Ultrafast Synthesis of Ni-MOF in One Minute by Ball Milling - MDPI

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Dec 18, 2018 - Keywords: metal-organic frameworks; ball milling; solvent-free; ... demonstrated to have great application prospects for ... [17] described a simple solution-phase method ... Mechanochemistry, i.e., chemical synthesis enabled or sustained by ..... FriÅ¡cic, T. Supramolecular concepts and new techniques in ...
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Ultrafast Synthesis of Ni-MOF in One Minute by Ball Milling Ren Zhang 1,2,† , Cheng-An Tao 2, *,† , Rui Chen 2 , Lifang Wu 1,2 , Xiaoxuan Zou 1, * and Jianfang Wang 2, * 1

2

* †

College of Chemistry, Key Laboratory of Environmental Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, China; [email protected] (R.Z.); [email protected] (L.W.) College of Liberal Arts and Science, National University of Defense Technology, Changsha 410073, China; [email protected] Correspondence: [email protected] (C.-A.T.); [email protected] (X.Z.); [email protected] (J.W.); Tel.: +86-731-8700-1822 (C.-A.T.); +86-731-8700-1801 (J.W.) These authors contributed equally to this work.

Received: 28 November 2018; Accepted: 16 December 2018; Published: 18 December 2018

 

Abstract: A mechanical ball milling method for ultrafast synthesis of a nickel-based metal organic framework (Ni-MOF) has been proposed. The Ni-MOF was successfully synthesized in merely one minute without any solvent, additives, or preliminary preparation. The effect of milling time, mechano-frequency, type of assistant liquid, and amount of assistant water were systematically explored. It was found that the product can be obtained even only at a mechano-frequency of 10 Hz within one minute without any external solvent-assist, which indicated that the crystal water present in the nickel precursor was sufficient to promote MOF formation. Increasing the milling time, raising the mechano-frequency, and the addition of assistant solvent could promote the reaction and increase the yield. The method is rapid, highly efficient, eco-friendly, and has great scalability. The product generated within merely one minute even exhibited high capacitance. Keywords: metal-organic frameworks; ball milling; solvent-free; mechanochemistry; liquid assisted grinding

1. Introduction Metal-organic frameworks (MOFs) [1,2] have attracted intense attention during the past two decades due to their intriguing structural properties, which led to many potential applications, including gas storage [3], separations [4], catalysis [5], and sensing [6–8], etc. Recently, MOFs have been proven to be useful for electrochemical energy storage and considered as potential electrode material for supercapacitors because of their very large surface area, adjustable pore size, controllable microporous structure, and special structures with potential pseudo-capacitive redox centers [9–12]. Among them, nickel-based MOFs have get more interest not only because they have been demonstrated to have great application prospects for supercapacitors, but also because they could be the template/precursor for unique metal oxide and carbon materials with specific structure, especially the Ni3 (BTC)2 ·12H2 O (BTC = 1,3,5-benzenrtricarboxylic acid). Kong’s group [13] demonstrated that the hydrothermally-synthesized Ni3 (BTC)2 ·12H2 O had high specific capacitance of 726 F/g. Wang et al. [14] took the Ni-MOF as a precursor to prepare mesoporous metal oxide by calcining the precursor in the air, and the prepared nickel oxide (NiO) had high-capacitance retention at high scan rate and exhibited excellent cycle-life stability due to its special mesoporous character with bimodal size distribution. Chen et al. [15] synthesized large-scale of multiwalled carbon nanotubes Nanomaterials 2018, 8, 1067; doi:10.3390/nano8121067

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carbon nanotubes using Ni3(BTC)2·12H2O as precursor. However, the synthesis processes of Ni-MOFs normally require bulk solvents, high temperatures, and longof reaction times. For instance, using Ni3 (BTC) However, the synthesis processes Ni-MOFs normally require 2 ·12H 2 O as precursor. Wang et al. [14] synthesized Ni3(BTC) 2·12Hreaction 2O in a Teflon-lined autoclave andetheated 200 °C for bulk solvents, high temperatures, and long times. For instance, Wang al. [14]atsynthesized ◦ C for 24 h, 24 Du et al. [16] synthesized Ni-BTC MOF and in a Teflon-lined and to 150 °C for 12 Ni3h, (BTC) a Teflon-lined autoclave heated at 200autoclave Duheated et al. [16] synthesized 2 ·12H 2 O in ◦ h in N,N-dimethyl formamideautoclave (DMF), and andheated Kong’sto group Ni-MOF at a lower Ni-BTC MOF in a Teflon-lined 150 C [13] for 12prepared h in N,N-dimethyl formamide temperature of 105 group °C in [13] DMF but for Ni-MOF longer time (up totemperature two days). of Although et al. (DMF), and Kong’s prepared at a lower 105 ◦ C inJin DMF but[17] for described simple solution-phase method foral.the Ni3(BTC) 2·12H2O under room longer timea (up to two days). Although Jin et [17]synthesis describedofa simple solution-phase method temperature in a short organic linker must be deprotonated, and an organic solvent was for the synthesis of Ni3time, (BTC)an · 12H O under room temperature in a short time, an organic linker 2 2 also Therefore, a simple, rapid, and was energy-efficient route to generate Ni-based mustused. be deprotonated, and angreen, organic solvent also used. Therefore, a simple, green, MOFs rapid, without high temperatures bulk solvents is still highly desirable. and energy-efficient route toorgenerate Ni-based MOFs without high temperatures or bulk solvents is Mechanochemistry, i.e., chemical synthesis enabled or sustained by mechanical force [18], has still highly desirable. been Mechanochemistry, recently introduced as an synthesis alternative to or conventional syntheses [19,20]. i.e., chemical enabled sustained byMOF mechanical force [18], has Mechanochemical synthesis MOFs enable avoiding bulk temperature, and/or been recently introduced as anofalternative to conventional MOFsolvents, syntheseshigh [19,20]. Mechanochemical corrosive employed in solvents, solution synthesis. Importantly, it is even possible to be synthesis reagents of MOFsfrequently enable avoiding bulk high temperature, and/or corrosive reagents more rapidemployed and efficient than solvent-based methods. it Since thepossible synthesis of more the moderately porous frequently in solution synthesis. Importantly, is even to be rapid and efficient copper(II) isonicotinate MOF in pioneering work reported by James’ porous group [21], millingisonicotinate procedures than solvent-based methods. Since the synthesis of the moderately copper(II) have successfully applied for by theJames’ synthesis of several popular MOF materials, such as zeolitic MOF been in pioneering work reported group [21], milling procedures have been successfully azolate frameworks metal−organic frameworks [24], isoreticular applied for the synthesis [22,23], of several rare-earth(III) popular MOF materials, such as zeolitic azolate frameworks [22,23], metal-organic frameworks (IRMOFs) [25],[24], iron(III) trimesate MIL-100 (MIL = Materials of Institut rare-earth(III) metal −organic frameworks isoreticular metal-organic frameworks (IRMOFs) [25], Lavoisier) [26], MOF-74 [27],(MIL Hong=Kong University of Science and Technology (HKUST)-1 [28], iron(III) trimesate MIL-100 Materials of Institut Lavoisier) [26], MOF-74 [27], Hong Kong University of Science and Technology (HKUST)-1 [28], Cu2 I2 (triphenylphosphine) (L) (n = 1, 2) [29], Cu 2I2(triphenylphosphine) 2(L)n (n = 1, 2) [29], copper-based MOF-505 [30], and UiO-66 (UiO = n 2 copper-based MOF-505 andderivatives UiO-66 (UiO = University of Oslo) and and UiO-67 derivatives [31–33]. University of Oslo) and [30], UiO-67 [31–33]. Previously, Pichon James [21] described a Previously, Pichon and James [21] described of2, the potential reactions between Ni(OAc) survey of the potential reactions betweena survey Ni(OAc) Ni(NO 3)2, NiSO 4 , and H3BTC under 2, Ni(NO3 )2 , NiSO4 , and H3 BTC under mechanochemical solvent-free and, unfortunately, mechanochemical solvent-free conditions and, unfortunately, only conditions grinding nickel sulfate with only grinding nickel reaction. sulfate with H3 BTC gave partialofreaction. Consequently, the Ni-MOFs ability of H 3BTC gave a partial Consequently, theaability mechanochemistry to access mechanochemistry to access Ni-MOFs rapidly, has remained insufficient explored. rapidly, has remained insufficient explored. we report the ultrafast synthesis synthesis of Ni-MOF by a mechanical ball milling method Herein, we could be obtained in merely one minute without without bulk solvent, (Figure 1). 1).Notably, Notably,Ni-MOF Ni-MOF could be obtained in merely one minute bulkadditives, solvent, or any preliminary preparation.preparation. The structure and the morphology of Ni-MOF were confirmed by additives, or any preliminary The structure and the morphology of Ni-MOF were powder X-ray and scanning electronand microscopy (SEM), respectively. Interestingly, confirmed bydiffraction powder (PXRD), X-ray diffraction (PXRD), scanning electron microscopy (SEM), the Ni-MOF was formed bythe fastNi-MOF crystallization withinby one minute, after which thereone wasminute, no apparent respectively. Interestingly, was formed fast crystallization within after changethere in thewas yield crystallinity, even after 180 and min.crystallinity, The effect of even milling time, which noand apparent change in the yield after 180mechano-frequency, min. The effect of kind of assistant liquid, amount of assistant water were systematically explored. The scalability the milling time, mechano-frequency, kind of assistant liquid, amount of assistant water of were ball milling method were also Ni-MOFs exhibit as high systematically explored. The investigated. scalability ofThe thegenerated ball milling method werehigh alsocapacitance investigated. The as 640 F/gNi-MOFs at the current density 1 A/g. Toas thehigh bestas of640 ourF/g knowledge, this isdensity the firstofexample of generated exhibit high of capacitance at the current 1 A/g. To ultrafast synthesis of Ni-MOF milling. the best of our knowledge, thisby is ball the first example of ultrafast synthesis of Ni-MOF by ball milling.

Figure of Ni Ni--MOF. Figure 1. 1. Schematic Schematic representation representation of of the the ball ball milling milling method method for for the the rapid rapid synthesis synthesis of MOF.

2. Materials Materials and and Methods Methods 2. 2.1. Materials 2.1. Materials Nickel(II) acetate tetrahydrate [Ni(OAc) ·4H O, AR] was purchased from Tianjin Fenchuan Nickel(II) acetate tetrahydrate [Ni(OAc)22·4H22O, AR] was purchased from Tianjin Fenchuan Technology Co. Ltd. (Tianjin, China) and benzene-1,3,5-tricarboxylic acid (H3 BTC, 99%) was purchased Technology Co. Ltd. (Tianjin, China) and benzene-1,3,5-tricarboxylic acid (H3BTC, 99%) was from J&K Scientific Ltd. (Beijing, China). DMF(AR) was purchased from Tianjin Hengxing Chemical purchased from J&K Scientific Ltd. (Beijing, China). DMF(AR) was purchased from Tianjin

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Preparation Co. Ltd. (Tianjin, China), methanol (MeOH, AR) was purchased from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China), and ethanol (EtOH, 99.7%) was purchased from General-Reagent (Shanghai, China). Pure water was homemade. 2.2. Synthesis of Ni-MOF Ni-MOFs were synthesized by ball milling of of 2:3 (molar ratio) mixtures of H3 BTC and Ni(OAc)2 ·4H2 O at varied frequency from 10 Hz to 50 Hz. In a typical reaction, about one gram of the H3BTC and metal precursor mixture was ball milled for various times (1, 5, 30, 60, and 180 min) in a agate vial (80 mL) using a QM-QX0.4L mill (Miqi Instrument Equipment Co., Ltd., Changsha, China) with the addition of specific amount of water (0, 0.5, 1, or 2 mL) or 1 mL of other organic solvent (MeOH, EtOH, and DMF) and increased the feed rate by three times and five times. The ball-to-powder mass ratio was consistently kept at about 5:1 for all experiments. The products were scraped off the jar walls, washed with water and ethanol trice, then the supernatant was removed by centrifugation, and the solid product was dried at 60 ◦ C in an oven for 12 h. The enlarged scale reaction was performed with three or five times of the H3 BTC and metal precursor mixture. The yield was calculated according to the following equation based on the number of moles of Ni(II). Yield =

nNi(II) in Ni−btc nNi(OAc)2

× 100%

2.3. Characterization FTIR spectra were recorded in the range of 400–4000 cm−1 on a PerkinElmer Spectra Two FT-IR spectrophotometer (Waltham, MA, USA) with an attenuated total reflectance (ATR) accessory. The milled samples were analyzed by powder XRD on a Tri III powder diffractometer (Rigaku, Tokyo, Japan) using Cu Kα radiation between 8◦ and 60◦ with a scan rate of 5◦ /min. Thermogravimetric analyses (TGA) were performed on a STA6000 thermal analyzer (PerkinElmer, Waltham, MA, USA) under N2 with a heating rate of 4 ◦ C/min. Nitrogen adsorption-desorption isotherms, pore size distributions and surface areas of the samples were measured via N2 adsorption-desorption at 77 K on a BEL SORP-mini II surface area and porosity analyzer (Bel Japan Inc., Osaka, Japan). Before measurement, the samples were activated at 60 ◦ C for 12 h. The morphology of the sample was observed by a SEM Model S-4800 (Hitachi, Tokyo, Japan). All electrochemical measurements were done in a three-electrode experimental setup. A platinum wire electrode and a saturated Ag/AgCl electrode were used as the counter and reference electrodes, respectively. All the electrochemical measurements were carried out in 6 mol/L KOH aqueous electrolyte using a CHI660C (Shanghai Chenhua Apparatus, Shanghai, China) electrochemical workstation. 3. Results and Discussion 3.1. Synthesis of Ni-MOF 3.1.1. Effect of Grinding Time Firstly, the Ni-MOF Ni3 (BTC)2 ·12H2 O was synthesized by grinding of 2:3 (molar ratio) mixtures of H3 BTC and Nickel(II) acetate tetrahydrate [Ni(OAc)2 ·4H2 O] with water-assist under mechano-grinding at 50 Hz for a specific time (1, 5, 30, 60, and 180 min). The products are denoted as Ni-BTC-1 m, Ni-BTC-5 m, Ni-BTC-30 m, Ni-BTC-60 m, and Ni-BTC-180 m, respectively. There is no apparent change in the yield, which only varied from 66% to 72% (Table S1). The Fourier transform infrared (FTIR) spectra of different Ni-MOFs are very similar. These spectra clearly show the vibrational bands of the waters around 3500 and 3200 cm−1 , which suggests there are crystallization waters in the product (Figure 2a). Additionally, there is no band at around 1710 cm−1 (which is the characteristic of protonated carboxylic groups) observed in Ni-MOFs curves, suggesting the absence of protonated carboxylic groups in the product.

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The XRD pattern shows the characteristic diffraction peaks of Ni 3(BTC)2·12H2O (Figure 2b), The XRD pattern shows characteristic diffraction peaks [14,16,17,34], of Ni3 (BTC)2 ·and 12H2the O (Figure 2b), which matches well with thethe result of the reported literature simulated which matches well with the result of the reported literature [14,16,17,34], and the simulated pattern pattern for [Ni3(BTC)2·12H2O] was based on single-crystal data from the Cambridge for [Ni3 (BTC)2 ·12H was based on single-crystal data from the Cambridge Crystallographic Data Crystallographic Data (CCDC) [35]. The generated product in only one minute has already 2 O]Centre Centre good (CCDC) [35]. The generated product in only one minute good crystals, crystallinity. shown crystallinity. To evaluate the effect of grinding timehas onalready the sizeshown of Ni-MOF the To evaluate the effect of grinding time on the size of Ni-MOF crystals, the full width at half maxima of full width at half maxima of peaks (FWHM) at 17.7° corresponding to the (220) lattice plane were ◦ peaks in (FWHM) at 17.7 corresponding to the (220) latticewas plane were listed in Table S2. average listed Table S2. The average size of Ni-MOF crystals inversely proportional to The the FWHM. size of Ni-MOF crystals slightly, was inversely proportional the FWHM. FWHM decreases slightly, The FWHM decreases at first, and then to increases, and The finally decreases along with at first, and thenindicating increases,that andthe finally decreases along with grinding time, indicating thewith average grinding time, average size of Ni-MOF crystals changed slightlythat along the size of Ni-MOF grinding time. crystals changed slightly along with the grinding time.

Figure 2. Characterizations of the Ni-BTC samples obtained at various reaction times. (a) FTIR spectra, Figure 2. Characterizations of the Ni-BTC samples obtained at various reaction times. (a) FTIR (b) XRD patterns, (c) TGA curve of Ni-BTC-1m, and (d) Nitrogen adsorption and desorption. spectra, (b) XRD patterns, (c) TGA curve of Ni-BTC-1m, and (d) Nitrogen adsorption and desorption.

To further confirm the composition of the Ni-MOF, the thermogravimetric analyses (TGA) were To further confirm the composition of ◦the Ni-MOF, the thermogravimetric analyses (TGA) performed under air with a heating rate of 4 C/min. There are two different stages of weight loss were performed under air with a heating rate of 4 °C/min. There are two different stages of weight in the TGA curve of Ni-MOF-1m, as shown in Figure 2c. In the first stage, weight loss of 27.35 wt % loss in the◦ TGA curve of Ni-MOF-1m, as shown in Figure 2c. In the first stage, weight loss of 27.35 from 100 C to 250 ◦ C could be ascribed to the loss of crystallization water molecules. The second wt % from 100 °C to 250 °C could◦ be ascribed to the loss of crystallization water molecules. The sharp weight loss started from 250 C and ended at 400 ◦ C, due to the decomposition of the organic second sharp weight loss started from 250 °C and ended at 400 °C, due to the decomposition of the frameworks, and the final residue was NiO [17]. These evidences further prove that the coordinated organic frameworks, and the final residue was NiO [17]. These evidences further prove that the or adsorbed H2 O molecules existence in the Ni-MOFs. This result is in conformity to the FTIR data. coordinated or adsorbed H2O molecules existence in the Ni-MOFs. This result is in conformity to Additionally, the weight loss is in agreement with the chemical formula Ni3 (C6 H3 (COO)3 )2 ·12H2 O. the FTIR data. Additionally, the weight loss is in agreement with the chemical formula In addition, nitrogen adsorption-desorption isotherms, the specific surface area and pore Ni3(C6H3(COO)3)2·12H2O. structures of Ni-MOFs were studied by surface area and porosity analyzer, and the results are shown In addition, nitrogen adsorption-desorption isotherms, the specific surface area and pore in Figure 2d and Table S3. The Brunauer–Emmett–Teller (BET) specific surface area of Ni-MOF-1m is structures of Ni-MOFs were studied by surface area and porosity analyzer, and the results are only 4.85 m2 /g, after the longer reaction time, the specific surface area of Ni-MOF-180m reaches the shown in Figure 2d 2and Table S3. The Brunauer–Emmett–Teller (BET) specific surface area of maximum of 10.08 m /g. The average pore sizes of Ni-MOFs are about 4.6 nm except Ni-MOF-180m Ni-MOF-1m is only 4.85 m2/g, after the longer reaction time, the specific surface area of (4.1 nm). According to the Barrett–Joyner–Halenda (BJH) analysis of the Ni-BTC samples obtained at Ni-MOF-180m reaches the maximum of 10.08 m2/g. The average pore sizes of Ni-MOFs are about various reaction times, as shown in Figure S1, the pore sizes in the micropore range are around 1.7 nm 4.6 nm except Ni-MOF-180m (4.1 nm). According to the Barrett–Joyner–Halenda (BJH) analysis of for all MOF samples. However, the pore sizes in the mesopore range are different, and it is inferred the Ni-BTC samples obtained at various reaction times, as shown in Figure S1, the pore sizes in the micropore range are around 1.7 nm for all MOF samples. However, the pore sizes in the mesopore

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that the comes the interparticle which is different to the varied morphology range areporosity different, and from it is inferred that the voids, porosity comes from thedue interparticle voids, which is of the MOFs. different due to the varied morphology of the MOFs. Morphological investigations using a field-emission SEM, andand the Morphological investigations of of Ni-MOFs Ni-MOFswere werecarried carriedout out using a field-emission SEM, SEM images are shown in Figure 3. Ni-MOF-1 m exhibits rod-like shapes, but the dispersion of their the SEM images are shown in Figure 3. Ni-MOF-1 m exhibits rod-like shapes, but the dispersion of sizes is veryisbroad. larger ones have ones a length andawidth 10 µm and 500 respectively, their sizes very The broad. The larger have lengthof and width of nm, 10 μm and 500while nm, the smaller ones have a length and width of about 500 nm and 100 nm (Figure 3a,b). The surface of respectively, while the smaller ones have a length and width of about 500 nm and 100 nm (Figure microcrystals of Ni-MOF-1 m looks very smooth. For the products with longer reaction time, there are 3a,b). The surface of microcrystals of Ni-MOF-1 m looks very smooth. For the products with longer no apparent change theno morphology (Figuresin 3c–f S2), suggesting the success of synthesis of reaction time, there inare apparent change theand morphology (Figure 3c–f and Figure S2), Ni-MOF in merely one minute. suggesting the success of synthesis of Ni-MOF in merely one minute.

Figure Figure 3. 3. SEM SEM images images of of the the Ni-BTC Ni-BTC samples samples obtained obtained at at various various reaction reaction times. times. (a,b) (a,b) Ni-BTC-1 Ni-BTC-1m, m, (c) Ni-BTC-5 m, (d) Ni-BTC-30m, (e) Ni-BTC-60 m, and (f) Ni-BTC-180 m. The bar in (a) represents (c) Ni-BTC-5 m, (d) Ni-BTC-30m, (e) Ni-BTC-60 m, and (f) Ni-BTC-180 m. The bar in (a) represents 55 μm, µm, and and the the bars bars in in (b–f) (b–f) to to represent represent 500 500 nm. nm.

3.1.2. Effect Effect of of Mechano-Frequency Mechano-Frequency of Grinding 3.1.2. In addition addition to tothe thegrinding grindingtime, time,the themechano-frequency mechano-frequency grinding also factor In ofof grinding is is also oneone keykey factor of of ball-milling conditions. With the assist of water, the products were obtained under different ball-milling conditions. With the assist of water, the products were obtained under different frequencies from 5050 HzHz by keeping the reaction time of only onefor minute, and theyand denoted frequencies from10 10Hz Hztoto by keeping the reaction time offor only one minute, they as Ni-BTC-10 Hz, Ni-BTC-20 Hz, Ni-BTC-30 Hz, Ni-BTC-40 Hz, and Ni-BTC-50 Hz, respectively. Their denoted as Ni-BTC-10 Hz, Ni-BTC-20 Hz, Ni-BTC-30 Hz, Ni-BTC-40 Hz, and Ni-BTC-50 Hz, XRD patterns were Figure 4a. All these patterns show the diffraction peaksthe of respectively. Theirshown XRD in patterns were shown in Figure 4a. characteristic All these patterns show Ni3 (BTC)2 ·12Hdiffraction product of 3Ni-BTC-10Hz. crystallinity of the obtained at characteristic peaks of Ni (BTC)2·12H2O, The even the product of products Ni-BTC-10Hz. The 2 O, even the 10 Hz and 20 Hz are also significantly lower than that obtained at higher frequencies. The yield of crystallinity of the products obtained at 10 Hz and 20 Hz are also significantly lower than that Ni-BTC-10Hz is only about 39% (Table S4). of When the mechano-frequency to 20 Hz,When the yield obtained at higher frequencies. The yield Ni-BTC-10Hz is only aboutincreases 39% (Table S4). the increases to about 60%. To further increase theyield milling frequency, the yield maintains at about 60–70%. mechano-frequency increases to 20 Hz, the increases to about 60%. To further increase the milling frequency, the yield maintains at about 60–70%.

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Figure 4. XRD patterns of the Ni-BTC samples obtained at various grinding frequencies (a), with Figure 4. XRD patterns of the Ni-BTC samples obtained at various grinding frequencies (a), with addition of different solvents (b), with addition of varied amount of water (c), and at enlarged scales (d). addition of different solvents (b), with addition of varied amount of water (c), and at enlarged scales 3.1.3.(d). Effect of Auxiliary Liquid

effect of the type of auxiliary liquid on the ball milling reaction was studied. In addition 3.1.3.The Effect of Auxiliary Liquid to water, the polar solvents MeOH, EtOH, and DMF, commonly used in MOF synthesis, were effect of theXRD typepatterns of auxiliary liquid onwere the ball milling reaction addition to also The explored. The of products shown in Figure 4b.was Allstudied. of themInhave similar water, the polar solvents MeOH, EtOH, and DMF, commonly used in MOF synthesis, were also patterns which match well with the simulated pattern for Ni3 (BTC)2 ·12H2 O, suggesting the success of explored. The patterns products were shown in of Figure 4b. All of them patterns preparation of XRD product in oneofminute whatever the kind auxiliary liquid. The have yieldssimilar of the products which match well with the simulated pattern for Ni 3 (BTC) 2 · 12H 2 O, suggesting the success of are between 60% to 70% (Table S5). Water is the best choice among them, considering its environment preparation product the in one whateverofthe kind ofalso auxiliary liquid. Unexpectedly, The yields of it the friendliness. of Moreover, effectminute of the quantity water was investigated. is products are between 60% to 70% (Table S5). Water is the best choice among them, considering its found that the product can be obtained without water, as shown in Figure 4c, and the yield can environment Moreover, thewater-assist effect of the quantity waterincrease was also investigated. achieve aboutfriendliness. 56% (Table S6). Under the condition, theofyields a little to around Unexpectedly, it is foundthe thataddition the product can is benot obtained without water, as shown 65%. This result implies of water necessary. Based on this result, in theFigure effects4c, of and the yield can achieve about 56% (Table S6). Under the water-assist condition, the yields mechano-frequency were explored again under no liquid-assist condition. In the absence of additives, increase little to around Thiscan result the addition of water not necessary. Based S3). on even at aa minimum of 10 65%. Hz, we stillimplies obtain a product with good is crystallization (Figure this result,the theyield effects of mechano-frequency were S7). explored underincreases, no liquid-assist However, drops to only about 29% (Table As theagain frequency the yieldcondition. increases In the absence of additives, even at a minimum of 10 Hz, we can still obtain a product with good slightly. Even if the frequency rises to 40 Hz, the yield is only about 40%. When the frequency is raised crystallization (Figure S3). However, the drops to only 29% to (Table As the frequency to 50 Hz (the maximum limit frequency of yield the instrument), the about yield rises 56%. S7). In contrast to the case increases, the yield increases slightly. Even if the frequency rises to 40 Hz, the yield is only about with liquid assist (Figure 5), it can be seen that the yield of product with liquid assistance is significantly 40%. When the frequency is raised to 50 Hz (the maximum limit frequency of the instrument), the higher than that without liquid assistance. Under the liquid assist condition, the maximum yield can yield rises toat56%. In the contrast to theiscase liquid assist (Figure 5), itassistance can be seen thatisthe of be achieved when frequency overwith 30 Hz, while without liquid there stillyield a large product with liquid assistance is significantly higher than that without liquid assistance. Under the increase in yield at 50 Hz. liquid assist condition, the maximum yield can be achieved at when the frequency is over 30 Hz, while without liquid assistance there is still a large increase in yield at 50 Hz.

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Figure yields of Ni-BTC samples obtained withwith and without addition of water (1 mL).(1 Figure5.5.Comparison Comparisonofof yields of Ni-BTC samples obtained and without addition of water mL).

3.1.4. Scalability

3.1.4. InScalability addition, we investigated the scalability and expandability of the method. We increased the feed rate by three times and five times, and foundand thatexpandability the crystallinity of the product basically In addition, we investigated the scalability of the method. Wewas increased the the same andand thefive yield was and slightly improved S8). This method canwas be used for feed rate(Figure by three4d), times times, found that the (Table crystallinity of the product basically bulk preparation MOFs at the gram scaleimproved (2.77 g in (Table a run). S8). In general, the method rapid, the same (Figureof4d), andeasily the yield was slightly This method can beisused for high-efficiency, eco-friendly, low cost, and has great scalability. bulk preparation of MOFs easily at the gram scale (2.77 g in a run). In general, the method is rapid, Recently, a few reports [17,36,37] have the rapid synthesis of MOFs (Table S9). high-efficiency, eco-friendly, low cost, andalso hasdemonstrated great scalability. Duan Recently, et al. [36] demonstrated the synthesis of hierarchical porous ZIF-8 materials within one min at a few reports [17,36,37] have also demonstrated the rapid synthesis of MOFs (Table room temperature by using organic amines as a supramolecular (organic amine-template), S9). Duan et al. [36] demonstrated the synthesis of hierarchicaltemplate porous ZIF-8 materials within one but the bulk organic solvent (methanol) and additives (organic amines) were still required. Jin et(organic al. [17] min at room temperature by using organic amines as a supramolecular template · synthesized Ni (BTC) 12H O by a solution-phase method under room temperature in short time, 3 2 amine-template), but2 the bulk organic solvent (methanol) and additives (organic amines) were but still the organic Jin linker to be deprotonated, and the organic solvent were also used. Huang et al. [37] required. et must al. [17] synthesized Ni3(BTC) 2·12H2O by a solution-phase method under room reported the synthesis F4 -UiO-66 100 s using grinding, however, metal source temperature in short of time, but the in organic linkerwater-assisted must to be deprotonated, and thethe organic solvent must be the pre-prepared zirconium clusters. Despite these developments, it should be noted that were also used. Huang et al. [37] reported the synthesis of F4-UiO-66 in 100 s using water-assisted the work described the first ultrafast synthesis of Ni-MOF ball milling grinding, however,here therepresents metal source mustexample be the of pre-prepared zirconium clusters.byDespite these without solvent or any preliminary preparation. developments, it should be noted that the work described here represents the first example of This fast reaction speed is due to the sufficient energy provided the mechanical force during ultrafast synthesis of Ni-MOF by ball milling without solvent or anyby preliminary preparation. ball milling. From a thermodynamic point of view, generally, the chemical potential of a in This fast reaction speed is due to the sufficient energy provided by the mechanicalsubstance force during the solid state is higher than that of the same one in the liquid state [30,38–40]. Thus, the driving force ball milling. From a thermodynamic point of view, generally, the chemical potential of a substance ofinthe is higher solution-based methodThus, [30]. the Therefore, theformation solid stateofisNi-MOF higher than that ofthan the that sameofone in the liquidsynthesis state [30,38–40]. driving the mechanochemical construction of Ni-MOF can be reacted within a short time. On the other force of the formation of Ni-MOF is higher than that of solution-based synthesis methodhand, [30]. the additional solvents (water, MeOH, EtOH, or DMF) had a good dissolution of the raw materials Therefore, the mechanochemical construction of Ni-MOF can be reacted within a short time. On the and, thus, they can boost molecular the reactants during Evendissolution if no addition of other hand, the additional solventsmobility (water, of MeOH, EtOH, or DMF)reaction. had a good of the solvent, a small amount of crystal water contained in nickel acetate separates out under the action raw materials and, thus, they can boost molecular mobility of the reactants during reaction. Evenofif ball acting an auxiliary Moreover, the acetate actsacetate as a base to catalyze no milling, additionthereby of solvent, a as small amount solvent. of crystal water contained in ion nickel separates out the deprotonation of the H BTC, thereby increasing the reaction rate. In the experiment, nickel chloride 3 under the action of ball milling, thereby acting as an auxiliary solvent. Moreover, the acetate ion and nickel nitrate were used as the metal source to carry out the reaction, and no detectable product acts as a base to catalyze the deprotonation of the H3BTC, thereby increasing the reaction rate. In the was found, which proved the inference. experiment, nickel chloride and nickel nitrate were used as the metal source to carry out the reaction, and no detectable product was found, which proved the inference. 3.2. Electrochemical Performance of Ni-MOF

to provePerformance that the Ni-MOF produced within one minute has considerable electrochemical 3.2. Finally, Electrochemical of Ni-MOF performance, cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) were performed Finally, to prove that the Ni-MOF produced within one minute has considerable using a three electrode system in 6 mol/L KOH electrolyte and the results were presented in Figure 6. electrochemical performance, cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) The CV was carried out with a potential range from 0 to 0.5 V at varied scan rates (5, 10, 25, 50, 75, were performed using a three electrode system in 6 mol/L KOH electrolyte and the results were and 100 mV/s). There are a couple of distinct redox peaks could be observed, which are correspond to presented in Figure 6. The CV was carried out with a potential range from 0 to 0.5 V at varied scan the reversible redox of the reversible intercalation and deintercalation of OH- ions [13]. These surface rates (5, 10, 25, 50, 75, and 100 mV/s). There are a couple of distinct redox peaks could be observed, which are correspond to the reversible redox of the reversible intercalation and deintercalation of OH- ions [13]. These surface faradic redox reactions lead to the typical pseudocapacitive behavior of

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faradic redox reactions lead to the typical pseudocapacitive behavior of Ni-MOFs. The GCD curves Ni-MOFs. The GCD curves under current a potential range of 4b. 0–0.45 V are under different current densities in different a potential range densities of 0–0.45 in V are shown in Figure It can be shown in Figure 4b. It can be found that there is a plateau in the potential from 0.2 to 0.25 V at found that there is a plateau in the potential from 0.2 to 0.25 V at different current densities from 1 to different current densities from 1to to A/g.reaction, Such a indicating plateau isthe ascribed to the redox reaction, 10 A/g. Such a plateau is ascribed the10 redox significant contribution of the indicating the significant contribution ofthe thephenomena pseudo-capacitance, which agrees capacitance well with C the pseudo-capacitance, which agrees well with of CV curves. The specific m phenomena of CV curves. The specific capacitance C m (F/g) is calculated from the GCD (F/g) is calculated from the GCD measurement (Figure 4b) according to the following equation [13]: measurement (Figure 4b) according to the following equation [13]: Cm = I∆t/∆E = i∆t/(m∆E) 𝐶m = 𝐼∆t/∆𝐸 = 𝑖∆𝑡/(𝑚∆𝐸) where where II is is the the discharge discharge current current density density calculated calculated using using II== i/m, i/m, i is the current and m is is the the active active mass mass of of the the electrode, electrode, ∆t Δt is is the the during during time time of of the the discharge discharge curve, curve, and and ∆E ΔE is is the the potential potential window window of of the the discharge discharge curve, curve, respectively. respectively. The The calculated calculated C Cm m values of Ni-MOF-1 are present in Figure S4. The specific capacitance can achieve 640 F/g at the current The specific capacitance can achieve 640 F/g at the current density density of of11A/g, A/g, which is comparable comparable to to the the performance performance of of the the hydrothermally-synthesized hydrothermally-synthesized one one[13]. [13].

Figure Figure 6. 6. (a) (a) Evolution Evolution of of CVs CVs of of Ni-BTC-1 Ni-BTC-1 at atvarious variousscan scanrates: rates: 5, 5,10, 10,25, 25,50, 50,75, 75,and and100 100mV/s; mV/s; and and (b) charge-discharge diagrams of Ni-BTC-1 at different current densities: 1, 2, 3, 5, 7, and 10 (b) charge-discharge diagrams of Ni-BTC-1 at different current densities: 1, 2, 3, 5, 7, and 10A/g. A/g.

4. Conclusions 4. Conclusions We have proposed a mechanical ball milling method for ultrafast synthesis of Ni-based metal We have proposed a mechanical ball milling method for ultrafast synthesis of Ni-based metal organic frameworks. The results of XRD show that there is no significant difference in the crystallinity organic frameworks. The results of XRD show that there is no significant difference in the of the products obtained at different reaction times (1–180 min), and the stable product can be obtained crystallinity of the products obtained at different reaction times (1–180 min), and the stable product even only at macheno-frequency of 10 Hz within one minute without any solvent-assist. Generally, can be obtained even only at macheno-frequency of 10 Hz within one minute without any increasing the milling time, raising the mechanical frequency, and the addition of assistant solvent will solvent-assist. Generally, increasing the milling time, raising the mechanical frequency, and the increase the yield, while the kind of assistant solvent has no evident effect on the yield. The water is addition of assistant solvent will increase the yield, while the kind of assistant solvent has no the best choice among the solvents, considering the environment friendliness. The method is rapid, evident effect on the yield. The water is the best choice among the solvents, considering the highly efficiency, eco-friendly, and has great scalability (at the gram scale in a run). environment friendliness. The method is rapid, highly efficiency, eco-friendly, and has great scalability (at the gram scale in a run).are available online at http://www.mdpi.com/2079-4991/8/12/1067/ Supplementary Materials: The following s1; Table S1. Yields of Ni-BTC samples obtained at various reaction times; Table S2. FWHM of Ni-BTC samples obtained at various reaction The times; Table S3.are Surface area online and pore of Ni-BTC samples obtained Supplementary Materials: following available at structures www.mdpi.com/link; Table S1. Yields at of various Figure S1. Thereaction BJH analysis the Ni-BTC samples obtained at various reaction times; Ni-BTC reaction samplestimes; obtained at various times;ofTable S2. FWHM of Ni-BTC samples obtained at various Figure S2. SEM images of the Ni-BTC samples obtained at various reaction times. (a) Ni-BTC-5 m, (b) Ni-BTC-30 reaction times; Table S3. Surface area and pore structures of Ni-BTC samples obtained at various reaction times; m, (c) Ni-BTC-60 m, and (d) Ni-BTC-180 m; Table S4. Yields of Ni-BTC samples obtained at various grinding Figure S1. The analysis thewater; Ni-BTC samples obtained at various reaction times; Figure S2. SEM images frequencies withBJH addition of 1ofmL Table S5. Yields of Ni-BTC samples obtained with addition of different of the Ni-BTC at various (a) Ni-BTC-5 m, (b)amount Ni-BTC-30 m, (c) Ni-BTC-60 m, solvents; Table samples S6. Yieldsobtained of Ni-BTC samplesreaction obtainedtimes. with addition of varied of water; Figure S3. XRD patterns the Ni-BTC obtained without water addition; Table S7. and (d) of Ni-BTC-180 m;samples Table S4. Yieldsatofvarious Ni-BTCgrinding samplesfrequencies obtained at variousthe grinding frequencies with Yields of Ni-BTC obtained at various grinding frequencies without addition water; Table S8. Yields addition of 1 mLsamples water; Table S5. Yields of Ni-BTC samples obtained withthe addition of of different solvents; Table of Ni-BTC samples obtained at enlarged scales; Table S9. Comparison of the synthesis of MOF in previously S6. Yields of Ni-BTC samples obtained with addition of varied amount of water; Figure S3. XRD patterns of the published reports; Figure S4. Diagram of the specific capacitance of materials at different current densities. Ni-BTC samples obtained at various grinding frequencies without the water addition; Table S7. Yields of Ni-BTC samples obtained at various grinding frequencies without the addition of water; Table S8. Yields of Ni-BTC samples obtained at enlarged scales; Table S9. Comparison of the synthesis of MOF in previously published reports; Figure S4. Diagram of the specific capacitance of materials at different current densities.

Author Contributions: Conceptualization: C.-A.T.; data curation: R.Z. and L.W.; formal analysis: R.Z. and R.C.; funding acquisition: C.-A.T.; investigation: R.Z. and R.C.; methodology: C.-A.T. and J.W.; project

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Author Contributions: Conceptualization: C.-A.T.; data curation: R.Z. and L.W.; formal analysis: R.Z. and R.C.; funding acquisition: C.-A.T.; investigation: R.Z. and R.C.; methodology: C.-A.T. and J.W.; project administration: J.W.; supervision: C.-A.T. and X.Z.; visualization: L.W.; writing—original draft: C.-A.T.; writing—review and editing: X.Z. and J.W. Funding: The National Natural Science Foundation of China (21573285), the Natural Science Foundation of Hunan Province (2018JJ3597), and a research project of National University of Defense Technology (ZK16-03-51). Conflicts of Interest: The authors declare no conflict of interest.

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