The increasingly sophisticated nature of modern, more environmentally friendly cementitious binders requires a better understanding and control particularly of the complex, dynamic processes involved in the early phase of cement hydration. In-situ monitoring of properties of a constantly changing system over a defined period of time calls for simple, sensitive, fast, and preferably also non-invasive methods like optical spectroscopy. Here, we exploit the time-dependent changes in the absorbance and fluorescence features of the negatively charged optical probe 2',7'-difluorofluorescein (DFFL) for the study of the hydration processes in pastes of white cement (WC), cubic tricalcium aluminate (C3A), and tricalcium silicate (C3S), the main phases of cement, and in pastes of quartz (Q) over 24 h after addition of the dye solution. For comparison, also conventional techniques like isothermal heat flow calorimetry were applied. Based upon the time-dependent changes in the spectroscopic properties of DFFL, that seem to originate mainly from dye aggregation and dye-surface interactions and considerably vary between the different pastes, molecular pictures of the hydration processes in the cement pastes are derived. Our results clearly demonstrate the potential of optical spectroscopy, i.e., diffuse reflectance, steady state and time-resolved fluorometry in conjunction with suitable optical reporters, to probe specific hydration processes and to contribute to a better understanding of the early hydration processes of cement at the molecular scale.

Polycarboxylate ethers (PCEs) are widely used in construction, but the exact nature of their interaction with cement is still debated. Aiming at a better understanding of the role of tricalcium aluminate (C(3)A) in cement hydration, we assessed the potential of optical spectroscopy in combination with a water-soluble fluorescent organic reporter dye (S0586) to monitor the early hydration of C(3)A in the presence of 26 wt% CaSO4 center dot 2H(2)O (C(3)A26G-S) with and without PCE. As optical methods, steady-state fluorescence and diffuse reflectance (UV-VisDR) spectroscopy were employed. Phase characterization and particle size distribution were performed with in-situ X-ray diffraction (in-situ XRD) and dynamic light scattering (DLS). Our results show that fluorescence and UV-VisDR spectroscopy can be used to monitor the formation of metastable phases by the disaggregation of the dye 50586 in a cement paste as well as changes in ettringite formation. Addition of PCE slowed down the disaggregation of the dye as reflected by the corresponding changes of the dyes absorption and fluorescence. This prolonged induction period is a well-known side effect of PCEs and agrees with previous reported calorimetric studies and the inhibition of gypsum dissolution observed by in-situ XRD. This demonstrates that fluorescence and UV-VisDR spectroscopy together with a suitable optical probe can provide deeper insights into the influence of PCE on C(3)A-gypsum hydration which could be e.g., utilized as screening method for comparing the influences of different types of PCEs. (C) 2020 Published by Elsevier Ltd.

Tricalcium aluminate (C3A) is found with less than 10% wt. of the total composition; however, during hydration, C3A plays an important role in the early hydration of cement in the presence of gypsum as a set retarder. The aim of this investigation is to assess the suitability of optical spectroscopy and a dye-based optical probe to monitor early hydration of C3A in the presence of gypsum and hemihydrate. Optical evaluation was performed using steady-state fluorescence and diffuses reflectance spectroscopy (UV-VisDR). Phase characterization during hydration was done with in-situ X-ray diffraction. UV-VisDR with a cyanine dye probe was used to monitor the formation of metastable phases and was employed together with fluorescence spectroscopy, to follow the aggregation and disaggregation of the dye during hydration. In conclusion, for the first time, a cyanine dye was identified as a feasible and stable probe to monitor C3A hydration changes in the presence of calcium sulfate.

In this study, a promising alternative method for addressing grain boundary issues in hematite (α‐Fe 2 O 3 )‐based photoanodes is presented. The porous α‐Fe 2 O 3 films are prepared by dip coating a polymerizable precursor onto fluorine‐doped tin oxide (FTO) substrates. The photoelectrochemical (PEC) performance of α‐Fe 2 O 3 photoanodes is characterized and optimized through controlling the annealing temperature and the number of deposition cycles. Samples of improved crystallinity consisting of a layer of hematite particles of 50 nm in diameter exhibit highest photoresponses of 1.04 mA cm −2 at 1.23 V versus a reversible hydrogen electrode (RHE), and 2.0 mA cm −2 before the dark current onset at 1.7 V RHE . This study indicates that the PEC performance of hematite can be enhanced greatly by an improved crystallinity, particle texture, and a better control of grain boundary effects.

In photosynthesis nature uses transition metal complexes as catalysts to evolve oxygen and hydrogen from water. In this process the catalytic centers are separated from the light capturing and absorbing co-factors in photosystem I and II integrated in the thylakoid membrane. To develop bio-inspired catalysts and to mimic e.g. the Mn3CaO3MnO complex in PSII, different manganese oxides as well as alkaline metal and earth alkali metal manganates were investigated with respect to the oxygen evolution reaction (OER) in the process of water oxidation. In this contribution special attention will be turned to the structure - morphology– function relationship of these materials. Manganese oxide electrodes have been prepared by reactive magnetron sputtering from a Mn target in an Ar/O2 atmosphere as well as by anodic electrodeposition and a subsequent annealing step. Besides amorphous MnOx obtained at low temperatures, crystallized oxides, such as γ-MnO2 and α-Mn2O3, were tested as electrocatalysts. Thin (60-70 nm) and dense layers were deposited by reactive magnetron sputtering using conductive glass (FTO) slides as substrates. Cross section transmission electron micrograph (Figure 1a) clearly revealed that the sputtered α-Mn2O3 layers consist of nanocrystals of 10 - 20 nm edge length. These layers show current densities of 10 mA/cm-2 at an overpotential of 370 mV (pH 13.8). Similar overvoltages were also obtained with electrodeposited α-Mn2O3 layers (10mA/cm-2 at 340 mV overvoltage). In contrast, the electrochemically deposited α-Mn2O3 layers are several 100 nm thick and appear highly porous (Figure 1b). Obviously, the specific activity related to the active surface area of the sputtered material is about one order of magnitude higher than the one of the electrodeposited samples. Nevertheless, higher current densities can be achieved with the electrodeposited material because high electrochemically active surface areas can be provided. This behavior will be discussed as a function of defect chemistry and will be compared with the structure – function relationship of other OER electrocatalysts. References [1] A. Ramírez, P. Hillebrand, D. Stellmach, M. May, P. Bogdanoff, S. Fiechter; J. Phys Chem. C, 118 (2014) 14073-14081 and SI. [2] M.M. Najafpour, T. Ehrenberg, M. Wiechen, P. Kurz; Angew. Chem. Int. Ed., 49 (2010) 2233-2237. Figure 1: α-Mn2O3 deposited a) by reactive magnetron sputtering and b) by electrochemical deposition. Figure 1

Due to the high energy density of hydrogen it could play an important role as energy storage medium replacing fossil fuels in the future. To produce it in large quantities “artificial leaf”-type structures could be used converting solar light into this fuel by electrochemical decomposition of water. Since this process is most efficient working under acidic conditions in an electrolyzing system semiconducting materials are required on the one hand efficiently absorbing sunlight and generating electron-hole pairs, the energy of which is high enough to split water. To reduce overvoltages on the other hand electro-catalysts are in demand lowering in addition the overvoltages at cathode and anode side. To replace costly platinum as most efficient HER catalyst for more than a decade attempts were made to replace this noble metal by molybdenum sulfides. It has been shown that nanoparticles of MoS 2 are most efficient consisting of three to four S-Mo-S slabs deposited on a suited support, e.g. MoO 3-x , multiwalled carbon nanotubes (MWCNT) and activated carbon paper [1, 2]. In this contribution, we report on the deposition of (NH 4 ) 2 Mo 3 S 13 particles on highly fractal MoS 2 layers deposited by reactive magnetron sputtering. Best results were obtained after drop casting a solution of ammonium thiomolybdate on a sputtered MoS 2 film as shown in Figure 1. The modified electrode exhibited an overvoltage of -220mV at a current density of 10mAcm -2 . Since it is known that the edges of the hexagonal MoS 2 crystallites of the sputtered film are containing the highly catalytically active sites this contribution will address the nano-architecture of the modified electrodes. Figure 1

A procedure based on sol-gel aggregation for the fabrication of mesoporous WO3 photoanaodes was adopted to produce the doped oxide films by admixing the tungstic acid sol with Keggin-type borotungstic acid (H5BW12O40) or the borotungstic acid–stabilized hematite (Fe2O3). Such physicochemical properties as structure, morphology and spectroscopic identity of the resulting hybrid (doped) WO3 films were assessed using X-ray diffraction, scanning electron microscopy, as well as UV-Vis and Raman spectrocopies. When using a solar cell system operating in 0.5 mol dm-3 H2SO4 in the three-electrode configuration, the doped WO3 films acting as photoanodes yielded (following illumination with visible light) significantly (up to 80%) larger water oxidation photocurrents (at potentials higher than 0.75 V vs. RHE), in comparison to the analogous undoped mesoporous tungsten oxide films. The effect was the most pronounced in a case of the WO3 film doped with borotungstic acid–stabilized hematite. The observed enhancement effects could be rationalized either in terms of appearance of new conduction band structures in doped WO3 films or a marked increase in the degree of hydration of the doped oxide structures regardless annealing at high temperatures (450 oC). Judging from our experiments in 0.5 mol dm-3 NaCl, the systems are also applicable in the sea-water-type environments. Present results are consistent with the view that the doped tungsten oxide structures do not exhibit significant undesirable electron-hole recombination effects. Optimization of the system is underway aiming at further improvement of the photocurrent efficiency and the systems' utility under different conditions. Any increase in the photoelectrochemical efficiency of WO3–based films is of importance to the development of photoanodes for the visible-light driven water splitting.

We report the deposition of ultra-thin alpha-Fe2O3 (hematite) films on fluorine-doped tin oxide (FTO) substrates using radio frequency (RF) sputtering, and the investigation of their photoelectrochemical (PEC) performance towards water oxidation. By varying the deposition pressure and time, the film microstructure and morphology could be optimized. The best hematite films having a thickness of about 50 nm exhibited a photocurrent density of 0.59 mA cm(-2) at U = 1.23 V vs. RHE and 1.92 mA cm(-2) at U = 1.85 V using a tungsten halogen lamp of 40 mW cm(-2) light intensity in the wavelength range from 300 to 600 nm. These values are comparable or even higher than those ever measured hematite films (undoped and having no co-catalyst deposited on top of the electrode). Further measurements were explored to investigate the limiting factors in our films for possibly approaching their predicted PEC properties. A detailed analysis reveals that a slow water oxidation reaction and a trapping of charges on the surface, especially at the potential below 1.4 V, are obviously the reasons for the limited PEC performance.

Different manganese oxide phases were prepared as thin films to elucidate their structure-function relationship with respect to oxygen evolution in the process of water splitting. For this purpose, amorphous MnOx films anodically deposited on F:SnO2/glass and annealed at different temperatures (to improve film adherence and crystallinity) were tested in neutral and alkaline electrolytes. Differential electrochemical mass spectroscopy showed that the anodic current correlated well with the onset of the expected oxygen evolution, where in 1 M KOH, the anodic current of crystalline alpha-Mn2O3 films was determined to onset at an overpotential (eta) of 170 mV(RHE) (at J = 0.1 mA/cm(2)) with current densities of ca. 20 mA/cm(2) at eta = 570 mVRHE. Amorphous MnOx films heated at 573 K (MnOx-573 K) were found to improve their adherence to F:SnO2/glass substrate after heat treatment with a slight crystallization detected by Raman spectroscopy. The onset of water oxidation of MnOx-573 K films was identified at eta = 230 mV(RHE) (at J = 0.1 mA/cm(2)) with current densities of ca. 20 mA/cm(2) at eta = 570 mV(RHE) (1 M KOH). The least active of the investigated manganese oxides was Mn3O4 with an onset at eta = 290 mVRHE (at J = 0.1 mA/cm2) and current densities of ca. 10 niA/cm(2) at eta = 570 mV(RHE) (1 M KOH). In neutral solution (1 M KPi, a similar tendency was observed with the lowest overpotential found for alpha-Mn2O3 followed by MnOx-573 K and Mn3O4. X-ray photoelectron spectroscopy revealed that after electrochemical treatment, the surfaces of the manganese oxide electrodes exhibited oxidation of Mn II and Mn III toward Mn IV under oxygen evolving conditions. In the case of alpha-Mn2O3 and MnOx-573 K, the manganese oxidation was found to be reversible in KPi when switching the potential above and below the oxygen evolution reaction (OER) threshold potential. Furthermore, scanning electron microscopy (SEM) images displayed the presence of an amorphous phase on top of all manganese oxide films here tested after oxygen evolution. The results indicate that structural changes played an important role in the catalytic activity of the manganese oxides, in addition to oxidation states, a large variety of Mn-O bond lengths and a high concentration of oxygen point defects. Thus, compared to Mn3O4, crystalline alpha-Mn2O3 and MnOx-573 K are the most efficient catalyst for water oxidation in the manganese oxygen system.

Manganese oxide films on conductive substrates were prepared by electrochemical deposition. Charge carrier kinetics in these films was studied by contactless transient photoconductivity measurements. These measurements revealed the generation of mobile charge carriers with a nonneglectable lifetime. The transport from charge carriers generated in the oxides to the substrate was observed. (C) 2013 Elsevier B.V. All rights reserved.

Colored Organic/Inorganic Hybrid Materials (OIHM) with reversible properties were prepared by a solid-state reaction between several montmorillonites (MMT) with different Si/Al ratio (Bentolite L®, Texas, Wyoming and Kunipia deposits) and a neutral organic dye (thioindigo) at 413K for nine hours. Spectroscopic and thermogravimetric analysis of the above mentioned interactions were evaluated. Results obtained by these techniques revealed an influence of Si/Al ratio in dehydrated MMT, where the intramolecular charge-transfer (IMCT) in thioindigo progressively red-shifted and increased in intensity in the following order: BentoT (Si/Al=8.2)>TexasT (Si/Al=7)>WyomingT (Si/Al=5.6)>KunipiaT (Si/Al=4.8). Moreover, a disturbance of thioindigo C=O at 1654cm−1 to lower frequencies occurred due to C=O---Lewis acid sites (LAS) and Brønsted (B) interaction in MMT with high Si/Al ratio. In the presence of water, a smaller C=O shift due to C=O---(H2O)LAS or B interaction was identified. In addition, displacement of the basal spacing (001) in all MMT confirmed the effect of water on the reversible color changes displayed by UV–Vis diffuse reflectance spectroscopy. The existence of different binding strengths in OIHM was also evaluated by TGA and UV–Vis spectroscopy of their ethanol Soxhlet extractions.

Presently our institute pursues the aim to realize a monolithic structure to split water from sunlight by immerging a membrane in an electrolyte consisting of photoelectrocatalytic electrode surfaces to evolve hydrogen and oxygen from water as demonstrated for the first time by Khaselev and Turner in 1998 [1]. For this type of device alternative photovoltaic materials, catalysts and electrode architectures are in demand. In nature, water splitting occurs in the thylakoid membrane of plants at catalytic centres located in photosystem I and II. In order to mimic these processes by artificial systems the most challenging task is to overcome the limiting kinetics of the four electron transfer of water oxidation by the development of stable and highly effective oxygen evolving catalysts. Since materials change physical and chemical properties when prepared in the nanoscale [2] the manufacturing of nanoparticles and nanocomposites has been envisaged. In addition to semiconducting RuS2 single crystals [3-4], thin layers have been prepared in a first approach using reactive sputtering from a metal target in presence of H2S. Among others, titanium foils were employed as support. The prepared layers were characterized by X-ray Diffractometry (XRD), Scanning Electron Microscopy (SEM), Rutherford Back Scattering (RBS), Cyclic Voltammetry (CV), Differential Electrochemical Mass Spectroscopy (DEMS) and in-situ X-ray Absorption Spectroscopy (XANES, EXAFS). The nanocrystalline layers possess an indirect and a direct band gap at 1.36eV and 2.2eV, respectively. For this reason, a photovoltage in the range from 600 to 900mV should be expected. However, possibly the presence of surface and bulk defects lower the photoeffect to values <200mV. It has been demonstrated that the photoelectrochemical behaviour is a function of film stoichiometry, crystallinity and morphology, which could be influenced by the sputter pressure, the sputter mode (DC or AC mode) and the substrate temperature, respectively. At high substrate temperatures (< 600°C), the films showed a S:Ru ratio of 2 measured by Rutherford Back Scattering (RBS), what is consistent with XRD analysis, where the cubic pyrite-type phase was detected. Using Differential Electrochemical Mass Spectroscopy (DEMS) an oxygen evolution has been observed at a potential ≥ 1.4V in the dark at pH 0 (Fig. 1). Under illumination the threshold could be lowered by ~100mV. Compared to sputtered RuO2 layers RuS2 films showed a higher corrosion stability. In a further approach, manganates were prepared by electrochemical deposition, hydrothermal and sol-gel techniques, among others, and tested as oxygen evolving catalysts. For this reason, CaMnO3 [5], FexMn1-xO [6] and CaMn2O4, Ca2MnO4, γ-Mn(O,OH)2 and α-Mn2O3 were prepared in amorphous and crystalline bulk form and as thin films on FTO. In the work, inspired by the oxygen evolving Mn4Ca cluster unit in PS II [7], it could be demonstrated that manganates are able to oxidise water by an overpotential of about 400mV at pH 1. Recently, Kurz [8] reported on the efficiency of CaMn2O4 (marokite) to evolve oxygen from water. The preparation conditions and the functionality of the catalyst will be discussed.

In this investigation, we address the question of how organic thioindigo binds to inorganic palygorskite to form a pigment similar to Maya Blue. We also address how such binding, if it occurs, might be affected by varying the proportion of dye relative to that of the mineral, and by varying the length of heating time used in preparation of the pigment. In addition to samples of palygorskite and thioindigo both alone, four synthetic pigment samples were prepared; two samples of 8 wt.% dye, one heated at 170 degrees C for 3 h and one at 170 degrees C for 9 h, and two samples of 16 wt.% dye, one heated at 170 degrees C for 3 h and one at 170 degrees C for 9 h. All samples were examined using Fourier transform-infrared (FT-IR) and FT-Raman spectroscopy. For the pigment samples, FT-IR peaks at 1627 cm(-1) are attributed to a downshifted C=O stretching mode of thioindigo due to dye-clay interaction. This interpretation is corroborated by FT-Raman C=O peaks with 14 cm(-1) shifts to lower wavenumber for the pigment relative to thioindigo alone. Additional Raman scattering between 550 cm(-1) and 650 cm(-1) also suggests dye-clay interaction through metal-oxygen bonding. We are thus led to the possibility of mostly hydrogen bonding between silanol and carbonyl at lower dye concentration, with a predominance of metal-oxygen bonding at higher dye concentration. Copyright (c) 2008 John Wiley & Sons, Ltd.