Introduction

Strontium is an alkaline earth metal that has two electrons in the outer valence shell. It has a very low electronegativity (1.0), thus tending to be readily ionized as Sr2+ to react with oxygen and H2O in air1. It is an important element as A-site dopant or ingredient in perovskite-type electrodes and catalysts2,3,4. For instance, in (La,Sr)MnO3 and (La,Sr)(Co,Fe)O3 perovskite cathodes, the Sr2+ substitution for La3+ provides a structure that is favorable to polaron hopping, improving the electrical conductivity in solid oxide fuel cells (SOFC)5,6,7,8.

Sr surface segregation frequently occurs in perovskite structures and is among important issues since the near-surface structure determines the overall properties of the materials. The relatively large size of Sr2+ (0.144 nm for 12 coordination) is regarded as the cause of the segregation9,10,11,12,13,14. In humid environments, the segregation is accelerated as the absorption of H2O distorts the lattice, allowing for facile migration of Sr ions toward the surface15,16,17. The segregated Sr is likely to exist as SrCO3 on the surface18.

Strontianite, or SrCO3, has a wide range of applications, such as in dye-sensitized solar cell19, thermochemical energy storage20, cataluminescence-based sensor21, eletrocatalyst22, and photocatalyst23,24,25, apart from raw material in industry. The morphology control of SrCO3 has attracted considerable interest as providing the opportunity to explore and develop novel properties: e.g., urchin-like SrCO3 particles showed enhanced specific capacitance26, and vertically-oriented SrCO3 nanorods exhibited photoluminescence (PL) quenching over the full solar spectrum range27.

In fabricating SrCO3 hierarchical superstructures (e.g., needle-, dumbbell-, and flower-like morphologies), various synthesis methods including hydrothermal route have been developed, where strontium nitrate, chloride, and acetate are frequently used as the Sr source (listed in Supplementary Table S1 online)26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42. Our recent work showed that the continuous Sr segregation induced by H2O absorption leads to the formation of SrCO3 (Supplementary Fig. S1 online), especially with nanorod morphology16. However, no report has been found for the SrCO3 formation via the H2O absorption induced Sr-segregation in ambient environment, which could be closely related to the biomineral carbonation in nature. Despite great advances in the synthesis and application, studies on the fundamental science of SrCO3 formation have lagged behind. For further design and development of one-dimensional (1D) architectures of SrCO3, the growth mechanism of SrCO3 nanorods needs to be understood.

The current study focuses on the structuring role of water in the growth of single-crystalline SrCO3 nanorod arrays. The evolution of hydrated strontium into SrCO3 is investigated by in situ Raman spectroscopy. It is found that the hydrated Sr has a strong tendency to absorb CO2 in air whereas non-hydrated Sr does not. The observation of the intermediate phase, as well as thermodynamic considerations, elucidates the reaction pathway for the formation of SrCO3. X-ray diffraction (XRD) simulation and transmission electron microscopy (TEM) analyses further reveal that the epitaxial growth of SrCO3 occurs along the c-axis, leaving stacking faults behind.

Results

SrCO3 vertical growth on hydrated strontium compounds

As an example, Sr9Ni7O21, which is one of Sr-enriched structures and subject to Sr segregation in humid environment, was used to investigate the growth of SrCO3 nanorods from hydrated strontium compounds. Figure 1a shows scanning electron microscopy (SEM) images of the surface of strontium nickel oxide (SNO; Sr9Ni7O21) exposed to a humid environment (2.7% H2O) for 0–48 h and 10 days. It is seen that Sr is readily segregated from SNO by absorbing moisture, leading to the growth of nanorods on the surface. Figure 1b displays the cross-sectional images (HAADF and elemental mapping) of the nanorods on the SNO surface. Sr-enrichment is observed in the surface nanorods and along the SNO grain boundaries. The nanorod is identified as SrCO3 (Pmcn; JCPDS No. 05-0418) by the fast-Fourier-transform (FFT) technique (Fig. 1c), thus demonstrating Sr-segregation and SrCO3 growth from a Sr-rich compound. It should be noted that the strontium, segregated by H2O absorption, would exist initially as Sr(OH)2·8H2O, according to our previous study16. The hydrated Sr-hydroxide is subject to reacting with CO2 in air, resulting in SrCO3, as per the reaction.

$${\text{Sr}}({\text{OH}})_{2} \cdot 8{\text{H}}_{2} {\text{O }} + {\text{ CO}}_{{2{\text{(g)}}}} \to {\text{ SrCO}}_{3} + \, 9{\text{H}}_{2} {\text{O}}$$
(1)
Figure 1
figure 1

(a) SEM images of the surface of strontium nickel oxide pellet before and after the exposure to a humid environment (2.7% H2O content) for 1 h, 3 h, 7 h, 19 h, 48 h, and 10 days. The SEM–EDS spectrum of the nanorod array is also presented in Supplementary Fig. S2 online. It is noted that these samples were dried in ambient air (0.3% H2O content) for 2 h after exposure to the humid environment. SEM images of the samples that were not dried are also displayed in Supplementary Fig. S3 online. (b) High-angle annular dark-field (HAADF) and elemental mapping images (Ni, Sr, and O) of the cross-section of the SNO pellet exposed to the humid environment for 5 days, analyzed using FIB and STEM-EDS. (c) High-resolution TEM images of the nanorod grown on the SNO surface and the corresponding FFT pattern indexed in the Pmcn space group of SrCO3. (d) Schematic representation of the structural evolution of SNO by hydration and the accompanying SrCO3 nanorod growth.

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It is interesting that the SrCO3 here grows in one-dimension, eventually forming SrCO3 nanorod arrays on the SNO surface (Fig. 1d). Such a phenomenon may occur in other alkaline-earth-metal compounds as well. For instance, SrCO3 nanowhiskers were found to grow from Sr4Mn3O10 in humid environment (see Supplementary Fig. S4 online), which have potential applications as Cr/S getters43 in high-temperature electrochemical systems. While water molecules are considered contributors to the directional growth, the role of water molecules in Sr carbonation is rarely studied. It remains to be discovered how Sr(OH)2·8H2O in air transforms into SrCO3 and how the resultant SrCO3 has 1D morphology. These questions will be answered in the following sections.

Effect of moisture on SrCO3 formation

In order to study the influence of moisture on the formation of SrCO3, as-received SrO and Sr(OH)2·8H2O powders were placed for 24 h in an ambient room air (1.2% H2O present; and 400 ppm CO2) or under a flow of CO2–air mixed gas (H2O absent; and 30% CO2). Their morphology and structure changes were analyzed by SEM and XRD.

Figure 2 displays XRD patterns and SEM images of the SrO exposed to the two different atmospheric conditions. Several distinguishing characteristics of the reactivity of SrO with air were revealed as follows. First, as-received SrO (Alfa Aesar, USA) was identified as Sr(OH)2·H2O with a small portion of Sr(OH)2 by XRD analysis (Fig. 2a) since SrO was readily hydrated by absorbing airborne moisture during the XRD analysis, indicating the hygroscopic nature of SrO. Second, it is interesting that SrO (and if any existing Sr(OH)2) does not react with airborne CO2 in the absence of moisture; that is, no SrCO3 formed under dry CO2–air flow, albeit containing a high concentration of CO2 (30%) (Fig. 2b,d). In other words, the reaction between SrO and CO2 occurs only in the presence of moisture; indeed, SrCO3 was produced in an ambient air containing moisture (1.2% H2O in this case) although the concentration of CO2 was as low as 400 ppm (Fig. 2c). Third, a morphological transformation into nanorods during the SrCO3 formation was observed (Fig. 2e). Further information to understand the anisotropic growth of SrCO3 is given in Fig. 3.

Figure 2
figure 2

XRD patterns of SrO before and after exposure to ambient room air (1.2% H2O present, and 400 ppm CO2) and a CO2-air mixed gas (H2O absent, and 30% CO2) for 24 h (a–c), and their corresponding SEM images (d,e). Note that the XRD pattern of SrO could not be properly recorded because SrO was readily hydrated by absorbing airborne moisture in the middle of XRD analysis; thus, Sr(OH)2 and Sr(OH)2·H2O were detected instead of SrO.

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Figure 3
figure 3

XRD patterns of Sr(OH)2·8H2O before and after exposure to an ambient room air (1.2% H2O present, and 400 ppm CO2) and a CO2-air mixed gas (H2O absent, and 30% CO2) for 24 h (a–c), and their corresponding SEM images (d,e).

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Figure 3 shows XRD patterns and SEM images of the Sr(OH)2·8H2O exposed to the two different atmospheric conditions. It is found that the exposure of hydrated strontium (i.e., Sr(OH)2·8H2O) to CO2, regardless of the presence of moisture, results in the formation of SrCO3 (Fig. 3a–c), corresponding to the above discussion. SEM observations reveal that the SrCO3 tends to grow in one-dimension when derived from Sr(OH)2·8H2O (Fig. 3d,e). This tendency is very prominent for the Sr(OH)2·8H2O exposed to air containing moisture (Fig. 3e). That is, quasi-vertically aligned SrCO3 nanorod arrays were produced from the Sr(OH)2·8H2O further hydrated in the presence of moisture (i.e., 1.2% H2O with 400 ppm CO2 in an ambient air) (Fig. 3e) whereas irregularly aligned SrCO3 nanorods with relatively small aspect ratios grew in the moisture-free environment (i.e., dry air with 30% CO2) (Fig. 3d).

The anisotropic growth of SrCO3 from Sr(OH)2·8H2O in air can be explained by considering the reaction kinetics. As per Eq. (1), in the very low partial pressure of CO2 (i.e., 400 ppm in air), the reaction rate for SrCO3 formation would be very slow, so that the clusters and nuclei of SrCO3 afford movement and rotation to assemble along their preferential orientation, resulting in the 1D-oriented growth of SrCO3. This is possible because moisture can hydrate the surface strontium and form an aqueous liquid layer over the solid crystalline Sr(OH)2·8H2O where SrCO3 nuclei can move and rotate freely; indeed, the presence of aqueous liquid layer on SNO surface was found (see Supplementary Fig. S3 online).

Preferred growth orientation of SrCO3

The preferred orientation for the growth of SrCO3 is further investigated by XRD simulations together with the measured XRD data. First, the XRD pattern of the quasi-vertically aligned SrCO3 nanorod arrays in Fig. 3c was closely inspected. The intensity of (111) peak is the strongest while that of (021) peak seems to be weakened, compared with Fig. 3b. This has often misled us to make an incorrect conclusion that the SrCO3 crystal grows mainly along the (111) face41,44. However, such XRD pattern (at ~ 25.5° in Fig. 3c) is likely due to the broadening and overlapping of (111) and (021) peaks, rather than to the (111) oriented growth, which is in good agreement with the XRD profile simulated as a function of SrCO3 crystallite sizes (see Supplementary Fig. S5 online). Since the peaks for (002), (113) and (023) planes, as shown in Fig. 3c, have much stronger intensities than those of typical SrCO3 (Supplementary Fig. S6 online), those planes can be nominated as the preferred orientation of the SrCO3 growth. Next, XRD simulations on the planes were performed using CrystalDiffract software to identify the preferred orientation plane. Figure 4a displays simulated XRD patterns of the SrCO3 with (002) preferred orientation as a function of the degree of alignment. It is found that the intensity of (113)/(023) peak increases, along with (002) peak, as shown in the XRD patterns of the SrCO3 with preferred orientation in (002) plane (Fig. 4a: right insets). This simulation result is well matched to the measured one in Fig. 3c, suggesting that the SrCO3 nanorod grows preferentially at \(\langle {001} \rangle\) direction (i.e., along c-axis).

Figure 4
figure 4

(a) Simulated XRD patterns of the SrCO3 (Pmcn; orthorhombic; and D = 14 nm) with the preferred orientation along (002) plane (i.e., c-axis) as a function of the degree of alignment. The degree of alignment is represented by a fraction, with 0 indicating a completely random-oriented structure, and 1 being a fully oriented structure (e.g., precisely parallelly aligned rods). (b) SEM and (c) TEM images of SrCO3 nanorods. (d,e) Low- and high-magnification TEM images of a single-crystalline SrCO3 nanorod. (f–h) FFT diffraction patterns of selected areas in (e), indexed to the orthorhombic SrCO3 (Pmcn; JCPDS No. 05-0418) (Supplementary Fig. S7 online).

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TEM analysis was performed to further validate the above results. Figure 4b,c show SEM and TEM images of nanorods grown from Sr(OH)2·8H2O in humid condition. The low- and high-resolution (HR) TEM images of a single nanorod in Fig. 4d,e show fringes parallel to the growth direction of the nanorod (marked by arrows), indicative of planar defects. The FFT patterns of selected areas in Fig. 4e are displayed in Fig. 4f–h. The diffraction spots are indexed to the orthorhombic SrCO3 (Pmcn; JCPDS No. 05-0418) where the streaks, lying parallel to the [010] direction, reflect local disorder of the atomic stacking (i.e., stacking faults) as being perpendicular to the fringes in the HR-TEM image (see Supplementary Fig. S7 online). The distance between the streaks, normal to the growth direction, corresponds to the (001) plane, i.e., 0.60 nm, demonstrating the growth direction of the nanorod to be \(\langle {001} \rangle\). Note that this kind of pattern appears in all regions of the nanorod (Fig. 4f–h). Hence, it is concluded that the SrCO3 nuclei tend to assemble along the c-axis leaving stacking faults behind, which is in good agreement with the result from XRD simulation.

Chemical structure evolution of hydrated SrO to SrCO3

The reaction pathway for the carbonation of hydrated SrO was investigated by in situ Raman spectroscopy. Figure 5 shows representative Raman spectra of hydrated SrO that is exposed to ambient air for up to 16 min. Vibration modes of the Raman spectra are displayed in Supplementary Table S2, according to literature45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62.

Figure 5
figure 5

(a) Raman spectra of hydrated SrO in ambient air, recorded over time (0, 4, 8, 12, and 16 min). (b) Superimposed Raman spectra at the range of 1020–1100 cm−1. (c) A structure of the transition-state intermediate, SrOH+⋯CO32−. Vibration modes of the Raman spectra are displayed in Supplementary Table S2.

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The initial Raman spectrum of SrO soaked with water (Fig. 5a: Hydrated SrO) is dominated by a broad band at 3000–3700 cm−1 owing to the stretching of O–H in water46, implying hydration of SrO. Over time, the peaks ascribable to SrCO3 (149, 182, 244, 701, and 1073 cm−1) rise, indicating the carbonation of hydrated strontium (Fig. 5a: 4–16 min). Careful observations on the peak for SrCO3 at 1020–1100 cm−1 (Fig. 5b) reveal that the growth of the SrCO3 peak is accompanied by the rise of the small band at ~ 1055 cm−1 (assigned to CO32− ions in aqueous solution51,52) which is not present in the normal spectrum of solid SrCO3 (see Supplementary Fig. S8 online). The results demonstrate that (i) hydrated strontium has a high tendency to absorb CO2 and (ii) the absorbed CO2 is present as CO32− ions which are supposed to react with strontium to form SrCO3. These results are supported by the facts that aqueous alkaline-earth-metal hydroxide solutions are strong bases (e.g., pH = 11.27–13.09 at 1–100 mM Sr(OH)2) and that the basic solution has the high solubility of CO2 (supposed to convert to CO32−) thus capturing airborne CO2 so as to meet the equilibrium condition. For details, see Supplementary Fig. S9 online.

While the dissolved carbon is present as CO32− in the basic aqueous solution, the chemical structure of strontium has not been completely determined so far; for instance, the Sr2+ coordination number surrounded by H2O/OH− was variously suggested to be six, or eight, etc.63,64,65,66,67,68,69,70,71,72,73. Aside from such discrepancy, there is a general consensus that hydrated Sr2+ clusters have flexible structures freely exchanging H2O and OH− between the first and second shells74,75. This indicates that Sr2+ hydroxide clusters with Sr2+–OH− interaction (mostly mono/di-hydroxides75) are always present in high pH aqueous solutions (OH− abundant); however, Sr2+ and OH− are not directly connected in both Sr(OH)2·8H2O and Sr(H2O)7(OH)+ solid phases (instead, only Sr2+–H2O interaction exists unless further hydrated)69,76.

This assumption is thermodynamically reasonable. Figure 6 displays Gibbs free energies for the plausible reactions of strontium hydroxide and carbon ions in aqueous solution. Based on the Gibbs free energy changes of the formation of SrOH+ and Sr2+ ions (red and orange curves in Fig. 6, respectively), it is energetically favorable for Sr2+ to be paired with OH−, namely SrOH+, which corresponds to the computational study64,77. After many attempts, SrOH+ was indeed observed by in situ Raman spectroscopy. The signals attributable to SrOH+ (bands at 361, 396, 518, and 539 cm−1) and to O–O stretching in Sr(O2)1−x–Ox (bands at 840–860 cm−1) were detected (Fig. 5a: 4 min). The intensity of the bands relating to SrOH+, Sr(O2)1−x–Ox, and SrCO3 increased simultaneously during the Sr-carbonation (Fig. 5a: 4–12 min). After completion of the carbonation (Fig. 5a: 16 min), the bands disappeared, leaving only those for SrCO3. These findings indicate that SrOH+–CO32−, or SrHO+–OCO22− (depicted in Fig. 5c), is an intermediate phase during the carbonation of hydrated strontium. It is thus evident that, in the Sr liquid droplets, the interaction of CO32− with SrOH+ in Sr(OH)x(H2O)y cluster orients the SrCO3 nuclei, and subsequent dehydration leaves SrCO3 nanorods.

Figure 6
figure 6

Gibbs free energy changes for the plausible reactions of strontium hydroxide and carbon ions in aqueous solution at 0–100 °C, plotted using HSC Chemistry software.

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Discussion

We have studied water-mediated Sr-precipitate and growth of strontianite nanorod arrays on the surface of strontium compounds in ambient conditions. As a case study, we choose to investigate Sr9Ni7O21, which is a Cr gettering material78. The compound is prone to Sr-segregation in air by absorbing airborne moisture and transforming the structure as per the reaction: Sr9Ni7O21 + nH2O → 7SrNiO3 + 2Sr(OH)2·8H2O. The segregated Sr(OH)2·8H2O subsequently converts to SrCO3 in ambient air, which interestingly has nanorod morphology. Time-dependent SEM analysis demonstrated that the growth of SrCO3 occurs in one direction when derived from hydrated SrO, such as Sr(OH)2·8H2O.

The effect of water molecules on the formation and growth of SrCO3 during the carbonation has been elucidated. In the absence of water molecules, SrO does not react with CO2 even under high CO2 partial pressure (30%), whereas SrO readily reacts with CO2 in ambient air (water molecule present) despite its low concentration (400 ppm). These results suggest that the carbonation of SrO at ordinary temperatures occurs via hydration in the presence of moisture, as illustrated in Supplementary Fig. S10 online. The driving force for the Sr-carbonation can be understood from the perspective of CO2 dissolution in aqueous alkaline solution. Basically, aqueous alkaline solutions have high solubility of carbonate ions due to high pH (i.e., high concentration of OH−). Likewise, the moisture absorption endows strontium with more alkaline character, and thus the hydrated strontium, such as Sr(OH)2·8H2O and Sr(OH)2·xH2O, becomes capable of absorbing CO2 in air. In the alkaline condition of hydrated strontium, Sr2+ is likely to be coupled with OH− (i.e., SrOH+), and the absorbed CO2 converts into CO32−. Subsequent interaction of the Sr and C ions leads to the formation of SrCO3 nuclei (Supplementary Fig. S10 online). Hence, the generally known fact “SrO is likely to react with CO2” can be better understood as “Once hydrated, SrO reacts with CO2”.

It is also found that the morphology of the resulting SrCO3 is affected by the kinetics of SrCO3 formation. The Sr-carbonation via hydration is known to occur following the reaction, Sr(OH)2·8H2O + CO2 → SrCO3 + 9H2O. It is thus implied that the kinetics of SrCO3 formation is slower under low CO2 and high H2O partial pressures (or humidity level). Experimental results in Figs. 2 and 3 showed the growth trend of SrCO3 under different atmospheric conditions. When SrCO3 forms under high pCO2 and low pH2O (i.e., favorable for fast reaction kinetics), it has spherical morphology, suggesting the isotropic growth of SrCO3. In contrast, when SrCO3 forms under low pCO2 and high pH2O (i.e., desirable for slow reaction kinetics), it is likely to have nanorod morphology, indicating the anisotropic growth of SrCO3. That is, the slower the reaction kinetics, the more the SrCO3 grows in one direction (Supplementary Fig. S11 online). Thus, it is considered that, when sufficient time is given for precursor ions to move and rotate, anisotropic assembly occurs particularly along [001] direction resulting in c-axis oriented SrCO3 nanorods.

The formation process of SrCO3 nanorod arrays from Sr-enriched phases by vapor–liquid–solid mechanism in ambient conditions is summarized in Fig. 7. First, the Sr-terminated surface, being hygroscopic, attracts and absorbs water molecules in air (Fig. 7a). This leads to the hydration and segregation of strontium onto the surface, thus forming Sr(OH)2·H2O, Sr(OH)2·8H2O, and Sr(OH)x·(H2O)y layers (Fig. 7b). The alkaline nature of the hydrated Sr-hydroxide layer causes the uptake of CO2 in air, which subsequently converts to HCO3− and CO32− (Fig. 7c). In the moistened medium, Sr and C ion complexes, including SrOH+ and CO32−, freely move, rotate, and interact with each other, leading to the assembly, nucleation, and growth of SrCO3 (Fig. 7d). Particularly, the growth occurs preferentially along the c-axis, leaving staking faults behind. In this way, quasi-vertically aligned SrCO3 nanorod arrays with (002) orientation can form at room temperature in ambient air without external addition of carbon source (Fig. 7e). This study paves the way for designing and developing 1D architecture of SrCO3 via H2O-mediated precipitation from Sr-rich compounds. This strategy can also be applicable to other alkaline-earth-metal compounds to fabricate 1D nanostructures.

Figure 7
figure 7

Schematic diagram for the process of the growth of SrCO3 nanorod arrays from hydrated strontium that is segregated from Sr-containing substrates.

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Methods

For the preparation of strontium nickel oxide (Sr9Ni7O21), the mixtures of Sr(OH)2·8H2O (Sigma-Aldrich, USA) and Ni(OH)2 (Sigma-Aldrich, USA) were heat-treated at 850–900 °C in air, where the molar ratio of Sr to Ni was 35:65 (see Ref.14). As-prepared SNO powder was uniaxially pressed into cylindrical pellets, followed by sintering at 900 °C. The SNO pellets were placed in a humid environment (i.e., a sealed container containing water at the bottom, 2.7% H2O content measured by a ThermoPro hygrometer). The pellet surface, which was exposed to the humid environment for 0–48 h and 10 days, was examined by a field-emission environmental scanning electron microscope (FE-ESEM; Quanta 250 FEG, FEI, USA) equipped with an energy dispersive X-ray spectrometer (EDS), for which the specimens were coated with gold either after being dried for 2 h in ambient air or immediately after being taken out from the humid condition. For scanning transmission electron microscopy (STEM) analysis, the SNO pellet, exposed to the humid environment for 5 days, was sliced using a focused ion beam (FIB; Helios Nanolab 460F1, FEI, USA). The specimen was analyzed using STEM (Talos F200X S/TEM, FEI, USA).

The structures and morphologies of as-received SrO (Alfa Aesar, USA) and Sr(OH)2·8H2O (Sigma-Aldrich, USA), exposed either to an ambient atmosphere (with 400 ppm CO2 and 1.2% H2O contents) or to CO2-air mixture (with 30% CO2 and ~ 0% H2O contents) at room temperature, were analyzed using SEM and X-ray diffractometer (XRD; D8 Advance, Bruker, Germany) with Cu-kα radiation (λ = 0.1542 nm). CrystalDiffract software (Version 6.8.2, CrystalMaker Software Ltd.) was used to identify and simulate XRD patterns of SrCO3 nanorods with different growth directions. The following parameters were used in the simulation: Pmcn (orthorhombic); lattice constants a = 5.107 Å, b = 8.414 Å, and c = 6.029 Å; and crystallite size = 14 nm. TEM analysis (Talos F200X S/TEM; FEI, USA) was performed on SrCO3 nanorods grown from Sr(OH)2·8H2O in humid condition. For TEM sample preparation, the nanorods were dispersed in ethanol and dropped onto a carbon-coated grid. TEM images were obtained at an accelerating voltage of 200 kV, followed by fast-Fourier-transformation analysis.

For in situ monitoring of the growth of SrCO3 from hydrated SrO, Raman spectroscopy (Ramanscope 2000, Renishaw, Gloucestershire, UK) was employed. As-received SrO particles were drenched in a droplet of DI water on a microscope glass slide to form an aqueous film of strontium hydroxide hydrate. Raman spectra of the film were then recorded over time with a laser of 514.5 nm wavelength. Gibbs free energy changes for the probable reactions between strontium and carbon ions at 0–100 °C were calculated from the database in HSC Chemistry 6 (Outotec, Finland).