Flat TiO2 Research Articles

Biofunctionalization of Porous Titanium Oxide through Amino Acid Coupling for Biomaterial Design.

Porous transition metal oxides are widely studied as biocompatible materials for the development of prosthetic implants. Resurfacing the oxide to improve the antibacterial properties of the material is still an open issue, as infections remain a major cause of implant failure. We investigated the functionalization of porous titanium oxide obtained by anodic oxidation with amino acids (Leucine) as a first step to couple antimicrobial peptides to the oxide surface. We adopted a two-step molecular deposition process as follows: self-assembly of aminophosphonates to titanium oxide followed by covalent coupling of Fmoc-Leucine to aminophosphonates. Molecular deposition was investigated step-by-step by Atomic Force Microscopy (AFM) and X-ray Photoemission Spectroscopy (XPS). Since the inherent high roughness of porous titanium hampers the analysis of molecular orientation on the surface, we resorted to parallel experiments on flat titanium oxide thin films. AFM nanoshaving experiments on aminophosphonates deposited on flat TiO2 indicate the formation of an aminophosphonate monolayer while angle-resolved XPS analysis gives evidence of the formation of an oriented monolayer exposing the amine groups. The availability of the amine groups at the outer interface of the monolayer was confirmed on both flat and porous substrates by the following successful coupling with Fmoc-Leucine, as indicated by high-resolution XPS analysis.

Substantial Na-Ion Storage at High Current Rates: Redox-Pseudocapacitance through Sodium Oxide Formation.

Batteries and supercapacitors, both governed by electrochemical processes, operate by different electrochemical mechanisms which determine their characteristic energy and power densities. Battery materials store large amounts of energy by ion intercalation. Electrical double-layer capacitors store charge through surface-controlled ion adsorption which leads to high power and rapid charging, but much smaller amounts of energy stored. Pseudocapacitive materials offer the promise to combine these properties by storing charge through surface-controlled, battery-like redox reactions but at high rates approaching those of electrochemical double-layer capacitors. This work compares the pseudo-capacitive charge storage characteristics of self-organized titanium dioxide (TiO2-x) nanotubes (NTs) to flat TiO2-x surface films to further elucidate the proposed charge storage mechanism within the formed surface films. By comparing TiO2-x NTs to flat TiO2-x surface films, having distinctively different oxide mass and surface area ratios, it is shown that NaO2 and Na2O2 formation, which constitutes the active surface film material, is governed by the metal oxide bulk. Our results corroborate that oxygen diffusion from the lattice oxide is key to NaO2 and Na2O2 formation.

Modulating noncovalent and covalent forces to control inverse phosphocholine lipid self-assembly on inorganic surfaces: Nanoarchitectonic design principles

While noncovalent forces typically drive lipid vesicle adsorption and rupture to form supported lipid bilayer (SLB) coatings on inorganic surfaces for various material science applications, this strategy only works on a few materials with suitable energetics such as SiO2. The use of coordination chemistry between inverse phosphocholine (PC) lipid headgroups and surfaces has emerged as a promising strategy to enable SLB formation on other materials such as TiO2 based on covalent forces. However, until now, a cohesive picture of how noncovalent and covalent forces jointly contribute to the SLB formation process on TiO2 has been lacking and addressing this gap is important to design functional lipid biointerfaces with tailored properties such as antifouling capabilities. Herein, we investigated inverse PC lipid vesicle adsorption onto TiO2 and SiO2 surfaces and discovered how adsorption pathways can be controlled by tuning the balance of noncovalent and covalent forces, which enabled us to establish molecular design rules to fabricate physically robust SLB coatings on macroscopically flat TiO2 surfaces. On TiO2, SLB formation depended on two key factors: (1) favorable noncovalent forces to facilitate initial vesicle adsorption; and (2) a critical density of lipid-TiO2 covalent bonds to enable sufficient vesicle deformation triggering fusion and rupture. In other cases, either no adsorption or intact vesicle adsorption without rupture occurred even when covalent bonds were present. Conversely, on SiO2, conditions were identified to support inverse PC lipid adsorption whereas vesicles were repelled otherwise. The experimental results were supported by interfacial force modeling and our findings demonstrate how a subtle interplay of noncovalent and covalent forces plays a deterministic role in modulating lipid self-assembly pathways. Specific conditions in which the physically stable, fabricated SLBs on TiO2 surfaces exhibit antifouling properties were also identified based on optimizing the lipid composition to enhance vesicle-surface interactions while preventing other nonspecific interaction events.

Fabrication of moth-eye patterned TiO2 active layers for high energy efficiency and current density of dye-sensitized solar cells

Past two decades have shown the unprecedentedly rapid emergence of dye-sensitized solar cells (DSSCs) that are efficient, inexpensive, and eco-friendly. However, structural studies of the internal solar cells have not made much progress because of the high cost and complexity of the fabrication. In this study, therefore, we designed and fabricated a moth-eye patterned TiO2 layer as a working electrode using a soft lithography method with polystyrene (PS) monolayer, and the moth-eye patterned TiO2 active layers were constructed using a polydimethylsiloxane (PDMS) stamp via inverse honeycomb patterning having various PS beads of 400, 800, 1200 and 1600 nm. For the 1600 nm patterned stamp, Jsc is higher for moth-eye patterned TiO2 (13.63 mA/cm2) than that for flat TiO2 (11.09 mA/cm2), which results in a higher power efficiency of 7.66% rather than 6.40%. This result found an increment of around 23% in current density and 20% in energy conversion efficiency, mainly because of greater light-harvesting effect by its structural anti-reflective property.

Direct Observation of Atomistic Reaction Process between Pt Nanoparticles and TiO2 (110)

The catalytic activity and selectivity of heterogeneous catalysts are governed by atomic and electronic structures at the heterointerface between noble metal nanoparticles (NPs) and oxide substrates. In specific chemical reactions, it is well-known that the catalytic activity is strongly suppressed by annealing in a reducing atmosphere, so-called strong metal-support interaction (SMSI). However, it is still unclear the formation process and atomistic origin of the SMSI. By preparing well-defined platinum (Pt) NPs supported on atomically flat TiO2 (110) substrate, we directly show the formation of chemically ordered Pt-Ti intermetallic NPs and impregnation of NPs into TiO2 substrate at high temperatures by using atomic-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy. Furthermore, we observed negative charge transfer from the Pt-Ti intermetallic NPs to the TiO2 surface, which would strongly affect the catalytic activities.

2D/2D Heterojunction of TiO2 Nanoparticles and Ultrathin G-C3N4 Nanosheets for Efficient Photocatalytic Hydrogen Evolution

Photocatalytic hydrogen evolution is considered one of the promising routes to solve the energy and environmental crises. However, developing efficient and low-cost photocatalysts remains an unsolved challenge. In this work, ultrathin 2D g-C3N4 nanosheets are coupled with flat TiO2 nanoparticles as face-to-face 2D/2D heterojunction photocatalysts through a simple electrostatic self-assembly method. Compared with g-C3N4 and pure TiO2 nanosheets, 2D/2D TiO2/g-C3N4 heterojunctions exhibit effective charge separation and transport properties that translate into outstanding photocatalytic performances. With the optimized heterostructure composition, stable hydrogen evolution activities are threefold and fourfold higher than those of pure TiO2, and g-C3N4 are consistently obtained. Benefiting from the favorable 2D/2D heterojunction structure, the TiO2/g-C3N4 photocatalyst yields H2 evolution rates up to 3875 μmol·g−1·h−1 with an AQE of 7.16% at 380 nm.

Adsorption of Polyions on Flat TiO2 Surface

In this study, the surface properties of Ti/TiO2 substrate before and after the adsorption of polyelectrolytes were investigated. As model polyelectrolytes, strongly charged polycation poly(diallyldimethylammonium) (PDADMA) and strongly charged polyanion poly(4-styrenesulfonate) (PSS) were used. Initially, the bare titanium substrate was characterized by means of ellipsometry, atomic force microscopy (AFM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and measurements of inner surface potential using crystal electrode (CrE). It was shown that the substrate surface is very smooth with the roughness of 3.5 nm and oxide layer thickness of 3.8 nm. After the adsorption of PDADMA and PSS, polyelectrolyte-coated titanium surface was examined using the above-mentioned methods under the same conditions. It was found that both PDADMA cations and PSS anions form a stable polymeric nanofilm on Ti/TiO2 surface that partially covers the surface, without significant impact on the surface roughness. The corrosion protection effectiveness values indicate that the corrosion properties were greatly enhanced upon polyion adsorption and polyelectrolyte coating formation on the flat TiO2 surface. The obtained results were additionally confirmed by inner surface potential measurements. According to the methods employed, PDADMA nanofilm modification offers enhanced corrosion protection to the underlying titanium material in sodium chloride electrolyte solution.

Quasi-Hexagonal to Lepidocrocite-like Transition in TiO2 Ultrathin Films on Cu(001)

The structure and growth mechanism of TiO2 ultrathin films on Cu(001) were investigated by experimental and theoretical studies. Multiple experimental techniques, including photoemission spectroscopies and scanning probe microscopy, were used to carefully characterize the chemical, electronic, and morphological structure of the metal oxide layer. A complete and flat TiO2 bilayer with a lepidocrocite-like structure is observed at the earliest stages of its growth on the Cu(001) surface under controlled experimental conditions. The employed multitechnique approach reveals that a transition between two different structures of the TiO2 film, that is, the quasi-hexagonal and lepidocrocite-like structure, occurs as a function of the coverage passing from one to two monolayers. Ab initio calculations indicate that the lepidocrocite-like structure is thermodynamically more stable on Cu(001) than the quasi-hexagonal one, thus supporting the observed transition.

Vertical 3D Printed Forest‐Inspired Hierarchical Plasmonic Superstructure for Photocatalysis

AbstractEfficient light‐harvesting is of significant importance to achieve high solar energy utilization efficiency for various solar‐driven technologies. Compared with a 2D planar structure, a 3D plasmonic structure can largely increase the light adsorption/interaction areas and also utilizes the plasmonic effect to achieve much higher light utilization efficiency. However, this remains challenging in terms of structural design, reliable manufacturing, and ability to scale up. Herein, inspired by the light absorption strategy of natural forests, a hierarchical plasmonic superstructure is demonstrated composed of vertical TiO2 pillar arrays (as tree trunks), dense nanorod arrays (as branches), and a large number of plasmonic Au nanoparticles (as leaves). Such a forest‐like plasmonic superstructure can effectively absorb light from the surface plasmonic resonance effects of Au nanoparticles and the multiple scattering of light in the hierarchical branched structure. The strong light absorption and abundant photocatalytic active sites help yield a 15‐fold higher nitrogen photo‐fixation activity than that of the flat TiO2 films decorated with Au nanoparticles. The study provides an effective strategy to construct 3D plasmonic superstructures with excellent light‐harvesting efficiency and high stability and can be readily applied to a range of light‐driven applications

Absorption mechanism of dopamine/DOPAC-modified TiO2 nanoparticles by time-dependent density functional theory calculations

Donor-modified TiO2 nanoparticles are interesting hybrid systems shifting the absorption edge of this semiconductor from the ultra-violet to the visible or infrared light spectrum, which is a benefit for several applications ranging from photochemistry, photocatalysis, photovoltaics, or photodynamic therapy. Here, we investigate the absorption properties of two catechol-like molecules, that is, dopamine and DOPAC ligands, when anchored to a spherical anatase TiO2 nanoparticle of realistic size (2.2 nm), by means of time-dependent density functional theory calculations. By the differential absorbance spectra with the bare nanoparticle, we show how it is possible to determine the injection mechanism. Since new low-energy absorption peaks are observed, we infer a direct charge transfer injection, which, unexpectedly, does not involve the lowest energy conduction band states. We also find that the more perpendicular the molecular benzene ring is to the surface, the more intense is the absorption, which suggests aiming at high molecular packing in the synthesis. Through a comparative investigation with a flat TiO2 surface model, we unravel both the curvature and coverage effects.

Mo Doping Assisting the CVD Synthesis of Size-Controlled, Uniformly Distributed Single-Layer MoS2 on Rutile TiO2(110)

Molybdenum disulfide (MoS2) has attracted considerable interests due to superior electronic and optical properties which have seen promising applications in optoelectronics and catalysis. Chemical vapor deposition (CVD) method has been successfully applied in synthesizing MoS2 on various substrates. However, it remains a great challenge to fabricate high-quality MoS2 sheets with well-controlled micro/nano size and homogeneous distribution over the functional substrates such as active metal oxides. Herein, we have developed a two-step synthetic strategy via depositing MoO3 first followed by subsequent vulcanization, thus to grow single-layer MoS2 on atomically flat rutile TiO2(110) (r-TiO2(110)) substrate. This method not only very well controls the size as well as the special distribution of MoS2 nanosheets over the TiO2 surface, but also averts the formation of contaminative species at the heterojunction while maintaining the atomic structure of the substrate surface. The extensive characterizations reveal that the formation of MoS2 derives from the sulphurization of the singly dispersed Mo6+ and Mo5+ species in the surface/subsurface region instead of the aggregated MoO3 patches on top of the TiO2 surface. Such mechanism may dictate a general way for synthesizing high-quality transition-metal dichalcogenides (TMDs) over a variety of functional substrates.

Molecular Structure and Electronic Properties ofpara-Hexaphenyl Monolayer on Atomically Flat Rutile TiO2(110)

The structure and stability of a molecular wetting layer of the rodlike organic semiconductor molecule para-hexaphenyl grown at room temperature on atomically flat titanium dioxide TiO2(110) surfac...

Dewetted Au Nanoparticles on TiO2 Surfaces: Evidence of a Size-Independent Plasmonic Photoelectrochemical Response

Flat TiO$_2$ layers are deposited by magnetron sputtering on Ti/Si wafers. The TiO$_2$ surfaces are then sputter-coated with thin Au films of a nominal thickness of 0.5-10 nm that are converted by solid-state dewetting into Au nanoparticles of tuneable size and spacing; the Au nanoparticle size can be tuned over a broad range, i.e. ca. 3-200 nm. The Au-decorated TiO$_2$ surfaces enable plasmonic photo-electrochemical water splitting under visible light illumination (450-750 nm). The water splitting performance reaches a maximum for TiO$_2$ layers decorated with ~ 30 nm-sized Au particles. As expected, optical absorption measurements show a red shift of the plasmonic extinction band with increasing the Au nanoparticle size. However, the plasmonic photocurrent is found to peak at ~ 600 nm regardless of the size of the Au nanoparticles, i.e. the plasmonic photocurrent band position is size-independent. Such a remarkable observation can be ascribed to a hot electron injection cut-off effect.

Engineering atomically flat rutile TiO2(100) over a centimeter scale

Atomically well-defined TiO2 is the prerequisite for both interfacial-physics engineering and photocatalytic-mechanism understanding. Here, we report the preparation of atomically flat rutile TiO2(100) over a centimeter scale by high-temperature annealing in ultrahigh vacuum and detailedly traced its microscopic evolution dynamics towards atomic smoothness. The reflection high-energy electron-diffraction pattern and the scanning topographic image both show an atomically flat surface over a sub–square-centimeter area. These results provide a microscopic understanding of the methodologies for obtaining atomically well-defined surfaces.

Pyramidal Metal–dielectric hybrid-structure geometry with an asymmetric TiO2 layer for broadband light absorption and photocatalytic applications

In this study, a pyramidal metal–dielectric hybrid-structure geometry with high broadband light absorption was prepared and applied as a photoelectrode for solar water oxidation. TiO2 was obliquely deposited on the pyramidal Au film, leading to asymmetrically thick pyramids. With the decoration of Au nanoparticles, the light absorption in the entire UV–visible region significantly increased to > 90%, which was examined by three-dimensional finite-difference time-domain simulations and confirmed by confocal spectral mapping techniques. By the introduction of Ti as the insertion layer, the alignment of bands at the TiO2/Au film interface was tuned, thereby promoting the separation of photogenerated carriers via the efficient transport of electrons to the Au film. This transport led to a remarkable enhancement in the photocurrent density (~0.16 mA/cm2) by 3.4 times compared to that observed for a flat TiO2 layer.

Optically Generated Free-Carrier Collection from an All Single-Walled Carbon Nanotube Active Layer.

Semiconducting single-walled carbon nanotubes' (SWCNTs) broad absorption range and all-carbon composition make them attractive materials for light harvesting. We report photoinduced charge transfer from both multichiral and single-chirality SWCNT films into atomically flat SnO2 and TiO2 crystals. Higher-energy second excitonic SWCNT transitions produce more photocurrent, demonstrating carrier injection rates are competitive with fast hot-exciton relaxation processes. A logarithmic relationship exists between photoinduced electron-transfer driving force and photocarrier collection efficiency, becoming more efficient with smaller diameter SWCNTs. Photocurrents are generated from both conventional sensitization and in the opposite direction with the semiconductor under accumulation and acting as an ohmic contact with only the p-type nanotubes. Finally, we demonstrate that SWCNT surfactant choice and concentration play a large role in photon conversion efficiency and present methods of maximizing photocurrent yields.

Physicochemical properties of new cellulose-TiO2 composites for the removal of water pollutants: Developing specific interactions and performances by cellulose functionalization

Cellulose-TiO2 composites are developed using microcrystalline cellulose (MCC). The closed MCC structure was solubilized by liquid-phase treatments with nitric (HNO3) or phosphoric acids (H3PO4) to improve the TiO2-dispersion. These pretreatments simultaneously generate different types of oxygen and phosphorus functionalities on the cellulose chains that determine the interactions with Ti-alkoxide precursor during impregnation, leading to composite materials with different Ti-loadings and physicochemical characteristics. The morphological, textural, chemical and crystalline properties of both cellulose derivatives and cellulose-TiO2 composites were studied by complementary techniques. Microporous composites with round-shaped anatase TiO2 microcrystals were obtained by using HNO3 treatments, while H3PO4 favours the formation of mesoporous composites with flat and non-crystalline TiO2 particles. The photocatalytic efficiency of the obtained cellulose-TiO2 composites always improve the results obtained with the commercial TiO2-photocatalysts used as reference materials; this behaviour was analysed taking into account the physicochemical characteristics of the samples.

The new generation of the biologicalengineering materials for applications in medical and dental implant-scaffolds

Purpose: The publication aims to find the relationship between the proliferation of surface layers of living cells and the deposition of thin atomic layers deposition ALD coatings on the pores internal surfaces of porous skeletons of medical and dental implant-scaffolds manufactured with the selective laser deposition SLS additive technology using titanium and Ti6Al4V alloy. Design/methodology/approach: The extensive review of the literature presents the state-of-the-art in the field of regenerative medicine and tissue engineering. General ageing of societies, increasing the incidence of oncological diseases and some transport and sports accidents, and also the spread of tooth decay and tooth cavities in many regions of the world has taken place nowadays. Those reasons involve resection of many tissues and organs and the need to replace cavities, among others bones and teeth through implantation, more and more often hybridized with tissue engineering methods. Findings: The results of investigations of the structure and properties of skeleton microporous materials produced from titanium and Ti6Al4V alloy powders by the method of selective laser sintering have been presented. Particularly valuable are the original and previously unpublished results of structural research using high-resolution transmission electron microscope HRTEM. Particular attention has been paid to the issues of surface engineering, in particular, the application of flat TiO2 and Al2O3 coatings applied inside micropores using the atomic layers deposition ALD method and hydroxyapatite applied the dip-coating sol-gel method, including advanced HRTEM research. The most important part of the work concerns the research of nesting and proliferation of live cells of osteoblasts the hFOB 1.19 (Human ATCC - CRL - 11372) culture line on the surface of micropores with surfaces covered with the mentioned layers. Research limitations/implications: The investigations reported in the paper fully confirmed the idea of the hybrid technology of producing microporous implants and implant-scaffolds to achieve original Authors’ biological-engineering materials. The surface engineering issues, including both flat-layered nonorganic coatings and interactions of those coverings with flat layers of living cells, play a crucial role. Originality/value: Materials commonly used in implantology and the most commonly used materials processing technologies in those applications have been described. Against that background, the original Authors' concept of implant-scaffolds and the application of microporous skeleton materials for this purpose have been presented.

(Invited) Influence of Nanoscale Surface Structure of TiO2 Single Crystal Electrode on Water Photooxidation Reaction Process

The oxygen photoevolution reaction on TiO2 and related metal oxides has attracted strong attention from the point of view of solar water splitting. In our previous study, we have proposed a new reaction model in which generated holes are consumed not only by the O2 evolution reaction but also by three competitive paths possibly occur at different surface sites; photoluminescence, surface roughening (i.e. photocorrosion), and nonradiative recombination [1,2]. Actually, we found that the branch ratio for these paths on TiO2 (110) changed by the density and the direction of steps (i.e. atomic leveled surface structure) [3]. On the other hand, it is expected that the nano-sized structure of the TiO2 surface also affect the branch ratio of 4 competitive photooxidation reactions. Thus, in this study, we investigated oxygen photoevolution efficiency on a nanostructured TiO2 electrode that exposes different facets. A Pt wire and an Ag/AgCl/sat.KCl electrode were used as a counter and a reference electrode, respectively. An n-type TiO2 (110) doped with 0.05wt% Nb was used as a working electrode. The j−U curves were measured in 0.1 M HClO4 (pH 1.1) under UV irradiation. Nanostructured TiO2(110) surfaces were prepared by photoetching in 0.05 M H2SO4 [4]. We obtained the nanostructured surfaces on which shallow grooves are regularly arranged. We should note that formed surface of the nanostructure is composed by (100) facet faces [4]. Photocurrent generated along with the O2 evolution from H2O on the nanostructured TiO2 (110) was apparently different from that on the flat TiO2 (100). The onset potential of nanostructured TiO2 electrode was shifted toward negative voltage, indicating that the activation energy of oxygen evolution reaction was smaller than that on the flat surface. In addition, the consumption ratio of the photogenerated holes by above mentioned 4 competitive reactions was largely different from that on the atomically flat (100) surface. Considering the fact that the photoetched TiO2(110) surface is composed by (100) microfacets[4], this result can be attributed to the nanostructuring effect. We also carried out the same experiment using the some other TiO2 electrode those have different nanostructure on their surfaces, and found that the nanostructuring effect (1. Branch ratio of 4 competitive photooxidation reactions, 2. Activation energy of O2 evolution reaction) depends on the nano-sized surface morphology of the TiO2 electrode. [1] A. Imanishi, T. Okamura, N. Ohashi, R. Nakamura, Y. Nakato, J. Am. Chem. Soc., 129 (2007) 11569. [2] A. Imanishi, K. Fukui, J. Phys. Chem. Lett., 5 (2014) 2108. [3] M. Kadono, Master’s thesis, Osaka Univ. (2013). [4] A. Imanishi, H. Suzuki, K. Murakoshi, Y. Nakato, J. Phys. Chem. B, 110 (2006) 21050.

Underwater wettability of oleic acid on TiO2 photocatalyst surface

Rough and flat titanium dioxide (TiO2) surfaces were prepared, and the effect of water pH on the underwater wettability of oleic acid on the TiO2 surfaces was investigated. The underwater contact angle of oleic acid on the relatively flat TiO2 surface depended on the pH of water in the range pH 3–5. The repellency of oleic acid on the relatively flat TiO2 surface increased with decreasing pH. On the other hand, the hierarchical rough TiO2 surface repelled oleic acid in water, regardless of the pH of water.