Hydrogenated anatase: strong photocatalytic dihydrogen evolution without the use of a co-catalyst

Black Titanium Oxide/Activated TaS2 Flakes Photoelectrode for Plasmon Assisted Hydrogen Evolution at Neutral pH at High Current Density

A heterojunction photo-electrode(s) consisting of porous black titanium oxide (bTiO2) and electrochemically self-activated TaS2 flakes is proposed and utilized for hydrogen evolution reaction (HER). The self-activated TaS2 flakes provide abundant catalytic sites for HER and the porous bTiO2, prepared by electrochemical anodization and subsequent reduction serves as an efficient light absorber, providing electrons for HER. Additionally, Au nanostructures are introduced between bTiO2 and TaS2 to facilitate the charge transfer and plasmon-triggering ability of the structure created. After structure optimization, high HER catalytic activity at acidic pH and excellent HER activity at neutral pH are achieved at high current densities. In particular, with the utilization of bTiO2@TaS2 photoelectrode (neutral electrolyte, sunlight illumination) current densities of 250 and 500 mA cm-2 are achieved at overpotentials of 433, and 689 mV, respectively, both exceeding the "benchmark" Pt. The addition of gold nanostructures further reduces the overpotential to 360 and 543 mV at 250 and 500 mA cm-2, respectively. The stability of the prepared electrodes is investigated and found to be satisfying within 24 h of performance at high current densities. The proposed system offers an excellent potential alternative to Pt for the development of green hydrogen production on an industrial scale.

Self-Modification of Defective TiO2 under Controlled H2/Ar Gas Environment and Dynamics of Photoinduced Surface Oxygen Vacancies

In recent years, defective TiO2 has caught considerable research attention because of its potential to overcome the limits of low visible light absorption and fast charge recombination present in pristine TiO2 photocatalysts. Among the different synthesis conditions for defective TiO2, ambient pressure hydrogenation with the addition of Ar as inert gas for safety purposes has been established as an easy method to realize the process. Whether the Ar gas might still influence the resulting photocatalytic properties and defective surface layer remains an open question. Here, we reveal that the gas flow ratio between H2 and Ar has a crucial impact on the defective structure as well as the photocatalyic activity of TiO2. In particular, transmission electron microscopy (TEM) in combination with electron energy loss spectroscopy (EELS) revealed a larger width of the defective surface layer when using a H2/Ar (50 %-50 %) gas mixture over pure H2. A possible reason could be the increase in dynamic viscosity of the gas mixture when Ar is added. Additionally, photoinduced enhanced Raman spectroscopy (PIERS) is implemented as a complementary approach to investigate the dynamics of the defective structures under ambient conditions which cannot be effortlessly realized by vacuum techniques like TEM.

In-situ construction of TiO2 polymorphic junction nanoarrays without cocatalyst for boosting photocatalytic hydrogen generation

There are significant challenges in developing technologies for high-yield photocatalytic hydrogen production reactions. Current photocatalytic materials face three key problems: low utilization of light, rapid recombination of photogenerated electron-hole pairs, and a limited number of active sites during photocatalytic reactions. As a result, these materials only improve one or two of the three steps involved in photocatalytic hydrogen production reactions. Consequently, achieving simultaneous multifunctional synergy to enhance the efficiency of all three processes is difficult. Here, we report an in situ dissolution-recrystallisation approach to design and fabricate a three-dimensional TiO2 rutile/anatase (AE-TiO2) array photocatalytic material for photocatalytic hydrolysis applications. It is shown that the unique 3D nanoarray structure and in situ fabrication of the AE-TiO2 homojunction with synergistic effects among the components lead to an increase in light harvesting efficiency, charge transport separation efficiency and surface active sites, which remarkably improve the photocatalytic hydrolysis performance. The prepared AE-TiO2 homojunction materials realizes a maximal photoactivity of 4 μmol cm-2·h-1, which is 39 times larger than that of pure TiO2 rutile nanorods.

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.

Amine-Functionalized A-Center Sphalerite for Selective and Efficient Destruction of Perfluorooctanoic Acid

Perfluorochemicals (PFCs), especially perfluorooctanoic acid (PFOA), have contaminated the ground and surface waters throughout the world. Efficient removal of PFCs from contaminated waters has been a major challenge. This study developed a novel UV-based reaction system to achieve fast PFOA adsorption and decomposition without addition of sacrificial chemicals by using synthetic photocatalyst sphalerite (ZnS-[N]) with sufficient surface amination and defects. The obtained ZnS-[N] has the capability of both reduction and oxidation due to the suitable band gap and photo-generated hole-trapping properties created by surface defects. The cooperated organic amine functional groups on the surface of ZnS-[N] play a crucial role in the selective adsorption of PFOA, which guarantee the efficient destruction of PFOA subsequently, and 1 μg L^-1 PFOA could be degraded to <70 ng L^-1 after 3 h in the presence of 0.75 g L^-1 ZnS-[N] under 500 W UV irradiation. In this process, the photogenerated electrons (reduction) and holes (oxidation) on the ZnS-[N] surface work in a synergistic manner to achieve complete defluorination of PFOA. This study not only provides promising green technology for PFC-pollution remediation but also highlights the significance of developing a target system capable of both reduction and oxidation for PFC degradation.

A Light(er) Approach for the Selective Hydrogenation of 5-Hydroxymethylfurfural to 2,5-Bis(hydroxymethyl)furan without External H2

The selective conversion of 5-hydroxymethyfurfural (HMF), a biomass-derived platform molecule, to value added chemicals can ease the burden on petroleum-based fine chemical synthesis. The active contribution of renewable energy sources along with low cost, environmental friendliness, and a simple reaction system must be adopted for better sustainability. In this context, photocatalytic selective hydrogenation of HMF to 2,5-bis(hydroxymethyl)furan (BHMF) was achieved over P25 titania nanoparticles without chemical squander. Simultaneously the photo-oxidation of p-methoxybenzyl alcohol (MeOBA) to p-methoxybenzaldehyde (MeOBaL), similar to biomass-derived vanillin, was carried out, abolishing the need of additional redox reagents. This system put forward the competent employment of photogenerated excitons for the valorization of lignocellulosic biomass to fine chemicals, which is an urgent requirement for sustainable chemical synthesis.

Light-Induced Migration of Spin Defects in TiO2 Nanosystems and their Contribution to the H2 Evolution Catalysis from Water

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 (V_O ^. ). 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.

Prospects of Synthesized Magnetic TiO2-Based Membranes for Wastewater Treatment: A Review

Global accessibility to clean water has stressed the need to develop advanced technologies for the removal of toxic organic and inorganic pollutants and pathogens from wastewater to meet stringent discharge water quality limits. Conventionally, the high separation efficiencies, relative low costs, small footprint, and ease of operation associated with integrated photocatalytic-membrane (IPM) technologies are gaining an all-inclusive attention. Conversely, photocatalysis and membrane technologies face some degree of setbacks, which limit their worldwide application in wastewater settings for the treatment of emerging contaminants. Therefore, this review elucidated titanium dioxide (TiO₂), based on its unique properties (low cost, non-toxicity, biocompatibility, and high chemical stability), to have great potential in engineering photocatalytic-based membranes for reclamation of wastewater for re-use. The environmental pathway of TiO₂ nanoparticles, membranes and configuration types, modification process, characteristics, and applications of IPMs in water settings are discussed. Future research and prospects of magnetized TiO₂-based membrane technology is highlighted as a viable water purification technology to mitigate fouling in the membrane process and photocatalyst recoverability. In addition, exploring life cycle assessment research would also aid in utilizing the concept and pressing for large-scale application of this technology.

Probing the formation and optical properties of Ti3+–TiO2 with (001) exposed crystal facet by ethanol-assisted fluorination

(001)-faceted TiO₂ with Ti³⁺ defects that are exclusively embedded in the bulk lattice near the surface was synthesized.

Enhanced Solar Photothermal Catalysis over Solution Plasma Activated TiO2

Colored wide-bandgap semiconductor oxides with abundant mid-gap states have long been regarded as promising visible light responsive photocatalysts. However, their catalytic activities are hampered by charge recombination at deep level defects, which constitutes the critical challenge to practical applications of these oxide photocatalysts. To address the challenge, a strategy is proposed here that includes creating shallow-level defects above the deep-level defects and thermal activating the migration of trapped electrons out of the deep-level defects via these shallow defects. A simple and scalable solution plasma processing (SPP) technique is developed to process the presynthesized yellow TiO2 with numerous oxygen vacancies (Ov), which incorporates hydrogen dopants into the TiO2 lattice and creates shallow-level defects above deep level of Ov, meanwhile retaining the original visible absorption of the colored TiO2. At elevated temperature, the SPP-treated TiO2 exhibits a 300 times higher conversion rate for CO2 reduction under solar light irradiation and a 7.5 times higher removal rate of acetaldehyde under UV light irradiation, suggesting the effectiveness of the proposed strategy to enhance the photoactivity of colored wide-bandgap oxides for energy and environmental applications.

Photocatalytic hydrogen evolution by co-catalyst-free TiO2/C bulk heterostructures synthesized under mild conditions

Hydrogen production by photocatalytic water splitting is one of the most promising sustainable routes to store solar energy in the form of chemical bonds. To obtain significant H2 evolution rates (HERs) a variety of defective TiO2 catalysts were synthesized by means of procedures generally requiring highly energy-consuming treatments, e.g. hydrogenation. Even if a complete understanding of the relationship between defects, electronic structure and catalytic active sites is far from being achieved, the band gap narrowing and Ti3+-self-doping have been considered essential to date. In most reports a metal co-catalyst (commonly Pt) and a sacrificial electron donor (such as methanol) are used to improve HERs. Here we report the synthesis of TiO2/C bulk heterostructures, obtained from a hybrid TiO2-based gel by simple heat treatments at 400 °C under different atmospheres. The electronic structure and properties of the grey or black gel-derived powders are deeply inspected by a combination of classical and less conventional techniques, in order to identify the origin of their photoresponsivity. The defective sites of these heterostructures, namely oxygen vacancies, graphitic carbon and unpaired electrons localized on the C matrix, result in a remarkable visible light activity in spite of the lack of band gap narrowing or Ti3+-self doping. The materials provide HER values ranging from about 0.15 to 0.40 mmol h-1 gcat -1, under both UV- and visible-light irradiation, employing glycerol as sacrificial agent and without any co-catalyst.

Construction of Defective Zinc-Cadmium-Sulfur Nanorods for Visible-Light-Driven Hydrogen Evolution Without the Use of Sacrificial Agents or Cocatalysts

Solar-driven H₂ evolution is an essential process for sustainable energy development. Currently, the greatest challenge is the development of efficient photocatalysts to drive this reaction, especially in pure water systems (without the use of a sacrificial agent). In this study, structural defects in Zn-Cd-S nanorod photocatalysts are found to increase charge separation efficiency significantly by sevenfold. Efficient H₂ evolution (352.7 μmol h^-1  g^-1 , 100 mg of catalyst) is achieved by using this defective Zn-Cd-S nanorod photocatalyst in the absence of sacrificial agents and precious metal cocatalysts under visible-light irradiation. Thus, this cocatalyst- and sacrificial-agent-free, visible-light-responsive system shows remarkable potential as a new artificial photosynthesis route for green H₂ production.

Sulfur and Ti3+ co-Doping of TiO2 Nanotubes Enhance Photocatalytic H2 Evolution Without the Use of Any co-catalyst

TiO₂ nanotubes were successfully co-doped with sulfur and Ti³⁺ states using a facile annealing treatment in H₂ /H₂ S 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 H₂ S atmosphere-this without the presence of any co-catalyst. It was found that sulfur in co-doped TiO₂ exists in the form of S^2- 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.

Self-Enhancing H2 Evolution from TiO2 Nanostructures under Illumination

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.

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

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

Black and white anatase, rutile and mixed forms: band-edges and photocatalytic activity

Polymorphs of “black” TiO₂ in their mixed phase forms provide efficient junctions for photocatalytic H₂ generation in absence of any external co-catalyst.

Core-shell structured titanium dioxide nanomaterials for solar energy utilization

Because of its unmatched resource potential, solar energy utilization currently is one of the hottest research areas. Much effort has been devoted to developing advanced materials for converting solar energy into electricity, solar fuels, active chemicals, or heat. Among them, TiO2 nanomaterials have attracted much attention due to their unique properties such as low cost, nontoxicity, good stability and excellent optical and electrical properties. Great progress has been made, but research opportunities are still present for creating new nanostructured TiO2 materials. Core-shell structured nanomaterials are of great interest as they provide a platform to integrate multiple components into a functional system, showing improved or new physical and chemical properties, which are unavailable from the isolated components. Consequently, significant effort is underway to design, fabricate and evaluate core-shell structured TiO2 nanomaterials for solar energy utilization to overcome the remaining challenges, for example, insufficient light absorption and low quantum efficiency. This review strives to provide a comprehensive overview of major advances in the synthesis of core-shell structured TiO2 nanomaterials for solar energy utilization. This review starts from the general protocols to construct core-shell structured TiO2 nanomaterials, and then discusses their applications in photocatalysis, water splitting, photocatalytic CO2 reduction, solar cells and photothermal conversion. Finally, we conclude with an outlook section to offer some insights on the future directions and prospects of core-shell structured TiO2 nanomaterials and solar energy conversion.

Preparation of a New Type of Black TiO2 under a Vacuum Atmosphere for Sunlight Photocatalysis

Black TiO₂ as a solar-driven photocatalyst has attracted enormous attention from scientists and engineers in water and wastewater treatment field. Most of the methods used for the preparation of black TiO₂ are thermal treatment under a hydrogen atmosphere. Nevertheless, it is well known that working with hydrogen is not safe and needs special maintenance. Here, for the first time, we prepared black TiO₂ by sintering P25 pellets at different temperatures (500-800 °C) under a vacuum atmosphere that showed the same performance with the prepared black TiO₂ under a hydrogen atmosphere. The samples were characterized by X-ray diffraction, Raman spectra field emission scanning electron microscopy, transmission electron microscopy, electron paramagnetic resonance, X-ray photoelectron spectroscopy, and ultraviolet-visible deep resistivity sounding techniques. The differences between the formation of oxygen vacancy density and color turning in sintered powder and pellet were also studied. The results showed that the color of the P25 powder became darker after sintering but not completely turning to black, whereas the P25 pellets completely turned black after sintering. The resultant black TiO₂ was used for the photocatalytic degradation of the acetaminophen (ACE) in aqueous solution under AM 1.5G solar light illumination; it was found that the P25 pellet sintered in 500 °C had the highest photocatalytic performance for ACE degradation under AM 1.5G solar light illumination. The photocatalytic activity of prepared black TiO₂ under vacuum and hydrogen atmospheres was also compared together; the results showed that photocatalytic activities of both samples were so close together. The existence of the oxygen vacancy after 6 months and long and short-term stability (by application for photocatalytic degradation of ACE in an aqueous solution) of the black TiO₂ pellets was also studied; the results showed that the TiO₂ pellets in aqueous phase had acceptable stability.

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

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

Intrinsically Activated SrTiO3: Photocatalytic H2 Evolution from Neutral Aqueous Methanol Solution in the Absence of Any Noble Metal Cocatalyst

Noble metal cocatalysts are conventionally a crucial factor in oxide-semiconductor-based photocatalytic hydrogen generation. In the present work, we show that optimized high-temperature hydrogenation of commercially available strontium titanate (SrTiO₃) powder can be used to engineer an intrinsic cocatalytic shell around nanoparticles that can create a photocatalyst that is highly effective without the use of any additional cocatalyst for hydrogen generation from neutral aqueous methanol solutions. This intrinsic activation effect can also be observed for SrTiO₃[100] single crystal as well as Nb-doped SrTiO₃[100] single crystal. For all types of SrTiO₃ samples (nanopowders and either of the single crystals), hydrogenation under optimum conditions leads to a surface-hydroxylated layer together with lattice defects visible by transmission electron microscopy, electron paramagnetic resonance (EPR), and photoluminescence (PL). Active samples provide specific defects identified by EPR, PL, and electron-energy loss spectroscopy as Ti³⁺ states in a defective matrix-this is in contrast to the inactive defects formed in other reductive atmospheres. In aqueous media, active SrTiO₃ samples show a significant negative shift of the flatband potential (in photoelectrochemical as well as in capacitance data) and a lower charge-transfer resistance for photoexcited electrons. We therefore ascribe the remarkable cocatalyst-free activation of the material to a synergy between thermodynamics (altered interface energetics induced by hydroxylation) and kinetics (charge transfer mediation by suitable Ti³⁺ states).

Multiple Heterojunction in Single Titanium Dioxide Nanoparticles for Novel Metal-Free Photocatalysis

Despite a longstanding controversy surrounding TiO₂ materials, TiO₂ polymorphs with heterojunctions composed of anatase and rutile outperform individual polymorphs because of the type-II energetic band alignment at the heterojunction interface. Improvement in photocatalysis has also been achieved via black TiO₂ with a thin disorder layer surrounding ordered TiO₂. However, localization of this disorder layer in a conventional single TiO₂ nanoparticle with the heterojunction composed of anatase and rutile has remained a big challenge. Here, we report the selective positioning of a disorder layer of controlled thicknesses between the anatase and rutile phases by a conceptually different synthetic route to access highly efficient novel metal-free photocatalysis for H₂ production. The presence of a localized disorder layer within a single TiO₂ nanoparticle was confirmed for the first time by high-resolution transmission electron microscopy with electron energy-loss spectroscopy and inline electron holography. Multiple heterojunctions in single TiO₂ nanoparticles composed of crystalline anatase/disordered rutile/ordered rutile layers give the nanoparticles superior electron/hole separation efficiency and novel metal-free surface reactivity, which concomitantly yields an H₂ production rate that is ∼11-times higher than that of Pt-decorated conventional anatase and rutile single heterojunction TiO₂ systems.

Efficient promotion of charge transfer and separation in hydrogenated TiO2/WO3 with rich surface-oxygen-vacancies for photodecomposition of gaseous toluene

Oxygen-deficient TiO₂/WO₃ constructed via the controllable temperature of hydrogen annealing is designed in view of combining the broad visible spectrum absorption with the prominent coupled semiconductor properties. Surface lattice disorder of TiO₂/WO₃ arises at hydrogen annealing temperature of 200 and 300°C, while critical phase transition from TiO₂/WO₃ to TiO₂/WO_2.9 occurs at 400°C, both of which can introduce oxygen vacancies. The hydrogenated TiO₂/WO₃ with rich surface-oxygen-vacancies exhibits much higher photocatalytic activity for decomposition of gaseous toluene than pristine TiO₂/WO₃ under visible-light illumination (λ>420nm). The photoelectrochemical analysis shows that the improved electronic properties of oxygen-deficient TiO₂/WO₃ enable dramatically efficient promotion of photoinduced charge transfer and separation, which is the key factor for the improved photocatalytic activity. It is hoped that the present work could boost ongoing interest for preparing various hydrogenated coupled semiconductors with enhanced activity for diverse photocatalytic applications.

Molten salt construction of stable oxygen vacancies on TiO2 for enhancement of visible light photocatalytic activity

The construction of defects on TiO₂ surface has attracted great interest due to their prominent effect on photocatalytic activity. However, most synthesis methods often lead to unstable oxygen vacancies which limits their effect for improvement of visible light photoactivity. In this work, stable oxygen vacancies were successfully introduced in commercial TiO₂ (P25) via one-step molten salt (MS) method. Due to the incomplete combination of trifluoroacetic acid (TFA) adsorbed on TiO₂ in MS, the lattice oxygen atoms of TiO₂ were consumed resulting in the formation of oxygen vacancies both on the surface and in the bulk of TiO₂. The optical adsorption edge of oxygen defective TiO₂ showed a substantial shift toward to visible light region combining with a color variation from white to dark blue. Meanwhile, the morphology and crystalline phase were also changed because of the presence of oxygen vacancies. As a result, the blue TiO₂ with rich oxygen vacancies exhibited a considerably enhanced photocatalytic activity for decomposition of rhodamine B (RhB) and selective oxidation of benzyl alcohol under visible light irradiation.

Stable blue TiO₂ with exposed (001) facets was synthesized from pristine white TiO₂via a one-step molten salt (MS) method.

Cobalt Phosphide Modified Titanium Oxide Nanophotocatalysts with Significantly Enhanced Photocatalytic Hydrogen Evolution from Water Splitting

Production of hydrogen from photocatalytic water splitting holds promise as an alternative energy source with superiority of cleanliness, environment friendliness, low price, and sustainability. Perfectly constructing the noble-metal-free and stable hybrid structure photocatalyst is quite essential; herein, for the first time the authors aim to use cobalt phosphide as the cocatalyst on titanium oxide to form a novel hybrid structure to enhance the utilization of the photoexcited electrons in redox reactions for improved photocatalytic H₂ evolution activity. Thus, the achieved significantly increased photocatalytic H₂ -evolution rate on the optimized CoP/TiO₂ (8350 µmol h^-1 g^-1 ) is 11 times higher than that of the pristine TiO₂ . Moreover, this work is expected to spur more insight into synthesizing such novel photofunctional systems, achieving high photocatalytic H₂ evolution activity and sufficient stability for solar-to-chemical conversion and utilization.

Noble-Metal-Free Photocatalytic Hydrogen Evolution Activity: The Impact of Ball Milling Anatase Nanopowders with TiH2

Ball milling TiO₂ anatase together with TiH₂ can create an effective photocatalyst. The process changes the lattice and electronic structure of anatase. Lattice deformation created by mechanical impact combined with hydride incorporation yield electronic gap-states close to the conduction band of anatase. These provide longer lifetimes of photogenerated charge carriers and lead to an intrinsic cocatalytic activation of anatase for H₂ evolution.

Black Magic in Gray Titania: Noble-Metal-Free Photocatalytic H2 Evolution from Hydrogenated Anatase

'Black' TiO₂ -in the widest sense, TiO₂ reduced by various treatments-has attracted tremendous scientific interest in recent years because of some outstanding properties; most remarkably in photocatalysis. While the material effects visible light absorption (the blacker, the better), black titania produced by high pressure hydrogenation was recently reported to show another highly interesting feature; noble-metal-free photocatalytic H₂ generation. In a systematic investigation of high-temperature hydrogen treatments of anatase nanoparticles, TEM, XRD, EPR, XPS, and photoelectrochemistry are used to characterize different degrees of surface hydrogenation, surface termination, electrical conductivity, and structural defects in the differently treated materials. The materials' intrinsic activity for photocatalytic hydrogen evolution is coupled neither with their visible light absorption behavior nor the formation of amorphous material, but rather must be ascribed to optimized and specific defect formation (gray is better than black). This finding is further confirmed by using a mesoporous anatase matrix as a hydrogenation precursor, which, after conversion to the gray state, even further enhances the overall photocatalytic hydrogen evolution activity.

Surface Localization of Defects in Black TiO2: Enhancing Photoactivity or Reactivity

In the past several years, surface-disordered TiO₂, which is referred to as black TiO₂ and can absorb both visible and near-infrared solar light, has triggered an explosion of interest for many important applications. Despite the excellent optical and electrical features of black TiO₂ for various photoelectrochemical (PEC) and photochemical reactions, the current understanding of the photocatalytic mechanism is unsatisfactory and incomplete. On the basis of previous studies, we present new insight into the surface localization of defects and perspectives on the liquid/solid interface. The future prospects for understanding black TiO₂ from this perspective suggest that defect engineering at the liquid/solid interface is a potential method of guiding nanomaterial design.

Photoanodes based on TiO2and α-Fe2O3for solar water splitting – superior role of 1D nanoarchitectures and of combined heterostructures

Solar driven photoelectrochemical water splitting represents a promising approach for a sustainable and environmentally friendly production of renewable energy vectors and fuel sources, such as H₂.

Templated dewetting: designing entirely self-organized platforms for photocatalysis

Formation and dispersion of metal nanoparticles on oxide surfaces in site-specific or even arrayed configuration are key in various technological processes such as catalysis, photonics, electrochemistry and for fabricating electrodes, sensors, memory devices, and magnetic, optical, and plasmonic platforms. A crucial aspect towards an efficient performance of many of these metal/metal oxide arrangements is a reliable fabrication approach. Since the early works on graphoepitaxy in the 70s, solid state dewetting of metal films on patterned surfaces has been much explored and regarded as a most effective tool to form defined arrays of ordered metal particles on a desired substrate. While templated dewetting has been studied in detail, particularly from a mechanistic perspective on lithographically patterned Si surfaces, the resulting outstanding potential of its applications on metal oxide semiconductors, such as titania, has received only limited attention. In this perspective we illustrate how dewetting and particularly templated dewetting can be used to fabricate highly efficient metal/TiO2 photocatalyst assemblies e.g. for green hydrogen evolution. A remarkable advantage is that the synthesis of such photocatalysts is completely based on self-ordering principles: anodic self-organized TiO2 nanotube arrays that self-align to a highest degree of hexagonal ordering are an ideal topographical substrate for a second self-ordering process, that is, templated-dewetting of sputter-deposited metal thin films. The controllable metal/semiconductor coupling delivers intriguing features and functionalities. We review concepts inherent to dewetting and particularly templated dewetting, and outline a series of effective tools that can be synergistically interlaced to reach fine control with nanoscopic precision over the resulting metal/TiO2 structures (in terms of e.g. high ordering, size distribution, site specific placement, alloy formation) to maximize their photocatalytic efficiency. These processes are easy to scale up and have a high throughput and great potential to be applied to fabricate not only (photo)catalytic materials but also a large palette of other functional nanostructured elements and devices.

Aligned metal oxide nanotube arrays: key-aspects of anodic TiO2 nanotube formation and properties

Over the past ten years, self-aligned TiO₂ nanotubes have attracted tremendous scientific and technological interest due to their anticipated impact on energy conversion, environment remediation and biocompatibility. In the present manuscript, we review fundamental principles that govern the self-organized initiation of anodic TiO₂ nanotubes. We start with the fundamental question: why is self-organization taking place? We illustrate the inherent key mechanistic aspects that lead to tube growth in various different morphologies, such as ripple-walled tubes, smooth tubes, stacks and bamboo-type tubes, and importantly the formation of double-walled TiO₂ nanotubes versus single-walled tubes, and the drastic difference in their physical and chemical properties. We show how both double- and single-walled tube layers can be detached from the metallic substrate and exploited for the preparation of robust self-standing membranes. Finally, we show how by selecting specific growth approaches to TiO₂ nanotubes desired functional features can be significantly improved, e.g., enhanced electron mobility, intrinsic doping, or crystallization into pure anatase at high temperatures can be achieved. Finally, we briefly outline the impact of property, modifications and morphology on functional uses of self-organized nanotubes for most important applications.

Photoemission Fingerprints for Structural Identification of Titanium Dioxide Surfaces

The wealth of properties of titanium dioxide relies on its various polymorphs and on their mixtures coupled with a sensitivity to crystallographic orientations. It is therefore pivotal to set out methods that allow surface structural identification. We demonstrate herein the ability of photoemission spectroscopy to provide Ti LMV (V = valence) Auger templates to quantitatively analyze TiO2 polymorphs. The Ti LMV decay reflects Ti 4sp-O 2p hybridizations that are intrinsic properties of TiO2 phases and orientations. Ti LMV templates collected on rutile (110), anatase (101), and (100) single crystals allow for the quantitative analysis of mixed nanosized powders, which bridges the gap between surfaces of reference and complex materials. As a test bed, the anatase/rutile P25 is studied both as received and during the anatase-to-rutile transformation upon annealing. The agreement with X-ray diffraction measurements proves the reliability of the Auger analysis and highlights its ability to detect surface orientations.

Efficient Photocatalytic H2 Evolution: Controlled Dewetting-Dealloying to Fabricate Site-Selective High-Activity Nanoporous Au Particles on Highly Ordered TiO2 Nanotube Arrays

Anodic self-organized TiO2 nanostumps are formed and exploited for self-ordering dewetting of Au-Ag sputtered films. This forms ordered particle configurations at the tube top (crown position) or bottom (ground position). By dealloying from a minimal amount of noble metal, porous Au nanoparticles are then formed, which, when in the crown position, allow for a drastically improved photocatalytic H2 production compared with nanoparticles produced by conventional dewetting processes.

Photoelectrochemical water splitting promoted with a disordered surface layer created by electrochemical reduction

The recent discovery of colored TiO2 indicated that the disordered surface layer over the TiO2 particles/photoelectrodes is beneficial for higher photocatalytic performance; however, the role of the disordered surface TiO2 layer is not well understood. Here, we report an electrochemical strategy for tuning the surface structure of TiO2 nanorod arrays (NRAs) and try to understand the role of the disordered surface TiO2 layer. Photoelectrodes of TiO2 NRAs with a disordered shell were prepared by an electrochemical reduction method. The photocurrent of the NRAs with a disordered shell can reach as high as ∼1.18 mA/cm(2) at 1.23 V, which is 2.2 times of that of the pristine TiO2 NRAs. Our results show that the surface disordered layer not only improves the bulk charge separation but also suppresses the charge recombination at the electrode/electrolyte interface, acting as an efficient water oxidation cocatalyst of photoelectrochemical cell for solar water splitting.

Highly-efficient cocatalyst-free H2-evolution over silica-supported CdS nanoparticle photocatalysts under visible light

A silica-supported CdS nanoparticle photocatalyst exhibits excellent visible-light driven H₂evolution activity without the use of a cocatalyst. The apparent quantum yield can reach 42% under 420 nm light illumination.