Rapid combustion synthesis of Cu 2Y2O5 as a precursor for CuYO 2 [610273]

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Rapid combustion synthesis of Cu 2Y2O5 as a precursor for CuYO 2

Andrei V. Racu1,2, Radu Baies1,3, Srinivas R. Popuri4, Mihai -Cosmin Pascariu1,5,
Mircea Niculescu3, Radu Banic a1,3*
1 National Institute o f Research & Development for Electrochemistry and Conde nsed Matter –
INCEMC Timi șoara , 144 Dr. Aurel Păunescu -Podeanu , RO-300569 Timișoara, Romania
2 Institute of Applied Physics of the Academy of Sciences of Moldova, 5 Academiei , MD-2028
Chișinău, Moldova
3 University Politehnica Timișoara, Faculty of Industrial Chemistry and Environ mental
Engineering, 6 Vasile Pârvan Blvd., RO -300223 Timișoara, Romania
4 Institute of Chemical Sciences and Centre for Advanced Energy Storage and Recovery, School
of Engineering and Physical Sciences, Heriot -Watt University, Edinburgh, EH14 4AS, UK
5 “Vasi le Goldiș” Western University of Arad, Faculty of Pharmacy, 86 Liviu Rebreanu, RO-
310414 Arad, Romania
* Corresponding author: [anonimizat]

Abstract
A rapid synthesis method for Cu2Y2O5 nanocrystals , achieved by combining the
nitrate /glycerol auto-combustion and the annealing process es, is reported for the first time . The
optimized conditions for the preparation of pure phase material in five minutes at 1000 °C is an
important improvement over the conventional solid state reaction procedures , which usual ly
require several hours. X-ray diffraction , scanning electron microscopy , high-resolution
transmission electron microscopy , Fourier transform Raman spectroscopy and diffuse reflectance
spectroscopy were all used to confirm the genesis and phase purity of Cu 2Y2O5 and the derived
CuYO 2 nanocrystals. A subtle influence of the thermal processing parameters on the final
product ’s morphology was noticed . High temperature X-ray diffraction confirmed the phase
transformation of Cu 2Y2O5 to CuYO 2-type delafossite . The optical band gap of Cu2Y2O5 was
found to be of 2.6 eV.
Keywords: Cu2Y2O5, CuYO 2, delafossite , auto-combustion reaction, short annealing , transparent
conductin g oxide .

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1. Introduction

Copper based ternary oxides , such as CuIMIIIO2 (of delafossite type) and CuII2MIII2O5, have
gained tremendous interest due to their great potential in several important technologies ,
including transparent electronics and superconduct ors. In the early 19 90s, the investigation of
Cu2R2O5 family of materials , where R is a rare-earth trivalent cation, begin , in direct relation
with the technology of quaternary superconducting compounds in the Cu -R-Ba-O system [1].
The Cu 2R2O5 compounds crystallize in an orthorhombic system with the space group P21nb,
consisting of an orthorhombic framework of RO 6 polyhedra , and with the copper ions being
surrounded by a highly deformed octahedron composed of oxygen ions [2, 3].
K.T. Jacob et al. and G.M. Kale et al. have made significant contribution s to the studies
regarding the phase relations in the Cu -R-O system and the decomposition temperatures of this
type of materials , in order to evaluate its oxygen stor age capacity. They have also investigated
the correlation of thermodynamic properties with the ionic radius of the trivalent rare -earth
cations in such compounds [4 -9]. The Gibbs free energies of formation and the phase equilibria
between Cu 2Y2O5 and CuYO 2 were investigated by W. Przybyło et al. [10] and R.A. Konetzki et
al. [11], respectively. Cu 2R2O5 materials , where R is Dy, Er or Y, were studied for their magnetic
[3, 16, 17] and optical [18] properties. Recently, Cu 2Y2O5 thin films were also shown to possess
antibacterial properties , which are probably a consequence of the release of copper ions [19, 20].
Based on th is feature , smart windows having antibacterial coatings on their surface can be
produced [19]. However , during the last decade , the most noticeable a pplication of Cu 2R2O5 was
as a precursor for the synthesis of some CuRO 2 (delafossite ) based P -type transparent conducting
oxides (TCO) [12 -14]. CuYO 2, for example , can be obtained by heating Cu 2Y2O5 in either
vacuum or an inert atmosphere [12, 15].
Cu2Y2O5 is usually synthesized u sing a solid state reaction process , by heating copper and
yttrium oxides at elevated temperatures , between 800 and 1200 °C [12, 14, 20, 21]. The total
synthesis time , required for the formation of the pure phase , is of the order of several hours, i.e. 3
h [14], 5 h [21], 6 h [20] or 24 h [12 ]. The wet -chemical synthesis methods , such as auto –
combustion, co -precipitation, sol-gel, hydrothermal and s olvothermal synthesis, ha ve proven to

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be more effective for obtain ing pure phase nanomaterials , because a lower reaction temperature
and/or duration is required .
In order t o reduce the overall duration , and to save energy during the preparation of
nanocrystalline Cu2Y2O5, we have developed a simple method which reduces the synthesis time
from a few hours to less than 10 minutes , by using a combination of two processes: auto-
combustion and annealing. In the present work , besides the investigation of the combustion
mechanism, we focus on the structural and optical properties of Cu2Y2O5 by using various
techniques: X-ray diffraction (XRD ), high-resolution transmission electron microscopy
(HRTEM ), scanning electron microscopy (SEM ), Raman spectroscopy and diffuse reflectance
spectroscopy (DRS ). Also, to confirm the usability of Cu 2Y2O5 as a rapid precursor for CuYO 2
TCO, we investigate d its stability by in-situ high temperature XRD under vacuum conditions.

2. Experimental procedure

2.1. Synthesis of Cu 2Y2O5
Analytical grad e metal nitrates and methanol were purchased from Sigma -Aldrich , while
glycerol was purchased from Chimreactiv (Romania). The s ynthesis starts with the preparation
of a 1.79 mol L-1 glycerol solution, using methanol as solvent. In the first step, stoichiometric
amounts of Y(NO 3)3·6H2O and Cu(NO 3)2·3H2O were dissolved in 5.0 m L of glycerol solution
under continuous stirring , after which the obtained deep blue solution was heated to reach the
ignition temperature. In the second step, the powder y product , obtained after the liquid ’s
combustion , was placed in quartz glass crucibles and heated in air at temperatures between 800
and 1000°C for 5, 10 or 30 minutes. The c rucibles were introduced directly in the furnace , which
was preheated at the working temperature. Following the annealing , the crucibles were slowly
cooled to room temperature. The thus prepared powder samples were used for further
characterization.

2.2. Analytical methods
The a uto-ignition temperature was determined using a PC interfaced PCE -892
thermometer with a K-type thermocouple. The phase purity of the as-synthesized powders was
investigated with an X’Pert PRO X -ray powder diffractometer , by using Ni-filtered CuKα

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radiation (λ = 1.5404 Å); additionally, the CuYO 2 phase formation from Cu2Y2O5 was studied
using the in-situ high temperature XRD technique . The Fourier t ransform Raman data w ere
collected with a SpectraPro -2500i spectrometer , using a 532 nm excitation wavelength . The DRS
data were acquired using a Lambda 950 UV/VIS/NIR s pectrophotometer , with a 150 mm
integrating sphere (Spectralon was used as reflecti on standard ). Electron micrographs obt ained
by HRTEM and SEM were acquired using a Titan G2 80 -200 and , respectively, a FEI Inspect S
microscope .

3. Results and discussion

3.1. Cu 2Y2O5 formation m echanism
As previously mentioned, the synthesis of Cu2Y2O5 was achie ved by auto -combustion of a
mixture consisting of yttrium and copper nitrates in a glycerol /methanol solution , followed by an
additional thermal treatment .
The XRD spectra of the synthesized powders , annealed at different temperatures , are
shown in Fig. 1 . The p ost-combustion reaction product consists of a mixture of very reactive
Y2O3 and Cu2O, with traces of copper (dark brown XRD pattern) . The morphology of the
mixture ( not shown here ) was studied using SEM , which revealed a foam -like aspect. The
combustion reaction process for the metal nitrates / glycerol mixture is represented in Fig. 2. The
mechanism of the combustion reaction is quite comp lex and invol ves several intermediate steps.
A major portion of methanol evaporates up to around 85 °C . Afterwards, a quantity of water
from the hydrated metal salts evaporates as well . During these process es, the solution viscosity
increases and the color of the solution changes from deep blue to greenish -blue. Foaming is
observed near the ignition point . The combustion reaction starts at 128 °C, as can be observed
from Fig. 2. A flame was present d uring the reaction.
The g lycerol combustion may be represented according to the following equation :
2 C3H8O3 + 7 O2 → 6 CO 2 + 8 H2O (Eq. 1)
In our particular case, the oxygen required for combustion is released during
decomposi tion of yttrium and copper nitrates, so the overall reaction between the nitrates and
glycerol can be written as:
14 Y(NO 3)3·6H2O + 14 Cu(NO 3)2·3H2O + 25 C3H8O3 →

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7 Cu2Y2O5 + 35 NO + 75 CO + 226 H 2O (Eq. 2)
A part of the hydration water from the metal nitrates vapori zes before reaching the ignition
temperature. Afterwards, the decomposition of copper and yttrium nitrates occurs at different
temperatures. As seen from literature, t he Y 2O3 formation by th ermal decomposition of
Y(NO 3)3·6H2O takes place in 9 stages [22]. The most endothermic processes take place at
temperatures between 300 and 500 șC and are associated with the loss of hydration water. The
Y2O3 compound is formed only at temperatures higher than 450 șC, according to the reaction
[22]:
Y4O5(NO 3)2 → 2Y2O3+N 2O5 (Eq. 3)
Thus, if we use a stoichiometric mixture of nitrates and glycerol at temperatures lower than
450 șC, the reducing conditions are present in the system. Due to this, the reduction of CuII to
CuI, followed by Cu0, occurs, as noticed from the XRD spectra (Fig. 1). This means that a part of
the oxygen released during the nitrates ’ (i.e. Y4O4(NO 3)4 and Y 4O5(NO 3)2) decomposition [22]
does not react with the fuel. According to Eq. 2, 48 moles of gases are released for each mole of
Cu2Y2O5. This high volume of gases hinders the access of oxygen from the surrounding air and
thus the reoxidation of metallic copper and Cu 2O to CuO. As a result of the combustion reaction,
the formation of Cu 2Y2O5 does not take place direct ly, due to a very short time ( under one
second) of exposure of precurs ors to an elevated temperature.
In order to compensate for the short exposure time of the precursors to high temperatures,
we performed a subsequent thermal treatment on the obta ined foam mixture. At a high
temperature, both Cu 2O and Cu oxidize to CuO , which further reacts with Y 2O3 in an oxidizing
atmosphere to form Cu 2Y2O5 [16]. From the change in the Gibbs free energy for the reaction
between Y 2O3 and CuO oxides, it is possible to conclude that a spontaneous reaction occurs at
temperatures higher than 673 °C [10]. It was also observed that orthorhombic Cu2Y2O5 is present
as a minority phase in the mixture with cubic Y 2O3 and CuO at 700 °C [23]. Because of the slow
reaction rate at 700 °C, the annealing of the reaction mixture was performed at 800 șC and 900
șC for 30 minutes and , respectively, at 900 șC and 1000 șC for 5 and 10 minutes,. After annealing
the post-combustion mixture at 800 °C for 30 min, the XRD spectra shows the mixture of
Cu2Y2O5 and Y 2O3 phases , indicat ing an incomplete reaction (Fig. 1, marked with a black arrow
29° 2θ? ).

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By increasing the annealing temperature to 900 °C, after 5 minutes of reaction, the Y 2O3
concentration is reduced ( Fig. 1 , gray arrow ). Traces of Y 2O3 can be seen in the XRD pattern
even after increasing the annealing time to 30 minutes at 900 °C (Fig. 1, light gray arrow ). In
order to get pure phase material, the temperature was further increased to 1000 °C and the
annealing time was reduc ed to 5 or 10 minutes. The later were found to be the optimal conditions
for obtaining Cu2Y2O5 as a pure phase .

3.2. Structural studies
At room temperature , all XRD reflections of the synthesized Cu 2Y2O5 closely match with
the ones from ICDD file 01-83-0341 . No trace s of othe r phases corresponding to any yttrium or
copper oxides were detected. The crystal structure was refined in order to determine the lattice
parameters , the XRD pattern of Cu2Y2O5 prepared at 1000 °C for 5 min being analyzed with
profile matching using pseudo -Voigt Axial divergence asymmetry pro file function in FullProf
suite (Fig. 3 ). All diffraction reflections were indexed according to the orthorhombic phase of
Cu2Y2O5 with space group Pna21 (33), with lattice parameters a = 10.8006 (1) Å, b = 3.4962 (1)
Å, c = 12.4539 (1) Å and V = 470.282 (8) Å. In order to verify this, the average crystallite size s
(L) of Cu 2Y2O5 powders were estimated using the Debye Scherrer equation.

cosKL

where K is a constant related to the crystallite shape and taken a s 0.9 , assuming that the
crystallites are spherical in shape, λ is the X -ray wavelength, β is the line broadening of the peak
at half of the maximum intensity (FWHM) after subtracting the instrumental line broadening (the
line broadening of the diffractometer was measured using a polycrystalline silicon standard ), and
θ is the Bragg angle. The calculated average crystallite size of Cu 2Y2O5 particles obtained at 800
°C / 30 min., 900 °C / 5 min., 900 °C / 30 min. and 1000 °C / 5 min. are 111, 109, 129 and ,
respectively, 180 nm. This shows that an increas e in the reaction t emperature or duration leads to
an enhancement in the crystallites sizes.

3.3. Morphological studies
The SEM morphology images of Cu 2Y2O5 product obtained at 1000 °C after 5 minutes of
thermal treatment shows nano particles having a foam aspect (Fig. 5a). Due to the high

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temperature combustion reaction, individual nanocrystals become attached to each other ,
form ing aggregates . The precursor ’s morphology looks very much like the morphology of the
annealed compound. This indicates that the short anneal ing time at 1000 °C has a minor
influence on the morphology of the final product. Using HR TEM micrograph, an interplanar
distance of  4.08 nm was measured for 10 atomic layers , as shown in Fig. 5b. This corresponds
to the (202) crystallographic plane of Cu2Y2O5.

3.4. CuYO 2 phase formation study
An in -situ high temperature XRD study on the obtained Cu 2Y2O5 was performed in order
to identify favorable formation conditions for preparation of CuYO 2 based TCO.
In air, Cu 2Y2O5 decomposes into Y 2O3, Cu 2O and CuO at 1100 °C, wi thout forming the
CuYO 2 phase [ 325]. This is attributed to the low thermodynamic driving force for CuYO 2
formation. Consequently, the reducing atmosphere is a necessary condition for delafossite phase
formation, in order to avoid its decomposition. I n our investigation we used a 10-2 mbar vacuum
and the 25-1000 °C temperature domain in order to study the phase transformation (Fig. 6). In
our case, Cu 2Y2O5 is stable until 500 °C and the phase transformation of Cu 2Y2O5 starts above
500 °C. Up to 700 °C, a phase coexistence of Cu 2Y2O5, CuYO 2 and Y 2O3 is noticed. Above 800
°C, only traces of CuYO 2 and Y 2O3 can be observed, which indicates the complete
transformation of Cu 2Y2O5 into CuYO 2 and Y 2O3 phases . This results suggest that further work
using inert atmospheres would be necessary in order to achieve pure phase CuYO 2 from
Cu2Y2O5, in accordance with the existing literature data [ 326].

3.5. Optical properties
The phase purity of Cu 2Y2O5 was also investigated by Raman spectroscopy. A typical
ambie nt temperature Fourier transform Raman spectr um of a sample ( 1000 °C / 5 min utes
variant ) is shown in Fig. 4. The Raman spectrum shows several sharp peaks corresponding to the
lattice vibrations, which indicates a good crystalline material behavior. The predominant Rama n
lines , located around 157, 173, 199, 210, 241, 264, 302, 328, 354, 393, 515, 562 and 586 cm-1,
are in agreement wi th literature reported values. The Raman spectr oscopy also confirm the phase
purity of Cu 2Y2O5, as there are no trace s of CuO (which would give a weak band at 296 cm-1) or
Y2O3 (which would give a strong band at 376 cm-1), in agreement with the XRD data.

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The o ptical properties of Cu 2Y2O5 were also investigated using DRS (Fig. 7). The DRS
curve s for the Cu 2Y2O5 sample s prepared at 900 °C and 1000 °C have similar shapes. The broad
peak in the 350-600 nm domain , with the maximum at 495 nm , shows a spectral shape close to
that reported for Cu 2Dy2O5, which possesses a crystallographic structure similar to that of
Cu2Y2O5 [18]. The bluish –green color of the samples is characteristic for compound s of this
type. The color in Cu 2Y2O5 is probably due to the crystal field splitting of Cu2+ d-orbitals and the
d-d electronic transition [18]. The DRS curve of the CuYO 2 sample prepared from the Cu2Y2O5
precursor shows a higher reflectance when compared with Cu2Y2O5, its intensity decreasing
rapidly at 40 0 nm. The curve shape is similar with that reported by [Reference ]. In order to
estimate the optical band gap , the Tauc plot, ( R(T)hν)2/n vs hν, was used , in which R(T) is
Kubelka -Munk absorbance , hν is the photon energy , and n = 1 for allowed direct optical
transition s. The optical band gap ( Eg) value was taken at the intersection point of the traced
tangent and the horizont al hν axis. As shown in the inset from Fig. 7 , in the case of Cu 2Y2O5
prepared at 1000 °C for 5 min , the direct band gap is estimated to be around 2.6 eV.

4. Conclusions

The efficiency of an original two-step technique for the produ ction of orthorhombic
Cu2Y2O5 nanocrystals was demonstrated . The first step of this technique , namely the auto –
combustion synthesis , leads to the formation of a very reactive mixture of Y 2O3, Cu 2O and
metallic copper . The second step consists in the annealing of the post -combustion mixture in the
temperature range of 800 -1000 °C , between 5 and 30 min. At this stage , the oxidation of Cu 2O
and metallic copper leads to the formation of CuO , which reacts with Y 2O3 and finally leads to
the formation of pure phase Cu 2Y2O5. The optimized preparation conditio ns for pure phase
Cu2Y2O5 involve the thermal treatment at 1000 °C for five minutes. The SEM studies indicate
that temperature and duration have a minor influence on the morphology of the final products.
The Fourier transform Raman and HR TEM further confirm the purity of the synthesized
materials. From optical st udies, the band gap of Cu2Y2O5 is found to be around 2. 6 eV. It was
also confirmed that the thus -obtained Cu 2Y2O5 can serve as a rapid precursor for preparation of
CuYO 2 based TCO , after a 1000 °C medium vacuum treatment .

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Acknowledgements : This work was partially supported b y the grant
POSDRU/159/1.5/S/137070 (2014) of the Ministry of National Education, Romania, co -financed
by the European Social Fund – Investing in People, within the Sectoral Operational Programme
Human Resources Development 2007 -2013. We also thank Dr. Olg a Iliasenco and Daniela
Drasovean for their help in drafting of the work.

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Figures:

Fig 1. XRD patterns of the post-combustion mixture and of the samples obtained at various
annealing temperatures and durations.

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Fig 2. Combustion reaction mechanism of copper and y ttrium nitrates dissolved in
glycerol/methanol

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Fig 3. Profile matching to the XRD pattern of Cu 2Y2O5 synthesized at 1000°C/5 min ; the
observed (red points), calculated (black solid line), Bragg positions (green bars) and difference
curve (bo ttom blue line) are shown ; the conventional reliability factors for profile matching are
Rp = 12.1 %, R wp = 9.50 % and χ2 = 1.787.

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Fig 4. Fourier transform Raman of Cu 2Y2O5 obtained at 1000 °C / 5 min

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Fig 5. SEM (a) and HR TEM (b) images of Cu 2Y2O5 annealed at 1000 °C / 5 min

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Fig 6. High temperature XRD patterns for decomposition of Cu 2Y2O5 in vacuum (10-2 mbar)

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Fig.7 DRS curve s: (1) CuYO 2 synthesized in vacuum and (2) Cu 2Y2O5 precursor ; inset:
determination of optical band gap for CuYO 2 and Cu 2Y2O5 using Tauc plots

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Graphical Abstract (for review)

Highlights (for re view)

 A simple , rapid and energy saving combustion technique was developed for preparation
of Cu2Y2O5 nanocrystals
 The optimal annealing conditions for obtaining pure phase Cu2Y2O5 is less than 10
minutes
 In-situ XRD studies revealed the thermal stabilit y of Cu 2Y2O5, and paved the way for
preparation of CuYO 2
 Optical band gaps of Cu 2Y2O5 and CuYO 2 nanocrystals were found to be 2. 6 eV and 3.5
eV respectively

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Cover Letter

Dr. Radu Banica,
Renewable Energies – Photovoltaic Laboratory
INCEMC, Str. Dr. A. P. Podeanu, rtr.l44,
Timisoara, 300569, Romania
benny_van_li @yahoo.com
Tel.: +407 2l 6827 5

Timi șoara, February 20th, 2015

Dear editor,

Please find an enclosed manuscript entitled “ Rapid Combustion Synthesis of Cu 2Y2O5: A
Precursor for Delafossite CuYO 2 based TCO ” by A .V. Racu, R. Baies, Srinivasa R. Popuri, R.
Banica, which we are submitting fo r exclusive consideration of publication as an article in
“Journal of solid state chemistry”.

As indicated in the title and abstract we have developed a novel synthesis method based on liquid
auto-combustion followed by short annealing for obtaining Cu 2Y2O5 nanocrystals. The resulted
powder product is of interest in: medicine – for antibacterial coatings, in optics – for greenish
blue pigments, and in optoelectronics – as precursor for copper delafossite p -type transparent
conducting oxides.
The novelty of our work is consists in: 1) Development of a green, energy and time saving
synthesis – a rapid method for preparation of Cu 2Y2O5; 2) Reduction of the synthesis time from
reported several hours to less than 10 minutes; 3) Identification of op timal annealing conditions
for obtaining a pure material; 4) Finding the thermal stability and phase transformation to
CuYO 2.

We hope that you will consider our manuscript for publication and are looking forward to
your response.

Sincerely,
Radu Banica