Hydrothermal Growth of Centimeter-Scale CuO Plates: Planar

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Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Hydrothermal Growth of Centimeter-Scale CuO Plates: Planar Chromium(III) Oligomer as a Facet-Directing Agent Yuan Zhang, Long Yuan, Xia Zhong, Keke Huang, and Shouhua Feng*,† †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China S Supporting Information *

Before the hydrothermal procedure, the mixed prereaction solution was black, resulting from the blue Cu(NO3)2 and the purple Cr(NO3)3. When the massive OH− was added, the transparent solution turned to a brown suspension. After the solution was stirred for 30 min and stood for 1 h, a claret powder precipitated from the suspension, while the solution turned back to dark green. Then the suspension was transported into the autoclave for the 72 h hydrothermal process. The resulting products consist of glossy black CuO plates and a dark-green solution. The whole experimental procedure is schematically illustrated in Figure 1. The corresponding digital images visualize the features of the products.

ABSTRACT: In this work, a simple hydrothermal method was developed to synthesize CuO plates in centimeter scale for the first time. Plates of up to 20 μm thickness and several square millimeters in area have been prepared. The unusual size was obtained under ultrahighconcentration NaOH and a planar chromium(III) oligomer, which served as a new kind of inorganic facetdirecting agent. The obtained CuO plates were glossy black, free-standing, and crack-free. The chromium(III) oligomer offered ideal chemically active sites for adsorbing and confining Cu2+ ions. They could be adsorbed on the surface of Cu(OH)42− clusters via hydrogen-bonding interaction, which thus modified the growth orientation. The as-synthesized centimeter-scale CuO plates could possibly serve as substrates and electronic materials with potential applications.

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uO is an excellent p-type semiconductor with a narrow band gap (1.2 eV).1 Plenty of articles reported the preparation of CuO plate-like crystals on micro- or nanoscale.2−4 However, most of the synthetic procedures of anisotropic materials require organic capping agents to block the addition of atoms to certain crystal facets. When the reaction finished, the capping agents need to be washed away or decomposition annealed. The synthesis of CuO plates on micro- or nano-scale without any surfactants or templates has also been reported.5−7 However, strategies for growing unique anisotropic crystal shapes based on inorganic structure-directing agents have not been established. With the development of nanotechnology, most materials were designed to reduce to nanometers. However, the synthesis of macroscopic crystals is still particularly valuable to semiconductor devices, nonlinear-optical materials, and crystallography. For CuO, a millimeter-above-sized crystal was obtained by the floating-zone method in an oxygen/argon atmosphere under high pressure.8,9 Control of the nucleation and growth of CuO in order to achieve macroscopic crystals via a simpler and cheaper method has significant meaning. The zur Loye group described a low-temperature hydroflux method to obtain crystals at the millimeter level.10−18 It is a hybrid approach between aqueous hydrothermal and molten hydroxide flux techniques. The method we used here, with ultraalkaline hydrothermal condition, is very close to the lowtemperature hydroflux method with a lower concentration of alkaline, a higher volume of autoclave, and a larger filling degree. © XXXX American Chemical Society

Figure 1. Whole experimental procedure. The insets are the digital images of the products.

The crystal structures of the as-prepared products were determined by X-ray diffraction (XRD; Figure S1). Notably, the claret sedimentation was identified as pure cubic Cu2O (JCPDS 05-0667). The black plates with a metallic luster after hydrothermal treatment is pure CuO crystallized in a monoclinic phase (space group C2/c) according to standard JCPDS (481548). Without the grinding process, it can be seen that only (002) and (004) peaks appeared in the profile because of the highly anisotropic crystal shape (Figure S2). After grinding, the peak intensity of the (002) facet is still significantly stronger than that of the (111) facet (Figure S1). This shows that CuO plates are mainly oriented on the [001] zone axis and grow preferentially along the [100] and [010] directions. Received: December 8, 2017

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DOI: 10.1021/acs.inorgchem.7b03095 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Morphological studies were carried out via a scanning electron microscopy (SEM) technique. Figure 2a shows the clear

3Cu 2O (dark‐red precipitation) + 2CrO4 2 − (yellow) + 11H 2O = 6Cu 2 + (blue) + 2Cr 3 + (purple) + 22OH−

(2)

The appearance of the Cu2O red precipitate clearly indicated the chemical reaction. To verify the proposed equations, a series of experiments varying the mass of Cr(NO3)3 in the synthetic processes were carried out (for details, see the Supporting Information, SI). This preliminarily verified that chromium(III) had been oxidized to chromium(VI), while copper(II) had been reduced to copper(I). Owing to the kinetic robustness of Cr3+, Cr3+ centers could form significant amounts of soluble oligomers at an early stage of the hydrolytic polymerization process.19 A complete series of oligomers could be formed ranging from dimer to hexamer in solutions. For example, the dimer consists of two octahedra connecting with an edge whose formula can be established as [Cr 2 (OH) 2 (H 2 O) 8 ] 4+ . 20 The trimeric complex, green [Cr3(OH)4(H2O)9]5+, consists of a trigonal array in which the three Cr ions share a common hydroxide bridge.21 The tetramer with various isomers is considerably more acidic than the trimer. One of the isomers of the tetramer, Cr4(OH)66+, could transform into Cr4O(OH)55+ through intramolecular condensation.22 We obtained CuO plates with Cr3+ and CuO microparticles without Cr3+. Therefore, certain forms of Cr3+ must have acted as the facet-directing agent in the hydrothermal process. The contrast experiment shows that CrO42− did not play a modifying or oxidizing role in the formation processes of CuO plates (for details, see the SI). On the basis of the above experimental observation and the behaviors of Cr3+ in the solution, we concluded that Cr3+ transformed into the Cr3+ oligomer under hyperconcentrated alkaline conditions. During the hydrothermal process, the chromium(III) oligomer acted as a morphology modifier, directing formation of the CuO plates. In order to probe whether the chromium(III) oligomer existed in the reaction solution, certain spectral analyses of the aqueous solution before and after the hydrothermal reaction were carried out (Figure 3). First, before and after the hydrothermal procedures, the dark-green solution suggested the existence of

Figure 2. (a) SEM of Cu2O particles obtained before the hydrothermal process. SEM images of CuO plates obtained after the hydrothermal process from above (b) and the side view (c). (d) SEM image of CuO microparticles obtained in the absence of Cr3+.

octahedral appearance of Cu2O precursors with varied sizes. The rough surfaces of the octahedra resulted from the etch effect of the high concentration alkalinity. Figure 2b gives the overall vision of the CuO plate from above; it is a flat shape, where the lateral dimension is centimeter-sized and the thickness is about 20 μm (Figure 2c). Tapping atomic force microscopy (AFM) indicated that the surface roughness of the plate was about 35 nm (Figure S3). A smooth surface is hard to achieve except by physical deposition, which could be applied via a substrate, on which a certain material could be deposited. A contrast experiment was carried out in the absence of Cr3+, while the remaining synthetic processes were identical. The XRD profile demonstrated that the product was pure CuO (Figure S2). The SEM image showed that they were microparticles of about 2 μm size (Figure 2d). The above results indicated that Cr3+ ions played a crucial role in the anisotropic growth of the CuO plates. A focused-ion-beam (FIB) technique was used for transmission electron microscopy (TEM) sample preparation. The resulting TEM and selected-area electron diffraction (SAED) images are listed in Figure S4. High-resolution TEM (HRTEM) images revealed that the CuO plate consisted of small CuO particles, which exposed the (002) facet. Considering that Cu2O and CrO42− are the stable forms for copper and chromium under high-pH conditions, the following equation was proposed based on the above experimental observations: 6Cu 2 + (blue) + 2Cr 3 + (purple) + 22OH− = 3Cu 2O (dark‐red precipitation) + 2CrO4 2 − (yellow) + 11H 2O

(1)

The reaction was carried out as eq 1 at room temperature; when the temperature and pressure were increased in the hydrothermal environment, the reaction proceeded as follows:

Figure 3. (a) UV−vis absorption spectra. (b) IR absorption spectra. (c) Raman scattering spectra of the solution before and after hydrothermal processes and the commercial K2CrO4 solution. B

DOI: 10.1021/acs.inorgchem.7b03095 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

sharing, a sheet of distorted Cu(OH)6 octahedra was formed. The 2D layers of these sheets parallel to the (001) plane were connected through hydrogen bonds. Then, hydrothermal heating led to the breaking of interplanar hydrogen bonds and consequently accelerated the dehydration rate. The chromium(III) trimer was in one plane attached on the {001} plane; thus, the ⟨001⟩ direction was protected and stopped growing. The exposed facets in the lateral dimension grew quickly to form centimeter-sized CuO plates. This is because the chromium(III) trimer stabilized the negatively charged (001) plane. Thus, we proposed that a planar chromium(III) trimer dominated the formation of CuO plates. As a consequence, this mechanism contributes to the rapid transformation process of Cu(OH)42− (orthorhombic structure) to CuO plates (monoclinic structure).24 Figure 4c gives a schematic illustration of the formation mechanism of the formation of the planar plate shape of CuO with developed {001} planes. In summary, we fabricated CuO plates at the centimeter level. The unusual size was obtained under an ultrahigh concentration of NaOH with the assistance of the chromium(III) trimer. The whole formation process involved two redox reactions between copper(II) and chromium(III). Our experiments have proven that the chromium(III) oligomer could be adsorbed on the surface of Cu(OH)42− clusters via electrostatic attraction between H+ and OH−, which thus modified the growth orientation. It is a self-assembly process by CuO microparticles. The synthesis method is simpler and cheaper, yielding CuO plates, which are promising candidates to be applied as proper substrates.

the chromium(III) oligomer because of the same color. The absorbance peaks in the UV−vis absorption spectra with maxima at 373 and 273 nm are typical for CrO42− in alkaline media. The additional peak at 227 nm in the presence of both reaction solutions but absent in the K2CrO4 spectrum was ascribed to the chromium(III) oligomer (Figure 3a). The peaks located at 3500 and 1660 cm−1 in the IR absorption spectra are attributed to CrO42− (Figure 3b). The additional peaks located at 1500 and 960 cm−1 are assigned to the chromium(III) oligomer. It is no surprise that the features of the Raman scattering spectra were also identified as those of the above spectra (Figure 3c). The peaks located at 760, 850, and 1160 cm−1 are attributed to CrO42−. The three peaks are associated with the Ag symmetric stretching vibrations of the Cr−O bond in the CrO42− mode. Additional peaks located at 1050 and 1250 cm−1 are assigned to the oligomer. The additional peaks verified newly appearing ions, which have been highlighted by gray boxes in Figure 3. Experiments of varying the mass of NaOH were also carried out. The results showed that the particular morphology of the CuO black plates is closely related to the coexistence of a high OH− concentration and chromium(III) (see also the SI). Parts a and b of Figure 4 give the crystal structures of the Cr3+ trimer. From the view of the ⟨100⟩ direction, all three Cr−O

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03095. Detailed synthetic procedures and characterization techniques, XRD patterns, AFM images, liquid/solid UV−vis absorption, IR absorption, and Raman scattering spectra, HRTEM, SAED, and SEM images, and XPS (PDF)

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Figure 4. (a and b) Crystal structures of the chromium(III) trimer viewed from different perspectives. (c) Schematic illustration of the formation mechanism of the CuO plates under hyperconcentrated alkaline conditions with the assistance of a chromium trimer.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

octahedra are in one plane. The green color of the solution is also consistent with the trimer. Moreover, the robustness and stability of the trimers are higher than those of the dimer. From the perspective of the above reasons, we deduced that the Cr3+ trimers were more likely to dominate the modification effect. Details of the formation of a CuO black plate are at the initial stage of the hydrothermal process: small Cu2O octahedra gradually dissolved in a high concentration of a NaOH solution. In a conventional flux method, nitrate as an oxy-acid-related flux tends to generate more O2− ions, which could possibly promote dissolution of metal oxides.23 In this work, NO3− within Cr(NO3)3 may also accelerate dissolution of a Cu2O precursor and promote crystal growth in the ⟨100⟩ and ⟨010⟩ directions. The resulting Cu2+ ions were surrounded by plenty of OH−, which quickly transformed into Cu(OH)42−. Through edge

Long Yuan: 0000-0002-3047-0295 Keke Huang: 0000-0002-8995-2176 Shouhua Feng: 0000-0002-6967-0155 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21427802, 21671076, and 21621001). C

DOI: 10.1021/acs.inorgchem.7b03095 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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(22) Stuenzi, H.; Marty, W. Early stages of the hydrolysis of chromium (III) in aqueous solution. 1. Characterization of a tetrameric species. Inorg. Chem. 1983, 22, 2145−2150. (23) Flood, H.; Förland, T.; Sillén, L. G.; Linnasalmi, A.; Laukkanen, P. The acidic and basic properties of oxides. Acta Chem. Scand. 1947, 1, 592−604. (24) Zhang, Z. P.; Sun, H. P.; Shao, X. Q.; Li, D.; Yu, H.; Han, M. Three-dimensionally oriented aggregation of a few hundred nanoparticles into monocrystalline architectures. Adv. Mater. 2005, 17, 42− 47.

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DOI: 10.1021/acs.inorgchem.7b03095 Inorg. Chem. XXXX, XXX, XXX−XXX