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Article

Preparation of a Graphene-Enhanced Hydroxyapatite Film on Dolomitic Marble by the Sol-Gel Method

by
Feng Wang
1,*,
Di Li
2,
Yaoqi Gu
1 and
Shuya Wei
1,*
1
Institute of Cultural Heritage and History of Science & Technology, University of Science and Technology Beijing, Beijing 100083, China
2
Beijing Stone Carving Art Museum, Beijing 100044, China
*
Authors to whom correspondence should be addressed.
Crystals 2023, 13(4), 642; https://doi.org/10.3390/cryst13040642
Submission received: 20 March 2023 / Revised: 4 April 2023 / Accepted: 6 April 2023 / Published: 9 April 2023
(This article belongs to the Special Issue Advanced Surface Engineering Materials: Characterization and Properties)
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Abstract

:
The preparation of continuous hydroxyapatite film on stone is a promising method of protecting marble from erosion. However, many methods negatively affect the calcium in the substrate and forming of struvite on the dolomite surface, leading to a heterogeneous coating and low efficiency. In this study, a continuous hydroxyapatite coating on dolomitic marble was achieved from graphene enhanced Ca(OH)2 nanoparticles as the calcium precursor using the sol-gel method. The morphology and the structure of the film was evaluated by a field emission scanning electron microscope coupled with energy dispersive spectroscopy (FESEM-EDS), an optical microscope, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and analytical techniques. Moreover, the color and the contact angle measurements, as well as the simulated acid rain test and freeze–thaw treatment, were performed to assess the chromatic aberration, hydrophilicity, reliability, and durability of the coating. A suppositional combination model among hydroxyapatite, graphene quantum dots, and dolomite were suggested based on structural similarities between the support material and components of the functional coating. The integrality and efficiency of the hydroxyapatite film was improved by compositing with graphene quantum dots.

1. Introduction

In the Beijing area, most stone carvings are made of a kind of dolomitic marble (DM) called Qingbaishi [1]. It has a very low total porosity (about 1.15%) and forced water absorption (0.31%), and is composed of dolomite (CaMg(CO3)2) and a small proportion of quartz, exhibiting a granuloblastic texture and quasi-blastobedding structure [2]. These marble artworks exposed outdoors suffer from deterioration caused by temperature difference [3,4], freeze–thaw cycling [5,6], rainwater [7,8], air pollutants [9,10], and so on. Different kinds of materials are applied to prevent further loss or erosion. The application of the organic consolidants based on epoxy resin, silicone, and acrylic and fluorinated polymers is one of the major solutions [11,12,13,14,15]. However, oxidation between polymeric materials and rocks will cause an irreversible alteration of the structure on the surface, resulting in low compatibility and residual strain after heating–cooling cycles [16,17], even being difficult to remove from the substrate. The photo-oxidative reaction-induced discoloration is also an adverse effect which needs to be solved [14,18]. Recently, further studies have been performed on biomimetic materials to improve efficiency and durability.
Among different kinds of biomimetic materials, calcium oxalate and hydroxyapatite (HAP, Ca10(PO4)6(OH)2) attracted more attention regarding stone conservation. Calcium oxalate has been applied to enhance the corrosion-resistant performance of marble, after it was found to have a good protection effect on the Parthenon [19]. Regarding the formation process, the biological hypothesis supports the idea that weddellite origins from the metabolic product of microorganisms colonized on the surface [20,21,22], while the chemical explanation is that the calcium oxalate is derived from the degradative oxidation product of organic conservation materials such as milk, egg, oils, and Arabic gum, used on the substrate in the past [23,24,25]. In addition, oxalic acid existing in the environment from fuel combustion has also been considered an alternative source [26,27]. Either way, it would take a long time to obtain a satisfactory coating on marble. At present, calcium oxalate film is usually prepared through the reaction of ammonium oxalate with calcite [28,29]; nevertheless, a uniform coating is difficult to form on the surface in short time. HAP, as the main component of bones, has been tested to consolidate carbonate stones by Enrico Sassoni et al. since 2011 [30]. Through the use of a poultice or brushing the diammonium hydrogen phosphate (DAP) on a marble surface, HAP or other calcium phosphates can be formed, strengthening the mechanical property [31,32,33]. Furthermore, the preparation of a continuous HAP coating is a promising method of protecting marbles from erosion caused by rainwater and pollutants [34,35]. However, the reaction between DAP and marble will damage the calcium in the host rock, and struvite could form on the dolomite which may result in a heterogeneous coating and low efficiency.
Sol-gel-derived HAP film on the implantable metallic substrate, such as Ti [36], Ti-6Al-4V [37], Ti-Zr-Nb [38], and 316L [39] has been successfully synthesized at high sintering temperatures up to and above 450 °C, improving the integrity of crystallized films. Table 1 displays the common precursors, solvents, and pH conditioners/additives for preparation of HAP sol, while the Ca(NO3)2∙4H2O is the most frequently used calcium source. Aiming to reduce the crack and improve the strength, multi-walled carbon nanotubes (MWCNTs), as a kind of carbon material, were added in the HAP sol-gel. Liu et al. successfully synthesized MWCNT-reinforced HAP composite film on the surface of Ti, and the bonding strength increased to 32.9 MPa from 22.2 MPa [40]. Park et al. demonstrated that the MWCNTs could stimulate the nucleation and crystallization of HAP by increasing the concentration of the MWCNTs up to 1 wt.% [41]. In our previous job, Graphene Quantum Dot (GQDs)-enhanced Ca(OH)2 nanoparticles were successfully prepared, and the composition mechanism was proved by HRTEM and calculations [42]. Reacting with HNO3, the Ca(OH)2/GQD nanocomposite can be transformed into Ca(NO3)2. Inspired by the positive effect of MWCNTs, graphene can be introduced into the HAP film through the sol-gel process for reducing the coating defects instead of sintering. In addition, in order to match the composition of DM and increase the hydrophobicity of film, TEOS and PDMS may be brought in the sol system [43].
In this work, a continuous HAP film on dolomitic marble was prepared from graphene-enhanced Ca(OH)2 nanoparticles as the calcium precursor with the sol-gel method. We characterized the morphology, structure features, chromatic aberration, hydrophilicity, anti-corrosion property, and freeze–thaw resistance of the film. For comparison, the behavior of the sample without graphene was performed at the same time.

2. Materials and Methods

2.1. Materials

Ammonia solution (Ammonia 20%) and HNO3 (≥70.0%) were purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD (Shanghai, China). CaCl2·2H2O (≥99.0%), NaOH (≥96.0%), Ca(OH)2 (≥95.0%), H2SO4 (95.0–98.0%), PDMS, and ethanol (EtOH, ≥99.7%) were obtained from Sinopharm Chemical Reagent Co. LTD (Beijing, China). (NH4)2HPO4 (≥99.9%), H3PO4 (≥85%), and tetraethyl orthosilicate (TEOS, C8H20O4Si ≥ 99.99%) were purchased from Macklin reagent. Graphene Quantum Dots solutions (0.10 mg/mL) were supplied by Yan Li’s lab through cyclic voltammograms method [51]. Ca(OH)2/GQD nanoparticles were obtained by a homogeneous synthetic method, and the average size was 67.6 nm [42]. Naturally weathered dolomitic marble samples derived from the Qing Dynasty for laboratory testing were cut into 25 × 25 × 10 mm pieces along with the surface layer, provided by the Beijing Stone Carving Art Museum. The marble specimens were cleaned in an ultrasonic apparatus for 15 min to remove fragments and dried in an air dry oven.

2.2. Preparation of Graphene-Enhanced HAP Film

2.252 g Ca(OH)2 or Ca(OH)2&GQDs powders were added into a 60.8 mL HNO3 solution (1.0 mol/L) under magnetic stirring in a water bath kettle at 40 °C. After 10 min, the (NH4)2HPO4 (1.0 mol/L) solution was dropped in until the Ca/P mole ratio was 1.67. At the same time, ammonia solution was added to keep the pH value above 9, and stirring was continued for 90 min until the HAP sol was achieved. TEOS was pre-hydrolyzed in a solution containing EtOH, H2O, and H3PO4 at room temperature under magnetic stirring for 140 min. In this solution, PDMS was added in drops, and the stirring was continued for another 15 min. The mole ratio of TEOS/EtOH/H2O/PDMS/H3PO4 was set as 1/4/4/0.04/0.000067. Finally, 4.7 mL silica sol was dropped into the HAP sol and, which was stirred for 24 h. The gel was brushed on the surface of dolomitic marble specimens until distinct refusal, and then placed in a natural environment for two weeks. Samples untreated and composited with and without GQDs were named DM, H1, and H2, respectively. The whole process is illustrated in Figure 1.

2.3. Characterization of HAP Film

The morphology of the sample surface was studied by an Hitachi SU8020 (Hitachi Ltd., Tokyo, Japan) field emission scanning electron microscope (FESEM) at 100 kV. Energy dispersive spectroscopy (EDS) was performed with an HORIBA EX-350 (HORIBA Scientific, Tokyo, Japan) in 133 eV resolution. Optical images were acquired by a VHX 5000 ultra-depth-of-field microscope (Keyence Corporation, Osaka, Japan). Raman spectra were obtained with an HORIBA XploRA PLUS spectrometer (HORIBA FRANCE SAS, Palaiseau, France), using 532 nm laser excitation. The infrared spectra were characterized with ThermoFisher Nicolet iS5 (Thermo Fisher Scientific, Waltham, MA, USA) Fourier transform infrared spectroscopy (FTIR) at 0.4 cm−1 resolution. The chromatic aberration of specimens was evaluated by measuring the CIE Lab color parameters with a 3nh NS800 colorimeter (3nh Ltd., Guangzhou, China) and calculated by Δ E = ( Δ L * ) 2 + ( Δ a * ) 2 + ( Δ b * ) 2 (L* = black-white, a* = green–red, b* = blue-yellow). The variation of chroma was obtained by Δ C = ( Δ a * ) 2 + ( Δ b * ) 2 . The hydrophilicity of film was evaluated by a POWEREACH JC2000DM (POWEREACH Ltd., Shanghai, China) contact angle meter. The acid resistance of coating was assessed by a dripping apparatus as described in Graziani’s article [34]. Considering the annual average rainfall in Beijing (about 600 mm), the average frequency of acid rain was about 20% from 2012 to 2017 [52]; a 750 mL solution composed of HNO3 and H2SO4 (1:1, pH = 4.0) was dripped on the sample (25 × 25 mm2) and the period of dripping was 2 h, corresponding to the amount of acid rain in decade. Dissolution of the specimens was evaluated by measuring the Ca2+ ion content in the runoff solution with Dionex ICS-2000 ion chromatography (ThermoFisher Scientific, Waltham, MA, USA). The heating–cooling treatment was carried out in a Shanghai ROJIDA DR-IV freeze–thaw chamber (ROJIDA Ltd., Shanghai, China) between −20 °C to 20 °C 30 times, in accordance with GB/T241-1994, DZT 0276.8-2015, and the temperature variation in Beijing’s winter. The coefficient of frost resistivity (f) was calculated by f = R d 2 / R d 1 , where Rd1 = the compressive strength of dry sample before cycling, and Rd2 = the compressive strength of dry sample after cycling. The mass loss ratio (Km) was evaluated by K m ( % ) = ( m 1 − m 2 ) × 100 / m 1 , where m1 = the mass of dry sample before freeze–thaw, m2 = the mass of dry sample after freeze–thaw. The geometrical configurations and charge density difference plots were illustrated with VESTA software (version 3.5.8) [53]. The models of Ca10(PO4)6(OH)2 and CaMg(CO3)2 were referenced from the Materials project website (https://materialsproject.org/, accessed on 11 March 2023).

3. Results and Discussion

3.1. Morphology and Structure of the Coatings

The morphology results of samples are illustrated in Figure 2. Mineral crystals can be observed on the surface of DM samples (Figure 2a), while microcracks and the step bordering of the crystal can be found in SEM images (Figure 2b). After GQD-composited sol-gel treatment, the surface of H1 becomes smooth and a continuous film grows, mixing with a few cuboid crystals (Figure 2c,d). Nevertheless, on the H2 sample, the one without GQDs, involving fewer microcracks and step bordering, can be recognized better than the DM sample; however, the coating is not distinctly uniform (Figure 2e,f). From the view of cross section in Figure 3a, an obvious film on H1 can be identified, whose thickness is about 15 μm, while no good-growth coating appears on H2 (3c). For further confirmation of the component of HAP films, EDS mapping has been applied. The distribution of phosphorous on H1 samples (Figure 3b) assigned to the HAP is consistent with the film observed in Figure 3a, while the content of calcium in this area is not as well-proportioned as that of the substrate. Oxygen mapping does not show a distinct difference between the coating and host rock. In addition, several bright particles in silicon mapping results are probably indexed to SiO2, resulting from the hydrolysis of TEOS. As for the H2 sample, little phosphorous is displayed in the red circle part, indicating no well-crystallized HAP film deposited (Figure 3d). Unlike the H1, the distribution of calcium and oxygen is quite uniform and cannot be an indicator for HAP coating. It is noticed that more silicon is discovered on H2, which may be due to the thin thickness of HAP film, and the fact that it is easier for silica sol to stay on the surface in this case.
As revealed by the Raman spectra of the samples in Figure 4a, the characteristic peaks of the HAP can be identified on H1 and H2, especially the 960 cm−1(vs) band and 1049 cm−1 (m) band [39]. Additionally, peaks from 2900 cm−1 to 3100 cm−1 and at 1430 cm−1 are assigned to the CH3 asymmetry stretching and bending modes of PDMS [54], which can increase the hydrophobicity of film, and are found on both specimens. Dolomite bonds at 1095 cm−1(vs), 173 cm−1(m), and 297 cm−1(m) are from the substrate, owing to laser penetration [55]. Furthermore, the Raman spectra of H1 also have the D-band (1368 cm−1) and G-band (1597 cm−1) of GQDs, and their ID/IG are around 0.45, close to Yan Li’s results [51]. The Raman frequency of NH4NO3 (near 1050 cm−1) [56], N–H bending (1140 cm−1), N–H stretching (3360 cm−1), or NH2 stretching (3500 cm−1) [57] are not presented, which may be due to decomposition during the aging process.
The FTIR spectra of specimens before and after treatment are revealed in Figure 4b. A total of 3 internal modes of CO3 groups from CaMg(CO3)2 were present in the FTIR results: 728 cm−1 (in-plane bending), 878 cm−1 (out-of-plane bending), and 1422 cm−1 (asymmetric stretching) [58]. The peaks at the 1110–928 cm−1 and 650–480 cm−1 ranges correspond to the asymmetric stretching and bending modes of PO43−, respectively. The band at about 1647 cm−1, and a series of bands varying from 3700 cm−1 to 3110 cm−1, are indexed to the vibration mode of OH− [37]. The HAP formation can be reconfirmed by the absorption peaks of PO43− and OH− functional groups in the FTIR data. In addition, the band at 1235 cm−1 is assigned to the CH3 symmetric stretching deformation of PDMS; the weak one at 1108 cm−1 is related to the Si-O-Si mode, while the absorption peak at 1163 cm−1 is probably from the alcohol bond [59,60]. Similar to Raman results, peaks related to N-H bonds are not observed, indicating that long-time aging may be an effective way to release ammonium salt.

3.2. Property Evaluation of the Film

Chromatic aberration is an important indicator of stone conservation. Considering the color changes of the treated specimens displayed in Figure 5, both cases are lower than the threshold of the JND (Just Noticeable Difference) in the CIE Lab space ( Δ E = 2.3 ) [61]. The coatings increase the grayscale and reduce the chroma, which is probably due to the white color of HAP, while the thin thickness of film does not have a great influence on the color.
Aiming to investigate the hydrophilicity of HAP film prepared with different formulations, water contact angles (CAs) were measured. As shown in Figure 6, the CA is about 52.9° on the untreated specimens, and increases to 86.5° when a discontinuous HAP film is deposited on the surface. In addition, the HAP film composited with GQDs shows better hydrophobicity when the CA is about 104.6° (Figure 6b). PDMS and well-crystallized coatings play a positive role in hydrophobic modification, and will improve the anticorrosive property caused by water issuing.
The Ca2+ release of samples with coatings is visibly lower than that of the untreated reference, shown in Figure 7a. According to the concentration of Ca2+ ions in the runoff solution, the GQD-enhanced film prevented the corrosion resulting from simulated acid rain efficaciously, and even the discontinuous HAP film could also improve the acid resistance. Thanks to the improved hydrophobicity, H1 exhibits the minimum mass loss ratio and highest coefficient of frost resistivity, which are consistent with better freeze–thaw resistance, as displayed in Figure 7b. Similarly, the film on H2 can improve the property to some extent.

3.3. Formation Mechanism

From the results of morphology and property characterization, the film-composited graphene exhibits a better performance for conservation. Compared with the H2 sample, GQDs play a key role in the modification process. GQDs are zero-dimensional carbon materials similar to the crystalline structure of single or few layers of graphene [62,63]. Owing to their small size (≤20 nm) and unique structure, abundant functional groups such as hydroxyl and carboxyl, comprising oxygen on the GQDs’ surface, provide beneficial channel for further functionalization [64]. Table 2 reveals the basic crystal structures of Ca10(PO4)6(OH)2 and CaMg(CO3)2. Although the crystal symmetry of Ca10(PO4)6(OH)2 is hexagonal, which is not exactly the same as CaMg(CO3)2, the lattice parameters are nonetheless very similar. The two crystals have the same axial angles, and the crystal axis at a or b of Ca10(PO4)6(OH)2 (9.42 Å) is nearly double that of CaMg(CO3)2 (4.86 Å/4.81 Å). A GQD is hexagonal structure, and there is a moderate deformation of the carbon skeleton. GQDs have a crystalline lattice parameter of 0.24 nm [51], and the length of the C-C bond inside can reduce from 1.42 Å inside to 1.29–1.33 Å on the outer edge [65]. The deformation could provide a transition layer between Ca10(PO4)6(OH)2 molecules and CaMg(CO3)2 crystals, matching the very small lattice difference. Additionally, functional groups such as C-O-C and hydroxyl can be easily formed on the surface of GQDs (Figure 8a) based on Yan Li’s results [51]. It is probable that metal atoms from Ca10(PO4)6(OH)2 or CaMg(CO3)2 combine with oxygen atoms on GQDs through strong ionic bonds, shown in Figure 8b. The GQDs, as a bridge, could provoke the combination between HAP and dolomite, leading to the continuous film.

4. Conclusions

A graphene-enhanced HAP film on dolomitic marble was successfully prepared through the Ca(OH)2/GQD nanocomposite with the sol-gel method. According to the morphology investigation, Raman, and FTIR analysis, continuous HAP film was synthesized via the participation of GQDs without residual ammonium salt. The noteworthy improvement of hydrophobicity, acid, and the freeze–thaw resistance property was demonstrated and compared with untreated dolomitic marble. Considering the moderate deformation of the carbon skeleton and functional groups in GQDs, a bridge could be formed through strong ionic bonds between the HAP and dolomite, as their lattice parameters are similar. However, more research is needed on the mechanism, as well as evaluating the reliability and durability of the HAP coating for in situ stone conservation.

Author Contributions

Conceptualization, methodology, formal analysis, data curation, writing—original draft preparation, F.W.; methodology, investigation, resources, funding acquisition, D.L.; validation, investigation, visualization, Y.G.; writing—review and editing, supervision, project administration, funding acquisition, S.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Beijing Municipal Cultural Heritage Bureau, grant number 11000023T000002095697.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We thank Yan Li’s lab (Department of Inorganic Nonmetallic Material, School of Materials Science and Engineering, University of Science and Technology Beijing) for offering GQDs for our experiments. We also thank the Beijing Stone Carving Art Museum for funding acquisition and project management.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 13 00642 g001 550
Figure 1. Preparation process of HAP film.
Figure 1. Preparation process of HAP film.
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Crystals 13 00642 g002 550
Figure 2. Optical images (倍率= magnification) and FESEM results of specimens’ surface: (a) Microscope image of DM surface; (b) FESEM micrograph of DM surface; (c) Microscope image of H1 surface; (d) FESEM micrograph of H1 surface; (e) Microscope image of H2 surface; (f) FESEM micrograph of H2 surface.
Figure 2. Optical images (倍率= magnification) and FESEM results of specimens’ surface: (a) Microscope image of DM surface; (b) FESEM micrograph of DM surface; (c) Microscope image of H1 surface; (d) FESEM micrograph of H1 surface; (e) Microscope image of H2 surface; (f) FESEM micrograph of H2 surface.
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Crystals 13 00642 g003 550
Figure 3. FESEM-EDS images of cross section (a) FESEM micrograph of H1 cross section; (b) Distribution of Ca, P, O, and Si at H1 cross section; (c) FESEM micrograph of H2 cross section; (d) Distribution of Ca, P, O, and Si at H2 cross section.
Figure 3. FESEM-EDS images of cross section (a) FESEM micrograph of H1 cross section; (b) Distribution of Ca, P, O, and Si at H1 cross section; (c) FESEM micrograph of H2 cross section; (d) Distribution of Ca, P, O, and Si at H2 cross section.
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Crystals 13 00642 g004a 550Crystals 13 00642 g004b 550
Figure 4. Structure analysis of the specimens: (a) Raman spectra; (b) FTIR results.
Figure 4. Structure analysis of the specimens: (a) Raman spectra; (b) FTIR results.
Crystals 13 00642 g004aCrystals 13 00642 g004b
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Figure 5. Chromatic aberration of H1 and H2 compared with DM.
Figure 5. Chromatic aberration of H1 and H2 compared with DM.
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Figure 6. Water contact angles of samples: (a) DM; (b) H1; (c) H2.
Figure 6. Water contact angles of samples: (a) DM; (b) H1; (c) H2.
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Figure 7. Reliability and durability of the coating: (a) The acid resistance of samples; (b) Freeze–thaw test results.
Figure 7. Reliability and durability of the coating: (a) The acid resistance of samples; (b) Freeze–thaw test results.
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Figure 8. Geometrical configurations and molecule models of involved materials: (a) GQD molecule configuration; (b) The suppositional combination model among Ca10(PO4)6(OH)2, GQDs and CaMg(CO3)2.
Figure 8. Geometrical configurations and molecule models of involved materials: (a) GQD molecule configuration; (b) The suppositional combination model among Ca10(PO4)6(OH)2, GQDs and CaMg(CO3)2.
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Table
Table 1. Chemical reagents for HAP sol preparation.
Table 1. Chemical reagents for HAP sol preparation.
PrecursorsSolventspH Conditioners/AdditivesReferences
CaCl2, Na3PO4∙12H2OWaterNaOH[38]
Ca(CH3COO)2∙H2O, H3PO4Water and 1,2-ethanediolEDTA, triethanolamine[44,45]
Ca(NO3)2∙4H2O, P2O5Ethanol-[39,46]
Ca(NO3)2∙4H2O, (NH4)2HPO4WaterNH4OH[37,47]
Ca(NO3)2∙4H2O, NH4H2PO4Ethanol-waterNH4OH[48]
Ca(NO3)2∙4H2O, P(OC2H5)3Ethanol-water-[49,50]
Table
Table 2. Crystal Structures of HAP and dolomite.
Table 2. Crystal Structures of HAP and dolomite.
MaterialCrystal SymmetryLattice Parameters
Ca10(PO4)6(OH)2Hexagonal, P63/ma = b = 9.42 Å, c = 6.88 Å, α = β = 90°, γ = 120° [66,67]
CaMg(CO3)2Trigonal, R 3 - a = b = 4.86 Å, c = 16.21 Å (Calculated); a = b = 4.81 Å, c = 16.05 Å (Experimental Parameters) α = β = 90°, γ = 120° [68,69]
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Wang, F.; Li, D.; Gu, Y.; Wei, S. Preparation of a Graphene-Enhanced Hydroxyapatite Film on Dolomitic Marble by the Sol-Gel Method. Crystals 2023, 13, 642. https://doi.org/10.3390/cryst13040642

AMA Style

Wang F, Li D, Gu Y, Wei S. Preparation of a Graphene-Enhanced Hydroxyapatite Film on Dolomitic Marble by the Sol-Gel Method. Crystals. 2023; 13(4):642. https://doi.org/10.3390/cryst13040642

Chicago/Turabian Style

Wang, Feng, Di Li, Yaoqi Gu, and Shuya Wei. 2023. "Preparation of a Graphene-Enhanced Hydroxyapatite Film on Dolomitic Marble by the Sol-Gel Method" Crystals 13, no. 4: 642. https://doi.org/10.3390/cryst13040642

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