CO2 doping of organic interlayers for perovskite solar cells.

In perovskite solar cells, doped organic semiconductors are often used as charge-extraction interlayers situated between the photoactive layer and the electrodes. The π-conjugated small molecule 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene (spiro-OMeTAD) is the most frequently used semiconductor in the hole-conducting layer1-6, and its electrical properties considerably affect the charge collection efficiencies of the solar cell7. To enhance the electrical conductivity of spiro-OMeTAD, lithium bis(trifluoromethane)sulfonimide (LiTFSI) is typically used in a doping process, which is conventionally initiated by exposing spiro-OMeTAD:LiTFSI blend films to air and light for several hours. This process, in which oxygen acts as the p-type dopant8-11, is time-intensive and largely depends on ambient conditions, and thus hinders the commercialization of perovskite solar cells. Here we report a fast and reproducible doping method that involves bubbling a spiro-OMeTAD:LiTFSI solution with CO2 under ultraviolet light. CO2 obtains electrons from photoexcited spiro-OMeTAD, rapidly promoting its p-type doping and resulting in the precipitation of carbonates. The CO2-treated interlayer exhibits approximately 100 times higher conductivity than a pristine film while realizing stable, high-efficiency perovskite solar cells without any post-treatments. We also show that this method can be used to dope π-conjugated polymers.

Conversion of chirality to twisting via sequential one-dimensional and two-dimensional growth of graphene spirals.

The properties of two-dimensional (2D) van der Waals materials can be tuned through nanostructuring or controlled layer stacking, where interlayer hybridization induces exotic electronic states and transport phenomena. Here we describe a viable approach and underlying mechanism for the assisted self-assembly of twisted layer graphene. The process, which can be implemented in standard chemical vapour deposition growth, is best described by analogy to origami and kirigami with paper. It involves the controlled induction of wrinkle formation in single-layer graphene with subsequent wrinkle folding, tearing and re-growth. Inherent to the process is the formation of intertwined graphene spirals and conversion of the chiral angle of 1D wrinkles into a 2D twist angle of a 3D superlattice. The approach can be extended to other foldable 2D materials and facilitates the production of miniaturized electronic components, including capacitors, resistors, inductors and superconductors.

Domino electroreduction of CO2 to methanol on a molecular catalyst.

Electrochemical carbon dioxide (CO2) reduction can in principle convert carbon emissions to fuels and value-added chemicals, such as hydrocarbons and alcohols, using renewable energy, but the efficiency of the process is limited by its sluggish kinetics1,2. Molecular catalysts have well defined active sites and accurately tailorable structures that allow mechanism-based performance optimization, and transition-metal complexes have been extensively explored in this regard. However, these catalysts generally lack the ability to promote CO2 reduction beyond the two-electron process to generate more valuable products1,3. Here we show that when immobilized on carbon nanotubes, cobalt phthalocyanine-used previously to reduce CO2 to primarily CO-catalyses the six-electron reduction of CO2 to methanol with appreciable activity and selectivity. We find that the conversion, which proceeds via a distinct domino process with CO as an intermediate, generates methanol with a Faradaic efficiency higher than 40 per cent and a partial current density greater than 10 milliamperes per square centimetre at -0.94 volts with respect to the reversible hydrogen electrode in a near-neutral electrolyte. The catalytic activity decreases over time owing to the detrimental reduction of the phthalocyanine ligand, which can be suppressed by appending electron-donating amino substituents to the phthalocyanine ring. The improved molecule-based electrocatalyst converts CO2 to methanol with considerable activity and selectivity and with stable performance over at least 12 hours.

Active Sites of Cobalt Phthalocyanine in Electrocatalytic CO2 Reduction to Methanol.

Many metal coordination compounds catalyze CO2 electroreduction to CO, but cobalt phthalocyanine hybridized with conductive carbon such as carbon nanotubes is currently the only one that can generate methanol. The underlying structure-reactivity correlation and reaction mechanism desperately demand elucidation. Here we report the first in situ X-ray absorption spectroscopy characterization, combined with ex situ spectroscopic and electrocatalytic measurements, to study CoPc-catalyzed CO2 reduction to methanol. Molecular dispersion of CoPc on CNT surfaces, as evidenced by the observed electronic interaction between the two, is crucial to fast electron transfer to the active sites and multi-electron CO2 reduction. CO, the key intermediate in the CO2 -to-methanol pathway, is found to be labile on the active site, which necessitates a high local concentration in the microenvironment to compete with CO2 for active sites and promote methanol production. A comparison of the electrocatalytic performance of structurally related porphyrins indicates that the bridging aza-N atoms of the Pc macrocycle are critical components of the CoPc active site that produces methanol. In situ X-ray absorption spectroscopy identifies the active site as Co(I) and supports an increasingly non-centrosymmetric Co coordination environment at negative applied potential, likely due to the formation of a Co-CO adduct during the catalysis.

Spatially separating redox centers on 2D carbon nitride with cobalt single atom for photocatalytic H2O2 production.

Redox cocatalysts play crucial roles in photosynthetic reactions, yet simultaneous loading of oxidative and reductive cocatalysts often leads to enhanced charge recombination that is detrimental to photosynthesis. This study introduces an approach to simultaneously load two redox cocatalysts, atomically dispersed cobalt for improving oxidation activity and anthraquinone for improving reduction selectivity, onto graphitic carbon nitride (C3N4) nanosheets for photocatalytic H2O2 production. Spatial separation of oxidative and reductive cocatalysts was achieved on a two-dimensional (2D) photocatalyst, by coordinating cobalt single atom above the void center of C3N4 and anchoring anthraquinone at the edges of C3N4 nanosheets. Such spatial separation, experimentally confirmed and computationally simulated, was found to be critical for enhancing surface charge separation and achieving efficient H2O2 production. This center/edge strategy for spatial separation of cocatalysts may be applied on other 2D photocatalysts that are increasingly studied in photosynthetic reactions.

Heterogeneous Molecular Catalysts of Metal Phthalocyanines for Electrochemical CO2 Reduction Reactions.

ConspectusMolecular catalysts, often deployed in homogeneous conditions, are favorable systems for structure-reactivity correlation studies of electrochemical reactions because of their well-defined active site structures and ease of mechanistic investigation. In pursuit of selective and active electrocatalysts for the CO2 reduction reactions which are promising for converting carbon emissions to useful fuels and chemical products, it is desirable to support molecular catalysts on substrates because heterogeneous catalysts can afford the high current density and operational convenience that practical electrolyzers require. Herein, we share our understanding in the development of heterogenized metal phthalocyanine catalysts for the electrochemical reduction of CO2. From the optimization of preparation methods and material structures for the electrocatalytic activity toward CO2 reduction to CO, we find that molecular-level dispersion of the active material and high electrical conductivity of the support are among the most important factors controlling the activity. The molecular nature of the active site enables mechanism-based optimization. We demonstrate how electron-withdrawing and -donating ligand substituents can be utilized to modify the redox property of the molecule and improve its catalytic activity and stability. Adjusting these factors further allows us to achieve electrochemical reduction of CO2 to methanol with appreciable activity, which has not been attainable by conventional molecular catalysts. The six-electron reduction process goes through CO as the key intermediate. Rapid and continuous electron delivery to the active site favors further reduction of CO to methanol. We also point out that, in homogeneous electrocatalysis where the catalyst molecules are dissolved in the electrolyte solution, even if the molecular structure remains intact, the actual catalysis may be dominated by molecules permanently adsorbed on the electrode surface and is thus heterogeneous in nature. This account uses our research on CO2 electroreduction reactions catalyzed by metal phthalocyanine molecules to illustrate our understanding about heterogeneous molecular electrocatalysis, which is also applicable to other electrochemical systems.

Electrochemical Reductive N-Methylation with CO2 Enabled by a Molecular Catalyst.

The development of benign methylation reactions utilizing CO2 as a one-carbon building block would enable a more sustainable chemical industry. Electrochemical CO2 reduction has been extensively studied, but its application for reductive methylation reactions remains out of the scope of current electrocatalysis. Here, we report the first electrochemical reductive N-methylation reaction with CO2 and demonstrate its compatibility with amines, hydroxylamines, and hydrazine. Catalyzed by cobalt phthalocyanine molecules supported on carbon nanotubes, the N-methylation reaction proceeds in aqueous media via the chemical condensation of an electrophilic carbon intermediate, proposed to be adsorbed or near-electrode formaldehyde formed from the four-electron reduction of CO2, with nucleophilic nitrogenous reactants and subsequent reduction. By comparing various amines, we discover that the nucleophilicity of the amine reactant is a descriptor for the C-N coupling efficacy. We extend the scope of the reaction to be compatible with cheap and abundant nitro-compounds by developing a cascade reduction process in which CO2 and nitro-compounds are reduced concurrently to yield N-methylamines with high monomethylation selectivity via the overall transfer of 12 electrons and 12 protons.

Enhanced Electrocatalytic Activity of a Zinc Porphyrin for CO2 Reduction: Cooperative Effects of Triazole Units in the Second Coordination Sphere.

The control of the second coordination sphere in a coordination complex plays an important role in improving catalytic efficiency. Herein, we report a zinc porphyrin complex ZnPor8T with multiple flexible triazole units comprising the second coordination sphere, as an electrocatalyst for the highly selective electrochemical reduction of carbon dioxide (CO2 ) to carbon monoxide (CO). This electrocatalyst converted CO2 to CO with a Faradaic efficiency of 99 % and a current density of -6.2 mA cm-2 at -2.4 V vs. Fc/Fc+ in N,N-dimethylformamide using water as the proton source. Structure-function relationship studies were carried out on ZnPor8T analogs containing different numbers of triazole units and distinct triazole geometries; these unveiled that the triazole units function cooperatively to stabilize the CO2 -catalyst adduct in order to facilitate intramolecular proton transfer. Our findings demonstrate that incorporating triazole units that function in a cooperative manner is a versatile strategy to enhance the activity of electrocatalytic CO2 conversion.

Acid-Base Interaction Enhancing Oxygen Tolerance in Electrocatalytic Carbon Dioxide Reduction.

Hybrid electrodes with improved O2 tolerance and capability of CO2 conversion into liquid products in the presence of O2 are presented. Aniline molecules are introduced into the pore structure of a polymer of intrinsic microporosity to expand its gas separation functionality beyond pure physical sieving. The chemical interaction between the acidic CO2 molecule and the basic amino group of aniline renders enhanced CO2 separation from O2 . Loaded with a cobalt phthalocyanine-based cathode catalyst, the hybrid electrode achieves a CO Faradaic efficiency of 71 % with 10 % O2 in the CO2 feed gas. The electrode can still produce CO at an O2 /CO2 ratio as high as 9:1. Switching to a Sn-based catalyst, for the first time O2 -tolerant CO2 electroreduction to liquid products is realized, generating formate with nearly 100 % selectivity and a current density of 56.7 mA cm-2 in the presence of 5 % O2 .

Dimensional reduction and ionic gating induced enhancement of superconductivity in atomically thin crystals of 2H-TaSe2.

The effects of dimensional reduction and ion intercalation on superconductivity (SC) in the presence of charge density waves (CDWs) in two-dimensional crystals of 2H-TaSe2 were characterized. We prepared atomically thin crystals by mechanical exfoliation and performed electrical transport measurements on devices made by photolithography. The superconducting transition temperature (T c SC ) was found to increase monotonically as the thickness decreased, changing from 0.14 K in the bulk to higher than 1.4 K for a 3-nm-thick crystal. The temperature dependence of upper critical field was found to be anomalous. The CDW transition temperature (T c CDW ) was found to decrease, but to a less extent than T c SC , from 120 K in the bulk to around 113 K for the 3-nm-thick crystal. In addition, ion intercalation was found to increase T c SC and suppress T c CDW in an atomically thin crystal of 2H-TaSe2. The implications of these findings are discussed. We suggest that dimensional reduction and ion intercalation are potentially effective ways to engineer material properties for layered transition metal chalcogenides.

An Integrated CO2 Electrolyzer and Formate Fuel Cell Enabled by a Reversibly Restructuring Pb-Pd Bimetallic Catalyst.

A single device combining the functions of a CO2 electrolyzer and a formate fuel cell is a new option for carbon-neutral energy storage but entails rapid, reversible and stable interconversion between CO2 and formate over a single catalyst electrode. We report a new catalyst with such functionalities based on a Pb-Pd alloy system that reversibly restructures its phase, composition, and morphology and thus alters its catalytic properties under controlled electrochemical conditions. Under cathodic conditions, the catalyst is relatively Pb-rich and is active for CO2 -to-formate conversion over a wide potential range; under anodic conditions, it becomes relatively Pd-rich and gains stable catalytic activity for formate-to-CO2 conversion. The bifunctional activity and superior durability of our Pb-Pd catalyst leads to the first proof-of-concept demonstration of an electrochemical cell that can switch between the CO2 electrolyzer/formate fuel cell modes and can stably operate for 12 days.

Introducing Fe2+ into Nickel-Iron Layered Double Hydroxide: Local Structure Modulated Water Oxidation Activity.

Exploring materials with regulated local structures and understanding how the atomic motifs govern the reactivity and durability of catalysts are a critical challenge for designing advanced catalysts. Herein we report the tuning of the local atomic structure of nickel-iron layered double hydroxides (NiFe-LDHs) by partially substituting Ni2+ with Fe2+ to introduce Fe-O-Fe moieties. These Fe2+ -containing NiFe-LDHs exhibit enhanced oxygen evolution reaction (OER) activity with an ultralow overpotential of 195 mV at the current density of 10 mA cm-2 , which is among the best OER catalytic performance to date. In-situ X-ray absorption, Raman, and electrochemical analysis jointly reveal that the Fe-O-Fe motifs could stabilize high-valent metal sites at low overpotentials, thereby enhancing the OER activity. These results reveal the importance of tuning the local atomic structure for designing high efficiency electrocatalysts.

Self-Cleaning Catalyst Electrodes for Stabilized CO2 Reduction to Hydrocarbons.

A surface-restructuring strategy is presented that involves self-cleaning Cu catalyst electrodes with unprecedented catalytic stability toward CO2 reduction. Under the working conditions, the Pd atoms pre-deposited on Cu surface induce continuous morphological and compositional restructuring of the Cu surface, which constantly refreshes the catalyst surface and thus maintains the catalytic properties for CO2 reduction to hydrocarbons. The Pd-decorated Cu electrode can catalyze CO2 reduction with relatively stable selectivity and current density for up to 16 h, which is one of the best catalytic durability performances among all Cu electrocatalysts for effective CO2 conversion to hydrocarbons. The generality of this approach of utilizing foreign metal atoms to induce surface restructuring toward stabilizing Cu catalyst electrodes against deactivation by carbonaceous species accumulation in CO2 reduction is further demonstrated by replacing Pd with Rh.

Electrochemical CO2 Reduction to Hydrocarbons on a Heterogeneous Molecular Cu Catalyst in Aqueous Solution.

Exploration of heterogeneous molecular catalysts combining the atomic-level tunability of molecular structures and the practical handling advantages of heterogeneous catalysts represents an attractive approach to developing high-performance catalysts for important and challenging chemical reactions such as electrochemical carbon dioxide reduction which holds the promise for converting emissions back to fuels utilizing renewable energy. Thus, far, efficient and selective electroreduction of CO2 to deeply reduced products such as hydrocarbons remains a big challenge. Here, we report a molecular copper-porphyrin complex (copper(II)-5,10,15,20-tetrakis(2,6-dihydroxyphenyl)porphyrin) that can be used as a heterogeneous electrocatalyst with high activity and selectivity for reducing CO2 to hydrocarbons in aqueous media. At -0.976 V vs the reversible hydrogen electrode, the catalyst is able to drive partial current densities of 13.2 and 8.4 mA cm(-2) for methane and ethylene production from CO2 reduction, corresponding to turnover frequencies of 4.3 and 1.8 molecules·site(-1)·s(-1) for methane and ethylene, respectively. This represents the highest catalytic activity to date for hydrocarbon production over a molecular CO2 reduction electrocatalyst. The unprecedented catalytic performance is attributed to the built-in hydroxyl groups in the porphyrin structure and the reactivity of the copper(I) metal center.

Spontaneous rotational symmetry breaking in KTaO3 heterointerface superconductors.

Broken symmetries play a fundamental role in superconductivity and influence many of its properties in a profound way. Understanding these symmetry breaking states is essential to elucidate the various exotic quantum behaviors in non-trivial superconductors. Here, we report an experimental observation of spontaneous rotational symmetry breaking of superconductivity at the heterointerface of amorphous (a)-YAlO3/KTaO3(111) with a superconducting transition temperature of 1.86 K. Both the magnetoresistance and superconducting critical field in an in-plane field manifest striking twofold symmetric oscillations deep inside the superconducting state, whereas the anisotropy vanishes in the normal state, demonstrating that it is an intrinsic property of the superconducting phase. We attribute this behavior to the mixed-parity superconducting state, which is an admixture of s-wave and p-wave pairing components induced by strong spin-orbit coupling inherent to inversion symmetry breaking at the heterointerface of a-YAlO3/KTaO3. Our work suggests an unconventional nature of the underlying pairing interaction in the KTaO3 heterointerface superconductors, and brings a new broad of perspective on understanding non-trivial superconducting properties at the artificial heterointerfaces.

Robust Weak Antilocalization Effect Up to ∼120 K in the van der Waals Crystal Fe5-xGeTe2 with Near-Room-Temperature Ferromagnetism.

The van der Waals Fe5-xGeTe2 is a 3d ferromagnetic metal with a high Curie temperature of 275 K. We report herein the observation of an exceptional weak antilocalization (WAL) effect that can persist up to 120 K in an Fe5-xGeTe2 nanoflake, indicating the dual nature with both itinerant and localized magnetism of 3d electrons. The WAL behavior is characterized by the magnetoconductance peak around zero magnetic field and is supported by the calculated localized nondispersive flat band around the Fermi level. The peak to dip crossover starting around 60 K in magnetoconductance is visible, which could be ascribed to temperature-induced changes in Fe magnetic moments and the coupled electronic band structure as revealed by angle-resolved photoemission spectroscopy and first-principles calculations. Our findings would be instructive for understanding the magnetic exchanges in transition metal magnets as well as for the design of next-generation room-temperature spintronic devices.

Fe-Intercalation Dominated Ferromagnetism of van der Waals Fe3 GeTe2.

Fe3 GeTe2 have proven to be of greatly intrigue. However, the underlying mechanism behind the varying Curie temperature (Tc ) values remains a puzzle. This study explores the atomic structure of Fe3 GeTe2 crystals exhibiting Tc values of 160, 210, and 230 K. The elemental mapping reveals a Fe-intercalation on the interstitial sites within the van der Waals gap of the high-Tc (210 and 230 K) samples, which are observed to have an exchange bias effect by electrical transport measurements, while Fe intercalation or the bias effect is absent in the low-Tc (160 K) samples. First-principles calculations further suggest that the Fe-intercalation layer may be responsible for the local antiferromagnetic coupling that gives rise to the exchange bias effect, and that the interlayer exchange paths greatly contribute to the enhancement of Tc . This discovery of the Fe-intercalation layer elucidates the mechanism behind the hidden antiferromagnetic ordering that underlies the enhancement of Tc in Fe3 GeTe2 .

Breaking Scaling Relationships in CO2 Reduction on Copper Alloys with Organic Additives.

Boundary conditions for catalyst performance in the conversion of common precursors such as N2, O2, H2O, and CO2 are governed by linear free energy and scaling relationships. Knowledge of these limits offers an impetus for designing strategies to alter reaction mechanisms to improve performance. Typically, experimental demonstrations of linear trends and deviations from them are composed of a small number of data points constrained by inherent experimental limitations. Herein, high-throughput experimentation on 14 bulk copper bimetallic alloys allowed for data-driven identification of a scaling relationship between the partial current densities of methane and C2+ products. This strict dependence represents an intrinsic limit to the Faradaic efficiency for C-C coupling. We have furthermore demonstrated that coating the electrodes with a molecular film breaks the scaling relationship to promote C2+ product formation.

Charge Density Wave Orders and Enhanced Superconductivity under Pressure in the Kagome Metal CsV3 Sb5.

Superconductivity in topological kagome metals has recently received great research interests. Here, charge density wave (CDW) orders and the evolution of superconductivity under various pressures in CsV3 Sb5 single crystal with V kagome lattice are investigated. By using high-resolution scanning tunneling microscopy/spectroscopy (STM/STS), two CDW orders in CsV3 Sb5 are observed which correspond to 4a × 1a and 2a × 2a superlattices. By applying pressure, the superconducting transition temperature Tc is significantly enhanced and reaches a maximum value of 8.2 K at around 1 GPa. Accordingly, CDW state is gradually declined as increasing the pressure, which indicates the competing interplay between CDW and superconducting state in this material. The broad superconducting transitions around 0.4-0.8 GPa can be related to the strong competition relation among two CDW states and superconductivity. These results demonstrate that CsV3 Sb5 is a new platform for exploring the interplay between superconductivity and CDW in topological kagome metals.