Protonation‐Assisted Exfoliation of N‐Containing 2D Conjugated Polymers

Ultrathin 2D conjugated polymer nanosheets are an emerging class of photocatalysts for solar‐to‐chemical energy conversion. Until now, the majority of ultrathin 2D polymer photocatalysts are produced through exfoliation of layered polymers. Unfortunately, it still remains a great challenge to exfoliate layered polymers into ultrathin nanosheets with high yields. In this work, a liquid‐phase protonation‐assisted exfoliation is demonstrated to enable remarkably improved exfoliation yields of various 2D N‐containing conjugated polymers such as g‐C₃N₄, C₂N, and aza‐CMP. The exfoliation yields are only 2–15% in pure water whereas they can be substantially improved to 41–56% in 12 m HCl. The exfoliated ultrathin nanosheets possess average thicknesses less than 5 nm and can be easily dispersed in aqueous solutions. More importantly, the exfoliated nanosheets exhibit significantly enhanced photocatalytic activity toward photocatalytic water splitting compared to their bulk counterparts. Further characterizations and computational calculations reveal that protonation of the heterocyclic nitrogen sites in the conjugated polymer frameworks can lead to strong hydrogen bonding between the polymer surfaces and water molecules, resulting in facilitated exfoliation of polymers into the liquid phase. This study unveils an important protocol toward producing ultrathin 2D N‐containing conjugated polymer nanosheets for future solar energy conversion.     

Mesoporous Polymeric Cyanamide‐Triazole‐Heptazine Photocatalysts for Highly‐Efficient Water Splitting

Conjugated polymers are promising light harvesters for water reduction/oxidation due to their simple synthesis and adjustable bandgap. Herein, both cyanamide and triazole functional groups are first incorporated into a heptazine‐based carbon nitride (CN) polymer, resulting in a mesoporous conjugated cyanamide‐triazole‐heptazine polymer (CTHP) with different compositions by increasing the quantity of cyanamide/triazole units in the CN backbone. Varying the compositions of CTHP modulates its electronic structures, mesoporous morphologies, and redox energies, resulting in a significantly improved photocatalytic performance for both H₂ and O₂ evolution under visible light irradiation. A remarkable H₂ evolution rate of 12723 µmol h^?1 g^?1 is observed, resulting in a high apparent quantum yield of 11.97% at 400 nm. In parallel, the optimized photocatalyst also exhibits an O₂ evolution rate of 221 µmol h^?1 g^?1, 9.6 times higher than the CN counterpart, with the value being the highest among the reported CN‐based bifunctional photocatalysts. This work provides an efficient molecular engineering approach for the rational design of functional polymeric photocatalysts.     

Hydrogen evolution from water splitting on nanocomposite photocatalysts

The photocatalytic production of H₂ in one step is potentially one of the most promising ways for the conversion and storage of solar energy. The paper overviews our recent studies on the photocatalysts splitting water into hydrogen under irradiation. The attention was mainly focused on the promotion effects of nanosized modifications in the interlayer and surface of photocatalysts for hydrogen evolution with visible light. The photocatalytic activity depended significantly on modification techniques, such as loading, proton exchange, and intercalation. The formation of a ‘‘nest’’ on the particle surface promoted a uniform distribution and strong combination of the nanosized particles on the surface of catalysts. By the methods of intercalation and pillaring as well as by selecting both host and guest, a large variety of molecular designed host–guest systems were obtained. Cadmium sulfide (CdS)-intercalated composites showed higher activity and stability. This activity of K₄Ce₂M₁₀O₃₀ (M = Ta, Nb) evolving H₂ under visible light irradiation was enhanced by the incorporation of Pt, RuO₂ and NiO as co-catalysts. Especially, the nanosized NiO_x (Ni–NiO double-layer structure) greatly prompted the photocatalytic H₂ evolution significantly.     

Amine Functionalized Metal–Organic Framework Coordinated with Transition Metal Ions: d–d Transition Enhanced Optical Absorption and Role of Transition Metal Sites on Solar Light Driven H2 Production

Design and development of efficient photocatalysts for H₂ production from water and sunlight have gained significant attention as the solar assisted approach is considered to be a promising approach for the generation of clean fuel. However, the poor charge carrier separation and light harvesting ability of existing photocatalysts limits the efficiency of photoconversion of water. In this work, the synthesis of transition metal ions (M²⁺ = Co²⁺, Cu²⁺, and Ni²⁺) coordinated with Ti‐metal organic frameworks (Ti‐MOFs) through a simple post‐synthetic coordination method for efficient solar light‐driven H₂ production is reported. Notably, coordination of M²⁺ ions with Ti‐MOF significantly improves the optical absorption by d–d transitions and the multimetal sites facilitate the fast charge carrier separation, thereby enhancing the solar light‐driven H₂ production activity. Very interestingly, the rate of solar light‐driven H₂ production is varied with respect to different metal ions coordination due to the position of d–d bands absorption in the solar spectrum, and the complexing tendency of M²⁺ ions with sacrificial electron donors. A maximum solar H₂ production rate of 1583.55 µmol h^?1 g^?1 is achieved with a Cu²⁺ coordinated Ti‐MOF, which is ≈13 fold higher than that of the pristine Ti‐MOF.     

Carbon nitride photocatalyst with internal electric field induced photogenerated carriers spatial enrichment for enhanced photocatalytic water splitting

Photocatalytic water splitting is a cost-effective way to convert sustainable solar energy into chemical energy. Among various photocatalytic systems, coupling the H₂- and O₂- evolving photocatalysts has been widely used in photocatalytic water splitting. However, due to the close spatial distance between surface electrons and surface holes, this heterogeneous material easily catalyzes the unwanted reverse reaction, limiting the solar energy conversion efficiency. Here we present a carbon nitride nanosheet (CNN) homojunction which possesses electrons-enriched region and holes-enriched region induced by the interfacial internal electric field. The reverse reactions are significantly suppressed by benefiting from the spatial separation of the oxidation (+2.21 V) and reduction (-1.19 V) regions. The homojunction exhibits efficient photocatalytic activity for H₂ and O₂ evolution (1270.5 and 36.0 μmol h⁻¹) with the scavenger. Meanwhile, the solar-to-hydrogen efficiency of overall water splitting was improved to 0.14%. This research provides a new way for semiconductor design in solar energy conversion applications.     

An Unusual Strong Visible‐Light Absorption Band in Red Anatase TiO2 Photocatalyst Induced by Atomic Hydrogen‐Occupied Oxygen Vacancies

Increasing visible light absorption of classic wide‐bandgap photocatalysts like TiO₂ has long been pursued in order to promote solar energy conversion. Modulating the composition and/or stoichiometry of these photocatalysts is essential to narrow their bandgap for a strong visible‐light absorption band. However, the bands obtained so far normally suffer from a low absorbance and/or narrow range. Herein, in contrast to the common tail‐like absorption band in hydrogen‐free oxygen‐deficient TiO₂, an unusual strong absorption band spanning the full spectrum of visible light is achieved in anatase TiO₂ by intentionally introducing atomic hydrogen‐mediated oxygen vacancies. Combining experimental characterizations with theoretical calculations reveals the excitation of a new subvalence band associated with atomic hydrogen filled oxygen vacancies as the origin of such band, which subsequently leads to active photo‐electrochemical water oxidation under visible light. These findings could provide a powerful way of tailoring wide‐bandgap semiconductors to fully capture solar light.     

Poly(1,4‐Diethynylbenzene) Gradient Homojunction with Enhanced Charge Carrier Separation for Photoelectrochemical Water Reduction

As appealing photoelectrode materials for photoeletrochemical hydrogen evolution reaction (PEC HER), conjugated polymers still show poor PEC HER performance as a result of their serious recombination of photogenerated electrons and holes. Herein, a novel design of gradient homojunction is demonstrated by controlled copolymerization of 1,4‐diethynylbenzene (DEB) and 1,3,5‐triethynylbenzene (TEB). The as‐built gradient distribution of TEB monomer in poly(1,4‐diethynylbenzene) (pDEB) leads to continuous band bending engineering, which constitutes a gradient homojunction. Under AM 1.5G irradiation and in 0.1 m Na₂SO₄ aqueous solution, the as‐fabricated pDEB gradient homojunction exhibits a charge separation efficiency of 0.27% at 0.3 V versus reversible hydrogen electrode (RHE), which is 3.4 and 1.7 times higher than those for pure pDEB and the traditionally designed pDEB homojunction. As a result, the photocurrent of the pDEB gradient homojunction unprecedentedly reaches 55 µA cm^?2 at 0.3 V versus RHE, which is much higher than 19 µA cm^?2 for pure pDEB, 32 µA cm^?2 for the pDEB homojunction, and state‐of‐the‐art organic photocathodes, e.g., g‐C₃N₄ (≈1?32 µA cm^?2). This work opens up a new window for the design of gradient homojunctions and will advance the exploration of high‐performance organic photoelectrodes.     

Black Phosphorus Revisited: A Missing Metal‐Free Elemental Photocatalyst for Visible Light Hydrogen Evolution

Metal‐free elemental photocatalysts for hydrogen (H₂) evolution are more advantageous than the traditional metal‐based inorganic photocatalysts since the nonmetal elements are generally cheaper, more earth‐abundant, and environmentally friendly. Black phosphorus (BP) has been attracting increasing attention in recent years based on its anisotropic 2D layered structure with tunable bandgap in the range of 0.3–2.0 eV; however, the application of BP for photocatalytic H₂ evolution has been scarcely reported experimentally although being theoretically predicted. Herein, for the first time, the visible light photocatalytic H₂ evolution of BP nanosheets prepared via a facile solid‐state mechanochemical method by ball‐milling bulk BP is reported. Without using any noble metal cocatalyst, the visible light photocatalytic hydrogen evolution rate of BP nanosheets reaches 512 µmol h^?1 g^?1, which is ≈18 times higher than that of the bulk BP, and is comparable or even higher than that of graphitic carbon nitrides (g‐C₃N₄).     

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.     

Structural, photophysical and photocatalytic properties of Bi2MTaO7 (M = La and Y)

Bi₂MTaO₇ (M = Y and La) were synthesized by solid-state reaction method and their structural and photocatalytic properties were investigated. The results indicated that these compounds crystallize in the pyrochlore-type structure, cubic system with space group Fd-3 m. In addition, the band gaps of Bi₂LaTaO₇ and Bi₂YTaO₇ were estimated to be about 2.17(3) and 2.22(7) eV, respectively. For the photocatalytic water splitting reaction, H₂ or O₂ evolution was observed from pure water respectively with Bi₂MTaO₇ (M = Y and La) as the photocatalysts under ultraviolet light irradiation. Photocatalytic degradation of aqueous methylene blue (MB) dye over these compounds was further investigated under visible light irradiation. Bi₂MTaO₇ (M = Y and La) showed markedly higher catalytic activity compared to P-25 for MB photocatalytic degradation under visible light irradiation. Complete removal of aqueous MB was observed after visible light irradiation for 190 min with Bi₂LaTaO₇ as the photocatalyst and for 200 min with Bi₂YTaO₇ as the photocatalyst. The decrease of the total organic carbon (TOC) and the formation of inorganic products, SO₄²⁻ and NO₃⁻, demonstrated the continuous mineralization of aqueous MB during the photocatalytic process.     

From UV to Near‐Infrared Light‐Responsive Metal–Organic Framework Composites: Plasmon and Upconversion Enhanced Photocatalysis

The exploitation of photocatalysts that harvest solar spectrum as broad as possible remains a high‐priority target yet grand challenge. In this work, for the first time, metal–organic framework (MOF) composites are rationally fabricated to achieve broadband spectral response from UV to near‐infrared (NIR) region. In the core–shell structured upconversion nanoparticles (UCNPs)‐Pt@MOF/Au composites, the MOF is responsive to UV and a bit visible light, the plasmonic Au nanoparticles (NPs) accept visible light, whereas the UCNPs absorb NIR light to emit UV and visible light that are harvested by the MOF and Au once again. Moreover, the MOF not only facilitates the generation of “bare and clean” Au NPs on its surface and realizes the spatial separation for the Au and Pt NPs, but also provides necessary access for catalytic substrates/products to Pt active sites. As a result, the optimized composite exhibits excellent photocatalytic hydrogen production activity (280 µmol g^?1 h^?1) under simulated solar light, and the involved mechanism of photocatalytic H₂ production under UV, visible, and NIR irradiation is elucidated. Reportedly, this is an extremely rare study on photocatalytic H₂ production by light harvesting in all UV, visible, and NIR regions.     

Facile preparation and enhanced photocatalytic H2-production activity of Cu(OH)2 nanospheres modified porous g-C3N4

A facile precipitation approach for the preparation of Cu(OH)₂/g-C₃N₄ composite photocatalysts with good porous structure was developed for the first time. The as-synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible light (UV–vis) absorbance spectra, photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). A photocatalytic water splitting reaction on the as-prepared photocatalysts were carried out under visible light irradiation. The results revealed that the prepared samples showed significantly enhanced photocatalytic activity. The optimal Cu(OH)₂ loading content was found to be 0.34 mol%, giving an H₂-production rate of 48.7 μmol h⁻¹ g⁻¹, which is higher 16.5 times than that of pure g-C₃N₄. This high photocatalytic H₂-production activity is attributed to the presence of Cu(OH)₂ clusters on the surface of the porous g-C₃N₄, which efficiently promotes the visible light absorption and separation of photogenerated electron–hole pairs.     

Smart Utilization of Carbon Dots in Semiconductor Photocatalysis

Efficient capture of solar energy will be critical to meeting the energy needs of the future. Semiconductor photocatalysis is expected to make an important contribution in this regard, delivering both energy carriers (especially H₂) and valuable chemical feedstocks under direct sunlight. Over the past few years, carbon dots (CDs) have emerged as a promising new class of metal‐free photocatalyst, displaying semiconductor‐like photoelectric properties and showing excellent performance in a wide variety of photoelectrochemical and photocatalytic applications owing to their ease of synthesis, unique structure, adjustable composition, ease of surface functionalization, outstanding electron‐transfer efficiency and tunable light‐harvesting range (from deep UV to the near‐infrared). Here, recent advances in the rational design of CDs‐based photocatalysts are highlighted and their applications in photocatalytic environmental remediation, water splitting into hydrogen, CO₂ reduction, and organic synthesis are discussed.     

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.     

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.     

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.     

Extended visible light harvesting and boosted charge carrier dynamics in heterostructured zirconate–FeS<Subscript>2</Subscript> photocatalysts for efficient solar water splitting

Limited visible light absorption, slow charge transference, and high recombination are some of the main problems associated with low efficiency in photocatalytic processes. For these reasons, in the present work, we develope novel zirconate–FeS₂ heterostructured photocatalysts with improved visible light harvesting, effective charge separation and high photocatalytic water splitting performance. Herein, alkali and alkaline earth metal zirconates are prepared by a solid state reaction and coupled to FeS₂ through a simple wet impregnation method. The incorporation of FeS₂ particles induces visible light absorption and electron injection in zirconates, while the appropriate coupling of the semiconductors in the heterostructure allows an enhanced charge separation and suppression of the recombination. The obtained heterostructures exhibit high and stable photocatalytic activity for water splitting under visible light, showing competitive efficiencies among other reported materials. The highest hydrogen evolution rate (4490 µmol g^?1 h^?1) is shown for BaZrO₃–FeS₂ and corresponds to more than 20 times the activity of the bare BaZrO₃. In summary, this work proposes novel visible light active heterostructures for efficient visible light photocatalytic water splitting.     

Metal‐Complex/Semiconductor Hybrid Photocatalysts and Photoelectrodes for CO2 Reduction Driven by Visible Light

CO₂ reduction to carbon feedstocks using heterogeneous photocatalysts is an attractive means of addressing both climate change and the depletion of fossil fuels. Of particular importance is the development of a photosystem capable of functioning in response to visible light, which accounts for the majority of the solar spectrum, representing a kind of artificial photosynthesis. Hybrid systems comprising a metal complex and a semiconductor are promising because of the excellent electrochemical (and/or photocatalytic) activity of metal complexes during CO₂ reduction and the ability of semiconductors to efficiently oxidize water to molecular O₂. Here, the development of hybrid photocatalysts and photoelectrodes for CO₂ reduction in combination with water oxidation is described.     

An Earth‐Abundant Catalyst‐Based Seawater Photoelectrolysis System with 17.9% Solar‐to‐Hydrogen Efficiency

The implementation of water splitting systems, powered by sustainable energy resources, appears to be an attractive strategy for producing high‐purity H₂ in the absence of the release of carbon dioxide (CO₂). However, the high cost, impractical operating conditions, and unsatisfactory efficiency and stability of conventional methods restrain their large‐scale development. Seawater covers 70% of the Earth's surface and is one of the most abundant natural resources on the planet. New research is looking into the possibility of using seawater to produce hydrogen through electrolysis and will provide remarkable insight into sustainable H₂ production, if successful. Here, guided by density functional theory (DFT) calculations to predict the selectivity of gas‐evolving catalysts, a seawater‐splitting device equipped with affordable state‐of‐the‐art electrocatalysts composed of earth‐abundant elements (Fe, Co, Ni, and Mo) is demonstrated. This device shows excellent durability and specific selectivity toward the oxygen evolution reaction in seawater with near 100% Faradaic efficiency for the production of H₂ and O₂. Powered by a single commercial III–V triple‐junction photovoltaic cell, the integrated system achieves spontaneous and efficient generation of high‐purity H₂ and O₂ from seawater at neutral pH with a remarkable 17.9% solar‐to‐hydrogen efficiency.     

Metallic Cobalt–Carbon Composite as Recyclable and Robust Magnetic Photocatalyst for Efficient CO2 Reduction

CO₂ conversion into value‐added chemical fuels driven by solar energy is an intriguing approach to address the current and future demand of energy supply. Currently, most reported surface‐sensitized heterogeneous photocatalysts present poor activity and selectivity under visible light irradiation. Here, photosensitized porous metallic and magnetic 1200 Co C composites (PMMCoCC‐1200) are coupled with a [Ru(bpy)₃]Cl₂ photosensitizer to efficiently reduce CO₂ under visible‐light irradiation in a selective and sustainable way. As a result, the CO production reaches a high yield of 1258.30 µL with selectivity of 64.21% in 6 h, superior to most reported heterogeneous photocatalysts. Systematic investigation demonstrates that the central metal cobalt is the active site for activating the adsorbed CO₂ molecules and the surficial graphite carbon coating on cobalt metal is crucial for transferring the electrons from the triplet metal‐to‐ligand charge transfer of the photosensitizer Ru(bpy)₃²⁺, which gives rise to significant enhancement for CO₂ reduction efficiency. The fast electron injection from the excited Ru(bpy)₃²⁺ to PMMCoCC‐1200 and the slow backward charge recombination result in a long‐lived, charge‐separated state for CO₂ reduction. More impressively, the long‐time stability and easy magnetic recycling ability of this metallic photocatalyst offer more benefits to the photocatalytic field.