Directional heat dissipation across the interface in anatase-rutile nanocomposites

High-throughput HSE study on the doping effect in anatase TiO2

Systematic study on the doping effects of anatase TiO₂ doped with 40 kinds of elements by high-throughput HSE06 calculations.

Conductive Black Titania Nanomaterials for Efficient Photocatalytic Degradation of Organic Pollutants

Abstract

Titanium dioxide (TiO2) as an important semiconductor is widely used in the fields of solar cell, solar thermal collectors, and photocatalysis, but the visible-light power harvest remains insufficient due to the little effective visible-light absorption and many carrier-recombination centers originating from the wide band gap structure. Herein, conductive black titania (BT) nanomaterials with crystalline-TiO2-core/amorphous-TiO2−x-shell structure prepared through two-zone Al-reduction route are found efficient in photocatalyzing the degradation of organic pollutants to environmentally friendly products under full solar and even visible light irradiation. The unique core–shell structure and numerous surface oxygen vacancies or Ti3+ species in the amorphous layer accompanying prominent physicochemical properties of narrow band gap, high carrier concentration, high electron mobility, and excellent separation and transportation of photoinduced e−−h+ pairs result in exceptional photocatalytic efficiency. The optimized BT-500 (pristine TiO2 treated at 500 °C during two-zone Al-reduction process) catalyst achieves superior photocatalytic degradation rates for toluene and ethyl acetate as well as an excellent photostability with high degradation efficiency of 93% for the 6th reuse.

Graphic Abstract

Modification of TiO2 Nanoparticles with Organodiboron Molecules Inducing Stable Surface Ti3+ Complex

As one of the most promising semiconductor oxide materials, titanium dioxide (TiO₂) absorbs UV light but not visible light. To address this limitation, the introduction of Ti³⁺ defects represents a common strategy to render TiO₂ visible-light responsive. Unfortunately, current hurdles in Ti³⁺ generation technologies impeded the widespread application of Ti³⁺ modified materials. Herein, we demonstrate a simple and mechanistically distinct approach to generating abundant surface-Ti³⁺ sites without leaving behind oxygen vacancy and sacrificing one-off electron donors. In particular, upon adsorption of organodiboron reagents onto TiO₂ nanoparticles, spontaneous electron injection from the diboron-bound O²⁻ site to adjacent Ti⁴⁺ site leads to an extremely stable blue surface Ti³⁺‒O^−· complex. Notably, this defect generation protocol is also applicable to other semiconductor oxides including ZnO, SnO₂, Nb₂O₅, and In₂O₃. Furthermore, the as-prepared photoelectronic device using this strategy affords 10³-fold higher visible light response and the fabricated perovskite solar cell shows an enhanced performance.

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Organodiborons are used to reshape the surface electronic state of semiconductor oxides

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Diboron adsorption leads to spontaneous charge transfer and reduced surface metal ions

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Photodetector based on diboron material affords 10³ fold higher visible light response

A symbiotic hetero-nanocomposite that stabilizes unprecedented CaCl2-type TiO2 for enhanced solar-driven hydrogen evolution reaction

Symbiotic hetero-nanocomposites prevail in many classes of minerals, functional substances and/or devices. However, design and development of a symbiotic hetero-nanocomposite that contains unachievable phases remain a significant challenge owing to the tedious formation conditions and the need for precise control over atomic nucleation in synthetic chemistry. Herein, we report a solution chemistry approach for a symbiotic hetero-nanocomposite that contains an unprecedented CaCl2-type titania phase inter-grown with rutile TiO2. CaCl2 structured TiO2, usually occurring when bulk rutile-TiO2 is compressed at an extreme pressure of several GPa, is identified to be a distorted structure with a tilt of adjacent ribbons of the c-axis of rutile. The structural specificity of the symbiotic CaCl2/rutile TiO2 hetero-nanocomposite was confirmed by Rietveld refinement, HRTEM, EXAFS, and Raman spectra, and the formation region (TiCl4 concentration vs. reaction temperature) was obtained by mapping the phase diagram. Due to the symbiotic relationship, this CaCl2-type TiO2 maintained a high stability via tight connection by edge dislocations with rutile TiO2, thus forming a CaCl2/rutile TiO2 heterojunction with a higher reduction capacity and enhanced charge separation efficiency. These merits endow symbiotic CaCl2/rutile TiO2 with a water splitting activity far superior to that of the commercial benchmark photocatalyst, P25 under simulated sunlight without the assistance of a cocatalyst. Our findings reported here may offer several useful understandings of the mechanical intergrowth process in functional symbiotic hetero-nanocomposites for super interfacial charge separation, where interfacial dislocation appears to be a universal cause.

A Critical Review on Energy Conversion and Environmental Remediation of Photocatalysts with Remodeling Crystal Lattice, Surface, and Interface

Solar energy is a renewable resource that can supply our energy needs in the long term. A semiconductor photocatalysis that is capable of utilizing solar energy has appealed to considerable interests for recent decades, owing to the ability to aim at environmental problems and produce renewal energy. Much effort has been put into the synthesis of a highly efficient semiconductor photocatalyst to promote its real application potential. Hence, we reviewed the most advanced methods and strategies in terms of (i) broadening the light absorption wavelengths, (ii) design of active reaction sites, and (iii) control of the electron-hole (e^--h⁺) recombination, while these three processes could be influenced by remodeling the crystal lattice, surface, and interface. Additionally, we individually examined their current applications in energy conversion (i.e., hydrogen evolution, CO₂ reduction, nitrogen fixation, and oriented synthesis) and environmental remediation (i.e., air purification and wastewater treatment). Overall, in this review, we particularly focused on advanced photocatalytic activity with simultaneous wastewater decontamination and energy conversion and further enriched the mechanism by proposing the electron flow and substance conversion. Finally, this review offers the prospects of semiconductor photocatalysts in the following three vital (distinct) aspects: (i) the large-scale preparation of highly efficient photocatalysts, (ii) the development of sustainable photocatalysis systems, and (iii) the optimization of the photocatalytic process for practical application.

Surface Assistant Charge Separation in PEC Cu2S-Ni/Cu2O Cathode

Fabrication of a high efficiency photocathode is a challenging issue in photoelectrocatalysis (PEC). In this work, a Cu₂S-Ni/Cu₂O photocathode was constructed via electrodeposition followed by a two-step overlayer deposition procedure including direct-current magnetron sputtering (DCMS) and ion exchange reaction. We found that the presence of Ni in the inner-layer could not only affect the morphology but also enhance the formation rate of the outer-layer Cu₂S. The XPS results indicate that the Ni exist as NiO_x instead of Ni⁰. The photocurrent of Cu₂S-Ni/Cu₂O achieved 2 times of it on the pristine Cu₂O. The charge dynamic characterizations, including electrochemical impedance spectroscopy (EIS), Tafel slopes, and photoluminescence (PL) spectra, demonstrated that the Ni can promote the hydrogen evolution reaction follow the Heyrovsky reaction, while Cu₂S shows a crucial role on the surface charge separation. At last, surface photovoltage microscopy (SPVM) technology was used to reveal the function of each overlayer. It gives direct evidence for the charge transportation pathway in the system and explains the function of each component.

Position-controlled laser-induced creation of rutile TiO2 nanostructures

For potential applications of nanostructures, control over their position is important. In this report, we introduce two continuous wave laser-based lithography techniques which allow texturing thin TiO₂ films to create a fine rutile TiO₂ structure on silicon via spatially confined oxidation or a solid-liquid-solid phase transition, for initial layers, we use titanium and anatase TiO₂, respectively. A frequency-doubled Nd:YAG laser at a wavelength of 532 nm is employed for the lithography process and the samples are characterized with scanning electron microscopy. The local orientation of the created rutile crystals is determined by the spatial orientation of hydrothermally grown rutile TiO₂ nanorods. Depending on the technique, we obtain either randomly aligned or highly ordered nanorod ensembles. An additional chemically inert SiO₂ cover layer suppresses the chemical and electronic surface properties of TiO₂ and is removed locally with the laser treatment. Hence, the resulting texture provides a specific topography and crystal structure as well as a high contrast of surface properties on a nanoscale, including the position-controlled growth of TiO₂ nanorods.

Amorphous TiO2 nanostructures: synthesis, fundamental properties and photocatalytic applications

In this review, we mainly highlight the advances made in the development of amorphous TiO₂ nanostructures for photocatalysts. Some perspectives on the challenges and new direction are also discussed.

Directly Probing Charge Separation at Interface of TiO2 Phase Junction

Phase junction is often recognized as an effective strategy to achieve efficient charge separation in photocatalysis and photochemistry. As a crucial factor to determine the photogenerated charges dynamics, there is an increasingly hot debate about the energy band alignment across the interface of phase junction. Herein, we reported the direct measurement of the surface potential profile over the interface of TiO₂ phase junction. A built-in electric field up to 1 kV/cm from rutile to anatase nanoparticle was detected by Kelvin Probe Force Microscopy (KPFM). Home-built spatially resolved surface photovoltage spectroscopy (SRSPS) supplies a direct evidence for the vectorial charge transfer of photogenerated electrons from rutile to anatase. Moreover, the tunable anatase nanoparticle sizes in TiO₂ phase junction leads to high surface photovoltage (SPV) by creating completely depleted space charge region (SCR) and enhancing the charge separation efficiency. The results provide a strong basis for understanding the impact of built-in electric field on the charge transfer across the interface of artificial photocatalysts.

Hydrogenated Cagelike Titania Hollow Spherical Photocatalysts for Hydrogen Evolution under Simulated Solar Light Irradiation

We synthesized the hydrogenated cagelike TiO2 hollow spheres through a facile sacrificial template method. After the hydrogenation treatment, the disordered surface layer and cagelike pores were generated on the shell of the hollow spheres. The spheres exhibit a high hydrogen evolution rate of 212.7 ± 10.6 μmol h(-1) (20 mg) under the simulated solar light irradiation, which is ∼12 times higher than the hydrogenated TiO2 solid spheres and is ∼9 times higher than the original TiO2 hollow spheres. The high activity results from the unique architectures and hydrogenation. Both the multiple reflection that was improved by the cagelike hollow structures and the red shift of the absorption edge that was induced by hydrogenation can enhance the ultraviolet and visible light absorption. In addition, the high concentration of oxygen vacancies, as well as the hydrogenated disordered surface layer, can improve the efficiency for migration and separation of generated charge carriers.

Understanding the anatase-rutile phase junction in charge separation and transfer in a TiO2 electrode for photoelectrochemical water splitting

New insight into junction-based designs for efficient charge separation is vitally important for current solar energy conversion research. Herein, an anatase-rutile phase junction is elaborately introduced into TiO2 films by rapid thermal annealing treatment and the roles of phase junction on charge separation and transfer are studied in detail. A combined study of transient absorption spectroscopy, electrochemical and photoelectrochemical (PEC) measurements reveals that appropriate phase alignment is essential for unidirectional charge transfer, and a junction interface with minimized trap states is crucial to liberate the charge separation potential of the phase junction. By tailored control of phase alignment and interface structure, an optimized TiO2 film with an appropriately introduced phase junction shows superior performance in charge separation and transfer, hence achieving ca. 3 and 9 times photocurrent density enhancement compared to pristine anatase and rutile phase TiO2 electrodes, respectively. This work demonstrates the great potential of phase junctions for efficient charge separation and transfer in solar energy conversion applications.

Self-induced synthesis of phase-junction TiO2 with a tailored rutile to anatase ratio below phase transition temperature

The surface phase junction of nanocrystalline TiO₂ plays an essential role in governing its photocatalytic activity. Thus, facile and simple methods for preparing phase-junction TiO₂ photocatalysts are highly desired. In this work, we show that phase-junction TiO₂ is directly synthesized from Ti foil by using a simple calcination method with hydrothermal solution as the precursor below the phase transition temperature. Moreover, the ratio of rutile to anatase in the TiO₂ samples could be readily tuned by changing the ratio of weight of Ti foil to HCl, which is used as the hydrothermal precursor, as confirmed by the X-ray diffraction analysis. In the photocatalytic reaction by the TiO₂ nanocomposite, a synergistic effect between the two phases within a certain range of the ratio is clearly observed. The results suggest that an appropriate ratio of anatase to rutile in the TiO₂ nanocomposite can create more efficient solid-solid interfaces upon calcination, thereby facilitating interparticle charge transfer in the photocatalysis.

Ni-doped rutile TiO2 nanoflowers: low-temperature solution synthesis and enhanced photocatalytic efficiency

A facile solution approach was developed to synthesize micrometre-sized aggregates of Ni-doped TiO₂ nanoflowers, which exhibited a rate constant four times that of commercial Degussa P25 TiO₂ nanoparticles under the UV + Vis illumination.

Hydrogen-treated BiFeO3 nanoparticles with enhanced photoelectrochemical performance

Compared with pristine BiFeO₃, hydrogen-treated BiFeO₃ nanoparticles exhibit higher photoelectrochemical performance.

In Situ Formation of Disorder-Engineered TiO2(B)-Anatase Heterophase Junction for Enhanced Photocatalytic Hydrogen Evolution

Hydrogenation of semiconductors is an efficient way to increase their photocatalytic activity by forming disorder-engineered structures. Herein, we report a facile hydrogenation process of TiO2(B) nanobelts to in situ generate TiO2(B)-anatase heterophase junction with a disordered surface shell. The catalyst exhibits an excellent performance for photocatalytic hydrogen evolution under the simulated solar light irradiation (∼580 μmol h(-1), 0.02 g photocatalyst). The atomically well-matched heterophase junction, along with the disorder-engineered surface shell, promotes the separation of electron-hole and inhibits their recombination. This strategy can be further employed to design other disorder-engineered composite photocatalysts for solar energy utilization.

Three-phase junction for modulating electron-hole migration in anatase-rutile photocatalysts

The heterophase solid-solid junction as an important type of structure unit has wide applications for its special mechanics and electronic properties. Here we present a first three-phase atomic model for the anatase-rutile TiO2 heterophase junction and determine its optical and electronic properties, which leads to resolution of the long-standing puzzles on the enhanced photocatalytic activity of anatase-rutile photocatalysts. By using a set of novel theoretical methods, including crystal phase transition pathway sampling, interfacial strain analysis and first principles thermodynamics evaluation of holes and electrons, we identify an unusual structurally ordered three-phase junction, a layer-by-layer "T-shaped" anatase/TiO2-II/rutile junction, for linking anatase with rutile. The intermediate TiO2-II phase, although predicted to be only a few atomic layers thick in contact with anatase, is critical to alleviate the interfacial strain and to modulate photoactivity. We demonstrate that the three-phase junction acts as a single-way valve allowing the photogenerated hole transfer from anatase to rutile but frustrating the photoelectron flow in the opposite direction, which otherwise cannot be achieved by an anatase-rutile direct junction. This new model clarifies the roles of anatase, rutile and the phase junction in achieving high photoactivity synergistically and provides the theoretical basis for the design of better photocatalysts by exploiting multi-phase junctions.

Design of a solar-driven TiO2 nanofilm on Ti foil by self-structure modifications

A novel solar-driven V_O–N–TiO₂ (A/R) nanofilm was designed. Its optical absorption can cover the ultraviolet, visible and near-infrared region.

Band gap engineering of early transition-metal-doped anatase TiO₂: first principles calculations

The thermal stability and electronic structures of anatase TiO2 doped with early transition metals (TM) (group III-B = Sc, Y and La; group IV-B = Zr and Hf; group V-B = V, Nb and Ta) have been studied using first principles calculations. It was found that all doped systems are thermodynamically stable, and their band gaps were reduced by 1-1.3 eV compared to pure TiO2. Doping with transition metals affects the strength of the hybrid orbital of TM-O bonding, and the band gap increases approximately linearly with the MP value of TM-O bonding.

A facile solution route to deposit TiO2 nanowire arrays on arbitrary substrates

A facile solution-based technique was developed to grow vertically aligned TiO2 nanowires with predominantly anatase phase on arbitrary substrates of stainless steel, glass, silicon wafer and carbon cloth at the low temperature of 80 °C and in an open atmosphere.

Formation of polycrystalline TiO2 on the ablated surfaces of RbTiOPO4 single crystals by thermal annealing

A combination of laser ablation and thermal annealing allows one to coat the surface of a RbTiOPO₄ crystal with TiO₂ nanoparticles.