Nanoporous thin films in optical waveguide spectroscopy for chemical analytics.

Spectroscopy with planar optical waveguides is still an active field of research for the quantitative analysis of various supramolecular surface architectures and processes, and for applications in integrated optical chip communication, direct chemical sensing, etc. In this contribution, we summarize some recent development in optical waveguide spectroscopy using nanoporous thin films as the planar substrates that can guide the light just as well as bulk thin films. This is because the nanoporosity is at a spacial length-scale that is far below the wavelength of the guided light; hence, it does not lead to an enhanced scattering or additional losses of the optical guided modes. The pores have mainly two effects: they generate an enormous inner surface (up to a factor of 100 higher than the mere geometric dimensions of the planar substrate) and they allow for the exchange of material and charges between the two sides of the solid thin film. We demonstrate this for several different scenarios including anodized aluminum oxide layers for the ultrasensitive determination of the refractive index of fluids, or the label-free detection of small analytes binding from the pore inner volume to receptors immobilized on the pore surface. Using a thin film of Ti metal for the anodization results in a nanotube array offering an even further enhanced inner surface and the possibility to apply electrical potentials via the resulting TiO2 semiconducting waveguide structure. Nanoporous substrates fabricated from SiNx thin films by colloid lithography, or made from SiO2 by e-beam lithography, will be presented as examples where the porosity is used to allow for the passage of ions in the case of tethered lipid bilayer membranes fused on top of the light-guiding layer, or the transport of protons through membranes used in fuel cell applications. The final example that we present concerns the replication of the nanopore structure by polymers in a process that leads to a nanorod array that is equally well suited to guide the light as the mold; however, it opens a totally new field for integrated optics formats for direct chemical and biomedical sensing with an extension to even molecularly imprinted structures. Graphical abstract.

Zirconium Oxycarbide: A Highly Stable Catalyst Material for Electrochemical Energy Conversion.

Metal carbides and oxycarbides have recently gained considerable interest due to their (electro)catalytic properties that differ from those of transition metals and that have potential to outperform them as well. The stability of zirconium oxycarbide nanopowders (ZrO0.31 C0.69 ), synthesized via a hybrid solid-liquid route, is investigated in different gas atmospheres from room temperature to 800 °C by using in-situ X-ray diffraction and in-situ electrical impedance spectroscopy. To feature the properties of a structurally stable Zr oxycarbide with high oxygen content, a stoichiometry of ZrO0.31 C0.69 has been selected. ZrO0.31 C0.69 is stable in reducing gases with only minor amounts of tetragonal ZrO2 being formed at high temperatures, whereas it decomposes in CO2 and O2 gas atmosphere. From online differential electrochemical mass spectrometry measurements, the hydrogen evolution reaction (HER) onset potential is determined at -0.4 VRHE . CO2 formation is detected at potentials as positive as 1.9 VRHE as ZrO0.31 C0.69 decomposition product, and oxygen is anodically formed at 2.5 VRHE , which shows the high electrochemical stability of this material in acidic electrolyte. This peopwery makes the material suited for electrocatalytic reactions at anodic potentials, such as CO and alcohol oxidation reactions, in general.

Fabrication of Ti substrate grain dependent C/TiO2 composites through carbothermal treatment of anodic TiO2.

Composite materials of titania and graphitic carbon, and their optimized synthesis are highly interesting for application in sustainable energy conversion and storage. We report on planar C/TiO2 composite films that are prepared on a polycrystalline titanium substrate by carbothermal treatment of compact anodic TiO2 with acetylene. This thin film material allows for the study of functional properties of C/TiO2 as a function of chemical composition and structure. The chemical and structural properties of the composite on top of individual Ti substrate grains are examined by scanning photoelectron microscopy and micro-Raman spectroscopy. Through comparison of these data with electron backscatter diffraction, it is found that the amount of generated carbon and the grade of anodic film crystallinity correlate with the crystallographic orientation of the Ti substrate grains. On top of Ti grains with ∼(0001) orientations the anodic TiO2 exhibits the highest grade of crystallinity, and the composite contains the highest fraction of graphitic carbon compared to Ti grains with other orientations. This indirect effect of the Ti substrate grain orientation yields new insights into the activity of TiO2 towards the decomposition of carbon precursors.

The Relevance of the Interfacial Water Reactivity for Electrochemical CO Reduction on Copper Single Crystals.

The electrochemical reduction of CO2 is an important electrolysis reaction that enables the conversion of a waste gas to fuels or value-added chemicals. To make this reaction viable, a profound understanding of central intermediate steps, such as the CO electroreduction, is required. On Cu, the CO reduction reaction (CORR) is intimately linked to the hydrogen evolution reaction (HER) that proceeds via the reduction of water in alkaline or neutral electrolytes. Here, we demonstrate that the interaction of water or more specifically the water reduction kinetics on differently smooth Cu(100) and Cu(111) surfaces during the CORR in alkaline media significantly governs the CORR. On Cu(111), faster HER kinetics and the highest CORR activity are observed, even though HER and CORR onsets are more negative. While on Cu(100) small Cu ad-island clusters form in the cathodic potential range only when CO is present, structural changes appear on a larger length scale on Cu(111) both under CORR conditions and when no CO is present. These differences in the reconstruction characteristics may be attributed to the dominance of either the CORR and its intermediates or the HER on the different Cu surfaces. Therefore, the interfacial water reactivity is considered an essential activity descriptor for the CORR on Cu in alkaline media.

Titanium Oxycarbide as Platinum-Free Electrocatalyst for Ethanol Oxidation.

The compound material titanium oxycarbide (TiOC) is found to be an effective electrocatalyst for the electrochemical oxidation of ethanol to CO2. The complete course of this reaction is one of the main challenges in direct ethanol fuel cells (DEFCs). While TiOC has previously been investigated as catalyst support material only, in this study we show that TiOC alone is able to oxidize ethanol to acetaldehyde without the need of expensive noble metal catalysts like Pt. It is suggested that this behavior is attributed to the presence of both undercoordinated sites, which allow ethanol to adsorb, and oxygenated sites, which facilitate the activation of water. This is a milestone in DEFC research and development and opens up innovative possibilities for the design of catalyst materials for intermediate temperature fuel cells.

Electroreduction of CO2 in a Non-aqueous Electrolyte-The Generic Role of Acetonitrile.

Transition metal carbides, especially Mo2C, are praised to be efficient electrocatalysts to reduce CO2 to valuable hydrocarbons. However, on Mo2C in an aqueous electrolyte, exclusively the competing hydrogen evolution reaction takes place, and this discrepancy to theory was traced back to the formation of a thin oxide layer at the electrode surface. Here, we study the CO2 reduction activity at Mo2C in a non-aqueous electrolyte to avoid such passivation and to determine products and the CO2 reduction reaction pathway. We find a tendency of CO2 to reduce to carbon monoxide. This process is inevitably coupled with the decomposition of acetonitrile to a 3-aminocrotonitrile anion. Furthermore, a unique behavior of the non-aqueous acetonitrile electrolyte is found, where the electrolyte, instead of the electrocatalyst, governs the catalytic selectivity of the CO2 reduction. This is evidenced by in situ electrochemical infrared spectroscopy on different electrocatalysts as well as by density functional theory calculations.

A laboratory-based multifunctional near ambient pressure X-ray photoelectron spectroscopy system for electrochemical, catalytic, and cryogenic studies.

A versatile multifunctional laboratory-based near ambient pressure x-ray photoelectron spectroscopy (XPS) instrument is presented. The entire device is highly customized regarding geometry, exchangeable manipulators and sample stages for liquid- and solid-state electrochemistry, cryochemistry, and heterogeneous catalysis. It therefore delivers novel and unique access to a variety of experimental approaches toward a broad choice of functional materials and their specific surface processes. The high-temperature (electro)catalysis manipulator is designed for probing solid state/gas phase interactions for heterogeneous catalysts including solid electrolyzer/fuel cell electrocatalysts at pressures up to 15 mbar and temperatures from room temperature to 1000 °C. The liquid electrochemistry manipulator is specifically designed for in situ spectroscopic investigations of polarized solid/liquid interfaces using aqueous electrolytes and the third one for experiments for ice and ice-like materials at cryogenic temperatures to approximately -190 °C. The flexible and modular combination of these setups provides the opportunity to address a broad spectrum of in situ and operando XPS experiments on a laboratory-based system, circumventing the limited accessibility of experiments at synchrotron facilities.

Who Does the Job? How Copper Can Replace Noble Metals in Sustainable Catalysis by the Formation of Copper-Mixed Oxide Interfaces.

Following the need for an innovative catalyst and material design in catalysis, we provide a comparative approach using pure and Pd-doped LaCu x Mn1-x O3 (x = 0.3 and 0.5) perovskite catalysts to elucidate the beneficial role of the Cu/perovskite and the promoting effect of Cu y Pd x /perovskite interfaces developing in situ under model NO + CO reaction conditions. The observed bifunctional synergism in terms of activity and N2 selectivity is essentially attributed to an oxygen-deficient perovskite interface, which provides efficient NO activation sites in contact with in situ exsolved surface-bound monometallic Cu and bimetallic CuPd nanoparticles. The latter promotes the decomposition of the intermediate N2O at low temperatures, enhancing the selectivity toward N2. We show that the intelligent Cu/perovskite interfacial design is the prerequisite to effectively replace noble metals by catalytically equally potent metal-mixed-oxide interfaces. We have provided the proof of principle for the NO + CO test reaction but anticipate the extension to a universal concept applicable to similar materials and reactions.

The Potential of Zero Charge and the Electrochemical Interface Structure of Cu(111) in Alkaline Solutions.

Copper (Cu) is a unique electrocatalyst, which is able to efficiently oxidize CO at very low overpotentials and reduce CO2 to valuable fuels with reasonable Faradaic efficiencies. Yet, knowledge of its electrochemical properties at the solid/liquid interface is still scarce. Here, we present the first two-stranded correlation of the potential of zero free charge (pzfc) of Cu(111) in alkaline electrolyte at different pH values through application of nanosecond laser pulses and the corresponding interfacial structure changes by in situ electrochemical scanning tunneling microscopy imaging. The pzfc of Cu(111) at pH 13 is identified at -0.73 VSHE in the apparent double layer region, prior to the onset of hydroxide adsorption. It shifts by (88 ± 4) mV to more positive potentials per decreasing pH unit. At the pzfc, Cu(111) shows structural dynamics at both pH 13 and pH 11, which can be understood as the onset of surface restructuring. At higher potentials, full reconstruction and electric field dependent OH adsorption occurs, which causes a remarkable decrease in the atomic density of the first Cu layer. The expansion of the Cu-Cu distance to 0.3 nm generates a hexagonal Moiré pattern, on which the adsorbed OH forms a commensurate (1 × 2) adlayer structure with a steady state coverage of 0.5 monolayers at pH 13. Our experimental findings shed light on the true charge distribution and its interrelation with the atomic structure of the electrochemical interface of Cu.

Operando Fourier-transform infrared-mass spectrometry reactor cell setup for heterogeneous catalysis with glovebox transfer process to surface-chemical characterization.

We describe a new type of operando Fourier transform infrared (FTIR)-mass spectrometry setup for surface-chemical and reactivity characterization of heterogeneous catalysts. On the basis of a sophisticated all-quartz FTIR reactor cell, capable of operating between room temperature and 1000 °C in reactive gas atmospheres, the setup offers a unique opportunity to simultaneously collect and accordingly correlate FTIR surface-chemical adsorption data of the active catalyst state and FTIR gas phase data with complementary reactivity data obtained via mass spectrometry in situ. The full set of catalytic operation modes (recirculating static and flow reactor conditions) is accessible and can be complemented with a variety of temperature-programmed reaction modes or thermal desorption. Due to the unique transfer process involving a home-built portable glovebox to avoid air exposure, a variety of complementary quasi in situ characterization methods for the pre- and post-reaction catalyst states become accessible. We exemplify the capabilities for additional x-ray photoelectron spectroscopy characterization of surface-chemical states, highlighting the unique strength of combining adsorption, electronic structure, and reactivity data to gain detailed insight into the reactive state of a Cu/ZrO2 heterogeneous catalyst during methanol steam reforming operation.

Interfacial Water Structure as a Descriptor for Its Electro-Reduction on Ni(OH)2-Modified Cu(111).

The hydrogen evolution reaction (HER) has been crucial for the development of fundamental knowledge on electrocatalysis and electrochemistry, in general. In alkaline media, many key questions concerning pH-dependent structure-activity relations and the underlying activity descriptors remain unclear. While the presence of Ni(OH)2 deposited on Pt(111) has been shown to highly improve the rate of the HER through the electrode's bifunctionality, no studies exist on how low coverages of Ni(OH)2 influence the electrocatalytic behavior of Cu surfaces, which is a low-cost alternative to Pt. Here, we demonstrate that Cu(111) modified with 0.1 and 0.2 monolayers (ML) of Ni(OH)2 exhibits an unusual non-linear activity trend with increasing coverage. By combining in situ structural investigations with studies on the interfacial water orientation using electrochemical scanning tunneling microscopy and laser-induced temperature jump experiments, we find a correlation between a particular threshold of surface roughness and the decrease in the ordering of the water network at the interface. The highly disordered water ad-layer close to the onset of the HER, which is only present for 0.2 ML of Ni(OH)2, facilitates the reorganization of the interfacial water molecules to accommodate for charge transfer, thus enhancing the rate of the reaction. These findings strongly suggest a general validity of the interfacial water reorganization as an activity descriptor for the HER in alkaline media.

True Nature of the Transition-Metal Carbide/Liquid Interface Determines Its Reactivity.

Compound materials, such as transition-metal (TM) carbides, are anticipated to be effective electrocatalysts for the carbon dioxide reduction reaction (CO2RR) to useful chemicals. This expectation is nurtured by density functional theory (DFT) predictions of a break of key adsorption energy scaling relations that limit CO2RR at parent TMs. Here, we evaluate these prospects for hexagonal Mo2C in aqueous electrolytes in a multimethod experiment and theory approach. We find that surface oxide formation completely suppresses the CO2 activation. The oxides are stable down to potentials as low as -1.9 V versus the standard hydrogen electrode, and solely the hydrogen evolution reaction (HER) is found to be active. This generally points to the absolute imperative of recognizing the true interface establishing under operando conditions in computational screening of catalyst materials. When protected from ambient air and used in nonaqueous electrolyte, Mo2C indeed shows CO2RR activity.

Substantially Improved Na-Ion Storage Capability by Nanostructured Organic-Inorganic Polyaniline-TiO2 Composite Electrodes.

Developing sodium (Na)-ion batteries is highly appealing because they offer the potential to be made from raw materials, which hold the promise to be less expensive, less toxic, and at the same time more abundant compared to state-of-the-art lithium (Li)-ion batteries. In this work, the Na-ion storage capability of nanostructured organic-inorganic polyaniline (PANI) titanium dioxide (TiO2) composite electrodes is studied. Self-organized, carbon-coated, and oxygen-deficient anatase TiO2-x -C nanotubes (NTs) are fabricated by a facile one-step anodic oxidation process followed by annealing at high temperatures in an argon-acetylene mixture. Subsequent electropolymerization of a thin film of PANI results in the fabrication of highly conductive and well-ordered, nanostructured organic-inorganic polyaniline-TiO2 composite electrodes. As a result, the PANI-coated TiO2-x -C NT composite electrodes exhibit higher Na storage capacities, significantly better capacity retention, advanced rate capability, and better Coulombic efficiencies compared to PANI-coated Ti metal and uncoated TiO2-x -C NTs for all current rates (C-rates) investigated.

Hybrid synthesis of zirconium oxycarbide nanopowders with defined and controlled composition.

A combined synthesis strategy involving a carbothermal reduction and gelation approach with glycine as gelating agent was used to obtain Zr-based (oxy)carbide materials with defined and controlled composition. A comparatively low temperature approach (1500 °C) allows exploration of the ZrC-ZrO2 phase diagram and reproducibly leads to zirconium (oxy)carbide phases with different C/Zr ratios, as confirmed by combined X-ray diffraction (XRD) and transmission electron microscopy (TEM) data. The latter also indicates a chemically very homogeneous distribution of oxygen and carbon throughout the sample bulk, a prerequisite for further characterization of its intrinsic physico-chemical properties. Due to the general variability of the synthesis procedure - variation of metal precursor, amount of gelating agent and carbon precursor source - it is expected that the method can be easily adapted and transferred to other metal - oxycarbide materials.

An ultra-flexible modular high vacuum setup for thin film deposition.

A modular high vacuum chamber dedicated to thin film deposition is presented. We detail the vacuum and gas infrastructure required to operate two highly flexible chambers simultaneously, with a focus on evaporation techniques (thermal and electron beam) and magnetron sputtering, including baking equipment to remove residual water from the chamber. The use of O-ring-sealed flat flanges allows a tool-free assembly process, in turn enabling rapid changes of the whole setup. This leads to a high flexibility regarding the deposition techniques as the chamber can be adapted to different sources within minutes, permitting the formation of multilayer systems by consecutive depositions onto the same substrate. The central piece of the chamber is a flat flange ground glass tube or cross. The glass recipient permits optical monitoring of the deposition process. Further equipment, such as for the introduction of gases, additional pressure gauges, or evaporators, can be incorporated via specifically designed stainless steel/aluminum interconnectors and blank flanges. In the end, we demonstrate the preparation of an unsupported thin film system consisting of electron-beam-evaporated platinum nanoparticles embedded in magnetron-sputtered zirconia (ZrO2), deposited onto NaCl single crystals, which subsequently can be removed by dissolution. These films are further analyzed by means of transmission electron microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy.

Photoelectrocatalytic Synthesis of Hydrogen Peroxide by Molecular Copper-Porphyrin Supported on Titanium Dioxide Nanotubes.

We report on a self-assembled system comprising a molecular copper-porphyrin photoelectrocatalyst, 5-(4-carboxy-phenyl)-10,15,20-triphenylporphyrinatocopper(II) (CuTPP-COOH), covalently bound to self-organized, anodic titania nanotube arrays (TiO2 NTs) for photoelectrochemical reduction of oxygen. Visible light irradiation of the porphyrin-covered TiO2 NTs under cathodic polarization up to -0.3 V vs. Normal hydrogen electrode (NHE) photocatalytically produces H2O2 in pH neutral electrolyte, at room temperature and without need of sacrificial electron donors. The formation of H2O2 upon irradiation is proven and quantified by direct colorimetric detection using 4-nitrophenyl boronic acid (p-NPBA) as a reactant. This simple approach for the attachment of a small molecular catalyst to TiO2 NTs may ultimately allow for the preparation of a low-cost H2O2 evolving cathode for efficient photoelectrochemical energy storage under ambient conditions.

Erratum: Substrate Grain-Dependent Chemistry of Carburized Planar Anodic TiO2 on Polycrystalline Ti.

[This corrects the article DOI: 10.1021/acsomega.6b00472.].

Preferentially Oriented TiO2 Nanotubes as Anode Material for Li-Ion Batteries: Insight into Li-Ion Storage and Lithiation Kinetics.

Self-organized TiO2 nanotubes (NTs) with a preferential orientation along the [001] direction are anodically grown by controlling the water content in the fluoride-containing electrolyte. The intrinsic kinetic and thermodynamic properties of the Li intercalation process in the preferentially oriented (PO) TiO2 NTs and in a randomly oriented (RO) TiO2 NT reference are determined by combining complementary electrochemical methods, including electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic cycling. PO TiO2 NTs demonstrate an enhanced performance as anode material in Li-ion batteries due to faster interfacial Li insertion/extraction kinetics. It is shown that the thermodynamic properties, which describe the ability of the host material to intercalate Li ions, have a negligible influence on the superior performance of PO NTs. This work presents a straightforward approach for gaining important insight into the influence of the crystallographic orientation on lithiation/delithiation characteristics of nanostructured TiO2 based anode materials for Li-ion batteries. The introduced methodology has high potential for the evaluation of battery materials in terms of their lithiation/delithiation thermodynamics and kinetics in general.

Substrate Grain-Dependent Chemistry of Carburized Planar Anodic TiO2 on Polycrystalline Ti.

Mixtures or composites of titania and carbon have gained considerable research interest as innovative catalyst supports for low- and intermediate-temperature proton-exchange membrane fuel cells. For applications in electrocatalysis, variations in the local physicochemical properties of the employed materials can have significant effects on their behavior as catalyst supports. To assess microscopic heterogeneities in composition, structure, and morphology, a microscopic multitechnique approach is required. In this work, compact anodic TiO2 films on planar polycrystalline Ti substrates are converted into carbon/titania composites or multiphase titanium oxycarbides through carbothermal treatment in an acetylene/argon atmosphere in a flow reactor. The local chemical composition, structure, and morphology of the converted films are studied with scanning photoelectron microscopy, micro-Raman spectroscopy, and scanning electron microscopy and are related with the crystallographic orientations of the Ti substrate grains by means of electron backscatter diffraction. Different annealing temperatures, ranging from 550 to 850 °C, are found to yield different substrate grain-dependent chemical compositions, structures, and morphologies. The present study reveals individual time scales for the carbothermal conversion and subsequent surface re-oxidation on substrate grains of a given orientation. Furthermore, it demonstrates the power of a microscopic multitechnique approach for studying polycrystalline heterogeneous materials for electrocatalytic applications.

Electrochemical Capture and Release of CO2 in Aqueous Electrolytes Using an Organic Semiconductor Electrode.

Developing efficient methods for capture and controlled release of carbon dioxide is crucial to any carbon capture and utilization technology. Herein we present an approach using an organic semiconductor electrode to electrochemically capture dissolved CO2 in aqueous electrolytes. The process relies on electrochemical reduction of a thin film of a naphthalene bisimide derivative, 2,7-bis(4-(2-(2-ethylhexyl)thiazol-4-yl)phenyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (NBIT). This molecule is specifically tailored to afford one-electron reversible and one-electron quasi-reversible reduction in aqueous conditions while not dissolving or degrading. The reduced NBIT reacts with CO2 to form a stable semicarbonate salt, which can be subsequently oxidized electrochemically to release CO2. The semicarbonate structure is confirmed by in situ IR spectroelectrochemistry. This process of capturing and releasing carbon dioxide can be realized in an oxygen-free environment under ambient pressure and temperature, with uptake efficiency for CO2 capture of ∼2.3 mmol g-1. This is on par with the best solution-phase amine chemical capture technologies available today.