Solvothermal/hydrothermal synthesis of metal oxides and metal powders with and without microwaves

Article abstract of DOI:10.1515/znb-2010-0809

Anatase and Ca, Sr and Ca_0.5Sr_0.5 hydroxyapatites were synthesized by conventional-hydrothermal (C-H) as well asmicrowave- hydrothermal (M-H)methods.Microwave-assisted reactions led to accelerated syntheses of anatase but no such acc

Full text of DOI:10.1515/znb-2010-0809

Solvothermal/Hydrothermal Synthesis of Metal Oxides
and Metal Powders with and without Microwaves
Sridhar Komarnenia, Young Dong Noha, Joo Young Kima, Seok Han Kima, and
Hiroaki Katsukib
a Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA
b Saga Ceramics Research Laboratory, 3037-7, Arita-machi, Saga 844-0024, Japan
Reprint requests to Prof. Dr. S. Komarneni. E-mail: komarneni@psu.edu
Z. Naturforsch. 2010, 65b, 1033 – 1037; received January 29, 2010
Anatase and Ca, Sr and Ca0.5Sr0.5 hydroxyapatites were synthesized by conventional-hydrothermal
(C-H) as well as microwave-hydrothermal (M-H) methods. Microwave-assisted reactions led to accel-
erated syntheses of anatase but no such acceleration of reactions could be detected with the syntheses
of hydroxyapatites because the crystallization of the latter materials occurred at very low temperature.
Cu and Au metal powders were produced by using glucose, fructose or sucrose as reducing agents un-
der C-H conditions at 160 ◦C, where fructose and sucrose were found to be stronger reducing agents
than glucose. The crystallinity of all the powders was characterized by powder X-ray diffraction, and
morphology and particle sizes were determined by scanning or transmission electron microscopy.
Key words: Nanoparticles, Anatase, Apatites, Copper, Gold, Microwave-Hydrothermal Technique
Introduction
hydrothermal/solvothermal process has been in use for
a long time, the hydrothermal/solvothermal process in
The solvothermal process including the hydrother- combination with electric, microwave and ultrasonic
mal process is one of the oldest “Green Chemistry” ap- fields is rather recent [10 – 18]. Starting about 1985, we
proaches, if not the oldest for materials synthesis. The developed the microwave-hydrothermal/solvothermal
hydrothermal/solvothermal process is a green process process [12] and employed it successfully for the syn-
because of the closed system conditions used in hy- thesis of phases in the wide arena of materials families
drothermal reactors, and it has been extensively used of interest including metal powders [12 – 17]. There
for growing single crystals, for example, quartz [1] are several obvious reasons to further develop, under-
and many kinds of metal oxide [2 – 5], metal [6, 7] stand, and exploit the impact of this technique in the
and semiconductor [8, 9] powders. The hydrother- synthesis of materials which are technologically im-
mal/solvothermal process has many advantages in ad- portant. Some significant advantages are: (i) it can in-
dition to being a green process: (i) it is energy efficient crease the reaction rate by one to two orders of mag-
because of low temperature synthesis, (ii) it is an en- nitude, (ii) it can lead to novel phases, (iii) it can
vironmentally friendly process because of closed sys- eliminate metastable phases, and (iv) it can lead to
tem conditions, (iii) unused components can be recy- a rapid heating. Rapid heating along with rapid ki-
cled, (iv) precipitants are not always needed, (v) high- netics can lead to energy savings. The objectives of
purity products can be synthesized, (vi) metastable the present study are to show that the microwave-
and new phases can be accessed, (vii) hydroxylated hydrothermal/solvothermal process is advantageous
and hydrated materials such as clays and zeolites can over the conventional hydrothermal process in synthe-
only be made by this process, and (viii) crystal size, sizing some metal oxide phases and to synthesize metal
morphology, composition, and polymorphism of the phases by the conventional hydrothermalprocess using
synthesized phases are easily controlled. Experimen- biomolecules.
tal parameters such as temperature and/or pressure,
pH, Eh (oxidation-reduction potential), concentration
of chemical species, and the use of inorganic additives
for homogeneous and heterogeneous nucleation can
Experimental Section
TiO syntheses
2
For the preparation of TiO2, a 0.5 M Ti solution was pre-
also be carefully controlled. Although the conventional pared from a titanium oxysulfate sulfuric acid complex (Alfa
c
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S. Komarneni et al. · Solvothermal/Hydrothermal Synthesis of Metal Oxides and Metal Powders
Table 1. Synthesis conditions
for metals with different metal
compounds and biomolecules
as reducing agents.
Run no. Metal compound Carbohydrate NaOH treatment pH before NaOH pH after NaOH
SK12 CuSO4 · 5H2O
SK13 CuSO4 · 5H2O
SK18 CuSO4 · 5H2O
SK20 CuSO4 · 5H2O
D-(+)-Glucose
D-(+)-Glucose
D-(–)-Fructose
D-(+)-Sucrose
n
y
n
n
n
y
4.3
4.3
4.3
4.7
1.7
1.7
–
9.9
–
–
–
SK30 HAuCl4 · xH2O D-(+)-Glucose
SK31 HAuCl4 · xH2O D-(+)-Glucose
10.8
Aesar). 20 mL of this solution was treated in Parr reactors at 20 mL of a 0.375 M (NH4)2HPO4 solution. Then, the pH
different temperatures for different durations under conven- was adjusted to 10 using NH4OH, and the reactants were
tional hydrothermal conditions. For comparison, microwave- heated in the microwave digestion system at precisely con-
hydrothermal reactions were carried out in Teflon vessels trolled temperatures in the range of 80 – 160 ◦C for 1 h. The
using a microwave-accelerated reaction system, MARS-5 resulting precipitates were washed several times using deion-
(CEM Corp.). The microwave system operates at a frequency ized water and dried at 60 ◦C in an oven prior to characteri-
of 2.45 GHz with a maximum power of 1200 W. The exper- zation.
iments were carried out in double-walled digestion vessels
Cu and Au metal particle synthesis using biomolecules
having an inner non-reactive Teflon PFA liner and an outer
Ultem polyetherimide shell of high mechanical strength.
Temperature was controlled in a single reference vessel with
a microwave-transparent fiber optic temperature probe (RTP-
300 Plus, CEM Corp.). Accurate pressure feedback can also
be achieved with the ESP-1500 Plus (CEM Corp.) for a sin-
gle reference vessel. Temperature and pressure probes al-
lowed the reaction to be controlled by precisely monitoring
the temperature and pressure within a control vessel. The re-
ported temperatures are measured temperatures in this mi-
crowave unit. The maximum operating temperature and pres-
sure for the system are 240 ◦C and 350 psi, respectively. Af-
ter the hydrothermal reactions, the solid and solution phases
were separated by centrifugation and the precipitates washed
several times with deionized water and dried at 60 ◦C.
Copper(II) sulfate pentahydrate (¿ 98 %, Alfa Aesar) and
tetrachloroauric acid hydrate (Au content = 49 %, Johnson
Matthey Inc.) were used as metal source and D-(+)-glucose
(99.5 %, Sigma), D-(–)-fructose (98 %, Sigma), and D-(+)-
sucrose (> 99.5 % Fluka) were used as reducing agents. In a
typical procedure, 0.64 mmol of the copper or the gold com-
pound and 0.10 g of the biomolecules were fully dissolved
in 10 g of distilled water with stirring at r. t. Aqueous NaOH
solution (1.0 M, Alfa Aesar) was added to the above solu-
tions to reach pH ∼ 10, or until no further change of color
was observed. The detailed conditions for each sample are
described in Table 1. Thereafter, each of the solutions was
transferred to a Teflon-lined stainless steel autoclave, and
◦
heated at 160 C under static conditions for 8 h. The prod-
ucts were washed with ethanol and separated by centrifuga-
Hydroxyapatite syntheses
tion. Finally, the samples were dried at 60 ◦C for 24 h.
For the synthesis of Ca, Sr, and Ca0.5Sr0.5 hydroxyap-
atites, Sr(NO3)2, (NH4)2HPO4, and Ca(NO3)2 containing
15.7 % H2O (Alfa Aesar) were used as precursors. The molar
ratios of Ca or Sr : P for Ca- and Sr-hydroxyapatite synthe-
ses was 5 : 3, and that of Ca : Sr : P for Ca0.5Sr0.5 hydroxy-
apatite synthesis 2.5 : 2.5 : 3. In a typical synthesis under the
conventional hydrothermal conditions, appropriate amounts
of Ca(NO3)2 and/or Sr(NO3)2, were dissolved in 20 mL of
deionized water, and then 40 mL of a 0.375 M (NH4)2HPO4
solution was added. After mixing, the pH was adjusted to 10
using NH4OH. The m◦ixture was treated hydrothermally in
autoclaves at 50 – 160 C for 2.5 – 4.5 h or 24 h. The result-
ing precipitates were washed several times with deionized
Characterization
The synthesized solid phases were characterized by X-ray
diffraction (XRD) using a Scintag X-ray diffraction unit with
CuKα radiation in the 2θ range of 5 – 40◦ at a scanning speed
of 5 ◦ min−1. The morphology and particle size were de-
termined by a field emission scanning electron microscope
(JSM-6700F, JEOL, Tokyo, Japan) on samples coated with a
very thin carbon film or via transmission electron microscopy
(TEM; Model 2010, JEOL, Tokyo, Japan).
Results and Discussion
◦
water and dried at 60 C. For room-temperature syntheses,
Syntheses of the anatase form of TiO2 by C-H and M-H
processes
some of the batches were stirred for 0.5 h after pH adjust-
ment to 10 and subsequently washed with deionized water
and dried at 60 ◦C.
For the synthesis of Ca, Sr, and CaSr hydroxyapatites un-
der microwave-hydrothermal conditions, 10 mL of deionized
water and suitable amounts of Ca(NO3)2 and/or Sr(NO3)2,
The anatase form of TiO2 was produced at all tem-
peratures investigated by both the conventional-hydro-
thermal (C-H) and microwave-hydrothermal (M-H)
were put into Teflon vessels followed by the addition of processes using the titanium oxysulfate sulfuric acid
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S. Komarneni et al. · Solvothermal/Hydrothermal Synthesis of Metal Oxides and Metal Powders
1035
Table 2. Yields of TiO2 (anatase) by conventional-
hydrothermal (C-H) and microwave-hydrothermal (M-H)
processes.
Temperature
Heating time
Anatase yield (%)
(◦C)
(h)
2
4
8
0.5
1
C-H
M-H
a
120
120
120
140
91.7
a
26.3
73.8
a
a
12.6
87.8
a
140
140
140
160
160
2
4
0.5
1
14.4
90.8
a
87.5
a
90.3
88.2
16.3
a
Syntheses not performed.
Fig. 2. Anatase prepared by a conventional-hydrothermal
process at 120 ◦C/8 h (A) and by a microwave-hydrothermal
process at 120 ◦C/2 h (B).
Fig. 1. Powder diffraction pattern (CuKα radiation) of
anatase prepared by a conventional-hydrothermal process
at 120 ◦C/4 h (A) and by a microwave-hydrothermal process Fig. 3. Anatase prepared by a microwave-hydrothermal pro-
at 120 ◦C/2 h (B).
cess at 140 ◦C/1 h without PVP (A) and with PVP (B).
complex (Table 2). The presence of sulfate led to the der M-H conditions (Fig. 1). Thus the M-H process
formation of the anatase polymorph unlike when tita- led to increased crystallization compared to the C-H
nium chloride was used previously, which led to ru- process supporting our previous results for other sys-
tile [13]. These results are supported by previous find- tems [21, 22]. The faster crystallization under M-H
ings under C-H [19] and M-H conditions [20]. The pre- conditions, however, led to more agglomeration of the
vious studies showed that Cl− ions led to rutile while powders compared to those prepared by the C-H pro-
2−
SO4 ions led to anatase, because chloride coordi- cess (Fig. 2). The faster crystallization is attributed
nates weakly to TiO species while the sulfate coordi- here to localized “super heating” [23] rather than any
nates strongly. Thus the Ti oxysulfate salt is a good pre- special microwave effect. We used polyvinylpyrroli-
cursor for the pure anatase polymorph, which is useful done (PVP) during M-H synthesis to see if we can pre-
in photocatalytic applications. Table 2 shows that the vent an aggregation of anatase. As can be seen from
M-H process led to higher yields in a shorter time at Fig. 3, the presence of PVP did not disperse anatase
all temperatures compared to the C-H process. For ex- but led to well rounded anatase aggregates. Both sam-
◦
ample at 120 C, the M-H process led to 92 % yield ples, however, showed high surface areas. The surface
in 2 h as compared to the C-H process which yielded areas of the two samples without and with PVP shown
only 26 % after 4 h of treatment. Powder X-ray diffrac- in Fig. 3 are 132 and 77 m2 g−1, as determined by BET-
tion analysis of anatase crystallized at 120 ◦C/4 h un- N2 analysis. This aggregation problem may be avoided
der C-H conditions showed broader and less intense by stirring during the M-H process as has been shown
◦
peaks compared to that crystallized at 120 C/2 h un- with BaTiO3 [24].
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S. Komarneni et al. · Solvothermal/Hydrothermal Synthesis of Metal Oxides and Metal Powders
Temperature ( ◦C) Time (h)
Ca apatite
Monetite formed
CaSr apatite
Monetite formed
Sr apatite
Phase crystallized + impurties
Table 3. The formation of
hydroxyapatites by con-
ventional-hydrothermal
and microwave-hydro-
thermal treatments.
25a
–
Conventional-hydrothermal method:
50
80
100
160
2.5, 4.5
2.5, 4.5
2.5, 4.5
2.5, 4.5
Phase crystallized Phase crystallized
Phase crystallized Phase crystallized
Phase crystallized Phase crystallized
Phase crystallized Phase crystallized
Phase crystallized
Phase crystallized
Phase crystallized
Phase crystallized
a
After adjustment to pH =
10, the mixtures were stirred
at r. t. for 0.5 h.
Microwave-hydrothermal method:
80
100
1
1
Phase crystallized Phase crystallized
Phase crystallized Phase crystallized
Phase crystallized
Phase crystallized
Fig. 4. Ca0.5Sr0.5 hydroxyapatites synthesized by
C-H (A) and M-H (B) processes at 80 ◦C/2.5 h and
Sr-hydroxyapatites synthesized by C-H (C) and M-H (D)
processes at 80 ◦C/2.5 h.
Syntheses of apatites by C-H and M-H processes
Table 3 shows the results of Ca, Sr and Ca0.5Sr0.5
hydroxyapatite syntheses. As can be seen, apatites
could be synthesized at very low temperatures. In fact,
Sr apatite was synthesized even at r. t. but not Ca and
Ca0.5Sr0.5 apatites. Because of the low temperatures
involved, we could not see the effect of microwaves
on reaction kinetics of the apatite syntheses. However,
the M-H process led to somewhat more agglomerated
powders than those obtained by the C-H process, as
can be seen from Fig. 4.
Fig. 5. Cu metal particles prepared by a conventional-
hydrothermal process at 160 ◦C/8 h: (A) CuSO4 plus glucose;
(B) CuSO4 plus glucose and adjusting the pH to about 10
using NaOH; (C) CuSO4 plus fructose; (D) CuSO4 plus su-
crose.
alkaline conditions using NaOH [25]. The objective
here was to see what kind of morphologies could be
obtained using biomolecules under higher temperature
conditions. Figs. 5A and 5B show the morphologies of
Cu particles using glucose with and without adjusti◦ng
the pH. Nanoparticles of Cu were obtained at 160 C
under acidic pH (Fig. 5A), but large hexagonal plates
and other structures were obtained under alkaline con-
ditions (Fig. 5B). Fructose and sucrose led to very large
Syntheses of Cu and Au metal particles by C-H process
Powder X-ray diffraction showed that both Cu and
Au crystallized under all the experimental conditions
listed in Table 1. Simple sugars acted as reducing
agents for both Cu and Au salts to form metal powders.
Glucose has been used previously as a reducing agent
at r. t. to prepare Au nanoparticles from HAuCl4 under
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S. Komarneni et al. · Solvothermal/Hydrothermal Synthesis of Metal Oxides and Metal Powders
1037
than glucose. Uniform particles of about 200 nm of
gold were formed under acidic conditions (Fig. 6A),
while non-uniform particles of different sizes were
formed under alkaline conditions (Fig. 6B). Further
studies are needed to control the size and shapes of
both Cu and Au metal particles under hydrothermal
conditions.
Conclusion
The presence of a microwave field during hydrother-
mal processes increases the kinetics of crystallization
of anatase, TiO2. An effect of microwaves on the crys-
tallization of Ca, Sr, and Ca0.5Sr0.5 hydroxyapatites
could not be detected because these phases crystal-
lized easily at low temperatures. Biomolecules such
as glucose, fructose and sucrose were shown to reduce
Cu and Au salts to metal powders under conventional-
hydrothermal conditions.
Fig. 6. Au metal particles prepared by a conventional-
hydrothermal process at 160 ◦C/8 h: (A) HAuCl4 ·xH2O plus
glucose; (B) HAuCl4 · xH2O plus glucose and adjusting the
pH to about 10 using NaOH.
Cu particles (Figs. 5C and 5D) under acidic conditions
unlike glucose which led to nanoparticles. Thus, fruc-
tose and sucrose appear to be stronger reducing agents
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