Hydrothermal Formation of Calcium Copper Tetrasilicate

Article abstract of DOI:10.1002/chem.201503364

We describe the first hydrothermal synthesis of CaCuSi₄O₁₀ as micron-scale clusters of thin platelets, distinct from morphologies generated under salt-flux or solid-state conditions. The hydrothermal reaction conditions are surprisingly specific: too cold, and instead of CaCuSi₄O₁₀, a porous calcium copper silicate forms; too hot, and calcium silicate (CaSiO₃) forms. The precursors also strongly impact the course of the reaction, with the most common side product being sodium copper silicate (Na₂CuSi₄O₁₀). Optimized conditions for hydrothermal CaCuSi₄O₁₀ formation from calcium chloride, copper(II) nitrate, sodium silicate, and ammonium hydroxide are 350 °C at 3000 psi for 72 h; at longer reaction times, competitive delamination and exfoliation causes crystal fragmentation. These results illustrate that CaCuSi₄O₁₀ is an even more unique material than previously appreciated.

Full text of DOI:10.1002/chem.201503364

DOI: 10.1002/chem.201503364
Communication
&
Crystal Engineering
Hydrothermal Formation of Calcium Copper Tetrasilicate
Darrah Johnson-McDaniel,[a] Sara Comer,[b] Joseph W. Kolis,[b] and Tina T. Salguero*[a]
Chem. Eur. J. 2015, 21, 17560 – 17564
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geology of these sites suggests that cuprorivaite forms in a hy-
drothermal environment. Indeed, the heavier congeners of Ca-
CuSi4O10, namely SrCuSi4O10 and BaCuSi4O10, can be synthesized
from the alkali earth chlorides, copper(II) oxide, and sodium sil-
icate in basic, aqueous conditions at a modest temperature
(2508C).[15] The hydrothermal synthesis of CaCuSi4O10 has re-
mained elusive, however. As stated by Warner in his mono-
graph on important inorganic materials, “Success in this area
would break the historic dependency on the use of a salt flux,
and may provide an insight to the conditions under which cu-
prorivaite forms in nature.”[2]
Abstract: We describe the first hydrothermal synthesis of
CaCuSi4O10 as micron-scale clusters of thin platelets, dis-
tinct from morphologies generated under salt-flux or
solid-state conditions. The hydrothermal reaction condi-
tions are surprisingly specific: too cold, and instead of Ca-
CuSi4O10, a porous calcium copper silicate forms; too hot,
and calcium silicate (CaSiO3) forms. The precursors also
strongly impact the course of the reaction, with the most
common side product being sodium copper silicate
(Na2CuSi4O10). Optimized conditions for hydrothermal Ca-
CuSi4O10 formation from calcium chloride, copper(II) ni-
trate, sodium silicate, and ammonium hydroxide are
3508C at 3000 psi for 72 h; at longer reaction times, com-
petitive delamination and exfoliation causes crystal frag-
mentation. These results illustrate that CaCuSi4O10 is an
even more unique material than previously appreciated.
We now report the conditions required for CaCuSi4O10 for-
mation in a hydrothermal environment. The key to success is
threefold: the copper source must be specifically copper(II) ni-
trate, the mineralizer is optimally ammonium hydroxide, and
the reaction temperature/pressure should be about 3508C/
3000 psi. In the optimized procedure, an aqueous mixture of
CaCl2·2H2O, Cu(NO3)2·2.5H2O, and [Na2O(SiO2)x(H2O)x] in a 1:2:4
molar ratio is adjusted to pH 12 with NH4OH. The resulting ge-
latinous material is heated at 3508C for 3 days. These condi-
tions result in complete conversion to highly crystalline CaCu-
Si4O10. The blue product (Figure 1b) consists of 40–60 mm crys-
tal clusters with a uniform flowerlike morphology (see Fig-
ure S1 in the Supporting Information for additional images and
a distribution analysis).
Calcium copper tetrasilicate (CaCuSi4O10) holds a special place
in the history of chemistry as the first synthetic inorganic ma-
terial produced on large scale.[1,2] During the past five millen-
nia, it has been used extensively as a blue pigment (Egyptian
blue), and recent research has focused on its remarkable near
infrared luminescence properties.[3] In addition to archaeologi-
cal imaging,[4] these investigations have included using Yb3+
doping to increase near infrared emission,[5] evaluating CaCu-
Si4O10 as the active material in optical sensors,[6] measuring the
effects of temperature on photoluminescence,[7] detailing the
electronic structure of the blue chromophore,[8] and utilizing
the upconversion capability of CaCuSi4O10 under laser excita-
tion to produce bright broadband light.[9]
Monitoring this reaction from 12 to 96 h by powder X-ray
diffraction (PXRD) (Figure 1a) shows that some CaCuSi4O10
forms early in the reaction, but SiO2 and calcium silicates like
Ca2(SiO3OH)OH and CaSi2O7 also are present. Scanning electron
microscopy (SEM) illustrates a progression from heterogeneous
morphologies at 12 and 24 h (Figures 1c and 1d, respectively)
to distinct 25 mm CaCuSi4O10 platelets after 48 h (Figure 1e). At
72 h, the product is pure CaCuSi4O10 in the form of well-devel-
oped clusters of square-shaped crystals (Figure 1f). At 96 h and
beyond, however, crystal delamination and fragmentation
become noticeable: the clusters are smaller, exhibit frayed
edges, and are covered in debris (Figure 1g). The previously re-
ported exfoliation of layered CaCuSi4O10 in 808C water is the
likely fragmentation mechanism here.[16] For this reason, 72 h is
the ideal length of reaction time for hydrothermal CaCuSi4O10
crystal growth.
Traditionally CaCuSi4O10 has been prepared by salt-flux
routes;[10] the necessary reaction temperature (ca. 8758C) was
accessible even in antiquity using wood-fired ovens.[11] Solid-
state synthesis also can be used to produce CaCuSi4O10 but re-
quires higher temperatures (>10008C).[12] Neither method pro-
vides control over product morphology or size, however,
which is an important consideration for modern applications
like imaging and sensing technologies. We are interested in
synthesizing CaCuSi4O10 using a hydrothermal process because
this method typically has the advantages of producing homo-
geneous and well-crystallized materials under relatively mild
temperatures and pressures.[13]
Varying the calcium and copper sources, as well as the min-
eralizer, has significant effects on the products of the reaction.
Table 1 summarizes the results of using CaCl2·2H2O versus
Ca(NO3)2·4H2O and CuCl2·4H2O versus Cu(NO3)2·2.5H2O with
either NH4OH or NaOH. First, these experiments show that
NaOH is a problematic mineralizer in this system because the
high concentration of Na+ leads to preferential formation of
Na2CuSi4O10, a known tubular silicate.[17] The use of NH4OH in-
stead of NaOH mitigates this reaction pathway to a large
extent, though [Na2O(SiO2)x(H2O)x] is another Na+ source pres-
ent in the system. We do not detect other previously reported
sodium copper silicates, such as Na4Cu2Si12O27(OH)2·2H2O or
Na2Cu2Si4O11·2H2O.[18]
Notably, CaCuSi4O10 occurs as a rare mineral, cuprorivaite, at
locations in Italy (Mount Vesuvius), Germany (the Sattelberg
volcanic cone, Eifel region), South Africa (Messina copper
mines), and the United States (Klamath County, Oregon).[14] The
[a] D. Johnson-McDaniel, Prof. T. T. Salguero
Department of Chemistry, The University of Georgia
140 Cedar Street, Athens, GA 30602 (USA)
E-mail: salguero@uga.edu
[b] Dr. S. Comer, Prof. J. W. Kolis
Department of Chemistry, Clemson University
219 Hunter Laboratories, Clemson, SC 29634 (USA)
Second, the reaction is relatively insensitive to the calcium
source but sensitive to the copper source: the use of
CuCl2·4H2O is consistently ineffective in producing CaCuSi4O10
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503364.
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CaCuSi4O10 also forms from Ca(NO3)2·4H2O+Cu(NO3)2·2.5H2O.
One expects little difference in chemistry between CaCl2 and
Ca(NO3)2 here, with both converting to Ca(OH)2 under these
conditions.[21] Curiously, a small amount of CaCuSi4O10 also is
generated from Ca(NO3)2·4H2O+Cu(NO3)2·2.5H2O in the pres-
ence of NaOH, despite Na2CuSi4O10 formation, which may re-
flect subtle effects related to the solubility and reactivity of cal-
cium and copper hydroxides when approaching the supercriti-
cal point of water.[22]
Reaction temperature/pressure is another key parameter in
this system. In general, a higher reaction temperature of 4508C
and corresponding pressure of 15000 psi disfavors CaCuSi4O10
formation (Table S1 in the Supporting Information). When
using NH4OH as the mineralizer, the dominant product at
4508C is CaSiO3 with minor amounts of CuSiO3. In close similar-
ity to 3508C reactions, Ca(NO3)2·2H2O+CuCl2·2.5H2O+NH4OH
yields Na2CuSi4O10, as do all reactions utilizing NaOH. Again,
some CaCuSi4O10 is generated when both nitrate salts are used
in the presence of NaOH at 4508C as at 3508C (Figure S7 in
the Supporting Information).
Figure 2. Micro-dumbbells formed at 2508C: a) Optical microscopy image,
b) SEM image, c) PXRD pattern, and d) experimental 29Si NMR spectrum
(black curve) with peak fitting (dashed curves).
Figure 1. Hydrothermal formation of CaCuSi4O10 at 3508C: a) PXRD patterns
of products from 12-96 h reactions, b) optical microscopy image of phase
pure CaCuSi4O10 produced with 72 h reaction time, and SEM images of prod-
ucts corresponding to c) 12, d) 24, e) 48, f) 72, and g) 96 h reaction times.
Of the eight reactions listed in Table 1, only one provides
an isolable product at 2508C with an autogenous pressure
of
about
1800 psi:
Ca(NO3)2·4H2O+Cu(NO3)2·2.5H2O+
compared to Cu(NO3)2·2.5H2O. This observation is surprising
because both CuCl2·4H2O and Cu(NO3)2·2.5H2O should convert
to Cu(OH)2, [Cu(NH3)4]2+, and mixed ligand species in the pres-
ence of excess NH4OH. In fact, prior studies have shown that
both CuCl2 and Cu(NO3)2 produce nanostructured CuO via pu-
tative Cu(OH)2 when treated with NH4OH under hydrothermal
conditions.[19] Based on established hydrothermal chemistry,
we speculate that strong CuÀCl bonding persists at 3508C,
which influences the formation of reactive intermediates.[20]
Although we observe the most phase pure CaCuSi4O10 prod-
uct from CaCl2·2H2O and Cu(NO3)2·2.5H2O starting materials,
[Na2O(SiO2)x(H2O)x]+NH4OH. As illustrated in Figure 2, the in-
teresting product consists of 0.1–0.2 mm bright blue crystals
with a dumbbell morphology. Their color is, in fact, a more
pure blue than CaCuSi4O10 (Figure S8 in the Supporting Infor-
mation), but this material does not luminesce in the infrared
range of 700–1400 nm (Figure S9 in the Supporting Informa-
tion). Elemental analysis (Table S2 in the Supporting Informa-
tion) provides an approximate stoichiometry of Ca2CuSi8O16. Al-
though several crystalline calcium copper silicate compositions
are known, including kinoite (Ca2Cu2Si3O10·2H2O),[23] stringha-
mite (CaCuSiO4·H2O),[24] and recently discovered diegogattaite
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Table 1. Hydrothermal experiments at 3508C and 3000 psi.
Reactants
Copper source
Mineralizer
Results (PXRD analysis)
Calcium source
CaCl2·2H2O
Ca(NO3)2·4H2O
CaCl2·2H2O
Ca(NO3)2·4H2O
CaCl2·2H2O
Ca(NO3)2·4H2O
CaCl2·2H2O
Cu(NO3)2·2.5H2O
Cu(NO3)2·2.5H2O
CuCl2·4H2O
NH4OH
NH4OH
NH4OH
NH4OH
NaOH
NaOH
NaOH
NaOH
bright blue crystals (CaCuSi4O10)
major: bright blue crystals (CaCuSi4O10); minor: larger clear crystals (SiO2)
pale blue fibrous network [Na2CuSi4O10, SiO2, and Na(Si2O4(OH))H2O]
pale blue fibrous network (Na2CuSi4O10 and SiO2)
CuCl2·4H2O
Cu(NO3)2·2.5H2O
Cu(NO3)2·2.5H2O
CuCl2·4H2O
pale blue fibrous network (Na2CuSi4O10)
major: pale blue fibrous network (Na2CuSi4O10); minor: bright blue crystals (CaCuSi4O10)
pale blue fibrous network (Na2CuSi4O10, quartz, and SiO2)
pale blue fibrous network (Na2CuSi4O10 and quartz)
Ca(NO3)2·4H2O
CuCl2·4H2O
(Na2CaCu2Si8O20·H2O),[25] the PXRD pattern of the micro-dumb-
bells (Figure 2c) does not match anything previously reported.
However, the low-angle data show features similar to data
from calcium silicates like gyrolite (Ca16Si24O60(OH)8·(14+x)H2O
and hillebrandite Ca2(SiO3)(OH)2.[26] Gyrolite in particular is
known to readily incorporate Cu2+ into its layered structure.[27]
We further investigated the structure of the dumbbell mate-
rial using solid-state 29Si magic angle spinning (MAS) nuclear
magnetic resonance (NMR) spectroscopy. The spectrum exhib-
its three overlapping peaks (Figure 2d) at À103, À109, and
À117 ppm, which we assign to one Q3 species [Si(OSi)3OH] and
two distinct Q4 species [Si(OSi)4]. Such a low silanol content is
consistent with a porous framework silicate and is similar to
data reported for Cu2+-doped zeolites and mesoporous silica
materials.[28] These results suggest that the dumbbells contain
coordinated Cu2+ within pores or channels formed by the sili-
cate structure.
NH4OH (ACS reagent, 28–30% NH3 basis), and 5m NaOH (ACS Re-
agent, were purchased from Sigma-Aldrich.
Cu(NO3)2·2.5H2O (certified ACS grade) was purchased from Fischer
Scientific. Ca(NO3)2·4H2O (certified ACS crystalline grade) was pur-
chased from J. T. Baker.
ꢀ97.0%)
Hydrothermal synthesis of CaCuSi4O10
: CaCl2·2H2O (0.196 g,
1.33 mmol) and Cu(NO3)2·2.5H2O (0.619 g, 2.66 mmol) or
CuCl2·2H2O (0.227 g, 1.33 mmol) were dissolved in deionized water
(30 mL). While monitoring the pH and stirring, sodium silicate solu-
tion [Na2O(SiO2)x(H2O)x] (1.27 mL; 5.31 mmol of SiO2) was added
drop-wise to the solution. The final pH of the solution was then
adjusted to 12 using 14.6m NH4OH (28–30% NH3 basis). Then
a sample of the resulting gelatinous mixture (0.8 mL) was placed
into a silver ampule, welded shut, placed in a high temperature au-
toclave (filled with deionized water to counter-pressure the am-
pules), and heated to 3508C for 12–96 h. Once the reaction time
was complete, the vessel was cooled to room temperature, the
solid product was washed with water and collected by vacuum fil-
tration. The isolated yield of blue crystals was 0.007 g (54%). Addi-
tional experiments to examine varying reaction conditions were
carried out in the same manner with Ca(NO3)2·4H2O (0.314 g,
1.33 mmol), CuCl2·2H2O (0.227 g, 1.33 mmol), and 5m NaOH at 350
or 4508C.
The micro-dumbbells convert to CaCuSi4O10 when heated at
3508C under hydrothermal conditions, with the cleanest reac-
tion obtained by heating isolated dumbbells in deionized
water. SEM analysis (Figure S11 in the Supporting Information)
shows that the product mixture contains relatively thick CaCu-
Si4O10 crystals, thin CaCuSi4O10 platelets, and needlelike and
globular morphologies that may correspond to unreacted ma-
terial.
Hydrothermal synthesis of calcium copper silicate hydrate
micro-dumbbells: Ca(NO3)2·4H2O (0.314 g, 1.33 mmol) and
Cu(NO3)2·2.5H2O (0.619 g, 2.66 mmol) were dissolved in deionized
water (30 mL). While monitoring the pH and stirring, sodium sili-
cate solution [Na2O(SiO2)x(H2O)x] (1.27 mL; 5.31 mmol of SiO2) was
added drop-wise to the solution. The final pH of the solution was
then adjusted to 12 using 14.6m NH4OH (28–30% NH3 basis). The
resulting gelatinous mixture was placed into a 45 mL capacity
Teflon-lined autoclave (Parr Instrument Co. acid digestion vessel)
and heated at 2508C for 96 h. The blue product (0.140 g) was iso-
lated by decanting the gelatinous supernatant and washing the
crystals with water. Additional experiments to examine varying re-
action conditions were carried out in the same manner with
CaCl2·2H2O (0.196 g, 1.33 mmol), CuCl2·2H2O (0.227 g, 1.33 mmol),
and 5m NaOH at 2508C.
In conclusion, the conditions for the hydrothermal formation
of CaCuSi4O10 are surprisingly specific. Although both SrCu-
Si4O10 and BaCuSi4O10 form in a range of temperatures and
from variable precursors, highly crystalline and phase pure Ca-
CuSi4O10 forms readily only at 3508C using copper nitrate and
ammonium hydroxide. This significant difference in chemistry
explains in part why cuprorivaite is not found in the same nat-
ural mineral deposits as wesselsite (SrCuSi4O10) or effenberger-
ite (BaCuSi4O10).[29] The final question is, why has it taken until
now for the hydrothermal formation of CaCuSi4O10 to be re-
ported? We suggest that the experimental challenge of con-
ducting hydrothermal reactions at temperatures >2508C
(beyond the capability of Teflon-lined pressure vessels) has
been the main impediment to developing this chemistry.
Characterization: Product composition was confirmed using
a Bruker D8-Advance powder x-ray diffractometer (CoKa radiation
source) operated at 40 mA and 40 kV. PXRD patterns were record-
ed in the 2q range of 10–708 with a scanning rate of 0.5 s per step.
The morphology of products was examined by FEI Inspect F field
emission gun scanning electron microscope (SEM) operated at
20 keV. The samples were prepared for SEM by sprinkling the pow-
ders on carbon sticky tape and then coating with gold. EDS data
were collected with an EDAX Inc. instrument. Solid-state 29Si magic
angle spinning (MAS) nuclear magnetic resonance (NMR) spectra
Experimental Section
General: CaCl2·2H2O (ACS reagent, ꢀ99% purity), sodium silicate
solution [Na2O(SiO2)x(H2O)x] (reagent grade purity, 26.5% SiO2),
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Published online on October 20, 2015
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