Symposium EM3 : Electronic and Ionic Dynamics at Solid-Liquid Interfaces

Conventional silicon based electronic computing devices use about one million times more energy to carry out a computing operation than does a mammalian brain. The devices, interconnections, and information processing paradigms in the latter are profoundly different from those used in today’s computers. Approaches to the development of extremely energy efficient computing will likely rely on devices that operate on entirely different principles, that are mutable, and which likely possess innately three dimensional structures and architectures. We discuss one possible approach that relies on the control of the conductivity of oxide thin films via tiny but reversible ionic currents of oxygen ions that are induced by very large electric fields at the interface with ionic liquids1. Removal of sub atomic percent concentrations of oxygen from structures that have open channels for the ready migration of oxygen gives rise to giant structural distortions2,3 and metallization of what were initially insulating layers. This may allow a path to innately mutable, cognitive switches.
1 Jeong, J. et al. Suppression of Metal-Insulator Transition in VO2 by Electric Field–Induced Oxygen Vacancy Formation. Science 339, 1402-1405, (2013).
2 Jeong, J. et al. Giant reversible, facet-dependent, structural changes in a correlated-electron insulator induced by ionic liquid gating. Proc. Natl. Acad. Sci. 112, 1013-1018, (2015).
3 Altendorf, S. G. et al. Facet-Independent Electric-Field-Induced Volume Metallization of Tungsten Trioxide Films. Adv. Mater. AOP, (2016).

Ferroelectrics are a classification of materials that spontaneously polarize, accumulating charge at interfaces, and have non-linear hysteretic polarization curves. Polarization switching fields required for ferroelectric materials are often very high, requiring thin insulating layers and high applied voltages. Film quality is extremely important as defects in the films can lead to shorts or depolarization fields that limit the functional properties and area that can be actively switched. Ionic liquids have recently received heavy interest for the formation of electronic double layers which lead to huge electric fields at interfacial regions with low applied biases. By utilizing this ion-induced charge accumulation at the interface of ferroelectric films, we will show that ionic liquid gating may offer the ideal solution to switch large regions of a ferroelectric film without limitations associated with film defects. This has great importance to practical applications and fundamental interface studies that require large sample regions to be uniformly polarized.

Due to unique fundamental behaviors of complex oxides, such as the subtle balance between competing electronic and magnetic phases, and their sensitivity to defects and doping, electric fields can be used to craft electronic order, modify chemistry, apply strain, and manipulate spin-orbit couplings (e.g. in the 5d oxides). This presents a promising opportunity to create novel functionalities, in principle, enabling device concepts that go far beyond what conventional semiconductor physics allows (i.e. adopting a scheme that emulates the neuron-circuit in the human brain). In particular, the very high charge density induced by an electric double layer (EDL) formed at an electrochemical solid-liquid interface has recently been used to induce or “gate” exotic phase transitions, therefore electronic ground states of strongly correlated oxides in the interfacial region, via ‘field-effect doping’. However, a number of intriguing aspects of the gate “knob” for researchers are still poorly defined and would be fertile ground for exploration in a broad range of oxides that exhibit such functionality. One outstanding argument to tackle: Is the EDL gating purely an electrostatic (electronic phenomena), or chemical redox effect (field-driven ionic motion), or due to other driven mechanism, or interplay of them?

To address this intriguing question, we carried out in-situ and real-time X-ray study to reveal structural basis (e.g. reconstruction and evolution) during EDL gating on various complex oxide heterostructures. The experimental findings from X-ray investigations illustrate distinct but contrasting structural responses to EDL field manipulation in these representative systems although most exhibit drastic modulation on transport due to the field effect. The structural behaviors, static and dynamic, in LAO/STO during EDL gating can be reconciled with the synergy of polar reconstruction, electrostriction and surface defects, which are intimately linked with the mechanism responsible for the establishment of 2DEG at this interface. In contrast, the structural evolution in nickelates/ruthenate/tungstate during EDL gating, respectively, is more consistent with controlling a metal-insulator transition by manipulating oxygen vacancies via redox chemistry. More interesting to note that shuttling oxygen vacancies could lead to emergent controllability on structural motif of complex oxides, such as creating metastable brownmiller-phase and dynamic tuning oxygen octahedral rotation.

A central problem in the modeling of materials at the atomic scale is the simulation of non-equilibrium phenomena involving charge transfer. An important example is the fundamental difficulty to describe the processes occurring during the discharge or the recharging of a Galvanic cell. Most methods, including DFT and conventional charge-equilibration methods, fail because they (must) assume the chemical potential to be constant at the beginning of the simulation. Such minimizations automatically annihilate all voltage in a full (nanoscale) Galvanic model cell.

In my talk, I present the split-charge equilibration method, which allows one to describe non-equilibrium redox reactions in force-field based simulations. This in turn enables one to simulate from atomistic principles the generic processes that occur during the discharge and the recharge of a Galvanic cell. An interesting side aspect of the split-charge method is that it can also be used to describe the dielectric response function of continuous media on coarse scales. This makes it a promising candidate for the multi-scale modeling of dielectric phenomena.

Electrical control of magnetism is an active research field that has strong potential in spintronic devices. Electrostatic doping of complex ferromagnetic (FM) oxides, such as manganites, is a promising approach to realize this control at interfaces of heterostructures. Recently, two directions in creating field effects have drawn particular attention, ferroelectric (FE) and ionic liquid (IL) gating. While for FE gating the electric polarization is the source of charge carriers, it is the electric double layer forming at IL interfaces that leads to charge accumulation orders higher than in conventional dielectric materials. However, in both cases direct probing of the field-induced changes in magnetic ground states and separating them from other origins has been challenging. For example, electrochemical changes due to oxygen migration have been found to cause drastic changes in IL gated manganites and possibly dominate over electrostatic doping. Here we present a combined approach of FE and IL gating to conclusively demonstrate reversible electric field control of interfacial magnetism in a La_0.8Sr_0.2MnO₃ (LSMO) / PbZr_0.2Ti_0.8O₃ (PZT) / IL system. While the PZT layer acts as an efficient barrier to IL induced oxygen migration, its FE polarization can be switched by applying a bias to the IL interfaced heterostructure. Using polarized neutron reflectivity we will show that the magnetization of an interfacial LSMO layer is reversibly controlled by the IL assisted FE switching, which is consistent with an electrostatic carrier modulation. These results demonstrate a new path to study magnetoelectric coupling mechanisms at FE/FM interfaces.

Although ionic liquids (ILs) have been used extensively in recent years as a high capacitance “dielectric” in electric double layer transistors (EDLTs), the dynamics of the double layer formation have remain relatively unexplored. Better understanding of the dynamics and relaxation processes involved in EDL formation will guide device optimization particularly with regard to switching speed. In this presentation, we explore the dynamical characteristics of an IL in a metal-ionic liquid-metal (M/IL/M) capacitor. In particular we examine an Au/IL/Au structure where the IL is 1-butyl-1-methyl pyrrolidinium tris-(pentafluoroethyl)trifluorophosphate ([P14]⁺[FAP]^-). The experiments consist of frequency-dependent impedance measurements and time dependent current vs. voltage measurements for applied linear voltage ramps and abrupt voltage steps. The parameters of an equivalent circuit model are determined by fits to the impedance vs. frequency data and subsequently verified by calculating the current vs. voltage characteristics for the applied potential profiles. The data analysis indicates that the dynamics of the structure are characterized by a wide distribution of relaxation times spanning the range of less than microseconds to longer than seconds.

Electric double layer (EDL) transistor is an incredibly rich playground to explore and manipulate physical properties of materials via electric-field since the EDL formed at an interface between electrolyte and solid channel can electrostatically induce a large amount of conducting charge carriers as large as 0.1 electron per atoms. Many exciting experiments in this decade has exhibited the tuning superconducting transition temperature (T_c) in single crystals of oxides [1] and transition chalcogenides [2], ferromagnetic properties [3], and metal-insulator transition in strongly correlated electron systems [4]. On the other hand, the electrochemical aspect of EDL, which has been long known as a cause of corrosion or used for coating a metal on solid surface, has been treated as an obstacle in electrostatic control of physical properties at the interface.
However, the electrochemical aspect of EDL can be a promising method to peel-off a van der Waals-type layered material. Layered materials has been intensively studied as a new platform for exploring novel two-dimensional quantum physical and chemical phenomena after the discovery of graphene field-effect-transistor (FET) [5]. Especially, cleaved a-few-layer FET structure often exhibits unique properties distinct from those of bulk form. Among them, the iron selenide (FeSe) superconductor exhibits very unique properties where the T_c is largely enhanced from bulk value (T_c ~ 8 K) when the monolayer is deposited on SrTiO₃ substrate. The first report of T_c ~ 60 K in monolayer FeSe using scanning tunnel spectroscopy (STS) [6] triggered an explosion of investigations which include STS, angle-resolved photoemission spectroscopy (ARPES) [7,8] and electrical transport measurements [6], searching for the role of FeSe / SrTiO₃ interface as an origin of high-T_c superconductivity. We demonstrate that the electrochemical reaction allows etching FeSe film layer-by-layer from thick film (over 20 monolayers) to one monolayer and thus, makes it possible to investigate systematic thickness dependence of superconductivity as well as electrostatic control of electrical properties [9] on various kinds of insulting substrate materials, which is difficult to be addressed by in-situ STS or ARPES measurements. In this talk, experimental schemes of both electrochemical etching of FeSe films and electrostatic control of the superconductivity will be presented in addition to the discussion on the origin of the high-T_c superconductivity.
[1] K. Ueno et al., J. Phys. Soc. Jpn. 83, 032001 (2014) and references therein. [2] W. Shi et al., Sci. Rep. 5, 12534 (2015) and references therein. [3] M. Weisheit et al., Science 315, 349 (2007). [4] M. Nakano et al., Nature 487, 459 (2012). [5] K. S. Novoselov et al., Science 306, 666 (2004). [6] Q. Y. Wang et al., Chin. Phys. Lett. 29, 037402 (2012). [7] S. He et al., Nat. Mater. 12, 605 (2013). [8] J. J. Lee et al., Nature 515, 245 (2014). [9] J. Shiogai et al., Nature Phys. 12, 42 (2016).

The field of autonomous systems is seeing a growth spurt. Intersections with the materials field includes topics such as neuromorphic electronics, exoskeleton design and adaptive components. These subjects draw from several fields including bio-inspired interfaces; collective behavior in nature such as swarms and evolutionary dynamics. Solid-liquid interfaces naturally occur in many of these themes. This serves as an inspiration to think broadly about interfacing liquids with tunable lattices and searching for emergent properties that arise when the system is perturbed from ground state. Starting from model systems of water-alumina interfaces, we will contrast properties of ionic liquid-complex oxide interfaces under strong electric fields. Ordering in liquids near solid interfaces, electrochemical instability driven massive electronic structure changes and ion exchange dynamics that mimics neural synapses will be considered.

To meet the continuous demand for more computing power, researchers are driven to shrink electrical circuitry to molecular-scale.[1] Toward making molecular-scale electronic device, one of the most important tasks is to look for molecules that mimic the electronic behavior of conventional silicon-based semiconductors (e.g. diode or rectifier). The predictability, diversity and programmability of DNA make it a leading candidate for the design of functional electronic devices.[2] Here we present a DNA-based single-molecule diode constructed by site-specific intercalation of small molecules (coralyne) into a custom-designed 11-base-pair DNA duplex (5’-CGCGAAACGCG-3’) which contains three mismatched A-A base pairs (bps) in the middle of the sequence.[3] We use scanning tunneling microscope break junction (STM-BJ) technique to wire DNA molecule to Au electrodes to form single-molecule junction and subsequently measure its conductance and current-voltage (I-V) characteristics.[4] The results of UV titration and circular dichroism experiments indicate that when adding coralyne into the studied DNA duplex, two coralyne molecules are able to intercalate into the DNA, forming a DNA-corlayne complex. Single-molecule conductance measurements of the DNA-corlayne complex junction show that its conductance under -0.9V almost triples that under 0.3V, a striking phenomenon at single-molecule level. We then measure the I-V curves of the DNA-coralyne complex and find that unexpectedly large rectification with sharp current increase under high negative bias occurs. The average rectification ratio of many DNA-coralyne complex junctions can reach up to 15 at 1.1V. To explain the observed rectification, we use a non-equilibrium Green's function-based model parameterized by density functional theory calculations. The calculation results reveal that the coralyne intercalation-induced spatial asymmetry in the electron state distribution is the source of the observed rectification, and this inherent asymmetry leads to changes in the coupling of the molecular HOMO−1 level to the electrodes when an external voltage is applied, resulting in an asymmetric change in transmission.
[1] D. Xiang, X. Wang, C. Jia, T. Lee, and X. Guo, Chem. Rev. 116, 4318 (2016).
[2] D. Cees and R. Mark, Physics World 14, 29 (2001).
[3] C. Guo, K. Wang, E. Zerah-Harush, J. Hamill, B. Wang, Y. Dubi, and B. Xu, Nature Chem. 8, 484 (2016).
[4] B. Q. Xu and N. J. Tao, Science 301, 1221 (2003).

Organic electronics has recently emerged as a powerful technology platform for bio-sensing applications. In many applications, the organic devices are operated in direct contact with an aqueous environment, thus resulting in strong coupling between charge transport in the organic solid and water dipoles, ions and cells in the liquid. However, a detailed microscopic characterization of the charge-ion-dipole interactions at the semiconductor-liquid interface is still lacking. In addition, the optical signatures of organic polymers, which are very sensitive to environmental variations and can be used to probe the electronic processes occurring at a molecular level, have not been fully utilized.
In this study, we fabricated a set of highly stable polymer based liquid-gated organic field effect transistors. Based on the device platform, we combined spectroscopic measurements (e.g. charge accumulation spectroscopy and charge modulation spectroscopy), electrical measurements and first-principle calculations to characterize the microscopic charge transport process at the semiconductor-liquid interface. Our results reveal that the effective polaronic absorption cross-section of the organic semiconductor reduces rapidly from bulk towards its interface with water, suggesting a highly disordered polymer monolayer at the interface region. It is also found that metal ions can selectively bound to polymer aggregates, resulting in a unique spectroscopic response to the ion diffusion. This study demonstrates for the first time the spectroscopic signatures of charge carriers at organic solid-liquid interface, and provides experimental and theoretical findings that can help to precisely interpret the concerted processes in a bioelectronics system.

The interaction of ions with charged solid-liquid interfaces is a critical feature for understanding a number of important phenomena, ranging from the transport of contaminants in the environment (e.g., at mineral-water interfaces) and an understanding of capacitive energy storage technologies (e.g., the organization of ions at the electrode electrolyte interface). The actual distribution of ions at the interfaces is normally obscured by the presence of the liquid phase. I will present recent work where we use X-ray based probes (e.g., x-ray reflectivity and resonant scattering) to observe the structures and interactions of ions at solid liquid interfaces through direct in-situ measurements. Examples will include: 1) metal ion adsorption at mineral-water interfaces with a focus on the muscovite-water. The results reveal the critical role of ion solvation in understanding structure and dynamics of cations adsorbed at mineral-water interfaces; and 2) the organization of room temperature ionic liquids at graphene surfaces, revealing the charge separated ion layers under applied potentials and insight into the unexpected slow processes and hysteresis observed with time-dependent potentials.

*This work was done in collaboration with Sang Soo Lee, Ahmet Uysal (Argonne National Laboratory), Kathryn Nagy and Neil Sturchio, (University of Illinois at Chicago) and others. Mineral-water interface work is supported by the DOE/BES/Geosciences Research Program, and the ionic liquid/graphene studies are funded by the Fluid Interface Structure Reactivity and Transport (FIRST) project, a DOE/BES Energy Frontier Research Center.

Electric double layers (EDLs) at liquid-solid interfaces have attracted much attention in soft-material electronics, because they can induce extremely large electric fields, through which effective carrier injection and charge separation can be realized. As representative EDL materials, ionic liquids possess excellent features for electronic application, such as a wide electrochemical window, low vapor pressure, and high chemical and physical stability. In this presentation, we describe our recent works [1] on organic field-effect transistors with ionic liquids as gate dielectrics. Secondly, we describe the opto-electronic conversion in organic photocells with a [Metal | Semiconductor | Electrolyte | Metal] structure, which can be used to effectively produce a polarization current under modulated light stimulus. We discuss their application to the information conversion in the NIR light range [2], as biosensors [3], and as AC photovoltaic cells based on the layered Perovskite compounds [4].

[1] T. Fujimoto, K. Awaga, Phys. Chem. Chem. Phys., 2013, 15, 8983.
[2] S. Dalgleish, M. M. Matsushita, L. Hu, B. Li, H. Yoshikawa, K. Awaga, J. Am. Chem. Soc., 2012, 134, 12742.
[3] S. Dalgleish, L. Reissig, Y. Sudo, K. Awaga, Chem. Commun., 2015, 51, 16401.
[4] S. Karak, C. Nanjo, M. Odaka, K. Yuyama, G. Masuda, M. M. Matsushita, K. Awaga, J. Mater. Chem. A, 2016, in press.

The traditional planar Si ISFET has been widely researched in the past decades. However, although the voltage sensitivity of ISFET nearly reaches the limit of 60mV/PH [1], the device still faces the issue of low reliability and limited current sensitivity. We propose with additional nanowire array in the device structure, the sensitivity area will increase significantly, so that the current sensitivity of the device is improved. The structure was first simulated and then also fabricated in the lab. The simulation and experimental results were consistent and showed that the current sensitivity of the device structure we introduced increased by 20% compared with the traditional ISFET devices. Based on these results, it is clearly demonstrated that the nanowire array ISFET is a promising device structure to further improve the performance of the ISFET device.

Understanding the structural, physical and chemical properties of the surface and interfaces of different metal-oxides and their possible applications in photocatalysis and biology is a very important emerging research field. Motivated in this direction, here we would like to enable understanding of how different fluids, particularly water, interact with oxide surfaces. We have studied the water contact angle of 3d transition metal oxide thin-films of SrTiO₃, and of 4f rare-earth oxide thin-films of Lu₂O₃. These metal oxides were grown using pulsed laser deposition and they’re atomically flat and with known orientation and explicitly characterized for their structure and composition. Further study was done on the effects of oxygen vacancies on the water contact angle of the 3d and 4f oxides. For 3d transition metal oxides with oxygen vacancies, we have observed an increase in hydroxylation with consequent increase of wettability which is in line with the previous reports whereas an interesting opposite trend was seen in the case of rare-earth oxides. Density function theory simulations of water interaction on the above mentioned systems have also been presented to further substantiate our experimental findings.

Understanding interaction of water molecules with the surface of different metal oxides has become a very interesting theme of research in last couple of years – for technological and environmental applications. In this work, recent claims about intrinsic hydrophobicity of Rare Earth Oxides (REOs) has been investigated in a more controlled manner. We have made an effort to measure the intrinsic behavior of oxides by means of conventional water contact angle (WCA) measurement and by a novel approach which uses force spectroscopy. We have prepared high quality single crystal epitaxial thin films of REOs and transition metal oxides with roughness (rms) below 1nm. It was found that, contrary to recent reports, almost all REO films are intrinsically extremely hydrophilic but undergo atmospheric stabilization when exposed to ambient environment. This causes the WCA to increase with time. With our new approach based on Force-Displacement spectroscopy, it was observed that pull-off forces on the surface of these films, decrease with time – another indication of increasing hydrophobicity. The nature of the evolution is similar for all the different oxides studied, albeit with different saturation values, suggesting that the atmospheric stabilization happening is of the same nature. We have used XPS to understand the surface chemistry of the oxides on exposure to atmosphere. It was found that air borne hydrocarbons and hydroxide species are present on the surface of the oxides on exposure to ambient atmosphere and the relative concentration evolves with time. We also employed a mass spectrometry based tool to identify methane as the main constituent of the hydrocarbon adsorbate.

The very high charge density induced by an electric double layer (EDL) formed at an electrochemical solid-liquid interface has recently been used to induce or “gate” exotic phase transitions, therefore electronic ground states of strongly correlated oxides in the interfacial region, via ‘field-effect doping’. Whether the EDL gating is purely an electrostatic (electronic phenomena), or chemical redox effect (field-driven ionic motion), or due to other driven mechanism, or interplay of them is under intense debate. To address this intriguing question, in this poster, we present in-situ and real-time X-ray surface scattering study of perovskite oxide heterostructure NdNiO₃ /NdGaO₃, as a model system, to reveal structural basis (e.g. reconstruction and evolution) during EDL gating. Significant structural responses to EDL field manipulation were observed by in-situ high-resolution surface X-ray diffraction while the drastic modulation on transport (e.g. resistive switching) was simultaneously monitored. The out-of-plane c-lattice spacing exhibited pronounced expansion with the maximal variation up to 0.13% as a positive gate voltage up to 2 V was applied. Meanwhile, the thin film displayed a well-shaped hysteresis loop on transverse resistance as a function of gate voltages, implying a bistable resistive switching induced by EDL gating. In contrast, the lattice showed negligible change at negative gate voltages when the critical temperature of insulator-to-metal transition in typical NdNiO₃ was effectively decreased by EDL gating. These X-ray findings can be reconciled with respect to the fundamental picture that electrostatic carrier modulation plays a dominant role in EDL gating while surface redox reaction is negligible (e.g. negative bias), whereas oxygen vacancy manipulation via redox chemistry can lead to the emergence of a strongly correlated insulating phase due to electron doping by oxygen vacancy (e.g. positive bias). Moreover, superlattice half-order thin film Bragg peaks (e.g. both intensities and positions) were also observed to be modulated accordingly upon EDL gating, suggesting a new controllability on structural motif of pervoskite oxides---dynamic tuning of oxygen octahedral rotation/tilting. Beyond typical ionic electrolytes in use, we are proposing to design a new category of ionic gels with multiple functional components that potentially incorporate more controllability via photo-electro-magnetic responses. We believe that this study provides some insightful understandings for exploration in a broad range of oxide heterostructures that exhibit exotic phenomena and multifaceted functionality enabled by EDL gating.

Oxide materials are studied for their unique properties, including superconductivity, colossal magnetoresistance, multi-ferroicity, and metal-insulator type transitions. Many of these properties can be controlled, enhanced, or suppressed through even slight changes to oxygen stoichiometry within the film. Tuning oxygen stoichiometry can be difficult to control and requires special attention to growth parameters and post process annealing. Ionic liquids have been used in many recent studies as an insulating gate medium for electric measurements on a wide variety of novel electronic materials. Here, the ionic liquids form electric double layers that give orders higher charge accumulation at interfaces than conventional dielectric materials, which have been used to control functionalities ranging from superconductivity to magnetic ordering. Importantly, the mechanism in these interfacial devices is still under debate as to the relative significance of electrostatic vs electrochemical changes to the underlying solid in dictating functionality. We present work describing the ability to finely control the oxygen stoichiometry of oxide thin films.

Ceria abrasives enables us to prepare extremely smooth glasses since ceria works as chemical polishing as well as mechanical polishing (chemical mechanical polishing, CMP). The mechanism of chemical polishing has still remained vague, but the redox of Ce⁴⁺/Ce³⁺ would play a major role for softening glass surface. This redox accompanies the process of the charge transfer at glass/slurry interface. Electron charge carrier would be important role in chemical polishing if the redox reaction occurs during polishing. The charge carrier profile would lead to clarify chemical polishing mechanism and quantitative guideline for developing innovative abrasives. The estimation of the interfacial area specific resistivity (ASR) at the glass/slurry interface is thus of great importance to clarify charge transfer mechanism and diffusion charge carrier during polishing. However, there are few data on charge carrier profiles during polishing. We then prepared the polishing model that it is possible to estimate slurry resistivity and the interfacial ASR. The interfacial ASR was measured using conventional ceria abrasive and soda-lime glass and the ASR was found to reflect amount of hydration layer by chemical polishing. Thus, the interfacial ASR would be quantitative index parameter of chemical polishing.
We also suppose that the chemical polishing has a time constant in the range between 70 ns and 700 ns by the results using SrZrO₃/ ZrO₂ nano-composite abrasives. Thus, we investigated the interfacial ASR with the nano-composite abrasives to clarify the time constant of chemical polishing. SrZrO₃/ ZrO₂ nano-composite particles were synthesized by spray pyrolysis and primary particle size of the composite was controlled by calcining the composite at various temperature. The glass/slurry interfacial ASR was detected by polishing with the composite abrasives at various relative velocities between glass and abrasives.

The GaN/ZnO alloy functions as a visible light photocatalyst for splitting water into hydrogen and oxygen when coupled with a co-catalyst for proton reduction. We have investigated computationally the microscopic structure of the aqueous interfaces of the (1010) and (1210) surfaces of the 1:1 GaN/ZnO alloy and compared them with the (1010) aqueous interfaces of pure GaN and ZnO.[1] The calculations were carried out using first principles density functional theory based molecular dynamics (DFT-MD). Water adsorption on all the surfaces was substantially dissociative through acid/base chemistry involving protonation of surface anions and hydroxylation of surface cations from dissociated water molecules. We further investigated the water oxidation mechanism on the prototypical GaN surface using a combined ab initio molecular dynamics and molecular cluster model approach taking into account the role of water dissociation and hydrogen bonding within the first solvation shell of the hydroxylated surface.[2] We also examined the sequential proton-coupled electron-transfer (PCET) steps with the proton transfer (PT) following the electron transfer (ET), and found that photo-generated holes prefer to localize on surface –N rather than –O sites after proton removal, i.e., the calculated free-energy changes indicate that PCET through –NH sites is thermodynamically more favorable than through –OH sites. However, proton transfer from –OH with subsequent localization of holes on the remaining oxygen atoms is kinetically favored (i.e., has a lower free-energy barrier) owing to hydrogen bonding interactions at the GaN–water interface. The catalytic water oxidation reaction proceeds through a sequence of four proton-coupled electron-transfer (PCET) steps starting with *OH^– and then through the intermediates *O^●–, *OOH^–, and *O₂^●– (where * represents a net +1 charged Ga surface site) before returning to the initial state by the elimination of O₂ and the addition of H₂O accompanied by the dissociation of a proton. The first step, *OH^– to *O^●–, requires the highest energy input.

More recently, we have been exploring the analogous dissociation of water molecules at the pristine rutile (110) and anatase (101) surfaces of TiO₂ as well as hole trapping in the bulk semiconductors and at their aqueous interfaces. We find extensive water dissociation (ca. 10 to 30%) on both surfaces that involves either the direct or bridging-water-mediated proton transfer from a Ti-bound water molecule to a bridging oxygen surface site. For rutile, the direct process dominates at room temperature, but both pathways are triggered by a hydration layer fluctuation.

[1] Ertem, M. Z.; Kharche, N.; Batista, V. S.; Hybertsen, M. S.; Tully, J. C.; Muckerman, J. T. ACS Catal. 2015, 5, 2317-2323.
[2] Kharche, N.; Hybertsen, M. S.; Muckerman, J. T. Phys. Chem. Chem. Phys. 2014, 16, 12057-12066.

The electronic structure of liquid water in the bulk has been extensively studied.^1,2 The interfacial water on various substrates, which attracts many attentions recently,^3,4 is, however, much less investigated. A liquid flow cell with gold-coated Si₃N₄ membrane, combined with x-ray absorption spectroscopy (XAS) taken by photon yield (FY) and electron yield (EY) detection techniques, has been proven to be an effective method to probe bulk and interfacial liquid species, respectively.³ The atoms from the interfacial species that interact directly with the gold electrode are expected to produce distinctive EY-XA signal when compared with the FY-XA signal that has major contributions from the bulk species upon the core-level resonant excitations. By adopting a similar measurement scheme as in ref. 3, we have studied a series of small liquid molecules, water (H-O-H), heavy water (D-O-D), diethylether (C-O-C), isopropanol (C-O-H) and acetone (C=O), in a flow cell by measuring partial fluorescence yield (PFY) and total electron yield (TEY) XA spectra at the O K-edge. The chemical symbols in parentheses for each molecule indicate the coordinated atoms bonded to the oxygen which is believed to be the anchor atom interacting directly with the gold electrode. Distinctive spectral features have been observed between the PFY- and TEY-XA spectra for each molecule indicating the presence of strong interactions of the oxygen with the gold electrode, and the characteristic spectral features are identified and then associated with the interfacial species. There are also spectral variations when comparing the TEY-XA spectra of different molecules, demonstrating the influence of local chemical environment around the oxygen atom on the interaction strength at the interface. This study provides a new insight into the electronic structure of interfacial species, and further promotes our knowledge of electrocatalysis and photochemistry which occur mainly at the liquid-electrode interface.

1. Wernet, P. et al. The Structure of the First Coordination Shell in Liquid Water. Science 304, 995–999 (2004).
2. Smith, J. D. et al. Probing the Local Structure of Liquid Water by X-ray Absorption Spectroscopy. J. Phys. Chem. B 110, 20038–20045 (2006).
3. Velasco-Velez, J.-J. et al. The structure of interfacial water on gold electrodes studied by x-ray absorption spectroscopy. Science 346, 831–834 (2014).
4. Winter, B. Interfaces: Scientists strike wet gold. Nat. Chem. 7, 192–194 (2015).

Oxides are a key material class for applications such as photo-catalysis or protective coating. In such applications the oxide’s growth and stability in electrochemical environment is a critical issue. Utilising the insight gained from our recently developed unified approach, which connects and “translates” concepts from semiconductor defect chemistry and electrochemistry [1], we construct ab-initio based surface Pourbaix diagrams [2] and defect stability phase diagrams [3,4] using ZnO as a prototype oxide considering the whole range of admissible chemical potential conditions. The obtained diagrams enable us to identify areas of interest in the context of electrochemical applications and reveal conditions under with the oxide becomes thermodynamically unstable with respect to its native point defects. The surface and bulk stability of ZnO will be discussed in the context of growth and dissolution of the oxide barrier layer forming in corrosive environment.

[1] M. Todorova and J. Neugebauer, Phys. Rev. Appl. 1 (2014) 014001.
[2] S. Yoo, M. Todorova and J. Neugebauer (in preparation).
[3] M. Todorova and J. Neugebauer, Surf. Sci. 631 (2015) 190-195.
[4] M. Todorova and J. Neugebauer, Farad. Discussions 180 (2015) 97-112.

The basic quantum chemical study of CO2 activation is presented. The molecular properties are discussed, describing the single molecule of CO2 on the surfaces, its geometry, wave functions, charges, and spectroscopic properties of vibrational frequencies, in vacuum and solutions. An answer to “What is the activation of CO2?” from the quantum chemical viewpoint is presented. Recent experimental results in hydrothermal condition are explained from the basic quantum chemical point of view, focusing on M-H and M-H-M speicies. Finally, the discussion will attempt to bridge these quantum chemical features and new proposal for CO2 activation.

Heterogeneous chemistry plays many vital roles, from the thermocatalytic, electrocatalytic and photocatalytic conversion of chemicals, to protecting the environment by removing unwanted and often toxic byproducts, to creating renewable forms of energy such as liquid fuels. Heterogeneous chemistry function by reactions on surfaces that provides alternative mechanisms for the making and breaking of chemical bonds. A series of intermediate chemical transformation steps at the surface and interface of solid promote and directs reactions through a selective chemical bond breaking and formation from one intermediate to the next. The catalytic transformation from reactants to products is a rare stochastic event. If we monitor the species at the surface and interface under operando conditions, we will observe the species limiting the overall reaction rate at high concentration due to the high barrier to the next step. The other intermediates, which may rule the overall selectivity, will be hard to see the overall concentration in operando conditions can be very low due to the low barrier to the next step. It is desired to capture information on all steps of forming and breaking bonds at surfaces and interfaces to achieve a predictive understanding of catalytic transformation. One can dream of use element-specific and site-specific probes of the electronic structure to follow the chemical bonding changes in real time during the chemical transformation. Ultrafast x-ray pulses from an x-ray free electron laser (FEL) is making a step closer to reality. In this talk, our recent studies of chemical transformation at surfaces using soft x-ray free electron laser on the ultrafast time scale by using soft X-ray resonant spectroscopy will be presented. [1-7]

References
1) M. Dell'Angela et al., Science 339, 1302 (2013)
2) M. Beye et al., Physical Review Letters 110, 186101 (2013)
3) T. Katayama et al., Journal of Electron Spectrosc, 9, 187, 2013.
4) H. Öström et al., Science 347, 978 (2015)
5) H. Xin et al., Physical Review Letters 114, 156101 (2015)
6) H. Öberg et al., Surface Science 640, 80 (2015)
7) M. Dell'Angela et al., Structural Dynamics 2, 025101 (2015)

Evaluation of water oxidation and carrier transfer between a single crystal n-type GaN photoelectrode and an electrolyte are useful to know the reaction mechanism of electrochemistry because GaN single crystal has less defect densities compared with the polycrystalline water oxidation catalyst. From the analysis of the photoluminescence and photoelectrochemical properties of GaN, it is clarified that the intermediate hole trap of GaN, which is the acceptor of the donor-acceptor (DA) complex related to the broad photoluminescence at around 2.2 eV (yellow luminescence: YL), plays an important role for the electron capture of water oxidation. The YL is also known to have very long life time of the excited state. This long life time is probably important to wait that the reacting molecule requires a certain time to change its shape to accept the next carrier.
The YL has also interesting properties that the luminescent intensity is proportional to the 1/3 of its excited light intensity when the excited intensity is about over the photoelectrochemical evaluations. From the comparison with YL and GaN band gap energies, the process is estimated to be 3 DA complex to 2 band edge free carrier Auger process. (The energy of 3 YLs (3 × 2.2 = 6.6 [eV]) is close to the energy of 2 free carriers (2 × 3.4 = 6.8 [eV]).) It is well known that the Auger process requires the reacted carriers have to localize within the region of the wave-function overlapping. For this reaction case, 3 DA complex requires to localize within this wave-function overlapping. However, the excited intensity is too small to generate the uniform Auger process for this case (about 1/10000).
The DA complex localized structures must be existed in the GaN photoelectrode from the result. Considering from the accumulation order of the DA complex, the complex need to localize near the dislocation lines with the complex of Ga vacancy hole trap and oxygen impurity donor [1]. Since the YL was reported to originate from the stacking fault in GaN crystal [2], the peripheral dislocation of the stacking fault is a candidate of the accumulation site of DA complex.
The “YL proportional to 1/3 of excitation” and the photoelectrochemical water oxidation are observed from MOVPE grown GaN, but are hardly observed from HVPE grown GaN. This shows that the DA complex localization is different with the dislocation property changing considering from the difference of stacking imperfectness between the GaN of MOVPE and HVPE observed from low-temperature PL. From these analyses, the important properties for photoelectrochemical water oxidation is probably not only the long life time of carrier excitation but also the high density of carrier localization.
[1] J. Elsner et al., Phys. Rev. B 58 (1998) 12571.
[2] Y. T. Rebane et al., Phys. Stat. Sol. (a) 164 (1997) 141.

Photogenerated electron holes are the key players in photoelectrochemical water oxidation. They provide the basis for direct solar fuel production in photoelectrochemical cells. The physics and the chemistry underlying the processes relevant for the water oxidation are probably the most complex known in physical chemistry. Therefore it is not surprising that the science of solar water splitting rests still on some speculative elements. Thanks to considerable progress in synchrotron radiation instrumentation, x-rays and electrons as fundamental probes for chemical and physical processes can now be used in complex electrocatalytic and photoelectrochemical experiments during actual device operation (1). The results of these studies which were impossible until very recently are indeed spectacular. We show (2-5) how we can assess with x-ray based ligand and valence band NEXAFS and AP-XPS spectroscopy the density of hole states in photoelectrodes as a function of electrochemical parameters and at the same time find quantitative information on the surface intermediates formed prior to, during and after water splitting. We are able to resolve the interaction of the photoelectrode with the electrolyte down to the Fe3d and O2p orbitals with bias parameterized energetic and spatial depth resolution, including the charge carrier accumulation layer, the electrode surface and the Helmholtz layer. The x-ray electronic structure data are in full alignment with the charge carrier dynamics probed with electroanalytical methods. Noteworthy is that we could verify and confirm a historically speculated second electron hole, which corresponds to a transition into the charge transfer band, which precedes the water splitting and coincides with the formation of a hydroxyl intermediate. Latter disappears when water oxidation sets on.

References
D. K. Bora et al. Between Photocatalysis and Photosynthesis: Synchrotron spectroscopy methods on molecules and materials for solar hydrogen generation, J. Electron Spectr. Rel. Phenom. 2013, 190 A, 93-105.
A Braun et al. The electronic, chemical and electrocatalytic processes and intermediates on iron oxide surfaces during photoelectrochemical water splitting, Catal. Today (2015) http://dx.doi.org/10.1016/j.cattod.2015.07.024
A. Braun et al. Direct observation of two electron holes in hematite during photo-electrochemical water splitting, J. Phys. Chem. C 2012, 116 (23) 16870-16875.
A. Braun et al. Iron resonant photoemission spectroscopy on anodized hematite points to electron hole doping during anodization, invited for Special Issue: Electrochemistry and Energy in ChemPhysChem 2012, 13(12), 2937-2944.
D.K. Bora et al. Evolution of an oxygen NEXAFS transition in the upper Hubbard band in α-Fe2O3 upon electrochemical oxidation, J. Phys. Chem. C, 2011, 115 (13), 5619–5625.

Tracking surface chemistry of a catalyst during catalysis is significant for fundamental understanding of catalytic performance of the catalyst since it allows for establishing an intrinsic correlation between surface chemistry of an active catalyst and its corresponding catalytic performance. Ambient pressure XPS (AP-XPS) can be used for in-situ studies of surfaces of different materials or devices in a gas. To simulate the gaseous environment of a catalyst in a fixed-bed reactor through in-situ/operando studies, a flowing gaseous environment of reactants around the catalyst is necessary. Here we developed a new flowing reaction cell for simulating in-situ study of a catalyst surface under a reaction condition in gas of one reactant or during catalysis in a mixture of reactants of a catalytic reaction. The reaction cell is installed in a high vacuum (HV) or ultrahigh vacuum (UHV) environment of a chamber. The flowing gas in the reaction cell is separated from the HV or UHV environment through well sealings at three interfaces between the reaction cell and X-ray window, sample door and aperture of front cone of an energy analyzer. With this cell, a catalyst at 800^oC in a flowing gas can be tracked readily. The capability of tracking surface chemistry of catalysts in gas of reactants under reaction conditions or during catalysis was demonstrated by the photoemission feature of Ag 3d of Ag thin film in 20 Torr N₂ with a flow rate of 4 ml/min.
In the second part of this talk, I will exemplify the in-situ studies of catalysts under reaction condition and during catalysis performed with AP-XPS. The focus of this part is the new information we could achieve from in-situ studies using AP-XPS and how the in-situ surface chemistry of catalysts uncovered with AP-XPS help fundamental understanding of catalytic performances at a molecular level.

Understanding the complex and inherently multi-scale interface between the battery electrode surface and an organic electrolyte would inform design of more efficient and less costly electrochemical energy storage devices. Joint density-functional theory (JDFT) [1,2] bridges the relevant length-scales by joining a fully ab initio description of the electrode with an accurate yet computationally efficient description of the liquid electrolyte structure, avoiding the costly statistical sampling of the liquid required by molecular dynamics calculations. We first present a microscopically informed continuum model for organic electrolyte which reproduces key solvation phenomena relevant to battery operation. Leveraging this solvation model within our framework to treat charged systems in periodic boundary conditions [1] we go on to predict how the voltage-dependent structure and energetics of the metal oxide cathode in Li-ion batteries change due to the presence of liquid electrolyte. Specifically, we find that compared to vacuum calculations [3], the voltage stability window of the Li-terminated (001) surfaces of the spinel LiMn₂O₄ battery cathode in solution is enhanced. We compare our predicted in situ surface structure and energetics with both molecular dynamics simulations and experiment, and discuss the broader impact of our findings upon reaction pathways for electrolyte decomposition and cathode dissolution.

Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

[1] K. Letchworth-Weaver and T.A. Arias, Phys. Rev. B. 86, 075140 (2012).
[2] D. Gunceler et al, Modelling Simul. Mater. Sci. Eng. 21,074005 (2013).
[3] R. Warburton et al, ACS Applied Materials and Interfaces, 8 (17), 11108–11121 (2016).

Single contact electrochemical molecular systems with self-assembled redox active monolayers are being widely investigated nowadays in various device applications including supercapacitors, sensors, batteries, solar cells, memristors etc. In these systems, electrochemistry plays a major role in determining the overall performance and efficiency of the device. The factors controlling the rate of electron transfer - such as the reorganization energy and the coupling strength may be obtained experimentally through Ultrafast voltammetric techniques. The high scan rates employed here requires efficient interfacial screening of the electrode potential in the solution. The resulting charging current associated with this ultrashort screening process is an important experimental determinant in finding both the reorganization energy and coupling strength through this method.

On the one hand, time-dependent decay of the charging current mitigates its impact on the current contribution from faradaic processes; while on the other hand, allowing substantial decay translates into a reduced upper-bound of applicable scan rates - which are crucial for ultrafast characterization. Analysis of the decay in the charging current suggests that the required screening can be achieved within the charging time constant. For weakly coupled systems, the scan rate corresponding to nanoscale charging time constants appears to be suitable for the ultrafast investigation of electron transfer characteristics. Moreover, the level of screening achieved at nanosecond decay times can change with the coverage of electrode surface by monolayers; where sharp changes in the time constant was calculated at successive saturation of interfacial layers by supporting ions.

These observations are expected to further our understanding of the relation of interfacial screening via packing density of a nanoelectrode surface with the ultimate monolayer characterization and faradaic efficiency in nano-electrochemical systems.

Spinel LiMn₂O₄ (LMO) is of considerable interest as a high voltage, low cost alternative to LiCoO₂ cathodes for Li-ion batteries. Mn³⁺ disproportionation at the electrode surface, however, leads to capacity fade process through Mn²⁺ dissolution into the electrode and deposition onto the counterelectrode.¹ Thin oxide films on the electrode surface effectively reduce Mn dissolution from LMO materials², but as yet, there is much fundamental knowledge to be gained regarding the electrode / film interface. Identifying these interfacial properties at the atomic-scale are critical to understanding and optimizing coated LMO electrodes.
Density Functional Theory (DFT) calculations have been performed to evaluate the thermodynamic stability of LMO surface structures. The surface energies of off-stoichiometric LMO surface terminations are calculated within a grand canonical ensemble, allowing for direct comparison to previously reported^3–5 surface structures and providing a thermodynamic basis for surface structures that are likely to be present on LMO nanoparticles. We have further extended this approach to select high index LMO surface facets to evaluate the stability of potential model systems for steps, edges, or defects that may be present on LMO particles. Results for the (111) surface are further compared with experimental X-ray reflectivity measurements on highly oriented LMO films.
In order to better understand the nature of the interface between LMO and protective films, DFT calculations are used in direct comparison with atomic layer deposition (ALD) experiments to describe the growth of ultrathin Al₂O₃ films on LMO, through alternating trimethylaluminum (TMA) and water exposures. Using previously identified stable surface terminations⁶, DFT-calculated demethylation kinetics suggest TMA is likely to lose its methyl groups on both low- and high-index facets of LMO in the first ALD half-reaction. Methyl fragments, coadsorbed on surface oxygen, inhibit further TMA adsorption, leading to sluggish film growth in early ALD cycles, as observed by quartz crystal microbalance (QCM) experiments. Such mechanistic analyses of the foundational layers of protective film growth can provide important structural insights toward an atomistic understanding of coated LMO performance.
References
(1) Zhan, C.; Lu, J.; Kropf, A. J.; Wu, T.; Jansen, A. N.; Sun, Y.-K.; Qiu, X.; Amine, K. Nat. Commun. 2013, 4, 2437.
(2) Park, J. S.; Meng, X.; Elam, J. W.; Hao, S.; Wolverton, C.; Kim, C.; Cabana, J. Chem. Mater. 2014, 26 (10), 3128–3134.
(3) Benedek, R.; Thackeray, M. M. Phys. Rev. B 2011, 83 (19), 195439.
(4) Karim, A.; Fosse, S.; Persson, K. A. Phys. Rev. B 2013, 87 (7), 075322.
(5) Kim, S.; Aykol, M.; Wolverton, C. Phys. Rev. B 2015, 92 (11), 115411.
(6) Warburton, R. E.; Iddir, H.; Curtiss, L. A.; Greeley, J. ACS Appl. Mater. Interfaces 2016, 8 (17), 11108–11121.

Energy harvesting from ambient water wave energy provides a viable off-grid, on-site and long-lasting power source for standalone and autonomous electronics. It can be potentially applied in a variety of circumstances wherever electricity is needed. Conventional approaches to harvesting water wave energy by using electromagnetic generators have drawbacks of bulky size and heavy weight. We develop a novel method for this purpose, which relies on the triboelectrification at the solid/liquid interface. A direct and reciprocating interaction between a dynamic water wave and a solid electrification thin-film produces interfacing charges that drive induced electrons to flow between the electrodes underneath the solid surface. In this process, the mechanical dynamic energy of the water wave is converted into electrical energy. In promoting the performance of the wave energy harvester, materials and device structures are exploited. The surface of the solid electrification layer is modified into an arrayed structure in nanoscale, which significantly enhances the solid/liquid contact area. Moreover, integration of electrode arrays is designed so that the device is applicable to diverse types of ambient waves, especially random one. Through the optimization, an output power at the scale of ~mW can be achieved.

The generated electricity is demonstrated in applications in two aspects. First, the produced electricity is stored and used to power wireless data transmission. A 10 × 6 × 0.02 cm³ device is sufficient to sustain intermittent RF transmission at an interval of about 30 s. This demonstration indicates applications of the wave energy harvester as a possible perpetual energy solution for wireless sensors in off-shore monitoring and surveillance. Second, we’ve also successfully utilized the produced electricity in environment-related applications, including metal corrosion protection and surface anti-biofouling. For example, the anti-adhesion efficiency against microbes is found to as high as 99.7% and proves to be effective for a variety of species.

References
G. Zhu et al. Harvesting Water Wave Energy by Asymmetric Screening of Electrostatic Charges on a Nanostructured Hydrophobic Thin-Film Surface. ACS Nano, 8, 2014, 6031-6037.
X. J. Zhao et al. Triboelectric Charging at the Nanostructured Solid/Liquid Interface for Area-Scalable Wave Energy Conversion and Its Use in Corrosion Protection. ACS Nano, 9, 2015, 7671-7677.
X. J. Zhao et al. Biocide-Free Antifouling on Insulating Surfaces by Wave-Driven Triboelectrification-Induced Potential Oscillation. Advanced Materials Interfaces, 2016, accepted.

Atomic force microscopes (AFMs) can now routinely measure single atomic point-defects at the solid-liquid interface. Recent advances have extended the AFM’s capabilities to measuring three-dimensional atomic-scale hydration structures above solid interfaces in aqueous media.
A detailed look at the mica/water interface using the latest methodology enables quantitative interpretation of the three-dimensional force field of the hydration structure. Atomic resolution is confirmed by the observed inhomogeneity caused by random substitution of Si by Al on the mica surface. Two distinct monotonic damping regimes are identified and attributed to the mesoscopic and nanoscopic tip apex structures. Combined with the observed dynamics of ions on the mica surface, these results paint a more comprehensive atomistic picture of the interactions between the tip and the sample as well as between the solvent and ions. This enables rigorous comparison between experimental data and theoretical predictions and simulations.
The necessity for photothermal excitation of the AFM cantilever to perform these experiments is justified by comparison to piezoacoustic excitation, and the discussion is extended to a broader scope of experiments. For example, recent results such as in situ cycling between calcite crystal growth and dissolution, solvation structures of highly viscous ionic liquids, and high-resolution imaging of DNA double helix were enabled by the stability and accuracy provided by photothermal excitation.
Lastly, existing limitations of AFMs for studies at the solid-liquid interface will be discussed and an outlook for future development will be explored. The use of small cantilevers for reducing noise and enabling fast scanning will be explained and backed by experiments along with simulations. Recent efforts in providing control over temperature, electrochemical potential, and solution exchange during atomic-scale imaging of the solid-liquid interfaces will be presented.

A large impact on understanding biological function results from new imaging modes. Atomic force microscopy (AFM) has been applied from its inception to imaging biological materials. In principle, besides structural and force imaging of biological materials and cells there has been a paucity of important functional AFM biological applications. Such applications include scanning electrochemical microscopy (SECM), nano-optical imaging of cell membranes (NSOM) or for that matter hybridization of AFM techniques with high impact areas such as patch clamping [1]. All highlight the importance of glass AFM sensors. This paper resolves issues that have prevented live cell imaging with these important methods that could image functional biological processes. Exemplary imaging with large force constants glass probes are reported with a refinement only achieved recently in AFM with ultrasoft cantilevers. These developments are extended to near-field optical live cell membrane imaging with new modes of contrast and the recording of cellular membrane potential locally in the near-field. The results portend an important impact on biology.

During the last decade a growing interest in the colloid and electrokinetic communities is emerged on a fundamental aspect of electrical double layers, namely, their capacitance and its dependence on the potential and the ionic strength [1]. Such interest is associated to the possibility of either obtaining energy or cleaning (desalinating) solutions based on the increased capacitance when the ionic strength is raised. In this work, we will focus on the second phenomenon, CDI or capacitive deionization [2], where the ions disolved in a salty solution are partially removed under the application of an electric potential when the solution fills the space between porous electrodes. Most previous works are based on the use of carbon electrodes for the electrodes either bare or in contact with ionic exchange membranes (MCDI). In this contribution we propose a new approach inspired in the electrokinetics of soft particles: a layer of polyelectrolyte (cationic on one electrode, anionic on the opposite one) coats the carbon electrodes, converting them in a sort of “soft” electrode pair [3]. The polyelectrolyte coatings play a similar role to the ionic exchange membranes, with the key advantage of cost, ease of preparation and, presumably, durability. We present a theoretical model and a set of experiments showing how soft electrodes perform in capacitive deionization in comparison with bare ones and with MCDI.

References
[1] Rica RA, Ziano, R, Salerno D, Mantegazza, F and Brogioli D. Thermodynamic Relation between Voltage-Concentration Dependence and Salt Adsorption in Electrochemical Cells. Phys Rev. Lett. 109 (2012) 156103.
[2] Porada S, Zhao R, van der Wal A, Presser V and Biesheuvel, PM. Review on the science and technology of water desalination by capacitive deionization. Progress in Mater. Sci. 58 (2013) 1388.
[3] Ahualli S, Jiménez ML, Fernández MM, Iglesias G, Brogioli D and Delgado A.V. Polyelectrolyte-coated carbons used in the generation of blue energy from salinity differences. Phys.Chem.Chem.Phys. 16 (2014) 25241.

Acknowledgement- The authors thank MINECO (Spain) and FEDER Funds (EU) for financial support through project FIS2013-47666-C3-1-R and S. Ahualli thanks to "Vicerrectorado de Investigación y Transferencia, Universidad de Granada" for her postdoctoral grant.

A molecular level understanding of the solid-liquid interface requires techniques using probes that can reach the interface and that are surface sensitive. Natural probes are photons, as they can penetrate transparent media, as is the case in many liquids. The surface sensitivity of photon techniques can be achieved based on symmetry selection rules, as in the case of sum-frequency generation. In my laboratory we have developed over x-ray based techniques that can provide electronic structure information of the interfaces. For x-ray absorption spectroscopy (XAS), to probe the empty density of states, or LUMO orbitals. We achieve this by collecting electrons emitted in the decay of the core hole at the electrode under study (deposited on the back of an x-ray transparent SiN membrane). Another technique that we have recently implemented is X'ray Photoelectron Spectroscopy (XPS), in a cell where a Si3N4 membrane is covered with ~ 1um diameter holes covered by a graphene layer. The graphene separates gas (up to 2-3 bar) or liquid from the vacuum chamber containing the electron analyzer. I will illustrate with the usefulness of these techniques by showing how the structure of water changes near Au and Pt electrodes in saline and acid electrolytes as a function of bias. I will also show examples of XPS gas phase at atmospheric pressure for catalysis studies.

Understanding the nature of the interface between the electrode and the electrolyte is essential in order to develop lithium ion batteries with enhanced cycle life and improved safety. While the electrolyte decomposition and formation of a Solid Electrolyte Interphase (SEI) at the negative electrode has been extensively studied [1,2], less is known on the reactivity at the positive electrode, usually a transition metal oxide, where a thin Electrode Electrolyte Interface (EEI) layer is also observed [3,4]. At voltages where the batteries usually operate the electrolyte is expected to be stable against oxidation and the formation of the EEI layer is attributed to chemical reactions of solvent molecules or solvated salt at the oxide surface.
To explore the chemical reactions leading to solvent decomposition we have studied the interaction of a commonly used solvent, ethylene carbonate (EC), with the surface of LiMO₂ (M = Mn, Co and Ni) layered oxides by using Hubbard-corrected Density Functional Theory (DFT+U) calculations. Different reaction mechanisms have been considered, including nucleophilic attack of lattice oxygen towards the solvent molecule and proton transfer between the solvent and the oxide. We show how the chemical reactivity at the oxide surface strongly depends on the transition metal and on the lithium content in the electrode. These findings are rationalized in terms of changes in the electronic structure of the oxide when the nature or the oxidation state of the transition metal cation is varied.

[1] E. Peled, J. Electrochem. Soc. 126, 2047 (1979).
[2] D. Aurbach et al., J. Power Sources 81–82, 95 (1999).
[3] K. Xu et al., Chem. Rev., 114, 11503 (2014).
[4] M. Gauthier, T. Carney, A. Grimaud et al., J. Phys. Chem. Lett. 6, 4653 (2015).

Solid-liquid interfaces play a vital role in various electrochemical and biological systems, but in situ characterization of such buried interfaces remains a major challenge. [1] Porphyrin molecules in solution assemble into ordered quasi-two dimensional films on various substrates, and thus offer a promising system for observing the interplay between molecule-molecule, molecule-substrate, molecule-solvent, and substrate-solvent interactions. Metalloporphyrin (i.e. porphyrins with metallic centers) complexes are particularly important and abundant in biological systems, as well as exhibiting electronic properties of interest in various nanotechnological applications.
Here we study the supramolecular self-assembly of copper (II) octaethylporphyrin (CuOEP) and octaethyporphyrin (H₂OEP) on graphitic surfaces immersed in organic solvents (dichlorobenzene, dodecane) using scanning tunneling microscopy (STM) and Raman spectroscopy. [2] Solvation effects on the self-assembly structure are clearly observed with STM. We find that dissolved molecular O₂ in DCB produces distinct changes in the molecular self-assembly of the molecules. Importantly no changes were observed on the H₂OEP molecules, confirming the important roles of the Cu metal center in binding of O₂. Raman spectroscopy measurements are performed using single-layer graphene grown by chemical vapor deposition,[3] in order to achieve surface sensitivity and so that small changes in doping can be resolved (compared to using bulk graphite). This reveals that the presence of oxygen within the DCB solution alters the molecule-substrate interaction, which we attribute to charge transfer from the graphitic substrate to facilitate oxygen adsorption on the Cu center of CuOEP. This same behavior was not observed with DD as the solvent, where the supramolecular assembly remains similar for H₂OEP and CuOEP whether molecular O₂ is dissolved in the solution or not, indicating that solvation effects dominate. We highlight that CuOEP adsorbed on graphitic surfaces is a promising model system relevant to the study of the transport and activation of oxygen by enzymes and other complexes.

(1) Wu et al. Phys. Chem. Chem. Phys. 2015, 17, 30229–30239.
(2) Hao et al. Langmuir 2016, 32, 5526–5531.
(3) Weatherup et al. J. Phys. Chem. Lett. 2016, 7, 1622–1627.

The adsorbed water thin layer on the two dimensional (2D) interface under ambient condition has different structural and dynamic behaviors, comparing to those of bulk water. It is also considered as a fundamental important field, due to its relevance in many aspects of daily life. However, the knowledge of the interfacial water adlayers are still lack and many findings and theories are still under debate.¹
I am interested in the structural and dynamic behaviors of the confined water layer between 2D material interfaces. In this presentation, I am going to review the recent results focusing on the exploration of the confined water between 2D materials and various surfaces under ambient conditions. Subsequently, I will introduce the recent results in my group, which studies the water adlayer growth and structures between hydrophilic and hydrophobic (graphene) interfaces.² The ice-like water adlayers have been identified, which are confined between hydrophobic graphene and hydrophilic substrate. Via varying the temperature, their nucleation process has been discussed. And it is found that, on one hand, the packing structures of the ice-like water adlayers are determined by that of hydrophilic interface; one the other hand, the graphene guides the orientation of the confined water domains. In the end, all of the obtained knowledge has been discussed, by comparing to the phenomena of the confined water between 2D hydrophilic interfaces, like MoS₂. These new finding potentially can be utilized to understand the boundary condition for water structures and dynamic behaviors at interfaces, and the aqueous interfacial chemistry
References:
[1] Two-Dimensional Material Confined Water, Li, Q.; Song, J.; Besenbacher F.; Dong, M.; Accounts of Chemical Research, 2015, doi: x.doi.org/10.1021/ar500306w
[2] Evidence of Stranski–Krastanov growth at the initial stage of atmospheric water condensation. Song, J.; Li, Q.; Wang, X. F.; Li, J. Y.; Zhang, S.; Kjems, J.; Besenbacher, F.; Dong, M.; Nature Communication, 2014, 5, 4837

Conventional density-functional theory (DFT) methods tend to energetically favour electron-density distributions with highly-delocalised, non-integer charges. This has significant implications for the computational modeling of charge transfer, solution-phase ions, and crystalline organic salts, to list a few examples. In this talk, a solvated-electron model system is used to illustrate an extreme case of density-driven delocalisation error. The application of classical polarisation models, such as continuum solvation, can serve to localise charges when used with DFT. However, this can lead to localisation of integer charges on less-stable sites in solution and prediction of spuriously-high barriers to charge transfer.

Extraction of crude oil from reservoirs is generally ineffective, as often the majority of the oil remains unrecovered. One of the limiting factors can be the adhesion of crude oil to the porous reservoir rocks. Our research aims to investigate crude oil/brine/rock (COBR) interactions, so that technologies can be developed to systematically influence adhesion and thus improve recovery efficiency. Force spectroscopy using an atomic force microscope (AFM) is a powerful tool to study these interactions on the individual mineral level, featuring nanometer spatial and piconewton force resolutions. In order to employ this technique at reservoir conditions, we have designed and commissioned a specialized AFM that can operate at temperatures up to 373 K (212 °F) and pressures up to 10 MPa (1450 psi) typically found in oil and gas reservoirs. We developed and manufactured a crude-oil functionalized AFM probe capable of operating at these conditions, and performed the first force spectroscopy measurements at elevated temperatures and pressures, successfully measuring the adhesion between the crude-oil probe and a mineral substrate. Our results show that temperature and pressure have significant impact on the oil/mineral interactions, which could translate into changing the response at bigger scales. Beyond this, the flexibility inherent in this design allows many variables to be manipulated with near-arbitrary precision, including temperature, pressure, ionic composition of the brine, and dissolved gases. Combined with the ability to use any crude oil or mineral surface, this system is capable of examining many interactions in unprecedented detail and thus inspiring the next generation of oil recovery enhancements.

Modeling the local distribution of charged ions and ionic defects at oxide/liquid solution interface is key to understanding related electrochemical processes, such as corrosion, hydrogen evolution, and oxygen reduction or evolution reactions. Based on the grand canonical approach which defines the electrochemical potential of individual charged species, a unified treatment of defects on the solid side and ions on water side can be established. This approach is compatible with first-principles calculations where the formation free energy of individual charged species can be calculated. In this work, we apply this framework to the system of ZrO₂/water interface, which is important in a wide range of applications including catalysis, biomaterials, and corrosion. Density functional theory calculations are performed to obtain defect formation energy in ZrO₂ and ab initio molecular dynamics is used to assess the formation free energy of H⁺ and OH^- ions in water at different distances from the ZrO₂ surface. The results are fed into a continuum model which produces the equilibrated distribution of these charged species. The continuum model considers explicitly both the ion adsorption and defect segregation in the vicinity of the interface, and the diffuse layer and space charge layer in the extended area. Such a unified description reveals the influence of solution chemistry on oxide defect chemistry, and vice versa. Our results show that ZrO₂ surface immersed in water is negatively charged with hydrogen substitutional defects and interstitials accumulates in the space charge layer. This framework based on the grand canonical approach allows easy inclusion of additional charged species into the system. The structure, defect chemistry and dynamical behaviors of the electric double layer and space charge layer are carefully analyzed with different pH, water chemistry and doping elements in zirconium oxide, which serves as the basis for modeling reaction and transport kinetics under these effects.

Recently, room temperature ionic liquid (RTIL) gating of transition metal oxides has enabled providing fundamental insight into physics of strongly correlated oxides. However, despite much research activity little is known on the structure of IL in contact with transition metal oxide surface and its evolution with gate voltage. In this second part, we use similar approach to investigate the structure of an IL at a semiconducting oxide interface during the gate biasing process.
The structure and dynamics of the solid/liquid interface are of fundamental interest for various energy related technologies involving RTIL, for example electrochemical capacitors (EC). Theories describing the electrical double layer (EDL) have been proposed; however, a full molecular level experimental description is lacking to fully understand its interfacial structures, dynamics and reactivity. Due to the characteristic sizes of EDLs, this requires characterization techniques capable of resolving nm length scales. Atomic Force Microscopy (AFM) has recently shown great interest for characterizing the electrochemical storage processes at the nanoscale, where local phenomena at the tip-sample junction can be measured and analyzed.
Here, we demonstrate the use of AFM based techniques to understand electrochemical processes at the solid/liquid interface and introduce statistical and theory coupled approaches. We will establish the technique to image EDL on HOPG model surfaces for EC applications and will show unexpected structural characteristics of the EDL. Then we will apply the technique to RTIL gated oxide devices. We will show that the transition between both ON and OFF states of the IL-gated amorphous indium gallium zinc oxide transistor is caused by a densification and preferential orientation of counter-ions at the oxide channel surface. This process occurs in three distinct steps, corresponding to regions of different electrical conductivity. In this case, the EDL thickness associated with the flat arrangement of cations at the surface results in extremely high charge density leading to high drain currents. Overall, the gating process is explained in terms of the interfacial IL structure.

For the past years, ionic liquids have been used as a major component in broad range of applications such as batteries, catalysts, transistors, etc. Therefore, a demand for deeper understanding of their dynamics and interactions with solid interface has also been on a rise. Together with a scaling trend of electronic devices, a tool which would allow extracting and resolving electrical, structural and spectroscopic information of the ionic liquid at the nanoscale range under operating condition of the device has become even more essential. On this note, the aberration-corrected low energy electron microscopy system (AC-LEEM) at Center for Functional Nanomaterials is a perfect tool for the task as it is capable of applying a lateral voltage in situ to the sample or passing a current through it. In other words, one can operate the device and examine structural, compositional or electrical changes in real time at the nanoscale domain. In this work, we present an in situ investigation of a few monolayer of 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide deposited on an insulating substrate, with patterned isolated and parallel gold bars serving as electrodes, under various in-plane biasing conditions. Long range and correlated ionic reconfigurations that occur near the electrodes were found to be a function of temperature and thickness which, in turn, relate to ionic mobility and different configuration for out-of-plane ordering near the electrode interfaces. These results also serve as a stepping stone toward developing a novel in operando experiment for Li-ion and Li-O₂ batteries with ionic liquid as electrolyte.

Acknowledgement: This research used resources of the Center for Functional Nanomaterials, which is the U.S. DOE Office of Science User Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

Light-emitting electrochemical cells (LEECs) utilizing small molecule emitters such as iridium complexes have great potential as low cost emissive devices. In these devices, ions rearrange during operation to facilitate carrier injection, bringing about efficient operation from simple, single layer devices. Recent work has shown that the luminance, efficiency, and responsiveness of iridium-based LEECs are greatly enhanced by the inclusion of small amounts of lithium salts (≤ 0.5%) into the active layer. However, the origin of this enhancement has yet to be demonstrated experimentally. Furthermore, although iridium-based devices have been the longstanding leader among small molecule LEECs, fundamental understanding of the ionic distribution in these devices under operation is lacking. Herein, we use scanning Kelvin probe microscopy to measure the in situ potential profiles and electric field distributions of planar iridium-based LEECs and clarify the role of ionic lithium additives. In pristine devices, it is found that ions do not pack densely at the cathode, and ionic redistribution is slow. Inclusion of small amounts of Li[PF₆] greatly increases ionic space charge near the cathode that doubles the peak electric fields and enhances electronic injection relative to pristine devices. This study confirms and clarifies a number of longstanding hypotheses regarding iridium-based LEECs and recent postulates concerning optimization of their operation.