ph value dependant growth of a - fe2o3 hierarchical nanostructures

Journal of Crystal Growth 294 (2006) 353–357 pH value-dependant growth of a-Fe 2 O 3 hierarchical nanostructures Chong Jia, Yao Cheng, Feng Bao, Daqin Chen, Yuansheng Wang à State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China Received 27 February 2006; received in revised form 27 April 2006; accepted 15 June 2006 Communicated by K. Nakajima Available online 25 July 2006 Abstract The controllable synthesis of two kinds of a-Fe 2 O 3 hierarchical nanostructures, i.e., three-dimentional (3D) houseleek-like and two- dimentional (2D) snowflake-like dendrites were achieved through a simple hydrothermal route by changing pH value. The growth of a-Fe 2 O 3 dendrites was proceeded by self-assembly through two different modes of oriented attachment (OA): when pHp6, primary a-Fe 2 O 3 nanoparticles attached preferentially along the six crystallographically equivalent h 1 100i directions, resulting in the formation of sixfold-symmetric dendrites. While at pHp5, the growth process involved two steps: firstly, primary nanoparticles aggregated to form round flakes with their up and bottom surfaces parallel to {0 0 0 1} plane. These flakes stacked face-to-face with each other along [0 0 0 1] direction to construct the single crystalline spindle-like a-Fe 2 O 3 , which then aggregated together at the tips to construct the 3D houseleek-like dendrites. As far as we know, this is the first time using different modes of OA to realize the morphology control of hierarchical structures in one reaction system. r 2006 Elsevier B.V. All rights reserved. PACS: 61.66.Fn; 61.82.Rx; 81.10.Dn Keywords: A1. Nanostructures; A2. Hydrothermal crystal growth 1. Introduction The synthesis of nanophase with controlled shapes, directional and shape dependent properties is an important goal of advanced materials chemistry [1–6]. Among the various tactics used to construct desirable nanostructures, the oriented attachment (OA) [7] based self-assembly of nanocrystals should be a successful one adopting the bottom–up strategy, as has been verified by many examples over the past few years. Pacholski et al. [8] reported the formation of high-quality single crystalline ZnO nanorods through OA of quasi-spherical nanoparticles along c-axis. The (1 1 1) plane OA of cubic ZnS initial nanocrystals led to the nanorods or various branched nanostructures [9]. Either length-multiplied 1D nanostructures or 2D crystal sheets and walls were obtained by self-attachment of nanorods or nanoribbons through stacking or lateral lattice fusion [10,11]. Furthermore, the much more com- plex but ordered 3D architectures could also be obtained through various OA-based self-assemblies, such as den- drites [12,13], hollow spheres [14] , hollow octahedrons [15] and so on. In our previous papers, we have demonstrated that OA between nanoparticles along specific directions could lead to the single crystalline dendrites [13], while two- step OA-based self-assembly constructed the plate-built cylinders [16]. As diversiform nanostructures could be acquired through various OA modes, the key to control the morphology of nanocrystal could be co nverted to the control of OA modes under this bottom–up self-assembl y mechanism. Among a variety of nanostructures, the hierarchical structures are promising candidates for new functional nanomaterials. So far, many hierarchical structures of high-symmetric crystal-system, including cubic PbS [17] and noble metals [18], hexagonal Fe 2 O 3 [19] and HgS [20] , tetragonal tungstate [12] and PbMoO 4 [13], and orthor- hombic Bi 2 S 3 [21], have been synthesized. However, it is ARTICLE IN PRESS www.elsevier.com/locate/jcrysgro 0022-0248/$ -see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.06.027 à Corresponding author. Tel./fax: +86 591 8370 5402. E-mail address: yswang@fjirsm.ac.cn (Y. Wang). still a challenge to control the morphology and understand the growth mechanism owing to the traditional lack of understanding of the growth history and shape evolution process. a-Fe 2 O 3 (Hematite), the most stable iron oxide under ambient conditions, is a kind of n-type semiconductor with a band gap of 2.1 ev [22]. Due to its good stability, high resistance to corrosion and low cost, a-Fe 2 O 3 is widely used as catalysts, pigments, gas sensors, and elect rode materials [23–25]. Stimulated by its potential applications, synthesis of a-Fe 2 O 3 nanophase with special morphology is a subject of considerable topical importance [26]. We have success- fully synthesized the flower-like a-Fe 2 O 3 nanostructures from nanoparticles through OA of different planes in solvothermal system before [27]. Herein, we report the synthesis of a-Fe 2 O 3 hierarchical structures with a series of novel morphologies in hydrothermal system at low temperature. By adjusting the pH value and thus the different modes of OA, we realize the control of product architectures. As we know by literatures, this is the first time using different modes of OA to realize the morphol- ogy control in one reaction system. 2. Results and discussion Fig. 1 shows the XRD patterns of samples obtained from the solutions with different pH value. The peaks of all samples are well indexed to the hexagonal a-Fe 2 O 3 with cell constants a ¼ 0.5038 nm and c ¼ 1.377 nm (JCPDS, No. 72-0469). No diffraction peaks other than those from a-Fe 2 O 3 are detected, indicating high purity of a-Fe 2 O 3 samples. It is noticeable that, compared to the standard pattern, the ð11 20Þ and ð3030Þ diffraction peaks in the patterns for pHp5 are very strong, while at pHX6 they become weak oppositely, implying the difference of preferential growth directions for a-Fe 2 O 3 nanophase synthesized under different conditions. The typical FE–SEM images of the as-made samples are shown in Fig. 2. The pH value of the original solution and the reaction duration for Fig. 2a–f are 3, 4, 5, 5.5, 6, 11, and 3, 6, 9, 12, 24, 48 h, respectively. At pH ¼ 3, the products are houseleek-like dendrites sized about 1–2 mm with several spindle-like ‘‘leaves’’ of several hundred nanometers in length. When pH is increased to 4, the ‘‘leaves’’ of the dendrites are somewhat slenderized. More detailed nanostructures appeared from the ‘‘leaves’’ when the pH value further increased to 5, as shown in Fig. 2c, where all the ‘‘leaves’’ consist of many parallel flakes with thickness of about 100 nm. The morphology for sample at pH ¼ 5.5 is somewhat complex (see Fig. 2d), some of the ‘‘leaves’’ evolve from spindle-like to trigonal pyramid-like with one arris tending flatter than the other two. When the pH value achieved 6, all the products are 2D snowflake-like dendrites with sixfold-symmetry, as shown in Fig. 2e. Each main branch of the dendrites consists of several levels of sub-branches. With the pH value changing from 6 to 11, the morphology of dendrites does not change too much except the size changing from 5 to 7 mm to about 15 mm. Such large snowflake-like dendrites would be unstable: a large amount of the main branches separate from each other, as exhibited in Fig. 2f, resulting from the drastic agitation of boiling solution under hydrothermal condi- tions. Fig. 3 demonstrates the TEM photographs of the two typical products, i.e., 3D houseleek-like and 2D snowflake- like dendrites with pH ¼ 4 and 6, respectively. Fig. 3b , the selected area electron diffraction (SAED) pattern recorded from the squared region in a ‘‘leaf’’ of the hous eleek-like dendrite in Fig. 3a, indexed to a-Fe 2 O 3 along ½2 1 10 zone axis, reveals the ‘‘leaf’’ a single crystal with its long axis along [0 0 0 1] direction. A HRTEM image (Fig. 3c) taken from the ‘‘leaf’’-tip of a houseleek-like dendrite presents the uniform lattice structure, free of detectable crystal defects. However, the SAED from an entire houseleek-like dendrite yields complex poly-crystalline pattern, indicating that the dendrite is fabricated by random aggregation of the ‘‘leaves’’ at the tips. Fig. 3d shows the TEM image of a sixfold-symmetric snowflake-like dendrite. The SAED pattern taken from the entire dendrite, shown in Fig. 3e, reveals it a single crystal with six main branches grown along the six crystallographically equivalent h1 100i directions, respectively, as has been reported by Z.L. Wang’s group previously [19]. To reveal the generation process of the dend rites, time- dependent experiments were carried out at different reaction stages in the cases of pH ¼ 5 and 6, respectively. For the system with pH ¼ 6, it was found that many particles sized about 100 nm formed after reacting for 3 h (Fig. 4a), and these primary particles were confirmed to be hexagonal Fe 2 O 3 by XRD analysis. When the reaction lasted to 6 h, these particles aggregated together to construct the sixfold dendrites with 1–3 mm in size, as ARTICLE IN PRESS Fig. 1. The XRD patterns of samples synthesized at different pH values: pH ¼ 3 for 3 h; pH ¼ 4 for 6 h; pH ¼ 5 for 9 h; pH ¼ 5.5 for 12 h; pH ¼ 6 and 7 for 24 h; pH ¼ 11 for 48 h. C. Jia et al. / Journal of Crystal Growth 294 (2006) 353–357354 shown in Fig. 4b. Further prolonging the rea ction time to 24 h enabled the evolution from the small dendrites to the snowflake-like hier archical structures sta ted ab ove (Fig. 2e). For the syst em with pH ¼ 5, when reacted for 2 h, some 100 nm primary par ticles and round flakes sized 0. 2–0.5 mm built up by the former were obse rved (Fig. 4c ). It is noticeable that, as indica ted by the arrow, some of the flakes hav e stacked fac e-to-face with each other at this sta ge. The self-assembly of single crystal dendrites and other hierarchical nanostructures have been widely investigated in recent years [8–13,16]. In our previous papers, the single crystal PbMoO 4 dendrites were verified to grow by self- assembly through OA of nanoparticles sharing a common crystallographic orientation and joining at the planar interfaces [13], and the two-step self-assembly process through OA was found for hexagonal LaF 3 nanophase, ARTICLE IN PRESS Fig. 2. FE–SEM images of the samples obtained from the solutions with different pH values and reaction durations: (a) pH ¼ 3 for 3 h; (b) pH ¼ 4 for 6 h; (c) pH ¼ 5 for 9 h; (d) pH ¼ 5.5 for 12 h; (e) pH ¼ 6 for 24 h; (f) pH ¼ 11 for 48 h. The insets of (c)–(f) present the enlarged images of the products. Fig. 3. TEM images of the as-made samples: (a) the micrograph of the houseleek-like dendrites; (b) SAED pattern taken from the squared region in 3a; (c) HRTEM image taken from the ‘‘leaf’’-tip of a houseleek-like dendrite; (d) and (e) TEM image and its corresponding SAED pattern of a snowflake-like dendrite. C. Jia et al. / Journal of Crystal Growth 294 (2006) 353–357 355 i.e., the primary LaF 3 nanoparticles aggregated together by coalescence mainly through f10 10g planes to form nanoplates, which were then stacked face-to-face with each other along the [0 0 0 1] direction to construct the cylinder- like single crystals [16]. In the present case, similar to those of single crystal PbMoO 4 and LaF 3 , the formation of snowflake-like dendrites and spindle-like a-Fe 2 O 3 could obviously be ascribed to self-assembly through OA of primary nanoparticles proceeding in different modes explained schematically in Fig. 5. At the first stage of reaction, primary a-Fe 2 O 3 nanoparticles precipitated from the solution. Further reaction was affected by pH value of the solution: when pHX6, these nanoparticles aggregated and attached each other along the six crystallographically equivalent h1 100i directions, just like the case of PbMoO 4 [13], to form snowflake-like single crystal dendrites; while for solution of pHp5, much more primary nanoparticles were produced at the first reaction stage due to the faster reaction velocity (which will be discussed later), and as a result, instead of the dendrites with six trunks, the round flakes (regarded as the space-crammed dendrites) with their up and bottom surfaces parallel to {0 0 0 1} plane were formed, which then stacked face-to-face with each other along the [0 0 0 1] direction to build-up the single crystalline spindle-like a-Fe 2 O 3 , similar to the case of two-step self- assembly of LaF 3 cylinders [16]. The 3D houseleek-like morphology was further constructed by the aggregation of several spindle-like crystals at the tips. Certainly, accom- panying with self-assembly, the traditional Ostwald ripen- ing mechanism also acted to form the primary nanoparticles and smooth the product morphology during the course of OA. In published literatures, it was a general situation that only one kind of OA occurs in one reaction system. Herein, it is notewo rthy that by simply changing pH value, the a-Fe 2 O 3 dendrites with different morphol- ogies were formed through different modes of OA in one reaction system, which may provide a route to access controlled manufacture of newfangled nanostructures probably with useful properties. The chemical reactions concerned in the form ation of a-Fe 2 O 3 could be proposed below: ½FeðCNÞ 6  3À aFe 3þ þ 6CN À ; (1) Fe 3þ þ 3OH À a À FeOOH þ H 2 O; (2) 2aFeOOHaFe 2 O 3 þ H 2 O, (3) CN À þ H þ dHCN: (4) Among the four equations, Eq. (4) is very important although it does not participate in the formation of a-Fe 2 O 3 . Without this reaction, the CN À concentration will increase continuously with the decreasing of Fe 3+ concentration, and the dissociation of [Fe(CN) 6 ] 3À will thus be strongly restricted, and as a result, the hydro- thermal reaction will last for only a short duration at the initial stage. From the above analysis, it could be concluded that the CN À ions play two main roles in the hydrothermal reaction: ligand of Fe 3+ and reactant of Hþ. The concentration of Fe 3+ ions is the key factor determining the velocity of the whole reaction due to the weak dissociation tendency of [Fe(CN) 6 ] 3À ions (K s ¼ 1.0  10 À42 ) [19]. As for the hydrolyzation of Fe 3+ ions, among the different factors that can make an effect, such as the reaction temperature, the reactants’ concentra- tion, and so on [19], the pH value would be the most significant one. From Eqs. (1) and (4), the concentration change of Fe 3+ ions brought by the change of pH value can be estimated approximately as followe d. With pH value minus 1, the H + concentration increases 10 times, which leads to the decrease of CN À concentration to one- tenth of before owing to the low ionic constant of HCN ARTICLE IN PRESS Fig. 4. FE–SEM images of a-Fe 2 O 3 dendrites obtained under different reaction stages: (a) pH ¼ 6 for 3 h; (b) pH ¼ 6 for 6 h; (c) pH ¼ 5 for 2 h. Fig. 5. Schematic illustration for the self-assembly of two kinds of a- Fe 2 O 3 dendrites. C. Jia et al. / Journal of Crystal Growth 294 (2006) 353–357356 (K a ¼ 6.2  10 À10 ), and in turn, the Fe 3+ concentration increases about 10 6 times calculated from K s of [Fe(CN) 6 ] 3À , resulting in a tremendous increase of the reaction velocity. This could exp lain the ch ange of the durat ion for complete formation of a-Fe 2 O 3 from 48 h (for pH ¼ 11)to3h(for pH ¼ 3) in our ex periment. Additionally, based on Eq. (4) , with increasing or decreasing of pH value, Eq. (1) moves toward left or right, respectively, which significantly affects the supply of Fe 3+ and thus the growth rate of a-Fe 2 O 3 ,and finally re sulting in the different growth modes and product morphologies. 3. Conclusion Using a simple hydrothermal route, we realized the morphology control of a-Fe 2 O 3 dendrites by changing pH value of the reaction solution. When pHX6, the 2D snowflake-like dendrites were formed by the self-assembly of primary a-Fe 2 O 3 nanoparticles through OA preferen- tially along the six crystallographically equivalent h1 10 0i directions. While at pHp5, the primary nanoparticles first aggregated through OA to form round flakes with their up and bottom surfa ces parallel to {0 0 0 1} plane, which were then stacked face-to-face with each other along the [0 0 0 1] direction to build the single crystalline spindle-like a- Fe 2 O 3 . Finally, the spindle-like crystals were further aggregated at the tips to construct the 3D houseleek-like morphology. 4. Experimental procedure a-Fe 2 O 3 hierarchical structures were synthesized by low- temperature hydrothermal reaction of the solution contain- ing 0.015 M K 3 [Fe(CN) 6 ] and 0.15 M acetic acid. The pH value of the solution was adjusted from 3 to 11 using 5 M ammonia. In a typical experiment, the above-mentioned solution of 50 mL with different pH value was transferred into a Teflon-sealed autoclave of 70 mL capacity, and maintained at 140 1 C for a suit able time. After the autoclave was quickly cooled down to room temperature by quenching in water, the products with different co lor (black for pH ¼ 3; brown with different degree for pH ¼ 4–5.5; and vermeil for pH ¼ 6–11) were filtered off, repeatedly washed with distilled water and absolute ethanol, and then dried in air at 50 1C for 4 h. 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Journal of Crystal Growth 294 (2006) 353–357 pH value-dependant growth of a-Fe 2 O 3 hierarchical nanostructures Chong Jia, Yao Cheng,. patterns of samples synthesized at different pH values: pH ¼ 3 for 3 h; pH ¼ 4 for 6 h; pH ¼ 5 for 9 h; pH ¼ 5.5 for 12 h; pH ¼ 6 and 7 for 24 h; pH ¼ 11