07) 3224–3228www.elsevier.com/locate/matlet

Materials Letters 61 (20

CuO nanocrystallites obtained by oxidationof copper arachidate LB multilayers

Sukhvinder Singh a, R.S. Srinivasa b, S.S. Talwar a, S.S. Major a,⁎

a Department of Physics, Indian Institute of Technology Bombay, Mumbai-400076, Indiab Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai-400076, India

Received 11 August 2006; accepted 8 November 2006Available online 27 November 2006

Abstract

Copper arachidate (CuA) Langmuir–Blodgett (LB) multilayers were transferred in the subphase pH range 4.6–5.7. FTIR studies of multilayersshow a gradual increase in CuA content as subphase pH is increased. X-ray reflection patterns of multilayers show presence of a single layeredstructure in all the multilayers transferred at different subphase pH. A continuous decrease in bilayer period from 52.5 Å to 47 Å is seen withdecrease in CuA content of the multilayers. These observations suggest mixing of CuA and arachidic acid molecules at the molecular level. Theprecursor CuA multilayers were oxidized at 300 °C–700 °C. The formation of CuO is confirmed by UV–Visible spectroscopy. Atomic forcemicrographs show the formation of CuO nanocrystallites and their clusters, with the average size, size distribution, height and density ofnanocrystallites depending strongly on subphase pH, number of monolayers and oxidation temperature. Typically, 7–11 monolayers CuAtransferred at subphase pH of 4.6 and oxidized at 700 °C resulted in isolated and nearly mono-disperse nanocrystallites of size 20–30 nm andheight ∼1 nm. Crystallites of size 2–10 nm along with few clusters were obtained by oxidation of a 3 monolayer CuA.© 2006 Elsevier B.V. All rights reserved.

Keywords: Nanomaterials; Langmuir–Blodgett films; Atomic force microscopy

1. Introduction

Cupric oxide (CuO) is a transition metal oxide withmonoclinic unit cell and square planar coordination. It hasbeen under active study owing to its structural similarity to highTc cuprate superconductors as well as for photoconductive,photo-thermal and photo-electrochemical applications. Recent-ly, it has also been investigated for gas sensing applications[1,2]. Nanostructured CuO has received attention for a varietyof applications such as gas sensors [3], electrodes in lithiumcells [4], field emitters [5,6] and magnetic storage media [7].Owing to this interest, the formation of CuO has been reportedin various low dimensional forms such as nanoparticles [8],mono-disperse nanocrystals [9], nanowires [6], nanotubes andnanorods [10–12].

⁎ Corresponding author. Tel.: +91 22 25767567; fax: +91 22 25767552.E-mail address: syed@phy.iitb.ac.in (S.S. Major).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.11.043

Oxidation of precursor Langmuir–Blodgett (LB) multilayershas been used to form ultrathin and homogenous metal oxidefilms. In most of the cases the precursor LB multilayer is heattreated with UV radiation in air, resulting in oxide formation andremoval of organic components. Using this method, ultrathinfilms of cupric oxide (CuO) were formed from precursor copperarachidate LB multilayer [13]. In a subsequent work, single-phase nanocrystalline CuO films have been reported by directoxidation of copper arachidate multilayers [14]. In the presentwork, the direct oxidation method has been exploited to formuniformly distributed, mono-disperse nanocrystallites of CuO.

2. Experimental details

LB multilayers of CuA were prepared in a KSV 3000 LBtrough. Arachidic acid dissolved in chloroform was spread onan aqueous subphase containing CuCl2. Dilute HCl and NaOHsolutions were added to achieve and maintain the desiredsubphase pH values. The substrates used for deposition of

Fig. 2. XR patterns of 25 ML as-deposited CuA multilayers on quartz substratestransferred at subphase pH of (a) 5.7, (b) 5.5, (c) 5.25, (d) 5.0 and (e) 4.6.

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multilayers were quartz, CaF2 and mica. The as-deposited CuAmultilayers were oxidized in a stream of oxygen at atmosphericpressure in the temperature range 300 °C–700 °C. FTIR spectrawere obtained with a Perkin Elmer Spectrum1 instrument andUV–Vis spectra with a Shimadzu UV-160A spectrophotometer.X-ray reflection (XR) studies were performed with PANalyticalX'Pert PRO diffractometer using Cu Kα radiation. Atomicforce microscopy (AFM) studies were carried out using DigitalInstruments Nanoscope IV SPM.

3. Results and discussion

The composition of CuA multilayers transferred on CaF2 substrateat subphase pH values in the range 4.6 to 5.7 has been studied by FTIRspectroscopy. 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 theasymmetric stretching vibrations of the carboxylate group. Thepresence of this band and the complete absence of C_O stretchingband of unionized carboxylic acid at ∼1700 cm− 1 show that the as-deposited multilayer consists of CuA and not a mixture of arachidicacid and salt. The bands at ∼1470 cm− 1, ∼1413 cm− 1 and∼1315 cm− 1 are assigned to CH2 scissoring, carboxylate symmetricstretch 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 CuAcontent of the multilayers is reduced and that of arachidic acid isenhanced. 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) 5.0and (f) 4.6.

supports the enhanced content of arachidic acid in the multilayerstransferred at lower subphase pH [15].

Fig. 2(a)–(e) shows the XR patterns of CuA multilayers on quartzsubstrates transferred at different subphase pH values from 5.7 to 4.6.The XR patterns of multilayers transferred at different subphase pHshow the presence of single layered structure in all the multilayers. Themultilayer transferred at pH 5.7 (Fig. 2(a)) shows Bragg peakscorresponding to bilayer period of 52.5 Å. As the subphase pH isdecreased to 4.6 (Fig. 2(b)–(e)), the bilayer period is found tocontinuously decrease from ∼52.5 Å to ∼47 Å, the latter valuecorresponding to that of pure arachidic acid multilayers (not shownhere). The intensities of the Bragg peaks are also found to diminishwith decreasing subphase pH. These observations suggest mixing ofCuA and AA molecules at molecular level and decrease of coppercontent with decrease in subphase pH.

Fig. 3 shows the transmission spectra of the oxidized CuAmultilayers on quartz substrates, transferred at different subphase pHvalues. The multilayers subjected to oxidation at 300 °C showabsorption edge at ∼850 nm (∼1.45 eV) in all the cases, which isattributed to the formation of CuO [16]. The absorption of the

Fig. 3. Transmission spectra of CuO obtained by oxidation at 300 °C of 25 MLCuA transferred at subphase pH of (a) 5.7, (b) 5.5, (c) 5.25, (d) 5.0 and (e) 4.6.

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. Theaverage 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 and10–30 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 inthe inset.

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completely oxidized films decreases with the increase in arachidic acidcontent in the precursor multilayer, as expected.

AFM studies have been carried out to investigate the surfacemorphology of the multilayers after oxidation. These studies show thatthe oxidized films consist of clusters of nanocrystallites. The lateralsize and size distribution of clusters as well as their heights dependstrongly on the subphase pH at which the precursor multilayer wastransferred. Fig. 4(a)–(e) shows the AFM images of the oxidized filmsobtained 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 5.7consists of clusters with mean size of 1.5±0.2 μm (Fig. 4(a)). A fewclusters as large as 15 μm and a very large number of clusters as smallas ∼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 multilayerwas ∼65 nm. This indicates that during melting of arachidate, thedroplets get accumulated to larger heights on the substrate surface dueto interfacial tension. As shown in Fig. 4, with decrease in subphasepH, the mean size of CuO clusters decreases. Their heights alsodecrease monotonically to well below 50 nm. It is also noticed thatthere is a reduction in the number of large sized clusters as well as adecrease in the maximum cluster size as the subphase pH is decreased.Thus for the case of the precursor multilayer deposited at subphase pHof 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 inthe average size and heights of clusters is attributed to the decrease indensity of copper atoms available for oxidation in the precursor. A

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 MLoxidized 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 MLCuA 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 heightprofile of a nanocrystallite.

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zoomed-in image of a typical cluster in Fig. 4(f) along with its heightprofile shows that it consists of an agglomerate of a large number ofnanocrystallites.

For further studies at higher temperatures, CuA multilayerstransferred at subphase pH 4.6 have been taken up. Typical resultsfor 15 ML CuA transferred on quartz substrates and subsequentlyoxidized at temperatures of 300 °C, 500 °C and 700 °C are shown inFig. 5. Fig. 5(a) shows a 20 μ×20 μ image of 15 ML CuA oxidized at300 °C. This image shows clusters with mean size of 0.35±0.28 μmand maximum size of 1.3 μm. The size distribution is shown as inset inFig. 5(a). When a similar CuA multilayer is oxidized at 500 °C, a largenumber of small clusters of size b200 nm appear (Fig. 5(b)). The meansize reduces to 0.20±0.16 μm, but large clusters of size 1.5 μm are alsoobserved. The heights of the clusters in both the above cases are in therange of 10–20 nm. Fig. 5(c) shows a similar CuA multilayer oxidizedat 700 °C. This image shows a large number of very smallnanocrystallites. To analyze the cluster size a zoomed-in image isshown in Fig. 5(d). Selected area image analysis shows the sizes of thecrystallites to be 20–40 nm. However, due to large roughness of quartzsubstrates the image analysis over complete image could not beperformed and reliable height profiles could not be obtained.

In order to obtain crystallites with smaller size and density, thecopper content of the precursor was further decreased by reducing thenumber of monolayers. Multilayers with 3, 7 and 11 monolayers havebeen transferred at subphase pH of 4.6 on freshly cleaved micasubstrates and oxidized at 700 °C. Fig. 5(e) shows an AFM image of aCuA multilayer (11 ML) oxidized at 700 °C. This image shows isolatednanocrystallites of average size 28±4 nm, with corresponding heightsof∼1.5 nm. The size distribution of the nanocrystallites is shown in theinset. It shows that the sizes of most of the nanocrystallites are in anarrow range of 20–35 nm. Fig. 5(f) shows the AFM image of CuA(7 ML) oxidized at 700 °C. This image shows uniformly distributedisolated nanocrystallites of average size 20±7 nm, as seen in the insetof Fig. 5(f). The corresponding heights are found to be ∼1 nm (shownin another inset). The AFM images of CuA (3 ML) oxidized undersimilar conditions are shown in Fig. 5(g) and (h). The images show thepresence of a large number of nanocrystallites of size 2–10 nm alongwith a few large clusters ∼50 nm. This may be attributed to theaggregation of closely formed large number of nanocrystallites.

4. Conclusions

Isolated CuO nanocrystallites with narrow size distributionwere obtained by optimizing the copper content of precursorCuA LB multilayers through a control of the subphase pH andnumber of monolayers as well as the oxidation temperature. TheCuO cluster size and heights were found to reduce bydecreasing the subphase pH and their size distribution becamenarrower. Further reduction in sizes and heights of clusters wasachieved by reducing the number of monolayers and increasing

the oxidation temperature. There were drastic changes in thesize distribution and the heights of the CuO clusters afteroxidation at higher temperatures (∼700 °C). The average size ofCuO nanocrystallites was found to decrease as the number ofprecursor CuA monolayers was reduced. 7 ML and 11 ML CuAmultilayers were found to be the optimum precursors forobtaining isolated CuO nanocrystallites with narrow sizedistribution. Typically, with a 7 ML precursor, CuO nanocrys-tallites with average size ∼20 nm and height ∼1 nm wereobtained.

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 forproviding the facilities for AFM studies.

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