Hydrothermal Synthesis and Characterization of Cucro2 Laminar

ARTICLE IN PRESS

Journal of Crystal Growth 310 (2008) 5375–5379

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/jcrysgro

Hydrothermal synthesis and characterization of CuCrO2 laminar nanocrystals Shu Zhou a, Xiaodong Fang a,Ã, Zanhong Deng a,DaLia, Weiwei Dong a, Ruhua Tao a, Gang Meng a, Tao Wang a, Xuebin Zhu b a Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China b Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China article info abstract

Article history: CuCrO2 laminar nanocrystals were successfully synthesized by a simple hydrothermal technique. The Received 15 July 2008 nanocrystals have a quasi-hexagonal shape with a wide range of size distribution varying from about 10 Received in revised form to 80 nm and thickness less than 10 nm. The results of selected-area electron diffraction (SAED) data and 19 September 2008 high-resolution transmission electron microscopy (HRTEM) indicate the formation of well-formulated, Accepted 24 September 2008 single-crystal CuCrO . The dominant growth of nuclei is driven along the faces of {0 1 2}, and the growth Communicated by H. Fujioka 2 Available online 14 October 2008 along the c-axis is very slow, which directly results in the formation of CuCrO2 laminar nanocrystals. Optical properties of CuCrO2 laminar nanocrystals were investigated by UV–vis–Nis absorption PACS: spectrum, and direct bandgap was estimated to be 2.95 eV. 61.46.Df & 2008 Elsevier B.V. All rights reserved. 78.67.Bf 81.16.Be

Keyword: A2. Hydrothermal

B1. CuCrO2 B1. Delafossite B2. Laminar nanocrystals

1. Introduction p-type conducting oxides [14]. From the papers available so far, various research groups are working on the synthesis of In recent years, the study of nanostructural materials have delafossite structural materials and also on their applications. attracted great interests because of their widespread potential Shannox [15] has reported the synthesis and properties of ABO2 application, including photonics, nanoelectronics, information delafossite compounds, Sheets et al. [16] have reviewed the storage devices and sensors [1–3]. Up to now, various nanos- synthesis of delafossite-type oxides by hydrothermal. Zhao et al. tructural materials [4–7] such as nanoparticles, nanowires, [17] reported the synthesis of single-crystal CuFeO2 by applying nanotubes and nanodisks of metals, semiconductors and insula- the traveling solvent ﬂoating-zone technique. Kawazoe et al. [14] tors have been prepared by various approaches such as vapor reported the synthesis of delafossite CuAlO2 p-type transparent transport methods, thermal deposition, laser-assist catalytic conducting thin ﬁlm by pulsed laser deposition (PLD). CuCrO2 growth and solution routine. The hydrothermal method as a with delafossite structure as the transparent p-type conducting powerful tool for synthesizing of nanomorphologies has several also shows low resistivity at room temperature (RT). When doped outstanding characteristics such as employment of water as with 5% Mg, it has the lowest p-type resistivity in the delafossite solvent which is friendly with environment, some quite un- oxides group of ABO2 (A=Cu or Ag, B=Al, Cr, Sc, Y, etc.). It has direct expected reactions will take place under special conditions, optical bandgap of 3.1 eV. Moreover, it is potentially diluted morphologies can usually be controlled by varying the parameters magnetic semiconductor and RT thermoelectric material [18]. in the reaction system [8,9]. A lot of effort has been devoted to the investigation of CuCrO2,

Delafossite structural ABO2 (A=Cu or Ag, B=Al, Cr, Sc, Y, etc.) polycrystalline CuCrO2 has been synthesized by techniques such have been studied intensely due to their application as catalysts as solid-state reaction [19], flux decomposition [20] and hydro- [10,11], luminescent materials [12], batteries [13] and transparent thermal method [14]. Thin films of CuCrO2 have been prepared by several methods including PLD [21] and rf sputtering [22]. For the synthesis of nanostructural delafossite oxides, nanostructural thin ÃCorresponding author. Tel.: +86 5515593508; fax: +86 5515593527. films have been made such as chemical vapor deposition [23] and E-mail address: [email protected] (X. Fang). sputtering [24], a relatively recent report has investigated the

5376 S. Zhou et al. / Journal of Crystal Growth 310 (2008) 5375–5379

synthesizing of CuAlO2 nanoparticles by biosynthesis [25] and it used fungus as the reducing agent. But these works merely focused on the synthesizing nanostructural of CuAlO2. To the best of our knowledge, few reports about synthesizing CuCrO2 nanocrystals. In this paper, we propose a facile hydrothermal processing for one-step synthesis of CuCrO2 laminar nanocrystals.

2. Experimental section

All chemical reagents in this experiment were of analytical grade and used without further puriﬁcation. Cuprous oxide

(Cu2O), chromium nitrate (Cr(NO3)3 9H2O) and sodium hydro- xide (NaOH) were purchased from Alfa Aesar. In the synthetic procedure, 6 mmol of Cr(NO3)3 9H2O was dissolved in 28 ml deionized water with magnetic stirring, followed by the addition Fig. 1. XRD patterns of the as-formed products: (a) CuCrO2 laminar nanocrystals, of 18 mmol NaOH and 3 mmol Cu2O with magnetic stirring. After (b) CuCrO2 bulk powders, and (c) the standard data for CuCrO2 (JPCDS 89-0539). stirring for 15 min, NaOH was introduced into the above solution until the concentration of NaOH was 2.5 M. Then the as-obtained mixed solution was transferred into a 40 ml Teﬂon-lined autoclave, which was sealed and maintained at 220 1C for 60 h. After that, the autoclave was cooled down to RT naturally. The precipitate was ﬁltered and washed with absolute alcohol and deionized water in sequence for several times. The product was dried in an oven at 80 1C for 8 h. The powder was collected.

Additionally, bulk CuCrO2 was synthesized by sol–gel according to the literature for comparing with the nanocrystals [21]. The powder X-ray diffraction (XRD) was performed on a MXP18 AHF X-ray diffractometer equipped with graphite-mono- chromatized Cu KR X-ray (l=1.5418 A˚ ) with a scanning rate of 1 81 min . The morphology of CuCrO2 laminar nanocrystals were observed by using a JEOL JSM-6330F ﬁeld emission scanning electron microscope (FE-SEM). Transmission electron microscopy Fig. 2. Plot of b cos y/l versus sin y/l for the samples. (TEM) and selected-area electron diffraction (SAED) analyses were performed on a JEOL 2010 high-resolution transmission electron relation [26]: microscope (HRTEM). The specimens for TEM observations were prepared by sonicating the synthesized materials in absolute b cos y=l ¼ 1=D þ sin y=l ethanol, a droplet was dispersed on a TEM microgrid and dried in where b is the measured FWHM in radians, y is the Bragg angle of air before observation. The X-ray photoelectron spectrometer the diffraction peak, l is the X-ray wavelength, A plot of b cos y/l (XPS) was collected on a PHI Quantum 2000 XPS system. RT versus sin y/l for the samples is shown in Fig. 2. There may be UV–vis–Nis spectroscopy was recorded on a UV-2550 spectro- some nonuniform strain and departure from uniform shape along photometer in the wavelength range of 200–1500 nm. All the the different crystallographic orientations. The valence state of Cu measurements were performed at RT. and Cr were investigated by the XPS, especially the binding energies of Cu 2p and Cr 2p were carefully inspected because different states of copper and chromium have different electron 3. Results and discussion binding energies. As shown in Fig. 3a, the peak position located at 932.47 and 952.4 eV, which is corrected with reference to C1s 3.1. Structure and morphology (284.6 eV), corresponding to the binding energies of Cu 2p3/2 and Cu 2p1/2, respectively, and are the standard peaks of Cu1+ which Phase purity of the sample was examined by the XRD pattern. conﬁrms the monovalent state of copper in the sample [27],

As shown in Fig. 1a, all the peaks can be indexed to CuCrO2 pure CuLMM peaks is shown in Fig. 3c. The signal of CuLMM peaks is phase (space group: R-3m (16 6); JCPDS # 89-0539). The relative closed to 917 eV, which is coincidence with the peak of Cu1+, stronger diffraction peaks at 31.31, 36.41 and 62.31 are further conﬁrms the monovalent state of copper in the sample. In corresponding to (0 0 6), (0 12) and (110) planes of CuCrO2, Fig. 3b, the peak located near 576.2 eV, corresponding to the 3/2 respectively. No peaks corresponding to CuO, Cu2O, Cr2O3 families binding energy of Cr2p , and conﬁrms the trivalent state of can be detected. The unit-cell parameters were calculated with chromium element in the sample [28]. Therefore, the results of CelRef, and obtained values are a=b=2.9746 A˚ , c=17.0952 A˚ , which XPS are further evidence for the purity and composition of the are in good agreement with the reference code: JCPDS 89-0539 product.

(a=b=2.9734 A˚ , c=17.1000 A˚ ). Compared with the XRD pattern of In Fig. 4a and b shows the SEM images of the CuCrO2 the bulk pure powder (Fig. 1b), it is noticed that the full-width at nanocrystals at low and high magniﬁcation, it is obvious that half-maximum (FWHM) in Fig. 1a are obviously broadened, which the product consists of laminar nanocrystals with similar is a typical feature of nanostructural materials. The information morphology, it has a wide range of size distribution varying on strain and the particle size of the nanocrystals could be from 10 to 80 nm and thickness of laminar nanocrytals less than obtained from the FWHM of the diffraction peaks. The FWHMs 10 nm. The structure and morphology were further investigated can be expressed as a linear combination of the contributions by TEM. Fig. 5a shows the TEM image of CuCrO2 laminar from the strain (e) and the particle size (D) through the following nanocrystals. From the image, it can be seen very thin laminar ARTICLE IN PRESS

S. Zhou et al. / Journal of Crystal Growth 310 (2008) 5375–5379 5377

Fig. 3. (a) XPS of Cu 2p level of the sample of CuCrO2 nanocrystals, (b) XPS of Cr2p of CuCrO2 nanocrystals and (c) Cu LMM.

well-formulated, single-crystal CuCrO2.InFig. 5c, a HRTEM image obtained from a portion of an individual CuCrO2 laminar nanocrystal is displayed in order to further conﬁrm the single-

crystal nature of our synthesized CuCrO2 samples. This image shows a clear lattice fringes domain with interplanar spacings of

about 2.457 A˚ , compatible with literature values for bulk CuCrO2 of 2.46 A˚ , (JCPDS no. 89-0539). The lattice spacings described above correspond to the {0 12} planes of a rhombohedral phase

CuCrO2 crystal, which is in agreement with XRD pattern. No obvious defects or dislocations were observed from the HRTEM

image of individual CuCrO2 laminar nanocrystals.

3.2. Formation mechanism

From the above experiment results, a possible formation

mechanism for the synthesis of CuCrO2 laminar nanocrystals is suggested as the following. As we know, the evolution of a solid from a solution phase involves two steps in crystallization: nucleation and growth [29], so a good understanding solubility 3+ for Cu2O and Cr under present conditions helps to understand the formation mechanism of CuCrO2 laminar nanocrystals. At RT, + Cu2O does not dissolve in aqueous solution to form the stable Cu . At elevated temperatures and under basic conditions, however, Cu2O dissolves to form the stable, aqueous soluble Cu(OH)2 species [30,31], and then concentrations of aqueous soluble 3+ Cu(OH)2 increases at elevated temperature. As for Cr , in aqueous solution, Cr3+ ions in alkaline conditions are stabilized by the formation of the aqueous soluble Cr(OH)4 species [32] and the concentrations of aqueous soluble Cr(OH)4 increase at elevated temperatures [33]. In the present synthetic procedure, ﬁrstly,

Fig. 4. SEM images of the CuCrO2 laminar nanocrystals with different magniﬁca- Cu2O and Cr(NO3)3 dissolved to form the stable, aqueous soluble tion. Cu(OH)2 and Cr(OH)4 species, and then hydrothermal conditions and excessive NaOH guarantees that the concentration of Cu(OH)2 nanocrystals and some laminar stack together, SAED data (Fig. 5b) and Cr(OH)4 species reach a critical supersaturation, resulting in taken from individual laminar nanocrystal show the presence of eruptible nucleation of CuCrO2. After the nuclei are formed from sharp diffraction spots, which are indicative of the formation of the solution, they grow via molecular addition, which relieves the ARTICLE IN PRESS

5378 S. Zhou et al. / Journal of Crystal Growth 310 (2008) 5375–5379

Fig. 5. TEM images of CuCrO2 laminar nanocrystals: (a) TEM image, (b) a selected area electron diffraction of a single laminar nanocrystal, (c) HRTEM image showing a single nanocrystal and (d) A single laminar nanocrystal.

supersaturated step. When the concentration of Cu(OH)2– and Cr(OH)4– drops below the critical level, nucleation stops and the particles continue to grow by molecular addition until the equilibrium concentration of species is reached, after that the

CuCrO2 laminar nanocrystals were believed to grow via Ostwald ripening in which large crystals grow at the expense of small ones, leading to the observed wide-size distribution. In the growth processing, from SEM and TEM images, CuCrO2 nanocrystals have low aspect ratio, it maybe the face along with the c-axis grows slowly because of anisotropic bonding in the CuCrO2 structure. From the Fig. 4c, it can be seen from HRTEM image the atoms arrange regularly to form a sixfold symmetric projected structure. Along these six symmetric directions indicated in Fig. 4(c), the d spacing is 0.246 nm, which corresponds to that of {0 12} planes. This indicates that the laminar nanocrystals grow mainly the six faces of (0 12), (10 2), (1¯ 0 2), (0 1¯ 2), (1¯ 12) and (11¯ 2), the growth along in c-axis was very slowly, so the higher growth rate of {0 12} induce the shrinking of the six {0 12} faces, while

(0 0 l) faces remained to formed quasi-hexagonal CuCrO2 laminar nanocrystals.

3.3. Optical property of the samples

To the best of our knowledge, few researchers have been working on the optical properties of CuCrO2 nanocrystals. The UV–vis–Nis absorption spectrum of CuCrO2 laminar nanocrystals is shown in Fig. 6a. As can be seen, there is broad absorption band in the range of UV area and weak absorption in the Nis area. According to the equation ( h )2=K(h E ) where is the a n n g a Fig. 6. (a) UV–vis–Nis absorption spectrum of CuCrO2 laminar nanocrystals and absorption coefﬁcient, K is constant, hn is the discrete photo (b) the corresponding (ahn)2 versus hn curve. ARTICLE IN PRESS

S. Zhou et al. / Journal of Crystal Growth 310 (2008) 5375–5379 5379

energy, and Eg is the bandgap energy. Tauc approach is employed References to estimate the bandgap of CuCrO2 laminar nanocrystals. The plot 2 of (ahn) versus hn based on the direct transition is shown in [1] M. Fiebig, T. Lottermoser, D. Fro¨hlich, A.V. Goltsev, R.V. Pisarev, Nature 419 (2002) 818. Fig. 6b. It can be seen that the bandgap of CuCrO2 laminar [2] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. nanocrystals is close to 2.95 eV. This value is slightly smaller than [3] H. Postma, A. Sellmeijer, C. Dekker, Adv. Mater. 17 (2000) 1299. the value of literature reported [21,22]. For the red-shift of Eg,it [4] Z.R. Dai, Z.W. Pan, Z.L. Wang, Adv. Funct. Mater. 13 (2002) 9. could be a result of an interfacial polaron effect arising from [5] Y.G. Yan, Y. Zhang, G.W. Meng, L.D. Zhang, J. Crystal Growth electron–phonon coupling. In principle, the electron–phonon (2006) 184. [6] X. Duan, Y. Huang, R. Agarwal, C.M. Lieber, Nature 421 (2003) 241. coupling coefficients are proposed to increase with decreasing [7] D.P. Yu, Q.L. Huang, Y. Ding, H.Z. Zhang, Z.G. Bai, J.J. Wang, Y.H. Zhou, Appl. size of semiconductors [34]. In certain systems, electron–phonon Phys. Lett. 73 (1998) 3076. coupling could be strong enough to overcome the spatial [8] G.F. Zou, H. Li, Y.J. Zhang, K. Xiong, Y.T. Qian, Nanotechnology 17 (2006) 313. [9] K. Byrappa, T. Adschiri, Prog. Cryst. Growth Charact. Mater. 53 (2007) 117. confinement to determine the energy of excitons. It determines [10] J.R. Monnier, M.J. Hanrahan, G.J. Apai, J. Catal. 92 (1985) 119. or modifies the effective mass of carriers and the style of carrier [11] Y. Bessekhouad, M. Trari, J.P. Doumerc, Int. J. Hydrogen Energy 28 (2003) 43. scattering by the lattice, leading to a red-shift of the bandgap. This [12] J.P. Doumerc, C. Parent, Z.J. Chao, G. Le Flem, Ammar, A.J. Le Flem, Less- Common Met. 148 (1989) 333. phenomenon has been observed in some semiconductor [13] T. Nagaura, Prog. Batteries Sol. Cells 4 (1982) 105. nanocrystals [35]. This may explain the red-shift of Eg. [14] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, Nature 389 (1997) 939. [15] R.D. Shannox, D.B. Rogers, C.T. Prewitt, Inorg. Chem. 10 (1971) 713. 4. Conclusion [16] W.C. Sheets, E. Mugnier, A. Barnabe, T.J. Marks, K.R. Poeppelmeier, Chem. Mater. 18 (2006) 7. [17] T.R. Zhao, M. Hasegawa, H. Takei, J. Crystal Growth 154 (1995) 322. [18] T. Okuda, N. Jufuku, S. Hidaka, N. Terada, Phys. Rev. B 72 (2005) 144403. In summary, we have first synthesized CuCrO2 laminar [19] W. Dannhauser, P.A. Vaughan, J. Am. Chem. Soc. 77 (1954) 896. nanocrystals by a single step, low-temperature hydrothermal [20] O. Crottaz, F. Kubel, H. Schmid, J. Solid State Chem. 122 (1996) 247. method. XRD spectrum confirms the purification of CuCrO2 phase [21] D. Li, X.D. Fang, Z.H. Deng, S. Zhou, X.B. Zhu, J. Phys. D 40 (2007) 4910. and small size (38 nm) of the crystals. XPS indicates the valence [22] R. Nagarajan, A.D. Draeseke, A.W. Sleight, J. Tate, J. Appl. Phys. 89 (2001) state of copper and chromium corresponding to univalency and 8022. [23] H. Gong, Y. Wang, Y. Luo, Appl. Phy. Lett. 76 (2000) 3959. trivalency, respectively. SEM and SAED show the morphology of [24] A.N. Banerjee, K.K. Chattopadhyay, J. Appl. Phys. Lett. 97 (2005) 084308. CuCrO2 nanocrystals are laminar crystals and individual laminar is [25] A. Ahmad, T. Jagadale, V. Dhas, S. Ogale, Adv. Mater. 19 (2007) 3295. single crystal, HRTEM indicates the higher growth rate faces of [26] P.l. Nowakowskia, J.P. Dallasa, J.R. Gavarri, J. Solid State Chem. 181 (2008) 1005. {0 12} induce the shrinking of the six {0 12} faces, while {0 0 l} [27] W.T. Wu, Y.S. Wang, L. Shi, W.M. Pang, Q.R. Zhu, G.Y. Xu, F. Lu, J. Phys. Chem. B faces remained to form quasi-hexagonal CuCrO2 laminar nano- 110 (2006) 14702. crystals. Comparing with past literature reported, UV–vis absorp- [28] D.H. Guo, Q.L. Guo, M.S. Altman, E.G. Wang, J. Phys. Chem. B 109 (2005) 20968. tion spectrum shown slightly smaller red-shift of the bandgap. [29] C. Burda, X.B. Chen, R. Narayanan, A. Mostafa, Chem. Rev. 105 (2005) 1025. [30] L.N. Varbyash, V.I. Rekharskii, Geokhimiya (1981) 683. [31] B. Beverskog, I. Puigdomenech, J. Electrochem. Soc. 144 (1997) 3476. Acknowledgments [32] J.L. Allen, K.R. Poeppelmeier, Polyhedron 13 (1994) 1301. [33] B. Beverskog, I. Puigdomenech, Corros. Sci. 39 (1997) 43. [34] K. Oshiro, K. Akai, M. Matsuura, Phys. Rev. B 59 (1999) 10850. We gratefully acknowledge financial support from the National [35] B.S. Zou, Asian J. Spectrosc. 6 (2002) 1. Natural Science Foundation of China (No. 50672097) and the Hundred Talents Program of the Chinese Academy of Science.