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Morphology-controlled synthesis of inorganic nanocrystals by ionic liquid assistance
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Morphology-controlled synthesis of inorganic nanocrystals by ionic liquid assistance Kezhen Qia,b and Wenjun Zhengb,c Ionic liquids (ILs) assisted synthesis is an effective way to control the preparation of inorganic materials. The unique property of ILs provides new opportunities and has demonstrated great potential for the design and production of the morphology controlled nanomaterials. This review is focused on several aspects of the synthesis mechanism, such as adsorption selectivity of ILs based on the geometric matching principle, utilization of ILs as reaction media for synthesis of concave nanostructures, and the adsorption model for ILs on crystals. The challenge and opportunity in this rapid developing area of IL-assisted synthesis nanomaterials are also discussed. Addresses a Institute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang, 110034, PR China b Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), and TKL of Metal and Molecule-Based Materials Chemistry, College of Chemistry, Nankai University, Tianjin 300071, PR China c Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, PR China Corresponding author: Zheng, Wenjun ([email protected])
weak interaction (hydrogen bond, and van der Waals force), pep stacking, and low interfacial tension. The unique characteristics of ILs can modify the surface chemical properties and the energetics of primary crystals. Until now, different morphologies of nano/micromaterials have been synthesized via various ILassisted routes, (e.g. TiO2 [16e20], ZnO [21,22], Al2O3 [23], CdSe [24], Au [25,26], Bi2S3 [27,28], and carbon materials [29]). However, at least two basic questions are exposed: (i) how to select the suitable ILs to produce inorganic crystals with the desired morphology, and (ii) how to understand the mechanism of adsorption selectivity of ILs on different crystal facets. In this paper, the model of interaction between ILs and inorganic nanomaterials is reviewed, hoping to improve the theoretical consideration, and to provide new ideas for the synthesis of specific functional nanomaterials.
Adsorption selectivity of ILs guided by geometric matching principle
Introduction
In 2009, our group has reported that the interaction between imidazolium cation and TiO6 octahedra could be the guiding factor for the formation of rutile phase in a water/1-ethyl-3-methylimidazolium bromide ([Emim] Br) system (Figure 1) [30]. A model was suggested that [Emim]Br acted as a capping agent based on its hydrogen-bonding and pep stacking interaction with the rutile (110) facet and accordingly controlling the phase and morphology of growing TiO2 crystals. This finding not only opened a new application field of ILs in inorganic synthesis, but also provided a new perspective for creating new nanostructures.
Ionic liquids (ILs), due to their unique and tunable properties, offer a good potential of preparing inorganic materials with controlled phase, morphology, structure and property, which is otherwise impossible to be prepared from conventional solvents [1e7]. The interaction between ILs and surface of growing crystals is the fundamental thrust for accomplishing such controls, and the latter is also one of the key scientific issues in this field. Many new opportunities for inorganic synthesis are brought by ILs [8e15] such as: (a) The strong dissolving power of ILs breaking through the limitation of the principle of similar phase dissolution; (b) the possibility of realizing the continuous adjustment of the medium polarity and ionic strength; (c) the participation with strong interaction (electrostatic interaction),
Density functional theory (DFT) calculation, as an effective tool in exploration of surface chemistry, has been widely used to study the mechanism about how the growth environments are affecting the crystal morphologies [31e34]. In Figure 2, the models of clean surface, and the Hþ, H2O and ILs adsorption surface were assumed for simulating the four related crystal growing environments, i.e. air, water, acidic condition, and ionic liquid assistance, respectively. [Emim]Br forms an electrostatic layer at TiO2 surface [35], which is relevant to the Derjaguin-Landau-Verwey-Overbeek (DLVO) model [36e38]. Figure 2c shows the adsorption energies of [Emim]Br on rutile facets, so that [Emim]Br prefers to adsorb on the (110) plane. On the
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Current Opinion in Green and Sustainable Chemistry 2017, 5:17–23
Current Opinion in Green and Sustainable Chemistry 2017, 5:17–23 This review comes from a themed issue on Green Solvents 2017 Edited by Charlotta Turner and Jianji Wang http://dx.doi.org/10.1016/j.cogsc.2017.03.011 2452-2236/© 2017 Elsevier B.V. All rights reserved.
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Figure 1
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(a) Surface structure of rutile cleaved along the [110] direction and schematic illustration of rutile (110)-c(2 × 2)-[Emim]+ original cell. (b) Schematic illustration of a projected view of [Emim]+ ions anchored onto rutile (110) plane to form tight coverage layer via the original cell [30].
Figure 2
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Optimized structures (a) Clean, (b) H+-, (c) [Emim]Br- (side view, to see the structure clearly, only one [Emim]+ ion was reserved) and (d) [Emim]Bradsorbed (top view) on rutile (110), (101), (001), and anatase (100), (101), (001) facets, (e) corresponding surface energies (g) [35]. Adsorption energy (eV) of [Emim]Br is shown in (c). Atoms are presented as gray (Ti), red (O), deep gray (C), sky-blue (N), white (H), deep red (Br). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
rutile (110) surface, the intermolecular distances between adsorbed [Emim]Br molecules can be 5.95, 6.55 ˚ , which are all close to the pep stacking and/or 7.20 A ˚ ) [39e41]. For distances of [Emim]Br (6.00e7.00 A anatase, the (101) surface is the most stable facet, but
the distance between bridging O atoms does not satisfy the mutual p-stacking between aromatic rings, restricting [Emim]þ ions to perpendicularly adsorbed on this plane. On the anatase (100) surface, the distances between bridge O atoms along [011] and [001]
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Morphology-controlled synthesis of inorganic nanocrystals Qi and Zheng
˚ , respectively, and the directions are 6.10 and 5.91 A intermolecular stacking distances of adsorbed [Emim] ˚ , respectively. Br along two directions are 6.23 and 7.10 A þ Thus, [Emim] could be allowed to perpendicularly adsorb on anatase (100), rather than (101) or (001). Based on these results, the authors proposed the match degree of geometry between the IL structure and the lattice of crystal facets which can be used in judging the adsorption selectivity of ILs. More experimental results have also supported this geometric matching principle. For example, Qi et al. [42], using IL as a capping agent, successfully prepared cube-shape anatase crystals with a high percentage of exposed (100) facets. Qi et al. [43] demonstrated that [Emim]Br not only induces a higher length-to-diameter ratio of rutile crystals but also increases the contribution of exposed (110) facets. Zhou et al. [44] proposed a model that possesses the hydrogen bonds formed between [BF4]e ions and SiO2 surface and the pep stacking interaction of neighboring imidazolium rings, which led to the mutual packing and formation of mesoporous SiO2. Similar phenomena were observed in other materials prepared, including Fe2O3 [45], ZnO [46], La(OH)3 [47], BiOCl [48], and so on. Therefore, a simple method is provided to predict the adsorption
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selectivity of ILs, that is, the interaction between adsorbed ILs and substrate is largely depended on the geometric matching degree at the intersurface of ILs/ substrate.
ILs as synthetic media By using ILs as the reaction media, one can change several important properties of the reaction solution, such as the solubility of gases (e.g., hydrogen, carbon dioxide, nitrogen, oxygen, etc.), and the pH of the solution [49e54]. Recently, the concave Cu2O crystals were prepared by a simple hydrothermal method under the assistance of [Emim]Br. Then the oxidative etching growth mechanism has been proposed (Figure 3). Without adding ILs to the reaction solution, the Cu2O crystals are octahedrals, indicating the diffusion-limited growth mechanism, however, as ILs adding into, the morphology of crystals shows serious facet-etched architectures, indicating the etching-limited growth. Oxidative etching is an important factor to affect the Cu2O morphologies [55e57]. Water solubility of oxygen at 25 C and pressure = 1 bar is about 40 mg/L water, while about 800 mg/L in IL solution [49]. It can be recognized that the addition of ILs will enhance the oxygen solubility and affect the Cu2O crystal etching
Figure 3
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Illustrations of (A) diffusion-limited growth, with no ILs assistance and (B) etching-limited growth, with ILs assistance. Optimized structures of [Emim]+ adsorbed on Cu2O (100) (C) and (111) facets (D). The calculated adsorption energy, Eads (eV). Brown: Cu; red: O; deep gray: C; sky blue: N; white: H. Schematic illustrations of the growth mechanism of Cu2O crystals without (E) and with ILs assistance (F). In press. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) www.sciencedirect.com
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growth. The (111) and (100) facet is Cu and O atom terminated, respectively. So, the (111) facet will be positively charged and interacts with negatively charged ions; by contrast, the (100) facet is negatively charged. It follows that oxygen will more likely attack the (111) facet, making the Cu2O (111) surfaces etched more. By the same reason, the [Emim]þ cations prefer to adsorb on Cu2O (100) than (111) facets (Figure 3B), and protect the (100) facet, forming the step-shape truncated octahedral. When [Emim]þ is absent, according to the crystal growing habit of Cu2O, there are branches extended at the vertices of these octahedrons, and fast growth in the [100] direction will take place, resulting the exposed (111) surfaces (Figure 3A).
in the formation of the ring structure. Whereas, Duan et al. [59] by adjusting the molar ratio of Fe(NO3)3$9H2O to 1-n-butyl-3-methylimidazolium dihydrogen phosphate ([Bmim][H2PO4]), have obtained truncated bipyramids microstructures exposed with dominated (111) facets and with carved (001) facets because of a cooperative effect between [Bmim]þ ions and additives (ethylene glycol and C4H9OH). The formation of carved bipyramid occurs through the preferential dissolution by Hþ ions attacking along the c axis, while the (111) facet is protected by [Bmim]þ ions. Therefore, it is an effective way to control the concave nanostructures through changing the solution environment by adding ILs.
Similar etching growth phenomena were observed in synthesis of other nanomaterials. Qi et al. [58] prepared ZnO nanorings with controllable ratio of length to width under ILs assistance, i.e. by using 1-propyl-3methylimidazolium bromide ([C3mim]Br) as capping agent to control the aspect ratio, which is mainly due to the protection of [C3mim]Br on the ZnO side (10-10) surface. Moreover, [C3mim]Br, as a kind of salt, will increase the ionization constant of the aqueous solution and thereby contribute to be the higher concentration of Hþ cations [51]. Because the (000-1) surface possesses higher defect density, it will be rapidly eroded resulting
Adsorption model for ILs on crystals The surface property of the growing crystal will directly affect its final morphology, thus the interaction model between ILs and crystal facet should be understood [60e62]. Duan et al. [63] utilized ammonium aluminum carbonate hydroxide (NH4-Dw) and gAlOOH, as the model system, to study the effect of ILs adsorption on the reaction process. And two different interaction mechanisms of ILs with crystal surface were proposed (Figure 4), such that the linear NH4-Dw and the flower-like g-AlOOH nanostructures can be obtained by adjusting the amount of 1-butyl-2,3-
Figure 4
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Scheme for the different effect models of [Bdmim]Cl in the synthesis of NH4-Dw and g-AlOOH [63]. Current Opinion in Green and Sustainable Chemistry 2017, 5:17–23
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Morphology-controlled synthesis of inorganic nanocrystals Qi and Zheng
dimethylimidazolium chloride ([Bdmim]Cl). The effective models are the cationic or the anionic dominant effective model, respectively, and determined by the different surface structure of the substrates. Therefore, according to the need of morphological control, the effect of ILs may be tuned by the appropriate choice of cations or anions of the ILs so as to match the effective adsorption model. Or, based on the isoelectric point balance requirement, tuning the pH value of the reaction medium also affects the adsorption mode of ILs on the crystal surface. By the way, this mechanism is also reflected in synthesis of other inorganic materials. Zhou et al. [44] synthesized mesoporous SiO2 using 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) as the template, and the [BF4]e anions interact with the silanol groups of SiO2 surface, which results in the orientation arrangement of [BF4]e ions along the pore walls. Similarly, Bockstaller et al. [64] prepared Au nanorods in 1-ethyl3-methylimidazolium ethylsulfate ([Emim][ES]), and suggested that the binding ability of imidazolium cation on different crystal facets of Au crystals is different, leading to the formation of Au nanorods. Wu et al. [65] prepared Zn(OH)F nanofibers under the assistance of 1,2,3-trimethylimidazolium tetrafluoroborate ([Tmim] [BF4]), and proposed the following interaction mechanism: the (110) facet of Zn(OH)F is suitable to form a full-coverage layer of the [BF4]e anions, and under the coulomb coupling force the [Tmim]þ cations will then form electrostatic adsorption layer. All in all, it is quite valuable to establish such models between ILs and target products for selecting rational IL media.
Conclusion and outlook In this paper, the mechanism of ILs-assisted synthesis of nanomaterials is reviewed, mainly focusing on how to select the proper ILs to control desired crystal morphology. The recent developments have been summarized as follows: (i) Primarily, using the geometric matching principle to predict the adsorption selectivity of ILs is discussed. (ii) The use of ILs as reaction media for synthesis of concave nanostructures and (iii) the adsorption model of ILs on crystals are discussed. In many cases, ILs are simply treated as surfactants or solvents, and their unique advantages have not been fully demonstrated. There are many factors, which need to be solved: (i) How to combine the various forces operating in the IL system to promote the synthesis of desired nanomaterials? (ii) How to develop the new synthetic methods based on the special properties of ILs? (iii) How to further study the interaction mechanism between ILs and substrates?
Notes
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Acknowledgments This work was supported by the Programs of National Natural Science Foundation of China (21371101, 51672135, 51602207 and 21421001) and MOE (B12015).
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59. Duan X, Li D, Zhang H, Ma J, Zheng W: Crystal-facet engi* neering of ferric giniite from ionic-liquid precursors and their enhanced photocatalytic performances under visible-light irradiation. Chem. A Eur. J. 2013, 19:7231–7242. The truncated bipyramids microstructures with dominated exposed facets {111} and with carved {001} facets were prepared, which is due to a cooperative effect between [Bmim]+ ions and additives (EG and C4H9OH). The formation of carved bipyramid occurs through a process of preferential dissolution by H+ ions attacked along the c axis and {111} facet is protected by [Bmim]+ ions. 60. Bouvy C, Baker GA, Yin H, Dai S: Growth of gold nanosheets and nanopolyhedra in pyrrolidinium-based ionic liquids: investigation of the cation effect on the resulting morphologies. Cryst. Growth Des. 2010, 10:1319–1322. 61. Duan X, Ma J, Lian J, Zheng W: The art of using ionic liquids in the synthesis of inorganic nanomaterials. CrystEngComm 2014, 16:2550–2559. 62. Mali SS, Betty CA, Bhosale PN, Devan RS, Ma Y-R, Kolekara SS, Patil PS: Hydrothermal synthesis of rutile TiO2 nanoflowers using brønsted acidic ionic liquid [BAIL]: synthesis, characterization and growth mechanism. CrystEngComm 2012, 14: 1920–1924. 63. Duan X, Kim T, Li D, Ma J, Zheng W: Understanding the effect * * models of ionic liquids in the synthesis of NH4-dw and gAlOOH nanostructures and their conversion into porous gAl2O3. Chem. A Eur. J. 2013, 19:5924–5937. Two different interaction mechanisms of ionic liquids with crystal surface were proposed, that is, under the cationic dominant regime, ionic liquids mainly show dispersion effect for NH4-Dw nanostructures meanwhile the anionic dominant model can induce g-AlOOH particles self-assembly to form hierarchical structures. 64. Ryu HJ, Sanchez L, Keul HA, Raj A, Bockstaller MR: Imidazo* lium-based ionic liquids as efficient shape-regulating solvents for the synthesis of gold nanorods. Angew. Chem. Int. Ed. 2008, 47:7639–7643. Au nanorods were prepared in [Emim][ES] and the authors proposed that the imidazolium cations have different binding ability to different crystal facets of Au crystals, resulting in the formation of Au nanorods. 65. Wu L, Lian J, Sun G, Kong X, Zheng W: Synthesis of zinc * hydroxyfluoride nanofibers through an ionic liquid assisted microwave irradiation method. Eur. J. Inorg. Chem. 2009, 2009: 2897–2900. Zn(OH)F nanofibers were synthesized using [Tmim][BF4] assistance and the authors proposed the interaction mechanism: the (110) plane of Zn(OH)F is suitable to form a coverage layer through a [BF4] – , and via the coulomb coupling force, the [Tmim]+ cations will then form Electrostatic adsorption layer.
Current Opinion in Green and Sustainable Chemistry 2017, 5:17–23
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Morphology-controlled synthesis of inorganic nanocrystals by ionic liquid assistance Kezhen Qia,b and Wenjun Zhengb,c Ionic liquids (ILs) assisted synthesis is an effective way to control the preparation of inorganic materials. The unique property of ILs provides new opportunities and has demonstrated great potential for the design and production of the morphology controlled nanomaterials. This review is focused on several aspects of the synthesis mechanism, such as adsorption selectivity of ILs based on the geometric matching principle, utilization of ILs as reaction media for synthesis of concave nanostructures, and the adsorption model for ILs on crystals. The challenge and opportunity in this rapid developing area of IL-assisted synthesis nanomaterials are also discussed. Addresses a Institute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang, 110034, PR China b Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), and TKL of Metal and Molecule-Based Materials Chemistry, College of Chemistry, Nankai University, Tianjin 300071, PR China c Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, PR China Corresponding author: Zheng, Wenjun ([email protected])
weak interaction (hydrogen bond, and van der Waals force), pep stacking, and low interfacial tension. The unique characteristics of ILs can modify the surface chemical properties and the energetics of primary crystals. Until now, different morphologies of nano/micromaterials have been synthesized via various ILassisted routes, (e.g. TiO2 [16e20], ZnO [21,22], Al2O3 [23], CdSe [24], Au [25,26], Bi2S3 [27,28], and carbon materials [29]). However, at least two basic questions are exposed: (i) how to select the suitable ILs to produce inorganic crystals with the desired morphology, and (ii) how to understand the mechanism of adsorption selectivity of ILs on different crystal facets. In this paper, the model of interaction between ILs and inorganic nanomaterials is reviewed, hoping to improve the theoretical consideration, and to provide new ideas for the synthesis of specific functional nanomaterials.
Adsorption selectivity of ILs guided by geometric matching principle
Introduction
In 2009, our group has reported that the interaction between imidazolium cation and TiO6 octahedra could be the guiding factor for the formation of rutile phase in a water/1-ethyl-3-methylimidazolium bromide ([Emim] Br) system (Figure 1) [30]. A model was suggested that [Emim]Br acted as a capping agent based on its hydrogen-bonding and pep stacking interaction with the rutile (110) facet and accordingly controlling the phase and morphology of growing TiO2 crystals. This finding not only opened a new application field of ILs in inorganic synthesis, but also provided a new perspective for creating new nanostructures.
Ionic liquids (ILs), due to their unique and tunable properties, offer a good potential of preparing inorganic materials with controlled phase, morphology, structure and property, which is otherwise impossible to be prepared from conventional solvents [1e7]. The interaction between ILs and surface of growing crystals is the fundamental thrust for accomplishing such controls, and the latter is also one of the key scientific issues in this field. Many new opportunities for inorganic synthesis are brought by ILs [8e15] such as: (a) The strong dissolving power of ILs breaking through the limitation of the principle of similar phase dissolution; (b) the possibility of realizing the continuous adjustment of the medium polarity and ionic strength; (c) the participation with strong interaction (electrostatic interaction),
Density functional theory (DFT) calculation, as an effective tool in exploration of surface chemistry, has been widely used to study the mechanism about how the growth environments are affecting the crystal morphologies [31e34]. In Figure 2, the models of clean surface, and the Hþ, H2O and ILs adsorption surface were assumed for simulating the four related crystal growing environments, i.e. air, water, acidic condition, and ionic liquid assistance, respectively. [Emim]Br forms an electrostatic layer at TiO2 surface [35], which is relevant to the Derjaguin-Landau-Verwey-Overbeek (DLVO) model [36e38]. Figure 2c shows the adsorption energies of [Emim]Br on rutile facets, so that [Emim]Br prefers to adsorb on the (110) plane. On the
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Current Opinion in Green and Sustainable Chemistry 2017, 5:17–23 This review comes from a themed issue on Green Solvents 2017 Edited by Charlotta Turner and Jianji Wang http://dx.doi.org/10.1016/j.cogsc.2017.03.011 2452-2236/© 2017 Elsevier B.V. All rights reserved.
18 Green Solvents 2017
Figure 1
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(a) Surface structure of rutile cleaved along the [110] direction and schematic illustration of rutile (110)-c(2 × 2)-[Emim]+ original cell. (b) Schematic illustration of a projected view of [Emim]+ ions anchored onto rutile (110) plane to form tight coverage layer via the original cell [30].
Figure 2
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Optimized structures (a) Clean, (b) H+-, (c) [Emim]Br- (side view, to see the structure clearly, only one [Emim]+ ion was reserved) and (d) [Emim]Bradsorbed (top view) on rutile (110), (101), (001), and anatase (100), (101), (001) facets, (e) corresponding surface energies (g) [35]. Adsorption energy (eV) of [Emim]Br is shown in (c). Atoms are presented as gray (Ti), red (O), deep gray (C), sky-blue (N), white (H), deep red (Br). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
rutile (110) surface, the intermolecular distances between adsorbed [Emim]Br molecules can be 5.95, 6.55 ˚ , which are all close to the pep stacking and/or 7.20 A ˚ ) [39e41]. For distances of [Emim]Br (6.00e7.00 A anatase, the (101) surface is the most stable facet, but
the distance between bridging O atoms does not satisfy the mutual p-stacking between aromatic rings, restricting [Emim]þ ions to perpendicularly adsorbed on this plane. On the anatase (100) surface, the distances between bridge O atoms along [011] and [001]
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Morphology-controlled synthesis of inorganic nanocrystals Qi and Zheng
˚ , respectively, and the directions are 6.10 and 5.91 A intermolecular stacking distances of adsorbed [Emim] ˚ , respectively. Br along two directions are 6.23 and 7.10 A þ Thus, [Emim] could be allowed to perpendicularly adsorb on anatase (100), rather than (101) or (001). Based on these results, the authors proposed the match degree of geometry between the IL structure and the lattice of crystal facets which can be used in judging the adsorption selectivity of ILs. More experimental results have also supported this geometric matching principle. For example, Qi et al. [42], using IL as a capping agent, successfully prepared cube-shape anatase crystals with a high percentage of exposed (100) facets. Qi et al. [43] demonstrated that [Emim]Br not only induces a higher length-to-diameter ratio of rutile crystals but also increases the contribution of exposed (110) facets. Zhou et al. [44] proposed a model that possesses the hydrogen bonds formed between [BF4]e ions and SiO2 surface and the pep stacking interaction of neighboring imidazolium rings, which led to the mutual packing and formation of mesoporous SiO2. Similar phenomena were observed in other materials prepared, including Fe2O3 [45], ZnO [46], La(OH)3 [47], BiOCl [48], and so on. Therefore, a simple method is provided to predict the adsorption
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selectivity of ILs, that is, the interaction between adsorbed ILs and substrate is largely depended on the geometric matching degree at the intersurface of ILs/ substrate.
ILs as synthetic media By using ILs as the reaction media, one can change several important properties of the reaction solution, such as the solubility of gases (e.g., hydrogen, carbon dioxide, nitrogen, oxygen, etc.), and the pH of the solution [49e54]. Recently, the concave Cu2O crystals were prepared by a simple hydrothermal method under the assistance of [Emim]Br. Then the oxidative etching growth mechanism has been proposed (Figure 3). Without adding ILs to the reaction solution, the Cu2O crystals are octahedrals, indicating the diffusion-limited growth mechanism, however, as ILs adding into, the morphology of crystals shows serious facet-etched architectures, indicating the etching-limited growth. Oxidative etching is an important factor to affect the Cu2O morphologies [55e57]. Water solubility of oxygen at 25 C and pressure = 1 bar is about 40 mg/L water, while about 800 mg/L in IL solution [49]. It can be recognized that the addition of ILs will enhance the oxygen solubility and affect the Cu2O crystal etching
Figure 3
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Illustrations of (A) diffusion-limited growth, with no ILs assistance and (B) etching-limited growth, with ILs assistance. Optimized structures of [Emim]+ adsorbed on Cu2O (100) (C) and (111) facets (D). The calculated adsorption energy, Eads (eV). Brown: Cu; red: O; deep gray: C; sky blue: N; white: H. Schematic illustrations of the growth mechanism of Cu2O crystals without (E) and with ILs assistance (F). In press. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) www.sciencedirect.com
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20 Green Solvents 2017
growth. The (111) and (100) facet is Cu and O atom terminated, respectively. So, the (111) facet will be positively charged and interacts with negatively charged ions; by contrast, the (100) facet is negatively charged. It follows that oxygen will more likely attack the (111) facet, making the Cu2O (111) surfaces etched more. By the same reason, the [Emim]þ cations prefer to adsorb on Cu2O (100) than (111) facets (Figure 3B), and protect the (100) facet, forming the step-shape truncated octahedral. When [Emim]þ is absent, according to the crystal growing habit of Cu2O, there are branches extended at the vertices of these octahedrons, and fast growth in the [100] direction will take place, resulting the exposed (111) surfaces (Figure 3A).
in the formation of the ring structure. Whereas, Duan et al. [59] by adjusting the molar ratio of Fe(NO3)3$9H2O to 1-n-butyl-3-methylimidazolium dihydrogen phosphate ([Bmim][H2PO4]), have obtained truncated bipyramids microstructures exposed with dominated (111) facets and with carved (001) facets because of a cooperative effect between [Bmim]þ ions and additives (ethylene glycol and C4H9OH). The formation of carved bipyramid occurs through the preferential dissolution by Hþ ions attacking along the c axis, while the (111) facet is protected by [Bmim]þ ions. Therefore, it is an effective way to control the concave nanostructures through changing the solution environment by adding ILs.
Similar etching growth phenomena were observed in synthesis of other nanomaterials. Qi et al. [58] prepared ZnO nanorings with controllable ratio of length to width under ILs assistance, i.e. by using 1-propyl-3methylimidazolium bromide ([C3mim]Br) as capping agent to control the aspect ratio, which is mainly due to the protection of [C3mim]Br on the ZnO side (10-10) surface. Moreover, [C3mim]Br, as a kind of salt, will increase the ionization constant of the aqueous solution and thereby contribute to be the higher concentration of Hþ cations [51]. Because the (000-1) surface possesses higher defect density, it will be rapidly eroded resulting
Adsorption model for ILs on crystals The surface property of the growing crystal will directly affect its final morphology, thus the interaction model between ILs and crystal facet should be understood [60e62]. Duan et al. [63] utilized ammonium aluminum carbonate hydroxide (NH4-Dw) and gAlOOH, as the model system, to study the effect of ILs adsorption on the reaction process. And two different interaction mechanisms of ILs with crystal surface were proposed (Figure 4), such that the linear NH4-Dw and the flower-like g-AlOOH nanostructures can be obtained by adjusting the amount of 1-butyl-2,3-
Figure 4
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Scheme for the different effect models of [Bdmim]Cl in the synthesis of NH4-Dw and g-AlOOH [63]. Current Opinion in Green and Sustainable Chemistry 2017, 5:17–23
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Morphology-controlled synthesis of inorganic nanocrystals Qi and Zheng
dimethylimidazolium chloride ([Bdmim]Cl). The effective models are the cationic or the anionic dominant effective model, respectively, and determined by the different surface structure of the substrates. Therefore, according to the need of morphological control, the effect of ILs may be tuned by the appropriate choice of cations or anions of the ILs so as to match the effective adsorption model. Or, based on the isoelectric point balance requirement, tuning the pH value of the reaction medium also affects the adsorption mode of ILs on the crystal surface. By the way, this mechanism is also reflected in synthesis of other inorganic materials. Zhou et al. [44] synthesized mesoporous SiO2 using 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) as the template, and the [BF4]e anions interact with the silanol groups of SiO2 surface, which results in the orientation arrangement of [BF4]e ions along the pore walls. Similarly, Bockstaller et al. [64] prepared Au nanorods in 1-ethyl3-methylimidazolium ethylsulfate ([Emim][ES]), and suggested that the binding ability of imidazolium cation on different crystal facets of Au crystals is different, leading to the formation of Au nanorods. Wu et al. [65] prepared Zn(OH)F nanofibers under the assistance of 1,2,3-trimethylimidazolium tetrafluoroborate ([Tmim] [BF4]), and proposed the following interaction mechanism: the (110) facet of Zn(OH)F is suitable to form a full-coverage layer of the [BF4]e anions, and under the coulomb coupling force the [Tmim]þ cations will then form electrostatic adsorption layer. All in all, it is quite valuable to establish such models between ILs and target products for selecting rational IL media.
Conclusion and outlook In this paper, the mechanism of ILs-assisted synthesis of nanomaterials is reviewed, mainly focusing on how to select the proper ILs to control desired crystal morphology. The recent developments have been summarized as follows: (i) Primarily, using the geometric matching principle to predict the adsorption selectivity of ILs is discussed. (ii) The use of ILs as reaction media for synthesis of concave nanostructures and (iii) the adsorption model of ILs on crystals are discussed. In many cases, ILs are simply treated as surfactants or solvents, and their unique advantages have not been fully demonstrated. There are many factors, which need to be solved: (i) How to combine the various forces operating in the IL system to promote the synthesis of desired nanomaterials? (ii) How to develop the new synthetic methods based on the special properties of ILs? (iii) How to further study the interaction mechanism between ILs and substrates?
Notes
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Acknowledgments This work was supported by the Programs of National Natural Science Foundation of China (21371101, 51672135, 51602207 and 21421001) and MOE (B12015).
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Morphology-controlled synthesis of inorganic nanocrystals Qi and Zheng
hydrogen, argon, and carbon monoxide in 1-butyl-3methylimidazolium tetrafluoroborate between temperatures 283 K and 343 K and at pressures close to atmospheric. J. Chem. Thermodyn. 2006, 38:490–502. This paper reported the experimental values for the solubility of gases (e.g., carbon dioxide, ethane, methane, oxygen, nitrogen, hydrogen, argon and carbon monoxide) in [bmim][BF4] as a function of temperature. 50. Luo X, Wang C: The development of carbon capture by functionalized ionic liquids. Curr. Opin. Green Sustain. Chem. 2017, 3:33–38. 51. Wagner M, Stanga O, Schroer W: The liquid–liquid coexistence of binary mixtures of the room temperature ionic liquid 1-methyl-3-hexylimidazolium tetrafluoroborate with alcohols. Phys. Chem. Chem. Phys. 2004, 6:4421–4431. 52. Qi L, Yu J, Jaroniec M: Enhanced and suppressed effects of ionic liquid on the photocatalytic activity of TiO2. Adsorption 2013, 19:557–561. 53. Wu W, Han B, Gao H, Liu Z, Jiang T, Huang J: Desulfurization of flue gas: SO2 absorption by an ionic liquid. Angew. Chem. Int. Ed. 2004, 43:2415–2417. 54. Xu A, Wang J, Wang H: Effects of anionic structure and lithium salts addition on the dissolution of cellulose in 1-butyl-3methylimidazolium-based ionic liquid solvent systems. Green Chem. 2010, 12:268–275. 55. Sui Y, Fu W, Zeng Y, Yang H, Zhang Y, Chen H, Li Y, Li M, Zou G: Synthesis of Cu2O nanoframes and nanocages by selective oxidative etching at room temperature. Angew. Chem. Int. Ed. 2010, 49:4282–4285. 56. Tang L, Du Y, Kong C, Sun S, Yang Z: One-pot synthesis of etched Cu2O cubes with exposed {110} facets with enhanced visible-light-driven photocatalytic activity. Phys. Chem. Chem. Phys. 2015, 17:29479–29482. 57. Shang Y, Sun D, Shao Y, Zhang D, Guo L, Yang S: A facile topdown etching to create a Cu2O jagged polyhedron covered with numerous {110} edges and {111} corners with enhanced photocatalytic activity. Chem. A Eur. J. 2012, 18:14261–14266. 58. Qi K, Yang J, Fu J, Wang G, Zhu L, Liu G, Zheng W: * Morphology-controllable ZnO rings: ionic liquid-assisted hydrothermal synthesis, growth mechanism and photoluminescence properties. CrystEngComm 2013, 15: 6729–6735. Ring-like ZnO nanocrystals with controllable ratio of length to width were successfully prepared under assistance of [C3mim]Br, which is mainly due to the protection of [C3mim]Br on the ZnO side (10-10) facet and H+ attack to etch to (0001) facet.
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59. Duan X, Li D, Zhang H, Ma J, Zheng W: Crystal-facet engi* neering of ferric giniite from ionic-liquid precursors and their enhanced photocatalytic performances under visible-light irradiation. Chem. A Eur. J. 2013, 19:7231–7242. The truncated bipyramids microstructures with dominated exposed facets {111} and with carved {001} facets were prepared, which is due to a cooperative effect between [Bmim]+ ions and additives (EG and C4H9OH). The formation of carved bipyramid occurs through a process of preferential dissolution by H+ ions attacked along the c axis and {111} facet is protected by [Bmim]+ ions. 60. Bouvy C, Baker GA, Yin H, Dai S: Growth of gold nanosheets and nanopolyhedra in pyrrolidinium-based ionic liquids: investigation of the cation effect on the resulting morphologies. Cryst. Growth Des. 2010, 10:1319–1322. 61. Duan X, Ma J, Lian J, Zheng W: The art of using ionic liquids in the synthesis of inorganic nanomaterials. CrystEngComm 2014, 16:2550–2559. 62. Mali SS, Betty CA, Bhosale PN, Devan RS, Ma Y-R, Kolekara SS, Patil PS: Hydrothermal synthesis of rutile TiO2 nanoflowers using brønsted acidic ionic liquid [BAIL]: synthesis, characterization and growth mechanism. CrystEngComm 2012, 14: 1920–1924. 63. Duan X, Kim T, Li D, Ma J, Zheng W: Understanding the effect * * models of ionic liquids in the synthesis of NH4-dw and gAlOOH nanostructures and their conversion into porous gAl2O3. Chem. A Eur. J. 2013, 19:5924–5937. Two different interaction mechanisms of ionic liquids with crystal surface were proposed, that is, under the cationic dominant regime, ionic liquids mainly show dispersion effect for NH4-Dw nanostructures meanwhile the anionic dominant model can induce g-AlOOH particles self-assembly to form hierarchical structures. 64. Ryu HJ, Sanchez L, Keul HA, Raj A, Bockstaller MR: Imidazo* lium-based ionic liquids as efficient shape-regulating solvents for the synthesis of gold nanorods. Angew. Chem. Int. Ed. 2008, 47:7639–7643. Au nanorods were prepared in [Emim][ES] and the authors proposed that the imidazolium cations have different binding ability to different crystal facets of Au crystals, resulting in the formation of Au nanorods. 65. Wu L, Lian J, Sun G, Kong X, Zheng W: Synthesis of zinc * hydroxyfluoride nanofibers through an ionic liquid assisted microwave irradiation method. Eur. J. Inorg. Chem. 2009, 2009: 2897–2900. Zn(OH)F nanofibers were synthesized using [Tmim][BF4] assistance and the authors proposed the interaction mechanism: the (110) plane of Zn(OH)F is suitable to form a coverage layer through a [BF4] – , and via the coulomb coupling force, the [Tmim]+ cations will then form Electrostatic adsorption layer.
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