Oxygen‐Vacancy‐Introduced BaSnO3−δ Photoanodes with Tunable Band Structures for Efficient Solar‐Driven Water Splitting

To achieve excellent photoelectrochemical water‐splitting activity, photoanode materials with high light absorption and good charge‐separation efficiency are essential. One effective strategy for the production of materials satisfying these requirements is to adjust their band structure and corresponding bandgap energy by introducing oxygen vacancies. A simple chemical reduction method that can systematically generate oxygen vacancies in barium stannate (BaSnO₃ (BSO)) crystal is introduced, which thus allows for precise control of the bandgap energy. A BSO photoanode with optimum oxygen‐vacancy concentration (8.7%) exhibits high light‐absorption and good charge‐separation capabilities. After deposition of FeOOH/NiOOH oxygen evolution cocatalysts on its surface, this photoanode shows a remarkable photocurrent density of 7.32 mA cm^?2 at a potential of 1.23 V versus a reversible hydrogen electrode under AM1.5G simulated sunlight. Moreover, a tandem device constructed with a perovskite solar cell exhibits an operating photocurrent density of 6.84 mA cm^?2 and stable gas production with an average solar‐to‐hydrogen conversion efficiency of 7.92% for 100 h, thus functioning as an outstanding unbiased water‐splitting system.     

Increasing Solar Absorption of Atomically Thin 2D Carbon Nitride Sheets for Enhanced Visible‐Light Photocatalysis

Atomically thin 2D carbon nitride sheets (CNS) are promising materials for photocatalytic applications due to their large surface area and very short charge‐carrier diffusion distance from the bulk to the surface. However, compared to their bulk counterpart, CNS' applications always suffer from an enlarged bandgap and thus narrowed solar absorption range. Here, an approach to significantly increase solar absorption of the atomically thin CNS via fluorination followed by thermal defluorination is reported. This approach can greatly increase the visible‐light absorption of CNS by extending the absorption edge up to 578 nm. The modulated CNS loaded with Pt cocatalyst as a photocatalyst shows a superior photocatalytic hydrogen production activity under visible‐light irradiation to Pt‐CNS. Combining experimental characterization with theoretical calculations shows that this approach can introduce cyano groups into the framework of CNS as well as the accompanied nitrogen vacancies at the edges, which leads to both narrowing the bandgap and changing the charge distribution. This study will provide an effective strategy to increase solar absorption of carbon‐nitride‐based photocatalysts for solar energy conversion applications.     

Photoassisted Construction of Holey Defective g‐C3N4 Photocatalysts for Efficient Visible‐Light‐Driven H2O2 Production

Holey defective g‐C₃N₄ photocatalysts, which are easily prepared via a novel photoassisted heating process, are reported. The photoassisted treatment not only helps to create abundant holes, endowing g‐C₃N₄ with more exposed catalytic active sites and crossplane diffusion channels to shorten the diffusion distance of both reactants from the surface to bulk and charge carriers from the bulk to surface, but also introduces nitrogen vacancies in the tri‐s‐triazine repeating units of g‐C₃N₄, inducing the narrowing of intrinsic bandgap and the formation of defect states within bandgap to extend the visible‐light absorption range and suppress the radiative electron–hole recombination. As a result, the holey defective g‐C₃N₄ photocatalysts show much higher photocatalytic activity for H₂O₂ production with optimized enhancement up to ten times higher than pristine bulk g‐C₃N₄. The newly developed synthetic strategy adopted here enables the sufficient utilization of solar energy and shows rather promising for the modification of other materials for efficient energy‐related applications.     

A Brown Mesoporous TiO2‐x/MCF Composite with an Extremely High Quantum Yield of Solar Energy Photocatalysis for H2 Evolution

A brown mesoporous TiO_2‐x/MCF composite with a high fluorine dopant concentration (8.01 at%) is synthesized by a vacuum activation method. It exhibits an excellent solar absorption and a record‐breaking quantum yield (Φ = 46%) and a high photon–hydrogen energy conversion efficiency (η = 34%,) for solar photocatalytic H₂ production, which are all higher than that of the black hydrogen‐doped TiO₂ (Φ = 35%, η = 24%). The MCFs serve to improve the adsorption of F atoms onto the TiO₂/MCF composite surface, which after the formation of oxygen vacancies by vacuum activation, facilitate the abundant substitution of these vacancies with F atoms. The decrease of recombination sites induced by high‐concentration F doping and the synergistic effect between lattice Ti³⁺–F and surface Ti³⁺–F are responsible for the enhanced lifetime of electrons, the observed excellent absorption of solar light, and the photocatalytic production of H₂ for these catalysts. The as‐prepared F‐doped composite is an ideal solar light‐driven photocatalyst with great potential for applications ranging from the remediation of environmental pollution to the harnessing of solar energy for H₂ production.     

A Full‐Spectrum Metal‐Free Porphyrin Supramolecular Photocatalyst for Dual Functions of Highly Efficient Hydrogen and Oxygen Evolution

A full‐spectrum (300–700 nm) responsive porphyrin supramolecular photocatalyst with a theoretical solar spectrum efficiency of 44.4% is successfully constructed. For the first time, hydrogen and oxygen evolution (40.8 and 36.1 µmol g^?1 h^?1) is demonstrated by a porphyrin photocatalyst without the addition of any cocatalysts. The strong oxidizing performance also presents an efficient photodegradation activity that is more than ten times higher than that of g‐C₃N₄ for the photodegradation of phenol. The high photocatalytic reduction and oxidation activity arises from a strong built‐in electric field due to molecular dipoles of electron‐trapping groups and the nanocrystalline structure of the supramolecular photocatalyst. The appropriate band structure of the supramolecular photocatalyst adjusted via the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of the porphyrin gives rise to thermodynamic driving potential for H₂ and O₂ evolution under visible light irradiation. Controlling the energy band structure of photocatalysts via the ordered assembly of structure‐designed organic molecules could provide a novel approach for the design of organic photocatalysts in energy and environmental applications.     

Self‐Sensitized Carbon Nitride Microspheres for Long‐Lasting Visible‐Light‐Driven Hydrogen Generation

A new type of metal‐free photocatalyst is reported having a microsphere core of oxygen‐containing carbon nitride and self‐sensitized surfaces by covalently linked polymeric triazine dyes. These self‐sensitized carbon nitride microspheres exhibit high visible‐light activities in photocatalytic H₂ generation with excellent stability for more than 100 h reaction. Comparing to the traditional g‐C₃N₄ with activities terminated at 450 nm, the polymeric triazine dyes on the carbon nitride microsphere surface allow for effective wide‐range visible‐light harvesting and extend the H₂ generation activities up to 600 nm. It is believed that this new type of highly stable self‐sensitized metal‐free structure opens a new direction of future development of low‐cost photocatalysts for efficient and long‐term solar fuels production.     

Creating Graphitic Carbon Nitride Based Donor‐π–Acceptor‐π–Donor Structured Catalysts for Highly Photocatalytic Hydrogen Evolution

Conjugated polymers with tailored donor–acceptor units have recently attracted considerable attention in organic photovoltaic devices due to the controlled optical bandgap and retained favorable separation of charge carriers. Inspired by these advantages, an effective strategy is presented to solve the main obstructions of graphitic carbon nitride (g‐C₃N₄) photocatalyst for solar energy conversion, that is, inefficient visible light response and insufficient separation of photogenerated electrons and holes. Donor‐π–acceptor‐π–donor polymers are prepared by incorporating 4,4′‐(benzoc 1,2,5 thiadiazole‐4,7‐diyl) dianiline (BD) into the g‐C₃N₄ framework (UCN‐BD). Benefiting from the visible light band tail caused by the extended π conjugation, UCN‐BD possesses expanded visible light absorption range. More importantly, the BD monomer also acts as an electron acceptor, which endows UCN‐BD with a high degree of intramolecular charge transfer. With this unique molecular structure, the optimized UCN‐BD sample exhibits a superior performance for photocatalytic hydrogen evolution upon visible light illumination (3428 µmol h^?1 g^?1), which is nearly six times of that of the pristine g‐C₃N₄. In addition, the photocatalytic property remains stable for six cycles in 3 d. This work provides an insight into the synthesis of g‐C₃N₄‐based D‐π–A‐π–D systems with highly visible light response and long lifetime of intramolecular charge carriers for solar fuel production.     

Homojunction of Oxygen and Titanium Vacancies and its Interfacial n–p Effect

The homojunction of oxygen/metal vacancies and its interfacial n–p effect on the physiochemical properties are rarely reported. Interfacial n–p homojunctions of TiO₂ are fabricated by directly decorating interfacial p‐type titanium‐defected TiO₂ around n‐type oxygen‐defected TiO₂ nanocrystals in amorphous–anatase homogeneous nanostructures. Experimental measurements and theoretical calculations on the cell lattice parameters show that the homojunction of oxygen and titanium vacancies changes the charge density of TiO₂; a strong EPR signal caused by oxygen vacancies and an unreported strong titanium vacancies signal of 2D ¹H TQ‐SQ MAS NMR are present. Amorphous–anatase TiO₂ shows significant performance regarding the photogeneration current, photocatalysis, and energy storage, owing to interfacial n‐type to p‐type conductivity with high charge mobility and less structural confinement of amorphous clusters. A new “homojunction of oxygen and titanium vacancies” concept, characteristics, and mechanism are proposed at an atomic‐/nanoscale to clarify the generation of oxygen vacancies and titanium vacancies as well as the interface electron transfer.     

Design for Highly Piezoelectric and Visible/Near‐Infrared Photoresponsive Perovskite Oxides

Defect‐engineered perovskite oxides that exhibit ferroelectric and photovoltaic properties are promising multifunctional materials. Though introducing gap states by transition metal doping on the perovskite B‐site can obtain low bandgap (i.e., 1.1–3.8 eV), the electrically leaky perovskite oxides generally lose piezoelectricity mainly due to oxygen vacancies. Therefore, the development of highly piezoelectric ferroelectric semiconductor remains challenging. Here, inspired by point‐defect‐mediated large piezoelectricity in ferroelectrics especially at the morphotropic phase boundary (MPB) region, an efficient strategy is proposed by judiciously introducing the gap states at the MPB where defect‐induced local polar heterogeneities are thermodynamically coupled with the host polarization to simultaneously achieve high piezoelectricity and low bandgap. A concrete example, Ni²⁺‐mediated (1–x)Na_0.5Bi_0.5TiO₃‐xBa(Ti_0.5Ni_0.5)O_3–δ (x = 0.02–0.08) composition is presented, which can show excellent piezoelectricity and unprecedented visible/near‐infrared light absorption with a lowest ever bandgap ≈0.9 eV at room temperature. In particular, the MPB composition x = 0.05 shows the best ferroelectricity/piezoelectricity (d₃₃ = 151 pC N^–1, Pr = 31.2 μC cm^–2) and a largely enhanced photocurrent density approximately two orders of magnitude higher compared with classic ferroelectric (Pb,La)(Zr,Ti)O₃. This research provides a new paradigm for designing highly piezoelectric and visible/near‐infrared photoresponsive perovskite oxides for solar energy conversion, near‐infrared detection, and other multifunctional applications.     

Enhanced visible light activity and mechanism of TiO2 codoped with molybdenum and nitrogen

Mo-N-codoped TiO₂ was synthesized by using ammonium molybdate tetrahydrate and ammonia water as the sources of Mo and N, respectively. The resulting materials were characterized by X-ray diffraction (XRD), X-photoelectron spectroscopy (XPS) and UV–vis light diffuse reflection spectroscopy (DRS). Furthermore, the activity enhanced-mechanism was proposed. XRD results indicated that codoping favored the formation of anatase and improved the anatase crystallinity. XPS analysis revealed that N was incorporated into the lattice of TiO₂ through substituting lattice O and coexisted in the substitutional forms. Meanwhile, Mo was incorporated into the lattice of TiO₂ through substituting Ti and coexisted in the forms of Mo⁶⁺ and Mo⁵⁺. DRS showed that the light absorption in visible region was improved by co-doping, leading to a narrower band gap and higher visible activity for the degradation of phenol than that of others. The enhanced activity was attributed to the high anatase crystallinity, large amount of surface oxygen vacancies, intense light absorption and narrow band gap.     

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.     

Noninvasively Modifying Band Structures of Wide‐Bandgap Metal Oxides to Boost Photocatalytic Activity

Although doping with appropriate heteroatoms is a powerful way of increasing visible light absorption of wide‐bandgap metal oxide photocatalysts, the incorporation of heteroatoms into the photocatalysts usually leads to the increase of deleterious recombination centers of photogenerated charge carriers. Here, a conceptual strategy of increasing visible light absorption without causing additional recombination centers by constructing an ultrathin insulating heterolayer of amorphous boron oxynitride on wide‐bandgap photocatalysts is shown. The nature of this strategy is that the active composition nitrogen in the heterolayer can noninvasively modify the electronic structure of metal oxides for visible light absorption through the interface contact between the heterolayer and metal oxides. The photocatalysts developed show significant improvements in photocatalytic activity under both UV–vis and visible light irradiation compared to the doped counterparts by conventional doping process. These results may provide opportunities for flexibly tailoring the electronic structure of metal oxides.     

Environmental Hazard Potential of Nano‐Photocatalysts Determined by Nano‐Bio Interactions and Exposure Conditions

Nano‐photocatalysts are known for their ability to degrade pollutants or perform water splitting catalyzed by light. Being the key functional ingredients of current and future products, the potential of nano‐photocatalysts releasing into the environment and causing unintended harm to living organisms warrants investigation. Risk assessment of these materials serves as an important step to allow safe implementation and to avoid irrational fear. Using TiO₂ and g‐C₃N₄ as representative nano‐photocatalysts, this study evaluates their hazard potential in zebrafish. Under simulated solar light, nano‐photocatalysts up to 100 mg L^?1 show no acute toxicity to zebrafish embryos due to the protection of chorions. The short‐lived reactive oxygen species generated by nano‐photocatalysts only exert injury to the hatched larvae at and above 50 mg L^?1. The input of solar energy, determined by the depth of water, irradiation time, and light intensity, greatly influences the toxicity outcome. Increasing concentrations of natural organic matters contribute positively to the hazard potential at 0–10 mg L^?1 while gradually diminishing the hazardous effect above 10 mg L^?1. This study demonstrates the importance of nano‐bio interactions and environmental exposure conditions in determining the safety profile of nano‐photocatalysts.     

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.     

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.     

ZnFe2O4 Leaves Grown on TiO2 Trees Enhance Photoelectrochemical Water Splitting

TiO₂ has excellent electrochemical properties but limited solar photocatalytic performance in light of its large bandgap. One important class of visible‐wavelength sensitizers of TiO₂ is based on ZnFe₂O₄, which has shown fully a doubling in performance relative to pure TiO₂. Prior efforts on this important front have relied on presynthesized nanoparticles of ZnFe₂O₄ adsorbed on a TiO₂ support; however, these have not yet achieved the full potential of this system since they do not provide a consistently maximized area of the charge‐separating heterointerface per volume of sensitizing absorber. A novel atomic layer deposition (ALD)‐enhanced synthesis of sensitizing ZnFe₂O₄ leaves grown on the trunks of TiO₂ trees is reported. These new materials exhibit fully a threefold enhancement in photoelectrochemical performance in water splitting compared to pristine TiO₂ under visible illumination. The new materials synthesis strategy relies first on the selective growth of FeOOH nanosheets, 2D structures that shoot off from the sides of the TiO₂ trees; these templates are then converted to ZnFe₂O₄ with the aid of a novel ALD step, a strategy that preserves morphology while adding the Zn cation to achieve enhanced optical absorption and optimize the heterointerface band alignment.     

Efficient visible-light driven photocatalysts: coupling TiO<Subscript>2</Subscript>(AB) nanotubes with g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> nanoflakes

The wide application of the titanium dioxide (TiO₂) as the photocatalysts is greatly hindered by its intrinsic large band gap and usually fast electron–hole recombination. Here, we reported the exploration of coupling g-C₃N₄ nanoflakes to TiO₂ nanotubes with the anatase and TiO₂(B) mixed phases (TiO₂(AB)) toward the efficient visible-light-driven hybrid photocatalyst. It is found that coupling TiO₂(AB) nanotubes with g-C₃N₄ nanoflakes could bring a profoundly extension the visible light adsorption capacity and enhanced photogenerated carrier separation. Accordingly, they exhibit much higher efficient photocatalytic activities toward the degradation of sulforhodamine B under the visible light irradiation, which is enhanced for nearly 15 times to those of the TiO₂(AB) and g-C₃N₄, suggesting their promising practical applications as novel and efficient semiconductor photocatalysts for the water purification.     

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.     

Efficient Photocatalytic Hydrogen Evolution via Band Alignment Tailoring: Controllable Transition from Type‐I to Type‐II

Considering the sizable band gap and wide spectrum response of tin disulfide (SnS₂), ultrathin SnS₂ nanosheets are utilized as solar‐driven photocatalyst for water splitting. Designing a heterostructure based on SnS₂ is believed to boost their catalytic performance. Unfortunately, it has been quite challenging to explore a material with suitable band alignment using SnS₂ nanomaterials for photocatalytic hydrogen generation. Herein, a new strategy is used to systematically tailor the band alignment in SnS₂ based heterostructure to realize efficient H₂ production under sunlight. A Type‐I to Type‐II band alignment transition is demonstrated via introducing an interlayer of Ce₂S₃, a potential photocatalyst for H₂ evolution, between SnS₂ and CeO₂. Subsequently, this heterostructure demonstrates tunability in light absorption, charge transfer kinetics, and material stability. The optimized heterostructure (SnS₂–Ce₂S₃–CeO₂) exhibits an incredibly strong light absorption ranging from deep UV to infrared light. Significantly, it also shows superior hydrogen generation with the rate of 240 µmol g⁻¹ h⁻¹ under the illumination of simulated sunlight with a very good stability.     

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.