes with tunable N-vacancy concentrations were controllably fabricated using an in situ soft-chemical method. The morphological evolution of g-C₃N₄, from the bulk to the porous tubular architecture, is discussed in detail with the aid of time-resolved hydrothermal experiments. We found that the NO removal ratio and apparent reaction rate constant of the g-C₃N₄ microtubes were 1.8 and 2.6 times higher than those of pristine g-C₃N₄, respectively. By combining detailed experimental characterization and density functional theory calculations, the effects of N-vacancies in the g-C₃N₄ microtubes on O₂ and NO adsorption activation, electron capture, and electronic structure were systematically investigated. These results demonstrate that surface N-vacancies act as specific sites for the adsorption activation of reactants and photoinduced electron capture, while enhancing the light-absorbing capability of g-C₃N₄. Moreover, the porous wall structures of the as-prepared g-C₃N₄ microtubes facilitate the diffusion of reactants, and their tubular architectures favor the oriented transfer of charge carriers. The intermediates formed during photocatalytic NO removal processes were identified by in situ diffuse reflectance infrared Fourier transform spectroscopy, and different reaction pathways over pristine and N-deficient g-C₃N₄ are proposed. This study provides a feasible strategy for air pollution control over g-C₃N₄ by introducing N-vacancy and porous tubular architecture simultaneously.

Tuning Pb(II) Adsorption from Aqueous Solutions on Ultrathin Iron Oxychloride (FeOCl) Nanosheets

Structural tunability and surface functionality of layered two-dimensional (2-D) iron oxychloride (FeOCl) nanosheets are critical for attaining exceptional adsorption properties. In this study, we combine computational and experimental tools to elucidate the distinct adsorption nature of Pb(II) on 2-D FeOCl nanosheets. After finding promising Pb(II) adsorption characteristics by bulk FeOCl sheets (B-FeOCl), we applied computational quantum mechanical modeling to mechanistically explore Pb(II) adsorption on representative FeOCl facets. Results indicate that increasing the exposure of FeOCl oxygen and chlorine sites significantly enhances Pb(II) adsorption. The (110) and (010) facets of FeOCl possess distinct orientations of oxygen and chlorine, resulting in different Pb(II) adsorption energies. Consequently, the (110) facet was found to be more selective toward Pb(II) adsorption than the (010) facet. To exploit this insight, we exfoliated B-FeOCl to obtain ultrathin FeOCl nanosheets (U-FeOCl) possessing unique chlorine- and oxygen-enriched surfaces. As we surmised, U-FeOCl nanosheets achieved excellent Pb(II) adsorption capacity (709 mg g^-1 or 3.24 mmol g^-1). Moreover, U-FeOCl demonstrated rapid adsorption kinetics, shortening adsorption equilibration time to one-third of the time for B-FeOCl. Extensive characterization of FeOCl-Pb adsorption complexes corroborated the simulation results, illustrating that increasing the number of Pb-O and Pb-Cl interaction sites led to the improved Pb(II) adsorption capacity of U-FeOCl.

Nanostructured TiO2 anatase-rutile-carbon solid coating with visible light antimicrobial activity

TiO₂ photocatalyst is of interest for antimicrobial coatings on hospital touch-surfaces. Recent research has focused on visible spectrum enhancement of photocatalytic activity. Here, we report TiO₂ with a high degree of nanostructure, deposited on stainless steel as a solid layer more than 10 μm thick by pulsed-pressure-MOCVD. The TiO₂ coating exhibits a rarely-reported microstructure comprising anatase and rutile in a composite with amorphous carbon. Columnar anatase single crystals are segmented into 15-20 nm thick plates, resulting in a mille-feuilles nanostructure. Polycrystalline rutile columns exhibit dendrite generation resembling pine tree strobili. We propose that high growth rate and co-deposition of carbon contribute to formation of the unique nanostructures. High vapor flux produces step-edge instabilities in the TiO₂, and solid carbon preferentially co-deposits on certain high energy facets. The equivalent effective surface area of the nanostructured coating is estimated to be 100 times higher than standard TiO₂ coatings and powders. The coatings prepared on stainless steel showed greater than 3-log reduction in viable E coli after 4 hours visible light exposure. The pp-MOCVD approach could represent an up-scalable manufacturing route for supported catalysts of functional nanostructured materials without having to make nanoparticles.

Photocatalysis with Reduced TiO2: From Black TiO2 to Cocatalyst-Free Hydrogen Production

Black TiO₂ nanomaterials have recently emerged as promising candidates for solar-driven photocatalytic hydrogen production. Despite the great efforts to synthesize highly reduced TiO₂, it is apparent that intermediate degree of reduction (namely, gray titania) brings about the formation of peculiar defective catalytic sites enabling cocatalyst-free hydrogen generation. A precise understanding of the structural and electronic nature of these catalytically active sites is still elusive, as well as the fundamental structure-activity relationships that govern formation of crystal defects, increased light absorption, charge separation, and photocatalytic activity. In this Review, we discuss the basic concepts that underlie an effective design of reduced TiO₂ photocatalysts for hydrogen production such as (i) defects formation in reduced TiO₂, (ii) analysis of structure deformation and presence of unpaired electrons through electron paramagnetic resonance spectroscopy, (iii) insights from surface science on electronic singularities due to defects, and (iv) the key differences between black and gray titania, that is, photocatalysts that require Pt-modification and cocatalyst-free photocatalytic hydrogen generation. Finally, future directions to improve the performance of reduced TiO₂ photocatalysts are outlined.

The Role of Surface Texture on the Photocatalytic H2 Production on TiO2

It has been often reported that an efficient and green photocatalytic dissociation of water under irradiated semiconductors likely represents the most important goal for modern chemistry. Despite decades of intensive work on this topic, the efficiency of the water photolytic process under irradiated semiconductors is far from reaching significant photocatalytic efficiency. The use of a sacrificial agent as hole scavenger dramatically increases the hydrogen production rate and might represent the classic “kill two birds with one stone”: on the one hand, the production of hydrogen, then usable as energy carrier, on the other, the treatment of water for the abatement of pollutants used as sacrificial agents. Among metal oxides, TiO2 has a central role due to its versatility and inexpensiveness that allows an extended applicability in several scientific and technological fields. In this review we focus on the hydrogen production on irradiated TiO2 and its fundamental and environmental implications.

Anisotropic photogenerated charge separations between different facets of a dodecahedral α-Fe2O3 photocatalyst

Hematite (α-Fe₂O₃) is regarded as one of the most promising photocatalysts, but its photocatalytic activities have always been limited to the pristine form because of poor charge separation efficiency.

Synthesis of mesoporous core-shell TiO2 microstructures with coexposed {001}/{101} facets: enhanced intrinsic photocatalytic performance

TiO₂ microstructures were synthesized via a facile one-step route for enhanced intrinsic photocatalytic performance. The prepared TiO₂ microstructures are featured by both mesoporous core-shell structures and coexposed {001}/{101} facets. Their intrinsic photocatalytic performance were remarkably enhanced due to their high specific surface area, coexposed {001}/{101} facets, and promoted separation of photogenerated carriers. Furthermore, the origin and detailed mechanism for diethylenetriamine (DETA) that served as a high efficient stabilizer of TiO₂ {001} facet have been theoretically investigated. Finally, a new DETA-modified Ostwald ripening mechanism was originally proposed when studying the growth mechanism of the mesoporous core-shell TiO₂ spherical microstructures with coexposed {001}/{101} facets.

Controllable dynamics of oxygen vacancies through extrinsic doping for superior catalytic activities

Due to its strong redox ability, high stability, cost effectiveness and non-toxicity, cerium oxide (CeO₂) has been extensively researched as an active photocatalyst material. The underlying photocatalytic reactions are mostly associated with the transportation of oxygen ions through vacancies, but the actual transport phenomenon had not been clearly understood. In this work, gadolinium (Gd) is sequentially doped into CeO₂ to investigate how extrinsic doping can modulate oxygen vacancies in CeO₂ and influence photocatalytic activities. From our investigations, it was found that the Gd doping may induce structural symmetry breaking leading to a pure CeO₂ fluorite structure that transforms mobile oxygen vacancies into clustered or immobile vacancies. When the vacancies were set as "mobile" (for Gd doping levels ≤15 at%), maximum photocatalytic activities were obtained. In contrast, suppressed photocatalytic efficiencies were noted for higher Gd doping levels (20 at% or more). The results reported in this research may provide an extra degree of freedom in the form of extrinsic doping to configure the oxygen vacancy defects and their mobility to achieve better catalytic efficiencies.

Pentacene/TiO2 Anatase Hybrid Interface Study by Scanning Probe Microscopy and First Principles Calculations

The understanding and control of the buried interface between functional materials in optoelectronic devices is key to improving device performance. We combined atomic resolution scanning probe microscopy with first-principles calculations to characterize the technologically relevant organic/inorganic interface structure between pentacene molecules and the TiO₂ anatase (101) surface. A multipass atomic force microscopy imaging technique overcomes the technical challenge of imaging simultaneously the corrugated anatase substrate, molecular adsorbates, monolayers, and bilayers at the same level of detail. Submolecular resolution images revealed the orientation of the adsorbates with respect to the substrate and allowed direct insights into interface formation. Pentacene molecules were found to physisorb parallel to the anatase substrate in the first contact layer, passivating the surface and promoting bulk-like growth in further organic layers. While molecular electronic states were not significantly hybridized by the substrate, simulations predicted localized pathways for molecule-surface charge injection. The localized states were associated with the molecular lowest unoccupied molecular orbital inside the oxide conduction band, pointing to efficient transfer of photo-induced electron charge carriers across this interface in prospective photovoltaic devices. In uncovering the atomic arrangement and favorable electronic properties of the pentacene/anatase interface, our findings testify to the maturity and analytic power of our methodology in further studies of organic/inorganic interfaces.

Structural evolution of titanium dioxide during reduction in high-pressure hydrogen

The excellent photocatalytic properties of titanium oxide (TiO₂) under ultraviolet light have long motivated the search for doping strategies capable of extending its photoactivity to the visible part of the spectrum. One approach is high-pressure and high-temperature hydrogenation, which results in reduced 'black TiO₂' nanoparticles with a crystalline core and a disordered shell that absorbs visible light. Here we elucidate the formation mechanism and structural features of black TiO₂ using first-principles-validated reactive force field molecular dynamics simulations of anatase TiO₂ surfaces and nanoparticles at high temperature and under high hydrogen pressures. Simulations reveal that surface oxygen vacancies created upon reaction of H₂ with surface oxygen atoms diffuse towards the bulk material but encounter a high barrier for subsurface migration on {001} facets of the nanoparticles, which initiates surface disordering. Besides confirming that the hydrogenated amorphous shell has a key role in the photoactivity of black TiO₂, our results provide insight into the properties of the disordered surface layers that are observed on regular anatase nanocrystals under photocatalytic water-splitting conditions.

Influence of surface atomic structure demonstrated on oxygen incorporation mechanism at a model perovskite oxide

Perovskite oxide surfaces catalyze oxygen exchange reactions that are crucial for fuel cells, electrolyzers, and thermochemical fuel synthesis. Here, by bridging the gap between surface analysis with atomic resolution and oxygen exchange kinetics measurements, we demonstrate how the exact surface atomic structure can determine the reactivity for oxygen exchange reactions on a model perovskite oxide. Two precisely controlled surface reconstructions with (4 × 1) and (2 × 5) symmetry on 0.5 wt.% Nb-doped SrTiO3(110) were subjected to isotopically labeled oxygen exchange at 450 °C. The oxygen incorporation rate is three times higher on the (4 × 1) surface phase compared to the (2 × 5). Common models of surface reactivity based on the availability of oxygen vacancies or on the ease of electron transfer cannot account for this difference. We propose a structure-driven oxygen exchange mechanism, relying on the flexibility of the surface coordination polyhedra that transform upon dissociation of oxygen molecules.

Calcium Phosphate Deposition on Planar and Stepped (101) Surfaces of Anatase TiO2: Introducing an Interatomic Potential for the TiO2/Ca-PO4/Water Interface

Titanium is commonly employed in orthopaedic and dental surgery, owing to its good mechanical properties. The titanium metal is usually passivated by a thin layer of its oxide, and in order to promote its integration with the biological tissue, it is covered by a bioactive material such as calcium phosphate (CaP). Here, we have investigated the deposition of calcium and phosphate species on the anatase phase of titanium dioxide (TiO₂) using interatomic potential-based molecular dynamics simulations. We have combined different force fields developed for CaP, TiO₂, and water, benchmarking the results against density functional theory calculations. On the basis of our study, we consider that the new parameters can be used successfully to study the nucleation of CaP on realistic anatase and rutile TiO₂ nanoparticles, including surface defects.

Water Dissociates at the Aqueous Interface with Reduced Anatase TiO2 (101)

Elucidating the structure of the interface between natural (reduced) anatase TiO2 (101) and water is an essential step toward understanding the associated photoassisted water splitting mechanism. Here we present surface X-ray diffraction results for the room temperature interface with ultrathin and bulk water, which we explain by reference to density functional theory calculations. We find that both interfaces contain a 25:75 mixture of molecular H2O and terminal OH bound to titanium atoms along with bridging OH species in the contact layer. This is in complete contrast to the inert character of room temperature anatase TiO2 (101) in ultrahigh vacuum. A key difference between the ultrathin and bulk water interfaces is that in the latter water in the second layer is also ordered. These molecules are hydrogen bonded to the contact layer, modifying the bond angles.

Quantitative Attachment of Bimetal Combinations of Transition-Metal Ions to the Surface of TiO2 Nanorods

We report the sequential, quantitative loading of transition-metal ions (Cr³⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺) onto the surface of rod-shaped anatase TiO₂ nanocrystals in bimetallic combinations (₆ C₂ = 15) to form M,M'-TiO₂ nanocrystals. The materials were characterized with transmission electron microscopy (TEM), powder X-ray diffraction (XRD), elemental analysis, X-ray photoelectron spectroscopy (XPS), and UV-visible spectroscopy. TEM and XRD data indicate that the sequential adsorption of metal ions occurs with the retention of the phase and morphology of the nanocrystal. Atomistic models of the M,M'-TiO₂ nanocrystals were optimized with density functional theory calculations. Calculated UV-visible absorption spectra and partial charge density maps of the donor and acceptor states for the electronic transitions indicate the importance of metal-to-metal charge transfer (MMCT) processes.

Combined pulsed laser deposition and non-contact atomic force microscopy system for studies of insulator metal oxide thin films

We have designed and developed a combined system of pulsed laser deposition (PLD) and non-contact atomic force microscopy (NC-AFM) for observations of insulator metal oxide surfaces. With this system, the long-period iterations of sputtering and annealing used in conventional methods for preparing a metal oxide film surface are not required. The performance of the combined system is demonstrated for the preparation and high-resolution NC-AFM imaging of atomically flat thin films of anatase TiO2(001) and LaAlO3(100).

Step edge structures on the anatase TiO2 (001) surface studied by atomic-resolution TEM and STM

Atomic arrangements in oxide surfaces can be uncovered by combining side view imaging using transmission electron microscopy and top view imaging using scanning tunnelling microscopy.

Synthesis and Enhanced Solar Light Photocatalytic Activity of a C/N Co-Doped TiO2/Diatomite Composite with Exposed (001) Facets

A C/N co-doped TiO2/diatomite composite with exposed (001) facet was prepared through a facile sol–gel method using tetrabutyl titanate as a titanium precursor and hexamethylenetetramine as C/N dopant. The as-prepared photocatalyst composites were characterised by X-ray diffraction (XRD), nitrogen adsorption–desorption isotherms, scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectra (DRS), photoluminescence spectroscopy (PL), as well as X-ray photoelectron spectroscopy (XPS). The TiO2 nanoparticles were immobilised and uniformly distributed on the surface of diatomite with a smaller grain size compared with pure TiO2. In addition, compared with pure TiO2 and the undoped TiO2/diatomite composite, the photocatalytic activity of the C/N co-doped TiO2/diatomite composite under solar light illumination was obviously enhanced. Results indicate that the introduction of a C/N dopant can effectively promote the growth of the highly active anatase (001) facet of TiO2. On the other hand, the N impurity was doped into the interstitial spaces of the TiO2 lattice, which accelerated the charge transfer and hindered the recombination of photogenerated electron–hole pairs. The results show that the as-prepared composite exhibited promising applications in dye wastewater degradation owing to its outstanding reusability and cost-effectiveness.

O2 Activation on Ceria Catalysts-The Importance of Substrate Crystallographic Orientation

An atomic-level understanding of dioxygen activation on metal oxides remains one of the major challenges in heterogeneous catalysis. By performing a thorough surface-science study of all three low-index single-crystal surfaces of ceria, probably the most important redox catalysts, we provide a direct spectroscopic characterization of reactive dioxygen species at defect sites on the reduced ceria (110) and (100) surfaces. Surprisingly, neither of these superoxo and peroxo species was found on ceria (111), the thermodynamically most stable surface of this oxide. Applying density functional theory, we could relate these apparently inconsistent findings to a sub-surface diffusion of O vacancies on (111) substrates, but not on the less-closely packed surfaces. These observations resolve a long standing debate concerning the location of O vacancies on ceria surfaces and the activation of O₂ on ceria powders.