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An experimental and theoretical study of RuO2/TiO2 heterostructures as photoelectrocatalysts for hydrogen evolution reaction (HER).

Heterostructured RuO2–Fe2O3 nanocomposite can be viewed as a bi-functional electrocatalyst for HER and OER because it exploits the synergistic effects of heterostructures and active sites at its interface.

The design of active and selective co-catalysts constitutes one of the major challenges in developing heterogeneous photocatalysts for energy conversion applications. This work provides a comprehensive insight into thermally induced bottom-up generation and transformation of a series of promising Cu-based co-catalysts. We demonstrate that the volcano-type HER profile as a function of calcination temperature is independent of the type of the Cu precursor but is affected by changes in oxidation state and location of the copper species. Supported by DFT modeling, our data suggest that low temperature (200 °C) do not affect the Cu oxidation state, but induce a gradual, temperature-dependent surface-to-bulk diffusion of Cu, which results in interstitial, tetra-coordinated Cu+ species. The disappearance of Cu from the surface and the introduction of new defect states is associated with a drop in HER performance. This work examines electronic and structural effects that are in control of the photocatalytic activity and can be transferred to other systems for further advancing photocatalysis., Calcination of Cu/TiO2 is detrimental to its photocatalytic HER performance. We relate this to heat-activated Cu diffusion into the TiO2 lattice, which decreases accessibility of the Cu sites on the surface and generates charge recombination centers.

In this study we investigate how electronic properties of buried semiconductor/ionic liquid interfaces may influence on charging effects observed by X-ray photoelectron spectroscopy (XPS). Droplets of the ionic liquid 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide ([EMIM][Tf2N]) were deposited on silicon and germanium semiconductor surfaces with different doping states. Core level spectra were recorded from the ionic liquid/ultra-high vacuum interface with and without irradiation of the droplets using low energy electrons from a flood gun. Changing electron intensity results in shifts of the binding energy of all XPS signals to lower values. Magnitude of shifts is found to directly depend on the intensity of the electron beam and to correlate with the nominal resistivity of the substrates. The observations are qualitatively explained by flood gun induced changes of the electronic properties of the [EMIM][Tf2N]/substrate interface and suggested to refer to space charge layer effects.

The ionic liquid [EMIM][TFSI] was investigated by X-ray Photoelectron Spectroscopy (XPS) on different substrates, ranging from metallic to semiconductor type materials. Parallel angle-resolved XPS was performed utilising a wide-angle lens hemispherical analyser with an acceptance angle of 60°. ARXPS data show no preferential orientation/accumulation of certain species at the ionic liquid/ultra-high vacuum interface. Charging caused by the electron emission process and electron flood gun experiments was recorded. The data indicate that the binding energies of the spectra depend on the properties of the substrate material, specifically on the substrate/ionic liquid interface. Observations are discussed by means of electric double layer and other processes that may effect on the potential of the ionic liquid.

We investigated the effect of different electrolyte additives such as vinyl carbonate (VC), succinic anhydride (SA), and lithium bis(oxalato)borate (LiBOB) on the chemical composition of Solid Electrolyte Interphase (SEI) layer and the electrochemical performance of Si/MC electrodes. The electrochemical behavior of the Si/MC electrodes was evaluated by galvanostatic charge/discharge and rate capability tests. It was found that various electrolyte compositions result in the formation and the resulting properties of the SEI of the electrodes with different compounds. From X-ray Photoelectron Spectroscopy (XPS) analysis, the electrodes cycled in SA- and LiBOB-containing electrolytes show a higher amount of lithium carbonates and a thicker SEI layer than those in standard and VC-containing electrolyte. The electrode with VC 5w% shows an excellent reversible capacity of ∼1100 mAh g −1 up to 100 charge-discharge cycles.

The aging behavior of electrochemical double layer capacitors (EDLCs) based on activated carbon electrodes bound with poly(tetrafluoroethylene) (PTFE) was tested in electrolyte solutions based on acetonitrile (AN) and propylene carbonate (PC) at a constant elevated cell voltage of 3.5 V. The aging was quantified in terms of capacitance loss and resistance increase for the full cell and the individual electrodes. It is shown that the enhanced aging rate of symmetric EDLCs in either solvent at elevated voltages is dominated by the aging of a single electrode, and that the polarity of this limiting electrode depends directly on the solvent. In AN, the positive electrode ages much more rapidly than the negative, while in PC the negative electrode exhibits faster aging than the positive. After aging, the electrodes were investigated by nitrogen adsorption and X-ray photoelectron spectroscopy, revealing significant modifications of the electrode surface and providing clear evidence for the deposition of electrolyte degradation products on the electrodes.

Simple tripeptides are scaffolds for the synthesis and further assembly of peptide/silver nanoparticle composites. Herein, we further explore peptide-controlled silver nanoparticle assembly processes. Silver nanoparticles with a pH-responsive peptide coating have been synthesized by using a one-step precipitation/coating route. The nature of the peptide/silver interaction and the effect of the peptide on the formation of the silver particles have been studied via UV/Vis, X-ray photoelectron, and surface-enhanced Raman spectroscopies as well as through electron microscopy, small angle X-ray scattering and powder X-ray diffraction with Rietveld refinement. The particles reversibly form aggregates of different sizes in aqueous solution. The state of aggregation can be controlled by the solution pH value. At low pH values, individual particles are present. At neutral pH values, small clusters form and at high pH values, large precipitates are observed.

The electrochemical doping of single-walled carbon nanotubes (SWCNTs) in 1 M Et4NBF4 in acetonitrile was investigated by in situ Raman spectroscopy. The capacitance was determined to be 82 F/g for the positive and 71 F/g for the negative SWCNT electrode, respectively, which approaches the typical values for microporous activated carbons used in supercapacitors. The changes in the Raman intensities and shifts of the D and G+ bands as well as of the radial breathing modes (RBMs) during electron and hole injection were studied as a function of the electrode potential. For the D and G+ bands, hole doping leads to strong upshifts which can be attributed to a stiffening of C–C bonds and the corresponding phonon modes. Electron doping results in much less pronounced changes in the band positions. The intensity attenuation of the RBM bands was found to be markedly different for semi-conducting and metallic SWCNTs, whereby sufficiently high doping leads to a loss of Raman intensity due to bleaching of electronic transitions. The main RBM bands upshift upon both electron and hole doping, which is attributed to changes in the chemical environment of individual SWCNTs upon charging and discharging of the electrochemical double layer within SWCNT bundles.

Passive layers on cobalt were prepared in 0.1 N NaOH (pH 13) under potentiostatic control with systematic variation of the relevant parameters, such as time and potential, and examined by x-ray photoelectron spectroscopy (XPS). The electrochemical preparation of the sputter-cleaned samples and their transfer to the ultrahigh vacuum was performed in a closed system under protection of purified argon. Evaluation of the XPS signals occurs on the basis of well-characterized standards. The results are compared with those obtained in borate buffer (pH 9.3), which have been published previously in this journal. Depending on the potential, cobalt forms two different passive layers. In 0.1 N NaOH (pH 13) the potential range of primary passivity goes from -0.4 to 0.3 V SHE . The passive layer consists of a pure Co(OH) 2 film that grows from 10 nm after 5 min of passivation up to 37 nm within 90 min. The layer forms most probably according to a dissolution precipitation mechanism. For E > 0.0 V SHE a thinner layer of only 4 nm (E = 0.2 V SHE for 5 min) is formed. In the range of secondary passivity at E = 0.3-0.8 V SHE Co(III) dominates the layer composition with contributions of Co 3 O 4 , CoOOH and Co 2 O 3 . After 5 min of passivation the layer thickness in this potential range increases slightly with increasing potential from 4 up to 7 nm. After 90 min the thickness reaches 9 nm, independent of the potential.

Passive layers on cobalt were prepared under potentiostatic control in borate buffer pH 9.3 and examined with x-ray photoelectron spectroscopy (XPS). The electrochemical preparation of the sputter-cleaned samples and their transfer to the ultrahigh vacuum was performed in a closed system under protection of purified argon. The XPS signals were evaluated quantitatively on the basis of standard spectra, which yields the composition of the passive layer, i.e. the contribution of the cobalt and oxygen species Co(0), Co(II), Co(III), OH−, O2− and H2O. Depending on the potential, cobalt forms two different passive layers. The primary passive film consists of Co(II) oxide and Co(II) hydroxide and is formed at low potentials. At high potentials a secondary passive layer is formed consisting of Co(II)/Co(III) mixed oxides. The oxidation to Co(III) is related to a significant increase of the layer thickness from 1 to 4 nm. Time-resolved XPS investigations show that the primary passive film grows within