Boosting Visible‐Light‐Driven Photocatalytic Hydrogen Evolution with an Integrated Nickel Phosphide–Carbon Nitride System

Small variation induces a big difference: the effect of polymerization kinetics of graphitic carbon nitride on its photocatalytic activity

Graphitic carbon nitride (g-CN) has emerged as a promising visible-light-responsive photocatalyst, and its activity is highly sensitive to synthesis conditions. In this work, we attempt to correlate the photocatalytic activity of g-CN with its production yield, which is kinetically determined by the specific condensation process. Bulk g-CN samples were synthesized by the conventional condensation procedure, but in static air and flowing air, respectively. The one synthesized in static air showed a lower production yield with an increased specific surface area and preferential surface chemical states, corresponding to a significantly improved activity for photocatalytic hydrogen evolution (PHE) and dye degradation. We further synthesized a series of g-CN samples by merely changing the synthetic atmosphere, the ramping rate, and the loading amount of the precursor, and the difference in their PHE performance was found to be as high as 7.05 times. The notable changes in their production yields as well as the photocatalytic activities have been discussed from the point of view of polymerization reaction kinetics, and the self-generated NH3 atmosphere plays a crucial role.

Photothermally driven decoupling of gas evolution at the solid-liquid interface for boosted photocatalytic hydrogen production

The slow mass transfer, especially the gas evolution process at the solid-liquid interface in photocatalytic water splitting, restricts the overall efficiency of the hydrogen evolution reaction. Here, we report a novel gas-solid photocatalytic reaction system by decoupling hydrogen generation from a traditional solid-liquid interface. The success relies on annealing commercial melamine sponge (AMS) for effective photothermal conversion that leads to rapid water evaporation. The vapor flows towards the photocatalyst covering the surface of the AMS and is split by the catalyst therein. This liquid-gas/gas-solid coupling system avoids the formation of photocatalytic bubbles at the solid-liquid interface, leading to significantly improved mass transfer and conversion. Utilizing CdS nanorods anchored by highly dispersed nickel atoms/clusters as a model photocatalyst, the highest hydrogen evolution rate from water splitting reaches 686.39 μmol h-1, which is 5.31 times that of the traditional solid-liquid-gas triphase system. The solar-to-hydrogen (STH) efficiency can be up to 2.06%. This study provides a new idea for the design and construction of efficient practical photocatalytic systems.

Ultrafast dynamics in polymeric carbon nitride thin films probed by time-resolved EUV photoemission and UV-Vis transient absorption spectroscopy

The ground- and excited-state electronic structures of four polymeric carbon nitride (PCN) materials have been investigated using a combination of photoemission and optical absorption spectroscopy. To establish the driving forces for photocatalytic water-splitting reactions, the ground-state data was used to produce a band diagram of the PCN materials and the triethanolamine electron scavenger, commonly implemented in water-splitting devices. The ultrafast charge-carrier dynamics of the same PCN materials were also investigated using two femtosecond-time-resolved pump-probe techniques: extreme-ultraviolet (EUV) photoemission and ultraviolet-visible (UV-Vis) transient absorption spectroscopy. The complementary combination of these surface- and bulk-sensitive methods facilitated photoinduced kinetic measurements spanning the sub-picosecond to few nanosecond time range. The results show that 400 nm (3.1 eV) excitation sequentially populates a pair of short-lived transient species, which subsequently produce two different long-lived excited states on a sub-picosecond time scale. Based on the spectro-temporal characteristics of the long-lived signals, they are assigned to singlet-exciton and charge-transfer states. The associated charge-separation efficiency was inferred to be between 65% and 78% for the different studied materials. A comparison of results from differently synthesized PCNs revealed that the early-time processes do not differ qualitatively between sample batches, but that materials of more voluminous character tend to have higher charge separation efficiencies, compared to exfoliated colloidal materials. This finding was corroborated via a series of experiments that revealed an absence of any pump-fluence dependence of the initial excited-state decay kinetics and characteristic carrier-concentration effects that emerge beyond few-picosecond timescales. The initial dynamics of the photoinduced charge carriers in the PCNs are correspondingly determined to be spatially localised in the immediate vicinity of the lattice-constituting motif, while the long-time behaviour is dominated by charge-transport and recombination processes. Suppressing the latter by confining excited species within nanoscale volumes should therefore affect the usability of PCN materials in photocatalytic devices.

Chemoenzymatic Photoreforming: A Sustainable Approach for Solar Fuel Generation from Plastic Feedstocks

Plastic upcycling through catalytic transformations is an attractive concept to valorize waste, but the clean and energy-efficient production of high-value products from plastics remains challenging. Here, we introduce chemoenzymatic photoreforming as a process coupling enzymatic pretreatment and solar-driven reforming of polyester plastics under mild temperatures and pH to produce clean H2 and value-added chemicals. Chemoenzymatic photoreforming demonstrates versatility in upcycling polyester films and nanoplastics to produce H2 at high yields reaching ∼103-104 μmol gsub-1 and activities at >500 μmol gcat-1 h-1. Enzyme-treated plastics were also used as electron donors for photocatalytic CO2-to-syngas conversion with a phosphonated cobalt bis(terpyridine) catalyst immobilized on TiO2 nanoparticles (TiO2|CotpyP). Finally, techno-economic analyses reveal that the chemoenzymatic photoreforming approach has the potential to drastically reduce H2 production costs to levels comparable to market prices of H2 produced from fossil fuels while maintaining low CO2-equivalent emissions.

Noble metal-free ternary cobalt-nickel phosphides for enhanced photocatalytic dye-sensitized hydrogen evolution and catalytic mechanism investigation

Transition metal phosphides have emerged as compelling alternatives to noble metal catalysts for photocatalytic hydrogen evolution, owing to their high efficiency, stability, ease of preparation, and low-cost-effectiveness. This study investigates a series of binary and ternary phosphides predominantly composed of cobalt and nickel employed for photocatalytic dye-sensitized hydrogen evolution. Under the optimal dye-to-catalyst mass ratio, CoNiP exhibited the highest hydrogen evolution activity (12.96 mmol g-1 h-1), demonstrating more significant and satisfactory performance than a variety of other reported materials. This can be attributed to the high conductivity and low hydrogen evolution overpotential of phosphides, which result from their metallic characteristics and the presence of free electrons, which promote efficient electron transfer between the catalyst and sensitizer. Density functional theory calculations revealed that the cobalt incorporation into the binary phosphides causes a negative shift in the average d-band center for CoNiP, weakening the adsorption affinity of the catalyst towards H2 molecules, thus effectively improving the hydrogen evolution rate compared to the pure binary phosphides. This work provides valuable insights for the development of low-cost and high-performance ternary phosphide photocatalysts.

V-doped Ni2P nanoparticle grafted g-C3N4 nanosheets for enhanced photocatalytic hydrogen evolution performance under visible light

Exploring low-cost and highly active photocatalysts with noble metal-free cocatalysts is of great significance for photocatalytic hydrogen evolution under simulated sunlight irradiation. In this work, a novel V-doped Ni₂P nanoparticle loaded g-C₃N₄ nanosheet is reported as a highly efficient photocatalyst for H₂ evolution under visible light irradiation. The results demonstrate that the optimized 7.8 wt% V-Ni₂P/g-C₃N₄ photocatalyst exhibits a high hydrogen evolution rate of 271.5 μmol g^-1 h^-1, which is comparable to that of the 1 wt% Pt/g-C₃N₄ photocatalyst (279 μmol g^-1 h^-1), and shows favorable hydrogen evolution stability for five successive runs within 20 h. The remarkable photocatalytic hydrogen evolution performance of V-Ni₂P/g-C₃N₄ is mainly due to the enhanced visible light absorption ability, the facilitated separation of photo-generated electron-hole pairs, the prolonged lifetime of photo-generated carriers and the fast transmission ability of electrons.

Floating Carbon Nitride Composites for Practical Solar Reforming of Pre-Treated Wastes to Hydrogen Gas

Solar reforming (SR) is a promising green-energy technology that can use sunlight to mitigate biomass and plastic waste while producing hydrogen gas at ambient pressure and temperature. However, practical challenges, including photocatalyst lifetime, recyclability, and low production rates in turbid waste suspensions, limit SR's industrial potential. By immobilizing SR catalyst materials (carbon nitride/platinum; CN_x |Pt and carbon nitride/nickel phosphide; CN_x |Ni₂ P) on hollow glass microspheres (HGM), which act as floating supports enabling practical composite recycling, such limitations can be overcome. Substrates derived from plastic and biomass, including poly(ethylene terephthalate) (PET) and cellulose, are reformed by floating SR composites, which are reused for up to ten consecutive cycles under realistic, vertical simulated solar irradiation (AM1.5G), reaching activities of 1333 ± 240 µmol_H2 m^-2 h^-1 on pre-treated PET. Floating SR composites are also advantageous in realistic waste where turbidity prevents light absorption by non-floating catalyst powders, achieving 338.1 ± 1.1 µmol_H2 m^-2 h^-1 using floating CN_x versus non-detectable H₂ production with non-floating CN_x and a pre-treated PET bottle as substrate. Low Pt loadings (0.033 ± 0.0013% m/m) demonstrate consistent performance and recyclability, allowing efficient use of precious metals for SR hydrogen production from waste substrates at large areal scale (217 cm² ), taking an important step toward practical SR implementation.

Design and application of g-C3N4-based materials for fuels photosynthesis from CO2 or H2O based on reaction pathway insights

Photocatalytic CO2 reduction reaction (CRR) and hydrogen evolution reaction (HER) based on graphitic carbon nitride (g-C3N4) that is regarded as the metal-free "holy grail" photocatalyst, provide promising strategies for producing next-generation fuels, contributing to achieving carbon neutrality, alleviating energy and environment crisis. However, the activity of CRR and HER over g-C3N4 leaves much to be desired. Therefore, numerous studies have sprung up to enhance photoactivity. A comprehensive understanding of the CRR and HER reaction pathways is crucial for designing g-C3N4-based materials, further promoting efficient fuel production. Different from previous reviews that focus on g-C3N4 modification from the viewpoint of material science. In this review, we divided the multistep processes of CRR and HER into five reaction pathways and summarized the latest advances for improving each pathway of fuels synthesis through CRR or HER. Meanwhile, the existing bottleneck issues of each step were also discussed. Finally, comprehensive conclusions, including the remaining challenges, outlooks, etc., for CRR and HER over g-C3N4 were put forward. We are sure that this review will conduce to the understanding of the structure-activity relationship between CRR, HER processes, and g-C3N4 structure, which can provide the reference for developing high-powered photocatalysts, not confined to g-C3N4.

Spark Plasma Sintering-Assisted Synthesis of Bi2Fe4O9/Bi25FeO40 Heterostructures with Enhanced Photocatalytic Activity for Removal of Antibiotics

Bismuth ferrite-based heterojunction composites have been considered as promising visible-light responsive photocatalysts because of their narrow band gap structure; however, the synthetic methods reported in the literature were usually time-consuming. In this study, we report a facile and quick preparation of bismuth ferrite-based composites by the hydrothermal method, combined with spark plasma sintering (SPS), a technique that is usually used for the high-speed consolidation of powders. The result demonstrated that the SPS-assisted synthesized samples possess significant enhanced photoelectric and photocatalytic performance. Specifically, the SPS650 (sintered at the 650 °C for 5 min by SPS) exhibits a 1.5 times enhancement in the photocurrent density and a 3.8 times enhancement in the tetracycline hydrochloride photodegradation activity than the unmodified bismuth ferrite samples. The possible influence factors of SPS on photoelectric and photocatalytic performance of bismuth ferrite-based composites were discussed carefully. This study provides a feasible method for the facile and quick synthesis of a highly active bismuth ferrite-based visible-light-driven photocatalyst for practical applications.

Intrinsic Lability of NiMoO4 to Excel the Oxygen Evolution Reaction

Nickel-based bimetallic oxides such as NiMoO₄ and NiWO₄, when deposited on the electrode substrate, show remarkable activity toward the electrocatalytic oxygen evolution reaction (OER). The stability of such nanostructures is nevertheless speculative, and catalytically active species have been less explored. Herein, NiMoO₄ nanorods and NiWO₄ nanoparticles are prepared via a solvothermal route and deposited on nickel foam (NF) (NiMoO₄/NF and NiWO₄/NF). After ensuring the chemical and structural integrity of the catalysts on electrodes, an OER study has been performed in the alkaline medium. After a few cyclic voltammetry (CV) cycles within the potential window of 1.0-1.9 V (vs reversible hydrogen electrode (RHE)), ex situ Raman analysis of the electrodes infers the formation of NiO(OH)_ED (ED: electrochemically derived) from NiMoO₄ precatalyst, while NiWO₄ remains stable. A controlled study, stirring of NiMoO₄/NF in 1 M KOH without applied potential, confirms that NiMoO₄ hydrolyzes to the isolable NiO, which under a potential bias converts into NiO(OH)_ED. Perhaps the more ionic character of the Ni-O-Mo bond in the NiMoO₄ compared to the Ni-O-W bond in NiWO₄ causes the transformation of NiMoO₄ into NiO(OH)_ED. A comparison of the OER performance of electrochemically derived NiO(OH)_ED, NiWO₄, ex-situ-prepared Ni(OH)₂, and NiO(OH) confirmed that in-situ-prepared NiO(OH)_ED remained superior with a substantial potential of 238 (±6) mV at 20 mA cm^-2. The notable electrochemical performance of NiO(OH)_ED can be attributed to its low Tafel slope value (26 mV dec^-1), high double-layer capacitance (C_dl, 1.21 mF cm^-2), and a low charge-transfer resistance (R_ct, 1.76 Ω). The NiO(OH)_ED/NF can further be fabricated as a durable OER anode to deliver a high current density of 25-100 mA cm^-2. Post-characterization of the anode proves the structural integrity of NiO(OH)_ED even after 12 h of chronoamperometry at 1.595 V (vs reversible hydrogen electrode (RHE)). The NiO(OH)_ED/NF can be a compatible anode to construct an overall water splitting (OWS) electrolyzer that can operate at a cell potential of 1.64 V to reach a current density of 10 mA cm^-2. Similar to that on NF, NiMoO₄ deposited on iron foam (IF) and carbon cloth (CC) also electrochemically converts into NiO(OH) to perform a similar OER activity. This work understandably demonstrates monoclinic NiMoO₄ to be an inherently unstable electro(pre)catalyst, and its structural evolution to polycrystalline NiO(OH)_ED succeeding the NiO phase is intrinsic to its superior activity.

Charge Separation by Creating Band Bending in Metal-Organic Frameworks for Improved Photocatalytic Hydrogen Evolution

Metal-organic frameworks (MOFs) have been intensively studied as a class of semiconductor-like materials in photocatalysis. However, band bending, which plays a crucial role in semiconductor photocatalysis, has not yet been demonstrated in MOF photocatalysts. Herein, a representative MOF, MIL-125-NH₂ , is integrated with the metal oxides (MoO₃ and V₂ O₅ ) that feature appropriate work functions and energy levels to afford the corresponding MOF composites. Surface photovoltage results demonstrate band bending in the MOF composites, which gives rise to the built-in electric field of MIL-125-NH₂ , boosting the charge separation. As a result, the MOF composites present 56 and 42 times higher activities, respectively, compared to the pristine MOF for photocatalytic H₂ production. Upon depositing Pt onto the MOF, ∼6 times higher activity is achieved. This work illustrates band bending of MOFs for the first time, supporting their semiconductor-like nature, which would greatly promote MOF photocatalysis.

Mechanism investigation for ultra-efficient photocatalytic water disinfection based on rational design of indirect Z-scheme heterojunction black phosphorus QDs/Cu2O nanoparticles

Photocatalysis has been regarded as a promising inactivation technology targeting to reduce drug-resistant bacteria contamination, but developing efficient photocatalysts with broad visible light harvesting capability is still a challenge. Here we report a MOFs-derived BPQDs/Cu₂O/N-doped hollow porous carbon (BP/CNC) with indirect Z-scheme heterojunctions (BPQDs/Cu₂O), which can inactivate 99.99999% Methicillin-resistant Staphylococcus aureus (MRSA) at a concentration of only 10 mg/L. Combining photoelectrochemical techniques and electrochemical measurements, the efficient inactivation process was attributed to the synergistic effect of enhanced light utilization and effective suppression of photogenerated carrier recombination. The mechanism of gradually damaged cell membrane for MRSA was studied by employing scanning electron microscopy (SEM), fluorescence staining and coagulase titer test to further decipher the changes in bacterial cells. We propose that reactive oxygen species (ROS) destroys the cell wall membrane and causes the leakage of cell contents, eventually leading to death. In addition, a series of in vitro and in vivo toxicity tests were conducted to evaluate the biocompatibility of the antibacterial system and its potential use in practice. This strategy of BPQDs/Cu₂O indirect heterojunction fabrication can spatially inhibit the recombination of photogenerated carriers, expands the light absorption range, providing a feasible method for disinfecting microbial contaminated water.

Development and Functionalization of Visible-Light-Driven Water-Splitting Photocatalysts

With global warming and the depletion of fossil resources, our fossil fuel-dependent society is expected to shift to one that instead uses hydrogen (H₂) as a clean and renewable energy. To realize this, the photocatalytic water-splitting reaction, which produces H₂ from water and solar energy through photocatalysis, has attracted much attention. However, for practical use, the functionality of water-splitting photocatalysts must be further improved to efficiently absorb visible (Vis) light, which accounts for the majority of sunlight. Considering the mechanism of water-splitting photocatalysis, researchers in the various fields must be employed in this type of study to achieve this. However, for researchers in fields other than catalytic chemistry, ceramic (semiconductor) materials chemistry, and electrochemistry to participate in this field, new reviews that summarize previous reports on water-splitting photocatalysis seem to be needed. Therefore, in this review, we summarize recent studies on the development and functionalization of Vis-light-driven water-splitting photocatalysts. Through this summary, we aim to share current technology and future challenges with readers in the various fields and help expedite the practical application of Vis-light-driven water-splitting photocatalysts.

Synergistic poly(lactic acid) photoreforming and H2 generation over ternary NixCo1-xP/reduced graphene oxide/g-C3N4 composite

Effective utilization of photoexcited electrons and holes is always a challenge in photocatalytic reactions. Herein, we reported ternary Ni_xCo_1-xP/reduced graphene oxide/g-C₃N₄ (Ni_xCo_1-xP/rGO/CN) composite as a photocatalyst for synergistic poly(lactic acid) photoreforming and H₂ generation in alkaline aqueous solution. The rate of H₂ production over the optimal 15Ni_0·1Co_0·9P/rGO/CN reached 576.7 μmol h^-1 g^-1, which is 3.6 times as high as binary 15Ni_0·1Co_0·9P/CN composite. The apparent quantum efficiency of the optimal 15Ni_0·1Co_0·9P/rGO/CN was 1.7% at λ = 420 nm monochromatic light. Mott-Schottky analysis suggested that the photogenerated electrons transfer along the pathway of CN→rGO→Ni_0·1Co_0·9P, where rGO and Ni_0·1Co_0·9P functioned as the medium for electron transporting and reaction site for H₂ generation, respectively. Meanwhile, poly(lactic acid) was photoreformed into formate and acetate by the photogenerated holes and hydroxyl radical. This work demonstrates that ternary Ni_xCo_1-xP/rGO/CN composite can be applied as a cheap and promising photocatalyst for synergistic plastic photoreforming and H₂ generation.

Synthesis of atomic form nickel co-catalysts on TiO2 for improved photocatalysis by RAFT technique

Using the high incompatibility between the neutral stabilizer body and the block polyelectrolyte to drive phase separation during polymerization-induced self-assembly, a modular approach to systematically adjust ionic monomer/polymer solubility was...

Robust visible-light photocatalytic H2 evolution on 2D RGO/Cd0.15Zn0.85In2S4–Ni2P hierarchitectures

Unique 2D ternary hierarchitectures constructed from reduced graphene oxide nanosheets grown with ultrathin Cd0.15Zn0.85In2S4 nanosheets and Ni2P nanoparticles exhibited an outstanding capability for visible-light photocatalytic H2 production.

Development of Ferromagnetic Materials Containing Co2P, Fe2P Phases from Organometallic Dendrimers Precursors

The development of synthesis methods to access advanced materials, such as magnetic materials that combine multimetallic phosphide phases, remains a worthy research challenge. The most widely used strategies for the synthesis of magnetic transition metal phosphides (TMPs) are organometallic approaches. In this study, Fe-containing homometallic dendrimers and Fe/Co-containing heterometallic dendrimers were used to synthesize magnetic materials containing multimetallic phosphide phases. The crystalline nature of the nearly aggregated particles was indicated for both designed magnetic samples. In contrast to heterometallic samples, homometallic samples showed dendritic effects on their magnetic properties. Specifically, saturation magnetization (M_s) and coercivity (H_c) decrease as dendritic generation increases. Incorporating cobalt into the homometallic dendrimers to prepare the heterometallic dendrimers markedly increases the magnetic properties of the magnetic materials from 60 to 75 emu/g. Ferromagnetism in homometallic and heterometallic particles shows different responses to temperature changes. For example, heterometallic samples were less sensitive to temperature changes due to the presence of Co₂P in contrast to the homometallic ones, which show an abrupt change in their slopes at a temperature close to 209 K, which appears to be related to the Fe₂P ratios. This study presents dendrimers as a new type of precursor for the assembly of magnetic materials containing a mixture of iron- and cobalt-phosphides phases with tunable magnetism, and provides an opportunity to understand magnetism in such materials.

Interfacial Electronic Effects in Co@N-Doped Carbon Shells Heterojunction Catalyst for Semi-Hydrogenation of Phenylacetylene

Metal-supported catalyst with high activity and relatively simple preparation method is given priority to industrial production. In this work, this study reported an easily accessible synthesis strategy to prepare Mott-Schottky-type N-doped carbon encapsulated metallic Co (Co@N_p+gC) catalyst by high-temperature pyrolysis method in which carbon nitride (g-C₃N₄) and dopamine were used as support and nitrogen source. The prepared Co@N_p+gC presented a Mott-Schottky effect; that is, a strong electronic interaction of metallic Co and N-doped carbon shell was constructed to lead to the generation of Mott-Schottky contact. The metallic Co, due to high work function as compared to that of N-doped carbon, transferred electrons to the N-doped outer shell, forming a new contact interface. In this interface area, the positive and negative charges were redistributed, and the catalytic hydrogenation mainly occurred in the area of active charges. The Co@N_p+gC catalyst showed excellent catalytic activity in the hydrogenation of phenylacetylene to styrene, and the selectivity of styrene reached 82.4%, much higher than those of reference catalysts. The reason for the promoted semi-hydrogenation of phenylacetylene was attributed to the electron transfer of metallic Co, as it was caused by N doping on carbon.

Emerging Cocatalysts on g-C3 N4 for Photocatalytic Hydrogen Evolution

Over the past few decades, graphitic carbon nitride (g-C₃ N₄ ) has arisen much attention as a promising candidate for photocatalytic hydrogen evolution reaction (HER) owing to its low cost and visible light response ability. However, the unsatisfied HER performance originated from the strong charge recombination of g-C₃ N₄ severely inhibits the further large-scale application of g-C₃ N₄ . In this case, the utilization of cocatalysts is a novel frontline in the g-C₃ N₄ -based photocatalytic systems due to the positive effects of cocatalysts on supressing charge carrier recombination, reducing the HER overpotential, and improving photocatalytic activity. This review summarizes some recent advances about the high-performance cocatalysts based on g-C₃ N₄ toward HER. Specifically, the functions, design principle, classification, modification strategies of cocatalysts, as well as their intrinsic mechanism for the enhanced photocatalytic HER activity are discussed here. Finally, the pivotal challenges and future developments of cocatalysts in the field of HER are further proposed.

Dynamically Monitoring the Photodeposition of Single Cocatalyst Nanoparticles on Semiconductors via Fluorescence Imaging

Loading of cocatalysts through photodeposition has been considered as one of the most promising methods to improve the photocatalytic activities of semiconductors, because of the advantages of intimate contact, easy preparation, and site-directed loading. While extensive efforts have been made to characterize the cocatalysts after synthesis, the growth kinetics of cocatalysts during photodeposition is largely a black box, thus leading to relatively empirical optimizations on the loading strategies of cocatalysts to date. Herein, we dynamically imaged the photodeposition of single cocatalysts on semiconductors via a wide-field fluorescence (FL) microscope, utilizing g-C₃N₄ sheets and CdS nanowires as models. This capability was based on the quenching effect of cocatalysts on the intrinsic FL emission of semiconductors. Single cocatalyst study revealed that FL emission of photocatalysts decayed monoexponentially during photodeposition, and cocatalysts possessed a self-limited growth. The significant heterogeneities (differences) of cocatalysts during photodeposition were also uncovered, regarding the apparent induction time, deposition rate and FL quenching amplitude. These informations were difficult to be accessed using the ex situ characterization. Programmable photodeposition and dissolution of Co_xP were also realized, utilizing a focused laser beam with a spot size of <1 μm. This work explored the hidden details of the growth of cocatalysts during photodeposition, opening up a new avenue to optimize photodeposition for rationally designing more efficient photocatalysts.

Protonated Imine-Linked Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution

Covalent organic frameworks (COFs) have emerged as an important class of organic semiconductors and photocatalysts for the hydrogen evolution reaction (HER)from water. To optimize their photocatalytic activity, typically the organic moieties constituting the frameworks are considered and the most suitable combinations of them are searched for. However, the effect of the covalent linkage between these moieties on the photocatalytic performance has rarely been studied. Herein, we demonstrate that donor‐acceptor (D‐A) type imine‐linked COFs can produce hydrogen with a rate as high as 20.7 mmol g⁻¹ h⁻¹ under visible light irradiation, upon protonation of their imine linkages. A significant red‐shift in light absorbance, largely improved charge separation efficiency, and an increase in hydrophilicity triggered by protonation of the Schiff‐base moieties in the imine‐linked COFs, are responsible for the improved photocatalytic performance.

P-Type Cobalt Phosphide Composites (CoP-Co2P) Decorated on Titanium Oxide for Enhanced Noble-Metal-Free Photocatalytic H2 Evolution Activity

Nowadays, transition-metal phosphides have been reported to function well in photocatalytic water splitting and possess great potential to substitute traditional noble-metal cocatalysts in the future. Herein, p-type cobalt phosphide (CoP-Co₂P) nanomaterials were synthesized by phosphating the solvothermally prepared Co(OH)₂ nanoflowers at a low temperature (300 °C). Then, we combined the phosphides with commercial TiO₂ through facile mechanical mixing to fabricate a useful noble-metal-free photocatalyst. The phosphating time that had an influence on the composition of phosphides was tuned, and 3 h was an ideal condition after comparison. The cobalt phosphide-modified TiO₂ at the optimal weight percentage (nominal 0.5%) exhibited the highest photocatalytic hydrogen rate of approximately 824.5 μmol g^-1 h^-1 under simulated sunlight irradiation, which was nearly equal to 160 times that of bare TiO₂ and 1.7 times that of single CoP-modified TiO₂. The CoP_x/TiO₂ heterojunction interfaces were studied using photoluminescence (PL), time-resolved PL, and photoelectrochemical methods, which revealed that the effective charge separation and transfer accelerated by the built-in electric field of p-n junction contributed significantly to the photocatalytic performance.

Experimental determination of charge carrier dynamics in carbon nitride heterojunctions

Carbon nitride (CN_x) is an emerging photocatalyst with the potential to efficiently produce solar fuels. CN_x heterojunctions often show significant photocatalytic activity improvements. We review the charge carrier dynamics in a range of CN_x heterojunctions including carbon-based material, black phosphorus, Ru complexes, molybdenum sulphide and metal phosphides. Time resolved photoluminescence (TRPL) and transient absorption spectroscopy (TAS) were the most common techniques employed for experimental charge carrier dynamics measurements. The low photoluminescence quantum yield of CN_x appeared to limit the depth of conclusions from TRPL, with both lengthening and shortening of the TRPL lifetimes observed and attributed to enhanced charge separation. Overall, the charge carrier dynamics studies often showed a relative lifetime change of ∼2-fold and an activity improvement of >10-fold. We highlight the need for the use of a wider range of techniques to monitor the charge carrier dynamics for conclusive determination of photophysics-activity relationships and elucidation of improvement mechanisms.

d10 or d0? Theoretical and experimental comparison between rutile GeO2 and TiO2 for photocatalytic water splitting

Rutile GeO₂ with d¹⁰ metal in octahedral coordination possesses both a high charge separation rate and carrier mobility because the partial charge density is dominated by Ge-O anti-bonding for CBM. GeO₂ is capable of photocatalytic water splitting, even visible light water splitting through combination with the sensitizer melon.

Promoting Photocatalytic Hydrogen Evolution Activity of Graphitic Carbon Nitride with Hole-Transfer Agents

Visible light-driven photocatalytic reduction of protons to H₂ is considered a promising way of solar-to-chemical energy conversion. Effective transfer of the photogenerated electrons and holes to the surface of the photocatalyst by minimizing their recombination is essential for achieving a high photocatalytic activity. In general, a sacrificial electron donor is used as a hole scavenger to remove photogenerated holes from the valence band for the continuation of the photocatalytic hydrogen (H₂ ) evolution process. Here, for the first time, the hole-transfer dynamics from Pt-loaded sol-gel-prepared graphitic carbon nitride (Pt-sg-CN) photocatalyst were investigated using different adsorbed hole acceptors along with a sacrificial agent (ascorbic acid). A significant increment (4.84 times) in H₂ production was achieved by employing phenothiazine (PTZ) as the hole acceptor with continuous H₂ production for 3 days. A detailed charge-transfer dynamic of the photocatalytic process in the presence of the hole acceptors was examined by time-resolved photoluminescence and in situ electron paramagnetic resonance studies.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

Ni–NiO heterojunctions: a versatile nanocatalyst for regioselective halogenation and oxidative esterification of aromatics

Herein, we report a facile method for the synthesis of Ni–NiO heterojunction nanoparticles, which we utilized for the nuclear halogenation reaction of phenol and substituted phenols using N-bromosuccinimide (NBS).

Design of charge transfer channels: defective TiO2/MoP supported on carbon cloth for solar-light-driven hydrogen generation

We successfully integrated MoP and TiO₂ on flexible carbon cloth (CC) to construct a panel photoreactor with efficient charge transfer channels, where CC acts as an electron collector and guides directional migration of electrons (TiO₂ → MoP → CC).

Cobalt-based metal–organic frameworks as functional materials for battery applications

Research progress on cobalt-based metal–organic frameworks as functional materials for battery applications has been presented.

Catching and killing of airborne SARS-CoV-2 to control spread of COVID-19 by a heated air disinfection system

Airborne transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) via air-conditioning systems poses a significant threat for the continued escalation of the current coronavirus disease (COVID-19) pandemic. Considering that SARS-CoV-2 cannot tolerate temperatures above 70 °C, here we designed and fabricated efficient filters based on heated nickel (Ni) foam to catch and kill SARS-CoV-2. Virus test results revealed that 99.8% of the aerosolized SARS-CoV-2 was caught and killed by a single pass through a novel Ni-foam-based filter when heated up to 200 °C. In addition, the same filter was also used to catch and kill 99.9% of Bacillus anthracis, an airborne spore. This study paves the way for preventing transmission of SARS-CoV-2 and other highly infectious airborne agents in closed environments.

A General Electrodeposition Strategy for Fabricating Ultrathin Nickel Cobalt Phosphate Nanosheets with Ultrahigh Capacity and Rate Performance

Transition-metal phosphates/phosphides possess promising theoretical electrochemical characteristics and exhibit great potential in advanced supercapacitors. Unfortunately, limited by the processing techniques and overall structure, their specific capacity and rate performance are still unsatisfactory. Herein, we report the fabrication of transition-metal phosphate electrodes with an ultrathin sheetlike array structure by one-step electrodeposition at room temperature. As a proof-of-concept, a transition-metal phosphate member of NiCo(HPO₄)₂·3H₂O with an ultrathin nanosheet structure (thickness ∼2.3 nm) was synthesized and investigated. The as-prepared NiCo(HPO₄)₂·3H₂O electrode showcases an ultrahigh specific capacity of 1768.5 C g^-1 at 2 A g^-1 (the highest value for transition-metal phosphates/phosphides reported to date), superb rate performance of 1144.8 C g^-1 at 100 A g^-1, and excellent electrochemical stability. Moreover, the transition-metal phosphate nanosheet array can be uniformly deposited on various conductive substrates, demonstrating the generality of our strategy. Therefore, this simple electrodeposition strategy provides an opportunity to fabricate ultrathin transition-metal phosphate nanosheet materials that can be used for energy storage/conversion, electrocatalysis, and other electrochemical energy-related devices.

Incorporating Transition-Metal Phosphides Into Metal-Organic Frameworks for Enhanced Photocatalysis

Metal-organic frameworks (MOFs) have been shown to be an excellent platform in photocatalysis. However, to suppress electron-hole recombination, a Pt cocatalyst is usually inevitable, especially in photocatalytic H₂ production, which greatly limits practical application. Herein, for the first time, monodisperse, small-size, and noble-metal-free transitional-metal phosphides (TMPs; for example, Ni₂ P, Ni₁₂ P₅ ), are incorporated into a representative MOF, UiO-66-NH₂ , for photocatalytic H₂ production. Compared with the parent MOF and their physical mixture, both TMPs@MOF composites display significantly improved H₂ production rates. Thermodynamic and kinetic studies reveal that TMPs, behaving similar ability to Pt, greatly accelerate the linker-to-cluster charge transfer, promote charge separation, and reduce the activation energy of H₂ production. Significantly, the results indicate that Pt is thermodynamically favorable, yet Ni₂ P is kinetically preferred for H₂ production, accounting for the higher activity of Ni₂ P@UiO-66-NH₂ than Pt@UiO-66-NH₂ .

Decorated nickel phosphide nanoparticles with nitrogen and phosphorus co-doped porous carbon for enhanced electrochemical water splitting

A novel free-standing electrode consisting of nickel phosphide (Ni₂P) nanoparticles on nitrogen and phosphorus co-doped porous carbon (NPC) are synthesized on carbon cloth (CC). Polyaniline (PANI) and nickel (Ni) are sequentially electro-deposited on the surface of CC, which are then transformed into NPC and Ni₂P by an in-situ carbonization-phosphorization combined process. The electrode surface is distributed with large amounts of uniform macropores, which could expose more active sites and enhance the interfacial exchange with the electrolyte. The Ni₂P@NPC@CC electrode delivers early overpotentials of 92 and 280 mV vs. Reversible Hydrogen Electrode (RHE) at 10 mA cm^-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline condition, respectively. The electrolytic cell with Ni₂P@NPC@CC electrode both as anode and cathode can achieve 10 mA cm^-2 at a small bias of 1.54 V for the overall water splitting. Density functional theory (DFT) calculation indicates that combination with Ni₂P and NPC can decrease Gibbs free energy for H* adsorption (ΔG_H*) and increase charge density on the interface, thus could lead to the enhanced activity for water splitting. The free-standing and noble-metal free Ni₂P@NPC@CC electrode is stable, highly active and cost effective, thus have great potential for the hydrogen production.