Calcium Silicate Phases Explained by High ... - ACS Publications

Article pubs.acs.org/Langmuir

Calcium Silicate Phases Explained by High-Temperature-Resistant Phosphate Probe Molecules Nicolas Giraudo,† Samuel Bergdolt,† Jonas Wohlgemuth,† Alexander Welle,†,‡ Rainer Schuhmann,†,§ Franz Königer,†,§ and Peter Thissen*,† †

Institut für Funktionelle Grenzflächen (IFG), ‡Karlsruhe Nano Micro Facility (KNMF), and §Competence Center for Material Moisture, Karlsruher Institut für Technologie (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ABSTRACT: In this work, high-temperature-resistant phosphate molecules are applied to characterize ultrathin (100 nm) calcium silicate (C−S) phases. These C−S phases are synthesized on silicon wafers, and the interaction of phosphates with the C−S phases is studied by means of in situ transmission Fourier transform infrared (FTIR) spectroscopy. At room temperature, the chemistry of the system is dominated by the formation of calcium phosphates (C−P). In the case of temperature rising to 1000 °C, the C−S phases are regenerated. FTIR results are analyzed on the basis of firstprinciples calculations and further supported by complementary time-offlight secondary ion mass spectrometry (ToF-SIMS) experiments. This study provides a detailed and self-consistent picture of the chemical and structural properties of interfaces such as the one between the atmosphere and ultrathin C−S phases (gas/C−S) and the one between them and silicon wafers (C−S/Si bulk). The material combination of ultrathin C−S phases grown on silicon wafers might in the future have great potential in selective chemistry, catalysis, and sensing technology as well as in semiconductor manufacturing.

■

INTRODUCTION The structure and chemical properties of calcium silicate (C− S) and calcium silicate hydrate (C−S−H) phases have been the subject of numerous experimental investigations.1−3 Today, the huge interest in these materials originates from their suitability as model systems for concrete, which is still the most important construction material used by mankind.4−8 Despite the importance of the surface chemistry of these materials in the context of corrosion, the rather complicated structure of these minerals has so far largely prohibited us from gaining a deeper understanding. As for many other minerals, information down to the atomic level of detail is rather limited.9 It is therefore desirable to employ the so-called surface science approach, which has been successful in understanding the surface chemistry of metals and, more recently, also of some metal oxides.6,10−12 Several issues have made difficult the investigation of well-defined mineral substrates under UHV conditions. Experimental problems and the fact that, until recently, infrared spectroscopy could not be applied to the surfaces of bulk dielectrics are examples of such issues. Consequently, further research into model reactions using spectroscopic methods has so far not been carried out. This lack of detailed experimental information prohibits a validation of theoretical predictions and makes a detailed understanding of even simple chemical processes on mineral surfaces and, in particular, on concrete virtually impossible. A new concept of research applicable to surfaces and interfaces is developed here. The implementation of probe © 2016 American Chemical Society

molecules will allow us to unravel the chemical structure of C− S phases. C−S phases can be synthesized on the surface of conventional silicon wafers.13,14 This synthesis produces two different interfaces to be investigated: the first between C−S phases and the atmosphere (gas/C−S) and the second between C−S phases and bulk silicon (C−S/Si bulk). The development of probe molecules became very popular in the past decade, principally to be used in fluorescence measurements.15,16 Furthermore, the implementation of probe molecules in the study of surfaces by means of Fourier transform infrared (FTIR) spectroscopy is also well-established in catalysis.17 Several molecules would be useful in analyzing the interface of the C−S phases and the environment, e.g., organic compounds. However, an obstacle to the application of these probe molecules is their limited thermal stability because most organic molecules cannot withstand the high temperatures necessary to induce significant changes in the C−S phases. Phosphorus compounds forming phosphates were found to be very suitable for these studies because they will not be decomposed and released into the environment during the heating steps but they will penetrate the ultrathin C−S phase to reach the silicon wafer.18,19 All steps in this migration will be analyzed by implementing transmission FTIR, and the Received: August 31, 2016 Revised: November 2, 2016 Published: November 29, 2016 13577

DOI: 10.1021/acs.langmuir.6b03218 Langmuir 2016, 32, 13577−13584

Article

Langmuir

Figure 1. Representation of the process of phosphates acting as probe molecules. (A) Initial calcium silicate (C−S) phases (gray) on a silicon wafer (blue). (B) Layer of phosphates (green) deposited on the C−S phases (gray) at room temperature. (C) Phosphates diffuse into the C−S phases (yellow) at 1000 °C. (D) Phosphorus doped into silicon (green) after several steps of heating at 1000 °C, regenerating the C−S phases (gray). calculation corresponding to a particular mineral was performed from the unit cell of the indicated mineral. Once the relaxation had converged, the vibrational modes were calculated. Synthesis of Calcium-Silicate-Hydrate (C−S−H) phases. A 3 cm × 1 cm (500-μm-thick and double-side polished) CZ silicon(111) wafer was first chemically cleaned with a 30 min exposure at 80 °C to a 1:3 solution of aqueous H2O2/18 M H2SO4 (piranha solution) prepared with 97% sulfuric acid used as received from Merck. The sample was then immersed in 10 mL of a 10 mM Ca(OH)2 solution of a solid with 96% obtained from Sigma-Aldrich in a closed container, at 65 °C for 6 h. Afterward, the sample was rinsed with deionized water and dried with N2. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR measurements were done in a N2-purged glovebox with a Bruker Vertex 70 and recorded at a nominal 4 cm−1 resolution. Spectra were collected from 400 to 4000 cm−1 in transmission mode with an angle of incidence of 64° with respect to the silicon surface normal. A roomtemperature pyroelectric detector (DTGS) was employed for data collection. In all experiments, 1024 scans were recorded. To perform the deposition of the desired probe molecule on the gas/C−S−H and gas/C−S interfaces, an aliquot of 4 μL of a 1 mM KH2PO4 aqueous solution was used. During the heating process, all measurements, before and after heating, were performed at 60 °C. After a successful implementation of the phosphates as a layer on the surface, heating is applied, thereby driving the probe molecules inside the structure. The silicon wafer is mounted on an IR holder designed to measure and heat up to 1000 °C (heating rate ∼50 °C/s) in situ. Time-of-Flight Secondary Ion Mass Spectrometry (ToFSIMS). The ToF-SIMS analysis was carried out on a gridless reflectron-based ToF-SIMS V (ION-TOF GmbH, Muenster, Germany) equipped with a bismuth-cluster ion source. All spectra were obtained using Bi+ primary ions at 25 keV energy in high-current bunched mode with a mass resolution of m/Δm = 6000. Depth profiling was performed by eroding the samples with a 1 keV Cs ion beam.

accumulation of phosphorus in the silicon structure of the wafer will be confirmed by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The aim of this study is the functionalization of C−S with phosphate probe molecules primarily to further strengthen the interpretation of the FTIR data and also to understand the interfacial chemistry of phosphate/silicate systems. It was therefore necessary to design a C−S system with the highest reproducibility and applicability, which is the ultrathin C−S phase grown on the silicon wafer. A representation of the process of phosphates acting as probe molecules is shown in Figure 1. These molecules allow for FTIR spectroscopic insights into the underlying transformations. The initial substrate consists of ultrathin C−S phases on a silicon wafer (gray, Figure 1A). Figure 1B represents the step after depositing the probe molecules, phosphates (green) on the C−S phases, where the phosphates of the solution form calcium phosphate on the surface.20 Figure 1C shows the transport mechanism of the phosphates at 1000 °C that are diffusing more deeply into the C−S phase (yellow), from the gas-phase/C−S interface to the C−S/Si-bulk interface. If the sample is heated continuously, then phosphates penetrate the silicon wafer and the C−S phases are regenerated (Figure 1D).

■

MATERIALS AND METHODS

Computational Methods. All ab-intio electronic structure calculations reported here were carried out using density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP).21 The electron−ion interaction is treated within the projector-augmented wave (PAW) method.22 The plane waves into which the valence electron wave functions are expanded has a kinetic energy cutoff of 360 eV. This energy limit yielded converged structures for all calculations. Brillouin zone sampling was completed with 1 × 1 × 1 meshes of Monkhorst−Pack k-points.23 The electron−electron exchange and correlation energy were approached by making use of the PW91 functional contained in the generalized gradient approximation (GGA).24 C−S minerals and phosphates were relaxed, and their vibrational modes were calculated. The aim of these calculations was to aid in the interpretation of the vibrational modes of such complex system. In the case of silicates, three different substrates were calculated: C2S (Ca2SiO4) for monotetrahedra, Jaffeite (Ca6Si2O7(OH)6) for the bitetrahedral configuration, and Wollastonite (CaSiO3) for an infinitely long chain.25−27 In the case of phosphates, hydroxyapatite was used as a model. Because this compound is monotetrahedral, the changes in the vibrational modes caused by the formation of multitetrahedra were supposed to be similar to those on the silicate compounds.27,28 Each

■

RESULTS AND DISCUSSION C−S−H Phases’ Characterization and Transformation to C−S Phases. A detailed description of the ultrathin C−S− H phases on silicon and their transformation to C−S phases as a function of temperature is given in Figures 2 and 3. In Figure 2, an IR spectrum of the synthesized C−S−H phases on a silicon wafer referenced to the piranha-cleaned wafer is depicted. As can be seen, the synthesis drives not only Ca to the surface but also carbonates that are identified by their vibrational modes (in νas(C−O) with a maximum at 1491 cm−1 and δ(O−C−O) at 878 cm−1).14,29−31 The spectrum shows two features related to the presence of OH: a broad band 13578


Article

Langmuir

temperature, the difference spectra do not show significant changes, which is taken as a reference. Every vibrational mode is correlated to a calculated vibrational mode, as shown in Table 1. The vibrational modes of specific compounds are correlated to the ones observed in the spectra of the present system to facilitate the interpretation of the changes occasioned in the structure. Even if the total wavenumbers do not agree exactly with the experimental ones, the order does. The most important fact of the transformation is that several new vibrations appear, which are associated with the formation of chains. Besides, the loss of OH groups is associated with the formation of CaO and the decomposition of Si−OH, named dehydroxylation.33,34 The remaining compounds react to build CaxSiyOz-chained structures. The sample was heated stepwise until a constant line was found in the difference spectra as a function of temperature, which means that further heating up to the same temperature does not change the surface chemistry, which is an adequate starting point for the probe molecules (Figure 1.A). Probe Molecules on C−S−H Phases. First, probe molecules were deposited on a sample with C−S−H phases before any heating treatment, and the corresponding spectrum of the resulting sample referenced to the C−S−H phases is shown in Figure 4. The OH stretching vibration of the dissociated water (ν(O−H) at 3740 cm−1) is associated with the presence of molecular water. These water molecules are observable by the corresponding stretching and bending vibrations (ν(O−H) between 3600 and 3400 cm−1 and δ(H−O−H) at 1620 cm−1). The interaction of the phosphates on the sample affects not only the silicates but also the carbonates. This is confirmed by the negative stretching and bending vibrations of the carbonates (νas(C−O) centered at 1420 cm−1 and δ(O−C−O) at 870 cm−1) and Si−O vibrational modes (νmono(Si−O) at 933 cm−1 and δmono(O−Si−O) at 570 and 475 cm−1) and the positive stretching vibrations of phosphates (ν(P−OH) at 1118 cm−1 and ν(P−O) at 1005 cm−1).35,36 As was observed in Figure 2, for the synthesis of the C−S−H phases on a silicon wafer it was seen that the Si−O stretching vibration was represented by a broad peak with a maximum at 970 cm−1. This wavenumber is influenced by the presence of water and carbonates on the sample but remains a unique vibrational mode. After the application of the probe molecules, only one vibrational mode is being affected by the formation of

Figure 2. IR spectrum of C−S−H phases referenced to the silicon wafer.

between 3100 and 3500 cm−1 corresponding to molecular water adsorbed and a sharp peak at 3690 cm−1 that confirms the presence of dissociated water as CaO−H.32 Other vibrational modes are observed after the synthesis, corresponding to Si−O (νmono(Si−O) with a maximum at 970 cm−1 and δmono(O−Si−O) with a maximum at 460 cm−1), which evidences the formation of the C−S−H phases. An important observation of Figure 2 is that no Si−O stretching or Si−O−Si bending vibrations corresponding to multitetrahedral compounds are observed. This is attributed here to the absence of long chains, i.e., all of the Si−O bonds of the tetrahedra present similar vibrational modes, and the explanation for that is that they are not highly interconnected. In the first heating step up to 900 °C, several changes are observed (Figure 3A). The removal of carbonates is underlined by the negative band corresponding to the stretching modes at 1491 cm−1 and the peak corresponding to the bending modes at 878 cm−1. Together with the elimination of carbonates, the OH species are being removed as well, denoted by the negative vibrational modes cited previously for such a compound (broad band between 3100 and 3500 cm−1 corresponding to ν(O−H) of free water and a sharp peak at 3690 cm−1 corresponding to dissociated water). The temperature changes the structure of the silicates. Vibrational modes evidence these changes on silicates (ν1,3−5,7−8(Si−O) at 1160, 1056, 1043, 1009, 940, and 840 cm−1). These vibrational modes remain active, being the same as shown in Figure 3B, but less intensive with every further step of heating. After 7 s of heating at the same

Figure 3. IR spectra of the transformation of C−S−H phases into C−S phases as a function of temperature. (A) Difference IR spectrum of the first heating step of the C−S−H phases up to 900 °C referenced to the C−S−H phases on a silicon wafer. (B) Difference IR spectrum of 2 s of heating at 900 °C referenced to the last step of heating (1 s). 13579


Article

Langmuir Table 1. Correlation of Experimental Vibrational Modes with Wavenumbers Calculated by Means of DFT

Figure 4. Difference IR spectrum of the phosphates molecules deposited on C−S−H phases referenced to the C−S−H spectrum.

Application of Probe Molecules to C−S Phases. Next, probe molecules are applied to C−S phases (as described in Figure 1). The difference IR spectra corresponding to the whole sensing process (resumed in Figure 1) are shown in Figure 5. The first spectrum corresponds to the detection of the phosphates on the C−S phases (Figure 5B-A). This spectrum differs strongly from the one shown in Figure 4, where the probe molecules are deposited on C−S−H phases (before heating). The positive P−O stretching vibration is present, and the bending vibrations of the phosphate also appear (broad ν1(P−O) peak with a maximum at 1074 cm−1 and δ1−2(O−P− O) at 714 and 625 cm−1).37 The negative peaks observed in the same figure, depicted as ν3−8(Si−O), correspond to the silicates that are modified by the formation of phosphates (Table 1, ν3−8(Si−O) at 1056, 1043, 1009, 979, 940, and 840 cm−1). There is only one positive peak of the Si−O vibrational modes in the spectrum (ν1(Si−O) at 1160 cm−1), and the meaning of this peak is explained by a new concept called frustrated chains. The concept of frustrated chains is reflected in a shift in the Si−O−Si vibrational mode to higher wavenumbers. This shift is related to the formation of calcium phosphates and causes a lower coordination of the silicate chains (lower coordination = frustrated). Because the vibrational modes of Si−O−Si are different in different structures, to isolate the effect of the presence of calcium this has to be analyzed in the same system. Therefore, a model of this concept is shown in Figure 6. This model confirms that the vibrational modes of longer (more highly coordinated) silicate chains shift to lower wavenumbers. In contrast, the vibrational modes of the frustrated (lower coordinated) chains shift to higher wavenumbers. Figure 6 shows that the Si−O−Si vibrational mode of a silicate chain shifts from 1010 to 969 cm−1 after bonding a second tetrahedron. This consideration confirms the hypothesis mentioned before that the 1160 cm−1 peak corresponds to less coordination of calcium in the tetrahedral chain. In Figure 3, it was observed that the peak at 1160 cm−1 appeared because of the formation of the silicate tetrahedral chain, i.e., the dehydroxylation of C−S−H phases. The principal reason for the formation of such chains is the release of CaO. The coordination of calcium, a product of the reaction of CaO with the dehydroxylated Si−O tetrahedral compounds, results in an increment of the chains length. Further heating produces the incorporation of more Ca into the structure of the C−S phase, thus incrementing the Ca/Si ratio, which is transduced in longer chains and at lower wavenumbers. The vibrational modes attributed to the frustrated silicate chains,

phosphates (ν(Si−O) at 930 cm−1). This confirms the statement made before: no long chains are formed before the heating of the C−S−H phases. The presence of chains is associated with the interconnection of the Si−O tetrahedral, as observed in several vibrational modes in Figure 2. 13580


Article

Langmuir

Figure 6. Model for the shift in wavenumbers of ν(Si−O) in silicate tetrahedral chains grafted to a silicon wafer with respect to the length of the chain.

Driving Force of the Diffusion. One possible approach to the supposed chemical driving force of the diffusion of phosphates through the C−S phases is presented. The black line in Figure 7 shows the spectrum of the first state of the

Figure 7. Difference IR spectra of C−S phases with a phosphate molecule. (Top) After phosphate molecule deposition at RT, (middle) after heating to 150 °C, and (bottom) after heating to 1000 °C.

phosphate molecule deposited on the C−S phase referenced to the heated and stable C−S phase on a wafer. As can be noticed, the phosphate on the surface is still hydroxylated (ν(O−H) at 3640 cm−1), which is here suggested to be CaHPO4. The mechanism of formation of this phosphate has already been understood and explained in earlier publications.38,39 The release of a proton by the dissociation of KH2PO4 induces the already-known metal−proton exchange reaction (MPER) and forms the first calcium hydrogen phosphate on the surface, as depicted in eq 1.20,40

Figure 5. Difference IR spectra of every step represented in Figure 1. (B-A) Phosphates on C−S phases. (C-B) After heating, probe molecules inside the C−S structure. (D-C) After the diffusion of phosphorus into the silicon wafer.

Ca 2 + + HPO4 2 − + H 2O ↔ CaHPO4 ·2H 2O

ν1(Si−O) at 1160 cm−1, appear once the sample is heated, and those of SiO4 of the C−S phase partially disappear. However, the vibrational modes of the phosphate remain unchanged. In Figure 5C-B, it can be seen how with further heating the peak corresponding to the frustrated chains keeps growing (ν1(Si− O) at 1160 cm−1). This phenomenon is explained by the diffusion of the phosphates in the calcium silicates. Once the phosphate has reached the interface of the wafer in Figure 5D-C, the bending vibrations of phosphate begin to disappear, and surprisingly the Si−O vibrations do the same, showing at the same time new vibrational modes that are attributed to the reconstruction of C−S phases, which after 8 s of heating at 1000 °C cease (ν2(Si−O) at 1095 cm−1 and δ3(O−Si−O) at 464 cm−1).

(1)

This compound is stable at room temperature, but after being heated to around 250 °C, its transformation occurs completely as described by eq 2. The formation of new phosphate compounds is driven by the higher concentration of calcium on the way down to the silicon wafer and the activation energy provided by the higher temperature. ∼ 250 ° C

2CaHPO4 ·2H 2O ⎯⎯⎯⎯⎯⎯⎯⎯→ Ca 2P2O7 + 3H 2O

(2)

In Figure 7, the spectrum of the system after being heated to 150 °C shows that the reaction described in eq 3 has already started. The negative band of ν(O−H) is the most remarkable change (ν(O−H) at 3640 cm−1). The shift observed in the 13581


Article

Langmuir stretching vibrations of P−OH (ν(P−OH) at 1118 to 1085 cm−1) corresponds to the change in free tetrahedra of the CaHPO4 to P−O stretching mode of P−O−P in Ca2P2O7. The frustrated chain vibrational mode disappears (Si−O systems react with CaO to form the C−S phase). After 1000 °C, PO4 has again advanced into the structure because of the calcium gradient proposed by eq 3. 1000 ° C

Ca 2P2O7 + CaO ⎯⎯⎯⎯⎯⎯⎯→ Ca3(PO4 )2

(3)

The heat of formation of every compound was calculated, validating the series of reactions proposed for the product depicted in eq 3.41,42 The driving force for the diffusion of phosphates toward the silicon wafer was explained as a gradient of calcium concentration increasing toward the silicon and a gradient of phosphorus decreasing in the same direction. Calcium phosphates are built on the way down, and these compounds take the calcium of the silicates, producing a stronger vibration of the Si−O−Si bonds bridging the chains, which was observed by means of FTIR. Several authors have observed this vibration in similar systems, attributing this vibrational mode to more linear, less cross-linked structures than the one presented on SiO2 networks.43−45 Others were even conscious of the influence of calcium in such a vibrational mode, relating it to Ca/Si < 1.46 But it was not evident until the probe molecules were applied, as will be explained. At the C/S−Si interface, the overall driving force was found to be the thermodynamic instability of P5+ in contact with silicon at high temperature, initiating a self-decomposition at 500 °C as described by Longo et al.36 After P diffused into the silicon wafer, CaO as an end product was obtained on the surface, which reacted again with the Si−O system to produce C−S phases (Figure 5D-C). ToF-SIMS Confirmation of Diffusion. Although the IR absorption spectrum provides evidence for the adsorption of phosphates, it is difficult to know if the silicon surface remains unoxidized because the SiO2 vibrational modes occur in the 1000−1250 cm−1 region, overlapping the Si−O and associated oxygen vibrations.47 Therefore, complementary ToF-SIMS measurements were applied to investigate the mass transport of phosphorus toward the C−S/Si-bulk interface and to estimate the layer thickness of the C−S−H phases, which was approximately of 100 nm. In Figure 8A,B, the sputter profiles of negative fragments most representative of oxidized silicon (gray line), elementary silicon (blue line), oxidized phosphorus (dark-green line), and reduced phosphorus (lightgreen line) are shown. Figure 8A depicts the situation after phosphate was applied (compare to the topmost IR spectrum in Figure 5). The mass transport of phosphates from the C−S− H/liquid interface is represented by the dark-green line, a PO2 fragment. Further fragments such as PO3 have been detected and follow the same line shape. The C−S/Si-bulk interface is found where the lines of oxidized silicon (gray line) and elementary silicon (blue line) cross. Note that before heating the phosphates reach the C−S/Si-bulk interface but they do not penetrate the silicon (dark-green line never goes behind the gray line). The situation completely changes after heating, as depicted in Figure 8B. Again, the C−S/Si-bulk interface is found where the lines of oxidized silicon (gray line) and elementary silicon (blue line) cross. Interestingly, the phosphates have been completely removed from the system. Instead, fragments of

Figure 8. ToF-SIMS measurements of (gray line) silicon oxide, (blue line) silicon, (light-green line) phosphorus, and (dark-green line) oxidized phosphorus (A) after the application of phosphates and (B) after heating to 1000 °C.

reduced phosphorus have been investigated that penetrate deeply into the silicon bulk. The maximum of the light-green line is found behind the C−S/Si-bulk interface. Overall, the ToF-SIMS measurement has complementarily confirmed the results of FTIR and given deeper insight into the mass transport from the C−S−H/liquid to the C−S/Si-bulk interface. Impact of the Results. In summary, phosphates applied to 100 nm thin calcium silicate phases are not desorbed from the surface when the temperature increases to 1000 °C. The powerful combination of FTIR and ToF-SIMS has unraveled the interface chemistry of the phosphate molecules. At room temperature, the chemistry of the system is dominated by the formation of calcium phosphates. In the case of temperature rising to 1000 °C, the calcium silicates are regenerated while phosphorus is diffusing into the silicon bulk. The detailed analysis of C−S and C−S−H phases is of broad interest in many research areas. C−S−H and C−S phases might be promising materials as sensors for phosphates in wastewater and even for other components that either need to be recovered or have to be urgently removed to avoid serious health problems, such as arsenates.48,49 Because the presence of phosphates on the surface was driven by the formation of calcium phosphates, the concentration of phosphorus (or other elements) can be determined for the problem solution by performing a simple calibration between the intensity of peaks associated with such compounds and their concentration. Besides, the synthesis of these compounds, such as calcium phosphate itself, is significant in the pharmaceutical industry because the phosphates were built on the surface without strict 13582


Article

Langmuir

Anhydrous and Hydrated Cements. J. Am. Chem. Soc. 2010, 132, 7321−7337. (5) Thomas, J. J.; Jennings, H. M.; Chen, J. J. Influence of Nucleation Seeding on the Hydration Mechanisms of Tricalcium Silicate and Cement. J. Phys. Chem. C 2009, 113, 4327−4334. (6) Allen, A. J.; Thomas, J. J.; Jennings, H. M. Composition and Density of Nanoscale Calcium-Silicate-Hydrate in Cement. Nat. Mater. 2007, 6, 311−316. (7) Plank, J. Applications of Biopolymers and Other Biotechnological Products in Building Materials. Appl. Microbiol. Biotechnol. 2004, 66, 1−9. (8) Juenger, M. C. G.; Winnefeld, F.; Provis, J. L.; Ideker, J. H. Advances in Alternative Cementitious Binders. Cem. Concr. Res. 2011, 41, 1232−1243. (9) Richardson, I. G. The Calcium Silicate Hydrates. Cem. Concr. Res. 2008, 38, 137−158. (10) Yu, P.; Kirkpatrick, R. J.; Poe, B.; McMillan, P. F.; Cong, X. Structure of Calcium Silicate Hydrate (C-S-H): Near-, Mid-, and FarInfrared Spectroscopy. J. Am. Ceram. Soc. 1999, 82, 742−748. (11) Sangodkar, R. P.; Smith, B. J.; Gajan, D.; Rossini, A. J.; Roberts, L. R.; Funkhouser, G. P.; Lesage, A.; Emsley, L.; Chmelka, B. F. Influences of Dilute Organic Adsorbates on the Hydration of LowSurface-Area Silicates. J. Am. Chem. Soc. 2015, 137, 8096−8112. (12) Sezen, H.; Shang, H.; Bebensee, F.; Yang, C.; Buchholz, M.; Nefedov, A.; Heissler, S.; Carbogno, C.; Scheffler, M.; Rinke, P.; Wöll, C. Evidence for Photogenerated Intermediate Hole Polarons in Zno. Nat. Commun. 2015, 6, 6901. (13) Ebbert, C.; Grundmeier, G.; Buitkamp, N.; Kroeger, A.; Messerschmidt, F.; Thissen, P. Toward a Microscopic Understanding of the Calcium-Silicate-Hydrates/Water Interface. Appl. Surf. Sci. 2014, 290, 207−214. (14) Giraudo, N.; Thissen, P. Carbonation Competing Functionalization on Calcium-Silicate-Hydrates: Investigation of Four Promising Surface-Activation Techniques. ACS Sustainable Chem. Eng. 2016, 4, 3985. (15) Briñas, R. P.; Troxler, T.; Hochstrasser, R. M.; Vinogradov, S. A. Phosphorescent Oxygen Sensor with Dendritic Protection and TwoPhoton Absorbing Antenna. J. Am. Chem. Soc. 2005, 127, 11851− 11862. (16) Bricks, J. L.; Kovalchuk, A.; Trieflinger, C.; Nofz, M.; Büschel, M.; Tolmachev, A. I.; Daub, J.; Rurack, K. On the Development of Sensor Molecules That Display Feiii-Amplified Fluorescence. J. Am. Chem. Soc. 2005, 127, 13522−13529. (17) Knözinger, H. Infrared Spectroscopy for the Characterization of Surface Acidity and Basicity. Handbook of Heterogeneous Catalysis; Wiley-VCH: Weinheim, Germany, 2008. (18) Thissen, P.; Seitz, O.; Chabal, Y. J. Wet Chemical Surface Functionalization of Oxide-Free Silicon. Prog. Surf. Sci. 2012, 87, 272− 290. (19) Raynaud, S.; Champion, E.; Bernache-Assollant, D.; Thomas, P. Calcium Phosphate Apatites with Variable Ca/P Atomic Ratio I. Synthesis, Characterisation and Thermal Stability of Powders. Biomaterials 2002, 23, 1065−1072. (20) Giraudo, N.; Krolla-Sidenstein, P.; Bergdolt, S.; Heinle, M.; Gliemann, H.; Messerschmidt, F.; Brüner, P.; Thissen, P. Early Stage Hydration of Wollastonite: Kinetic Aspects of the Metal-Proton Exchange Reaction. J. Phys. Chem. C 2015, 119, 10493−10499. (21) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561. (22) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (23) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rew. B 1976, 13, 5188−5192. (24) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple (Vol 77, Pg 3865, 1996). Phys. Rev. Lett. 1997, 78, 1396−1396.

chemical requirements that are very important to the bioactivity of substrates used in such fields.50 Finally, the doping of phosphorus in silicon wafers using this mechanism seems to be very promising for semiconductor manufacturing because the phosphorus was found to reside deep in the silicon, which is very complicated and expensive to achieve by applying state-ofthe-art doping processes.36,51−54

■

CONCLUSIONS C−S phases were investigated using phosphates as probe molecules to analyze their chemical structure, as evidenced by FTIR. At room temperature, the interaction is dominated by the formation of calcium phosphates, but in the case of an increment in temperature up to 1000 °C, the calcium silicate phases are regenerated. The phosphate molecules, instead of being decomposed or evaporated, penetrate the structure driven by the concentration gradients and dope the silicon wafer. The FTIR results were analyzed on the basis of firstprinciples calculations and further supported by complementary time-of-flight secondary ion mass spectrometry (ToF-SIMS) experiments. Exclusive information on the presented system was obtained from the in situ deposition of probe molecules and heating: (1) Long chains are not formed in the synthesized C−S−H phases on the silicon wafer until dehydroxylation takes place. (2) The formation of such chains can be understood on the basis of the interpretation of Si−O−Si vibrational modes. (3) Si−O−Si vibrational modes shift to lower wavenumbers when more calcium is incorporated into the structure, thereby increasing the Ca/Si ratio. The length of the chains plays a very important role in the structure of calcium silicates not only because of the mechanical properties associated with it but also because of the chemical corrosion produced by the metal−proton exchange reaction.20,55,56

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +49 721 608-28223. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The results presented in this article have been obtained within DFG-funded project TH 1566/3-2. The authors acknowledge the Texas Advanced Computing Center (TACC) for computational resources.

■

REFERENCES

(1) Nonat, A. The Structure and Stoichiometry of C-S-H. Cem. Concr. Res. 2004, 34, 1521−1528. (2) Garrault, S.; Behr, T.; Nonat, A. Formation of the C-S-H Layer During Early Hydration of Tricalcium Silicate Grains with Different Sizes. J. Phys. Chem. B 2006, 110, 270−275. (3) Schott, J.; Pokrovsky, O. S.; Spalla, O.; Devreux, F.; Gloter, A.; Mielczarski, J. A. Formation, Growth and Transformation of Leached Layers During Silicate Minerals Dissolution: The Example of Wollastonite. Geochim. Cosmochim. Acta 2012, 98, 259−281. (4) Rawal, A.; Smith, B. J.; Athens, G. L.; Edwards, C. L.; Roberts, L.; Gupta, V.; Chmelka, B. F. Molecular Silicate and Aluminate Species in 13583


Article

Langmuir

brid Organic−Inorganic Materials. Chem. Mater. 1999, 11, 1672− 1679. (45) Tänzer, R.; Buchwald, A.; Stephan, D. Effect of Slag Chemistry on the Hydration of Alkali-Activated Blast-Furnace Slag. Mater. Struct. 2015, 48, 629−641. (46) Hidalgo, A.; Petit, S.; Domingo, C.; Alonso, C.; Andrade, C. Microstructural Characterization of Leaching Effects in Cement Pastes Due to Neutralisation of Their Alkaline Nature: Part I: Portland Cement Pastes. Cem. Concr. Res. 2007, 37, 63−70. (47) Tian, R.; Seitz, O.; Li, M.; Hu, W.; Chabal, Y. J.; Gao, J. Infrared Characterization of Interfacial Si−O Bond Formation on Silanized Flat Sio2/Si Surfaces. Langmuir 2010, 26, 4563−4566. (48) Lim, S.-F.; Zheng, Y.-M.; Chen, J. P. Organic Arsenic Adsorption onto a Magnetic Sorbent. Langmuir 2009, 25, 4973−4978. (49) Zhang, J.; Stanforth, R. Slow Adsorption Reaction between Arsenic Species and Goethite (A-Feooh): Diffusion or Heterogeneous Surface Reaction Control. Langmuir 2005, 21, 2895−2901. (50) Hurle, K.; Christel, T.; Gbureck, U.; Moseke, C.; Neubauer, J.; Goetz-Neunhoeffer, F. Reaction Kinetics of Dual Setting A-Tricalcium Phosphate Cements. J. Mater. Sci.: Mater. Med. 2016, 27, 1. (51) Mori, K.; Yamaguchi, K.; Hara, T.; Mizugaki, T.; Ebitani, K.; Kaneda, K. Controlled Synthesis of Hydroxyapatite-Supported Palladium Complexes as Highly Efficient Heterogeneous Catalysts. J. Am. Chem. Soc. 2002, 124, 11572−11573. (52) McKelvie, I. D.; Peat, D. M.; Worsfold, P. J. Analytical Perspective. Techniques for the Quantification and Speciation of Phosphorus in Natural Waters. Anal. Proc. 1995, 32, 437−445. (53) Prati, C.; Gandolfi, M. G. Calcium Silicate Bioactive Cements: Biological Perspectives and Clinical Applications. Dent. Mater. 2015, 31, 351−370. (54) Vonlanthen, P.; Bittner, D.; Hudson, A. G.; Young, K. A.; Muller, R.; Lundsgaard-Hansen, B.; Roy, D.; Di Piazza, S.; Largiader, C. R.; Seehausen, O. Eutrophication Causes Speciation Reversal in Whitefish Adaptive Radiations. Nature 2012, 482, 357−362. (55) Hou, D.; Zhang, J.; Li, Z.; Zhu, Y. Uniaxial Tension Study of Calcium Silicate Hydrate (C−S−H): Structure, Dynamics and Mechanical Properties. Mater. Struct. 2015, 48, 3811−3824. (56) Giraudo, N.; Weidler, P. G.; Laye, F.; Schwotzer, M.; Lahann, J.; Wöll, C.; Thissen, P. Corrosion of Concrete by Water-Induced MetalProton Exchange. J. Phys. Chem. C 2016, 120, 22455.

(25) Ohashi, Y.; Finger, L. W. The Role of Octahedral Cations in Pyroxenoid Crystal Chemistry; I, Bustamite, Wollastonite, and the Pectolite-Schizolite-Serandite Series. Am. Mineral. 1978, 63, 274−288. (26) Yamnova, N.; Sarp, K.; Egorov-Tismenko, Y. K.; Pushcharovski, D.; Dasgupta, G. Crystal Structure of Jaffeite. Cryst. Rep. 1993, 38, 464−467. (27) Yamnova, N. A.; Zubkova, N. V.; Eremin, N. N.; Zadov, A. E.; Gazeev, V. M. Crystal Structure of Larnite Beta-Ca2SiO4 and Specific Features of Polymorphic Transitions in Dicalcium Orthosilicate. Crystallogr. Rep. 2011, 56, 210−220. (28) Downs, R. T.; Hall-Wallace, M. The American Mineralogist Crystal Structure Database. Am. Mineral. 2003, 88, 247−250. (29) Hirai, T.; Hodono, M.; Komasawa, I. The Preparation of Spherical Calcium Phosphate Fine Particles Using an Emulsion Liquid Membrane System. Langmuir 2000, 16, 955−960. (30) Andrews, L. D.; Deng, H.; Herschlag, D. Isotope-Edited Ftir of Alkaline Phosphatase Resolves Paradoxical Ligand Binding Properties and Suggests a Role for Ground-State Destabilization. J. Am. Chem. Soc. 2011, 133, 11621−11631. (31) Reyes-Gasga, J.; Martínez-Piñeiro, E. L.; Rodríguez-Á lvarez, G.; Tiznado-Orozco, G. E.; García-García, R.; Brès, E. F. Xrd and Ftir Crystallinity Indices in Sound Human Tooth Enamel and Synthetic Hydroxyapatite. Mater. Sci. Eng., C 2013, 33, 4568−4574. (32) Du, Q.; Superfine, R.; Freysz, E.; Shen, Y. R. Vibrational Spectroscopy of Water at the Vapor/Water Interface. Phys. Rev. Lett. 1993, 70, 2313−2316. (33) Alarcon-Ruiz, L.; Platret, G.; Massieu, E.; Ehrlacher, A. The Use of Thermal Analysis in Assessing the Effect of Temperature on a Cement Paste. Cem. Concr. Res. 2005, 35, 609−613. (34) Gasparini, E.; Tarantino, S. C.; Ghigna, P.; Riccardi, M. P.; Cedillo-González, E. I.; Siligardi, C.; Zema, M. Thermal Dehydroxylation of Kaolinite under Isothermal Conditions. Appl. Clay Sci. 2013, 80−81, 417−425. (35) Elzinga, E. J.; Sparks, D. L. Phosphate Adsorption onto Hematite: An in Situ Atr-Ftir Investigation of the Effects of pH and Loading Level on the Mode of Phosphate Surface Complexation. J. Colloid Interface Sci. 2007, 308, 53−70. (36) Longo, R. C.; Cho, K.; Schmidt, W. G.; Chabal, Y. J.; Thissen, P. Monolayer Doping Via Phosphonic Acid Grafting on Silicon: Microscopic Insight from Infrared Spectroscopy and Density Functional Theory Calculations. Adv. Funct. Mater. 2013, 23, 3471−3477. (37) Klähn, M.; Mathias, G.; Kötting, C.; Nonella, M.; Schlitter, J.; Gerwert, K.; Tavan, P. Ir Spectra of Phosphate Ions in Aqueous Solution: Predictions of a DFT/MM Approach Compared with Observations. J. Phys. Chem. A 2004, 108, 6186−6194. (38) Okano, K.; Miyamaru, S.; Kitao, A.; Takano, H.; Aketo, T.; Toda, M.; Honda, K.; Ohtake, H. Amorphous Calcium Silicate Hydrates and Their Possible Mechanism for Recovering Phosphate from Wastewater. Sep. Purif. Technol. 2015, 144, 63−69. (39) Kim, D. W.; Cho, I.-S.; Kim, J. Y.; Jang, H. L.; Han, G. S.; Ryu, H.-S.; Shin, H.; Jung, H. S.; Kim, H.; Hong, K. S. Simple Large-Scale Synthesis of Hydroxyapatite Nanoparticles: In Situ Observation of Crystallization Process. Langmuir 2010, 26, 384−388. (40) Sanna, S.; Schmidt, W. G.; Thissen, P. Formation of Hydroxyl Groups at Calcium-Silicate-Hydrate (C-S-H): Coexistence of Ca−OH and Si−OH on Wollastonite(001). J. Phys. Chem. C 2014, 118, 8007− 8013. (41) Martin, R. I.; TenHuisen, K. S.; Leamy, P.; Brown, P. W. Enthalpies of Formation of Compounds in the P2O5−CaO−H2O System. J. Phys. Chem. B 1997, 101, 9375−9379. (42) Hurle, K.; Neubauer, J.; Bohner, M.; Doebelin, N.; GoetzNeunhoeffer, F. Calorimetry Investigations of Milled A-Tricalcium Phosphate (A-TCP) Powders to Determine the Formation Enthalpies of A-TCP and X-Ray Amorphous Tricalcium Phosphate. Acta Biomater. 2015, 23, 338−346. (43) Lenza, R. F. S.; Vasconcelos, W. L. Structural Evolution of Silica Sols Modified with Formamide. Mater. Res. 2001, 4, 175−179. (44) Innocenzi, P.; Brusatin, G.; Guglielmi, M.; Bertani, R. New Synthetic Route to (3-Glycidoxypropyl)Trimethoxysilane-Based Hy13584


Calcium Silicate Phases Explained by High ... - ACS Publications

Recommend Documents