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Herein, the growth of transparent TiO2 nanotube (NT) layers is investigated by complete self‐organized anodization of a metallic Ti layer on fluorine‐doped tin oxide glass, deposited by electron beam evaporation. An initiation‐free open‐top tube morphology can be obtained for such transparent TiO2 NTs using an optimized second anodization approach combined with a post‐ultrasonication process. The photoelectrochemical properties of open‐top tubes exhibit notable enhancement, primarily attributed to their rapid electron‐transfer kinetics, with a ≈33% increase in the incident‐photon‐to‐electron conversion efficiency value (at 350 nm wavelength) in comparison to classical (initiation‐covered) NTs with a comparable morphology. [ABSTRACT FROM AUTHOR]

Titanium dioxide (TiO2) is the material of choice for photocatalytic and electrochemical applications owing to its outstanding physicochemical properties. However, its wide bandgap and relatively low conductivity limit its practical application. Modifying TiO2 with carbon species is a promising route to overcome these intrinsic complexities. In this work, we propose a facile method to modify TiO2 nanotubes (NTs) based on the remnant organic electrolyte retained inside the nanotubes after the anodization process, that is, without removing it by immersion in ethanol. Carbon‐modified TiO2 NTs (C‐TiO2 NTs) showed enhanced H2 evolution in photocatalysis under UV illumination in aqueous solutions. When the C‐TiO2 NTs were subjected to UV light illumination, the carbon underwent modification, resulting in higher measured photocurrents in the tube layers. After UV illumination, the IPCE of the C‐TiO2 NTs was 4.4‐fold higher than that of the carbon‐free TiO2 NTs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Single atoms (SAs) represent not only a new frontier in classic heterogeneous catalysis but are also increasingly investigated as co‐catalysts in photocatalytic reactions. In contrast to classic catalysis, many photocatalytic platforms require only a very low SA loading density to reach a saturated photocatalytic activity. As a result, an optimized light harvesting/carrier transport combination in the supporting semiconductor becomes the key aspect for the overall photocatalytic efficiency. In this work, it is demonstrated that Grätzel type mesoporous TiO2 layers represent an ideal host for Pt single‐atoms (SAs) that allow for a highly effective photocatalytic H2 generation. Using a layer with an optimized geometry, structure, as well as Pt SA loading, a photocatalytic H2 production is achieved of up to ≈2900 µL h−1 (under irradiation at λ = 365 nm and I = 65 mW cm−2) – a performance that is far superior to previous Pt SA/TiO2 structures based on TiO2 nanotubes, nanosheets, or metal organic frameworks. Moreover, such SA/substrate combination provides a highly stable H2 production over time. The present work thus introduces the use of this classic TiO2 nanostructure as the most effective host for Pt SAs and its use for highly efficient photocatalytic H2 production from aqueous solutions. [ABSTRACT FROM AUTHOR]

TiO2 used as a light‐absorbing semiconductor represents the classic benchmark for photocatalytic solar energy conversion and many other photocatalytic reactions. Various strategies are developed to improve the photoresponse of TiO2‐based materials, such as bandgap engineering or surface sensitization in combination with nanostructuring and geometry optimization. The present feature article is focused on direct and indirect approaches involving bulk or surface fluorine used to tune the surface chemistry, electronic structure, and the morphology of TiO2 photocatalysts. A comprehensive overview is provided on fluorine effects on TiO2, involving morphology modifications, and how surface or bulk fluorine affect the photocatalytic performance of TiO2. After outlining some basic interaction principles of F and TiO2, characterization techniques for different fluorine species are discussed. It is reviewed how fluorine during crystal growth mediates the morphology of TiO2; then how surface fluorination and doping fluorine effects can be beneficially utilized in photocatalysis are discussed. Finally, synergistic effects between fluorine and cocatalyst are discussed, for example, the use of lattice fluorine species to stabilize Pt single‐atom cocatalysts. Finally, the challenges and outlook on further advancing the development of fluorine effects are highlighted. [ABSTRACT FROM AUTHOR]

Studies on single‐atom catalysts (SACs) with individually isolated metal atoms anchored on specific supports have gained great interest in photocatalysis due to their enhanced catalytic activity and optimal atom utilization. By providing an optimized number of active sites and enhancing their intrinsic activity, SACs afford a distinctive platform for photocatalysis at the atomic level. In this study, we investigate the photocatalytic H2 generation of Pt single atoms (SAs) anchored on CdS‐sensitized single crystalline anatase TiO2 nanoflakes (ATNF) in the visible spectral range. Vertically‐aligned ATNF were synthesized on fluorine‐doped tin oxide substrates by a hydrothermal process, which were further sensitized by CdS nanoislands (NIs) using the successive ionic layer adsorption and reaction (SILAR) technique. Finally, a reactive‐deposition approach was used to successfully anchor Pt SAs on CdS‐sensitized ATNF. Under optimized conditions, the highest photocatalytic H2 evolution on Pt‐anchored single atom CdS sensitized ATNF was 17.8 μL h−1 under visible light illumination, which is 15.8, 7.5, and 6.7‐fold higher than bare CdS/FTO, PtSA/CdS/FTO, and ATNF, respectively. Overall, the density of Pt SAs plays a vital role via strong trapping of the photogenerated electrons and significantly improves the efficiency of electron‐hole separation, making PtSA/ATNF efficient solar‐driven photocatalysts. [ABSTRACT FROM AUTHOR]

In this work, NiCu co‐catalysts on TiO2 are studied for photocatalytic hydrogen evolution. NiCu co‐catalyst films are deposited at room temperature by argon plasma sputtering on high aspect‐ratio anodic TiO2 nanotubes. To tune the Ni : Cu atomic ratio, alloys of various compositions were used as sputtering targets. Such co‐catalyst films are found to be amorphous with small nanocrystalline domains. A series of parameters is investigated, i. e., i) Ni : Cu relative ratio in the sputtered films, ii) NiCu film thickness, and iii) thickness of the TiO2 nanotube layers. The highest photocatalytic activity is obtained with 8 μm long TiO2 nanotubes, sputter‐coated with a 10 nm‐thick NiCu films with a 1 : 1 Ni : Cu atomic ratio. This photocatalyst reaches a stable hydrogen evolution rate of 186 μL h−1 cm−2, 4.6 and 3 times higher than that of Ni‐ and Cu‐TiO2, respectively, demonstrating a synergistic co‐catalytic effect of Ni and Cu in the alloy co‐catalyst film. [ABSTRACT FROM AUTHOR]

In the present work, it is shown that anodic TiO2 nanotubes (NTs) can be decorated with Pt, Pd, Rh, and Au single atoms (SAs) by a simple "dark deposition" approach. Such TiO2 NTs with surface trapped noble metal SAs provide a high activity for photocatalytic H2 generation from pure water, i.e., in absence of a sacrificial agent. However, noble metals also act as active centers in the undesired hydrogen back‐oxidation (H2 + O2 → H2O), leading to a decrease in the overall photocatalytic H2 production efficiency. Here it is reported that the use of noble metal co‐catalysts, in the form of single atoms, can inhibit this recombination. From the different noble‐metal SAs investigated, Pd SAs yield the highest H2 production rate of 0.381 µmol h−1 cm−1 at a density of 0.41 × 105 Pd atoms µm−2. Overall, the results provide a path to a highly efficient photocatalytic performance for water splitting by the suppression of the H2/O2 recombination, which can be effectively achieved using Pd in the form of SAs as photocatalytic co‐catalysts. [ABSTRACT FROM AUTHOR]

Single crystal anatase TiO2 nanosheets (TNS) were grown on a fluorine‐doped tin oxide (FTO) substrate and then modified via multiple TiCl4 treatments to produce a hierarchical TiO2 structure. This high surface area hierarchical structure was then decorated with CdS (as a visible light absorber) using SILAR treatments. The resulting photoanodes provide a greatly enhanced photoelectrochemical H2 generation capability. For the optimized TiO2 and CdS decorated TNS electrode, a photocurrent magnitude of 3.5mA/cm2 at 0.5V Ag/AgCl (under AM 1.5G, 100mW/cm2) was obtained, and an applied bias to photoconversion efficiency (ABPE) of 2.5% could be reached. The increase in the specific surface was provided by the hierarchical TiO2 structure responsible for the improved performance in comparison to the plain, non‐hierarchical electrode with an ABPE=0.8%. Even after particle decoration, the rapid electron flow through the single‐crystalline scaffold was maintained. Consequently, the photoanode presented here best integrates in a synergistic manner, a larger surface area, a fast electron transport, and effective visible light harvesting properties. [ABSTRACT FROM AUTHOR]

For more than 20 years, Pt/TiO2 represents the benchmark photocatalyst/co‐catalyst platform for photocatalytic hydrogen (H2) generation. Here, single atom (SA) Pt is decorated on different polymorphs of TiO2 (anatase, rutile, and the mixed phase of P25) using a simple immersion anchoring approach. On P25 and anatase, Pt SAs act as highly effective co‐catalyst for pure water splitting with a photocatalytic H2 evolution activity (4600 µmol h−1 g−1)—on both polymorphs, SA deposition yields a significantly more active photocatalyst than those decorated with classic Pt nanoparticles or conventional SA deposition approaches. On rutile, Pt SAs provide hardly any co‐catalytic effect. Most remarkable, for P25, the loading of Pt SAs from precursor solution with a very low concentration (<1 ppm Pt) leads already to a maximized co‐catalytic effect. This optimized efficiency is obtained at 5.3 × 105 atoms µm−2 (at macroscopic loading of 0.06 at%)—for a higher concentration of Pt (a higher density of SAs), the co‐catalytic efficiency is significantly reduced due to H2/O2 recombination. The interactions of the SA Pt with the different polymorphs that lead to this high co‐catalytic activity of SA Pt at such low concentrations are further discussed. [ABSTRACT FROM AUTHOR]

In the present work, we report a preparation strategy for hematite phase‐pure photoanodes consisting of Sn‐doped hematite nanoflakes/hematite thin film bilayer nanostructure (Sn‐HB). This approach is based on a two‐step annealing process of pure iron films deposited on fluorine doped tin oxide (FTO) substrates by advanced magnetron sputtering. While the high density hematite ultrathin nanoflakes (HNs) with detrimental iron oxide layers (Fe3O4 and/or FeO) are generated during the first annealing step at 400 °C for two hours, the second thermal treatment at 800 °C for 15 minutes oxidises all the undesired iron oxide phases to a photoactive hematite layer as well as is providing efficient Sn doping of a drop‐casted SnCl4 in order to increase the conductivity. The optimized Sn‐HB shows an around 11 times higher photocurrent density (0.71 mA cm−2 at 1.23 VRHE) compared with a reference hematite photoanode produced from iron foil under the same conditions. [ABSTRACT FROM AUTHOR]

The use of single atoms (SAs) has become a highly investigated topic in heterogeneous catalysis, electrocatalysis, and most recently also in photocatalysis. In the field of photocatalysis, Pt SAs on TiO2 have been reported to be a highly efficient co‐catalyst in solar H2 production. Herein, the deposition of Pt SAs and nanoparticles on titania nanosheets is investigated. In particular, the Pt species responsible for the high co‐catalytic activity using a cyanide leaching process is elucidated. It is shown that neither Pt0 species nor the majority of Pt SAs do significantly contribute to the co‐catalytic activity of platinum on TiO2. In fact, >90% of the Pt of a standard deposition are non‐active and can be removed by cyanide leaching without activity loss—as a consequence, the remaining Pt SAs amount to a remarkable turnover frequency of 4.87 × 105 h−1 for H2 evolution. [ABSTRACT FROM AUTHOR]

Herein we report a one‐step, low cost, photocatalytic method for the synthesis of graphene oxide (GO) from commercial graphite using grey TiO2 as a photocatalyst. GO formation is achieved by UV‐illumination of a slurry of well‐dispersed grey TiO2 and commercial graphite. Light‐induced valence‐band holes from grey TiO2 then lead to an oxidation and exfoliation of graphite, resulting in the formation of visible‐light‐active GO. An optimal level of graphite oxidation can be established by controlling the UV illumination time. Moreover, the resulting GO‐decorated TiO2 is directly active as a visible‐light‐active photocatalyst that can, for example, be used for pollution degradation. [ABSTRACT FROM AUTHOR]

We examine comparatively the performance of sputtered TiO2 rutile and anatase thin films as an electron transport layer (ETL) in MAPbI3‐based perovskite solar cells. Both anatase and rutile TiO2 ETLs are deposited (on fluorine‐doped tin oxide [FTO] substrates) by magnetron sputtering in the form of nanocrystalline thin films. We systematically investigate the role of crystallographic phase composition of TiO2 ETLs on the photovoltaic performance of perovskite solar cells. The champion power conversion efficiencies (PCEs) of 18.4% and 17.7% under reverse scan mode are obtained for perovskite solar cells based on TiO2 anatase and TiO2 rutile ETL, respectively. The results show that the magnetron sputtering deposited ETLs differ from each other only in their phase composition while the overall performance of the devices is not greatly affected by the crystallographic phase of the TiO2 ETLs. Our results point to an important fact that for a proper and reliable comparison between the performance of TiO2 anatase and rutile ETLs, it is crucial to investigate films of similar morphology and structure that are synthesize under similar conditions. [ABSTRACT FROM AUTHOR]

Titanium dioxide (TiO2) and, in particular, its anatase polymorph, is widely studied for photocatalytic H2 production. In the present work, we examine the importance of reactive facets of anatase crystallites on the photocatalytic H2 evolution from aqueous methanol solutions. For this, we synthesized anatase TiO2 nanocrystals with a large amount of either {001} facets, that is, nanosheets, or {101} facets, that is, octahedral nanocubes, and examined their photocatalytic H2 evolution and then repeated this procedure with samples where Pt co‐catalyst is present on all facets. Octahedral nanocubes with abundant {101} facets produce >4 times more H2 than nanosheets enriched in {001} facets if the reaction is carried out under co‐catalyst‐free conditions. For samples that carry Pt co‐catalyst on both {001} and {101} facets, faceting loses entirely its significance. This demonstrates that the beneficial role of faceting, namely the introduction of {101} facets that act as electron transfer mediator is relevant only for co‐catalyst‐free TiO2 surfaces. [ABSTRACT FROM AUTHOR]

The photocatalytic activity for H2 production from water, without presence of hole scavengers, of thermally reduced TiO2 nanoparticles (H‐500, H‐700) and neat anatase were followed by in‐situ continuous‐wave light‐induced electron paramagnetic resonance technique (CW‐LEPR), in order to correlate the H2 evolution rates with the electronic fingerprints of the photoexcited systems. Under UV irradiation, photoexcited electrons moved from the deep lattice towards the superficially exposed Ti sites. These photogenerated redox sites mediated (e−+h+) recombination and were the crucial electronic factor affecting catalysis. In the best‐performant system (H‐500), a synergic combination of mobile electrons was observed, which dynamically created diverse types of Ti3+ sites, including interstitial Ti3+, and singly ionized electrons trapped in oxygen vacancies (VO.). The interplay of these species fed successfully surface exposed Ti4+ sites, which became a catalytically active, fast reacting Ti4+⇄Ti3+ state that was key for the H2 evolution process. [ABSTRACT FROM AUTHOR]

Single atom (SA) catalysis, over the last 10 years, has become a forefront in heterogeneous catalysis, electrocatalysis, and most recently also in photocatalysis. Most crucial when engineering a SA catalyst/support system is the creation of defined anchoring points on the support surface to stabilize reactive SA sites. Here, a so far unexplored but evidently very effective approach to trap and stabilize SAs on a broadly used photocatalyst platform is introduced. In self‐organized anodic TiO2 nanotubes, a high degree of stress is incorporated in the amorphous oxide during nanotube growth. During crystallization (by thermal annealing), this leads to a high density of Ti3+‐Ov surface defects that are hardly present in other common titania nanostructures (as nanoparticles). These defects are highly effective for SA iridium trapping. Thus a SA‐Ir photocatalyst with a higher photocatalytic activity than for any classic co‐catalyst arrangement on the semiconductive substrate is obtained. Hence, a tool for SA trapping on titania‐based back‐contacted platforms is provided for wide application in electrochemistry and photoelectrochemistry. Moreover, it is shown that stably trapped SAs provide virtually all photocatalytic reactivity, with turnover frequencies in the order of 4 × 106 h−1 in spite of representing only a small fraction of the initially loaded SAs. [ABSTRACT FROM AUTHOR]

Wetting is a surface phenomenon that commonly occurs in nature and has an enormous influence on human life. Slippery liquid‐infused porous surfaces have recently been developed to support the growing demand for anti‐fouling coatings. While short‐chain fluorinated compounds, commonly used to reduce the surface energy of substrates, are banned due to environmental toxicity, silane‐based compounds are expensive and barely scalable. In this sense, silicone‐based chemistry may match the gap as a real alternative. However, the grafting approaches demonstrated so far suffered from either slow binding kinetics or are applied under harsh conditions. Here, it is demonstrated that polydimethylsiloxanes graft to virtually any substrate when illuminated by UV light serving simultaneously as a reducing surface energy agent and infusing lubricant. This procedure is applied on metals, metal oxides, and ceramics of various surface morphologies. The proposed approach is simple, fast, scalable, environmentally friendly, and of low‐cost, yet forms stable lubricant‐infused slippery surfaces by a one‐pot process. Due to the biocompatibility of silicone‐based compounds, the process is examined on plain medically applicable substrates such as scalpel blades and glass lenses that display enhanced corrosion resistance, reduced friction through incision, and repel blood staining and bacterial adhesion without deteriorating their mechanical and optical characteristics. [ABSTRACT FROM AUTHOR]

Herein, highly ordered TiO2 nanotube (NT) arrays on a Ti substrate is synthesized in a fluoride-containing electrolyte, using the electrochemical anodization method, which yields amorphous oxide tubes. The effects of different thermal annealing profiles for the crystallization of the amorphous TiO2 NTs are studied. It is found that the temperature ramping rate has a significant impact on the magnitude of the resulting photocurrents (incident photon-to-current conversion efficiency [IPCE]) from the tubes. No appreciable changes are observed in the crystal structure and morphology of the TiO2 NTs for different annealing profiles (to a constant temperature of 450 °C). The electrochemical properties of the annealed TiO2 NTs are investigated using intensity-modulated photocurrent spectroscopy (IMPS), open-circuit potential decay, and Mott-Schottky analysis. The results clearly show that the annealing ramping rate of 1 °C s-1 leads to the highest IPCE performance. This beneficial effect can be ascribed to a most effective charge separation and electron transport (indicating the least amount of trapping states in the tubes). Therefore, the results suggest that controlling the annealing ramping rate is not only a key factor affecting the defect structure but also a powerful tool to tailor the physical properties, and photocurrent activity of TiO2 NTs. [ABSTRACT FROM AUTHOR]

Polypropylene (PP) powders are coated with silica nanoparticles in a fluidized bed to improve the flow behavior of the powders and the processability in powder bed fusion. The nanoparticles are produced in situ via dusty plasma‐enhanced chemical vapor deposition (PECVD) in an atmospheric‐pressure Ar/O2 plasma jet fixed at the distributor plate of the fluidized bed. Hexamethyldisiloxane is used as a precursor of the nanoparticles. The influence of the oxygen concentration in the plasma gas and the number of treatment cycles on the chemical composition of the nanoparticles, the amount of nanoparticles deposited, and the flow properties of the coated PP powders is investigated. The chemical composition of the formed silica particles is determined by X‐ray photon spectroscopy and infrared spectroscopy. The results reveal that the composition of the nanoparticles is SiOxCy, that is, the portion of organic residues introduced by the precursor can be controlled by changing the oxygen concentration in the plasma gas. The mass of nanoparticles deposited on the polymer powder's surface, as determined by inductively coupled optical emission spectroscopy, shows a linear dependence of the number of cycles and the oxygen concentration in the plasma gas. A considerable improvement of the flow behavior of the PP powders is observed after PECVD treatment. [ABSTRACT FROM AUTHOR]

In recent years, photocatalytic reactions on anodic TiO2 nanotubes have been intensively investigated. In order to show photocatalytic activity, anodically formed nanotubes need to be crystallized to anatase. This is conventionally done by thermal annealing in air at temperatures 400–600 °C. Recently, a so‐called "water annealing" treatment has been reported to be effective to also create a highly active form for photocatalysis. Here we report on the feasibility of using a water annealing treatment of TiO2 nanotubes to create a photocatalyst for H2 production that is as active as conventional thermal annealing. If the water‐annealed samples are additionally hydrogenated to a so‐called "grey" modification, a further significant improvement of the photocatalytic activity for H2 evolution can be achieved – this without the use of any noble metal co‐catalyst. A combination of water annealing, thermal annealing, followed by hydrogenation can deliver a H2 generation activity that is more than five times higher than that achieved by thermal annealing of anodic TiO2 nanotubes. [ABSTRACT FROM AUTHOR]

Herein, the growth of substoichiometric tungsten oxide (WO3–x) nanowires (NWs) via thermal oxidation of W films (a few 10 to a few 100 nm thick) deposited by Ar‐plasma sputtering on fluorine‐doped tin oxide (FTO) substrates is reported. A thermal treatment at relatively low temperature (525–550 °C) in Ar at atmospheric pressure leads to the conversion of the W films into W suboxide (WO3–x) NW arrays. Such NWs have a length of ≈500 nm and their diameter ranges from 10 to 40 nm depending on the duration of the annealing process. These growth conditions for the formation of WO3–x NWs are significantly milder and more straightforward than those reported in previous literature. This avoids high temperatures (e.g., above 600 °C), vacuum conditions, or the addition of catalysts. A thermal‐oxidative approach of these sputtered films is suitable to directly form WO3–x NWs on various surfaces, including conductive substrates to fabricate photoelectrodes or large‐scale supports, e.g., for smart windows. [ABSTRACT FROM AUTHOR]

Titanium dioxide has been the benchmark semiconductor in photocatalysis for more than 40 years. Full water splitting, that is, decomposing water into H2 and O2 in stoichiometric amounts and with an acceptable activity, still remains a challenge, even when TiO2‐based photocatalysts are used in combination with noble‐metal co‐catalysts. The bottleneck of anatase‐type TiO2 remains the water oxidation, that is, the hole transfer reaction from pristine anatase to the aqueous environment. In this work, we report that "grey" (defect engineered) anatase can provide a drastically enhanced lifetime of photogenerated holes, which, in turn, enables an efficient oxidation reaction of water to peroxide via a two‐electron pathway. As a result, a Ni@grey anatase TiO2 catalyst can be constructed with an impressive performance in terms of photocatalytic splitting of neutral water into H2 and a stoichiometric amount of H2O2 without the need of any noble metals or sacrificial agents. The finding of long hole lifetimes in grey anatase opens up a wide spectrum of further photocatalytic applications of this material. [ABSTRACT FROM AUTHOR]

In this work, we describe a simple anodization process of titanium in a hot phosphoric acid/ethylene glycol electrolyte that forms a hierarchically structured photocatalytically active TiO2 layer that consists directly of anatase nanocrystallites. Such anodic layers show, without any annealing, a high photocatalytic degradation activity for AO7 (an azo‐dye used as model pollutant) that is higher than the activity of comparable fully crystallized TiO2 nanotube layers. Furthermore, the hierarchically structured layers have a remarkable mechanical bending stability. These features make these novel anodic layers highly promising for applications in flexible substrate based photocatalytic devices. [ABSTRACT FROM AUTHOR]

Over the past years, so‐called water annealing of TiO2 nanotube (NT) arrays has attracted considerable attention as a tool to enhance the nanotubes' photocatalytic properties. The present work investigates the effect of different water‐annealing treatments on TiO2 nanotubes and compares it to thermally annealed tubes. We find that the most effective water annealing is water immersion of TiO2 NT arrays at 70 °C for several days. The two main effects are i) a partial crystallization of the tubes to anatase, and ii) an etching of the tube walls that leads to a roughening of the walls and, thus, to a higher surface area. Under the best conditions, water annealing is almost as effective in providing enhanced photocatalytic AO7 degradation as a classic thermal annealing of the tubes. We, however, find that water annealed tubes – due to their only partial crystallization – do not perform well when used as photoelectrodes. For photocatalysis, most efficient is a combination of water annealing, followed by thermal annealing, which provides both a larger surface area and a high crystallinity. [ABSTRACT FROM AUTHOR]

Black TiO2 in various forms has been investigated for numerous photochemical applications. In photocatalytic water splitting, "grey" titania forms have been reported to reach considerable H2 generation rates without using a noble metal co‐catalyst. Up to now, a variety of anatase powders or other morphologies has been investigated in grey and black forms. Here we describe that hydrothermal titanate/anatase nanotubes can show a strong noble metal free photocatalytic activity. For optimized "blackening" conditions, a drastically higher photocatalytic H2 production can be obtained than for other nanoscale morphologies. This effect can further be improved with a very mild Pt doping that again shows a clearly stronger photocatalytic H2 production than comparably loaded nanopowders. [ABSTRACT FROM AUTHOR]

In this research, we investigate the effect of alkali metal cations including Li, Na and Cs in hydrothermal solution on the morphology, stability, and photoactivity of nanostructured TiO2 nanoflakes as a photoanode. The TiO2 nanoflakes are formed through hydrothermal treatment of Ti foil in 1.0 M LiOH, NaOH or CsOH at 100 °C for 3 h. By subsequent thermal reduction of the structure in an optimized Ar/H2 environment, conductive TiO2 nanoflakes were formed. The reduction treatment remarkably improves the photocurrent density of the TiO2 nanoflakes and has the highest impact on the sample treated in the NaOH alkali solution. For the nanoflakes produced in NaOH alkali solution, the bandgap is shifted to a lower value and the structure shows the most stable morphology after thermal treatment compared to nanoflakes formed in other alkali solutions. Such reduced hydrothermally treated nanoflakes formed in NaOH can generate a photocurrent density of approximately 1 mA/cm−2 vs. Ag/AgCl in 1.0 M KOH solution, which is six times higher than for pristine TiO2. [ABSTRACT FROM AUTHOR]

The engineering of the electron transport layer (ETL)/light absorber interface is explored in perovskite solar cells. Single‐crystalline TiO2 nanorod (NR) arrays are used as ETL and methylammonium lead iodide (MAPI) as light absorber. A dual ETL surface modification is investigated, namely by a TiCl4 treatment combined with a subsequent PC61BM monolayer deposition, and the effects on the device photovoltaic performance were evaluated with respect to single modifications. Under optimized conditions, for the combined treatment synergistic effects are observed that lead to remarkable enhancements in cell efficiency, from 14.2% to 19.5%, and to suppression of hysteresis. The devices show JSC, VOC, and fill factor as high as 23.2 mA cm−2, 1.1 V, and 77%, respectively. These results are ascribed to a more efficient charge transfer across the ETL/perovskite interface, which originates from the passivation of defects and trap states at the ETL surface. To the best of our knowledge, this is the highest cell performance ever reported for TiO2 NR‐based solar cells fabricated with conventional MAPI light absorber. Perspective wise, this ETL surface functionalization approach combined with more recently developed and better performing light absorbers, such as mixed cation/anion hybrid perovskite materials, is expected to provide further performance enhancements. [ABSTRACT FROM AUTHOR]

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‐Fe2O3) 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‐Fe2O3 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‐Fe2O3 even at a low temperature of 400 °C and provide an excellent PEC performance compared with classic air annealing. The photocurrent of the α‐Fe2O3 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‐Fe2O3 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‐Fe2O3. Additionally, we demonstrate the photocurrent of α‐Fe2O3 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‐Fe2O3 photoanodes. [ABSTRACT FROM AUTHOR]

Pt nanoparticles are typically decorated as co‐catalyst on semiconductors to enhance the photocatalytic performance. Due to the low abundance and high cost of Pt, reaching a high activity with minimized co‐catalyst loadings is a key challenge in the field. We explore a dewetting‐dealloying strategy to fabricate on TiO2 nanotubes nanoporous Pt nanoparticles, aiming at improving the co‐catalyst mass activity for H2 generation. For this, we sputter first Pt‐Ni bi‐layers of controllable thickness (nm range) on highly ordered TiO2 nanotube arrays, and then induce dewetting‐alloying of the Pt‐Ni bi‐layers by a suitable annealing step in a reducing atmosphere: the thermal treatment causes the Pt and Ni films to agglomerate and at the same time mix with each other, forming on the TiO2 nanotube surface metal islands of a mixed PtNi composition. In a subsequent step we perform chemical dealloying of Ni that is selectively etched out from the bimetallic dewetted islands, leaving behind nanoporous Pt decorations. Under optimized conditions, the nanoporous Pt‐decorated TiO2 structures show a>6 times higher photocatalytic H2 generation activity compared to structures modified with a comparable loading of dewetted, non‐porous Pt. We ascribe this beneficial effect to the nanoporous nature of the dealloyed Pt co‐catalyst, which provides an increased surface‐to‐volume ratio and thus a more efficient electron transfer and a higher density of active sites at the co‐catalyst surface for H2 evolution. [ABSTRACT FROM AUTHOR]

In this work, we report on a facile and novel method for decorating titanium dioxide (TiO2) nanotubes with Rh nanoparticle‐nanonetworks that act as co‐catalysts in photocatalytic H2 generation. In a first step, a Ti−Rh (0.2 at%) alloy is etched in Kroll's solution leading to a dealloyed surface decorated with a Rh nanoparticle‐network of adjustable geometry and loading. By subsequent anodization of the alloy samples, Rh:TiO2 nanotubes can be grown where the tube mouths are strongly decorated with the Rh nanoparticle network (RhNw). As evident from X‐ray photoelectron spectroscopy (XPS) analysis, these Rh oxide Nws are converted to metallic Rh under UV irradiation. As a result, with time a steady increase of the H2 evolution from the RhNw decorated TiO2 nanotubes is observed. Nanotubes carrying RhNw yield in photocatalytic experiments a 5‐times higher H2 evolution activity in comparison with nanotubes decorated by conventional Rh‐sputtering (the same loading), and show a 228 times higher activity than pristine TiO2 nanotubes. [ABSTRACT FROM AUTHOR]

In the present work, we investigate the efficiency of TiO2 photoelectrode layers on boron‐doped diamond foil (BDDF) in comparison with a classic conducting glass (fluorine‐doped tin oxide, FTO) back contact. Crystalline thin TiO2 layers were prepared on the substrates by two different methods: (i) deposition of metallic Ti thin films followed by thermal oxidation to form TiO2 (TO‐TiO2), (ii) reactive sputter deposition of TiO2 thin films and crystallization of these layers (SP‐TiO2). The optimized layers show that TO‐TiO2 films on BDDF deliver a significantly higher incident photon to current efficiency (IPCE) compared to directly sputtered SP‐TiO2 layers and these layers on BDDF also outperform FTO as a back contact. We ascribe this beneficial effect of the BDDF back contact to the formation of an intermediate conductive phase of Ti carbides at the TO‐TiO2/BDDF interface. [ABSTRACT FROM AUTHOR]

TiO2 nanotubes were successfully co‐doped with sulfur and Ti3+ states using a facile annealing treatment in H2/H2S gas mixture. The obtained nanotubes were investigated for their photocatalytic performance and characterized by SEM, XRD, XPS, EPR, IPCE, IMPS and Mott‐Schottky measurements. The synthesized co‐doped TiO2 nanotubes show an enhanced photocatalytic hydrogen production rate compared to tubes that were treated only in pure H2 or H2S atmosphere—this without the presence of any co‐catalyst. It was found that sulfur in co‐doped TiO2 exists in the form of S2− and a small quantity of S4+/S6+, which leads to a narrowing of the band gap. However, the enhanced absorption of light in the visible range is not the key reason for the improved photocatalytic performance. We ascribe the enhanced photocatalytic activity to a synergetic effect of S mid‐gap states and disordered Ti3+ defects that facilitate photo generated electron transfer. [ABSTRACT FROM AUTHOR]

Anatase nanowires were synthesized in solution by using a simple mixing of titanium diisopropoxide bis(acetylacetonate), lactic acid and sodium hydroxide at room temperature. We discuss effects of reaction parameters and post treatment (annealing) on the nanowire morphology, surface area, and crystallinity, as well as the competing morphology directing effects of lactic acid and sodium hydroxide. Then the room temperature nanowires were directly grown onto fluoride doped tin oxide (FTO) glass to form photoanodes. Photoelectrochemical measurements of the different nanowires were performed and compared to conventional nanowires produced by high temperature synthesis. Clearly the nanowires introduced in this work show a significant increase in the maximum photocurrent, compared to classic hydrothermal nanowire layers. [ABSTRACT FROM AUTHOR]

Illumination of anatase in an aqueous methanolic solution leads to the formation of Ti3+ sites that are catalytically active for the generation of dihydrogen (H2). With increasing illumination time, a light‐induced self‐amplification of the photocatalytic H2 production rate can be observed. The effect is characterized by electron paramagnetic resonance (EPR) spectroscopy, reflectivity, and photoelectrochemical techniques. Combined measurements of H2 generation rates and in situ EPR spectroscopic observation over the illumination time with AM 1.5G or UV light establish that the activation is accompanied by the formation of Ti3+ states, which is validated through their characteristic EPR resonance at g=1.93. This self‐activation and amplification behavior can be observed for anatase nanoparticles and nanotubes. [ABSTRACT FROM AUTHOR]

Cu doping in titania is usually detrimental to the material's photoconductivity, which prevents the use of this combination in photoanodes. In this work, we produce TiO2 nanotube arrays intrinsically doped with copper and establish sufficient conductivity to use them as efficient photoanodes for methanol oxidation in a photoelectrochemical hydrogen generation setting. Firstly, Cu‐doped TiO2 nanotubes were produced by anodizing a Ti−Cu binary alloy. By subsequent thermal reduction of the structure in an Ar/H2 environment, conductive copper‐doped TiO2 nanotubes (TiCuTN−Ar/H2) can be achieved with an approximately 103 times higher conductivity than the non‐reduced material. When these reduced Cu‐doped TiO2 nanotubes are used as photoanode, copper species embedded in the TiO2 wall catalyze the methanol oxidation reaction. As a result of the combined effect of conductivity and catalytic effect of Cu, such reduced Cu:TiO2 nanotubes can generate a photocurrent of 0.76 mA cm−2 at 1 V vs. RHE, under AM1.5 (100 mW/Cm2) irradiation – in a 50 : 50 MeOH/water solution – this is 33 times higher than for pristine Cu:TiO2 nanotubes. Copper bottomed: TiO2 nanotube arrays intrinsically doped with copper are prepared as photoanode for methanol oxidation. Ar/H2‐treated Cu:TiO2 nanotubes can generate 33 times higher photocurrent in comparison to pristine Cu:TiO2 nanotubes under AM1.5 (100 mW/Cm2) irradiation in 0.1 M Na2SO4 containing 10 % methanol aqueous solution. [ABSTRACT FROM AUTHOR]

In this paper we prepare facetted "black" TiO2 nanotube arrays (NTAs) with a bamboo‐type structure which exhibits an enhanced performance for open‐circuit H2 generation without the use of any noble metal co‐catalyst. The TiO2 nanotubes were formed by applying alternating positive and negative voltages. During the electrochemical formation, not only a bamboo structure is formed but the tubes are reduced in the cathodic cycle. After a rapid annealing treatment of these tubes in air at 500 °C for 10 s, the bamboo rings (knots) provide (001) facets preferentially located at the knot, while (101) planes prevail on the smooth part of tube between knots, as shown by TEM. The short annealing time keeps Ti3+/oxygen vacancies (VO) as co‐catalytically active centers still preserved in the TiO2 NTAs. The results of intensity modulated photocurrent spectroscopy (IMPS) and open‐circuit potential decay measurements (VOC) indicate that these faceted and reduced TiO2 nanotube arrays provide an efficient separation and reaction of photo‐generated charge carriers, leading to an improved photocatalytic H2 evolution performance. A bamboo nano garden: facetted "black" TiO2 nanotube arrays with bamboo‐like structures are prepared as catalysts for hydrogen evolution. Bamboo rings provide different facets than the rest of the tube. These Ti3+‐containing nanotube arrays show an efficient separation and reaction of photo‐generated charge carriers, leading to an improved photocatalytic hydrogen evolution performance. [ABSTRACT FROM AUTHOR]

We investigate two distinctly different anodic TiO2 nanotubular morphologies, spaced and close‐packed arrays. While the close‐packed tubular arrays are formed in ethylene glycol, spaced nanotubes (NTs), which have a regular gap between individual NTs, grow in a diethylene glycol or dimethyl sulfoxide‐based electrolyte. Depending on the electrolyte used for anodization, the morphology, crystal structure, and chemical composition of the resulting nanotubular layer vary from one another. This influences the electrochemical and photoelectrochemical activity of the NTs. Overall, we find that spaced NTs can not only provide enhanced charge‐transfer characteristics but can also show beneficial light absorption characteristics when used as a photo‐ or light‐harvesting electrode. Give them space: Regular spacing between TiO2 nanotubes (NTs) leads to better light‐trapping characteristics and faster charge transfer behavior. [ABSTRACT FROM AUTHOR]

In the present work, we report the use of TiO2 nanotube (NT) layers with a regular intertube spacing that are decorated by Pt nanoparticles through the atomic layer deposition (ALD) of Pt. These Pt‐decorated spaced (SP) TiO2 NTs are subsequently explored for photocatalytic H2 evolution and are compared to classical close‐packed (CP) TiO2 NTs that are also decorated with various amounts of Pt by using ALD. On both tube types, by varying the number of ALD cycles, Pt nanoparticles of different sizes and areal densities are formed, uniformly decorating the inner and outer walls from tube top to tube bottom. The photocatalytic activity for H2 evolution strongly depends on the size and density of Pt nanoparticles, driven by the number of ALD cycles. We show that, for SP NTs, a much higher photocatalytic performance can be achieved with significantly smaller Pt nanoparticles (i.e. for fewer ALD cycles) compared to CP NTs. Personal space: TiO2 nanotubes with a regular spacing lead to higher photocatalytic H2 generation at a lower number of atomic layer deposition cycles in comparison to close‐packed TiO2 nanotubular arrays. [ABSTRACT FROM AUTHOR]

Abstract: Over the past years, α‐Fe2O3 (hematite) has re‐emerged as a promising photoanode material in photoelectrochemical (PEC) water splitting. In spite of considerable success in obtaining relatively high solar conversion efficiency, the main drawbacks hindering practical application of hematite are its intrinsically hampered charge transport and sluggish oxygen evolution reaction (OER) kinetics on the photoelectrode surface. In the present work, we report a strategy that synergistically addresses both of these critical limitations. Our approach is based on three key features that are applied simultaneously: i) a careful nanostructuring of the hematite photoanode in the form of nanorods, ii) doping of hematite by Sn4+ ions using a controlled gradient, and iii) surface decoration of hematite by a new class of layered double hydroxide (LDH) OER co‐catalysts based on Zn–Co LDH. All three interconnected forms of functionalization result in an extraordinary cathodic shift of the photocurrent onset potential by more than 300 mV and a PEC performance that reaches a photocurrent density of 2.00 mA cm−2 at 1.50 V vs. the reversible hydrogen electrode. [ABSTRACT FROM AUTHOR]