# Silica-Copper Oxide Composite Thin Films as Solar  Selective Coatings Prepared by Dipping Sol Gel

Authors: E. Barrera-Calva; J. Méndez-Vivar; M. Ortega-López; L. Huerta-Arcos; J.   Morales-Corona; R. Olayo-González
Journal: Research Letters in Materials Science
 (2008)

---

## Abstract

Silica-copper oxide (silica-CuO) composite thin films were prepared by a dipping sol-gel route using ethanolic solutions comprised TEOS and a copper-propionate complex. Sols with different TEOS/Cu-propionate (Si/Cu) molar ratios were prepared and applied on stainless steel substrates using dipping process. During the annealing process, copper-propionate complexes developed into particulate polycrystalline CuO dispersed in a partially crystallized silica matrix, as indicated by the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. The gel thermal analysis revealed that the prepared material might be stable up to400°C. The silica-CuO/stainless steel system was characterized as a selective absorber surface and its solar selectivity parameters, absorptance (α), and emittance (ε) were evaluated from UV-NIR reflectance data. The solar parameters of such a system were mostly affected by the thickness and phase composition of the SiO2-CuO film. Interestingly, the best solar parameters (α = 0.92 and ε = 0.2) were associated to the thinnest films, which comprised a CuO-Cu2O mixture immersed in the silica matrix, as indicated by XPS.


---

## Body

## 1. Introduction

Transition metal oxides display a wide variety of physical properties,
which make them useful for a number of technological applications. In last
years, they have received an especial attention as solar selective coatings for
photothermal conversion because, besides their physical properties, they have
demonstrated to be chemically and thermally stables in air. In last years, a
number of transition metal oxides, such as Cr2O3 [1], Co3O4 [2], and TiNOx [3, 4], have proven to be efficient and durable selective
coatings showing very suitable absorptance (0.95) and thermal emittance at 100°C
(0.05) values. However, there exists the interest in developing novel materials
with improved durability and manufacturability and at reduced cost.Black copper oxide (CuO) appears to be a good
candidate as solar selective coating because it is inexpensive and easy
processable. Furthermore, it has demonstrated to have the desirable optical
properties for solar photothermal conversion, such as a high absorptance (α≥0.90) and low emittance (ε≤0.2). However, earlier studies demonstrated that CuO was
susceptible to degradation [3], which has hindered for a more extended
application.Nowadays, a great amount of work on selective
coatings has been done by previous groups using several deposition methods
including magnetron sputtering, electrodeposition, vacuum evaporation, and sol
gel [2–4]. The present work reports our early attempts to prepare stable
CuO thin films for photothermal applications. Our experimental approach
consists in protecting it with more stable oxides, besides using sol gel, which
is an inexpensive and convenient technique to prepare a wide variety of
materials. Here, we report on the synthesis and physicochemical characterization
of particulate CuO dispersed in a silica matrix. It is shown that such a
composite possesses suitable optical properties for being used as a selective
coating.


## 2. Experimental

### 2.1. Film Preparation

Tetraethyl orthosilicate (TEOS, Sigma-Aldrich, USA), CuCl2⋅2H2O
(Baker), absolute ethanol
(Baker), HCl (Baker), and propionic acid (Baker) were used as the starting
materials.The sol was prepared starting from solutions of
TEOS and Cu-propionate separately dissolved in ethanol. The Cu-propionate
solution was prepared first dissolving CuCl2⋅2H2O in ethanol, and
then adding propionic acid. The precursor sol was obtained by adding dropwise
the Cu-propionate solution in a previously prepared TEOS solution, and then
adding HCl as acid catalyst. The reaction mixture was then stirred for 24 hours
at room temperature. In all experiments, the TEOS/HCl molar ratio and CuCl2⋅2H2O
concentration were kept at 10 and 0.05 M, respectively. Two sols with TEOS/Cu-propionate
(Si/Cu) molar ratios of 0.5 and 1 were essayed.The films were grown on stainless steel
substrates using a dipping process. Prior to deposition process, the metal
substrates were first mirror-like polished using a commercial buff. After
polished, they were cleaned with detergent, rinsed in distillated water, and
finally soaked in ethanol under ultrasonic agitation.The film deposition was carried out in a closed
chamber under controlled humidity (55%). For this, the substrate was quickly
immersed in the sol, taken out at the withdraw rate of 1 mm/s, dried at room
temperature, and then baked in air at 450°C for 30 minutes. Films with
different thickness were prepared by repeating the immersion, withdrawing,
drying, and baking cycle several times. In all cases, the final film was
further annealed in air at 450°C for 4 hours.


### 2.2. Characterization Techniques

The decomposition and pyrolysis of the gel was
analyzed in the 25–850°C
range, using a Netzsch STA 409 EP TG-DTA apparatus at a heating rate of 10ºC/min in air. Before the thermal measurements the gel was dried
overnight at 60°C, and then powdered to obtain a fine-grained
product; about 75 mg of the final product was loaded in the apparatus crucible.
The film crystalline properties were analyzed by X-ray diffraction (XRD), using
the CuKα radiation
of a D-500 Siemens diffractometer.The film chemical composition was analyzed by X-ray
photoelectron spectroscopy (XPS), using a UHV VG Microtech MultiLab ESCA 2000
equipped with an AlKα X-ray
source (hv=1486.6 eV) and a CLAM4 MCD detector. The surface chemical composition was determined from the orbital peak
area, using the atomic sensitivity factors reported by Scofield [5] and corrected with the analyzer transmission function, taking CuO-mesh and SiO2 as the reference
materials. In all cases, the film surface was etched during 2 minutes with 3.0 kV Ar+ to eliminate contaminating carbon. The background for the
high resolution and the survey spectra was that of Shirley and lineal,
respectively.The absorptance and thermal emittance of the
silica-CuO/SS system were evaluated from its reflectance spectrum, which was
measured from 0.3 to 25 microns using a Nicolet 750 spectrophotometer.


## 2.1. Film Preparation

Tetraethyl orthosilicate (TEOS, Sigma-Aldrich, USA), CuCl2⋅2H2O
(Baker), absolute ethanol
(Baker), HCl (Baker), and propionic acid (Baker) were used as the starting
materials.The sol was prepared starting from solutions of
TEOS and Cu-propionate separately dissolved in ethanol. The Cu-propionate
solution was prepared first dissolving CuCl2⋅2H2O in ethanol, and
then adding propionic acid. The precursor sol was obtained by adding dropwise
the Cu-propionate solution in a previously prepared TEOS solution, and then
adding HCl as acid catalyst. The reaction mixture was then stirred for 24 hours
at room temperature. In all experiments, the TEOS/HCl molar ratio and CuCl2⋅2H2O
concentration were kept at 10 and 0.05 M, respectively. Two sols with TEOS/Cu-propionate
(Si/Cu) molar ratios of 0.5 and 1 were essayed.The films were grown on stainless steel
substrates using a dipping process. Prior to deposition process, the metal
substrates were first mirror-like polished using a commercial buff. After
polished, they were cleaned with detergent, rinsed in distillated water, and
finally soaked in ethanol under ultrasonic agitation.The film deposition was carried out in a closed
chamber under controlled humidity (55%). For this, the substrate was quickly
immersed in the sol, taken out at the withdraw rate of 1 mm/s, dried at room
temperature, and then baked in air at 450°C for 30 minutes. Films with
different thickness were prepared by repeating the immersion, withdrawing,
drying, and baking cycle several times. In all cases, the final film was
further annealed in air at 450°C for 4 hours.


## 2.2. Characterization Techniques

The decomposition and pyrolysis of the gel was
analyzed in the 25–850°C
range, using a Netzsch STA 409 EP TG-DTA apparatus at a heating rate of 10ºC/min in air. Before the thermal measurements the gel was dried
overnight at 60°C, and then powdered to obtain a fine-grained
product; about 75 mg of the final product was loaded in the apparatus crucible.
The film crystalline properties were analyzed by X-ray diffraction (XRD), using
the CuKα radiation
of a D-500 Siemens diffractometer.The film chemical composition was analyzed by X-ray
photoelectron spectroscopy (XPS), using a UHV VG Microtech MultiLab ESCA 2000
equipped with an AlKα X-ray
source (hv=1486.6 eV) and a CLAM4 MCD detector. The surface chemical composition was determined from the orbital peak
area, using the atomic sensitivity factors reported by Scofield [5] and corrected with the analyzer transmission function, taking CuO-mesh and SiO2 as the reference
materials. In all cases, the film surface was etched during 2 minutes with 3.0 kV Ar+ to eliminate contaminating carbon. The background for the
high resolution and the survey spectra was that of Shirley and lineal,
respectively.The absorptance and thermal emittance of the
silica-CuO/SS system were evaluated from its reflectance spectrum, which was
measured from 0.3 to 25 microns using a Nicolet 750 spectrophotometer.


## 3. Results and Discussion

As mentioned above, we have prepared a number
of the samples starting from two sols whose Si/Cu molar ratio was 0.5 and 1.
However, we present only representative results in order to illustrate the
general trends.


### 3.1. Thermal Analysis

The thermal analysis (TGA/DTA) was carried out
on representative gels. Figure1 shows representative TGA/DTA results, which
were obtained from a gel at Si/Cu=1:1. The DTA curve in Figure 1 shows two
endothermic peaks at 130°C and 470°C, both accompanied by a noticeable mass
loss, as indicated by TGA. The first mass loss (18.4%) corresponds to a loss of
water and solvent molecules, which were only adsorbed in material (50–140°C), while the second one (12.6 %) was
related to the pyrolysis of some organic groups, the subsequent transformation
of the pyrolysis products into oxides and their further crystallization (420–480°C). As observed in Figure 1, the DTA
curve also exhibits a broad exothermic peak at 300°C probably connected to
condensation reactions involving the alkoxy and hydroxyl groups, while the accompanying
mass loss (3.7%) was associated to the removal of residual water and solvent
molecules trapped in micropores (200–400°C).Figure 1
TGA and DTA curves of a dried gel prepared from a sol at Si:Cu = 1.It is worth noting that the major
mass loss (26%) occurred at 540–780°C, and entailed no noticeable heat
exchange. We believe that such mass loss might be associated to the partial
decomposition of CuO, but further studies are needed here to elucidate this
point.


### 3.2. Structural Characterization

In order to obtain well-crystallized
samples, we have chosen 450°C as the annealing temperature because, as above pointed out, the copper oxide
crystallization occurs at temperatures in 420–480°C range. The thermal treatment duration
was determined from our FTIR studies (not reported here), which indicated that
organic-free samples could be obtained after being annealed for about 3 hours.   Figure 2 shows the XRD pattern of a sample
prepared at Si/Cu=1:1 for
5 dipping cycles. All prepared samples displayed similar XRD patterns, revealing
diffraction peaks corresponding to SiO2 (cristobalite), CuO
(tenorite), and stainless steel (substrate). By using the Scherrer formula, a crystal
size about 5 nm was estimated for CuO, indicating that it exists as a
particulate material dispersed in a partially crystallized SiO2 matrix. An unexpected result was the silica crystallization at a temperature as
low as 450°C.Figure 2
XRD spectrum of a SiO2-CuO thin
film deposited on stainless steel plate. The sample was prepared using a sol at
Si/Cu=1 and 3 dipping cycles. Diffraction peaks corresponding to CuO (copper
oxide, tenorita), SiO2 (silicon oxide, cristobalita), and the
stainless steel (SS) are observed.


### 3.3. XPS Analysis

XPS is a valuable technique to determine the
chemical composition of materials as well as the oxidation state of their
constituent elements. In all analyzed samples, contaminant carbon was detected
at their surface, probably coming from ambient atmosphere and was eliminated by
Ar etching.Figure3 displays the XPS spectra of samples
deposited at various dipping cycles (1, 3, and 5) after being Ar+ etched. Figure 3(a) shows the O 1s core level XPS spectra along with their
deconvolution; for comparison, the top and bottom panels display the SiO2 and CuO standard spectra, respectively. In the deconvoluting process, the
binding energies for the Cu2+ core
level and its satellite were fixed at 530.21 and
532.06 eV, respectively, while the Si4+ core level was kept at 532.58 eV. Figures 3(b) and 3(c) show the XPS spectra corresponding to Cu2+ 2p and Si4+ 2p core levels,
respectively.Figure 3
XPS spectra of samples deposited on SS
at 1, 3, and 5 dipping cycles (labeled
as 1i, 3i, and 5i), using a sol atSi/Cu=1: (a) O 1s core level XPS
spectra, (b) Cu 2p core level XPS spectra,  and (c) Si 2p core level XPS spectra. For
comparison, Figure 3(a) displays the XPS standards of SiO2 (top
panel) and CuO  (bottom panel).From Figures3(a) and 3(b), it is seen that with
the exception of the sample at 1 dipping cycle, the rest displays similar XPS
spectra. In all of them appear peaks associated to Cu2+ and Si4+,
testifying that copper and silicon oxides coexist as CuO and SiO2 in
such samples. On the other hand, the O 1s XPS spectrum of the sample at 1
dipping cycle (Figure 3(a)1i) exhibits a broad band that could be analyzed as
composed by three superposed bands, which were identified to correspond to Cu1+,
Cu2+, and Si4+, indicating the formation of CuO and SiO2 as well as Cu2O as an extra phase. The
presence of Cu in the +1 oxidation state is also evidenced in Figure 3(b)(1i), where a chemical shift is observed (i.e., the Cu1+ binding energy peak is slightly displaced to higher energies, with respect to the binding energy of the Cu2+ 2p orbital).


### 3.4. Optical Characterization

The absorptance (α) and emittance (ε) of the silica-CuO/SS system were determined
from its reflectance spectrum as described by Duffie and Beckman [6]. Because
of their relationship with the reflectance, a low spectral reflectance implies
a high absorptance and vice versa. For this, the total reflectance was measured
as a wavelength function, and the above parameters were obtained after
calculating the area under the reflectance curve, in the  0.3≤λ≤2.5⁢μm and 2.5≤λ≤25⁢μm ranges for absorptance and emittance, respectively.According to the AFM analysis, the
RMS roughness of silica-CuO films deposited on SS increased from 205 to 700 nm,
as the dipping cycle number increased from 1 to 7. On the other hand, it was
noted that the reflectance of the silica-CuO/SS system was affected for both,
the thickness and the phase composition of the silica-CuO films; for
illustrating the salient features of the reflectance, Figure4 shows the
reflectance spectra of two samples deposited at (a) 1 and (b) 3 dipping cycles,
using a sol with Si/Cu=0.5. It is seen that the reflectance strongly
decreased with the dipping cycle number in the wavelength range, where the
emittance was estimated (2.5≤λ≤25⁢μm), while a minor variation was observed at 0.3–2.5⁢μm (see Figure 4 (inset)), where absorptance was evaluated. Consequently,
the system emittance slightly increased with the silica-CuO thickness, while
absorptance remained practically constant.Figure 4
Total reflectance as a
function of the wavelength (0.2–25μm) of SiO2-CuO thin films deposited
on SS, at (a) 1 dipping and (b) 3 dipping cycles.Figure5 shows plots of the solar
parameters, α, ε, and the solar selectivity (S=α/ε) as a function of dipping cycle number. From
this figure, it is seen that absorptance remains at 0.94 yet up to seven
dipping cycles; emittance slightly increases with the dipping cycle number from
0.2 to 0.52; and  selectivity decreases
as dipping cycle number does. Notice that, we have assumed the film thickness
to increase with the dipping cycle number because we were unable for evaluating
it.Figure 5
Solar parameters of a
representative SiO2-CuO/SS system as a function of the number of
dipping cycles: solar absorptance (α), thermal emittance at 100°C (ε),
and selectivity (S=α/ε).We believe that our results are
encouraging ones because thin films with good solar parameter values [1–3] can
be achieved using an inexpensive and simple technique as sol gel. More
interestingly is that films showing acceptable solar parameter can be obtained
by means of only one dipping cycle.


## 3.1. Thermal Analysis

The thermal analysis (TGA/DTA) was carried out
on representative gels. Figure1 shows representative TGA/DTA results, which
were obtained from a gel at Si/Cu=1:1. The DTA curve in Figure 1 shows two
endothermic peaks at 130°C and 470°C, both accompanied by a noticeable mass
loss, as indicated by TGA. The first mass loss (18.4%) corresponds to a loss of
water and solvent molecules, which were only adsorbed in material (50–140°C), while the second one (12.6 %) was
related to the pyrolysis of some organic groups, the subsequent transformation
of the pyrolysis products into oxides and their further crystallization (420–480°C). As observed in Figure 1, the DTA
curve also exhibits a broad exothermic peak at 300°C probably connected to
condensation reactions involving the alkoxy and hydroxyl groups, while the accompanying
mass loss (3.7%) was associated to the removal of residual water and solvent
molecules trapped in micropores (200–400°C).Figure 1
TGA and DTA curves of a dried gel prepared from a sol at Si:Cu = 1.It is worth noting that the major
mass loss (26%) occurred at 540–780°C, and entailed no noticeable heat
exchange. We believe that such mass loss might be associated to the partial
decomposition of CuO, but further studies are needed here to elucidate this
point.


## 3.2. Structural Characterization

In order to obtain well-crystallized
samples, we have chosen 450°C as the annealing temperature because, as above pointed out, the copper oxide
crystallization occurs at temperatures in 420–480°C range. The thermal treatment duration
was determined from our FTIR studies (not reported here), which indicated that
organic-free samples could be obtained after being annealed for about 3 hours.   Figure 2 shows the XRD pattern of a sample
prepared at Si/Cu=1:1 for
5 dipping cycles. All prepared samples displayed similar XRD patterns, revealing
diffraction peaks corresponding to SiO2 (cristobalite), CuO
(tenorite), and stainless steel (substrate). By using the Scherrer formula, a crystal
size about 5 nm was estimated for CuO, indicating that it exists as a
particulate material dispersed in a partially crystallized SiO2 matrix. An unexpected result was the silica crystallization at a temperature as
low as 450°C.Figure 2
XRD spectrum of a SiO2-CuO thin
film deposited on stainless steel plate. The sample was prepared using a sol at
Si/Cu=1 and 3 dipping cycles. Diffraction peaks corresponding to CuO (copper
oxide, tenorita), SiO2 (silicon oxide, cristobalita), and the
stainless steel (SS) are observed.


## 3.3. XPS Analysis

XPS is a valuable technique to determine the
chemical composition of materials as well as the oxidation state of their
constituent elements. In all analyzed samples, contaminant carbon was detected
at their surface, probably coming from ambient atmosphere and was eliminated by
Ar etching.Figure3 displays the XPS spectra of samples
deposited at various dipping cycles (1, 3, and 5) after being Ar+ etched. Figure 3(a) shows the O 1s core level XPS spectra along with their
deconvolution; for comparison, the top and bottom panels display the SiO2 and CuO standard spectra, respectively. In the deconvoluting process, the
binding energies for the Cu2+ core
level and its satellite were fixed at 530.21 and
532.06 eV, respectively, while the Si4+ core level was kept at 532.58 eV. Figures 3(b) and 3(c) show the XPS spectra corresponding to Cu2+ 2p and Si4+ 2p core levels,
respectively.Figure 3
XPS spectra of samples deposited on SS
at 1, 3, and 5 dipping cycles (labeled
as 1i, 3i, and 5i), using a sol atSi/Cu=1: (a) O 1s core level XPS
spectra, (b) Cu 2p core level XPS spectra,  and (c) Si 2p core level XPS spectra. For
comparison, Figure 3(a) displays the XPS standards of SiO2 (top
panel) and CuO  (bottom panel).From Figures3(a) and 3(b), it is seen that with
the exception of the sample at 1 dipping cycle, the rest displays similar XPS
spectra. In all of them appear peaks associated to Cu2+ and Si4+,
testifying that copper and silicon oxides coexist as CuO and SiO2 in
such samples. On the other hand, the O 1s XPS spectrum of the sample at 1
dipping cycle (Figure 3(a)1i) exhibits a broad band that could be analyzed as
composed by three superposed bands, which were identified to correspond to Cu1+,
Cu2+, and Si4+, indicating the formation of CuO and SiO2 as well as Cu2O as an extra phase. The
presence of Cu in the +1 oxidation state is also evidenced in Figure 3(b)(1i), where a chemical shift is observed (i.e., the Cu1+ binding energy peak is slightly displaced to higher energies, with respect to the binding energy of the Cu2+ 2p orbital).


## 3.4. Optical Characterization

The absorptance (α) and emittance (ε) of the silica-CuO/SS system were determined
from its reflectance spectrum as described by Duffie and Beckman [6]. Because
of their relationship with the reflectance, a low spectral reflectance implies
a high absorptance and vice versa. For this, the total reflectance was measured
as a wavelength function, and the above parameters were obtained after
calculating the area under the reflectance curve, in the  0.3≤λ≤2.5⁢μm and 2.5≤λ≤25⁢μm ranges for absorptance and emittance, respectively.According to the AFM analysis, the
RMS roughness of silica-CuO films deposited on SS increased from 205 to 700 nm,
as the dipping cycle number increased from 1 to 7. On the other hand, it was
noted that the reflectance of the silica-CuO/SS system was affected for both,
the thickness and the phase composition of the silica-CuO films; for
illustrating the salient features of the reflectance, Figure4 shows the
reflectance spectra of two samples deposited at (a) 1 and (b) 3 dipping cycles,
using a sol with Si/Cu=0.5. It is seen that the reflectance strongly
decreased with the dipping cycle number in the wavelength range, where the
emittance was estimated (2.5≤λ≤25⁢μm), while a minor variation was observed at 0.3–2.5⁢μm (see Figure 4 (inset)), where absorptance was evaluated. Consequently,
the system emittance slightly increased with the silica-CuO thickness, while
absorptance remained practically constant.Figure 4
Total reflectance as a
function of the wavelength (0.2–25μm) of SiO2-CuO thin films deposited
on SS, at (a) 1 dipping and (b) 3 dipping cycles.Figure5 shows plots of the solar
parameters, α, ε, and the solar selectivity (S=α/ε) as a function of dipping cycle number. From
this figure, it is seen that absorptance remains at 0.94 yet up to seven
dipping cycles; emittance slightly increases with the dipping cycle number from
0.2 to 0.52; and  selectivity decreases
as dipping cycle number does. Notice that, we have assumed the film thickness
to increase with the dipping cycle number because we were unable for evaluating
it.Figure 5
Solar parameters of a
representative SiO2-CuO/SS system as a function of the number of
dipping cycles: solar absorptance (α), thermal emittance at 100°C (ε),
and selectivity (S=α/ε).We believe that our results are
encouraging ones because thin films with good solar parameter values [1–3] can
be achieved using an inexpensive and simple technique as sol gel. More
interestingly is that films showing acceptable solar parameter can be obtained
by means of only one dipping cycle.


## 4. Conclusions

In summary, silica-copper oxide composite thin
films were prepared using a dipping sol gel-route, and preliminary results on
the physicochemical characterization were presented. It was found that the film
phase composition is strongly affected by the whole deposition process (dipping
cycle number and thermal treatments), being composed of either CuO or CuO-CuO2 mixtures, and embedded in a partially crystallized silica matrix. Films
deposited on stainless steel substrates exhibited good solar selective
parameters (α=0.92 and ε=0.2), including that deposited at only one
dipping cycle. Taking into account the manufacturability and processing cost,
the film preparation process, proposed here, provides encouraging results
because selective coatings could be easy prepared. More work is in progress to
test the long-term stability of the prepared material.



---

*Source: 190920-2008-03-24.xml*