The Role of TiO2 Doping on RuO2-Coated Electrodes for the Water Oxidation Reaction

Electrochemical water splitting into H2 and O2 presents a significant and challenging energy loss due to the high overpotential required at the anode. Today, in industrially relevant applications, dimensionally stable anodes (DSA) based on the electrocatalytic active RuO2 are conventionally utilized. To enhance the resistance against corrosion, incorporation of TiO2 in the RuO2-coated electrodes is widely employed. In the present work we have used scanning electrochemical microscopy (SECM) to demonstrate that TiO2-doped RuO2-coated electrodes, in addition to being more durable, also show an electrocatalytic activity that is, on average, 13% higher as compared to the pure RuO2-coated electrodes. We also demonstrate that cracks in the pure RuO2 coating are the most active zones, probably because Ti from the Ti support has diffused into the first applied layer of the RuO2 coating. To reveal the nature of this enhanced activity for water oxidation displayed on TiO2-doped RuO2 electrodes, we have employed X-ray photoelectron spectroscopy (XPS) for material characterization. The results show that the electrocatalytic activity enhancement displayed on the mixed (Ru1–x:Tix)O2 coating is promoted through a charge transfer from the RuO2 to the TiO2, which provides new and more reactive sites designated as activated RuO2δ+.This study has partly been carried out in the framework of the European Commission FP7 Initial Training Network “ELCAT”, Grant Agreement No. 214936-2. Portions of this research were performed at SPring-8 with the approval of Japan Synchrotron Radiation Research Institute as Nanotechnology Support Project of the Ministry of Education, Culture, Sports, Science and Technology (Proposal No. 2007A2005 and 2008A1671/BL-47XU)

Hydrogenation of O and OH on Pt(111): A comparison between the reaction rates of the first and the second hydrogen addition steps

The formation of water through hydrogenation of oxygen on platinum occurs at a surprisingly low reaction rate. The reaction rate limited process for this catalytic reaction is, however, yet to be settled. In the present work, the reaction rates of the first and the second hydrogen addition steps are compared when hydrogen is obtained through intense synchrotron radiation that induces proton production in a water overlayer on top of the adsorbed oxygen species. A substantial amount of the produced hydrogen diffuses to the platinum surface and promotes water formation at the two starting conditions O/Pt(111) and (H 2O+OH)/Pt(111). The comparison shows no significant difference in the reaction rate between the first and the second hydrogen addition steps, which indicates that the rate determining process of the water formation from oxygen on Pt(111) is neither the first nor the second H addition step or, alternatively, that both H addition steps exert rate control. © 2014 AIP Publishing LLC

Probing unoccupied electronic states in aqueous solutions

Water is one of the most common compounds on earth and is essential for all biological activities. Water has, however, been a mystery for many years due to the large number of unusual chemical and physical properties, e.g. decreased volume during melting and maximum density at 4 °C. The origin of the anomalies behavior is the nature of the hydrogen bond. This thesis will presented an x-ray absorption spectroscopy (XAS) study to reveal the hydrogen bond structure in liquid water. The x-ray absorption process is faster than a femtosecond and thereby reflects the molecular orbital structure in a frozen geometry locally around the probed water molecules. The results indicate that the electronic structure of liquid water is significantly different from that of the solid and gaseous forms. The molecular arrangement in the first coordination shell of liquid water is actually very similar as the two-hydrogen-bonded configurations at the surface of ice. This discovery suggests that most molecules in liquid water have two-hydrogen-bonded configurations with one donor and one acceptor hydrogen bond compared to the four-hydrogen-bonded tetrahedral structure in ice. This result is controversial since the general picture is that the structure of liquid water is very similar to the structure of ice. The results are, however, consistent with x-ray and neutron diffraction data but reveals serious discrepancies with structures based on current molecular dynamics simulations. The two-hydrogen-bond configuration in liquid water is rigid and heating from 25 °C to 90 °C introduce a minor change in the hydrogen-bonded configurations. Furthermore, XAS studies of water in aqueous solutions show that ion hydration does not affect the hydrogen bond configuration of the bulk. Only water molecules in the close vicinity to the ions show changes in the hydrogen bond formation. XAS data obtained with fluorescence yield are sensitive enough to resolved electronic structure of water molecules in the first hydration sphere and to distinguish between different protonated species. Hence, XAS is a useful tool to provide insight into the local electronic structure of a hydrogen-bonded liquid and it is applied for the first time on water revealing unique information of high importance

The Origin of Ti 1s XANES Main Edge Shifts and EXAFS Oscillations in the Energy Storage Materials Ti2CTx and Ti3C2T x MXenes

A potential application of two-dimensional (2D) MXenes, such as Ti2CTx and Ti3C2Tx, is energy storage devices, such as supercapacitors, batteries, and hydride electrochemical cells, where intercalation of ions between the 2D layers is considered as a charge carrier. Electrochemical cycling investigations in combination with Ti 1s x-ray absorption spectroscopy have therefore been performed with the objective to study oxidation state changes during potential variations. In some of these studies Ti3C2Tx has shown main edge shifts in the Ti 1s x-ray absorption near-edge structure. Here we show that these main edge shifts originate from the Ti 4p orbital involvement in the bonding between the surface Ti and the termination species at the fcc-sites. The study further shows that the t2g–eg crystal field splitting (10Dq) observed in the pre-edge absorption region indicate weaker Ti–C bonds in Ti2CTx and Ti3C2Tx compared to TiC and the corresponding MAX phases. The results from this study provide information necessary for improved electronic modeling and subsequently a better description of the materials properties of the MXenes. In general, potential applications, where surface interactions with intercalation elements are important processes, will benefit from the new knowledge presented.Funding agencies: the Swedish Research council under contract 2018-07152, the Swedish Governmental Agencyfor Innovation Systems under Contract 2018-04969,and Formas under Contract 2019-02496. We also thank the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-MatLiU No. 2009 00971). M M acknowledges financial support from the Swedish Energy Research (Grant No. 43606-1) and the Carl Tryggers Foundation (CTS20:272, CTS16:303, CTS14:310). The FEFF calculations of scattering paths were performed using supercomputer resources provided by the National Academic Infrastructure for Supercomputing in Sweden (NAISS) at Linköping University funded by the Swedish Research Council through GrantAgreement No. 2022-06725</p

XPS spectra curve fittings of Ti3C2Tx based on first principles thinking

MXenes are an interesting family of 2D materials that have the potential to meet challenges in many applications. A useful tool in the work of understanding the nature of the MXenes, as well as exploring their capabilities, is Xray photoelectron spectroscopy (XPS). In analyzing XPS spectra it might be necessary to use curve fitting to extract valuable information. However, approaches toward the curve fitting procedure have been different in many studies and introductions of questionable assumptions, unverified feature assignments, and inconsistent curve fitting have led to contrasting conclusions from XPS analysis. It is therefore motivated to show curve fittings of F 1s, O 1s, Ti 2p, and C 1s XPS spectra obtained from high quality Ti3C2Tx that are based on fundamental knowledge applied step by step through the strategy of first principles thinking. With the use of first principles thinking the curve fittings and the subsequent analysis became more realistic compared to what have been presented in recent studies. The results of the curve fittings presented in this work are well founded and can be used as a model for future curve fittings of MXenes. The strategy of first principles thinking is advantageous in XPS curve fittings in general.Funding Agencies|Swedish Foundation for Strategic Research (SSF) [EM16-0004]; Swedish Research Council (VR) [642-2013-8020]; KAW Fellowship/Scholar program</p

Local chemical bonding and structural properties in Ti3AlC2 MAX phase and Ti3C2Tx MXene probed by Ti 1s x-ray absorption spectroscopy

The chemical bonding within the transition-metal carbide materials MAX phase Ti3AlC2 and MXene Ti3C2Txis investigated by x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine-structure(EXAFS) spectroscopies. MAX phases are inherently nanolaminated materials that consist of alternating layersof Mn+1Xn and monolayers of an A-element from the IIIA or IVA group in the Periodic Table, where M is atransition metal and X is either carbon or nitrogen. Replacing the A-element with surface termination speciesTx will separate the Mn+1Xn-layers forming two-dimensional (2D) flakes of Mn+1XnTx. For Ti3C2Tx the Tx corresponds to fluorine (F) and oxygen (O) covering both sides of every single 2D Mn+1Xn-flake. The Ti K-edge(1s) XANES of both Ti3AlC2 and Ti3C2Tx exhibit characteristic preedge absorption regions of C 2p-Ti 3dhybridization with clear crystal-field splitting while the main-edge absorption features originate from the Ti1s → 4p excitation, where only the latter shows sensitivity toward the fcc-site occupation of the terminationspecies. The coordination numbers obtained from EXAFS show that Ti3AlC2 and Ti3C2Tx are highly anisotropicwith a strong in-plane contribution for Ti and with a dynamic out-of-plane contribution from the Al monolayersand termination species, respectively. As shown in the temperature-dependent measurements, the O contributionshifts to shorter bond length while the F diminishes as the temperature is raised from room temperature up to 750 °C.Funding agencies: Swedish Research CouncilSwedish Research Council [2018-07152]; Swedish Governmental Agency for Innovation SystemsVinnova [2018-04969]; FormasSwedish Research Council Formas [2019-02496]; Swedish Research Council (VR) LiLi-NFM Linnaeus EnvironmentSwedish R</p

Chemical bonding in carbide MXene nanosheets

tThe chemical bonding in the carbide core and the surface chemistry in a new group of transition-metalcarbides Tin+1Cn-Tx(n = 1,2) called MXenes have been investigated by surface-sensitive valence bandX-ray photoelectron spectroscopy. Changes in band structures of stacked nano sheets of different thick-nesses are analyzed in connection to known hybridization regions of TiC and TiO2that affect elastic andtransport properties. By employing high excitation energy, the photoelectron cross-section for the C 2s– Ti 3d hybridization region at the bottom of the valence band is enhanced. As shown in this work, theO 2p and F 2p bands strongly depend both on the bond lengths to the surface groups and the adsorptionsites. The effect of surface oxidation and Ar+sputtering on the electronic structure is also discussed

X-ray Photoelectron Spectroscopy of Ti3AlC2, Ti3C2Tz, and TiC Provides Evidence for the Electrostatic Interaction between Laminated Layers in MAX-Phase Materials

The inherently nanolaminated Ti3AlC2 is one of the most studied MAX-phase materials. MAX-phases consists of two-dimensional Mn+1Xn-layers (e.g., T3C2-layers) with strong internal covalent bonds separated by weakly interacting A-layers (e.g., Al-layers), where the repetitive stacking of the Mn+1Xn-layers and the A-layers suggests being the foundation for the unusual but attractive material properties of the MAX-phases. Although being an important parameter, the nature of the bonding between the Mn+1Xn-layers and the A-layers has not yet been established in detail. The X-ray photoelectron spectroscopy data presented in this paper suggest that the weak interaction between the Ti3C2-layers and the Al-layers in Ti3AlC2 is through electrostatic attraction facilitated by a charge redistribution of the delocalized electrons from the Ti3C2-layers to the Al-layers. This charge redistribution is of the same size and direction as between Ti atoms and Al atoms in TiAl alloy. This finding opens up a pathway to predict and improve MAX-phase materials properties through A-layer alloying, as well as to predict new and practically feasible MXene compounds.Funding Agencies|Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [EM160004]; Swedish Research Council (VR)Swedish Research Council [6422013-8020]; KAW Fellowship/Scholar program</p

Chemical Bonding of Termination Species in 2D Carbides Investigated through Valence Band UPS/XPS of Ti3C2Tx MXene

MXenes are technologically interesting 2D materials that show potential in numerous applications. The properties of the MXenes depend at large extent on the selection of elements that build the 2D MX-layer. Another key parameter for tuning the attractive material properties is the species that terminate the surfaces of the MX-layers. Although being an important parameter, experimental studies on the bonding between the MX-layers and the termination species are few and thus an interesting subject of investigation. Here we show that the termination species fluorine (F) bonds to the T3C2-surface mainly through Ti 3p – F 2p hybridization and that oxygen (O) bonds through Ti 3p – O 2p hybridization with a significant contribution of Ti 3d and Ti 4p. The study further shows that the T3C2-surface is not only terminated by F and O on the threefold hollow face-centered-cubic (fcc) site. A significant amount of O sits on a bridge site bonded to two Ti surface atoms on the T3C2-surface. In addition, the results provide no support for hydroxide (OH) termination on the T3C2-surface. On the contrary, the comparison of the valence band intensity distribution obtained through ultraviolet- and x-ray photoelectron spectroscopy with computed spectra by density of states, weighed by matrix elements and sensitivity factors, reveals that OH cannot be considered as a inherent termination species in Ti3C2Tx. The results from this study have implications for correct modeling of the structure of MXenes and the corresponding materials properties. Especially in applications where surface composition and charge are important, such as supercapacitors, Li-ion batteries, electrocatalysis, and fuel- and solar cells, where intercalation processes are essential.Funding agencies: Proposals 20190625, 20191107, and 20200582, the Swedish ResearchCouncil under contract 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496, the Swedish Research Council through Grant Agreement No.2016-07213, the Swedish Research Council (VR) LiLi-NFM Linnaeus Environmentand Project Grant No. 621-2009-5258, the SwedishGovernment Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009-00971), the Swedish Energy Research (GrantNo. 43606-1) and the Carl Tryggers Foundation (CTS16:303,CTS14:310, CTS20:272).</p