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Morphology control of copper oxalate polycrystalline particles involving an etching process
ARTICLE IN PRESS
Journal of Crystal Growth 306 (2007) 366–372 www.elsevier.com/locate/jcrysgro
Morphology control of copper oxalate polycrystalline particles involving an etching process Xiufeng Zhaoa,b, Jiaguo Yua, a
State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, PR China b Department of Chemical Engineering, Changji University, Changji, Xingjiang 831100, PR China Received 13 January 2007; received in revised form 14 May 2007; accepted 21 May 2007 Communicated by K. Nakajima Available online 24 May 2007
Abstract The morphogenesis of copper oxalate polycrystalline pitted and patterned tablets were investigated in a simple precipitation reaction between copper sulfate and oxalic acid solutions. The formation of the pit or pattern on the surfaces of the tablets could be related to an etching or partial dissolution process at the central region of the tablets during aging. The presence of poly(ethylene glycol) (PEG), acidity of the reaction mixture and temperature exhibited significant effect on the morphology of the products, which could be rationally associated with different etching processes under different conditions. r 2007 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 81.10.Dn Keywords: A1. Crystal morphology; A3. Etch pit; A3. Pattern; B1. Copper oxalate; B1. Poly(ethylene glycol)
1. Introduction The synthesis of nano- or micro-sized materials with a specific morphology and texture structure has been an active field in materials science [1–7]. Many researches have shown that the control over the morphology and organization of materials provides an important method in tailoring their physical and chemical properties [8–10]. Numerous imaginative methods have been developed to realize the novel structures on nano- or micro-scale. Some traditional methods such as altering variables including supersaturation, pH value, temperature, pressure and solvent (e.g. various hydrothermal and solvothermal methods, etc.) have shown significant effects not only on size but also on morphology and organization of materials. It has long been known that the presence of impurities in a system can have profound effects on the nucleation and growth of a crystal in industry. However, deliberate addition of some Corresponding author. Tel.: +86 27 87871029; fax: +86 27 87879468.
E-mail address: [email protected] (J. Yu). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.05.038
specific ‘‘impurities’’ in a system has now proved to be one of the most important approaches to achieve the exquisite control on nucleation, growth and assembly of various materials. For example, a great number of polymers, surfactants, self-assembled supermolecular structures, reverse micelles, bicontinuous microemulsions, emulsions, etc. have been introduced to reaction systems, and a wide range of materials with novel morphologies and structures have been synthesized [11–17]. Recently, a catalytic solution route for preparing octahedral Cu2O nanocages undergoing an evacuation process by etching has been reported [18], suggesting that etching or partial dissolution can become an alternative approach to achieve novel structured materials. Copper oxalate is a material with an unusual antiferromagnetic character, which are related to its polymer-like structure described as a stacking of yCu(C2O4)Cu (C2O4)y ribbons [19]. Bowen and co-workers reported copper oxalate polycrystalline particles with cushion- and rod-liked shapes produced by a precipitation reaction, and a rational self-organization mechanism for the formation
ARTICLE IN PRESS X. Zhao, J. Yu / Journal of Crystal Growth 306 (2007) 366–372
2. Experimental section Poly(ethylene glycol) (PEG) (Mw ¼ 400) (chemically pure) was purchased from Shanghai Reagent Company. Copper sulfate pentahydrate (CuSO4 5H2O) and oxalic acid dihydrate (H2C2O4 2H2O) were obtained from Beijing Chemical Company. All the chemicals were used in the experiments without further purification. The preparation of copper oxalate CuC2O4 xH2O (with 0pxo1) was carried out by a precipitation reaction between CuSO4 and H2C2O4 solutions in a glass bottle (at a temperature lower than 100 1C) or a Teflon-line stainless autoclave (at a temperature higher than 100 1C). Briefly, 5 ml of CuSO4 solution (0.1 M) was firstly added into 40 ml of PEG solution under magnetic stirring. Then, 5 ml of H2C2O4 solution (0.1 M) was added, and pH value was immediately adjusted to 2 using 1 M H2SO4 and NaOH solutions. The initial Cu2+ and C2O2 4 concentrations in reaction mixture were 0.01 M. The mixture was stirred for another 1 min, and then kept under static conditions at a given temperature for 48 h (or specified otherwise). The blue precipitate was collected by centrifugation and dried in vacuum at 60 1C for 6 h. To study the effect of much stronger acidity (pHo1), a H2SO4 solution was firstly mixed with a PEG solution before CuSO4 and H2C2O4 solution were successively added. Powder X-ray diffraction (XRD) was performed using an HZG41B-PC X-ray diffractometer with monochromatied Cu Ka radiation (l ¼ 1.5406 A˚). Scanning electron microscopy (SEM) observation was conducted on a JSM5610LV scanning electron microscope at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were recorded on a JEM 2100 highresolution transmission electron microscope at an accelerating voltage of 200 kV. 3. Results and discussion The crystalline structures of all the products prepared under the different synthetic conditions were characterized by XRD. Some typical XRD profiles of obtained products are presented in Fig. 1. All the products can be indexed as orthorhombic crystalline structure with the Pmnn space group and the cell parameters determined are a ¼ 5.402 A˚, b ¼ 5.570 A˚, and c ¼ 2.546 A˚, in agreement with the literature value of copper oxalate crystal (JCPDS file no. 21-297). The mean crystallite sizes of all the samples are estimated from the half-width of (1 0 0) peak using Scherrer
121
111
220
(c)
011
120
100 Intensity (a.u.)
of such polycrystalline particles was proposed [20]. Herein, we demonstrate the morphogenesis of copper oxalate polycrystalline tablets with unusually pitted or patterned surfaces. Our study is focused on the formation of the pit or pattern on the surfaces of the tabular polycrystals, and an etching process provides a reasonable understanding for these phenomena.
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2θ (degree) Fig. 1. Powder XRD pattern of prepared copper oxalate by aging at room temperature for 48 h: (a) [PEG] ¼ 20%, (b) [PEG] ¼ 4%, (c) [PEG] ¼ 0; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
equation and the values range from 60 to 79 nm. In comparison with the micrometer sizes of obtained particles (see follows), the crystallite sizes are very small, suggesting that all the samples are polycrystalline. This is consistent with the result documented by Bowen and co-workers [20]. Fig. 2a presents the SEM images of the copper oxalate sample prepared with PEG concentration (vol%) of 4% in the reaction mixture at room temperature. It can be seen that rectangular tabular particles of ca. 1 mm size with a deep pit on their surfaces were formed. A higher magnification SEM image (inset in Fig. 2a) clearly exhibits the rugged surfaces of the tablets. The mean crystallite size calculated from XRD analysis is ca. 65 nm (Fig. 1b), implying the polycrystalline nature of the sample. The growth process of this sample was carefully followed by examining the crystal morphology at early stages. As shown in Fig. 2b, non-pitted rectangular convex tablets of ca. 1 mm were formed after an aging time of 8 h, quite similar to the copper oxalate cushion-like polycrystalline particles reported in the literature [20]. The higher magnification SEM image (inset in Fig. 2b) shows the smooth rounded surfaces of the tablets, in comparison with the rough pitted surfaces of the sample obtained by aging for 48 h. It is known that polycrystalline growth may not only render the crystals mechanically weak, but may even make the crystals thermodynamically instable. This usually leads to etching or partially dissolution occurring at sharp edges and apexes, i.e. at the regions of very small radius due to Gibbs–Thomson effect, and causes the crystal to achieve a rounded shape [21]. Crystallization always occurs at a high supersaturation whereas dissolution takes place at a low one [21]. In the present case, the pit in the center of the tablet surface and the rugged surface obviously arose from an etching or partial dissolution procedure at the
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Fig. 2. SEM images of copper oxalate particles obtained with PEG concentration of 4% by aging at room temperature for: (a) 48 h and (b) 8 h; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
Fig. 3. SEM images of copper oxalate particles obtained in the absence of PEG by aging at room temperature for: (a) 48 h and (b) 8 h; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
apexes of the convex tablets during aging, similar to the formation process of the hollow vaterite convex discs in the presence of poly(ethylene oxide)-block-poly(methacrylic acid) (PEO-b-PMAA) and sodium dodecylsulfate (SDS) [13]. The reason why such etching process essentially occurred at the late stage of crystal growth, as evidenced by our experiment mentioned above, could be related to the decreased level of supersaturation of copper oxalate when Cu2+ and C2O2 4 ions in the mother liquor were gradually depleted with prolonging the aging time. The etching selectively occurred in the central region of the tablets probably due to smaller size of the nanocrystallites located in the central part of the polycrystalline particles [22]. Considering the Gibbs–Thomson effect that solubility increases exponentially with reduction in crystallite size [21], the central parts of the copper oxalate tablets are more soluble, and then favor the etch. To investigate the effect of polymer PEG in the reaction mixture, controlled experiment was carried out in the absence of PEG. As shown in Fig. 3a, the obtained product consists of rectangular tabular particles with a shallow pit on their surfaces. Notably, the higher magnification SEM image (inset in Fig. 3a) clearly shows the relatively rough and loose surfaces in the central regions of the tablets. As expected, these pitted tablets were also evolved from nonpitted tabular particles via central etching on surfaces
during aging (see Fig. 3b and inset). The corresponding mean crystallite size of this sample is ca. 79 nm evaluated from the XRD analysis (Fig. 1c), larger than that of the sample produced at PEG concentration of 4% (65 nm). This was probably due to adsorption of PEG macromolecules on to the growing crystal surfaces, which consequently impeded crystal growth. Compared with the sample obtained in the presence of PEG (Fig. 2a), it is clear that, at the PEG concentration of 4%, the dissolution at the central regions of the tablets is substantially favored. The augmented etching at the PEG concentration of 4% could be associated with the decrease in crystallite size in the polycrystalline particles. With increasing PEG concentration to 10%, there is no considerable change in the crystallite size (62 nm, XRD data not shown) and morphology of the product (SEM image, not shown here) compared to those with the sample obtained at PEG concentration of 4%. However, when the concentration of PEG was further increased to 20%, thicker tablets with rounded shapes were obtained (Fig. 4), though the crystallite size did not change remarkably (60 nm, Fig. 1a). It is interesting to note that for most particles etching at the central regions did not lead to pitting in this case, but resulted in a rugged sponge-like central region on their surfaces, similar to the case in the absence of PEG (see Fig. 3a and inset). Obviously, the etching process was
ARTICLE IN PRESS X. Zhao, J. Yu / Journal of Crystal Growth 306 (2007) 366–372
retarded at such high PEG concentration of 20% in comparison with the cases that PEG concentrations were 4% and 10%, albeit the differences of the crystallite sizes in these three samples were very small. A plausible explanation for this result would be related to the presumption that the polycrystalline particles were almost completely coated by the polymer macromolecules at the PEG concentration of 20%, which as a result hindered the intensive etching from occurring. Thus, it can be inferred that at room temperature, a low PEG concentration (e.g. 4% or 10%) favors the etching process due to smaller crystallite size; in contrast, a high PEG concentration (e.g. 20%) inhibits this etching process probably because of excessive polymer coated on the particles surfaces. To obviate the possible formation of copper hydroxide, all the experiments in this study were carried out under an acidic condition (i.e. pH 2 in the foregoing discussion). It should be noted that further increasing acidity in the reaction system will inevitably increase the solubility of copper oxalate because oxalic acid is a weak acid, and thus the precipitation–dissolution equilibrium of copper oxalate is sensitive to the concentration of present hydrogen ions,
Fig. 4. SEM images of copper oxalate particles obtained with PEG concentrations of 20% by aging at room temperature for 48 h; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
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as given in the following scheme: 2 CuC2 O4 ðsÞ Ð Cu2þ ðaqÞ þ C2 O4ðaqÞ þ C2 O2 4ðaqÞ þ H ðaqÞ Ð HC2 O4ðaqÞ þ HC2 O 4ðaqÞ þ HðaqÞ Ð H2 C2 O4ðaqÞ .
It follows that an acidic condition in reaction mixture favors the etching process. Hence, at a constant PEG concentration of 10% and diverse H2SO4 concentrations, copper oxalate precipitation experiments were carried out. Fig. 5a presents the SEM image of the sample obtained at H2SO4 concentration of 0.06 M (initial pHo1). As shown, quasi-rectangular convex tabular particles with thickness of ca. 500 nm and edge length of ca. 1 mm were produced. The deep etch pit on the surfaces of the tablets makes them seem like tabular cages. With increasing H2SO4 concentration to 0.08 M, smaller rectangular pitted tablets were obtained, and the central pits were relatively large (Fig. 5b). Comparing Fig. 5a with Fig. 5b, it is obvious that the enhanced acidity favors the etching process, as expected. It was reported that classical means for controlling crystallization by changing parameters such as temperature was sufficient for size control, but had limited influence on the crystal morphology or texture of a growing crystal [23]. However, owing to the etching process occurring in the present cases, and the temperature dependence of solubility of copper oxalate, it was expected that temperature would exhibit considerable influence on the morphology of copper oxalate polycrystals. The effect of temperature was firstly investigated in the absence of PEG. As shown in the SEM image in Fig. 6a, with increasing aging temperature to 60 1C from room temperature, the obtained copper oxalate tablets are made up of a loose and porous pattern enwrapped by a densely continuous rectangular frame. The irregular interstices within the pattern can be clearly seen. The TEM image of the sample (Fig. 6b) shows that the tablets consist of irregular shaped subcrystals with nanometer size. Electron diffraction (ED) pattern (inset in
Fig. 5. SEM images of copper oxalate particles obtained with H2SO4 concentration of (a) 0.06 M and (b) 0.08 M by aging at room temperature for 48 h; [PEG] ¼ 10%; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M.
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Fig. 6. SEM images (a), TEM image (b), ED pattern (inset in b) and HRTEM image (c) of copper oxalate sample obtained in the absence of PEG by aging at 60 1C for 48 h; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
Fig. 6b) confirms that the sample is polycrystalline. Fig. 6c presents the HRTEM image of this sample. In this image, the direction of the lattice fringes of the adjacent subcrystals are randomly orientated from each other, suggesting that the subcrystals in the polycrystalline copper oxalate tablets are arbitrarily attached. Clearly, such polycrystalline particles contain much more defects than those constructed by oriented attachment of subcrysrtals, and are subject to etch. As expected, an etching process was confirmed again because smooth non-patterned convex quasi-rectangular tablets were observed at early stage (data not shown). It was clear that a higher temperature significantly facilitated such etching process. Unlike the foregoing cases at room temperature, because of decrease in solubility of copper oxalate when the reaction mixture was gradually cooled to room temperature, random deposition or recrystallization onto the partially dissolved surfaces of the tablets probably occur. This might be the main reason why patterned, rather than pitted tablets were formed at an elevated temperature, though the etching process occurred at both the higher and the room temperature. Additionally, it was found that similar patterned tablets could be also obtained at the temperatures ranging from 60 to 100 1C. Fig. 7a and b presents the SEM images of the samples prepared at 80 and 100 1C, respectively. However, when the temperature was elevated to 140 1C, as illustrated in Fig. 7c, the surfaces of obtained tablets became relatively smooth and even, and for some tablets the frames became less distinct. This was probably resulted from excessive dissolution during aging at such the
high temperature, and subsequently considerable deposition occurring on the surfaces of the tablets with the reaction mixture cooling to room temperature, which almost completely recovered the interstices on partially dissolved surfaces. Interestingly, although the presence of PEG exerted appreciable effect on the etching process at room temperature, it seemed that this effect was impaired at a higher temperature. As shown in Fig. 8a, patterned tablets of ca. 1 mm were obtained at 80 1C, to some extent analogous to the samples obtained without addition of PEG at high temperatures (Fig. 7a and b). The patterns on the tablet surfaces were proved to be caused by etching as illustrated in Fig. 8b. This result could be rationally related to the desoption of PEG macromolecules from the solid surfaces with raising temperature, which results in the similar result to the ones in which no PEG was added. 4. Conclusions In summary, copper oxalate micro-sized polycrystalline tablets with pitted and patterned surfaces could be produced via a facile precipitation reaction between copper sulfate and oxalic acid solutions. The formation of the pit on the surfaces of the tablets at room temperature was ascribed to an etching process occurring during aging. The etching process at room temperature was favorable when appropriate amount of PEG was present in reaction mixture due to smaller crystallite size, and inhibited at a very high PEG concentration owing to polymer coating on
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Fig. 7. SEM images of copper oxalate particles obtained in the absence of PEG by aging at: (a) 80 1C, (b) 100 1C, and (c) 140 1C for 48 h; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
Fig. 8. SEM images of copper oxalate particles with PEG concentration of 20% by aging at 80 1C for: (a) 48 h and (b) 8 h; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
the solid surfaces. Increasing acidity in reaction system could promote the etching procedure. A higher aging temperature led to not only a stronger etching, but also to random deposition of copper oxalate on the surfaces of the tablets when the reaction system was cooled to room temperature. This resulted in patterned copper oxalate tablets at higher temperatures. The effect of PEG was weakened at a higher temperature because a higher temperature would be disadvantageous to the adsorption of PEG macromolecules on solid surfaces. Acknowledgments This work was partially supported by the National Natural Science Foundation of China (20473059 and
50625208). This work was also financially supported by the Key Research Project of Chinese Ministry of Education (No. 106114) and PCSIRT (No. IRT0547). References [1] S. Mann, Angew. Chem. Int. Ed. 39 (2000) 3392. [2] X. Feng, D.C. Sayle, Z. Wang, M.S. Paras, B. Santora, A.C. Sutorik, T.X.T. Sayle, Y. Yang, Y. Ding, X. Wang, Y.-S. Her, Science 312 (2006) 1504. [3] Y. Xiong, B. Wiley, J. Chen, Z.-Y. Li, Y. Yin, Y. Xia, Angew. Chem. Int. Ed. 44 (2005) 7913. [4] M. Li, H. Co¨lfen, S. Mann, J. Mater. Chem. 14 (2004) 2269. [5] J. Yu, H. Guo, S.A. Davis, S. Mann, Adv. Funct. Mater. 16 (2006) 2035. [6] E. Dujardin, S. Mann, Adv. Eng. Mater. 14 (2002) 460.
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[7] O. Pujol, P. Bowen, P.A. Stadelmann, H. Hofmann, J. Phys. Chem. B 108 (2004) 13128. [8] M. Li, H. Schnablegger, S. Mann, Nature 402 (1990) 393. [9] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [10] C.N.R. Rao, A.K. Cheetham, J. Mater. Chem. 11 (2001) 2887. [11] X. Zhao, T.K. Li, Y.Y. Xi, D.H.L. Ng, J. Yu, Crystal Growth Des. 6 (2006) 2210. [12] X. Zhao, T.L.Y. Cheung, X. Zhang, D.H.L. Ng, J. Yu, J. Am. Ceram. Soc. 89 (2006) 2960. [13] L. Qi, J. Li, J. Ma, Adv. Mater. 14 (2002) 300. [14] D.D. Archibald, S. Mann, Nature 364 (1993) 430.
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H. Shi, L. Qi, J. Ma, N. Wu, Adv. Funct. Mater. 15 (2005) 442. D. Walsh, J.D. Hopwood, S. Mann, Science 264 (1994) 1576. C.L. Zoldesi, A. Imhof, Adv. Mater. 17 (2005) 924. C. Lu, L. Qi, J. Yang, X. Wang, D. Zhang, J. Xie, J. Ma, Adv. Mater. 17 (2005) 2562. A. Michalowicz, J.J. Girerd, J. Goulon, Inorg. Chem. 18 (1979) 3004. N. Jongen, P. Bowen, J. Lemaıˆ tre, J.-C. Valmalette, H. Hofmann, J. Colloid Interface Sci. 226 (2000) 189. J.W. Mullin, Crystallization, Butterworth-Heinemann, Oxford, 1992, 257, 241, 103. H.C. Zeng, J. Mater. Chem. 16 (2006) 649. E. Matijevic, MRS Bull. 14 (1989) 18.
Journal of Crystal Growth 306 (2007) 366–372 www.elsevier.com/locate/jcrysgro
Morphology control of copper oxalate polycrystalline particles involving an etching process Xiufeng Zhaoa,b, Jiaguo Yua, a
State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, PR China b Department of Chemical Engineering, Changji University, Changji, Xingjiang 831100, PR China Received 13 January 2007; received in revised form 14 May 2007; accepted 21 May 2007 Communicated by K. Nakajima Available online 24 May 2007
Abstract The morphogenesis of copper oxalate polycrystalline pitted and patterned tablets were investigated in a simple precipitation reaction between copper sulfate and oxalic acid solutions. The formation of the pit or pattern on the surfaces of the tablets could be related to an etching or partial dissolution process at the central region of the tablets during aging. The presence of poly(ethylene glycol) (PEG), acidity of the reaction mixture and temperature exhibited significant effect on the morphology of the products, which could be rationally associated with different etching processes under different conditions. r 2007 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 81.10.Dn Keywords: A1. Crystal morphology; A3. Etch pit; A3. Pattern; B1. Copper oxalate; B1. Poly(ethylene glycol)
1. Introduction The synthesis of nano- or micro-sized materials with a specific morphology and texture structure has been an active field in materials science [1–7]. Many researches have shown that the control over the morphology and organization of materials provides an important method in tailoring their physical and chemical properties [8–10]. Numerous imaginative methods have been developed to realize the novel structures on nano- or micro-scale. Some traditional methods such as altering variables including supersaturation, pH value, temperature, pressure and solvent (e.g. various hydrothermal and solvothermal methods, etc.) have shown significant effects not only on size but also on morphology and organization of materials. It has long been known that the presence of impurities in a system can have profound effects on the nucleation and growth of a crystal in industry. However, deliberate addition of some Corresponding author. Tel.: +86 27 87871029; fax: +86 27 87879468.
E-mail address: [email protected] (J. Yu). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.05.038
specific ‘‘impurities’’ in a system has now proved to be one of the most important approaches to achieve the exquisite control on nucleation, growth and assembly of various materials. For example, a great number of polymers, surfactants, self-assembled supermolecular structures, reverse micelles, bicontinuous microemulsions, emulsions, etc. have been introduced to reaction systems, and a wide range of materials with novel morphologies and structures have been synthesized [11–17]. Recently, a catalytic solution route for preparing octahedral Cu2O nanocages undergoing an evacuation process by etching has been reported [18], suggesting that etching or partial dissolution can become an alternative approach to achieve novel structured materials. Copper oxalate is a material with an unusual antiferromagnetic character, which are related to its polymer-like structure described as a stacking of yCu(C2O4)Cu (C2O4)y ribbons [19]. Bowen and co-workers reported copper oxalate polycrystalline particles with cushion- and rod-liked shapes produced by a precipitation reaction, and a rational self-organization mechanism for the formation
ARTICLE IN PRESS X. Zhao, J. Yu / Journal of Crystal Growth 306 (2007) 366–372
2. Experimental section Poly(ethylene glycol) (PEG) (Mw ¼ 400) (chemically pure) was purchased from Shanghai Reagent Company. Copper sulfate pentahydrate (CuSO4 5H2O) and oxalic acid dihydrate (H2C2O4 2H2O) were obtained from Beijing Chemical Company. All the chemicals were used in the experiments without further purification. The preparation of copper oxalate CuC2O4 xH2O (with 0pxo1) was carried out by a precipitation reaction between CuSO4 and H2C2O4 solutions in a glass bottle (at a temperature lower than 100 1C) or a Teflon-line stainless autoclave (at a temperature higher than 100 1C). Briefly, 5 ml of CuSO4 solution (0.1 M) was firstly added into 40 ml of PEG solution under magnetic stirring. Then, 5 ml of H2C2O4 solution (0.1 M) was added, and pH value was immediately adjusted to 2 using 1 M H2SO4 and NaOH solutions. The initial Cu2+ and C2O2 4 concentrations in reaction mixture were 0.01 M. The mixture was stirred for another 1 min, and then kept under static conditions at a given temperature for 48 h (or specified otherwise). The blue precipitate was collected by centrifugation and dried in vacuum at 60 1C for 6 h. To study the effect of much stronger acidity (pHo1), a H2SO4 solution was firstly mixed with a PEG solution before CuSO4 and H2C2O4 solution were successively added. Powder X-ray diffraction (XRD) was performed using an HZG41B-PC X-ray diffractometer with monochromatied Cu Ka radiation (l ¼ 1.5406 A˚). Scanning electron microscopy (SEM) observation was conducted on a JSM5610LV scanning electron microscope at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were recorded on a JEM 2100 highresolution transmission electron microscope at an accelerating voltage of 200 kV. 3. Results and discussion The crystalline structures of all the products prepared under the different synthetic conditions were characterized by XRD. Some typical XRD profiles of obtained products are presented in Fig. 1. All the products can be indexed as orthorhombic crystalline structure with the Pmnn space group and the cell parameters determined are a ¼ 5.402 A˚, b ¼ 5.570 A˚, and c ¼ 2.546 A˚, in agreement with the literature value of copper oxalate crystal (JCPDS file no. 21-297). The mean crystallite sizes of all the samples are estimated from the half-width of (1 0 0) peak using Scherrer
121
111
220
(c)
011
120
100 Intensity (a.u.)
of such polycrystalline particles was proposed [20]. Herein, we demonstrate the morphogenesis of copper oxalate polycrystalline tablets with unusually pitted or patterned surfaces. Our study is focused on the formation of the pit or pattern on the surfaces of the tabular polycrystals, and an etching process provides a reasonable understanding for these phenomena.
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2θ (degree) Fig. 1. Powder XRD pattern of prepared copper oxalate by aging at room temperature for 48 h: (a) [PEG] ¼ 20%, (b) [PEG] ¼ 4%, (c) [PEG] ¼ 0; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
equation and the values range from 60 to 79 nm. In comparison with the micrometer sizes of obtained particles (see follows), the crystallite sizes are very small, suggesting that all the samples are polycrystalline. This is consistent with the result documented by Bowen and co-workers [20]. Fig. 2a presents the SEM images of the copper oxalate sample prepared with PEG concentration (vol%) of 4% in the reaction mixture at room temperature. It can be seen that rectangular tabular particles of ca. 1 mm size with a deep pit on their surfaces were formed. A higher magnification SEM image (inset in Fig. 2a) clearly exhibits the rugged surfaces of the tablets. The mean crystallite size calculated from XRD analysis is ca. 65 nm (Fig. 1b), implying the polycrystalline nature of the sample. The growth process of this sample was carefully followed by examining the crystal morphology at early stages. As shown in Fig. 2b, non-pitted rectangular convex tablets of ca. 1 mm were formed after an aging time of 8 h, quite similar to the copper oxalate cushion-like polycrystalline particles reported in the literature [20]. The higher magnification SEM image (inset in Fig. 2b) shows the smooth rounded surfaces of the tablets, in comparison with the rough pitted surfaces of the sample obtained by aging for 48 h. It is known that polycrystalline growth may not only render the crystals mechanically weak, but may even make the crystals thermodynamically instable. This usually leads to etching or partially dissolution occurring at sharp edges and apexes, i.e. at the regions of very small radius due to Gibbs–Thomson effect, and causes the crystal to achieve a rounded shape [21]. Crystallization always occurs at a high supersaturation whereas dissolution takes place at a low one [21]. In the present case, the pit in the center of the tablet surface and the rugged surface obviously arose from an etching or partial dissolution procedure at the
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Fig. 2. SEM images of copper oxalate particles obtained with PEG concentration of 4% by aging at room temperature for: (a) 48 h and (b) 8 h; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
Fig. 3. SEM images of copper oxalate particles obtained in the absence of PEG by aging at room temperature for: (a) 48 h and (b) 8 h; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
apexes of the convex tablets during aging, similar to the formation process of the hollow vaterite convex discs in the presence of poly(ethylene oxide)-block-poly(methacrylic acid) (PEO-b-PMAA) and sodium dodecylsulfate (SDS) [13]. The reason why such etching process essentially occurred at the late stage of crystal growth, as evidenced by our experiment mentioned above, could be related to the decreased level of supersaturation of copper oxalate when Cu2+ and C2O2 4 ions in the mother liquor were gradually depleted with prolonging the aging time. The etching selectively occurred in the central region of the tablets probably due to smaller size of the nanocrystallites located in the central part of the polycrystalline particles [22]. Considering the Gibbs–Thomson effect that solubility increases exponentially with reduction in crystallite size [21], the central parts of the copper oxalate tablets are more soluble, and then favor the etch. To investigate the effect of polymer PEG in the reaction mixture, controlled experiment was carried out in the absence of PEG. As shown in Fig. 3a, the obtained product consists of rectangular tabular particles with a shallow pit on their surfaces. Notably, the higher magnification SEM image (inset in Fig. 3a) clearly shows the relatively rough and loose surfaces in the central regions of the tablets. As expected, these pitted tablets were also evolved from nonpitted tabular particles via central etching on surfaces
during aging (see Fig. 3b and inset). The corresponding mean crystallite size of this sample is ca. 79 nm evaluated from the XRD analysis (Fig. 1c), larger than that of the sample produced at PEG concentration of 4% (65 nm). This was probably due to adsorption of PEG macromolecules on to the growing crystal surfaces, which consequently impeded crystal growth. Compared with the sample obtained in the presence of PEG (Fig. 2a), it is clear that, at the PEG concentration of 4%, the dissolution at the central regions of the tablets is substantially favored. The augmented etching at the PEG concentration of 4% could be associated with the decrease in crystallite size in the polycrystalline particles. With increasing PEG concentration to 10%, there is no considerable change in the crystallite size (62 nm, XRD data not shown) and morphology of the product (SEM image, not shown here) compared to those with the sample obtained at PEG concentration of 4%. However, when the concentration of PEG was further increased to 20%, thicker tablets with rounded shapes were obtained (Fig. 4), though the crystallite size did not change remarkably (60 nm, Fig. 1a). It is interesting to note that for most particles etching at the central regions did not lead to pitting in this case, but resulted in a rugged sponge-like central region on their surfaces, similar to the case in the absence of PEG (see Fig. 3a and inset). Obviously, the etching process was
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retarded at such high PEG concentration of 20% in comparison with the cases that PEG concentrations were 4% and 10%, albeit the differences of the crystallite sizes in these three samples were very small. A plausible explanation for this result would be related to the presumption that the polycrystalline particles were almost completely coated by the polymer macromolecules at the PEG concentration of 20%, which as a result hindered the intensive etching from occurring. Thus, it can be inferred that at room temperature, a low PEG concentration (e.g. 4% or 10%) favors the etching process due to smaller crystallite size; in contrast, a high PEG concentration (e.g. 20%) inhibits this etching process probably because of excessive polymer coated on the particles surfaces. To obviate the possible formation of copper hydroxide, all the experiments in this study were carried out under an acidic condition (i.e. pH 2 in the foregoing discussion). It should be noted that further increasing acidity in the reaction system will inevitably increase the solubility of copper oxalate because oxalic acid is a weak acid, and thus the precipitation–dissolution equilibrium of copper oxalate is sensitive to the concentration of present hydrogen ions,
Fig. 4. SEM images of copper oxalate particles obtained with PEG concentrations of 20% by aging at room temperature for 48 h; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
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as given in the following scheme: 2 CuC2 O4 ðsÞ Ð Cu2þ ðaqÞ þ C2 O4ðaqÞ þ C2 O2 4ðaqÞ þ H ðaqÞ Ð HC2 O4ðaqÞ þ HC2 O 4ðaqÞ þ HðaqÞ Ð H2 C2 O4ðaqÞ .
It follows that an acidic condition in reaction mixture favors the etching process. Hence, at a constant PEG concentration of 10% and diverse H2SO4 concentrations, copper oxalate precipitation experiments were carried out. Fig. 5a presents the SEM image of the sample obtained at H2SO4 concentration of 0.06 M (initial pHo1). As shown, quasi-rectangular convex tabular particles with thickness of ca. 500 nm and edge length of ca. 1 mm were produced. The deep etch pit on the surfaces of the tablets makes them seem like tabular cages. With increasing H2SO4 concentration to 0.08 M, smaller rectangular pitted tablets were obtained, and the central pits were relatively large (Fig. 5b). Comparing Fig. 5a with Fig. 5b, it is obvious that the enhanced acidity favors the etching process, as expected. It was reported that classical means for controlling crystallization by changing parameters such as temperature was sufficient for size control, but had limited influence on the crystal morphology or texture of a growing crystal [23]. However, owing to the etching process occurring in the present cases, and the temperature dependence of solubility of copper oxalate, it was expected that temperature would exhibit considerable influence on the morphology of copper oxalate polycrystals. The effect of temperature was firstly investigated in the absence of PEG. As shown in the SEM image in Fig. 6a, with increasing aging temperature to 60 1C from room temperature, the obtained copper oxalate tablets are made up of a loose and porous pattern enwrapped by a densely continuous rectangular frame. The irregular interstices within the pattern can be clearly seen. The TEM image of the sample (Fig. 6b) shows that the tablets consist of irregular shaped subcrystals with nanometer size. Electron diffraction (ED) pattern (inset in
Fig. 5. SEM images of copper oxalate particles obtained with H2SO4 concentration of (a) 0.06 M and (b) 0.08 M by aging at room temperature for 48 h; [PEG] ¼ 10%; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M.
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Fig. 6. SEM images (a), TEM image (b), ED pattern (inset in b) and HRTEM image (c) of copper oxalate sample obtained in the absence of PEG by aging at 60 1C for 48 h; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
Fig. 6b) confirms that the sample is polycrystalline. Fig. 6c presents the HRTEM image of this sample. In this image, the direction of the lattice fringes of the adjacent subcrystals are randomly orientated from each other, suggesting that the subcrystals in the polycrystalline copper oxalate tablets are arbitrarily attached. Clearly, such polycrystalline particles contain much more defects than those constructed by oriented attachment of subcrysrtals, and are subject to etch. As expected, an etching process was confirmed again because smooth non-patterned convex quasi-rectangular tablets were observed at early stage (data not shown). It was clear that a higher temperature significantly facilitated such etching process. Unlike the foregoing cases at room temperature, because of decrease in solubility of copper oxalate when the reaction mixture was gradually cooled to room temperature, random deposition or recrystallization onto the partially dissolved surfaces of the tablets probably occur. This might be the main reason why patterned, rather than pitted tablets were formed at an elevated temperature, though the etching process occurred at both the higher and the room temperature. Additionally, it was found that similar patterned tablets could be also obtained at the temperatures ranging from 60 to 100 1C. Fig. 7a and b presents the SEM images of the samples prepared at 80 and 100 1C, respectively. However, when the temperature was elevated to 140 1C, as illustrated in Fig. 7c, the surfaces of obtained tablets became relatively smooth and even, and for some tablets the frames became less distinct. This was probably resulted from excessive dissolution during aging at such the
high temperature, and subsequently considerable deposition occurring on the surfaces of the tablets with the reaction mixture cooling to room temperature, which almost completely recovered the interstices on partially dissolved surfaces. Interestingly, although the presence of PEG exerted appreciable effect on the etching process at room temperature, it seemed that this effect was impaired at a higher temperature. As shown in Fig. 8a, patterned tablets of ca. 1 mm were obtained at 80 1C, to some extent analogous to the samples obtained without addition of PEG at high temperatures (Fig. 7a and b). The patterns on the tablet surfaces were proved to be caused by etching as illustrated in Fig. 8b. This result could be rationally related to the desoption of PEG macromolecules from the solid surfaces with raising temperature, which results in the similar result to the ones in which no PEG was added. 4. Conclusions In summary, copper oxalate micro-sized polycrystalline tablets with pitted and patterned surfaces could be produced via a facile precipitation reaction between copper sulfate and oxalic acid solutions. The formation of the pit on the surfaces of the tablets at room temperature was ascribed to an etching process occurring during aging. The etching process at room temperature was favorable when appropriate amount of PEG was present in reaction mixture due to smaller crystallite size, and inhibited at a very high PEG concentration owing to polymer coating on
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Fig. 7. SEM images of copper oxalate particles obtained in the absence of PEG by aging at: (a) 80 1C, (b) 100 1C, and (c) 140 1C for 48 h; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
Fig. 8. SEM images of copper oxalate particles with PEG concentration of 20% by aging at 80 1C for: (a) 48 h and (b) 8 h; [Cu2+] ¼ [C2O2 4 ] ¼ 0.01 M, pH 2.
the solid surfaces. Increasing acidity in reaction system could promote the etching procedure. A higher aging temperature led to not only a stronger etching, but also to random deposition of copper oxalate on the surfaces of the tablets when the reaction system was cooled to room temperature. This resulted in patterned copper oxalate tablets at higher temperatures. The effect of PEG was weakened at a higher temperature because a higher temperature would be disadvantageous to the adsorption of PEG macromolecules on solid surfaces. Acknowledgments This work was partially supported by the National Natural Science Foundation of China (20473059 and
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