CVD synthesis of Cu2O films for catalytic application

Article abstract of DOI:10.1039/c5ra05635g

Pure Cu₂O was synthesized at 270 °C by pulsed-spray evaporation chemical vapor deposition. The results indicate that Cu₂O is effective for the complete oxidation of VOCs with good reusability and reproducibility. The lattice and adsorbed oxygen as well as the hollow ball-like geometry are dedicated to the catalytic processes.

Full text of DOI:10.1039/c5ra05635g

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DOI: 10.1039/C5RA05635G
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CVD synthesis of Cu2O films for catalytic application
Guan-Fu Pan,a,b Shi-Bin Fan,a,b Jing Liang,a,c Yue-Xi Liua,b and Zhen-Yu Tiana,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Pure Cu2O was synthesized at 270 °C by pulsed-spray generally obtained by those methods and synthesis of pure Cu2O was
evaporation chemical vapor deposition. The results indicate that scarce. In recent years, pulsed-spray evaporation chemical vapor
Cu2O is effective for the complete oxidation of VOCs with good deposition (PSE-CVD) has been successfully used to prepare a series
22
reusability and reproducibility. The lattice and adsorbed oxygen of TMOs, e.g., Co3O4 and Mn3O4.23 Compared to the above-
as well as the hollow ball-like geometry are dedicated to the mentioned techniques, PSE-CVD is easy to control the thickness and
catalytic processes.
quality of the films without any further treatment. Thus, PSE-CVD
exhibits great potential to prepare Cu2O thin films.
1. Introduction
In the current work, the Cu2O thin films were synthesized by a
home-made PSE-CVD system. Among the VOCs, unsaturated
hydrocarbons are often encountered emissions from hydrocarbon
flames. Since the unsaturated hydrocarbons such as C2H2 and C3H6
are quite difficult to be oxidized, especially C2H2 is an important
precursor to form soot during the combustion of fossil fuel, C2H2
and C3H6 were selected as representatives for the deep oxidation of
VOCs existed in the exhaust emissions. The prepared Cu2O samples
were characterized in terms of structure, morphology and
composition as well as catalytic performance.
Volatile organic compounds (VOCs) are the main air pollutants
emitted from the combustion of fossil fuels in many industry
processes and transportation activities.1, 2 VOCs are associated with
various health-related problems.3 Catalytic combustion was
commonly used for the abatement of VOCs in the past decades.4 The
involved catalysts are mainly composed of noble metals such as Au,
Pt and Rh5-7 and transition metal oxides (TMOs).8, 9 Noble metals
generally exhibit good catalytic performance, but they are expensive
and easy to be poisoned. For these reasons, catalytic oxidation of
VOCs over TMOs has captured increasing attention, and
considerable efforts have been devoted to the synthesis and
application of TMOs for such purposes.10 Specifically, Mn3O4, CuO,
Fe2O3 and Co3O4 exhibited good performance as active catalysts for
VOCs treatment.4, 11-14
2. Material and methods
2.1 Preparation of Cu2O thin films
The Cu2O thin films were synthesized in a PSE-CVD system
combined with cold-wall stagnation point-flow CVD reactor and
waste collector, as presented in Fig. 1. The preparation process
consists of four steps. Firstly, copper acetylacetonate (Cu(acac)2)
was dissolved in ethanol at a concentration of 2.5 mM, which was
delivered as liquid feedstock by a PSE unit. The delivery frequency
was 4 Hz and the opening time of PSE was 2.5 ms. The feeding rate
was 1.03 mL/min. The 30 cm long evaporation chamber was kept at
3.0 kPa and 180 °C. Secondly, the resulting vapor was transported
into the deposition chamber with O2 and N2 with flowrates of 1.0
and 0.5 standard liter per minute (SLM), respectively. Thirdly, the
Cu2O thin films were formed on different substrates (planar glass,
stainless steel and mesh grid) kept at 270 °C for characterization
purposes. Finally, the waste vapor was collected by a liquid nitrogen
trap. The details of the optimized conditions are summarized in
Table ESI S1.
Compared to other TMOs, Cu2O has excellent catalytic
applications for VOCs.15 Rostami and Jafari16 used Cu2O as an
efficient catalyst to remove aromatic compounds. Kim and Shim15
investigated the catalytic characteristic of Cu2O and concluded that
Cu2O was active for the removal of VOCs.
Several methods were involved in the production of Cu2O, such
as sol-gel,17 top-down approach with nanosecond and picosecond
lasers,18 honey aided solution synthesis,19 electrochemical
synthesis20
and
photonic
crystal
template-assisted
electrodeposition.21 However, a mixture of Cu, Cu2O and CuO was
a.Institute of Engineering Thermophysics, Chinese Academy of Sciences, 11
Beisihuanxi Road, Beijing 100190, China.
b.University of Chinese Academy of Sciences, Beijing 100049, China.
c.School of Energy, Power and Mechanical Engineering, North China Electric Power
University, Beijing 102206, China.
2.2 Characterization
The obtained Cu2O films were characterized in terms of structure,
morphology and chemical composition with X-ray diffraction
† Electronic Supplementary Information (ESI) available: experimental conditions,
some XPS spectra and outlet profiles of CO and CO2 during the catalytic tests.
See DOI: 10.1039/x0xx00000x
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(XRD), Scanning electron microscopy (SEM) and X-ray rate of Cu2O is calculated to be 1.6 nm/min. AccordViniegw AtorticTleiOannlineet
photoelectron spectroscopy (XPS), respectively. XRD analysis was al.,25 the increase of the pressure may promote full surface coverage
DOI: 10.1039/C5RA05635G
performed on Bruker D8 Focus with Cu Kα radiation, scan angle of by adsorbing more Cu(acac)2 and O2, which could accelerate the
5-90° and scanning step of 0.02°. The microstructure was examined nucleation step and result in a high growth rate. In this work, the
using SEM (S-4800 Hitachi) with the resolution of 1.5 nm (15 KV). pressure for the synthesis of Cu2O films was optimized to be 3.0 kPa.
The chemical composition was identified by the XPS (ESCALAB According to the previous results,25 the substrate temperature should
250Xi) with pass energy of 20 eV and energy step size of 0.050 eV. be lower than 275 °C for the synthesis of pure Cu2O. In the
The XPS results were analyzed by Avantage which is one of the experiments, the temperature for the synthesis of Cu2O was fixed at
most widely used softwares for XPS analysis. The elemental 270 °C. Similar to the same strategy,25 the other parameters such as
contents in the samples was quantitative analyzed by non-linear least the concentration of the precursor in the liquid feedstock, the pulse
square fitting method and the oxygen was analyzed by Unimodal width and frequency of the precursor delivery have also been
fitting method.24
optimized (see Table ESI S1). The results also indicate that the
synthesis of Cu2O with PSE-CVD system can be tailored with
respect to thickness.
Fig. 3 Linear growth of Cu2O thin films on stainless steel.
Fig. 1 Schematic diagram of the PSE-CVD system.
3.2 Phase identification
2.3 Catalytic test
The XRD pattern of the obtained thin film is shown in Fig. 4.
The performances of the Cu2O films for C2H2 and C3H6 The well-defined diffraction peaks are observed at 36.41°, 42.24°,
conversion were investigated by a fixed bed quartz reactor, as shown and 61.88°, which can be well attributed to (111), (200) and (220)
in Fig. 2. A 60 cm long alundum tube (8.0 mm inner diameter) was orientations of Cu2O in the literature (JCPDS Nr. 05-0667). The two
fixed in a digital electrical furnace. 20 mg of the Cu2O supported on weak peaks located at 29.73° and 73.81° fit well with the (110) and
grid mesh of stainless steel was located at the isothermal area. A K- (311) planes of Cu2O. No characteristic peaks of any other
thermocouple was used to measure the temperature of Cu2O film. A impurities were observed in the XRD patterns, indicating the
gas mixture consisted of 1 % fuel gas (C2H2 or C3H6), 10 % O2 and formation of monoclinic Cu2O phase.26, 27 In the work by Medina-
89 % Ar was introduced into the alundum tube with a total flow rate Valtierra et al.,26 they prepared Cu2O film by CVD with the same
of 15 ml/min, corresponding to the gaseous hourly space velocity precursor. However, no film of Cu2O was formed when the
(ghsv) of 45,000 ml g−1 h−1. The temperature of the furnace was deposition temperature was lower than 320 °C. For sample prepared
cat
rised with a ramp of 2 °C/min. Finally, the exhausting gas was at 320 °C, only (111) and (200) planes were observed.26 However,
measured by a gas chromatograph (Agilent, GC3000) for qualitative the XRD analysis indicates that pure Cu2O thin films can be easily
and quantitative analysis.
prepared by PSE-DVD at as low as 270 °C in this work.
Fig. 2 Experimental setup of catalytic test.
Fig. 4 XRD pattern of the Cu2O thin film prepared on glass at 270 °C.
3.3 Microstructural studies
3. Results and discussion
The sharp and strong reflection peaks suggest that the as-
prepared samples are well crystallized. The crystallite size and the
micro-strain of Cu2O thin films were calculated to be 29 nm and
0.06% by applying Scherrer's formula: D = 0.9λ/β cos θ and the
equation ε = β/2cotθ to the most intense diffraction peak, where λ =
3.1 Growth
By measuring the weight difference of the substrates before and
after deposition, the growth of the prepared films can be estimated.
As displayed in Fig. 3, a linear behavior is observed and the growth
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0.154056 nm, β and θ represent the full width at half maximum and which further confirm the purity of Cu2O synthesized in this study.
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diffraction angle of the observed peak, respectively.28
The binding energies of O 1s were observedDtoObI:e105.21093.695/Ca5nRdA0553613.54G5
The formation of Cu2O can be preceded with the adsorption of eV (see Fig. 7b), corresponding to the lattice oxygen (OL) and
the precursor on the heated substrate, ligands removal, formation of adsorbed oxygen (Oads), respectively. The fitted profiles agree well
Cu metal and oxidation of Cu to Cu2O. Both temperature and with the measured profiles, as shown in Fig. 7b.
pressure play important roles in the deposition processes. With the
same precursor, a pure phase of CuO phase was obtained at 300 °C
by Tian et al.,25 while Cu2O was formed at 320 °C by Medina-
Valtierra et al.26 Such high temperature for Cu2O formation could be
due to the usage of low flowrate of O2 flow (30 ml/min) in their
work.
SEM inspection was carried out to reveal the morphology of the
as-obtained Cu2O samples. The representative SEM images with
different magnifications are shown in Fig. 5. The SEM images
indicate that the film is homogeneous and is composed of a large Fig. 6 Entire XPS spectrum (a) and Cu 2p spectrum (b) of Cu2O thin film.
quantity of small ball-like particles. The average grain size is
estimated to be 30 nm, which matches well with the crystallite size
calculated by the XRD results. The calculated size is much smaller
than that prepared with electrodeposition (500-700 nm) by Yu et
al.29 Besides the ball-like shapes, a number of hollows are also
observed. The small grain size and hollows could absorb more
oxygen than the smooth surface, which would benefit for the
catalytic oxidation of VOCs.
Fig. 7 Cu LM2 (a) and O 1s signals (b) of a representative Cu2O thin film.
3.5 Catalytic performance
The catalytic performance of the Cu2O grown on mesh of
stainless steel was tested for the complete oxidation of C2H2 and
C3H6 at atmospheric pressure. The background effect of the stainless
steel element on the oxidation was examined by carrying out the
experiments on non-coated mesh (NCM) under the same conditions.
The catalytic tests were carried out three times for the same sample
and the results are quite close, demonstrating that the prepared Cu2O
has good reusability with reproduced results. Figure 8 compares the
temperature-dependent conversion ratio of C2H2 and C3H6 with
Cu2O-coated mesh and non-coated mesh. Compared to the non-
coated mesh condition, the complete oxidation of C2H2 decreased
Fig. 5 SEM images of thin Cu2O films at different magnifications.
from 450 to 300 °C and for C3H6 decreased from 675 to 425 °C over
3.4 Surface composition
Cu2O. It should be mentioned that during the oxidation of C2H2 and
In order to explore the chemical composition of the Cu2O films, C3H6 over Cu2O-coated mesh, CO was not detected. However, a
an ex situ XPS analysis was performed on both bare and etched number of CO was measured over non-coated mesh. Compared to
22
surfaces of Cu2O films prepared on stainless steel. Figure 6a presents other TMOs such as Mn3O423, Co3O4 and Co–Mn oxides31 for the
the entire spectrum and Table ESI S2 lists the elemental contents for oxidation of C2H2 and C3H6, Cu2O exhibits better catalytic
the prepared samples. Based on the XPS results, the Cu:O atomic performances.
ratio of 95 nm etched layer was estimated to be about 2:1, which is
With an Arrhenius expression,14 apparent activation energies
consistent with the form of Cu2O obtained with XRD analysis. (Eappa) of less than 15% of C2H2 and C3H6 conversion were deduced.
Besides the presence of copper and oxygen, carbon was also found The Eappa of C2H2 and C3H6 oxidation over non-coated mesh are 93.5
on the surfaces of the prepared samples (see Fig. ESI S1-S3). After and 92.0 kJ/mol, while these values shift to 51.7 and 57.0 kJ/mol
95 nm etching, the content of C 1s that mainly comes from ambient with Cu2O-coated samples (average values for three times),
air or the decomposition of the precursor is negligible.
respectively. Compared to Eappa obtained with other TMOs, such as
Co3O4 (128.9 kJ/mol for C2H2, 127.1 KJ/mol for C3H6)14 and Mn3O4
As exhibited in Fig. 6b, the Cu 2p peak is centered at 952.25
1/2
(84.7 KJ/mol for C3H6),23 the reaction with Cu2O shows lower Eappa
.
eV and the Cu 2p
peaks at 932.35 eV.30 Since Cu 2p can hardly
3/2
differentiate Cu2+ from Cu1+, it is necessary to involve analysis of
auger spectrum. The Cu auger spectrum of the obtained thin film
was shown in Fig. 7a. The obvious peak located at 916.80 eV is
perfectly coinciding with the standard auger spectrum of Cu2O,
The abundant hollows revealed by the SEM images are expected to
absorb more oxygen and reduce the activation barrier. Moreover, the
high Cu:O ratio on the surfaces of the prepared samples could
provide more active sites. Thus, the relatively low Eappa may
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DOI: 10.1039/C5RA05635G
It is generally accepted that the oxidation of low-rank
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Fig. 8 Outlet profiles of C2H2 and C3H6 oxidation over NCM and
mesh grid of stainless steel coated with Cu2O.
4. Conclusions
This work presents a detailed introduction of facile synthesis of
Cu2O thin films by using a home-made PSE-CVD system for
catalytic oxidation of C2H2 and C3H6. XRD, SEM and XPS were
employed to characterize the physicochemical properties of the
deposited films. The catalytic oxidation of C2H2 and C3H6 over
Cu2O samples was tested at atmospheric pressure in a fixed-bed
quartz reactor. XRD analysis indicates that the prepared films are
pure Cu2O. The results show that the Cu2O leads the complete
oxidation decreased by 175 °C for C2H2 and 250 °C for C3H6
relative to the non-coated mesh. According to the microstructure and
XPS results, the lattice and adsorbed oxygen as well as the hollow
ball-like geometry could benefit for the deep oxidation of C2H2 and
C3H6. These results reveal that Cu2O can be easily prepared and
show good potential in the catalytic abatement of VOCs.
Acknowledgements
Prof. ZYT thanks the financial support from the Recruitment
Program of Global Youth Experts (Grant No. Y41Z024BA1).
Notes and references
* Corresponding author. Tel/Fax: +86-10 82543184, E-mail:
tianzhenyu@iet.cn.
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