CuO nanocrystallites obtained by oxidation
of copper arachidate LB multilayers
Sukhvinder Singh a , R.S. Srinivasa b , S.S. Talwar a , S.S. Major a,⁎
a
b
Department of Physics, Indian Institute of Technology Bombay, Mumbai-400076, India
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai-400076, India
Abstract
Copper arachidate (CuA) Langmuir–Blodgett (LB) multilayers were transferred in the subphase pH range 4.6–5.7. FTIR studies of multilayers
show a gradual increase in CuA content as subphase pH is increased. X-ray reflection patterns of multilayers show presence of a single layered
structure in all the multilayers transferred at different subphase pH. A continuous decrease in bilayer period from 52.5 Å to 47 Å is seen with
decrease in CuA content of the multilayers. These observations suggest mixing of CuA and arachidic acid molecules at the molecular level. The
precursor CuA multilayers were oxidized at 300 °C–700 °C. The formation of CuO is confirmed by UV–Visible spectroscopy. Atomic force
micrographs show the formation of CuO nanocrystallites and their clusters, with the average size, size distribution, height and density of
nanocrystallites depending strongly on subphase pH, number of monolayers and oxidation temperature. Typically, 7–11 monolayers CuA
transferred at subphase pH of 4.6 and oxidized at 700 °C resulted in isolated and nearly mono-disperse nanocrystallites of size 20–30 nm and
height ∼1 nm. Crystallites of size 2–10 nm along with few clusters were obtained by oxidation of a 3 monolayer CuA.
Keywords: Nanomaterials; Langmuir–Blodgett films; Atomic force microscopy
1. Introduction
Cupric oxide (CuO) is a transition metal oxide with
monoclinic unit cell and square planar coordination. It has
been under active study owing to its structural similarity to high
Tc cuprate superconductors as well as for photoconductive,
photo-thermal and photo-electrochemical applications. Recently, it has also been investigated for gas sensing applications
[1,2]. Nanostructured CuO has received attention for a variety
of applications such as gas sensors [3], electrodes in lithium
cells [4], field emitters [5,6] and magnetic storage media [7].
Owing to this interest, the formation of CuO has been reported
in various low dimensional forms such as nanoparticles [8],
mono-disperse nanocrystals [9], nanowires [6], nanotubes and
nanorods [10–12].
⁎
Oxidation of precursor Langmuir–Blodgett (LB) multilayers
has been used to form ultrathin and homogenous metal oxide
films. In most of the cases the precursor LB multilayer is heat
treated with UV radiation in air, resulting in oxide formation and
removal of organic components. Using this method, ultrathin
films of cupric oxide (CuO) were formed from precursor copper
arachidate LB multilayer [13]. In a subsequent work, singlephase nanocrystalline CuO films have been reported by direct
oxidation of copper arachidate multilayers [14]. In the present
work, the direct oxidation method has been exploited to form
uniformly distributed, mono-disperse nanocrystallites of CuO.
2. Experimental details
LB multilayers of CuA were prepared in a KSV 3000 LB
trough. Arachidic acid dissolved in chloroform was spread on
an aqueous subphase containing CuCl2. Dilute HCl and NaOH
solutions were added to achieve and maintain the desired
subphase pH values. The substrates used for deposition of
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multilayers were quartz, CaF2 and mica. The as-deposited CuA
multilayers were oxidized in a stream of oxygen at atmospheric
pressure in the temperature range 300 °C–700 °C. FTIR spectra
were obtained with a Perkin Elmer Spectrum1 instrument and
UV–Vis spectra with a Shimadzu UV-160A spectrophotometer.
X-ray reflection (XR) studies were performed with PANalytical
X'Pert PRO diffractometer using Cu Kα radiation. Atomic
force microscopy (AFM) studies were carried out using Digital
Instruments Nanoscope IV SPM.
3. Results and discussion
The composition of CuA multilayers transferred on CaF2 substrate
at subphase pH values in the range 4.6 to 5.7 has been studied by FTIR
spectroscopy. Fig. 1 shows the FTIR spectra in the region 1350 cm− 1–
1750 cm− 1 for CuA multilayers.
The spectrum of the as-deposited multilayer at subphase pH of 5.7
(Fig. 1(a)) shows intense band at 1587 cm− 1, which is assigned to the
asymmetric stretching vibrations of the carboxylate group. The
presence of this band and the complete absence of C_O stretching
band of unionized carboxylic acid at ∼ 1700 cm− 1 show that the asdeposited multilayer consists of CuA and not a mixture of arachidic
acid and salt. The bands at ∼ 1470 cm− 1, ∼ 1413 cm− 1 and
∼ 1315 cm− 1 are assigned to CH2 scissoring, carboxylate symmetric
stretch and CH2 wagging modes of arachidate salt, respectively [15].
The relative intensities of the bands at 1587 cm− 1 and 1702 cm− 1
(Fig. 1(b)–(f)) show that as the subphase pH value decreases, the CuA
content of the multilayers is reduced and that of arachidic acid is
enhanced. The splitting of CH2 scissoring band into a doublet further
Fig. 1. FTIR transmission spectra of CuA multilayers in the range 1350–
1750 cm− 1, transferred at subphase pH of (a) 5.7, (b) 5.55, (c) 5.4, (d) 5.2, (e) 5.0
and (f) 4.6.
Fig. 2. XR patterns of 25 ML as-deposited CuA multilayers on quartz substrates
transferred at subphase pH of (a) 5.7, (b) 5.5, (c) 5.25, (d) 5.0 and (e) 4.6.
supports the enhanced content of arachidic acid in the multilayers
transferred at lower subphase pH [15].
Fig. 2(a)–(e) shows the XR patterns of CuA multilayers on quartz
substrates transferred at different subphase pH values from 5.7 to 4.6.
The XR patterns of multilayers transferred at different subphase pH
show the presence of single layered structure in all the multilayers. The
multilayer transferred at pH 5.7 (Fig. 2(a)) shows Bragg peaks
corresponding to bilayer period of 52.5 Å. As the subphase pH is
decreased to 4.6 (Fig. 2(b)–(e)), the bilayer period is found to
continuously decrease from ∼ 52.5 Å to ∼ 47 Å, the latter value
corresponding to that of pure arachidic acid multilayers (not shown
here). The intensities of the Bragg peaks are also found to diminish
with decreasing subphase pH. These observations suggest mixing of
CuA and AA molecules at molecular level and decrease of copper
content with decrease in subphase pH.
Fig. 3 shows the transmission spectra of the oxidized CuA
multilayers on quartz substrates, transferred at different subphase pH
values. The multilayers subjected to oxidation at 300 °C show
absorption edge at ∼ 850 nm (∼ 1.45 eV) in all the cases, which is
attributed to the formation of CuO [16]. The absorption of the
Fig. 3. Transmission spectra of CuO obtained by oxidation at 300 °C of 25 ML
CuA transferred at subphase pH of (a) 5.7, (b) 5.5, (c) 5.25, (d) 5.0 and (e) 4.6.
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Fig. 4. AFM images of CuO clusters obtained by oxidation at 300 °C of 25 ML CuA transferred at subphase pH of (a) 5.7, (b) 5.5, (c) 5.25, (d) 5.0 and (e) 4.6. The
average sizes of the clusters are 1.5 ± 0.2, 3.2 ± 0.4, 1.3 ± 1.2, 1.0 ± 0.8 and 0.9 ± 0.5 μm and the height ranges are 50–150 nm, 50–100 nm, 30–50 nm, 30–50 nm and
10–30 nm, corresponding to (a), (b), (c), (d) and (e) respectively. (f) Shows a typical zoomed-in (1 μm × 1 μm) image of a CuO cluster along with its height profile in
the inset.
completely oxidized films decreases with the increase in arachidic acid
content in the precursor multilayer, as expected.
AFM studies have been carried out to investigate the surface
morphology of the multilayers after oxidation. These studies show that
the oxidized films consist of clusters of nanocrystallites. The lateral
size and size distribution of clusters as well as their heights depend
strongly on the subphase pH at which the precursor multilayer was
transferred. Fig. 4(a)–(e) shows the AFM images of the oxidized films
obtained from precursor CuA multilayers consisting of 25 monolayers
(ML), transferred at subphase pH values of 5.7, 5.5, 5.25, 5.0 and 4.6,
respectively. CuO obtained from the precursor transferred at pH 5.7
consists of clusters with mean size of 1.5 ± 0.2 μm (Fig. 4(a)). A few
clusters as large as 15 μm and a very large number of clusters as small
as ∼ 0.1 μm are observed. The heights of the larger clusters range
between 100 and 150 nm, while those of the smaller clusters are
∼50 nm. It may be noted that the thickness of the precursor multilayer
was ∼ 65 nm. This indicates that during melting of arachidate, the
droplets get accumulated to larger heights on the substrate surface due
to interfacial tension. As shown in Fig. 4, with decrease in subphase
pH, the mean size of CuO clusters decreases. Their heights also
decrease monotonically to well below 50 nm. It is also noticed that
there is a reduction in the number of large sized clusters as well as a
decrease in the maximum cluster size as the subphase pH is decreased.
Thus for the case of the precursor multilayer deposited at subphase pH
of 4.6 (Fig. 4(e)), the average cluster size is found to be 0.9 ± 0.5 μm,
with the heights in the range of 10–30 nm. The significant decrease in
the average size and heights of clusters is attributed to the decrease in
density of copper atoms available for oxidation in the precursor. A
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Fig. 5. AFM images of CuO nanocrystallites obtained by oxidation of CuA multilayers transferred at subphase pH 4.6: (a) 15 ML oxidized at 300 °C, (b) 15 ML
oxidized at 500 °C, (c) and (d) 15 ML oxidized at 700 °C (at different scales), (e) 11 ML CuA oxidized 700 °C, (f) 7 ML CuA oxidized at 700 °C and (g) and (h) 3 ML
CuA oxidized at 700 °C (at different scales). The insets (wherever given) show the size distributions of nanocrystallites and the second inset in (f) shows the height
profile of a nanocrystallite.
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zoomed-in image of a typical cluster in Fig. 4(f) along with its height
profile shows that it consists of an agglomerate of a large number of
nanocrystallites.
For further studies at higher temperatures, CuA multilayers
transferred at subphase pH 4.6 have been taken up. Typical results
for 15 ML CuA transferred on quartz substrates and subsequently
oxidized at temperatures of 300 °C, 500 °C and 700 °C are shown in
Fig. 5. Fig. 5(a) shows a 20 μ × 20 μ image of 15 ML CuA oxidized at
300 °C. This image shows clusters with mean size of 0.35 ± 0.28 μm
and maximum size of 1.3 μm. The size distribution is shown as inset in
Fig. 5(a). When a similar CuA multilayer is oxidized at 500 °C, a large
number of small clusters of size b 200 nm appear (Fig. 5(b)). The mean
size reduces to 0.20 ± 0.16 μm, but large clusters of size 1.5 μm are also
observed. The heights of the clusters in both the above cases are in the
range of 10–20 nm. Fig. 5(c) shows a similar CuA multilayer oxidized
at 700 °C. This image shows a large number of very small
nanocrystallites. To analyze the cluster size a zoomed-in image is
shown in Fig. 5(d). Selected area image analysis shows the sizes of the
crystallites to be 20–40 nm. However, due to large roughness of quartz
substrates the image analysis over complete image could not be
performed and reliable height profiles could not be obtained.
In order to obtain crystallites with smaller size and density, the
copper content of the precursor was further decreased by reducing the
number of monolayers. Multilayers with 3, 7 and 11 monolayers have
been transferred at subphase pH of 4.6 on freshly cleaved mica
substrates and oxidized at 700 °C. Fig. 5(e) shows an AFM image of a
CuA multilayer (11 ML) oxidized at 700 °C. This image shows isolated
nanocrystallites of average size 28 ± 4 nm, with corresponding heights
of ∼1.5 nm. The size distribution of the nanocrystallites is shown in the
inset. It shows that the sizes of most of the nanocrystallites are in a
narrow range of 20–35 nm. Fig. 5(f) shows the AFM image of CuA
(7 ML) oxidized at 700 °C. This image shows uniformly distributed
isolated nanocrystallites of average size 20 ± 7 nm, as seen in the inset
of Fig. 5(f). The corresponding heights are found to be ∼1 nm (shown
in another inset). The AFM images of CuA (3 ML) oxidized under
similar conditions are shown in Fig. 5(g) and (h). The images show the
presence of a large number of nanocrystallites of size 2–10 nm along
with a few large clusters ∼ 50 nm. This may be attributed to the
aggregation of closely formed large number of nanocrystallites.
4. Conclusions
Isolated CuO nanocrystallites with narrow size distribution
were obtained by optimizing the copper content of precursor
CuA LB multilayers through a control of the subphase pH and
number of monolayers as well as the oxidation temperature. The
CuO cluster size and heights were found to reduce by
decreasing the subphase pH and their size distribution became
narrower. Further reduction in sizes and heights of clusters was
achieved by reducing the number of monolayers and increasing
the oxidation temperature. There were drastic changes in the
size distribution and the heights of the CuO clusters after
oxidation at higher temperatures (∼ 700 °C). The average size of
CuO nanocrystallites was found to decrease as the number of
precursor CuA monolayers was reduced. 7 ML and 11 ML CuA
multilayers were found to be the optimum precursors for
obtaining isolated CuO nanocrystallites with narrow size
distribution. Typically, with a 7 ML precursor, CuO nanocrystallites with average size ∼ 20 nm and height ∼ 1 nm were
obtained.
Acknowledgements
One of the authors (Sukhvinder Singh) is thankful to CSIR,
New Delhi for Senior Research Fellowship. FIST (Physics)IRCC Central SPM Facility of IIT Bombay is acknowledged for
providing the facilities for AFM studies.
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