Hydrogen-Induced Reduction Improves the Photoelectrocatalytic Performance of Titania.

One of the main challenges to expand the use of titanium dioxide (titania) as a photocatalyst is related to its large band gap energy and the lack of an atomic scale description of the reduction mechanisms that may tailor the photocatalytic properties. We show that rutile TiO2 single crystals annealed in the presence of atomic hydrogen experience a strong reduction and structural rearrangement, yielding a material that exhibits enhanced light absorption, which extends from the ultraviolet to the near-infrared (NIR) spectral range, and improved photoelectrocatalytic performance. We demonstrate that both magnitudes behave oppositely: heavy/mild plasma reduction treatments lead to large/negligible spectral absorption changes and poor/enhanced (×10) photoelectrocatalytic performance, as judged from the higher photocurrent. To correlate the photoelectrochemical performance with the atomic and chemical structures of the hydrogen-reduced materials, we have modeled the process with in situ scanning tunneling microscopy measurements, which allow us to determine the initial stages of oxygen desorption and the desorption/diffusion of Ti atoms from the surface. This multiscale study opens a door toward improved materials for diverse applications such as more efficient rutile TiO2-based photoelectrocatalysts, green photothermal absorbers for solar energy applications, or NIR-sensing materials.

Smart Electrode Surfaces by Electrolyte Immobilization for Electrocatalytic CO2 Conversion.

The activity and selectivity of molecular catalysts for the electrochemical CO2 reduction reaction (CO2RR) are influenced by the induced electric field at the electrode/electrolyte interface. We present here a novel electrolyte immobilization method to control the electric field at this interface by positively charging the electrode surface with an imidazolium cation organic layer, which significantly favors CO2 conversion to formate, suppresses hydrogen evolution reaction, and diminishes the operating cell voltage. Those results are well supported by our previous DFT calculations studying the mechanistic role of imidazolium cations in solution for CO2 reduction to formate catalyzed by a model molecular catalyst. This smart electrode surface concept based on covalent grafting of imidazolium on a carbon electrode is successfully scaled up for operating at industrially relevant conditions (100 mA cm-2) on an imidazolium-modified carbon-based gas diffusion electrode using a flow cell configuration, where the CO2 conversion to formate process takes place in acidic aqueous solution to avoid carbonate formation and is catalyzed by a model molecular Rh complex in solution. The formate production rate reaches a maximum of 4.6 gHCOO- m-2 min-1 after accumulating a total of 9000 C of charge circulated on the same electrode. Constant formate production and no significant microscopic changes on the imidazolium-modified cathode in consecutive long-term CO2 electrolysis confirmed the high stability of the imidazolium organic layer under operating conditions for CO2RR.

Transition mechanism of the coverage-dependent polymorphism of self-assembled melamine nanostructures on Au(111).

Molecular self-assembled films have recently attracted increasing attention within the field of nanotechnology as they offer a route to obtain new materials. However, careful selection of the molecular precursors and substrates, as well as exhaustive control of the system evolution is required to obtain the best possible outcome. The three-fold rotational symmetry of melamine molecules and their capability to form hydrogen bonds make them suitable candidates to synthesize this type of self-assembled network. In this work, we have studied the polymorphism of melamine nanostructures on Au(111) at room temperature. We find two coverage-dependent phases: a honeycomb structure (α-phase) for submonolayer coverage and a close-packed structure (ß-phase) for full monolayer coverage. A combined scanning tunnel microscopy and density functional theory based-calculations study of the transition regime where both phases coexist allows describing the mechanism underlying this coverage driven phase transition in terms of the changes in the molecular lateral tension.

Epitaxial growth and characterization of SnSe phases on Au(111).

Two-dimensional (2D) layered group IV-VI semiconductors attract great interest due to their potential applications in nanoelectronics. Depending on the dimensionality, different phases of the same material can present completely different electronic and optical properties, expanding its applications. Here, we present a combined experimental and theoretical study of the atomic structure and electronic properties of epitaxial SnSe structures grown on a metallic Au(111) substrate, forming almost defect-free 2D layers. We describe a coverage-dependent transition from a metallicß-SnSe to a semiconductingα-SnSe phase. The combination of scanning tunneling microscopy/spectroscopy, non-contact atomic force microscopy, x-ray photoelectron spectroscopy/diffraction and angle-resolved photoemission spectroscopy, complemented by density functional theory, provides a comprehensive study of the geometric and electronic structure of both phases. Our work demonstrates the possibility to grow two distinct SnSe phases on Au(111) with high quality and on a large scale. The strong interaction with the substrate allows the stabilization of the previously experimentally unreportedß-SnSe, while the ultra-thin films of orthorhombicα-SnSe are structurally and electronically equivalent to bulk SnSe.

Enabling circular economy by N-recovery: Electrocatalytic reduction of nitrate with cobalt hydroxide nanocomposites on copper foam treating low conductivity groundwater effluents.

Fertilizers play a vital role in the food-energy-water nexus. The traditional method of artificial nitrogen fixation to produce ammonia is a high-energy intensive centralized process that has caused an imbalance of the N-cycle due to the release of N-species to water. Electrocatalytic nitrate reduction (ENR) to ammonia is a promising N-resource recovery alternative that can enable the circular reuse of ammonia in decentralized settings. However, the primary challenge is identifying selective and affordable electrocatalysts. Identifying electrodes that rely on something other than platinum-group metals is required to surpass barriers associated with using expensive and endangered elements. In this study, an earth-abundant bimetallic catalyst, Cu/Co(OH)x, prepared and optimized by electrodeposition, demonstrates superior ammonia production. Under environmentally relevant conditions of 30 mg NO3--N L-1, Cu/Co(OH)x showed higher ammonia production than pristine Cu foam with 0.7 and 0.3 mmol NH3 gcat-1 h-1, respectively. The experimental evaluation demonstrated direct reduction and catalytic hydrogenation mechanisms in Cu/Co(OH)x sites. Leaching analyses suggest that Cu/Co(OH)x has outstanding stability with negligible metal concentration below the maximum contaminant level for both Cu and Co. These results provide a framework for using earth-abundant materials in ENR with comparable efficiency and energy consumption to platinum-group materials.

On-surface synthesis of metal-organic frameworks: the critical role of the reaction conditions.

Two different metal-organic frameworks with either a honeycomb or Kagome structure were grown on Cu(111) using para-aminophenol molecules and native surface adatoms. Although both frameworks are made up from the same chemical species, they are structurally different emphasizing the critical role being played by the reaction conditions during their growth. This work highlights the importance of the balance between thermodynamics and kinetics in the final structure of surface-supported metal-organic networks.

The Role of Metal Adatoms in a Surface-Assisted Cyclodehydrogenation Reaction on a Gold Surface.

Dehydrogenation reactions are key steps in many metal-catalyzed chemical processes and in the on-surface synthesis of atomically precise nanomaterials. The principal role of the metal substrate in these reactions is undisputed, but the role of metal adatoms remains, to a large extent, unanswered, particularly on gold substrates. Here, we discuss their importance by studying the surface-assisted cyclodehydrogenation on Au(111) as an ideal model case. We choose a polymer theoretically predicted to give one of two cyclization products depending on the presence or absence of gold adatoms. Scanning probe microscopy experiments observe only the product associated with adatoms. We challenge the prevalent understanding of surface-assisted cyclodehydrogenation, unveiling the catalytic role of adatoms and their effect on regioselectivity. The study adds new perspectives to the understanding of metal catalysis and the design of on-surface synthesis protocols for novel carbon nanomaterials.

Electrocatalytic Conversion of CO2 to Formate at Low Overpotential by Electrolyte Engineering in Model Molecular Catalysis.

An electrolyte engineering strategy was developed for CO2 reduction into formate with a model molecular catalyst, [Rh(bpy)(Cp*)Cl]Cl, by modifying the solvent (organic or aqueous), the proton source (H2 O or acetic acid), and the electrode/solution interface with imidazolium- and pyrrolidinium-based ionic liquids (ILs). Experimental and theoretical density functional theory investigations suggested that π+ -π interactions between the imidazolium-based IL cation and the reduced bipyridine ligand of the catalyst improved the efficiency of the CO2 reduction reaction (CO2 RR) by lowering the overpotential, while granting partial suppression of the hydrogen evolution reaction. This allowed tuning the selectivity towards formate, reaching for this catalyst an unprecedented faradaic efficiency (FEHCOO -) ≥90 % and energy efficiency of 66 % in acetonitrile solution. For the first time, relevant CO2 conversion to formic acid/formate was reached at low overpotential (0.28 V) using a homogeneous catalyst in acidic aqueous solution (pH=3.8). These results open up a new strategy based on electrolyte engineering for enhancing carbon balance in CO2 RR.

Overcoming barriers for nitrate electrochemical reduction: By-passing water hardness.

Water matrix composition impacts water treatment performance. However, matrix composition impacts have rarely been studied for electrochemical water treatment processes, and the correlation between the composition and the treatment efficiency is lacking. This work evaluated the electrochemical reduction of nitrate (ERN) using different complex water matrices: groundwater, brackish water, and reverse osmosis (RO) concentrate/brine. The ERN was conducted using a tin (Sn) cathode because of the high selectivity towards nitrogen evolution reported for Sn electrocatalysts. The co-existence of calcium (Ca2+), magnesium (Mg2+), and carbonate (CO32-) ions in water caused a 4-fold decrease in the nitrate conversion into innocuous nitrogen gas due to inorganic scaling formation on the cathode surface. XRF and XRD analysis of fouled catalyst surfaces detected brucite (Mg(OH)2), calcite (CaCO3), and dolomite (CaMg(CO3)2) mineral scales formed on the cathode surface. Surface scaling created a physical barrier on the electrode that decreased the ERN efficiency. Identifying these main sources of ERN inhibition was key to devising potential fouling mitigation strategies. For this reason, the chemical softening pre-treatment of a real brackish water was conducted and this significantly increased nitrate conversion and faradaic efficiency during subsequent ERN treatment, leading to a lower electric energy consumption per order. Understanding the ionic foulant composition responsible for influencing electrochemically-driven technologies are the first steps that must be taken to move towards niche applications such as decentralized ERN. Thus, we propose either direct ERN implementation in regions facing high nitrate levels in soft waters, or a hybrid softening/nitrate removal system for those regions where high nitrate and high-water hardness appear simultaneously.

On-Surface Thermal Stability of a Graphenic Structure Incorporating a Tropone Moiety.

On-surface synthesis, complementary to wet chemistry, has been demonstrated to be a valid approach for the synthesis of tailored graphenic nanostructures with atomic precision. Among the different existing strategies used to tune the optoelectronic and magnetic properties of these nanostructures, the introduction of non-hexagonal rings inducing out-of-plane distortions is a promising pathway that has been scarcely explored on surfaces. Here, we demonstrate that non-hexagonal rings, in the form of tropone (cycloheptatrienone) moieties, are thermally transformed into phenyl or cyclopentadienone moieties upon an unprecedented surface-mediated retro-Buchner-type reaction involving a decarbonylation or an intramolecular rearrangement of the CO unit, respectively.

Role of the metal surface on the room temperature activation of the alcohol and amino groups of p-aminophenol.

We present a comparative study of the room-temperature adsorption of p-aminophenol (p-AP) molecules on three metal surfaces, namely Cu(110), Cu(111) and Pt(111). We show that the chemical nature and the structural symmetry of the substrate control the activation of the terminal molecular groups, which result in different arrangements of the interfacial molecular layer. To this aim, we have used in-situ STM images combined with synchrotron radiation high resolution XPS and NEXAFS spectra, and the results were simulated by DFT calculations. On copper, the interaction between the molecules and the surface is weaker on the (111) surface crystal plane than on the (110) one, favouring molecular diffusion and leading to larger ordered domains. We demonstrate that the p-AP molecule undergoes spontaneous dehydrogenation of the alcohol group to form phenoxy species on all the studied surfaces, however, this process is not complete on the less reactive surface, Cu(111). The Pt(111) surface exhibits stronger molecule-surface interaction, inducing a short-range ordered molecular arrangement that increases overtime. In addition, on the highly reactive Pt(111) surface other chemical processes are evidenced, such as the dehydrogenation of the amine group.

On-surface Synthesis of a Chiral Graphene Nanoribbon with Mixed Edge Structure.

Chiral graphene nanoribbons represent an important class of graphene nanomaterials with varying combinations of armchair and zigzag edges conferring them unique structure-dependent electronic properties. Here, we describe the on-surface synthesis of an unprecedented cove-edge chiral GNR with a benzo-fused backbone on a Au(111) surface using 2,6-dibromo-1,5-diphenylnaphthalene as precursor. The initial precursor self-assembly and the formation of the chiral GNRs upon annealing are revealed, along with a relatively small electronic bandgap of approximately 1.6 eV, by scanning tunnelling microscopy and spectroscopy.

On-Surface Driven Formal Michael Addition Produces m-Polyaniline Oligomers on Pt(111).

On-surface synthesis is emerging as a highly rational bottom-up methodology for the synthesis of molecular structures that are unattainable or complex to obtain by wet chemistry. Here, oligomers of meta-polyaniline, a known ferromagnetic polymer, were synthesized from para-aminophenol building-blocks via an unexpected and highly specific on-surface formal 1,4 Michael-type addition at the meta position, driven by the reduction of the aminophenol molecule. We rationalize this dehydrogenation and coupling reaction mechanism with a combination of in situ scanning tunneling and non-contact atomic force microscopies, high-resolution synchrotron-based X-ray photoemission spectroscopy and first-principles calculations. This study demonstrates the capability of surfaces to selectively modify local molecular conditions to redirect well-established synthetic routes, such as Michael coupling, towards the rational synthesis of new covalent nanostructures.

Scanning electrochemical microscopy screening of CO2 electroreduction activities and product selectivities of catalyst arrays.

The electroreduction of CO2 is one of the most investigated reactions and involves testing a large number and variety of catalysts. The majority of experimental electrocatalysis studies use conventional one-sample-at-a-time methods without providing spatially resolved catalytic activity information. Herein, we present the application of scanning electrochemical microscopy (SECM) for simultaneous screening of different catalysts forming an array. We demonstrate the potential of this method for electrocatalytic assessment of an array consisting of three Sn/SnOx catalysts for CO2 reduction to formate (CO2RF). Simultaneous SECM scans with fast scan (1 V s-1) cyclic voltammetry detection of products (HCOO-, CO and H2) at the Pt ultramicroelectrode tip were performed. We were able to consistently distinguish the electrocatalytic activities of the three compositionally and morphologically different Sn/SnOx catalysts. Further development of this technique for larger catalyst arrays and matrices coupled with machine learning based algorithms could greatly accelerate the CO2 electroreduction catalyst discovery.

Copper-assisted oxidation of catechols into quinone derivatives.

Catechols are ubiquitous substances often acting as antioxidants, thus of importance in a variety of biological processes. The Fenton and Haber-Weiss processes are thought to transform these molecules into aggressive reactive oxygen species (ROS), a source of oxidative stress and possibly inducing degenerative diseases. Here, using model conditions (ultrahigh vacuum and single crystals), we unveil another process capable of converting catechols into ROSs, namely an intramolecular redox reaction catalysed by a Cu surface. We focus on a tri-catechol, the hexahydroxytriphenylene molecule, and show that this antioxidant is thereby transformed into a semiquinone, as an intermediate product, and then into an even stronger oxidant, a quinone, as final product. We argue that the transformations occur via two intramolecular redox reactions: since the Cu surface cannot oxidise the molecules, the starting catechol and the semiquinone forms each are, at the same time, self-oxidised and self-reduced. Thanks to these reactions, the quinone and semiquinone are able to interact with the substrate by readily accepting electrons donated by the substrate. Our combined experimental surface science and ab initio analysis highlights the key role played by metal nanoparticles in the development of degenerative diseases.

Differential pulse voltammetric determination of the carcinogenic diamine 4,4'-oxydianiline by electrochemical preconcentration on a MoS2 based sensor.

An electrochemical sensor for the carcinogen 4,4'-oxydianiline (Oxy) is described. The method is based on the ability of MoS2 nanosheets to preconcentrate Oxy. A glassy carbon electrode (GCE) was covered, by drop-casting, with MoS2 nanosheets that were obtained by exfoliation. X-Ray photoemission spectroscopy indicates that Oxy accumulates on the MoS2 nanosheets through an electropolymerization process similar to that reported for aniline. Both electrochemical impedance spectroscopy and atomic force microscopy were used to characterize the electrode surface at the different stages of device fabrication. Employing the current measured at +0.27 V vs. Ag/AgCl after Oxy adsorption, the modified GCE enables the voltammetric detection of Oxy at 80 nM levels with relative errors and relative standard deviations of <8.3 and <5.6%, respectively, at all the concentrations studied. The method was applied to the selective determination of Oxy in spiked river water samples. Very good selectivity and recoveries of around 95% in average are found. Graphical abstractSchematic representation of 4,4-oxydianiline electrochemical polymerization and preconcentration onto molybdenum disulfide nanosheets for the diamine determination in river waters.

On-Surface Synthesis and Characterization of Acene-Based Nanoribbons Incorporating Four-Membered Rings.

A bottom up method for the synthesis of unique tetracene-based nanoribbons, which incorporate cyclobutadiene moieties as linkers between the acene segments, is reported. These structures were achieved through the formal [2+2] cycloaddition reaction of ortho-functionalized tetracene precursor monomers. The formation mechanism and the electronic and magnetic properties of these nanoribbons were comprehensively studied by means of a multitechnique approach. Ultra-high vacuum scanning tunneling microscopy showed the occurrence of metal-coordinated nanostructures at room temperature and their evolution into nanoribbons through formal [2+2] cycloaddition at 475 K. Frequency-shift non-contact atomic force microscopy images clearly proved the presence of bridging cyclobutadiene moieties upon covalent coupling of activated tetracene molecules. Insight into the electronic and vibrational properties of the so-formed ribbons was obtained by scanning tunneling microscopy, Raman spectroscopy, and theoretical calculations. Magnetic properties were addressed from a computational point of view, allowing us to propose promising candidates to magnetic acene-based ribbons incorporating four-membered rings. The reported findings will increase the understanding and availability of new graphene-based nanoribbons with high potential in future spintronics.

Versatile Graphene-Based Platform for Robust Nanobiohybrid Interfaces.

Technologically useful and robust graphene-based interfaces for devices require the introduction of highly selective, stable, and covalently bonded functionalities on the graphene surface, whilst essentially retaining the electronic properties of the pristine layer. This work demonstrates that highly controlled, ultrahigh vacuum covalent chemical functionalization of graphene sheets with a thiol-terminated molecule provides a robust and tunable platform for the development of hybrid nanostructures in different environments. We employ this facile strategy to covalently couple two representative systems of broad interest: metal nanoparticles, via S-metal bonds, and thiol-modified DNA aptamers, via disulfide bridges. Both systems, which have been characterized by a multitechnique approach, remain firmly anchored to the graphene surface even after several washing cycles. Atomic force microscopy images demonstrate that the conjugated aptamer retains the functionality required to recognize a target protein. This methodology opens a new route to the integration of high-quality graphene layers into diverse technological platforms, including plasmonics, optoelectronics, or biosensing. With respect to the latter, the viability of a thiol-functionalized chemical vapor deposition graphene-based solution-gated field-effect transistor array was assessed.

Characterization of LiCoO2 nanoparticle suspensions by single collision events.

Transient electrochemical experiments associated with the collisions between hydrothermally synthesized LiCoO2 (LCO) nanoparticles/aggregates of different sizes and a polarized gold ultramicroelectrode (UME) were used as a new additive-free analytical tool applied to Li ion insertion compounds. The size of the LCO nanoparticles/aggregates, ranging from 75 to 450 nm, the diffusion coefficient of the LCO nanoparticles/aggregates in suspension (∼8 × 10-9 cm2 s-1), and the Li ion diffusion coefficient within crystalline LCO nanoparticles (∼1.3 × 10-11 cm2 s-1) were estimated from single collision events. Interestingly, the charge exchanged during each nanoparticle collision was related to the size of the corresponding LCO aggregate, which enables electrochemical sizing distribution measurement displaying evident concordance with optical techniques, including dynamic light scattering (DLS) and cryo-transmission electron microscopy (cryo-TEM). Studying the nanoparticle collision frequency on the UME surface as a function of the LCO nanoparticle concentration allows estimation of the diffusion coefficient of LCO nanoparticles/aggregates in suspension. Finally, from the current decay observed in chronoamperometry after LCO nanoparticle collision on the polarized UME surface, which corresponds to the LCO oxidation (i.e. the Li+ deinsertion reaction), the Li ion diffusion coefficient within the host crystalline material is estimated. This is a key parameter, which controls the cycle lifetime and charge rate in Li ion battery performance. This new approach thus allows a fine description of the nanoparticle properties, which includes sizing as well as estimation of the Li ion diffusion coefficient within the host crystalline material.

On-Surface Hydrogen-Induced Covalent Coupling of Polycyclic Aromatic Hydrocarbons via a Superhydrogenated Intermediate.

The activation, hydrogenation, and covalent coupling of polycyclic aromatic hydrocarbons (PAHs) are processes of great importance in fields like chemistry, energy, biology, or health, among others. So far, they are based on the use of catalysts which drive and increase the efficiency of the thermally- or light-induced reaction. Here, we report on the catalyst-free covalent coupling of nonfunctionalized PAHs adsorbed on a relatively inert surface in the presence of atomic hydrogen. The underlying mechanism has been characterized by high-resolution scanning tunnelling microscopy and rationalized by density functional theory calculations. It is based on the formation of intermediate radical-like species upon hydrogen-induced molecular superhydrogenation which favors the covalent binding of PAHs in a thermally activated process, resulting in large coupled molecular nanostructures. The mechanism proposed in this work opens a door toward the direct formation of covalent, PAH-based, bottom-up synthesized nanoarchitectures on technologically relevant inert surfaces.