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1.
ACS Appl Mater Interfaces ; 12(5): 6565-6572, 2020 Feb 05.
Artigo em Inglês | MEDLINE | ID: mdl-31825591

RESUMO

Conformationally rigid multipodal molecules should control the orientation and packing density of functional head groups upon self-assembly on solid supports. Common tripods frequently fail in this regard because of inhomogeneous bonding configuration and stochastic orientation. These issues are circumvented by a suitable tetrapodal diazatriptycene moiety, bearing four thiol-anchoring groups, as demonstrated in the present study. Such molecules form well-defined self-assembled monolayers (SAMs) on Au(111) substrates, whereby the tetrapodal scaffold enforces a nearly upright orientation of the terminal head group with respect to the substrate, with at least three of the four anchoring groups providing thiolate-like covalent attachment to the surface. Functionalization by condensation chemistry allows a large variety of functional head groups to be introduced to the tetrapod, paving the path toward advanced surface engineering and sensor fabrication.

2.
Inorg Chem ; 58(24): 16609-16617, 2019 Dec 16.
Artigo em Inglês | MEDLINE | ID: mdl-31769972

RESUMO

The use of low-temperature solution synthesis followed by a brief annealing step allows metastable single-phase Co3B nanoparticles to be obtained, with sizes ranging from 11 to 22 nm. The particles are ferromagnetic with a saturation magnetization of 91 A m2 kg-1 (corresponding to 1.02 µB/Co) and a coercive field of 0.14 T at 5 K, retaining the semihard magnetic properties of bulk Co3B. They display a magnetic blocking temperature of 695 K and a Curie temperature near 710 K, but the measurement of these high-temperature properties was complicated by decomposition of the particles during heating in the magnetometer. Additionally, the nanoparticles of Co3B were investigated as an electrocatalyst in the oxygen evolution reaction and showed a low onset potential of 1.55 V vs RHE. XPS measurements were performed before and after the electrocatalytic measurements to study the surface of the catalyst, to pinpoint what appear to be the active surface species.

3.
Phys Chem Chem Phys ; 21(9): 5086-5096, 2019 Feb 27.
Artigo em Inglês | MEDLINE | ID: mdl-30762849

RESUMO

The formation of heterostructures has proven to be a viable way to achieve high photoelectrochemical water splitting efficiencies with BiVO4 based photoanodes. Especially, cobalt and nickel based oxides are suitable low cost contact materials. However, the exact role of these contact materials is not yet completely understood because of the difficulty to individually quantify the effects of surface passivation, charge carrier separation and catalysis on the efficiency of a heterostructure. In this study, we used photoelectron spectroscopy in combination with in situ thin film deposition to obtain direct information on the interface structure between polycrystalline BiVO4 and NiO, CoOx and Sn-doped In2O3 (ITO). Strong upwards band bending was observed for the BiVO4/NiO and BiVO4/CoOx interfaces without observing chemical changes in BiVO4, while limited band bending and reduction of Bi and V was observed while forming the BiVO4/ITO interface. Thus, the tunability of the Fermi level position within BiVO4 seems to be limited to a certain range. The feasibility of high upwards band bending through junctions with high work function (WF) compounds demonstrate that nickel oxide and cobalt oxide are able to enhance the charge carrier separation in BiVO4. Similar studies could help to identify whether new photoelectrode materials and their heterostructures would be suitable for photoelectrochemical water splitting.

4.
Nat Commun ; 9(1): 4794, 2018 11 12.
Artigo em Inglês | MEDLINE | ID: mdl-30420751

RESUMO

The original version of this Article contained an error in Fig. 2b in which the bottom of the pink-shaded conduction band region was incorrectly positioned at a value of 1.75 eV. The correct conduction band minimum has a value of 2.2 eV. This has now been corrected in both the PDF and HTML versions of the Article.

5.
Nat Commun ; 9(1): 4309, 2018 10 17.
Artigo em Inglês | MEDLINE | ID: mdl-30333488

RESUMO

The optical band gap is a major selection criterion for an absorber in photocatalytic water splitting. Due to its ideal value hematite has been intensively investigated without reaching the expectation, yet. In this work, the Fermi level positions in hematite due to doping and contact formation are investigated. An upper boundary for the Fermi level position at 1.8 eV above the valence band maximum due to the formation of polarons is identified. This results in a different concept of the effective band gap for hematite which we believe is transferable to any material with competing polaron formation after optical excitation: the optical band gap of 2.2 eV deviates from an effective electronic band gap of 1.75 eV. The polaron state acts as a limit in (quasi-)Fermi level shift, restricting the potential of charge transfer reactions. Additionally, it has led to an incorrect determination of the band edge positions of hematite in electrochemical contacts.

6.
Nanomaterials (Basel) ; 8(9)2018 Sep 06.
Artigo em Inglês | MEDLINE | ID: mdl-30200568

RESUMO

In this work, the fabrication of zinc oxide (ZnO) nanowire networks is presented. By combining ion-track technology, electrochemical deposition, and atomic layer deposition, hierarchical and self-supporting three-dimensional (3D) networks of pure ZnO- and TiO2-coated ZnO nanowires were synthesized. Analysis by means of high-resolution transmission electron microscopy revealed a highly crystalline structure of the electrodeposited ZnO wires and the anatase phase of the TiO2 coating. In photoelectrochemical measurements, the ZnO and ZnO/TiO2 nanowire networks, used as anodes, generated higher photocurrents compared to those produced by their film counterparts. The ZnO/TiO2 nanowire network exhibited the highest photocurrents. However, the protection by the TiO2 coatings against chemical corrosion still needs improvement. The one-dimensionality of the nanowires and the large electrolyte-accessible area make these 3D networks promising photoelectrodes, due to the improved transport properties of photogenerated charge carriers and faster redox reactions at the surface. Moreover, they can find further applications in e.g., sensing, catalytical, and piezoelectric devices.

7.
Phys Chem Chem Phys ; 20(32): 21144-21150, 2018 Aug 15.
Artigo em Inglês | MEDLINE | ID: mdl-30079411

RESUMO

High-resolution surface-sensitive synchrotron radiation photoelectron spectroscopy was used to study the interaction of water with the p-GaInP2(100) surface covered with submonolayer residual native oxide in order to get insight into water dissociation at the solar water-splitting photocathodes in real liquid environment. In the surface-sensitive valence band spectra features related to Ga-OH, In-OH, and H-In-OH bonds appear after emersion of the p-GaInP2(100) surface from liquid water at room temperature. Indium core levels remain intact after emersion, while the gallium core levels indicate transformation of gallium oxides to hydroxides, as well as the accumulation of metallic gallium. Surface sensitive P 2p core level spectra indicate formation of P-H bonds after emersion. These changes of the surface chemical bonds can be attributed to the dissociation of the water molecules on the p-GaInP2(100) surface, leading to the subsequent transformation of surface oxides to hydroxides. Interaction of water with the p-GaInP2(100) surface covered with submonolayer residual native oxide causes an increase in the work function by 80 meV and a modification of the valence band edge spectrum, which is evidence of a change of the surface dipole due to the charge redistribution induced by the transformation of the surface oxides to hydroxides.

8.
Rev Sci Instrum ; 89(7): 073104, 2018 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-30068114

RESUMO

A model all-solid-state battery cell with a thin film NaxCoO2 cathode was assembled under ultra-high vacuum conditions and cycled inside the vacuum chamber, using a dedicated sample holder. We present in-operando x-ray photoelectron spectroscopy measurements of a NaxCoO2 cathode at different charging states. During battery operation, the change in sodium content, the change in cobalt oxidation state, and the evolution of the O1s and VB emissions could be monitored. Comparison with a conventional post-mortem analysis technique showed that the new measurement technique produces comparable results regarding the oxidation state of the transition metal, but sodium and oxygen results show differences due to cathode/electrolyte interfacial reactions for conventional analysis. By using surface layer-free samples in the presented techniques, we could circumvent such reactions and obtain reliable spectra for the pure bulk-like active cathode material.

9.
ChemSusChem ; 11(18): 3150-3156, 2018 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-30047577

RESUMO

High-performance catalysts for the oxygen-evolution reaction in water electrolysis are usually based on expensive and rare elements. Herein, mixed-metal borides are shown to be competitive with established electrocatalysts like noble metal oxides and other transition-metal(oxide)-based catalysts. Iron incorporation into nanoscale dicobalt boride results in excellent activity and stability in alkaline solutions. (Co0.7 Fe0.3 )2 B shows an overpotential of η=0.33 V (1.56 V vs. RHE) at 10 mA cm-2 in 1 m KOH with a very low onset potential of ≈1.5 V vs. RHE, comparable to the performance of IrO2 and RuO2 . XPS shows that the original catalyst is modified under the reaction conditions and indicates that CoOOH and Co(OH)2 are formed as active surface species, whereas the Fe remains in the catalyst, contributing to an improved catalyst performance. The nanoscale borides are obtained by a one-step solution synthesis, calcined, and characterized by XRD, energy-dispersive X-ray spectroscopy, and SEM. Single crystals of (Co1-x Fex )2 B grown under chemical transport conditions were used for an unambiguous specification of the nanostructured particles by relating the cobalt/iron ratio to the lattice parameters.

10.
J Phys Chem Lett ; 9(13): 3731-3737, 2018 Jul 05.
Artigo em Inglês | MEDLINE | ID: mdl-29923411

RESUMO

Studying the structure-property relations of tailored dipolar phenyl and biphenylphosphonic acids, we report self-assembled monolayers with a significant decrease in the work function (WF) of indium-tin oxide (ITO) electrodes. Whereas the strengths of the dipoles are varied through the different molecular lengths and the introduction of electron-withdrawing fluorine atoms, the surface energy is kept constant through the electron-donating N, N-dimethylamine head groups. The self-assembled monolayer formation and its modification of the electrodes are investigated via infrared reflection absorption spectroscopy, contact angle measurements, and photoelectron spectroscopy. The WF decrease in ITO correlates with increasing molecular dipoles. The lowest ever recorded WF of 3.7 eV is achieved with the fluorinated biphenylphosphonic acid.

11.
ACS Appl Mater Interfaces ; 10(14): 11414-11419, 2018 Apr 11.
Artigo em Inglês | MEDLINE | ID: mdl-29557162

RESUMO

We tune the Fermi level alignment between the SnO x electron transport layer (ETL) and Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 and highlight that this parameter is interlinked with current-voltage hysteresis in perovskite solar cells (PSCs). Furthermore, thermally stimulated current measurements reveal that the depth of trap states in the ETL or at the ETL-perovskite interface correlates with Fermi level positions, ultimately linking it to the energy difference between the Fermi level and conduction band minimum. In the presence of deep trap states, charge accumulation and recombination at the interface are promoted, affecting the charge collection efficiency adversely, which increases the hysteresis of PSCs.

12.
ChemSusChem ; 11(5): 948-958, 2018 03 09.
Artigo em Inglês | MEDLINE | ID: mdl-29227580

RESUMO

Ni nanoparticles (NPs) consisting of Ni, NiO, and Ni(OH)2 were formed on Ti substrates by electrodeposition as electrocatalysts for the hydrogen evolution reaction (HER) in alkaline solution. Additionally, the deposition parameters including the potential range and the scan rate were varied, and the resulting NPs were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy. The chemical composition of the NPs changed upon using different conditions, and it was found that the catalytic activity increased with an increase in the amount of NiO. From these data, optimized NPs were synthesized; the best sample showed an onset potential of approximately 0 V and an overpotential of 197 mV at a cathodic current density of 10 mA cm-2 as well as a small Tafel slope of 88 mV dec-1 in 1 m KOH, values that are comparable to those of Pt foil. These NPs consist of approximately 25 % Ni and Ni(OH)2 each, as well as approximately 50 % NiO. This implies that to obtain a successful HER electrocatalyst, active sites with differing compositions have to be close to each other to promote the different reaction steps. Long-time measurements (30 h) showed almost complete transformation of the highly active catalyst compound consisting of Ni0 , NiO, and Ni(OH)2 into the less active Ni(OH)2 phase. Nevertheless, the here-employed electrodeposition of nonprecious metal/metal-oxide combination compounds represents a promising alternative to Pt-based electrocatalysts for water reduction to hydrogen.


Assuntos
Galvanoplastia/métodos , Hidrogênio/química , Nanopartículas Metálicas/química , Níquel/química , Catálise , Técnicas Eletroquímicas
13.
ACS Appl Mater Interfaces ; 9(45): 39821-39829, 2017 Nov 15.
Artigo em Inglês | MEDLINE | ID: mdl-29052974

RESUMO

Nickel oxide (NiO) is a widely used material for efficient hole extraction in optoelectronic devices. However, its surface characteristics strongly depend on the processing history and exposure to adsorbates. To achieve controllability of the electronic and chemical properties of solution-processed nickel oxide (sNiO), we functionalize its surface with a self-assembled monolayer (SAM) of 4-cyanophenylphosphonic acid. A detailed analysis of infrared and photoelectron spectroscopy shows the chemisorption of the molecules with a nominal layer thickness of around one monolayer and gives an insight into the chemical composition of the SAM. Density functional theory calculations reveal the possible binding configurations. By the application of the SAM, we increase the sNiO work function by up to 0.8 eV. When incorporated in organic solar cells, the increase in work function and improved energy level alignment to the donor does not lead to a higher fill factor of these cells. Instead, we observe the formation of a transport barrier, which can be reduced by increasing the conductivity of the sNiO through doping with copper oxide. We conclude that the widespread assumption of maximizing the fill factor by only matching the work function of the oxide charge extraction layer with the energy levels in the active material is a too narrow approach. Successful implementation of interface modifiers is only possible with a sufficiently high charge carrier concentration in the oxide interlayer to support efficient charge transfer across the interface.

14.
ACS Appl Mater Interfaces ; 8(33): 21824-31, 2016 Aug 24.
Artigo em Inglês | MEDLINE | ID: mdl-27452037

RESUMO

Energy band alignments at heterointerfaces play a crucial role in defining the functionality of semiconductor devices, yet the search for material combinations with suitable band alignments remains a challenge for numerous applications. In this work, we demonstrate how changes in deposition conditions can dramatically influence the functional properties of an interface, even within the same material system. The energy band alignment at the heterointerface between Cu2O and ZnO was studied using photoelectron spectroscopy with stepwise deposition of ZnO onto Cu2O and vice versa. A large variation of energy band alignment depending on the deposition conditions of the substrate and the film is observed, with valence band offsets in the range ΔEVB = 1.45-2.7 eV. The variation of band alignment is accompanied by the occurrence or absence of band bending in either material. It can therefore be ascribed to a pinning of the Fermi level in ZnO and Cu2O, which can be traced back to oxygen vacancies in ZnO and to metallic precipitates in Cu2O. The intrinsic valence band offset for the interface, which is not modified by Fermi level pinning, is derived as ΔEVB ≈ 1.5 eV, being favorable for solar cell applications.

15.
ACS Nano ; 10(6): 5999-6007, 2016 06 28.
Artigo em Inglês | MEDLINE | ID: mdl-27228558

RESUMO

Recently developed organic-inorganic hybrid perovskite solar cells combine low-cost fabrication and high power conversion efficiency. Advances in perovskite film optimization have led to an outstanding power conversion efficiency of more than 20%. Looking forward, shifting the focus toward new device architectures holds great potential to induce the next leap in device performance. Here, we demonstrate a perovskite/perovskite heterojunction solar cell. We developed a facile solution-based cation infiltration process to deposit layered perovskite (LPK) structures onto methylammonium lead iodide (MAPI) films. Grazing-incidence wide-angle X-ray scattering experiments were performed to gain insights into the crystallite orientation and the formation process of the perovskite bilayer. Our results show that the self-assembly of the LPK layer on top of an intact MAPI layer is accompanied by a reorganization of the perovskite interface. This leads to an enhancement of the open-circuit voltage and power conversion efficiency due to reduced recombination losses, as well as improved moisture stability in the resulting photovoltaic devices.

16.
ACS Appl Mater Interfaces ; 8(7): 4940-5, 2016 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-26829619

RESUMO

Multilayer solution-processed devices in organic electronics show the tendency of intermixing of subsequently deposited layers. Here, we synthesize naphthalene tetracarboxydiimide (NDI)-based n-type semiconducting polymers with thermally cleavable side chains which upon removal render the polymer insoluble. Infrared and photoelectron spectroscopy were performed to investigate the pyrolysis process. Characterization of organic field-effect transistors provides insight into charge transport. After the pyrolysis homogeneous films could be produced which are insoluble in the primary solvent. By varying curing temperature and time we show that these process parameters govern the amount of side chains in the film and influence the device performance.

17.
Phys Chem Chem Phys ; 18(16): 10751-7, 2016 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-26823011

RESUMO

To achieve an overall efficient solar water splitting device, not only the efficiencies of photo-converter and catalyst are decisive, but also their appropriate coupling must be considered. In this report we explore the origin of a voltage loss occurring at the interface between a thin film amorphous silicon tandem cell and the TiO2 corrosion protection layer by means of XPS. We find that the overall device can be disassembled into its primary constituents and that they can be analyzed separately, giving insight into the device structure as a whole. Thus, a series of model experiments were conducted, each representing a part of the complete device. We finally arrive at the conclusion, that the formation of a SiO2 interfacial layer between the TiO2 protection layer and the silicon cell gives rise to the voltage loss observed for the whole device.

18.
Phys Chem Chem Phys ; 18(16): 10708-18, 2016 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-26694730

RESUMO

To reduce energy losses in water electrolysers a fundamental understanding of the water oxidation reaction steps is necessary to design efficient oxygen evolution catalysts. Here we present CoOx/Ti electrocatalytic films deposited by thermal and plasma enhanced chemical vapor deposition (CVD) onto titanium substrates. We report electrochemical (EC), photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) measurements. The electrochemical behavior of the samples was correlated with the chemical and electronic structure by recording XPS spectra before and after each electrochemical treatment (conditioning and cyclovoltammetry). The results show that the electrochemical behavior of CoOx/Ti strongly depends on the resulting electronic structure and composition. The thermal deposition leads to the formation of a pure Co(II)Ox which transforms to a mixed Co(II)Co(III)Ox during the OER. This change in oxidation state is coupled with a decrease in overpotential from η = 0.57 V to η = 0.43 V at 5 mA cm(-2). Plasma deposition in oxygen leads to a Co(III)-dominated mixed CoOx, that has a lower onset potential as deposited due to a higher Co(III) content in the initial deposited material. After the OER XPS results of the CoOx/Ti indicate a partial formation of hydroxides and oxyhydroxides on the oxide surface. Finally the plasma deposition in air, results in a CoOxOH2 surface, that is able to completely oxidizes during OER to an oxyhydroxide Co(III)OOH. With the in situ formed CoOOH we present a highly active catalyst for the OER (η = 0.34 at 5 mA cm(-2); η = 0.37 V at 10 mA cm(-2)).

19.
Langmuir ; 31(37): 10303-9, 2015 Sep 22.
Artigo em Inglês | MEDLINE | ID: mdl-26315142

RESUMO

In this work we examine small conjugated molecules bearing a thiol headgroup as self assembled monolayers (SAM). Functional groups in the SAM-active molecule shift the work function of gold to n-channel semiconductor regimes and improve the wettability of the surface. We examine the effect of the presence of methylene linkers on the orientation of the molecule within the SAM. 3,4,5-Trimethoxythiophenol (TMP-SH) and 3,4,5-trimethoxybenzylthiol (TMP-CH2-SH) were first subjected to computational analysis, predicting work function shifts of -430 and -310 meV. Contact angle measurements show an increase in the wetting envelope compared to that of pristine gold. Infrared (IR) measurements show tilt angles of 22 and 63°, with the methylene-linked molecule (TMP-CH2-SH) attaining a flatter orientation. The actual work function shift as measured with photoemission spectroscopy (XPS/UPS) is even larger, -600 and -430 meV, respectively. The contact resistance between gold electrodes and poly[N,N'-bis(2-octyldodecyl)-naphthalene-1,4:5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene) (Polyera Aktive Ink, N2200) in n-type OFETs is demonstrated to decrease by 3 orders of magnitude due to the use of TMP-SH and TMP-CH2-SH. The effective mobility was enhanced by two orders of magnitude, significantly decreasing the contact resistance to match the mobilities reported for N2200 with optimized electrodes.

20.
Phys Chem Chem Phys ; 17(9): 6588-96, 2015 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-25659362

RESUMO

Organic materials are of interest as ion battery cathode materials because they offer advantages over inorganic cathodes such as abundant resources and a low ecological footprint. However, they suffer from slow kinetics and a comparatively low potential. In this paper, we have investigated alkali induced changes in the electronic structure of tetracyanoquinodimethane (TCNQ) to be used as cathode material in Li-ion batteries. Lithium was inserted stepwise into TCNQ thin films by exposure to lithium vapour and analysis by photoemission (PES) was performed. The evolution of core levels, electronic structure and Fermi-level with increasing lithium insertion into TCNQ was monitored. The results show that lithium insertion takes place under integer charge transfer and polaron formation. We find no indication of deterioration of the material. The consequences of evolution of electronic structure and polaron formation for electrode potential and kinetic properties of the material are discussed.

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