Thomas Schedel-Niedrig

Solar hydrogen evolution using metal-free photocatalytic polymeric carbon nitride/CuInS2 composites as photocathodes

Polymeric carbon nitride (g-C3N4) films were synthesized on polycrystalline semiconductor CuInS2 chalcopyrite thin film electrodes by thermal polycondensation and were investigated as photocathodes for the hydrogen evolution reaction (HER) under photoelectrochemical conditions. The composite photocathode materials were compared to g-C3N4 powders and were characterized with grazing incidence X-ray diffraction and X-ray photoemission spectroscopy as well as Fourier transform infrared and Raman spectroscopies. Surface modification of polycrystalline CuInS2 semiconducting thin films with photocatalytically active g-C3N4 films revealed structural and chemical properties corresponding to the properties of g-C3N4 powders. The g-C3N4/CuInS2 composite photocathode material generates a cathodic photocurrent at potentials up to +0.36 V vs. RHE in 0.1 M H2SO4 aqueous solution (pH 1), which corresponds to a +0.15 V higher onset potential of cathodic photocurrent than the unmodified CuInS2 semiconducting thin film photocathodes. The cathodic photocurrent for the modified composite photocathode materials was reduced by almost 60% at the hydrogen redox potential. However, the photocurrent generated from the g-C3N4/CuInS2 composite electrode was stable for 22 h. Therefore, the presence of the polymeric g-C3N4 films composed of a network of nanoporous crystallites strongly protects the CuInS2 semiconducting substrate from degradation and photocorrosion under acidic conditions. Conversion of visible light to hydrogen by photoelectrochemical water splitting can thus be successfully achieved by g-C3N4 films synthesized on polycrystalline CuInS2 chalcopyrite electrodes.

Surface and bulk properties of CuGaSe2 thin films

Abstract Using complementary techniques, namely X-ray fluorescence (XRF) and X-ray photoelectron spectroscopy (XPS), we present a comparative study of the bulk and surface composition in device grade CuGaSe2 (CGSe) thin films. The films were deposited in two stages by an open-tube chemical vapor deposition (CVD) process. The first stage leads to a nearly stoichiometric polycrystalline CGSe film of approximately 1.5 μm thickness. During the second stage the film is annealed in a Ga- and Se-rich atmosphere. While the XRF-data show a nearly stoichiometric integrated film composition, the surface composition, as determined by XPS analysis, is Cu-poor, pointing towards a highly non-stoichiometric surface layer. In addition, sodium was found at the film surfaces. The data are discussed in the framework of an ordered defect compound formation and the formation of a (Cu,Na)–Ga–Se compound at the surface of the CuGaSe2 films. Complementary ultraviolet photoelectron- and inverse photoelectron spectroscopy investigations of the film surface derive a widening of the surface energy band gap up to 2.2xa0eV in comparison with a bulk energy band gap around 1.65xa0eV (obtained by optical transmission analysis). The observed data are consistent with our model of a two layer film structure containing a defect-rich near-surface region and a defect-poor bulk.

Metal-Free Photocatalytic Graphitic Carbon Nitride on p-Type Chalcopyrite as a Composite Photocathode for Light-Induced Hydrogen Evolution

Recently, it has been shown that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradiation in the presence of a sacrificial donor. We present herein the preparation and characterization of graphitic carbon nitride (g-C(3)N(4)) films on p-type semiconducting CuGaSe(2) chalcopyrite thin-film substrates by thermal condensation of a dicyandiamide precursor under inert-gas conditions. Structural and surface morphological studies of the carbon nitride films suggest a high porosity of g-C(3)N(4) thin films consisting of a network of nanocrystallites. Photoelectrochemical investigations show light-induced hydrogen evolution upon cathodic polarization for a wide range of proton concentrations in the aqueous electrolyte. Additionally, synchrotron radiation-based photoelectron spectroscopy has been applied to study the surface/near-surface chemical composition of the utilized g-C(3)N(4) film photocathodes. For the first time, it has been shown that g-C(3)N(4) films coated on p-type CuGaSe(2) thin films can be successfully applied as new photoelectrochemical composite photocathodes for light-induced hydrogen evolution.

A Molecular Approach to Self‐Supported Cobalt‐Substituted ZnO Materials as Remarkably Stable Electrocatalysts for Water Oxidation

In regard to earth-abundant cobalt water oxidation catalysts, very recent findings show the reorganization of the materials to amorphous active phases under catalytic conditions. To further understand this concept, a unique cobalt-substituted crystalline zinc oxide (Co:ZnO) precatalyst has been synthesized by low-temperature solvolysis of molecular heterobimetallic Co(4-x)Zn(x) O4 (x = 1-3) precursors in benzylamine. Its electrophoretic deposition onto fluorinated tin oxide electrodes leads after oxidative conditioning to an amorphous self-supported water-oxidation electrocatalyst, which was observed by HR-TEM on FIB lamellas of the EPD layers. The Co-rich hydroxide-oxidic electrocatalyst performs at very low overpotentials (512u2005mV at pHu20057; 330u2005mV at pHu200512), while chronoamperometry shows a stable catalytic current over several hours.

Complementing Graphenes: 1D Interplanar Charge Transport in Polymeric Graphitic Carbon Nitrides.

Charge transport in polymeric graphitic carbon nitrides is shown to proceed via diffusive hopping of electron and hole polarons with reasonably high mobilities >10(-5) cm(2) V(-1) s(-1). The power-law behavior of the ultrafast luminescence decay exhibits that the predominant transport direction is perpendicular to the graphitic polymer sheets, thus complementing 2D materials like graphene.

Radiative recombination in CVT-grown CuGaSe2 single crystals and thin films

Abstract The theoretically expected high open circuit voltage of CuGaSe 2 (CGSe) based thin film solar cells is partly limited by their high deep defect density. To gain more insight on this aspect, we have analyzed the radiative recombination mechanisms of intrinsic CGSe single crystals and polycrystalline films with various compositions using the steady state photoluminescence spectroscopy technique. The single crystals were grown by chemical vapor transport (CVT) in a closed system using iodine as transport agent and polycrystalline CGSe as the raw charge. The CGSe thin films were grown by an open-tube chemical vapor deposition (CVD) technique using Cu 2 Se and Ga 2 Se 3 as source materials. Iodine and chlorine were used as transport agents for source materials in the growth process. The observed low temperature photoluminescence is described by a well-known defect model based on two shallow acceptors and one shallow donor. However, the model is widened in this study to include additional deep luminescence transitions, observed approximately at h ν=1.3 eV in nearly stoichiometric CGSe grown with halogen support. The characteristic chemical reactions of the CVD/CVT growth process, analyzed with thermodynamic equilibrium calculations, are shown to be responsible for the presence of these deep defects.

Hopping conductivity in p-CuGaSe2 films

The results of resistivity measurements on p-type CuGaSe2 films are presented and analyzed within the framework of different hopping conductivity models. Both the Mott [N. Mott and E. A. Davies, Electron Processes in Non-Crystalline Materials (Clarendon, Oxford, 1979); N. F. Mott, Metal-Insulator Transitions (Taylor & Francis, London, 1990)] and the Shklovski-Efros [Electronic Properties of Doped Semiconductors (Springer, Berlin, 1984)] regimes of variable-range hopping are observed. The values of the characteristic and transition temperatures as well as the complete set of parameters describing the properties of the localized holes (the localization radius, the dielectric permeability, the width of the Coulomb gap, and the values of density of states at the Fermi level) are determined.

Temperature dependence of the exciton gap in monocrystalline CuGaSe2

Near-band-edge photoluminescence properties of as-grown undoped CuGaSe2 single crystals have been investigated in the range between 10 K and room temperature. The temperature dependence of the exciton gap energy Eg was studied by means of a three-parameter thermodynamic model, the Einstein model, Varshnis model and the Passler model. The values of the band gap energy at T = 0 K, the effective phonon energy and the Einstein temperature, the cut-off phonon energy of the phonon spectrum as well as a dimensionless constant related to the electron–phonon coupling were estimated. Comparing all applied models points out that a minimum set of four parameters is required for a complete analytical description of the experimentally determined temperature dependence of the exciton gap in CuGaSe2.

Active and Stable Nickel-Based Electrocatalysts Based on the ZnO:Ni System for Water Oxidation in Alkaline Media

The synthesis of monodisperse, surfactant‐free, Ni‐substituted ZnO nanocrystallites (ZnO:Ni) by the mild solvolysis of heterobimetallic Ni4−xZnxO4 cubane‐like precursors (x=1–3) in benzylamine is reported. Ni4−xZnxO4 was grafted by electrophoretic deposition onto fluorine‐doped tin oxide glass substrates and used as an active and stable working electrode for water oxidation. Upon the application of a voltage at the electrodes, the ZnO:Ni precatalyst leads to an active composite material that can oxidize water (>15u2005h) with an increasing catalytic current. In contrast, the performance of homometallic NiO reference materials decreases rapidly over time and is surpassed by the composite from the ZnO:Ni precatalyst in terms of both stability and activity. Extensive characterization of the as‐prepared and activated ZnO:Ni precatalyst by using hard X‐ray photoelectron spectroscopy revealed that the excellent performance of the electrode material is because of the formation of a unique self‐supported turbostratically disordered mixture of γ‐NiOOH/α‐Ni(OH)2‐like phases from the rapid dissolution of ZnII in the ZnO:Ni precatalyst into the electrolyte during activation.

Graphene: Complementing Graphenes: 1D Interplanar Charge Transport in Polymeric Graphitic Carbon Nitrides (Adv. Mater. 48/2015)

On page 7993, C. Merschjann and co-workers use transient photoluminescence to study charge transport in semiconducting polymeric graphitic carbon nitrides, finding that electrons and holes show surprisingly high mobilities. Furthermore, they move predominantly along channels perpendicular to the graphitic sheet structure. Thus, carbon nitrides effectively complement 2D materials like graphene, opening potential routes for novel organic electronic devices.