Boron and Sodium Doping of Polymeric Carbon Nitride Photoanodes for Photoelectrochemical Water Splitting.

Polymeric carbon nitride is a promising photoanode material for water-splitting and organic transformation-based photochemical cells. Despite achieving significant progress in performance, these materials still exhibit low photoactivity compared to inorganic photoanodic materials because of a moderate visible light response, poor charge separation, and slow oxidation kinetics. Here, the synthesis of a sodium- and boron-doped carbon nitride layer with excellent activity as a photoanode in a water-splitting photoelectrochemical cell is reported. The new synthesis consists of the direct growth of carbon nitride (CN) monomers from a hot precursor solution, enabling control over the monomer-to-dopant ratio, thus determining the final CN properties. The introduction of Na and B as dopants results in a dense CN layer with a packed morphology, better charge separation thanks to the in situ formation of an electron density gradient, and an extended visible light response up to 550 nm. The optimized photoanode exhibits state-of-the-art performance: photocurrent densities with and without a hole scavenger of about 1.5 and 0.9 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE), and maximal external quantum efficiencies of 56% and 24%, respectively, alongside an onset potential of 0.3 V.

The Implications of Coupling an Electron Transfer Mediated Oxidation with a Proton Coupled Electron Transfer Reduction in Hybrid Water Electrolysis.

Invited for this month's cover are the groups of Menny Shalom at the Ben-Gurion University of the Negev, Israel and Dr. Biswajit Mondal at Indian Institute of Technology Gandhinagar, India. The image shows the connection between two half-cells: an electron transfer-mediated [(2,2,6,6-tetramethyl-1-piperidin-1-yl)oxyl] (TEMPO)-catalyzed benzylamine oxidation at the anode and a proton-coupled electron transfer at the cathode for hydrogen generation. The difference in pH dependence of the anodic and cathodic processes enables hybrid water electrolysis at low cell potential (∼1.0 V) by adjusting only the pH value of the electrolytic medium. The Research Article itself is available at 10.1002/cssc.202202271.

Developing extended visible light responsive polymeric carbon nitrides for photocatalytic and photoelectrocatalytic applications.

Polymeric carbon nitride (CN) has emerged as an attractive material for photocatalysis and photoelectronic devices. However, the synthesis of porous CNs with controlled structural and optical properties remains a challenge, and processable CN precursors are still highly sought after for fabricating homogenous CN layers strongly bound to a given substrate. Here, we report a general method to synthesize highly dispersed porous CN materials that show excellent photocatalytic activity for the hydrogen evolution reaction and good performance as photoanodes in photoelectrochemical cells (PEC): first, supramolecular assemblies of melem and melamine in ethylene glycol and water are prepared using a hydrothermal process. These precursors are then calcined to yield a water-dispersible CN photocatalyst that exhibits beneficial charge separation under illumination, extended visible-light response attributed to carbon doping, and a large number of free amine groups that act as preferential sites for a Pt cocatalyst. The optimized CN exhibits state-of-the-art HER rates up to 23.1 mmol h-1 g-1, with an AQE of 19.2% at 395 nm. This unique synthetic route enables the formation of a homogeneous precursor paste for substrate casting; consequently, the CN photoanode exhibits a low onset potential, a high photocurrent density and good stability after calcination.

The Implications of Coupling an Electron Transfer Mediated Oxidation with a Proton Coupled Electron Transfer Reduction in Hybrid Water Electrolysis.

Electrolysis of water is a sustainable route to produce clean hydrogen. Full water-splitting requires a high applied potential, in part because of the pH-dependency of the H2 and O2 evolution reactions (HER and OER), which are proton-coupled electron transfer (PCET) reactions. Therefore, the minimum required potential will not change at different pHs. TEMPO [(2,2,6,6-tetramethyl-1-piperidin-1-yl)oxyl], a stable free-radical that undergoes fast electro-oxidation by a single-electron transfer (ET) process, is pH-independent. Here, we show that the combination of PCET and ET processes enables hydrogen production from water at low cell potentials below the theoretical value for full water-splitting by simple pH adjustment. As a case study, we combined the HER with the oxidation of benzylamine by anodically oxidized TEMPO. The pH-independent electrocatalytic oxidation of TEMPO permits the operation of a hybrid water-splitting cell that shows promise to perform at a low cell potential (≈1 V) and neutral pH conditions.

Light-Induced Hypoxia in Carbon Quantum Dots and Ultrahigh Photocatalytic Efficiency.

Carbon quantum dots (CQDs) represent a class of carbon materials exhibiting photoresponse and many potential applications. Here, we present a unique property that dissolved CQDs capture large amounts of molecular oxygen from the air, the quantity of which can be controlled by light irradiation. The O2 content can be varied between a remarkable 1 wt % of the CQDs in the dark to nearly half of it under illumination, in a reversible manner. Moreover, O2 depletion enhances away from the air-solution interface as the nearby CQDs quickly regain them from the air, creating a pronounced concentration gradient in the solution. We elucidate the role of the CQD functional groups and show that excitons generated under light are responsible for their tunable adsorbed-oxygen content. Because of O2 enrichment, the photocatalytic efficiency of the CQDs toward oxidation of benzylamines in the air is the same as under oxygen flow and far higher than the existing photocatalysts. The findings should encourage the development of a new class of oxygen-enricher materials and air as a sustainable oxidant in chemical transformations.

Projected urban exposure to extreme precipitation over South Asia.

Urbanization is one of the pivotal aspects of socioeconomic advancement which is critically vulnerable to climatic extremes. Extreme precipitation and urbanization are largely interlinked. Estimating the extreme precipitation-induced urban area exposure is the fundamental aspect of urban risk assessment for precipitation-related floods. In this study, future urban area exposure to extreme precipitation and associated influential factors are investigated over South Asia under 1.5 °C, 2.0 °C, 3.0 °C, and 4.0 °C global warming thresholds. In this regard, we used newly released 20 up-to-date climate models outputs, and five Integrated Assessment Models (IAMs) based urban land-use products under four combined scenarios of the Shared Socioeconomic Pathways and Representative Concentration Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) from the sixth phase of Coupled Model Intercomparison Project (CMIP6). Extreme precipitation is characterized by adopting 20-, 50-, and 100-year return periods of annual maximum daily precipitation. Results reveal a massive urban area expansion over South Asia which is the utmost by 186.4% under SSP3-7.0 than the reference period (1995-2014). The variations in projected urban areas mainly occur in Indo-Gangetic Plain (IGP) region among scenarios. In relative terms, extreme precipitation frequency and associated urban area exposure are prospective to increase with continued global warming. The exposed urban area varies 4.5- to 7.4-fold higher under different warming thresholds than the reference period. The leading increase is estimated (7.4-fold) under 4.0 °C. Notably, for global warming targets set out by the Paris Agreement (1.5 °C, and 2.0 °C), exposed urban area is intended to be 10.2% higher under 2.0 °C than 1.5 °C. Spatially, the exposed urban area will be dominant in the southeast region relative to the reference period. Importantly, the interaction effect (simultaneous change in climate-urban land) is the principal contributor to the changes in urban area exposure to extreme precipitation over South Asia. However, this study's findings strongly support the accomplishment of the Paris Agreement target and provide a scientific basis for formulating urban land-use policy interventions.

Defect-rich, negatively-charged SnS2 nanosheets for efficient photocatalytic Cr(VI) reduction and organic dye adsorption in water.

Nanostructures of layered materials have gained increasing attention in photocatalytic and water-treatment processes. Herein, we report on sub-30 nm SnS2 nanosheets (NSs) which can perform photocatalytic reduction of Cr(VI) to Cr(III) quite efficiently on one hand, while removes large quantities of toxic organic dye molecules by choosing an adsorption mode of operation over photo-degradation on the other hand, unlike most other SnS2 nanostructures. The NSs have a highly extended crystallinity growing perpendicular to the (001) lattice direction but exhibit poor X-ray diffraction for the 10 l (1 = 1,2,3…) lattice planes. With such defects, the NSs have a narrow bandgap of 2.21 eV and exhibit a significant photocurrent density at near band-edge illumination. Cr(VI) photo-reduction using the SnS2 NSs follows a first-order reaction kinetics (rate constant of 0.10 min-1), five-fold higher than commercial TiO2 (P-25). Furthermore, the NSs adsorb Rhodamine B dye molecules from an aqueous solution by forming a monolayer of dye molecules following a pseudo-second-order kinetic model and exhibit an adsorption capacity of ∼ 53.28 mg/g. We show that the NSs have a Zeta potential of ∼ -22 eV and preferably adsorb cationic dyes only. Thus the SnS2 NSs can be effective for Cr(VI) contaminated waste-water treatment in a photocatalytic manner and can also act as a potential adsorbent for polluting dye molecules either in the presence or absence of sunlight. While both these activities are known for SnS2 as well as other materials, the competitive nature of the two mechanisms while each of them is a possibility has never been investigated. Therefore, besides the high activities, the study highlights the presence of different active sites on the material surface that can respond preferentially to either inorganic or organic impurities.

Projected changes in temperature, precipitation and potential evapotranspiration across Indus River Basin at 1.5-3.0 °C warming levels using CMIP6-GCMs.

The projections of mean temperature, precipitation (P), and potential evapotranspiration (PET) reflect the probabilities of long-term changes of hydrologic processes and induced extreme events. In this paper, we investigated the future changes in some pivotal climatic variables (mean temperature, precipitation, and potential evapotranspiration) under 1.5 °C, 2.0 °C, and 3.0 °C specific warming levels (SWLs) across the Indus River Basin of South Asia. The seven global climate models output under seven different emission scenarios (SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP4-3.4, SSP4-6.0, and SSP5-8.5) from the latest Sixth phase of Coupled Model Intercomparison Project (CMIP6) are used for this purpose. The Penman-Monteith approach is applied to estimate PET, and the water balance equation is for reflecting water surplus/deficit. Results indicate that except for precipitation, the greater increases in temperature and PET are inclined to happen with continued global warming. The highest increase in temperature is accounted for 14.6% (2.4 °C), and the enhanced PET is estimated at 5.2% higher than the reference period (1995-2014) under 3.0 °C SWL. While the precipitation is projected to increase by the highest 4.8% for 2.0 °C warming level. The differences in regional climate for an additional 0.5 °C (2.0-1.5 °C) and 1.0 °C (3.0-2.0 °C) of warming, the temperature is projected to increase by 0.4 °C and 0.9 °C in the entire IRB respectively. The highest increase in mean temperature (5.1%) and PET (2.4%) in the IRB are predicted to intensify for an additional 1.0 °C than that of 0.5 °C of warming, but precipitation is intended to decrease by 0.4%. Spatially, the increase in temperature, precipitation, and PET are dominated towards high elevation in the upper basin (north) under all the SWLs. The increased variability in climatological parameters across IRB depicts an evident occurrence of both wet events (upper basin) as well as dry events (lower basin) with the increase in global average temperature rise. However, these findings provide an insightful basis for water resource management as well as initiating mitigation and adaptation measures in the IRB related to water surplus (floods) and water deficit (droughts).

Doubling of the population exposed to drought over South Asia: CMIP6 multi-model-based analysis.

Drought has a substantial socioeconomic impact under the changing climate. The estimation of population exposure to drought could be the pivotal signal to predict future water scarcity in the climate hotspot of South Asia. This study examines the changing population exposure to drought across South Asia using 20 climate model ensembles from the latest CMIP6 and demographic data under shared socioeconomic pathways (SSPs). Underpinning the latest version of the IPCC 6th Assessment Report (AR6), this paper focuses on the 2021-2040 (near-term), 2041-2060 (mid-term), and 2081-2100 (long-term) periods to project population exposure changes relative to the reference period (1995-2014) under four SSP-RCP scenarios. Drought events are detected by adopting the standardized precipitation evapotranspiration index (SPEI) and run theory method. Model validation suggests that CMIP6-GCM performs well in projecting climate variables and capturing drought events. The results show that the projected increases in frequent drought events and affected areal coverage are stronger during the early part of the century and weaker at the end under all scenario combinations. In relative terms, the projected increase in the number of people exposed to drought is dominant (>1.5-fold) in the near-term and mid-term periods but decreases in the long-term period. Compared to the reference period, the leading increase in population exposure (2.3-fold) is projected under the newly designed gap scenario (SSP3-7.0) in the mid-term period. A surprising decline in the number of exposed populations was estimated to be 18.8% under SSP5-8.5 by the end of the century. The mitigating effect of the predicted heavy precipitation will decrease droughts in the late future. Spatially, increasing exposure will become more pronounced across India and Afghanistan. Furthermore, the population change effect is mainly responsible for the exposure changes in South Asia. However, this study strongly recommends future 'plausible world' regional rivalry pathways (SSP3) scenario-combinations into consideration for policymaking in regard to water management as well as migration planning over South Asia.

Facile d-band tailoring in Sub-10 nm Pd cubes by in-situ grafting on nitrogen-doped graphene for highly efficient organic transformations.

We demonstrate for the first time the in-situ synthesis of Pd nanocubes (PdNC) on nitrogen-doped reduced graphene oxide (NRGO) for facile organic transformations wherein the cubic morphology of Pd can only be realized by precision-controlled acid additions in the tune of 0.02 pH variations in the reaction medium. Due to the intimate contact arising from atom-by-atom addition of Pd on NRGO, the composite has exhibited a pronounced catalyst to support charge transfer effect, shift in the d-band center, and lowering of charge-transfer resistance when compared with PdNC-NRGO ex-situ composites prepared by mixing of the preformed components of PdNC and NRGO or PdNCs alone. The activities of these catalysts were tested for the Suzuki coupling and nitroarene reduction reactions using water as an industry-friendly solvent. In both, the in-situ deposited sample exhibited substantially higher catalytic activity as well as stability when compared with an ex-situ sample or pure PdNCs. We show that a very high turnover frequency of ~31300 h-1 and ~900 h-1 are achievable by using the in-situ deposited PdNC-NRGO composite for Suzuki coupling reactions and nitroarene reduction respectively, better than the state-of-the-art catalysts developed recently, in addition to high recyclability.

3D Porous Polymeric-Foam-Supported Pd Nanocrystal as a Highly Efficient and Recyclable Catalyst for Organic Transformations.

The efficient recovery of noble metal nanocrystals used in heterogeneous organic transformations has remained a significant challenge, hindering their use in industry. Herein, highly catalytic Pd nanoparticles (NPs) were first prepared having a yield of >98% by a novel hydrothermal method using PVP as the reducing cum stabilizing agent that exhibited excellent turnover frequencies of ∼38,000 h-1 for Suzuki-Miyaura cross-coupling and ∼1200 h-1 for catalytic reduction of nitroarene compounds in a benign aqueous reaction medium. The Pd NPs were more efficient for cross-coupling of aryl compounds with electron-donating substituents than with electron-donating ones. Further, to improve their recyclability, a strategy was developed to embed these Pd NPs on mechanically robust polyurethane foam (PUF) for the first time and a "dip-catalyst" (Pd-PUF) containing 3D interconnected 100-500 µm pores was constructed. The PUF was chosen as the support with an expectation to reduce the fabrication cost of the "dip-catalyst" as the production of PUF is already commercialized. Pd-PUF could be easily separated from the reaction aliquot and reused without any loss of activity because the leaching of Pd NPs was found to be negligible in the various reaction mixtures. We show that the Pd-PUF could be reused for over 50 catalytic cycles maintaining a similar activity. We further demonstrate a scale-up reaction with a single-reaction 1.5 g yield for the Suzuki-Miyaura cross-coupling reaction.

Upconversion Nanodevice-Assisted Healthy Molecular Photocorrection.

Mushrooms are rich in ergosterol, a precursor of ergocalciferol, which is a type of vitamin D2. The conversion of ergosterol to ergocalciferol takes place in the presence of UV radiation by the cleavage of the "B-ring" in the ergosterol. As the UV radiation cannot penetrate deep into the tissue, only minimal increase occurs in sunlight. In this study, upconversion nanoparticles with the property to convert deep-penetrating near-infrared radiation to UV radiation have been cast into a disk to use sunlight and emit UV radiation for vitamin D conversion. An engineered upconversion nanoparticle (UCNP) disk with maximum particles and limited clusters demonstrates ∼2.5 times enhanced vitamin D2 conversion.

Nanocrystalline Ag3PO4 for Sunlight- and Ambient Air-Driven Oxidation of Amines: High Photocatalytic Efficiency and a Facile Catalyst Regeneration Strategy.

Selective oxidation of amines to imines using sunlight as clean and renewable energy source is an important but challenging chemical transformation because of high reactivity of the generated imines and lack of visible light-responsive materials with high conversion rates. In addition, oxygen gas has to be purged in the reaction mixture in order to increase the reaction efficiency which, in itself, is an energy-consuming process. Herein, we report, for the first time, the use of Ag3PO4 as an excellent photocatalyst for the oxidative coupling of benzyl amines induced by ambient air in the absence of any external source of molecular oxygen at room temperature. The conversion efficiency for the selective oxidation of benzyl amine was found to be greater than 95% with a selectivity of >99% after 40 min of light irradiation indicating an exceptionally high conversion efficiency with a rate constant of 0.002 min-1, a turnover frequency of 57 h-1, and a quantum yield of 19%, considering all of the absorbed photons. Ag3PO4, however, is known for its poor photostability owing to a positive conduction band position and a favorable reduction potential to metallic silver. Therefore, we further employed a simple catalyst regeneration strategy and showed that the catalyst can be recycled with negligible loss of activity and selectivity.

High and reversible oxygen uptake in carbon dot solutions generated from polyethylene facilitating reactant-enhanced solar light harvesting.

Solar-driven photocatalysis is emerging as a key chemical transformation strategy due to its favourable energy economy. However, in photocatalytic oxidation reactions where molecular oxygen (O2) is a reactant, achieving higher efficiency requires an O2-saturated environment in order to maintain a high oxygen level on the catalyst surface, necessitating an additional energy-consuming step of O2 separation from air. Here we show that in the presence of carbon quantum dots (CQDs), the oxygen content and the ability of O2 to diffuse in water increase significantly. We first demonstrate a novel strategy to convert several grams of polyethylene, a stubborn pollutant, into highly photoactive CQDs by stepwise dehydrogenation and graphitization. In a typical CQD concentration of ∼1 mg ml-1, the oxygen level in water reaches ∼640 µM, double that of pure water inferring an extremely high O2 content of ∼1 wt% associated with CQDs under ambient conditions. Therefore, when the CQDs were used to catalyze photo-oxidation of aromatic alcohols by sunlight, the efficiency was found higher than previous instances despite those employing high oxygen pressure, temperature and expensive materials. Besides waste polyethylene utilization, the uniqueness of oxygen enrichment in CQD solutions may offer immense prospects including those in photo-oxidation reactions.

Synthesis of Bi3TaO7-Bi4TaO8Br composites in ambient air and their high photocatalytic activity upon metal loading.

Herein, we show that composites of Bi3TaO7-Bi4TaO8X (X = Cl, Br), two important Bi- and Ta-based light-responsive phases, can be prepared by high temperature, ambient air treatment of the precursors including easily oxidizable BiOX that retain the halide phases in excess of 60% and exhibit high photocatalytic activity. Furthermore, when these phases were loaded with less than 1% noble metals (Pd, Pt, Ag), nearly complete separation of the photogenerated excitons was observed, leading to a significant enhancement in the photocatalytic activity.

Self-immobilized Pd nanowires as an excellent platform for a continuous flow reactor: efficiency, stability and regeneration.

Despite extensive use of Pd nanocrystals as catalysts, the realization of a Pd-based continuous flow reactor remains a challenge. Difficulties arise due to ill-defined anchoring of the nanocrystals on a substrate and reactivity of the substrate under different reaction conditions. We demonstrate the first metal (Pd) nanowire-based catalytic flow reactor that can be used across different filtration platforms, wherein, reactants flow through a porous network of nanowires (10-1000 nm pore sizes) and the product can be collected as filtrate. Controlling the growth parameters and obtaining high aspect ratio of the nanowires (diameter = ∼13 nm and length > 8000 nm) is necessary for successful fabrication of this flow reactor. The reactor performance is similar to a conventional reactor, but without requiring energy-expensive mechanical stirring. Synchrotron-based EXAFS studies were used to examine the catalyst microstructure and Operando FT-IR spectroscopic studies were used to devise a regenerative strategy. We show that after prolonged use, the catalyst performance can be regenerated up to 99% by a simple wash-off process without disturbing the catalyst bed. Thus, collection, regeneration and redispersion processes of the catalyst in conventional industrial reactors can be avoided. Another important advantage is avoiding specific catalyst-anchoring substrates, which are not only expensive, but also non-universal in nature.

Emerging Materials in Heterogeneous Electrocatalysis Involving Oxygen for Energy Harvesting.

Water-based renewable energy cycle involved in water splitting, fuel cells, and metal-air batteries has been gaining increasing attention for sustainable generation and storage of energy. The major challenges in these technologies arise due to the poor kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), besides the high cost of the catalysts. Attempts to address these issues have led to the development of many novel and inexpensive catalysts as well as newer mechanistic insights, particularly so in the last three-four years when more catalysts have been investigated than ever before. With the growing emphasis on bifunctionality, that is, materials that can facilitate both reduction and evolution of oxygen, this review is intended to discuss all major families of ORR, OER, and bifunctional catalysts such as metals, alloys, oxides, other chalcogenides, pnictides, and metal-free materials developed during this period in a single platform, while also directing the readers to specific and detailed review articles dealing with each family. In addition, each section highlights the latest theoretical and experimental insights that may further improve ORR/OER performances. The bifunctional catalysts being sufficiently new, no consensus appears to have emerged about the efficiencies. Therefore, a statistical analysis of their performances by considering nearly all literature reports that have appeared in this period is presented. The current challenges in rational design of these catalysts as well as probable strategies to improve their performances are presented.