Graphene Wrapped TiO2 Based Catalysts with Enhanced Photocatalytic Activity

Reduced graphene oxide (RGO) wrapped titanium dioxide nanocrystals (TiO₂ NCs@RGO) with oxygen vacancies (Vo) and Ti³⁺ defects have been synthesized by electrostatically wrapping GO around TiO₂ NCs followed by thermal annealing at 400 °C. Transmission electron microscope observations have shown that TiO₂ NCs@RGO has a unique crystalline core/crystalline shell structure, which is different from the original amorphous TiO₂ covered TiO₂ NCs. Raman spectroscopy, X‐ray photoelectron spectroscopy, and electron paramagnetic resonance have demonstrated that Vo‐Ti³⁺ species are more readily formed in TiO₂ NCs@RGO than in TiO₂ NCs. As a result, TiO₂ NCs@RGO exhibits enhanced optical absorption in a wide wavelength range from visible light to near IR and red‐shifted absorption edge. In the photocatalytic degradation reaction of methyl orange, the photodegradation rate constant for TiO₂ NCs@RGO is 2.4 times higher than that of TiO₂ NCs. The enhanced photocatalytic performance can be attributed to the improved charge separation at the interface of TiO₂ NCs and RGO layer and the enhanced optical absorption in visible light region due to the donor levels of the defects such as Vo‐Ti³⁺ species. This work establishes a new method for preparing Vo defect contained TiO₂ catalysts.     

Solar photocatalysts from Gd–La codoped TiO2 nanoparticles

Gd–La codoped TiO₂ nanoparticles with diameter of 10 nm were successfully synthesized via a sol–gel method. The photocatalytic activity of the Gd–La codoped TiO₂ nanoparticles evaluated by photodegrading methyl orange has been significantly enhanced compared to that of undoped or Gd or La monodoped TiO₂. Ti⁴⁺ may substitute for La³⁺ and Gd³⁺ in the lattices of rare earth oxides to create abundant oxygen vacancies and surface defects for electron trapping and dye adsorption, accelerating the separation of photogenerated electron–hole pairs and methyl orange photodegradation. The formation of an excitation energy level below the conduction band of TiO₂ from the binding of electrons and oxygen vacancies decreases the excitation energy of Gd–La codoped TiO₂, resulting in versatile solar photocatalysts. The results suggest that Gd–La codoped TiO₂ nanoparticles are promising for future solar photocatalysts.     

Synthesis,characterization and photocatalytic activity of neodymium,nitrogen and neodymium–nitrogen doped TiO2

TiO₂ doped with 0.1–10% Nd³⁺ or 1.8% nitrogen or co-doped with both dopants as well as Nd₂Ti₂O₇ are synthesized by a sol–gel. The phase composition, crystal structure, morphology, optical properties and binding energies of the doped titania were investigated. The photocatalytic activity was tested for malachite green dye degradation and compared with the results obtained by dopant-free TiO₂ synthesized using the same procedure and with the commercial Degussa P25 product. The influence of the dopant type and relative content, temperature of ignition in the course of the synthesis, pH of the reaction medium and the type of the illumination light on the efficiency of the prepared catalysts were determined. A positive effect of Nd- and N-doping on the apparent degradation rate constant under UV irradiation and negative one of Nd-presence at solar illumination is found. Nd–N-codoping has a negative effect on the photocatalytic activity.     

Enhancement of Photocatalytic Activity of Bi2O3–BiOI Composite Nanosheets through Vacancy Engineering

Vacancy engineering is an effective strategy to enhance solar‐driven photocatalytic performance of semiconductors. It is highly desirable to improve the photocatalytic performance of composite nanomaterials by the introduction of vacancies, but the role of vacancies and the heterostructure in the photocatalytic process is elusive to the composite nanomaterials. Herein, the introduction of I vacancies can significantly enhance the photocatalytic activity of Bi₂O₃–BiOI composite nanosheets in a synergistic manner. The excellent photocatalytic performance of the Bi₂O₃–BiOI composites is attributed to the combination of Bi₂O₃ and BiOI and the existence of I vacancies in Bi₂O₃–BiOI composites. Specifically, density functional theory calculation shows that the existence of I vacancies would create a new electric states vacancy band below the conduction band of BiOI and thus can reduce the bandgap of BiOI nanosheets. This greatly facilitates the scavenging of the photogenerated electron on the surface of BiOI by Bi₂O₃, therefore, enhancing the overall photocatalytic activity of the composites. The enhanced photocatalytic efficiency is demonstrated by the degradation of tetracycline (TC), which reaches 96% after 180 min and by the high total organic carbon (TOC) removal (89% after 10 h visible light irradiation). This study provides a novel approach for the design of high‐performance composite catalysts.     

Visible-light photocatalysis in nitrogen-carbon-doped TiO2 films obtained by heating TiO2 gel-film in an ionized N2 gas

To use solar irradiation or interior lighting efficiently, we sought a photocatalyst with high reactivity under visible light. Nitrogen and carbon doping TiO₂ films were obtained by heating a TiO₂ gel in an ionized N₂ gas. The as-synthesized TiO_2−x−yN_xC_y films have shown an improvement over titanium dioxide in optical absorption and photocatalytic activity such as photodegradation of methyl orange under visible light. The process of the oxygen atom substituted by nitrogen and carbon was discussed. Oxygen vacancy induced by the formation of Ti³⁺ species and nitrogen and carbon doped into substitution sites of TiO₂ have been proven to be indispensable for the enhance of photocatalytic activity, as assessed by UV-Vis Spectroscopy and X-ray photoemission spectroscopy.     

A Photoresponsive Rutile TiO2 Heterojunction with Enhanced Electron–Hole Separation for High‐Performance Hydrogen Evolution

Rutile titanium dioxide (TiO₂) is a promising photocatalyst due to its high thermodynamic stability and few intragrain defects. However, it has not yet achieved photocatalytic activity comparable to that of anatase TiO₂ owing to its higher recombination rate of electron–hole pairs. To effectively separate the electron–hole pairs in rutile TiO₂, a facet heterojunction (FH) structure to prolong the lifetime of the photogenerated electrons is proposed. Ultrathin TiO₂ nanosheets with different facets are coated in situ onto TiO₂ nanorod (NR) substrates, where FHs are built among the nanosheets as well as between the nanosheets and NR substrates. The as‐prepared rutile TiO₂, with an FH structure (FH‐TiO₂), serves as an effective photocatalyst for water splitting. More than 45 and 18 times higher photogenerated current density and H₂ production rate, respectively, are obtained compared to those of pure rutile TiO₂ NRs. Moreover, FH‐TiO₂ delivers a 0.566 mmol g^?1 h^?1 H₂ production rate even in pure water. This study offers important insights into the rational design of rutile TiO₂ structures for highly efficient photocatalytic reactions.     

Ni‐Nanocluster Modified Black TiO2 with Dual Active Sites for Selective Photocatalytic CO2 Reduction

One of the key challenges in artificial photosynthesis is to design a photocatalyst that can bind and activate the CO₂ molecule with the smallest possible activation energy and produce selective hydrocarbon products. In this contribution, a combined experimental and computational study on Ni‐nanocluster loaded black TiO₂ (Ni/TiO_2[Vo]) with built‐in dual active sites for selective photocatalytic CO₂ conversion is reported. The findings reveal that the synergistic effects of deliberately induced Ni nanoclusters and oxygen vacancies provide (1) energetically stable CO₂ binding sites with the lowest activation energy (0.08 eV), (2) highly reactive sites, (3) a fast electron transfer pathway, and (4) enhanced light harvesting by lowering the bandgap. The Ni/TiO_2[Vo] photocatalyst has demonstrated highly selective and enhanced photocatalytic activity of more than 18 times higher solar fuel production than the commercial TiO₂ (P‐25). An insight into the mechanisms of interfacial charge transfer and product formation is explored.     

Synthesis of core/shell Ti/TiOx photocatalyst via single-mode magnetic microwave assisted direct oxidation of TiH2

Submicron core/shell Ti/TiO_x photocatalyst is successfully synthesized via single-mode magnetic microwave (SMMW) assisted direct oxidation of planetary ball-milled TiH₂. The thickness of TiOx shell including highly concentrated defects such as Ti³⁺ and/or oxygen vacancies is controllable in the range from 6 to over 18 nm by varying the treatment time in the SMMW assisted reaction. In addition to its quite narrow optical bandgap (1.34–2.69 eV) and efficient visible-light absorption capacity, the submicron Ti/TiO_x particle exhibits superior photocatalytic performance towards H₂ production from water under both UV and visible-light irradiation to compare with a commercial TiO₂ photocatalyst (P-25). Such excellent performance can be achieved by the synergetic effect of enhancement in visible light absorption capacity and photo-excited carrier separation because of the highly concentrated surface defects and the specific Ti/TiO_x core/shell structure, respectively.     

2D Porous TiO2 Single‐Crystalline Nanostructure Demonstrating High Photo‐Electrochemical Water Splitting Performance

Porous single crystals are promising candidates for solar fuel production owing to their long range charge diffusion length, structural coherence, and sufficient reactive sites. Here, a simple template‐free method of growing a selectively branched, 2D anatase TiO₂ porous single crystalline nanostructure (PSN) on fluorine‐doped tin oxide substrate is demonstrated. An innovative ion exchange–induced pore‐forming process is designed to successfully create high porosity in the single‐crystalline nanostructure with retention of excellent charge mobility and no detriment to crystal structure. PSN TiO₂ film delivers a photocurrent of 1.02 mA cm^?2 at a very low potential of 0.4 V versus reversible hydrogen electrode (RHE) for photo‐electrochemical water splitting, closing to the theoretical value of TiO₂ (1.12 mA cm^?2). Moreover, the current–potential curve featuring a small potential window from 0.1 to 0.4 V versus RHE under one‐sun illumination has a near‐ideal shape predicted by the Gartner Model, revealing that the charge separation and surface reaction on the PSN TiO₂ photoanode are very efficient. The photo‐electrochemical water splitting performance of the films indicates that the ion exchange–assisted synthesis strategy is effective in creating large surface area and single‐crystalline porous photoelectrodes for efficient solar energy conversion.     

Enhanced H2 Generation of Au‐Loaded,Nitrogen‐Doped TiO2 Hierarchical Nanostructures under Visible Light

Hierarchical N‐doped TiO₂ nanostructured catalysts with micro‐, meso‐, and macroporosity are synthesized by a facile self‐formation route using ammonia and titanium isopropoxide precursor. UV–vis diffuse reflectance spectra confirm the red shift and band gap narrowing due to the doping of N species in the TiO₂ nanoporous catalyst. Hierarchical macroporosity with fibrous channel patterning is observed and well preserved even after calcination at 800 °C, indicating thermal stability, whereas micro‐ and mesoporosity are lost after calcination at 500 °C. The photocatalytic activity of hiearchical N doped TiO₂ catalysts loaded with Au is evaluated for H₂ production reaction in visible light. The enhanced photocatalytic activity is attributed to the combined synergetic effect of N doping for visible light absorption, micro‐ and mesoporosity for an increase of effective surface area and light harvestation, and hierarchical macroporous fibrous structure for multiple reflection and effective charge transfer.     

Structures and properties of titania thin films annealed under different atmosphere

The effects of crystallinity, phase and oxygen vacancies on optical and photocatalytic properties of titania (TiO₂) thin films were systematically studied. The as-deposited amorphous titania films were prepared by reactive sputtering titanium metal targets in argon–oxygen plasma at 100 °C and subsequently annealed at various temperatures of 400–800 °C in air, vacuum and H₂ atmosphere. The results indicate that in general the crystallinity of the annealed films is enhanced with the increasing annealing temperature. At the same temperature, the H₂ annealed films achieve better crystallinity but containing more oxygen vacancies than the films annealed in air and in vacuum. In H₂ or in vacuum, the concentration of oxygen vacancies in the annealed films increases with increasing temperature, while in air it remains constant. Oxygen vacancies in titania film not only facilitate phase transformation but also lower the band gap of titania, and make the film visible-light responsive. Photocatalytic properties of the TiO₂ films were characterized in UV and visible light irradiation by following the Ag reduction and degradation of methylene blue. The films annealed at 600–700 °C in H₂ possess the best film crystallinity and the proper concentration of oxygen vacancies and exhibit the best photocatalytic performance under both UV and visible light.     

In Situ Irradiated X‐Ray Photoelectron Spectroscopy Investigation on a Direct Z‐Scheme TiO2/CdS Composite Film Photocatalyst

Inspired by nature, artificial photosynthesis through the construction of direct Z‐scheme photocatalysts is extensively studied for sustainable solar fuel production due to the effectiveness in enhancing photoconversion efficiency. However, there is still a lack of thorough understanding and direct evidence for the direct Z‐scheme charge transfer in these photocatalysts. Herein, a recyclable direct Z‐scheme composite film composed of titanium dioxide and cadmium sulfide (TiO₂/CdS) is prepared for high‐efficiency photocatalytic carbon dioxide (CO₂) reduction. In situ irradiated X‐ray photoelectron spectroscopy (ISI‐XPS) confirms the direct Z‐scheme charge‐carrier migration pathway in the photocatalytic system. Furthermore, density functional theory simulation identifies the intrinsic cause for the formation of the direct Z‐scheme heterojunction between the TiO₂ and the CdS. Thanks to the significantly enhanced redox abilities of the charge carriers in the direct Z‐scheme system, the photocatalytic CO₂ reduction performance of the optimized TiO₂/CdS is 3.5, 5.4, and 6.3 times higher than that of CdS, TiO₂, and commercial TiO₂ (P25), respectively, in terms of methane production. This work is a valuable guideline in preparation of highly efficient recyclable nanocomposite for photoconversion applications.     

3D FTO/FTO‐Nanocrystal/TiO2 Composite Inverse Opal Photoanode for Efficient Photoelectrochemical Water Splitting

A 3D fluorine‐doped SnO₂ (FTO)/FTO‐nanocrystal (NC)/TiO₂ inverse opal (IO) structure is designed and fabricated as a new “host and guest” type of composite photoanode for efficient photoelectrochemical (PEC) water splitting. In this novel photoanode design, the highly conductive and porous FTO/FTO‐NC IO acts as the “host” skeleton, which provides direct pathways for faster electron transport, while the conformally coated TiO₂ layer acts as the “guest” absorber layer. The unique composite IO structure is fabricated through self‐assembly of colloidal spheres template, a hydrothermal method and atomic layer deposition (ALD). Owing to its large surface area and efficient charge collection, the FTO/FTO‐NC/TiO₂ composite IO photoanode shows excellent photocatalytic properties for PEC water splitting. With optimized dimensions of the SnO₂ nanocrystals and the thickness of the ALD TiO₂ absorber layers, the 3D FTO/FTO‐NC/TiO₂ composite IO photoanode yields a photocurrent density of 1.0 mA cm^?2 at 1.23 V versus reversible hydrogen electrode (RHE) under AM 1.5 illumination, which is four times higher than that of the FTO/TiO₂ IO reference photoanode.     

Interfacial Lattice‐Strain‐Driven Generation of Oxygen Vacancies in an Aerobic‐Annealed TiO2(B) Electrode

Oxygen vacancies play crucial roles in defining physical and chemical properties of materials to enhance the performances in electronics, solar cells, catalysis, sensors, and energy conversion and storage. Conventional approaches to incorporate oxygen defects mainly rely on reducing the oxygen partial pressure for the removal of product to change the equilibrium position. However, directly affecting reactants to shift the reaction toward generating oxygen vacancies is lacking and to fill this blank in synthetic methodology is very challenging. Here, a strategy is demonstrated to create oxygen vacancies through making the reaction energetically more favorable via applying interfacial strain on reactants by coating, using TiO₂(B) as a model system. Geometrical phase analysis and density functional theory simulations verify that the formation energy of oxygen vacancies is largely decreased under external strain. Benefiting from these, the obtained oxygen‐deficient TiO₂(B) exhibits impressively high level of capacitive charge storage, e.g., ≈53% at 0.5 mV s^?1, far surpassing the ≈31% of the unmodified counterpart. Meanwhile, the modified electrode shows significantly enhanced rate capability delivering a capacity of 112 mAh g^?1 at 20 C (≈6.7 A g^?1), ≈30% higher than air‐annealed TiO₂ and comparable to vacuum‐calcined TiO₂. This work heralds a new paradigm of mechanical manipulation of materials through interfacial control for rational defect engineering.     

Au Nanoclusters Sensitized Black TiO2−x Nanotubes for Enhanced Photodynamic Therapy Driven by Near‐Infrared Light

The low reactive oxygen species production capability and the shallow tissue penetration of excited light (UV) are still two barriers in photodynamic therapy (PDT). Here, Au cluster anchored black anatase TiO_2?x nanotubes (abbreviated as Au₂₅/B‐TiO_2?x NTs) are synthesized by gaseous reduction of anatase TiO₂ NTs and subsequent deposition of noble metal. The Au₂₅/B‐TiO_2?x NTs with thickness of about 2 nm exhibit excellent PDT performance. The reduction process increased the density of Ti³⁺ on the surface of TiO₂, which effectively depresses the recombination of electron and hole. Furthermore, after modification of Au₂₅ nanoclusters, the PDT efficiency is further enhanced owing to the changed electrical distribution in the composite, which forms a shallow potential well on the metal–TiO₂ interface to further hamper the recombination of electron and hole. Especially, the reduction of anatase TiO₂ can expend the light response range (UV) of TiO₂ to the visible and even near infrared (NIR) light region with high tissue penetration depth. When excited by NIR light, the nanoplatform shows markedly improved therapeutic efficacy attributed to the photocatalytic synergistic effect, and promotes separation or restrained recombination of electron and hole, which is verified by experimental results in vitro and in vivo.     

TiO2@Perylene Diimide Full‐Spectrum Photocatalysts via Semi‐Core–Shell Structure

A semi‐core–shell structure of perylene diimide (PDI) self‐assembly coated with TiO₂ nanoparticles is constructed, in which nanoscale porous TiO₂ shell is formed and PDI self‐assembly presented 1D structure. A full‐spectrum photocatalyst is obtained using this structure to resolve a conundrum—TiO₂ does not exhibit visible‐light photocatalytic activity while PDI does not exhibit ultraviolet photocatalytic activity. Furthermore, the synergistic interaction between TiO₂ and PDI enables the catalyst to improve its ultraviolet, visible‐light, and full‐spectrum performance. The interaction between TiO₂ and PDI leads to formation of some new stacking states along the Π–Π stacking direction and, as a consequence, electron transfer from PDI to TiO₂ suppresses the recombination of e^?/h⁺ and thus improves photocatalytic performance. But the stronger interaction in the interface between TiO₂ and PDI is not in favor of photocatalytic performance, which leads to rapid charge recombination due to more disordered stacking states. The study provides a theoretical direction for the study of core–shell structures with soft materials as a core, and an idea for efficient utilization of solar energy.     

Identifying the Origin of Ti3+ Activity toward Enhanced Electrocatalytic N2 Reduction over TiO2 Nanoparticles Modulated by Mixed-Valent Copper

The ambient electrocatalytic N₂ reduction reaction (NRR) enabled by TiO₂ has attracted extensive recent attention. Previous studies suggest the formation of Ti³⁺ in TiO₂ can significantly improve the NRR activity, but it still remains unclear what kinds of Ti³⁺ are effective. Herein, it is demonstrated that mixed-valent Cu acts as an effective dopant to modulate the oxygen vacancy (V_O) concentration and Ti³⁺ formation, which markedly improves the electrocatalytic NRR performance. In 0.5 m LiClO₄, this electrocatalyst attains a high Faradic efficiency of 21.99% and a large NH₃ yield of 21.31 µg h⁻¹ mg_cat.⁻¹ at –0.55 V vs reversible hydrogen electrode, which even surpasses most reported Ti-based NRR electrocatalysts. Using density function theory calculations, it is evidenced that mixed-valent Cu ions modulate the TiO₂ (101) surface with multiple oxygen vacancies, which is beneficial for generating different Ti³⁺ 3d¹ defect states localized below the Fermi energy. N₂ activation and adsorption are effectively strengthened when Ti³⁺ 3d¹ defect states present the splitting of e_g and t_2g orbitals, which can be modulated by its coordination structure. The synergistic roles of the three ion pairs formed by the V_O defect, including Cu¹⁺–Ti⁴⁺, Ti³⁺–Ti⁴⁺ and Ti³⁺–Ti³⁺, are together responsible for the enhanced NRR performance.     

Multichannel Porous TiO2 Hollow Nanofibers with Rich Oxygen Vacancies and High Grain Boundary Density Enabling Superior Sodium Storage Performance

TiO₂ as an anode for sodium‐ion batteries (NIBs) has attracted much recent attention, but poor cyclability and rate performance remain problematic owing to the intrinsic electronic conductivity and the sluggish diffusivity of Na ions in the TiO₂ matrix. Herein, a simple process is demonstrated to improve the sodium storage performance of TiO₂ by fabricating a 1D, multichannel, porous binary‐phase anatase‐TiO₂–rutile‐TiO₂ composite with oxygen‐deficient and high grain‐boundary density (denoted as a‐TiO_2?x /r‐TiO_2?x ) via electrospinning and subsequent vacuum treatment. The introduction of oxygen vacancies in the TiO₂ matrix enables enhanced intrinsic electronic conductivity and fast sodium‐ion diffusion kinetics. The porous structure offers easy access of the liquid electrolyte and a short transport path of Na⁺ through the pores toward the TiO₂ nanoparticle. Furthermore, the high density of grain boundaries between the anatase TiO₂ and rutile TiO₂ offer more interfaces for a novel interfacial storage. The a‐TiO_2?x /r‐TiO_2?x shows excellent long cycling stability (134 mAh g^?1 at 10 C after 4500 cycles) and superior rate performance (93 mAh g^?1 after 4500 cycles at 20 C) for sodium‐ion batteries. This simple and effective process could serve as a model for the modification of other materials applied in energy storage systems and other fields.     

Effects of the co-addition of Zn and sodium dodecylbenzenesulfonate on photocatalytic activity and wetting performance of anatase TiO2 nanoparticle films

In this paper, doped and undoped anatase TiO₂ nanoparticle films on indium tin oxide glasses have been fabricated by spin coating sols containing Zn²⁺ or Zn²⁺ and sodium dodecylbenzenesulfonate (DBS), respectively. The effects of the co-addition of Zn²⁺ and DBS on the photocatalysis performance and wetting properties of the resulting TiO₂ nanoparticle films were investigated. The results showed that the addition of Zn²⁺ improved both the photocatalytic activity and the hydrophilicity, which was attributed to surface oxygen vacancies. The co-addition of Zn²⁺ and DBS resulted in an important increase of the surface roughness, resulting in films showing a superhydrophilic behavior. However, the photocatalytic activity was slightly decreased by co-adding Zn²⁺ and DBS. The DBS addition resulted in changes in the surface microstructure of the TiO₂ films, changing the photocatalytic activity and wetting performance.     

Photocatalytic degradation of formaldehyde by nanostructured TiO2 composite films

Interest in the photocatalytic oxidation of formaldehyde from contaminated wastewater is growing rapidly. The photocatalytic activity of the nanocrystalline Fe³⁺/F^? co-doped TiO₂–SiO₂ composite film for the degradation of formaldehyde solution under visible light was discussed in this study. The films were characterised by field emission scanning electron microscopy (FE-SEM) equipped with energy-dispersive spectroscopy, X-ray diffraction (XRD), BET surface area, UV–Vis absorption spectroscopy, and photoluminescence spectroscopy. The FE-SEM results revealed that the Fe³⁺/F^? co-doped TiO₂–SiO₂ film was composed of uniform round-like nanoparticles or aggregates with the size range of 5–10 nm. The XRD results indicated that only the anatase phase was observed in the film. Compared with a pure TiO₂ film and a singly modified TiO₂ film, the Fe³⁺/F^? co-doped TiO₂–SiO₂ composite film showed the best photocatalytic properties due to its strong visible light adsorption and diminished electrons-holes recombination.