Synthesis of high surface area CaSO4·0.5H2O nanorods using calcium ethoxide as precursor

Article abstract of DOI:10.1039/d1cc02014e

We report a novel solvothermal route for the production of bassanite (CaSO₄·0.5H₂O) nanoparticles using amorphous Ca-ethoxide as a precursor. Bassanite nanorods, 120-200 nm in length, with the highest specific surface area reported so far (54 m²g^?1) and enhanced reactivity, are obtained at 78 °C and 1 atm. Such nanoparticles may find application in several fields, including biomaterials, drug delivery, and cultural heritage conservation.

Full text of DOI:10.1039/d1cc02014e

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Synthesis of high surface area CaSO4ꢀ0.5H2O
nanorods using calcium ethoxide as precursor†
Cite this: Chem. Commun., 2021,
57, 7304
Miguel Burgos-Ruiz,
Gloria Pelayo-Punzano,
Encarnacion Ruiz-Agudo,
Received 15th April 2021,
Accepted 30th June 2021
Kerstin Elert and Carlos Rodriguez-Navarro
*
DOI: 10.1039/d1cc02014e
rsc.li/chemcomm
We report a novel solvothermal route for the production of bassanite crystals with varying aspect ratios. Recently, efforts have been
(CaSO4ꢀ0.5H2O) nanoparticles using amorphous Ca-ethoxide as a made to produce more reactive bassanite nanoparticles (length o
precursor. Bassanite nanorods, 120–200 nm in length, with the high- 100–500 nm) for improved applications. This is, however,
est specific surface area reported so far (54 m2
reactivity, are obtained at 78 8C and 1 atm. Such nanoparticles may
find application in several fields, including biomaterials, drug delivery, a non-classical multi-step sequence involving amorphous
g
ꢁ1) and enhanced challenging.
Interestingly, precipitation in the CaSO4–H2O system follows
and cultural heritage conservation.
calcium sulfate and metastable bassanite precursor nano-
particles prior to stable gypsum,1,14 which might enable access
Calcium sulfates are abundant in nature, currently being to intermediate (meta)stable nanophases.15 Following this
among the most demanded and massively produced inorganic approach, Wang and Meldrum stabilized bassanite nanocrys-
materials worldwide.1 They find a broad range of applications tals using different additives,16 while Colfen and co-workers
¨
in construction,2 heritage conservation,3 bone regeneration,4 obtained hemihydrate nanorods 50–200 nm in size by quench-
dental prosthetics,5 and drug delivery.4a,6
ing diluted Na2SO4 and CaCl2 solutions in different alcohols.17
Different solid calcium sulfate phases exist: gypsum (CaSO4ꢀ Recently, Chen et al.18 used CaCl2, (NH4)2SO4 and Na2EDTA
2H2O), bassanite or hemihydrate (a- and b-CaSO4ꢀ0.5H2O; note ethylene glycol–water solutions to obtain ellipsoidal hemihy-
that the existence of two distinct bassanite polymorphs is drate mesocrystals 300–500 nm in length, made up of nanorods
currently under debate),1,7 and anhydrite (a-, b- and g-CaSO4). with lengths of 30–80 nm and widths of 10–20 nm, which
Among them, hemihydrate is of particular interest due to its disaggregated upon EDTA removal. However, the precipitation
high reactivity, excellent biocompatibility, and optimal mechan- of NaCl or NH4Cl as by-products or the presence of residual
ical properties once hydrated to gypsum:1–8 set b-hemihydrate additives in the above-mentioned routes raises the problem of
(standard plaster of Paris) has a compressive strength o20 MPa, their later elimination, which is typically done by washing
and set a-CaSO4ꢀ0.5H2O (e.g., dental gypsum) can reach compres- cycles using water, thus enabling coarsening by Ostwald ripen-
sive strength 440 MPa.8 Plaster of Paris is produced by heating ing. Byproducts-free bassanite nanoparticles with specific sur-
gypsum at 110–220 1C and 1 atm under dry conditions.7,8 Multiple face area (SSA) of up to 33.3 m2 gꢁ1 (to our knowledge, the
energy intensive/complex routes for a-hemihydrate have been highest SSA reported so far) were obtained following freeze
proposed, including dehydration of gypsum in electrolyte (acid) drying of a saturated CaSO4 solution and oven-heating of the
solutions, in water (and water/alcohol) or in steam (autoclave),9 obtained powder at 150 1C for 300.19 However, this synthesis
oxidation of calcium sulfite,10 microwave-assisted synthesis,11 route is energy intensive, and the yield is very small due to the
electrochemical deposition,12 or reaction of Ca2+ and SO4 ions low solubility of CaSO4 (B2 g Lꢁ1 at 20 1C).
2ꢁ
in aqueous (or alcohol/water) solutions in the presence/absence of
Here we report on a novel, high-yield, one-pot mild route for
organic additives/surfactants, with and without ultrasonication.13 the synthesis of phase-pure CaSO4ꢀ0.5H2O nanorods with no
All of these routes typically produce micrometer sized bassanite residual additive or background electrolyte and showing the
highest SSA achieved to date. The reaction proceeds according
to (for details see Materials and methods in ESI†):
Dept. Mineralogy and Petrology, University of Granada, Fuentenueva s/n,
Ca0 + 2C2H5OH - Ca(OC2H5)2 + H2m
(1)
(2)
Granada, 18002, Spain. E-mail: carlosrn@ugr.es
† Electronic supplementary information (ESI) available: Details of the experi-
mental protocol, analytical techniques, kinetic model, and supplementary fig-
ures. See DOI: 10.1039/d1cc02014e
Ca(OC2H5)2 + 2H2O $ Ca(OH)2 + 2C2H5OH
7304 | Chem. Commun., 2021, 57, 7304–7307
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350 to 25 ꢂ 2 nm at 600. In contrast, a commercial (control)
b-hemihydrate (plaster of Paris) obtained after calcination of
gypsum (see Materials and methods in ESI†) showed a crystal-
lite size of 49 ꢂ 2 nm.
FTIR of synthesis products collected at 450, 550, and 600
showed moderate intensity bands at 3554 and 3610 cmꢁ1
ascribed to the n1 symmetric and n3 antisymmetric stretch of
water molecules, respectively, along with the d-OH bending
band at 1618 cmꢁ1, indicative of a single type of crystalline
water in bassanite (Fig. 1c).21 The observed moderate intensity
n1 (SO4) symmetric stretch band at 1006 cmꢁ1 and the n3 (SO4)
antisymmetric stretch band at B1100 cmꢁ1, clearly split into
three peaks, are all characteristic of bassanite.21 The solid
sample collected after 350 showed an additional weak shoulder
at 3650 cmꢁ1 and a weak and broad doublet at 1412–1485 cmꢁ1
(n3 asymmetric CO3 stretch) indicative, respectively, of the
existence of trace amounts of Ca(OH)2 and amorphous calcium
carbonate (likely formed following the carbonation of Ca(OH)2
upon contact with atmospheric CO2 during sample handling).
Very weak bands at 2975, 2937 and 2884 cmꢁ1 corresponding to
–OCH2CH3 groups were also detected in this latter sample.
Fig. 1 Characterization of precursor and product phases. (a) TEM image
of Ca-ethoxide (flake-like aggregates). The SAED pattern (inset) shows
diffuse haloes demonstrating the amorphous nature of this precursor
phase; (b) XRD patterns of product obtained at 350 and 600. Note the
presence, in addition of the intense bassanite peaks, of a small broad peak
corresponding to 001 portlandite (arrow) in the sample collected at 350;
(c) FTIR spectra of products obtained at 350 and 600. The inset shows a
detail of the O–H stretching of crystal water in bassanite and the shoulder
at 3650 cmꢁ1 of portlandite (after 350); (d) TG/DSC traces of products
obtained after 350 and 600 reaction time.
These results show that
a small fraction of unreacted
Ca-ethoxide and Ca(OH)2 precursors was present after 350
reaction time.
Simultaneous thermogravimetry and differential scanning
calorimetry (TG-DSC) analyses showed that samples collected at
450, 550, and 600 reaction time display a weight loss of B8 wt%
and an endothermic peak at B160 1C, corresponding to the
Ca(OH)2 + H2SO4 - CaSO4ꢀ0.5H2O + 1.5H2O
(3)
First, the irreversible redox reaction between metallic calcium removal of adsorbed water (1.8 wt%) and crystal water of
and a molar excess of absolute ethanol yields a homogeneous bassanite (6.2 wt%) (Fig. 1d). In addition, the DSC trace shows
whitish alcoholic suspension of amorphous calcium ethoxide, as a small exothermic peak at B200 1C, demonstrating the
shown by transmission electron microscopy (TEM) and selected presence of a-hemihydrate.21 This phase transformed into
area electron diffraction (SAED) (Fig. 1a), as well as infrared g-CaSO4 upon dehydration, which in turn transforms into
spectroscopy analysis (FTIR) (Fig. S1, ESI†). Subsequently, bassa- insoluble anhydrite (b-CaSO4) at such a T.22 However, we also
nite nanoparticles precipitate through a two-step mechanism observed an exothermic peak at 345 1C, reportedly corres-
upon addition of toluene and a H2SO4 solution in a 1 : 1 Ca0 : ponding to the transformation g-CaSO4 - b-CaSO4, after
2ꢁ
SO4 molar ratio. The toluene- and acid-catalyzed hydrolysis of dehydration of b-hemihydrate.22 This shows that the product
calcium ethoxide leads to the formation of calcium hydroxide is a mixture of a- and b-hemihydrate. No other weight loss was
(eqn (2)), which serves as precursor for the precipitation of CaSO4ꢀ observed, which allowed us to estimate a reaction yield Z99%.
0.5H2O nanorods via an acid–base neutralization (eqn (3)). Addi- TG-DSC analysis of samples obtained after 350 showed a weight
tion of non-polar toluene results in an increase of water activity loss of 1.0 wt% at o100 1C, due to adsorbed water, followed by
and hydration rate due to a decrease in the intensity of electro- a marked weight loss of 6.2 wt% ascribed to bassanite dehydra-
static intermolecular forces.20 In the absence of toluene, the tion, as well as a single exothermic band at B190 1C, demon-
formation of B1/1 (wt/wt) anhydrite/bassanite mixtures is favored strating the sole presence of a-hemihydrate (Fig. 1d). In
(Fig. S2–S4, ESI†). Modulating both the amount of water and addition, a weight loss of 0.9 wt% at B400 1C and of 1.9 wt%
reaction time helps prevent the conversion of bassanite into at B600 1C, assigned to the dehydroxylation of calcium hydro-
gypsum, and limits Ostwald ripening phenomena (see below).
xide (Ca(OH)2 - CaO + H2O) and the thermal decomposition of
X-Ray diffraction (XRD) patterns of powder samples calcium carbonate (CaCO3 - CaO + CO2), respectively, indicate
obtained at different reaction times (35, 45, 55 and 60 min) the presence of 3.43 wt% portlandite and 3.97 wt% calcium
showed that in all cases a monoclinic (space group I2) hemi- carbonate due to an incomplete reaction.
hydrate phase was obtained (Fig. 1b). However, at 350 charac-
TEM analysis of the sample collected at 350 showed multiple
teristic Bragg peaks of portlandite were detected, indicating non-oriented aggregates of rod-like hemihydrate nanocrystals
that Ca(OH)2 preceded the precipitation of hemihydrate. Por- 25–50 nm in length and 15–20 nm thick, along with larger
tlandite was no longer observed after Z450 reaction time. The shapeless or globular amorphous particles (i.e., unreacted
crystallite sizes of bassanite determined (for the 200 Bragg Ca-ethoxide and Ca(OH)2) (Fig. 2a). After 450 the shapeless
peak) using the Scherrer equation ranged from 17 ꢂ 2 nm at and amorphous globular particles were no longer detected,
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Fig. 3 PSD and N2 sorption isotherms of bassanite nanorods: (a) represen-
tative TEM image of bassanite nanorods formed after 450 reaction time;
(b) PSD of the particles in (a) determined by direct measurement of the length
(continuous line) and width (dashed line) of individual crystals; (c) DLS-derived
PSD curves for nanobassanite synthesized at 350, 450 and 600 reaction time;
(d) sorption isotherms of nanobassanite synthesized at 450 reaction time (red
curves) as compared to standard (control) b-hemihydrate (blue curves).
Fig. 2 TEM-SAED analysis of nanobassanite: (a) aggregates of bassanite
nanorods (o200 nm in size) obtained after 350, along with larger
(300–500 nm) shapeless particles corresponding to aggregates of pre-
cursor Ca-ethoxide and portlandite (white arrows). The inset shows a
detail of aggregated bassanite nanorods; (b) bassanite nanorods after 450
reaction time; (c) bassanite nanorods after 550. The orientation of the
c-axis of the red-circled nanorod is indicated (based on SAED results
shown in (d)); (d) SAED pattern of the red-circled nanorod in (c). Indexing
have been performed considering the structure (I2 space group) of ref. 23.
hemihydrate reported so far. Nano-bassanite displays a Type II
isotherm with H3 hysteresis (Fig. 3d), corresponding to non-
indicating the full consumption of precursors. In parallel, abun- microporous solids with interparticle (slit-shaped) meso- and
dant hemihydrate nanorods, which preferentially grew along the macro-pores.24 This is consistent with the features of hemihydrate
c axis, were observed (Fig. 2b and c). Crystal length increased with nanorod aggregates observed with TEM (Fig. 2b).
reaction time from B120 nm at 450 up to B500 nm at 600, while
The formation of phase-pure bassanite nanoparticles using this
thickness did not vary significantly. Fig. 2d shows the SAED pattern synthesis route can be rationalized considering that there is a large
of the rod-like nanoparticles with features of the reciprocal lattice amount of ethanol in the synthesis medium, which reduces the
and d-spacings corresponding to bassanite with space group I2.23 system’s water activity, hence increasing supersaturation with
Analysis of particle size distribution (PSD) using TEM images respect to any solid product.17 A very high nucleation rate14 and
showed a narrow Log-normal distribution, as exemplified for the limited growth of metastable lower hydrates will thus be
450 run where particles have mean length of 120 nm and width of favored.1,15,17 The fact that anhydrite forms in the absence of toluene
22 nm (Fig. 3a and b). TEM observations are in good agreement shows that this solvent, which speeds Ca–alkoxide hydrolysis20 but
with dynamic light scattering (DLS) analysis (Fig. 3c), showing does not contribute to a reduction in water activity, is crucial to
average particle size of 135 and 210 nm after 450 and 600 reaction balance these two critical parameters, fostering nanobassanite
time, respectively. After 350, however, an average particle size of formation.
330 nm is observed consistent with the presence of the larger
Considering that a main application of bassanite involves its
aggregates of unreacted amorphous Ca-ethoxide and Ca(OH)2 conversion to gypsum via hydration (e.g., used as a cementing
observed using TEM. At longer reaction times (Z450) and complete agent in construction or biomaterial applications), we evaluated
precursor(s) transformation, bassanite nanorods were the only its hydration kinetics, as compared with those of standard
particles contributing to the PSD observed with DLS. The change b-hemihydrate (plaster of Paris), with SSA of 3.72 m2 gꢁ1 and particle
in mean PSD between 450 and 600 reaction time is consistent with a size of 1–2 mm (details in ESI†). Note that such SSA and particle size
coarsening mechanism (Ostwald ripening). These results show that values are standard for commercial a- and b-hemihydrate.19 Solids
a careful control of the reaction time is critical to achieve maximum were subjected to (details in ESI†): (i) through-solution liquid-phase
conversion of precursors and minimum increase in crystal size.
(LP) hydration (relevant for the reactivity – dissolution and setting –
N2 sorption analysis (BET method) shows that the product of nanobassanite used as a drug carrier and in orthopedics applica-
obtained at 350 had a SSA of 78 m2 gꢁ1. The actual contribution tions);4–6 and (ii) vapor-phase (VP) hydration (relevant for alcohol
of the hemihydrate vs. the untransformed precursors to such a huge dispersions of nanobassanite, similar to nanolimes,25 to be used as
SSA is, however, unclear. The SSA of phase-pure bassanite collected a potential consolidant for historic gypsum plaster).
after 450 was 54.2 m2 gꢁ1, and decreased to 44 and 31 m2 gꢁ1 after
Fig. 4a and b show the evolution of the fractional conver-
550 and 600, respectively, due to the observed coarsening. To our sion of (nano)bassanite into gypsum during LP and VP hydra-
knowledge, 54.2 m2 gꢁ1 is the highest SSA value for pure tion, respectively. Fig. 4c and d show the good fitting
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We acknowledge financial support by the Spanish Govern-
´
ment (grant RTI2018-099565-B-I00), Junta de Andalucıa
(Research Group RNM-179) and University of Granada, UGR
(Research Excellence Unit UCE-PP2016-05 ‘‘Carbonates’’). We
´
also thank the personnel of the Centro de Instrumentacion
´
Cientıfica, UGR, for their help during TEM and TG/DSC ana-
lyses. MBR acknowledges a doctoral research contract funded
by the Spanish Government (PRE2019-090256).
Conflicts of interest
There are no conflicts to declare.
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solvothermal route that involves the formation of amorphous
Ca-ethoxide and its transformation into nanobassanite after acid–
base neutralization using concentrated sulfuric acid. The hydration
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