Hexagonal polytype of CuCrO2 nanocrystals obtained

J Nanopart Res (2012) 14:1110 DOI 10.1007/s11051-012-1110-3

RESEARCH PAPER

Hexagonal polytype of CuCrO2 nanocrystals obtained by hydrothermal method M. Miclau • D. Ursu • S. Kumar • I. Grozescu

Received: 11 March 2012 / Accepted: 31 July 2012 Springer Science+Business Media B.V. 2012

Abstract We had first synthesized hexagonal 2H-CuCrO2 nanocrystals by hydrothermal method. The stability diagram for Cu–Cr–Na–H2O system allows preparing of nanocrystalline CuCrO2, either 2H or 3R pure phase, by a single step, low-temperature hydrothermal synthesis. 2H- and 3R-CuCrO2 quasihexagonal shape nanocrystals have shown a wide range of size distribution varying from about 20 to 40 nm. An unexpected result is that 2H-CuCrO2 phase was a low temperature phase in the hydrothermal synthesis. Also, a formation mechanism for the synthesis of CuCrO2 nanocrystal, 2H- and 3R-polytypes was proposed. Keywords Nanocrystal 2H-polytype Delafossite Hydrothermal Thermoelectric

Introduction In the past decade, delafossite-type oxides CuMO2 have received considerable attention as promising candidates for a thermoelectric material (Singh et al. 2009; Kawazoe et al. 1997; Kumar et al. 2012). Delafossite-type oxide AMO2 is a p-type semiconductor oxide with interesting physical and chemical properties that make it suitable for various technological applications such as catalyst (Monnier et al. 1985), batteries (Nazri et al. 2000), and transparent p-type conducting oxides (Yanagi et al. 2001). The potential of a material for thermoelectric applications is determined in large part by a measure of the material’s figure of merit, ZT: ZT ¼

M. Miclau (&) D. Ursu S. Kumar I. Grozescu National Institute for Research and Development in Electrochemistry and Condensed Matter Timisoara, Str. Plautius Andronescu, No. 1, 300224 Timisoara, Romania e-mail: [email protected] D. Ursu Politehnica University of Timisoara, Pta Victoriei No. 2, 300006 Timisoara, Romania

a2 rT k

ð1Þ

where a is the Seebeck coefficient, r is the electrical conductivity, T is temperature, and j denotes total thermal conductivity (k = kL ? kE, the lattice and electronic contributions, respectively). Fundamental to the field of thermoelectric materials is the need to optimize a variety of conflicting properties. To maximize the thermoelectric figure of merit (ZT) of a material, a large thermopower (absolute value of the Seebeck coefficient), high electrical conductivity, and low thermal conductivity are required. Much of the recent interest in thermoelectrics materials from theoretical and experimental evidences of greatly enhanced ZT in nanostructured thin-films and wires

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due to enhanced Seebeck and reduced thermal conductivity. Also, the use of bulk (mm3) nanostructured materials would avoid detrimental electrical and thermal losses and use the existing fabrication routes. The challenge for any nanostructured bulk material system is electron scattering at interfaces between randomly oriented grains leading to a concurrent reduction of both the electrical and thermal conductivities (Sharma et al. 2010). One of delafossite-type oxides, CuCrO2 exhibits good thermoelectric properties like low resistivity, high thermopower, and low thermal conductivity. The delafossite structure can be visualized as consisting of two alternate layers: a planar layer of A cation in a triangular pattern and a layer of edge-sharing BO6 octahedra flattened with respect to the c-axis. Depending on the orientation of each layer in stacking, two crystalline forms can exist. By stacking the double layers with alternate A layers oriented 180 relative to each other, the hexagonal 2H type is formed which has P63/mmc space group symmetry. If the double layers are stacked with the A layers oriented in the same direction relative to one another but offset from each other in a three layer sequence, the rhombohedral 3R type is formed that has the space group symmetry of R3m (Marquardt et al. 2006). Previous studies had indicated that single crystals CuCrO2 could be prepared in either the 2H or 3R form. Metastable single crystal of 2H-CuCrO2 were prepared from 3R crystals which were rapidly heated to 1,100 C and then quenched into water. It should be noted that it was not possible to obtain a powder with a pure 2H phase, probably because of the insufficient quenching speed (Crottaz et al. 1996). In fact, it is challenging to find synthesis conditions that yield either 2H or 3R CuCrO2 nanocrystals in a pure form. Hydrothermal synthesis offers many advantages over conventional and nonconventional ceramic synthesis methods. The low reaction temperature avoids other problems encountered with high temperature processes such as poor stoichiometry control due to volatilization of components. Moreover, the ability to precipitate the powders directly from solution regulates the rate and uniformity of nucleation and growth which affords size, morphology, and aggregation control that is not possible with many synthesis processes (Yoshimura and Byrappa 2008). In the hydrothermal synthesis of 3R-CuCrO2 (Zhou et al. 2008; Sheets et al. 2006), no phase equilibria

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studies were reported so that we felt that phase equilibria studies would be particularly useful to obtain 2H-CuCrO2 nanopowder pure phase. To our knowledge, there have been no reports on 2H-CuCrO2 pure nanocrystals prepared by hydrothermal method. In order to discover conditions for the hydrothermal synthesis of 2H-CuCrO2 phase, we will construct the stability diagram for Cu–Cr–Na–H2O system with Cu2O, Cr(OH)3 as precursors and NaOH as a mineralizer from XRD results. We studied the influence of the concentration mineralizer, the temperature, the

Fig. 1 The UV absorption spectrum of Cr(OH)3 powders

Fig. 2 The stability diagram for Cu–Cr–Na–H2O system with Cu2O, Cr(OH)3 as precursors and NaOH as a mineralizer at 60 h


reaction time, and the ratio between Cu2O:Cr(OH)3 in the formation of desired phase, 3R or 2H.

Experimental The delafossite-type oxide of CuCrO2 nanocrystals was prepared by hydrothermal method. All of chemical reagents used in this experimental were of analytical grade. Cuprous oxide (Cu2O) and sodium hydroxide (NaOH) were purchased from SigmaAldrich. Cr(OH)3 amorphous was obtained from Cr(SO4)312H2O and NaOH. The powder obtained is light green and characterization of Cr(OH)3 was made by UV absorption spectra. The UV absorption

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spectrum of Cr(OH)3 (Fig. 1) shows two strong absorption peaks at 430 and 600 nm, most probably originating from the 4Ag2g ? 4T2g and 2Ag2g ? 2 T1g, 2E2g transition of metal Cr(III) ion in the crystal field, in accordance with the previous work (Xu et al. 2004). In typical synthesis process, the desired Cu2O and Cr(OH)3 quantities were dissolved in NaOH solution. The obtained solution was transferred into Teflon-line autoclave with a volume of 68 mL. The degree of filling the autoclave is 70 %. Samples were prepared in the temperature range from 180 to 270 C, reaction time 24, 48, and 60 h in 0.3/3.5 M of NaOH in an aqueous solution. The autoclave was cooled down to room temperature naturally. The precipitate was filtered and washed with deionized water and then, the product was dried at 80 C for 5 h. The structure of products was determined by powder X-ray diffraction (XRD) PW 3040/60 X’Pert ˚ ), in PRO using Cu-Ka radiation with (k = 1.5418 A the range 2h = 10–80, at room temperature. The data were analyzed by using FULLPROF program (Rodriguez-Carvajal 1993). A Scanning Electron Microscope Inspect S (SEM) and SAXS were used to observe the size and shape of the as synthesized nanocrystals. The UV absorption spectrum was obtained using a Lambda 950 UV–Vis–NIR Spectrophotometer.

Results and discussion

Fig. 3 The stability diagram for Cu–Cr–Na–H2O system with Cu2O, Cr(OH)3 as precursors and NaOH as a mineralizer at 48 h

The stability diagram for Cu–Cr–Na–H2O system with Cu2O, Cr(OH)3 as precursors and NaOH as a mineralizer was built using XRD patterns. The results are

Fig. 4 Room temperature XRD patterns of a 3RCuCrO2 and b 2H-CuCrO2 nanocrystals

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summarized in Figs. 2 and 3. Phase purity of 2H- and 3R-CuCrO2 phases was examined by the XRD pattern. As shown in Fig. 4, all the peaks can be indexed to 3RCuCrO2 phase (space group: R-3m; JCPDS 00-0390247) and 2H-CuCrO2 phase (space group: P63/mmc; JCPDS 01-089-0540). The unit-cell parameters were calculated with FullProof, and the obtained values ˚ , c = 17.154(2) A ˚ for 3Rare a = b = 2.979(1) A ˚, c = CuCrO2 and, respectively, a = b = 2.972(1) A ˚ for 2H-CuCrO2. 11.537(3) A The crystalline size can be calculated using the Scherrer formula: Dhkl ¼

Fig. 5 SEM images of a 2H-CuCrO2 nanocrystals and b 3RCuCrO2 nanocrystals, c SAXS pattern of 2H-CuCrO2 nanocrystals

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kk Bcosh

ð2Þ

where k is a constant (*1), B is the full width at half maximum (FWHM), k is the wavelength of X-ray, and h is the diffraction angle. The crystalline size of both phases, 2H and 3R, are about 20–30 nm. The SAXS measurement (Fig. 5) confirms the same approximate value. SEM images of 2H- and 3R-CuCrO2 nanocrystals are shown in Fig. 5, it is obvious that the powders consist of quasi-hexagonal shape nanocrystals with a wide range of size distribution varying from about 20 to 40 nm. The electron microprobe analysis was performed at different points in the sample, and the uniformity and the closely stoichiometric ratio Cu:Cr (50:50) were confirmed. The results indicate that the hydrothermal method is useful for preparing nanocrystalline CuCrO2, either 2H or 3R pure phase. The study of the stability diagram for Cu–Cr–Na–H2O system emphasizes the influence of the concentration mineralizer, the temperature, the reaction time, and the ratio between Cu2O:Cr(OH)3 in the formation of desired phase, 3R or 2H. The role of the NaOH in the synthesis CuCrO2 phase is to increase the solubility of the metal complexes by increasing the hydroxide concentration. Whatever the NaOH amount, at 180 C, no CuCrO2 was found. We expected that by increasing the temperature above 190 C, it will determine the stability of aqueous Cu?1 species because the more the decreased water dielectric constant destabilizer, the more the highly charged Cu2? ion. Indeed, the Cu2? species disappear but only for high NaOH concentration (above 1 M). In the low NaOH concentration domain, Cu1? ions disproportionate to Cu2? ions and the CuO phase was found even at 250 C.


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Fig. 6 Room temperature XRD pattern of sample obtained at (a) 220 C and 48 h in 2.5 M NaOH solution using the fast quenching into water, (b) 220 C and 48 h in 2.5 M NaOH solution using normal cooling, and (c) 220 C and 60 h in 2.5 M NaOH solution using normal cooling

When synthesis time was fixed at 60 h and the ratio Cu2O:Cr (OH)3 at 1 mmol:2 mmol, 2 domains of stability diagram, one of low temperature and high NaOH molarity (I), and another of high temperature and low NaOH molarity (II), establish the 2H-CuCrO2 formation. In first domain, below 200 C, 2H-CuCrO2 is never found as a single phase, but as part of a multiphase mixture which consists of 2H-CuCrO2 and Cu2O precursor. At 190 C, increasing of NaOH concentration from 2.5 to 3 M, no modification of the ratio between 2H-CuCrO2 and Cu2O phases was observed. This behavior can be explained by the limitation of solubility of Cu2O in function by NaOH molarity of solution. The different behaviors were observed at 200 C where the stability of 2H phase was improved by increasing NaOH amount, starting as a major phase at 1.5 M and establishes a pure phase at 2.5 M. In this strongly basic medium, chromium hydroxide Cr(OH)3 and Cu2O dissolves to form the stable aqueous soluble species, tetrahydroxochromate (III) ion Cr(OH)4 and, respectively, Cu(OH)2 species. For 200 C and 2.5 M, the concentration of Cu(OH)2 and Cr(OH)4 species reach a critical supersaturation, resulting in nucleation of 2H-CuCrO2. In the second domain, a multiphase mixture which consists of 2HCuCrO2, CuO, and Cu2O precursor appears for 0.3 M

NaOH. Increasing of NaOH molarity reduces precursor phase and for 0.5 M, the results have shown only 2H-CuCrO2 and minority CuO phase. In this domain, it was impossible to obtain 2H-CuCrO2 pure phase, because 2H–3R phase transition takes place very abruptly, 3R-CuCrO2 phase becomes major phase when the NaOH molarity reaches 0.6 M. After 1 M NaOH, 3R-CuCrO2 pure phase is presented. The stability diagram for Cu–Cr–Na–H2O system has shown that 3R-CuCrO2 pure phase is built quite easily if the temperatures vary between 210 and 250 C and, respectively, the NaOH molarity from 1 to 3.5 M. It can be observed that temperature and molarity are involved in 3R-CuCrO2 formation, but temperature and molarity effect disappear when 3R phase became pure. For example, at 2.5 M, increasing of the temperature from 210 to 250 C did not affect the crystallinity or sizes of the particles. This behavior can be explained by the saturation of hydrated phases at 210 C or 1 M when temperature is fixed at 250 C. After 250 C, 3R-CuCrO2 is destabilized and metallic copper phase is presented. Below 1 mmol:2 mmol Cu2O:Cr(OH)3 ratio, the results are identically with above results. But, increasing of Cu2O:Cr(OH)3 ratio kept the stoichiometric ratio, did not allow to establish a 3R-CuCrO2 pure phase, even for 250 C and 3.5 M.

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Fig. 7 Room temperature XRD pattern of sample obtained at a 250 C and 60 h in 2.5 M NaOH solution and b 200 C and 60 h in 2.5 M NaOH solution

It can be assumed that the Cu(OH)2 and Cr(OH)4 species are completely established up to 1 mmol:2 mmol Cu2O:Cr(OH)3 ratio.

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In terms of schematic representation of the delafossite structure, for the 2H-polytype with AaBbAa.. stacking sequence along c-axis, in the low temperature


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range (up to 200 C) thermodynamically is favored to build two sequences. The increasing of the temperature facilitated the apparition of the third sequence, Cc, and thus the 3R-polytype with AaBbCcAa… is established (Fig. 4). The same evolution of the polytype delafossite as a function of the temperature 1170 C

1190 C

synthesis 2H ! 2H þ 3R ! 3R was observed in CuYO2 delafossite obtained by complex solid-state reaction technique (Van Tendeloo et al. 2001). In order to study the formation mechanism, the time reaction was reduced to 48 h (Fig. 3). In these samples, CuO phase was presented and can be assumed as an intermediary phase for 2H- and 3RCuCrO2 pure phases obtained after 60 h. It is an unexpected result, but in both cases, the evolution through the pure phases was accompanied by CuO phase. In order to exclude the formation of CuO phase throughout the normal cooling process, some experiments, for the mixture phases and pure phase, were repeated using the fast quenching into water. In this way, the formation mechanism was ‘‘frozen’’ before the normal cooling process. We found CuO phase in the mixture phases’ experiments, for the both ways of cooling process (Fig. 6) and Cu2O phase for the pure phase experiments (Fig. 7). Throughout the normal cooling process, Cu2O phase is reduced and delafossite phase becomes majority or pure phase. Also, it was observed that the first crystalline planes formatted of the 3R delafossite structure are (006), (110), and (116). From the above experiment results, a possible formation mechanism for the synthesis of CuCrO2 nanocrystal, 2H- and 3R-polytypes is proposed as follows. At elevated temperature and NaOH molarity, in the beginning of formation process, copper (I) ions was partially oxidize to Cu2?, Cu(OH)2 , and Cu(OH)species had been formed (Beverskog and 3 Puigdomenech 1997a, b). It seems that, the increasing of stability area of the Cu1? is determined not only by the temperature but also by the reaction time, at expenses of the Cu2? ions. Also, in the strong alkaline environments, Cr(OH)3 (s) was dissolved and formed the hydroxo species, Cr(OH)4 formed (Beverskog, I. Puigdomenech 1997). Thus, for certain temperature, reaction time and molarity, the stability of Cu(OH)2 and Cr(OH)4 species is established, the suprasaturation is reached and 3R- or 2H-polytype is established. The Cu2O (s) (second and third hydrolysis steps) and

Cr(OH)3(s) reactions involving these species are given in the following equations: þ Cu1þ þ 2H2 OðlÞ ! CuðOHÞ 2 þ2H

ð3Þ

þ Cu2þ þ 3H2 OðlÞ ! CuðOHÞ 3 þ3H

ð4Þ

þ CrðOHÞ3 ðsÞ þ H2 O ! CrðOHÞ 4 þH CuðOHÞ 2 ðaqÞ þ CrðOHÞ4 ðaqÞ ! CuCrO2

ð5Þ þ 4H2 O ð6Þ

The formation process of the desired phases is continuous and Cu2O dissolves to form the stable, soluble Cu(OH)2 species even throughout the cooling process (Fig. 7).

Conclusions To summarize, we had first synthesized hexagonal 2HCuCrO2 nanocrystals by hydrothermal method. The stability diagram for Cu–Cr–Na–H2O system allows preparing of nanocrystalline CuCrO2, either 2H or 3R pure phase, by a single step, low-temperature hydrothermal synthesis. An unexpected result is that 2H-CuCrO2 phase was a low temperature phase in the hydrothermal synthesis. Also, a formation mechanism for the synthesis of CuCrO2 nanocrystal, 2H- and 3R-polytypes was proposed. So far, CuO phase was considered as an inhibitory phase in the formation of CuCrO2 delafossite phase and for preventing the aqueous disproportionation of Cu? species to Cu2? species at lower temperatures, it has developed the Teflon pouch synthesis, which involves that the Teflon pouch remaining impermeable to water until 150 C (Sheets et al. 2006). Our results have shown contrary that CuO phase can be assumed as an intermediary phase for 2H- and 3R-CuCrO2 pure phases. In future investigations, our attention will be focused on the study of thermoelectric properties of 2H- and 3R-CuCrO2 nanocrystals and the influence of nanostructuring and polytype of CuCrO2 on the thermoelectric figure of merit (ZT) of a material. Acknowledgments This study was supported by the strategic grant POSDRU ID77265 (2010), co-financed by the European Social Fund-Investing in People, within the Sectorial Operational Programme Human Resources Development 2007–2013 and Initial Training Network SOPRANO 214040, supported by the EU Seventh Framework Programme.

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