Pt Single Atoms on TiO2 Can Catalyze Water Oxidation in Photoelectrochemical Experiments.

Photoelectrochemical water splitting on n-type semiconductors is highly dependent on catalysis of the rate-determining reaction of O2 evolution. Conventionally, in electrochemistry and photoelectrochemistry O2 evolution is catalyzed by metal oxide catalysts like IrO2 and RuO2, whereas noble metals such as Pt are considered unsuitable for this purpose. However, our study finds that Pt, in its single-atom form, exhibits exceptional cocatalytic properties for photoelectrochemical water oxidation on a TiO2 photoanode, in contrast to Pt in a nanoparticle form. The decoration of Pt single atoms onto TiO2 yields a remarkable current density of 5.89 mA cm-2 at 1.23 VRHE, surpassing bare TiO2 (or Pt nanoparticle decorated TiO2) by 2.52 times. Notably, this enhancement remains consistent over a wide pH range. By accompanying theoretical work, we assign this significant enhancement to an improved charge transfer and separation efficiency along with accelerated kinetics in the oxygen evolution reaction facilitated by the presence of Pt single atoms on the TiO2 surface.

Pt Single Atoms Loaded on Thin-Layer TiO2 Electrodes: Electrochemical and Photocatalytic Features.

Recently, the use of Pt in the form of single atoms (SA) has attracted considerable attention to promote the cathodic hydrogen production reaction from water in electrochemical or photocatalytic settings. First, produce suitable electrodes by Pt SA deposition on Direct current (DC)-sputter deposited titania (TiO2) layers on graphene-these electrodes allow to characterization of the electrochemical properties of Pt single atoms and their investigation in high-resolution HAADF-STEM. For Pt SAs loaded on TiO2, electrochemical H2 evolution shows only a very small overpotential. Concurrent with the onset of H2 evolution, agglomeration of the Pt SAs to clusters or nanoparticles (NPs) occurs. Potential cycling can be used to control SA agglomeration to variable-size NPs. The electrochemical activity of the electrode is directly related to the SA surface density (up to reaching the activity level of a plain Pt sheet). In contrast, for photocatalytic H2 generation already a minimum SA density is sufficient to reach control by photogenerated charge carriers. In electrochemical and photocatalytic approaches a typical TOF of ≈100-150 H2 molecules per second per site can be reached. Overall, the work illustrates a straightforward approach for reliable electrochemical and photoelectrochemical investigations of SAs and discusses the extraction of critical electrochemical factors of Pt SAs on titania electrodes.

Long-term stability of aerophilic metallic surfaces underwater.

Aerophilic surfaces immersed underwater trap films of air known as plastrons. Plastrons have typically been considered impractical for underwater engineering applications due to their metastable performance. Here, we describe aerophilic titanium alloy (Ti) surfaces with extended plastron lifetimes that are conserved for months underwater. Long-term stability is achieved by the formation of highly rough hierarchically structured surfaces via electrochemical anodization combined with a low-surface-energy coating produced by a fluorinated surfactant. Aerophilic Ti surfaces drastically reduce blood adhesion and, when submerged in water, prevent adhesion of bacteria and marine organisms such as barnacles and mussels. Overall, we demonstrate a general strategy to achieve the long-term stability of plastrons on aerophilic surfaces for previously unattainable underwater applications.

Photocatalytic H2 Generation: Controlled and Optimized Dispersion of Single Atom Co-Catalysts Based on Pt-TCPP Planar Adsorption on TiO2.

When using single atoms (SAs) as a co-catalyst in photocatalytic H2 generation, achieving a well-dispersed, evenly distributed and adjustable SA surface density on a semiconductor surface is a challenging task. In the present work we use the planar adsorption of tetrakis-(4-carboxyphenyl)-porphyrin (TCPP) and its platinum coordinated analogue, Pt-TCPP, onto anatase TiO2 surfaces to establish a spatially controlled decoration of SAs. We show that the surface Pt SA density can be very well controlled by co-adsorption of Pt-TCPP and TCPP in the planar monolayer regime, and by adjusting the Pt-TCPP to TCPP ratio a desired well dispersed surface density of SAs up to 2.6×105  atoms µm-2 can be established (which is the most effective Pt SA loading for photocatalysis). This distribution and the SA state are maintained after a thermal treatment in air, and an optimized SA density as well as a most active form of Pt for photocatalytic H2 evolution can be established and maintained.

2D Metal-Organic Framework Nanosheets based on Pd-TCPP as Photocatalysts for Highly Improved Hydrogen Evolution.

In this report, a 2D MOF nanosheet derived Pd single-atom catalyst, denoted as Pd-MOF, was fabricated and examined for visible light photocatalytic hydrogen evolution reaction (HER). This Pd-MOF can provide a remarkable photocatalytic activity (a H2 production rate of 21.3 mmol/gh in the visible range), which outperforms recently reported Pt-MOFs (with a H2 production rate of 6.6 mmol/gh) with a similar noble metal loading. Notably, this high efficiency of Pd-MOF is not due to different chemical environment of the metal center, nor by changes in the spectral light absorption. The higher performance of the Pd-MOF in comparison to the analogue Pt-MOF is attributed to the longer lifetime of the photogenerated electron-hole pairs and higher charge transfer efficiency.

Metastable Ni(I)-TiO2-x Photocatalysts: Self-Amplifying H2 Evolution from Plain Water without Noble Metal Co-Catalyst and Sacrificial Agent.

Decoration of semiconductor photocatalysts with cocatalysts is generally done by a step-by-step assembly process. Here, we describe the self-assembling and self-activating nature of a photocatalytic system that forms under illumination of reduced anatase TiO2 nanoparticles in an aqueous Ni2+ solution. UV illumination creates in situ a Ni+/TiO2/Ti3+ photocatalyst that self-activates and, over time, produces H2 at a higher rate. In situ X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy show that key to self-assembly and self-activation is the light-induced formation of defects in the semiconductor, which enables the formation of monovalent nickel (Ni+) surface states. Metallic nickel states, i.e., Ni0, do not form under the dark (resting state) or under illumination (active state). Once the catalyst is assembled, the Ni+ surface states act as electron relay for electron transfer to form H2 from water, in the absence of sacrificial species or noble metal cocatalysts.

Spontaneous Dewetting of Au-Thin Layers on Oxide- and Fluorine-Terminated Single Crystalline Anatase and Efficient Use in Photocatalytic H2 Production.

In the present work, the spontaneous dewetting of thin Au layers on single crystalline anatase nanosheets into narrow-disperse Au nanoparticles is investigated. Patterns of the Au particles can be formed on the main facets of anatase that provide a high co-catalytic activity for photocatalytic generation of H2 . Dewetting is distinctly influenced by the respective facets (001) and (101), the deposit thickness, and secondary thermal dewetting, but most strongly by the surface termination of the nanosheets. Fluoride termination not only leads to an enhanced Au-phobic behavior but strongly affects the co-catalytic activity for photocatalytic generation of H2 . While fluoride termination with or without Au decoration is detrimental for hole transfer, the interplay of the Au co-catalyst and surface fluoride yields highly beneficial effect for electron transfer. This results in a three-times higher photocatalytic H2 production for the F-terminated surface. The findings suggest that dewetting of Au on surface fluorinated TiO2 is an effective way to modulate surface dewetting and achieve a strongly enhanced photocatalytic activity.

Optimized Pt Single Atom Harvesting on TiO2 Nanotubes-Towards a Most Efficient Photocatalyst.

In the present work the authors show that anodic TiO2 nanotubes (NT) show excellent harvesting properties for Pt single atoms (Pt SAs) from highly dilute Pt solutions. The tube walls of anodic nanotubes, after adequate annealing to anatase, provide ample of suitable trapping sites-that is, surface Ti3+ -Ov (Ov : oxygen vacancy) defects that are highly effective to extract and accumulate Pt in the form of SAs. A saturated (maximized) SA density can be achieved by an overnight immersion of a TiO2 NT layer to a H2 PtCl6 solution with a concentration that is as low as 0.01 mm Pt. Such TiO2 NTs with surface trapped Pt SAs provide a maximized high activity for photocatalytic H2 generation (reaching a turnover frequency (TOF) of 1.24 × 106 h-1 at a density of 1.4 × 105 Pt atoms µm-2 )-a higher loading with Pt nanoparticles does not further increase the photocatalytic activity. Overall, these findings show that anodic TiO2 nanotubes provide a remarkable substrate for Pt extraction and recovery from very dilute solutions that directly results in a highly efficient photocatalyst, fabricated by a simple immersion technique.

Macrophage-like Cells Are Responsive to Titania Nanotube Intertube Spacing-An In Vitro Study.

With the introduction of a new interdisciplinary field, osteoimmunology, today, it is well acknowledged that biomaterial-induced inflammation is modulated by immune cells, primarily macrophages, and can be controlled by nanotopographical cues. Recent studies have investigated the effect of surface properties in modulating the immune reaction, and literature data indicate that various surface cues can dictate both the immune response and bone tissue repair. In this context, the purpose of the present study was to investigate the effects of titanium dioxide nanotube (TNT) interspacing on the response of the macrophage-like cell line RAW 264.7. The cells were maintained in contact with the surfaces of flat titanium (Ti) and anodic TNTs with an intertube spacing of 20 nm (TNT20) and 80 nm (TNT80), under standard or pro-inflammatory conditions. The results revealed that nanotube interspacing can influence macrophage response in terms of cell survival and proliferation, cellular morphology and polarization, cytokine/chemokine expression, and foreign body reaction. While the nanostructured topography did not tune the macrophages' differentiation into osteoclasts, this behavior was significantly reduced as compared to flat Ti surface. Overall, this study provides a new insight into how nanotubes' morphological features, particularly intertube spacing, could affect macrophage behavior.

Solar Thermoplasmonic Nanofurnace for High-Temperature Heterogeneous Catalysis.

Most of existing solar thermal technologies require highly concentrated solar power to operate in the temperature range 300-600 °C. Here, thin films of refractory plasmonic TiN cylindrical nanocavities manufactured via flexible and scalable process are presented. The fabricated TiN films show polarization-insensitive 95% broadband absorption in the visible and near-infrared spectral ranges and act as plasmonic "nanofurnaces" capable of reaching temperatures above 600 °C under moderately concentrated solar irradiation (∼20 Suns). The demonstrated structures can be used to control nanometer-scale chemistry with zeptoliter (10-21 L) volumetric precision, catalyzing C-C bond formation and melting inorganic deposits. Also shown is the possibility to perform solar thermal CO oxidation at rates of 16 mol h-1 m-2 and with a solar-to-heat thermoplasmonic efficiency of 63%. Access to scalable, cost-effective refractory plasmonic nanofurnaces opens the way to the development of modular solar thermal devices for sustainable catalytic processes.

Activation of α-Fe2 O3 for Photoelectrochemical Water Splitting Strongly Enhanced by Low Temperature Annealing in Low Oxygen Containing Ambient.

Photoelectrochemical (PEC) water splitting is a promising method for the conversion of solar energy into chemical energy stored in the form of hydrogen. Nanostructured hematite (α-Fe2 O3 ) is one of the most attractive materials for a highly efficient charge carrier generation and collection due to its large specific surface area and the short minority carrier diffusion length. In the present work, the PEC water splitting performance of nanostructured α-Fe2 O3 is investigated which was prepared by anodization followed by annealing in a low oxygen ambient (0.03 % O2 in Ar). It was found that low oxygen annealing can activate a significant PEC response of α-Fe2 O3 even at a low temperature of 400 °C and provide an excellent PEC performance compared with classic air annealing. The photocurrent of the α-Fe2 O3 annealed in the low oxygen at 1.5 V vs. RHE results as 0.5 mA cm-2 , being 20 times higher than that of annealing in air. The obtained results show that the α-Fe2 O3 annealed in low oxygen contains beneficial defects and promotes the transport of holes; it can be attributed to the improvement of conductivity due to the introduction of suitable oxygen vacancies in the α-Fe2 O3 . Additionally, we demonstrate the photocurrent of α-Fe2 O3 annealed in low oxygen ambient can be further enhanced by Zn-Co LDH, which is a co-catalyst of oxygen evolution reaction. This indicates low oxygen annealing generates a promising method to obtain an excellent PEC water splitting performance from α-Fe2 O3 photoanodes.

Photocatalytic reduction and scavenging of Hg(ii) over templated-dewetted Au on TiO2 nanotubes.

Gold-decorated TiO2 nanotubes were used for the photocatalytic abatement of Hg(ii) in aqueous solutions. The presence of dewetted Au nanoparticles induces a strong enhancement of photocatalytic reduction and scavenging performances, with respect to naked TiO2. In the presence of chlorides, a massive formation of Hg2Cl2 nanowires, produced from Au nanoparticles, was observed using highly Au loaded photocatalysts to treat a 10 ppm Hg(ii) solution. EDS and XPS confirmed the nature of the photo-produced nanowires. In the absence of chlorides and/or at lower Hg(ii) starting concentrations, the scavenging of mercury proceeds through the formation of Hg-Au amalgams. Solar light driven Hg(ii) abatements up to 90% were observed after 24 h. ICP-MS analysis revealed that the removed Hg(ii) is accumulated on the photocatalyst surface. Regeneration of Hg-loaded exhaust photocatalysts was easily performed by anodic stripping of Hg(0) and Hg(i) to Hg(ii). After four catalytic-regeneration cycles, only a 10% decrease of activity was observed.

Sb-Doped SnO2 Nanorods Underlayer Effect to the α-Fe2 O3 Nanorods Sheathed with TiO2 for Enhanced Photoelectrochemical Water Splitting.

Here, a Sb-doped SnO2 (ATO) nanorod underneath an α-Fe2 O3 nanorod sheathed with TiO2 for photoelectrochemical (PEC) water splitting is reported. The experimental results, corroborated with theoretical analysis, demonstrate that the ATO nanorod underlayer effect on the α-Fe2 O3 nanorod sheathed with TiO2 enhances the PEC water splitting performance. The growth of the well-defined ATO nanorods is reported as a conductive underlayer to improve α-Fe2 O3 PEC water oxidation performance. The α-Fe2 O3 nanorods grown on the ATO nanorods exhibit improved performance for PEC water oxidation compared to α-Fe2 O3 grown on flat fluorine-doped tin oxide glass. Furthermore, a simple and facile TiCl4 chemical treatment further introduces TiO2 passivation layer formation on the α-Fe2 O3 to reduce surface recombination. As a result, these unique nanostructures show dramatically improved photocurrent density (139% higher than that of the pure hematite nanorods).

TiO2 nanotubes with different spacing, Fe2O3 decoration and their evaluation for Li-ion battery application.

In the present work, we report on the use of organized TiO2 nanotube (NT) layers with a regular intertube spacing for the growth of highly defined α-Fe2O3 nano-needles in the interspace. These α-Fe2O3 decorated TiO2 NTs are then explored for Li-ion battery applications and compared to classic close-packed (CP) NTs that are decorated with various amounts of nanoscale α-Fe2O3. We show that NTs with tube-to-tube spacing allow uniform decoration of individual NTs with regular arrangements of hematite nano-needles. The tube spacing also facilitates the electrolyte penetration as well as yielding better ion diffusion. While bare CP NTs show a higher capacitance of 71 µAh cm-2 compared to bare spaced NTs with a capacitance of 54 µAh cm-2, the hierarchical decoration with secondary metal oxide, α-Fe2O3, remarkably enhances the Li-ion battery performance. Namely, spaced NTs with α-Fe2O3 decoration have an areal capacitance of 477 µAh cm-2, i.e. they have nearly ∼8 times higher capacitance. However, the areal capacitance of CP NTs with α-Fe2O3 decoration saturates at 208 µAh cm-2, i.e. is limited to ∼3 times increase.

Photoanodes based on TiO2 and α-Fe2O3 for solar water splitting - superior role of 1D nanoarchitectures and of combined heterostructures.

Solar driven photoelectrochemical water splitting (PEC-WS) using semiconductor photoelectrodes represents a promising approach for a sustainable and environmentally friendly production of renewable energy vectors and fuel sources, such as dihydrogen (H2). In this context, titanium dioxide (TiO2) and iron oxide (hematite, α-Fe2O3) are among the most investigated candidates as photoanode materials, mainly owing to their resistance to photocorrosion, non-toxicity, natural abundance, and low production cost. Major drawbacks are, however, an inherently low electrical conductivity and a limited hole diffusion length that significantly affect the performance of TiO2 and α-Fe2O3 in PEC devices. To this regard, one-dimensional (1D) nanostructuring is typically applied as it provides several superior features such as a significant enlargement of the material surface area, extended contact between the semiconductor and the electrolyte and, most remarkably, preferential electrical transport that overall suppress charge carrier recombination and improve TiO2 and α-Fe2O3 photoelectrocatalytic properties. The present review describes various synthetic methods and modifying concepts of 1D-photoanodes (nanotubes, nanorods, nanofibers, nanowires) based on titania, hematite, and on α-Fe2O3/TiO2 heterostructures, for PEC applications. Various routes towards modification and enhancement of PEC activity of 1D photoanodes are discussed including doping, decoration with co-catalysts and heterojunction engineering. Finally, the challenges related to the optimization of charge transfer kinetics in both oxides are highlighted.

Highly Conducting Spaced TiO2 Nanotubes Enable Defined Conformal Coating with Nanocrystalline Nb2 O5 and High Performance Supercapacitor Applications.

Establishing self-organized spacing between TiO2 nanotubes allows for highly conformal Nb2 O5 deposition that can be adjusted to optimized supercapacitive behavior.

Large-Diameter TiO2 Nanotubes Enable Wall Engineering with Conformal Hierarchical Decoration and Blocking Layers for Enhanced Efficiency in Dye-Sensitized Solar Cells (DSSC).

Herein, we grew anodic TiO2 nanotube layer with a tube diameter >500 nm and an open tube mouth. We use this morphology in dye-sensitized solar cells (DSSCs) and show that these tubes allow the construction of hybrid hierarchical photoanode structures of nanotubes with a defined and wall-conformal TiO2 nanoparticle decoration. At the same time, the large diameter allows the successful establishment of an additional (insulating) blocking layer of SiO2 or Al2 O3 on the tube wall. We showed that this combination of hierarchical structure and blocking layer significantly enhances the solar-cell efficiency by suppressing recombination reactions. In such a DSSC structure, the solar-cell efficiency under back side illumination with AM1.5 illumination was enhanced from 3 % for the neat tube to 5 % for the hierarchical structure to 7 % when additionally a blocking layer was attached.

Photoelectrochemical H2 Generation from Suboxide TiO2 Nanotubes: Visible-Light Absorption versus Conductivity.

In the present work we report on the key factors dictating the photoelectrochemical (PEC) performance of suboxide titania (TiOx ) nanotubes. TiOx nanotubes were produced by a systematic variation of reduction heat treatments of TiO2 in Ar/H2 . The properties of the TiOx tubes were investigated by electron paramagnetic resonance (EPR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), solid-state conductivity, reflectivity measurements, photocurrent spectroscopy, and photoelectrochemical hydrogen evolution. In line with earlier literature, these suboxide tubes show a drastically improved photoelectrochemical water-splitting performance compared to non-reduced anatase TiO2 tubes. In this work we show that the key improvement in water-splitting performance is due to the strongly improved conductivity of TiOx semimetalic tubes, reaching 13.5 KΩ per tube compared to 70 MΩ (for non-reduced anatase), and is not due to the enhanced visible-light absorbance.

Tuning Anatase Surface Reactivity toward Carboxylic Acid Anchor Groups.

The effect of different post-treatments on TiO2 anatase surface reactivity was investigated in order to obtain the best techniques for enhancing anatase performance in diverse applications, e.g., in photocatalysis and especially as photoelectrodes for dye-sensitized solar cells (DSSCs). Different post-treatments of compact anodic anatase TiO2 were compared, including O2 plasma, UV irradiation, immersion in H2O2, vapor thermal treatment, and post-anodization, evaluating the increase of the amount of OH reactive groups on the surface and removal of surface contamination. In XPS spectra, the increase of OH groups is evident by the O 1s peak at higher binding energy. ToF-SIMS principal component analysis demonstrated that treatments performed in aqueous media led to a cleaner surface, with substantial removal of electrolyte residues. Stearic acid and the organic dye N719 were adsorbed to the differently post-treated anatase, and adsorption was evaluated by contact angle and dye desorption measurements. A higher loading with molecules containing carboxylic acid functionalities was confirmed by both techniques on the treated samples. The post-treatments that presented the highest amounts of dye were used to prepare photoelectrodes, and these were tested in DSSCs where the efficiency values doubled in comparison with the non-post-treated electrode.

Noble-Metal-Free Photocatalytic H2 Generation: Active and Inactive 'Black' TiO2 Nanotubes and Synergistic Effects.

Over the past five years a number of different synthesis approaches has been reported to obtain so-called 'black' titania. One of the outstanding features of the material is that certain synthesis processes lead to the formation of an intrinsic co-catalytic center and thus enable noble-metal free photocatalytic H2 -generation. In the present work, using TiO2 nanotube layers, we compare three common 'blackening' approaches, namely i) the original high-pressure hydrogenation (HPT-H2 ), ii) a classic high temperature reduction in Ar, and iii) an electrochemical (cathodic) reduction. We demonstrate that except for high pressure hydrogenation also cathodic reduction leads to an activation of TiO2 - that is, it exhibits noble-metal-free photocatalytic H2 generation. Moreover, we show that a combination of cathodic reduction/high pressure hydrogenation leads to a synergistic effect, that is, a significant enhancement of the combined co-catalytic activity.