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Preparation of flaky dihydrate zinc oxalate particles by controlled chelating double-jet precipitation
Advanced Powder Technology 30 (2019) 1941–1949
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Preparation of flaky dihydrate zinc oxalate particles by controlled chelating double-jet precipitation Chen Xing, Huang Kai ⇑, Wang Chengyan School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Xueyuan Rd. 30, Haidian District, 100083 Beijing, China
a r t i c l e
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Article history: Received 22 June 2017 Received in revised form 12 May 2019 Accepted 14 June 2019 Available online 24 June 2019 Keywords: Zinc oxalate particles Flaky shape Citrate ligand Controlled double-jet precipitation
a b s t r a c t Well-dispersed flaky ZnC2O42H2O particles were synthesized via a controlled double-jet precipitation process from the relatively concentrated solutions of Zn(NO3)2 (0.05 mol/L) and Na2C2O4 (0.06 mol/L) in the presence of sodium citrate. The effects of concentration of the reactants, reaction time, pH values and feeding rates on the size and shape of the final particles are investigated. It was found that asprepared dihydrate zinc oxalate particles are formed by aggregation of tiny microsized subunits, and the feeding rate as well as the citrate concentration is quite key to the morphological evolution of the hydrate zinc oxalate particles. Based on experimental results and the thermodynamic equilibrium calculation, it is confirmed that the presence of citrate ligand plays essential roles to the whole precipitation process, which may play the multiple roles of controllable release of zinc ions, crystal habit tunning agent, and dispersing surfactant. The controllable synthesis idea of well-dispersed particles in present research work has the potential of extensive application in the preparation of other similar particles. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Colloidal ZnO is a typical functional material that can find many applications in the fields of optics, pigments, cosmetics, electrics, adsorption, catalysis, and so on [1–5]. Due to the strong dependence of material properties on its size and shape, the scrupulous controlling of the size, shape and dispersity is a key in the particle preparation [6–10]. In the past four decades various methods have been developed to prepare the well-dispersed particles, in which the homogeneous precipitation was frequently employed due to its simple operation and good reproducibility [11–13]. While homogeneous precipitation has quite low yield in the practical process, the double-jet precipitation process was therefore usually proposed due to its relatively high concentration of reactants and good dispersity in the obtained products [14–17]. Comparing with the particles size tailoring, the controlling of particle shape is more difficult because there are not so many theories emerged in the past decades [18–20]. Lamer model, as a typical empirical formula, was usually used to explain the formation of dispersity or size distribution of the precipitated particles [21,22], and Weimarn law can illustrate the particle size variation based on the relative ratio of nucleation to growth rates and mass
⇑ Corresponding author.
balance [23,24]. While for the particle shape or morphology, there are quite few theories to give the convincing proofs and reasons. In the precipitation system, the growth model by diffusion of monomers to the nuclei is responsible for the formation of the crystals with various shapes in a large number of experimental reports [21,22,25,26]. The sufficient exposure of their crystal facets under this growth model, which usually occurred in the slow precipitation system at the extremely dilute solution, is believed to be the main reason for the formation mechanism of the speciallyshaped particles [25–27]. But there are also many proofs indicating that after nucleation the tiny subunits could aggregate into larger particles with special shape or morphology, such as spherical [28–30], needle-like [31–33], flower-like [34,35], flaky [36,37], rod-like [38,39] shape and so on. And in most cases, it was also found that the Oswald ripening may do some modification on the shape tailoring such as making the particle surface smoother via a dissolution and redeposition or recrystallization process [26,40,41]. And another measure frequently taken is to add the polymer to modify the shape and morphology of the precipitated particles. Considering the extensive charging of the surface of the precipitated colloids, the ionic polymers were usually chosen like organic acid or compounds, e.g., citric acid [19,28,42], ethylenediaminetetraacetic acid (EDTA)[43], hexadecyl trimethyl ammonium bromide (CTAB) [19,34], and so on. It is well known that citrate is a typical oxalate precipitation inhibitor, for example, in the
E-mail address: [email protected] (H. Kai). https://doi.org/10.1016/j.apt.2019.06.013 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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formation of calcium oxalate, citrate is believed to effectively inhibit the calcium oxalate nucleation by interacting with all the early stage CaC2O4 species including polynuclear stable complexes and amorphous precursors [42]. And during this process, the obtained calcium oxalate precipitated into different size and morphologies according to the designing strategy. The related experience of tailoring the oxalate precipitate particles provided quite good reference for the similar synthesis systems. Anyway, though many papers have been published about the controllable preparation of various inorganic particles with different shapes or morphologies, the controlling rules are still far from the size tailoring skills during the precipitation system. So most of the preparation of the particles with special morphology is still lying in the stage of ‘‘Trial and Error” and far from the stage of ‘‘Orientating design”, and anyway under the guide of some fundamental principles and experiences to synthesize various particles with unique shape or morphology is still an endeavor of this research filed. The reported work has been carried out to prepare the zinc oxalate particles by chemical precipitation processes, and most of them have the shapes of being flower-like [35,44], rod-like [45], wire-like [46], cuboid [47], though they built different media systems like ether-water [35,45,46], electrochemical synthesis [44], microemulsion [47], but it was never mentioned about the flakelike zinc oxalate particles in the literature. In our study, the flaky dihydrate zinc oxalate subunits as well as their hierachical aggregation particles are easily prepared via a controlled double-jet precipitation. Therefore, the present study demonstrated a specific flaky zinc oxalate precipitate for the first time, that would provide a unique and novel shape for zinc oxalate particles. 2. Experimental
tube is made of PTFE (polytetrafluoroethylene) with an inner diameter of 3 mm, which is also the nozzle size of the feeding tube. In a typical run, a 0.05 mol/l Zn(NO3)2 (A) solution and a 0.06 mol/l Na2C2O4 (B) solution in concentrations were simultaneously introduced into a cylindrical container fabricated by ourselves (a waterjacket steel bucket with the characteristic size of U17 cm 25 cm), containing 1 L of deionized water (C) kept at 30 °C, at a constant flow rate of 1–10 ml/min, followed by aging at the same temperature for various periods of time at a stirring speed of 500 rpm. A PTFE-coated steel propeller with the length of 9 cm and a width of 3 cm was set at a distance of 4.6 cm from the bottom of the reactor. The stock solutions of 0.5 mol/L NaOH and 0.5 mol/L HNO3 were used to adjust the pH of the mixing solutions. The asprepared suspensions were immediately separated from the mother liquor by centrifugation, repeatedly washed with deionized water, and finally dried at 60 °C for overnight. And the pH was measured with a suspension electrode. To investigate the particle growth mechanism, suspensions were sampled at different reaction time during the precipitation process and immediately filtered through a 0.2 lm-pore size polycarbonate membranes. The latter were then cut and pasted on copper stubs for the examination of the deposited solids by SEM observation. 2.3. Characterization The obtained particles were characterized with various instruments. The collected powder samples were observed by FESEM (Zeiss Supra55, Germany), FTIR (Nicolet IS10, Thermo Fisher, USA), XRD (Rigaku FEN-100, Japan), and TG/DSC (Netzsch STA-449, Germany). The sizes of the dispersed ZnC2O42H2O particles were obtained by randomly measuring at least 100 particles in the SEM images by using a software named as Image J.
2.1. Materials Sodium citrate (Aldrich) was used as the zinc chelator and the crystalline modifier, and all the other chemicals were of analytical grade, and their solutions were filtered through a 0.2 lm-pore size membrane to eliminate any possible solid impurities. 2.2. Preparation of zinc oxalate particles The experimental setup for the controlled double-jet precipitation (CDJP) has been described by many researchers, and a simple schematic illustration was shown in Fig. 1. In our work, the round
Fig. 1. Illustration diagram of the preparation of well-dispersed zinc oxalate particles by CDJP method (Citrate reagent was added in the solutions A, B and C in advance of the feeding).
3. Results and discussion 3.1. Effect of sodium citrate concentration In order to study the effect of sodium citrate on the formation of zinc oxalate particles, sodium citrate was added into each solution in advance of the feeding, including not only the feeding solutions of A and B, but also the bottom solution C at the same concentration of sodium citrate, as shown in Fig. 1. And with the proceeding of the feeding, the concentration of citrate in the reaction solution is kept at an almost constant level if ignoring the loss of citrate in the coprecipitation. Fig. 2 shows the SEM photos of the precipitated zinc oxalate particles obtained after double-jet feeding 10 h, and it can be seen that at the low concentration of citrate, i.g. 1 mM, the particles are aggregated obviously by fine flaky subunits according to some regular orientation to form some kind of special hierarchical structure approaching the flower shape. While by increasing amount of the citrate till to 5 mM, the aggregation disappeared and the individual flaky particles with rectangle shape are produced with average flat size around 20 lm defined by the maximum Feret’s diameter. The effect of citrate on the morphology of the precipitated particles is very drastic, as shown in Fig. 2, and it plays a very key role not only the chelating reagent with zinc ions but also an effective dispersing agent for the particles. It can be found that even without adding of citrate, the obtained particles also exhibited as the flaky subunits, while the presence of citrate improves the dispersity of them obviously probably due to its excellent steric hindrance of polymers that the citrate molecules adhered on the subunits would prevent them from random or uncontrolled aggregation into larger particles. The cheating affinity of citrate ligand with zinc ions may help its orientating packing to produce the well-dispersed flaky
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Fig. 2. SEM photos of the dihydrate zinc oxalate particles obtained from the suspension with the sodium citrate concentration of 0 (a), 1 mM (b) and 5 mM (c) after dipping 10 h of solution A (initial pH = 3) and solution B (initial pH = 6) into 1 L solution C (initial pH = 5) at a double-jet feeding rate of 1 ml/min.
Fig. 3. SEM photos of the calcination products (air, 400 °C, 1 h) of the dihydrate zinc oxalate particles obtained from the suspension described in Fig. 2.
particles. In the real experimental process, it is found that the pH of the solution C, i.e., the reaction zone of the precipitation, is kept at 5.0 constantly due to the strong pH buffer effect of citrate ligands, which is obviously very good for the constant control of the parameters in the precipitation process to tailor the particle morphology reproducibly.
The above obtained particles were further used for SEM observation after calcination at 400 °C for 1 h after increasing the temperature from 25 °C to 400 °C in 2 h, and the results were shown in Fig. 3. It can be found that the high temperature caused the fairly severe crackes due to the decomposition as well as the huge porous structures left in the particles. Obviously, the calcinated products
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reaction of ZnC2O42H2O = ZnC2O4 + 2H2O" (19.0% mass loss in theoretical calculation) and ZnC2O4 + 0.5O2 = ZnO + 2CO2" (38.1% mass loss in theoretical calculation). It is interesting to find that with the increasing of citrate amount in the precipitation system, the first decomposition peak occurs at the lower temperature corresponding to the less mass loss, i.e., 150.5 °C (19.8%), 143.6 °C (18.8%) and 133.1 °C (17.4%) for the cases of 0 mM, 1 mM and 5 mM citrate, respectively, suggesting that the presence of citrate will substitute some crystalline water molecules in the zinc oxalate hydrate and may lead to the looser crystal stacking structure for easier decomposition. The decomposition percentages at the temperature near 390 °C for three samples were 36.7%, 38.1% and 35.2%, all close to the theoretical decomposition ratio of 38.1%, indicating that the products after thermolysis at 400 °C in air should be the pure ZnO phase. 3.3. FTIR Above three samples were further tested by FTIR instrument, and the results were shown in Fig. 5. It can be seen that the three samples exhibit almost the same patterns except for the three peaks centered at 1635 cm1, 1364 cm1 and 1319 cm1 respectively, which can be ascribed to the vibration of ACOOA bonds, become much stronger in the presence of citrate during the precipitation process. It is clear to see that the introduction of citrate leads to the ACOOA bond stronger, suggesting that the citrate ligand must have become one part of the precipitated particles and its own ACOOH ligands causes the intensification of ACOOA bonds. 3.4. XRD analysis The above prepared precipitated particles were analyzed by XRD instrument, and it can be seen that all the precipitates were determined to be dihydrate zinc oxalate. While it is also very clear to see that the presence of citrate during the precipitation process causes the orientating growth on the different crystalline planets, 0 2i, as shown in Fig. 6, the growth of the crystalline facets of h2 0 2i were inhibited drastically to cause the weakenh0 0 2i and h4 ing of their peaks. While some other peaks such as h0 2 0i, h0 2 1i, 1 2i, h1 3 0i and h1 1 3i were all enhanced to become more h1 intensive. It is believed that exact these crystalline planets, having different growth rate under the presence of citrate ligand due to its
Fig. 4. TG/DSC curves of the dihydrate zinc oxalate particles obtained by the typical preparation route described in Fig. 2.
obtained at 400 °C probably had been transferred into ZnO, which would be discussed in details in the latter section. 3.2. TG/DSC and their thermolysis products The particle samples prepared under the conditions shown in Fig. 2, were used to examine their thermal decomposition behavior in the flowing air at a rate of 10 ml/min and 10 °C/min. The results demonstrate that the three samples show almost the same thermolysis behavior in Fig. 4, that is, twice mass loss occur at the temperature of 130–150 °C and 390 °C respectively, corresponding to the decomposition of the hydrate zinc oxalate according to the
Fig. 5. FTIR curves of the dihydrate zinc oxalate particles obtained from the suspension described in Fig. 2.
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tated adsorption of citrate on the special crystalline facets to lead to the flaky shape finally. 3.5. Effect of feeding rates
Fig. 6. XRD patterns of the prepared particles of the dihydrate zinc oxalate particles obtained from the suspension with the citric acid concentration of 0 (a), 0.001 mol/L (b), 0.005 mol/L (c) after dipping 10 h of solution A (initial pH = 3) and solution B (initial pH = 6) into 1 L solution C (initial pH = 5) at a simultaneous feeding rate of 1 ml/min, and the same citric acid of 0.005 mol/L while the different initial pH for solution A (pH 6.0), B (pH 6.5) and C (pH 6.5) (d).
selective absorption onto above-mentioned planets, leads to the different growth rates as well as the different crystalline shapes. The detailed mechanism can be explained by a key index, i.e., the growth ratio of deposition rate and surface diffusion rate of the precipitation precursors [48]. For the nuclei and the precursor clusters of zinc oxalate, when in the case of Vsurface deposition > Vsurface diffusion, the nuclei will just provide the surface of dominant deposition of zinc oxalate precursors to form the aggregated particles, as shown in Fig. 2a,b, while in the case of Vsurface deposition < Vsurface diffusion, the dominant growth step is surface diffusion of precursors which will lead to the flaky shape as shown in Fig. 2c. So the presence of citrate ligand was further verified by XRD, and its effect on the particle morphology can be ascribed to the orien-
According to the Lamer model [22], the slow feeding rate should be good for the formation of well-dispersed particles due to the smaller supersaturation to avoid the secondary nucleation and the too high nuclei population in the reaction zone. So it is easy to understand that at the larger feeding rate, the obtained precipitates are much easier to form the aggregation and the basic shape of the subunits would also be transformed from the rectangle to the prismatic, as shown in the Fig. 7 that the obvious change of the morphology of the flaky subunits has occurred. It was also found in the figure that the rectangular and prismatic shaped particles were obtained in the slow and fast feeding rates respectively, and the confirmed mechanics for the formation of different morphologies for two cases were not clear, but it could be deduced that the different supersaturation leaded to their respective growth habits. And larger supersaturation is favorable for the formation of prismatic particles corresponding to the 5 ml/min feeding rate, while lower one is beneficial to the rectangular particles corresponding to the 1 ml/min feeding rate. As for the confirmed clear mechanism, more detailed information was required to disclose it in the next-step study. 3.6. Effect of reaction time The effect of reaction time shows quite drastic positive promotion for the growth of the dihydrate zinc oxalate particles, as shown in Fig. 8, the flaky particles become larger and larger in the radial size with the prolonging of feeding time, and the size distribution width also grows broader correspondingly. As demonstrated in Fig. 8-a,b,c,d, the particles obtained by aging for 3 h, 5 h, 10 h and 15 h have the size distribution width of 0–14 lm, 0–20 lm, 0–32 lm and 0–50 lm, respectively, showing a quick growth behavior of particle size and size distribution.
Fig. 7. SEM photos of the dihydrate zinc oxalate particles obtained from the suspension obtained at the different feeding rate of 1 ml/min (a) and 5 ml/min (b, 500; c, 2000) respectively, and the other experimental conditions were the same as that of the Fig. 2c.
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Fig. 8. SEM photos of the dihydrate zinc oxalate particles obtained from the suspension obtained at the different feeding time: a-3 h; b-5 h; c-10 h and d-15 h at the feeding rate of 1 ml/min under the completely same experimental conditions as that of the Fig. 2c.
The primary crystals, produced by a burst of nuclei in the early period of the precipitation, grew quickly by swallowing the precursors of precipitate monomers provided by the continuous feeding. The growth of the well-dispersed particles suggests that the secondary nucleation has been inhibited or avoided effectively under the as-designed experimental conditions. It could be confirmed by that fact the finer particles did not appear due to the secondary nucleation in the experimental observations. By statistical estimation of the particles size, it was surprisingly found in Fig. 8e that the average radial size of the precipitate increases linearly with the reaction time, and the average size were 8.62 lm (3 h), 10.96 lm (5 h), 19.34 lm (10 h) and 25.66 lm (15 h) respectively. On the basis of these data, linear equation was functioned as d = 1.454 t + 4.14 lm (d denotes the radial size of the flaky particles, lm; and t is the feeding time, h), demonstrating the positive correlation between the particle size and the growth time. The slope of the linear equation is calculated to be 1.4545 lm/h, which gave an apparent growth rate of the radial size of the particles in the investigated precipitation system and conditions. The strong linear dependence of the radial growth of precipitated crystals on the reaction time provided a chance for the particle size controlling in the precipitation process. It can be deduced that different pre-
cipitating systems should have their own characteristic growth rates, which provides the more chances of preparing the various inorganic ultrafine particles with controlled size. 3.7. Effect of pH Based on the same precipitation conditions, effect of the pH values of the feeding solutions were investigated by increasing to much higher as described in the caption of Fig. 9, and it can be found that at higher pH the induction period time will be prolonged drastically that even after 10 h of feeding the solution still kept transparent evenly. And till 12 h, a light turbidity comes out, and so the precipitate sample was collected at 15 h. The high pH of the solution suggests that the precipitate occurs under the condition of much stronger chelating effect, suggesting that much slower precipitating process that may be good for the formation of well-dispersed particles due to the controllable release of zinc ions for the slow precipitation and inhibition of secondary nucleation. And the quantitative explain of chelating effect of citrate on the precipitation process could be performed in the following based on the thermodynamic equilibrium equations and basic calculations.
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Fig. 9. SEM photos of the dihydrate zinc oxalate particles (a, 200; b, 1000) obtained from the suspension obtained at the different initial pH for solution A (pH 6.0), B (pH 6.5) and C (pH 6.5) respectively at 15 h reaction, and the other experimental conditions were the same as that of the Fig. 2c.
In order to learn more about the effect of pH on the precipitation process, the thermodynamic diagrams of Zn-oxalate-H2O and Zn-citrate-oxalate-H2O were calculated based on the simultaneous equilibrating principles [49,50] (as shown in the supplementary information in details), as illustrated in Fig. 10, in which the equilibrium concentrations of zinc ions and oxalate were set at
Fig. 10. Species distribution of zinc in the solution Zn-oxalate-H2O (total zinc and oxalate concentration is 0.1 mmol/L and 0.12 mmol/L respectively (a) and Zncitrate-oxalate-H2O (total zinc and oxalate concentration is 0.1 mmol/L and 0.12 mmol/L respectively with the total concentration of citrate is 1 mmol/L, b, and 5 mmol/L c). Notes: Zn(ox), soluble complex species of zinc ions and oxalate anions; Zn(ox)(s), insoluble precipitate of zinc oxalate; ZnO(cr), crystalline precipitate of zinc hydroxide; Zn(cit) and Zn(cit)4 2 , complex species of zinc ions and citrate; Zn(Hcit), complex species of zinc ions and protonated citrate; 4 Zn2(cit)2(OH)2 , complex species of zinc ions, citrate and hydroxyl; Zn(OH)+, 2 Zn(OH)2, Zn(OH) 3 , and Zn(OH)4 , complex species of zinc ions, and hydroxyl.
extremely low to simulate the precipitation reaction conditions in the bottom solution. The typical three diagrams respectively correspond to the zinc oxalate precipitation system with zero, 1 mmol/L and 5 mmol/L citrate contained in the solutions. In these solution systems, the zinc ions may react with oxalate anions to 4 form the complexes like ZnC2O4, Zn(C2O4)2 2 and Zn(C2O4)3 or Zn 4 (ox), Zn(ox)2 2 and Zn(ox)3 ; or coordinate with hydroxyl ligands 2 to form Zn(OH)+, Zn(OH)2, Zn(OH) 3 , and Zn(OH)4 under the alkaline conditions; or react with citrate to produce the chelating com0 4 pounds like Zn(cit) and Zn(cit)4 2 , Zn(Hcit) and Zn2(cit)2(OH)2 . In above systems, two solid phases of zinc oxalate or zinc hydroxide may occur to precipitate under a certain condition. Obviously, the competition among these species would occur usually depending on the concentration of zinc ions, oxalate, citrate and pH, and finally a thermodynamic equilibrium state would appear, i.e., all the species attained a simultaneous equilibrium. Based on these calculation results, combining with the concrete experimental conditions, it is good to learn the solution details for the whole precipitation process, that will be much better to help tailor the size and shape of the precipitated particles in a scrupulously controllable way. As shown in Fig. 10(a), it clearly shows that zinc ions and oxalate will form the precipitate solid in the pH range of 3–8, as well as their complex, and small part of zinc will form its hydrate species of Zn(OH)+ from pH 6.0. Experiment was designed under the conditions of double-jet feeding of 0.05 mol/L Zn2+ and 0.06 mol/L C2O2 4 at a rate of 1 ml/min into 500 ml water at pH 5, it was found that the precipitate occurred after around 30 min feeding that could be regarded as the nucleation induction period. While in the presence of 1 mmol/L citrate, the nucleation induction period would become longer to 60 min, and 5 mmol/L citrate would make it to 120 min which could further prolong to more 10 h in the case of pH 6.5. Obviously, the presence of citrate plays a key role for all these results due to its strong affinity to zinc ions. Fig. 10(b) shows that even a very amount of citrate can cause the great variation of zinc species in the pH below 8, the predominance zone of zinc ions and oxalate via precipitation and coordination both shrank due to the formation of complex compounds of zinc and citrate in the species of Zn(Hcit)0 and Zn(cit), and it would become more remarkable in the presence of 5 mmol/L citrate as illustrated in Fig. 10(c). Therefore, it is the strong chelating affinity of citrate to zinc ions makes the zinc release much slower and precipitate by oxalate in a controllable way, which leads to the growth of the nuclei have a chance to follow its own crystallization habit, probably according to the growth mode of Vsurface deposition < Vsurface diffusion. 3.8. Formation mechanism of different morphologies Based on above results and discussion, it is confirmed that the presence of citrate is the key to influencing the dispersity and
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precipitation process by coordinating the nanoparticles via binding zinc ions on the surface of particles, or via H-bonding with structural water in the zinc-oxalate nanoparticles. It is easy to understand that the COO groups from citrate molecules could coordinate with Zn2+ in the Zn(ox)(s), and H-bonding interactions could be formed between OH ligands in citrate and carboxylic ions in oxalate groups of Zn(ox)(s). These tiny Zn(ox)(s) particles will aggregate into larger crystals, and on this occasion, the surface charging state will strongly affect the aggregation direction via the selective deposition on the specific facets of the crystals. Under the experimental conditions, at pH near 6.0, the citrate is negatively charged as shown in the Fig. 11, thus its binding to the surface of Zn(ox)(s) will be more preferential to the specific facets under the electrostatic interactions. Combined with the XRD analysis, it could be deduced that the selective absorption of citrate 0 2i, h0 0 2i and ligand onto the certain crystalline facets of h2 0 2i, inhibiting drastically and causing the weakening of their h4 XRD peaks, and no absorption onto the other facets such as o 1 2i, h1 3 0i and h1 1 3i with enhanced peaks. It h0 2 0i, h0 2 1i, h1 is easy to find that the tiny particles will of course finally grow into the flake-like shape via such a 2-dimensional growth mode as described above. Fig. 11. Schematic illustration of the formation mechanism of the dihydrate zinc oxalate with different morphologies (Remarks: the whiskers surrounding the subunits denoted the citrate polymers adhered on the subunits.)
morphology, which may play a multiple role as the chelating reagent to control the release rate of zinc ions, dispersing for the precipitated particles due to the steric hindrance, and tuning of crystalline directional growth via selective absorption on the specific facets. Therefore, the morphological evolution of the zinc oxalate hydrate particles could be illustrated in Fig. 11, in which the feeding rate and concentration of citrate in the precipitation system were regarded as the main factors to affect the final particle morphologies. In the precipitation system described in the Fig. 1, when the feeding rate is controlled to be quite slow, e.g., 1 ml/ min, and the citrate concentration is 5 mM, the dispersed hydrate zinc oxalate particles with flaky and rectangle shape were obtained, while if increasing the feeding rate or decreasing the citrate concentration, then the aggregation would occur seriously. So the slow feeding rate and higher amount of citrate in the precipitation system is the key parameters to prepare the well-dispersed hydrate zinc oxalate particles. Obviously, it is easy to understand that the fast feeding will lead to the greater supersaturation and the number of the precursors for nuclei growth will be much larger than that of the surface growth, causing the aggregation. While at the slower feeding rate and higher citrate concentration, the release rate of the free zinc ions will be controlled quite fit to the nuclei growth and produced the well-dispersed particles finally. As for the formation mechanism of the rectangular and prismatic shaped particles obtained in the slow and fast feeding rates respectively, we thought it might be due to the different supersaturation that leaded to their respective growth habits. Anyway, both particles were flake-like. As for the formation of well dispersed flakelike shape for the precipitated zinc oxalate particles, the role of citrate has been proven indispensable. By referring to the related study on the calcium oxalate precipitate under the effect of citrate [42], it can be assumed that the Zn2+ and C2O2 4 formed the polynuclear stable complexes like Zn(ox) that aggregate into larger assemblies (clusters), from which the zinc oxalate nucleates. When the supersaturation of the Zn(ox) is high enough that causes the nucleation, and a lot of tiny particles will be produced. In the early period of nucleation, citrate shows its inhibitory effect on the formation of zinc oxalate complexes, as shown in Fig. 10a and b. While after nucleation, citrate will still participate in the
4. Conclusion The experimental results presented above offer the example of the advantages of the controlled double-jet precipitation (CDJP) technique for the preparation of uniform dihydrate zinc oxalate particles. The presence of citrate in the precipitation system including the feeding reagents and the bottom solutions at the same levels was to enhance the controllability of the reaction process to form the dispsered particles. Citrate ligand plays the role of not only the controllable release of zinc ions from their coordinated complex as well as the morphological tunning agent for the crystal orientated growth, but also the dispersing surfacant during the whole precipitation process. The controllable aggregation of many microsized subunits into a larger particle was proposed as the main formation mechanism of well-dispersed flaky ZnC2O42H2O particles. It can be deduced that by choosing different chelating reagents the zinc oxalate particles with various morphologies and shapes will be produced, and this synthesis idea also can find more extensive application in the preparation of any other dispersed particles according to the similar setups and operational procedures, which will expand and enrich the current dispersed particles gallery. Acknowledgement The authors want to show their thanks to the Dr. Li for his help in SEM observation from Institute of Automation, Chinese Academy of Sciences. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2019.06.013. References [1] Q. Nie, L. Yang, C. Cao, Y. Zeng, G. Wang, C. Wang, S. Lin, Interface optimization of ZnO nanorod/CdS quantum dots heterostructure by a facile two-step lowtemperature thermal treatment for improved photoelectrochemical water splitting, Chem. Eng. J. 325 (2017) 151–159. [2] R. Kumar, A. Umar, G. Kumar, H.S. Nalwa, Antimicrobial properties of ZnO nanomaterials: a review, Ceram. Int. 43 (2017) 3940–3961. [3] D.T. Nguyen, K.-S. Kim, Structural evolution of highly porous/hollow ZnO nanoparticles in sonochemical process, Chem. Eng. J. 276 (2015) 11–19.
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Preparation of flaky dihydrate zinc oxalate particles by controlled chelating double-jet precipitation Chen Xing, Huang Kai ⇑, Wang Chengyan School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Xueyuan Rd. 30, Haidian District, 100083 Beijing, China
a r t i c l e
i n f o
Article history: Received 22 June 2017 Received in revised form 12 May 2019 Accepted 14 June 2019 Available online 24 June 2019 Keywords: Zinc oxalate particles Flaky shape Citrate ligand Controlled double-jet precipitation
a b s t r a c t Well-dispersed flaky ZnC2O42H2O particles were synthesized via a controlled double-jet precipitation process from the relatively concentrated solutions of Zn(NO3)2 (0.05 mol/L) and Na2C2O4 (0.06 mol/L) in the presence of sodium citrate. The effects of concentration of the reactants, reaction time, pH values and feeding rates on the size and shape of the final particles are investigated. It was found that asprepared dihydrate zinc oxalate particles are formed by aggregation of tiny microsized subunits, and the feeding rate as well as the citrate concentration is quite key to the morphological evolution of the hydrate zinc oxalate particles. Based on experimental results and the thermodynamic equilibrium calculation, it is confirmed that the presence of citrate ligand plays essential roles to the whole precipitation process, which may play the multiple roles of controllable release of zinc ions, crystal habit tunning agent, and dispersing surfactant. The controllable synthesis idea of well-dispersed particles in present research work has the potential of extensive application in the preparation of other similar particles. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Colloidal ZnO is a typical functional material that can find many applications in the fields of optics, pigments, cosmetics, electrics, adsorption, catalysis, and so on [1–5]. Due to the strong dependence of material properties on its size and shape, the scrupulous controlling of the size, shape and dispersity is a key in the particle preparation [6–10]. In the past four decades various methods have been developed to prepare the well-dispersed particles, in which the homogeneous precipitation was frequently employed due to its simple operation and good reproducibility [11–13]. While homogeneous precipitation has quite low yield in the practical process, the double-jet precipitation process was therefore usually proposed due to its relatively high concentration of reactants and good dispersity in the obtained products [14–17]. Comparing with the particles size tailoring, the controlling of particle shape is more difficult because there are not so many theories emerged in the past decades [18–20]. Lamer model, as a typical empirical formula, was usually used to explain the formation of dispersity or size distribution of the precipitated particles [21,22], and Weimarn law can illustrate the particle size variation based on the relative ratio of nucleation to growth rates and mass
⇑ Corresponding author.
balance [23,24]. While for the particle shape or morphology, there are quite few theories to give the convincing proofs and reasons. In the precipitation system, the growth model by diffusion of monomers to the nuclei is responsible for the formation of the crystals with various shapes in a large number of experimental reports [21,22,25,26]. The sufficient exposure of their crystal facets under this growth model, which usually occurred in the slow precipitation system at the extremely dilute solution, is believed to be the main reason for the formation mechanism of the speciallyshaped particles [25–27]. But there are also many proofs indicating that after nucleation the tiny subunits could aggregate into larger particles with special shape or morphology, such as spherical [28–30], needle-like [31–33], flower-like [34,35], flaky [36,37], rod-like [38,39] shape and so on. And in most cases, it was also found that the Oswald ripening may do some modification on the shape tailoring such as making the particle surface smoother via a dissolution and redeposition or recrystallization process [26,40,41]. And another measure frequently taken is to add the polymer to modify the shape and morphology of the precipitated particles. Considering the extensive charging of the surface of the precipitated colloids, the ionic polymers were usually chosen like organic acid or compounds, e.g., citric acid [19,28,42], ethylenediaminetetraacetic acid (EDTA)[43], hexadecyl trimethyl ammonium bromide (CTAB) [19,34], and so on. It is well known that citrate is a typical oxalate precipitation inhibitor, for example, in the
E-mail address: [email protected] (H. Kai). https://doi.org/10.1016/j.apt.2019.06.013 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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formation of calcium oxalate, citrate is believed to effectively inhibit the calcium oxalate nucleation by interacting with all the early stage CaC2O4 species including polynuclear stable complexes and amorphous precursors [42]. And during this process, the obtained calcium oxalate precipitated into different size and morphologies according to the designing strategy. The related experience of tailoring the oxalate precipitate particles provided quite good reference for the similar synthesis systems. Anyway, though many papers have been published about the controllable preparation of various inorganic particles with different shapes or morphologies, the controlling rules are still far from the size tailoring skills during the precipitation system. So most of the preparation of the particles with special morphology is still lying in the stage of ‘‘Trial and Error” and far from the stage of ‘‘Orientating design”, and anyway under the guide of some fundamental principles and experiences to synthesize various particles with unique shape or morphology is still an endeavor of this research filed. The reported work has been carried out to prepare the zinc oxalate particles by chemical precipitation processes, and most of them have the shapes of being flower-like [35,44], rod-like [45], wire-like [46], cuboid [47], though they built different media systems like ether-water [35,45,46], electrochemical synthesis [44], microemulsion [47], but it was never mentioned about the flakelike zinc oxalate particles in the literature. In our study, the flaky dihydrate zinc oxalate subunits as well as their hierachical aggregation particles are easily prepared via a controlled double-jet precipitation. Therefore, the present study demonstrated a specific flaky zinc oxalate precipitate for the first time, that would provide a unique and novel shape for zinc oxalate particles. 2. Experimental
tube is made of PTFE (polytetrafluoroethylene) with an inner diameter of 3 mm, which is also the nozzle size of the feeding tube. In a typical run, a 0.05 mol/l Zn(NO3)2 (A) solution and a 0.06 mol/l Na2C2O4 (B) solution in concentrations were simultaneously introduced into a cylindrical container fabricated by ourselves (a waterjacket steel bucket with the characteristic size of U17 cm 25 cm), containing 1 L of deionized water (C) kept at 30 °C, at a constant flow rate of 1–10 ml/min, followed by aging at the same temperature for various periods of time at a stirring speed of 500 rpm. A PTFE-coated steel propeller with the length of 9 cm and a width of 3 cm was set at a distance of 4.6 cm from the bottom of the reactor. The stock solutions of 0.5 mol/L NaOH and 0.5 mol/L HNO3 were used to adjust the pH of the mixing solutions. The asprepared suspensions were immediately separated from the mother liquor by centrifugation, repeatedly washed with deionized water, and finally dried at 60 °C for overnight. And the pH was measured with a suspension electrode. To investigate the particle growth mechanism, suspensions were sampled at different reaction time during the precipitation process and immediately filtered through a 0.2 lm-pore size polycarbonate membranes. The latter were then cut and pasted on copper stubs for the examination of the deposited solids by SEM observation. 2.3. Characterization The obtained particles were characterized with various instruments. The collected powder samples were observed by FESEM (Zeiss Supra55, Germany), FTIR (Nicolet IS10, Thermo Fisher, USA), XRD (Rigaku FEN-100, Japan), and TG/DSC (Netzsch STA-449, Germany). The sizes of the dispersed ZnC2O42H2O particles were obtained by randomly measuring at least 100 particles in the SEM images by using a software named as Image J.
2.1. Materials Sodium citrate (Aldrich) was used as the zinc chelator and the crystalline modifier, and all the other chemicals were of analytical grade, and their solutions were filtered through a 0.2 lm-pore size membrane to eliminate any possible solid impurities. 2.2. Preparation of zinc oxalate particles The experimental setup for the controlled double-jet precipitation (CDJP) has been described by many researchers, and a simple schematic illustration was shown in Fig. 1. In our work, the round
Fig. 1. Illustration diagram of the preparation of well-dispersed zinc oxalate particles by CDJP method (Citrate reagent was added in the solutions A, B and C in advance of the feeding).
3. Results and discussion 3.1. Effect of sodium citrate concentration In order to study the effect of sodium citrate on the formation of zinc oxalate particles, sodium citrate was added into each solution in advance of the feeding, including not only the feeding solutions of A and B, but also the bottom solution C at the same concentration of sodium citrate, as shown in Fig. 1. And with the proceeding of the feeding, the concentration of citrate in the reaction solution is kept at an almost constant level if ignoring the loss of citrate in the coprecipitation. Fig. 2 shows the SEM photos of the precipitated zinc oxalate particles obtained after double-jet feeding 10 h, and it can be seen that at the low concentration of citrate, i.g. 1 mM, the particles are aggregated obviously by fine flaky subunits according to some regular orientation to form some kind of special hierarchical structure approaching the flower shape. While by increasing amount of the citrate till to 5 mM, the aggregation disappeared and the individual flaky particles with rectangle shape are produced with average flat size around 20 lm defined by the maximum Feret’s diameter. The effect of citrate on the morphology of the precipitated particles is very drastic, as shown in Fig. 2, and it plays a very key role not only the chelating reagent with zinc ions but also an effective dispersing agent for the particles. It can be found that even without adding of citrate, the obtained particles also exhibited as the flaky subunits, while the presence of citrate improves the dispersity of them obviously probably due to its excellent steric hindrance of polymers that the citrate molecules adhered on the subunits would prevent them from random or uncontrolled aggregation into larger particles. The cheating affinity of citrate ligand with zinc ions may help its orientating packing to produce the well-dispersed flaky
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Fig. 2. SEM photos of the dihydrate zinc oxalate particles obtained from the suspension with the sodium citrate concentration of 0 (a), 1 mM (b) and 5 mM (c) after dipping 10 h of solution A (initial pH = 3) and solution B (initial pH = 6) into 1 L solution C (initial pH = 5) at a double-jet feeding rate of 1 ml/min.
Fig. 3. SEM photos of the calcination products (air, 400 °C, 1 h) of the dihydrate zinc oxalate particles obtained from the suspension described in Fig. 2.
particles. In the real experimental process, it is found that the pH of the solution C, i.e., the reaction zone of the precipitation, is kept at 5.0 constantly due to the strong pH buffer effect of citrate ligands, which is obviously very good for the constant control of the parameters in the precipitation process to tailor the particle morphology reproducibly.
The above obtained particles were further used for SEM observation after calcination at 400 °C for 1 h after increasing the temperature from 25 °C to 400 °C in 2 h, and the results were shown in Fig. 3. It can be found that the high temperature caused the fairly severe crackes due to the decomposition as well as the huge porous structures left in the particles. Obviously, the calcinated products
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reaction of ZnC2O42H2O = ZnC2O4 + 2H2O" (19.0% mass loss in theoretical calculation) and ZnC2O4 + 0.5O2 = ZnO + 2CO2" (38.1% mass loss in theoretical calculation). It is interesting to find that with the increasing of citrate amount in the precipitation system, the first decomposition peak occurs at the lower temperature corresponding to the less mass loss, i.e., 150.5 °C (19.8%), 143.6 °C (18.8%) and 133.1 °C (17.4%) for the cases of 0 mM, 1 mM and 5 mM citrate, respectively, suggesting that the presence of citrate will substitute some crystalline water molecules in the zinc oxalate hydrate and may lead to the looser crystal stacking structure for easier decomposition. The decomposition percentages at the temperature near 390 °C for three samples were 36.7%, 38.1% and 35.2%, all close to the theoretical decomposition ratio of 38.1%, indicating that the products after thermolysis at 400 °C in air should be the pure ZnO phase. 3.3. FTIR Above three samples were further tested by FTIR instrument, and the results were shown in Fig. 5. It can be seen that the three samples exhibit almost the same patterns except for the three peaks centered at 1635 cm1, 1364 cm1 and 1319 cm1 respectively, which can be ascribed to the vibration of ACOOA bonds, become much stronger in the presence of citrate during the precipitation process. It is clear to see that the introduction of citrate leads to the ACOOA bond stronger, suggesting that the citrate ligand must have become one part of the precipitated particles and its own ACOOH ligands causes the intensification of ACOOA bonds. 3.4. XRD analysis The above prepared precipitated particles were analyzed by XRD instrument, and it can be seen that all the precipitates were determined to be dihydrate zinc oxalate. While it is also very clear to see that the presence of citrate during the precipitation process causes the orientating growth on the different crystalline planets, 0 2i, as shown in Fig. 6, the growth of the crystalline facets of h2 0 2i were inhibited drastically to cause the weakenh0 0 2i and h4 ing of their peaks. While some other peaks such as h0 2 0i, h0 2 1i, 1 2i, h1 3 0i and h1 1 3i were all enhanced to become more h1 intensive. It is believed that exact these crystalline planets, having different growth rate under the presence of citrate ligand due to its
Fig. 4. TG/DSC curves of the dihydrate zinc oxalate particles obtained by the typical preparation route described in Fig. 2.
obtained at 400 °C probably had been transferred into ZnO, which would be discussed in details in the latter section. 3.2. TG/DSC and their thermolysis products The particle samples prepared under the conditions shown in Fig. 2, were used to examine their thermal decomposition behavior in the flowing air at a rate of 10 ml/min and 10 °C/min. The results demonstrate that the three samples show almost the same thermolysis behavior in Fig. 4, that is, twice mass loss occur at the temperature of 130–150 °C and 390 °C respectively, corresponding to the decomposition of the hydrate zinc oxalate according to the
Fig. 5. FTIR curves of the dihydrate zinc oxalate particles obtained from the suspension described in Fig. 2.
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tated adsorption of citrate on the special crystalline facets to lead to the flaky shape finally. 3.5. Effect of feeding rates
Fig. 6. XRD patterns of the prepared particles of the dihydrate zinc oxalate particles obtained from the suspension with the citric acid concentration of 0 (a), 0.001 mol/L (b), 0.005 mol/L (c) after dipping 10 h of solution A (initial pH = 3) and solution B (initial pH = 6) into 1 L solution C (initial pH = 5) at a simultaneous feeding rate of 1 ml/min, and the same citric acid of 0.005 mol/L while the different initial pH for solution A (pH 6.0), B (pH 6.5) and C (pH 6.5) (d).
selective absorption onto above-mentioned planets, leads to the different growth rates as well as the different crystalline shapes. The detailed mechanism can be explained by a key index, i.e., the growth ratio of deposition rate and surface diffusion rate of the precipitation precursors [48]. For the nuclei and the precursor clusters of zinc oxalate, when in the case of Vsurface deposition > Vsurface diffusion, the nuclei will just provide the surface of dominant deposition of zinc oxalate precursors to form the aggregated particles, as shown in Fig. 2a,b, while in the case of Vsurface deposition < Vsurface diffusion, the dominant growth step is surface diffusion of precursors which will lead to the flaky shape as shown in Fig. 2c. So the presence of citrate ligand was further verified by XRD, and its effect on the particle morphology can be ascribed to the orien-
According to the Lamer model [22], the slow feeding rate should be good for the formation of well-dispersed particles due to the smaller supersaturation to avoid the secondary nucleation and the too high nuclei population in the reaction zone. So it is easy to understand that at the larger feeding rate, the obtained precipitates are much easier to form the aggregation and the basic shape of the subunits would also be transformed from the rectangle to the prismatic, as shown in the Fig. 7 that the obvious change of the morphology of the flaky subunits has occurred. It was also found in the figure that the rectangular and prismatic shaped particles were obtained in the slow and fast feeding rates respectively, and the confirmed mechanics for the formation of different morphologies for two cases were not clear, but it could be deduced that the different supersaturation leaded to their respective growth habits. And larger supersaturation is favorable for the formation of prismatic particles corresponding to the 5 ml/min feeding rate, while lower one is beneficial to the rectangular particles corresponding to the 1 ml/min feeding rate. As for the confirmed clear mechanism, more detailed information was required to disclose it in the next-step study. 3.6. Effect of reaction time The effect of reaction time shows quite drastic positive promotion for the growth of the dihydrate zinc oxalate particles, as shown in Fig. 8, the flaky particles become larger and larger in the radial size with the prolonging of feeding time, and the size distribution width also grows broader correspondingly. As demonstrated in Fig. 8-a,b,c,d, the particles obtained by aging for 3 h, 5 h, 10 h and 15 h have the size distribution width of 0–14 lm, 0–20 lm, 0–32 lm and 0–50 lm, respectively, showing a quick growth behavior of particle size and size distribution.
Fig. 7. SEM photos of the dihydrate zinc oxalate particles obtained from the suspension obtained at the different feeding rate of 1 ml/min (a) and 5 ml/min (b, 500; c, 2000) respectively, and the other experimental conditions were the same as that of the Fig. 2c.
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Fig. 8. SEM photos of the dihydrate zinc oxalate particles obtained from the suspension obtained at the different feeding time: a-3 h; b-5 h; c-10 h and d-15 h at the feeding rate of 1 ml/min under the completely same experimental conditions as that of the Fig. 2c.
The primary crystals, produced by a burst of nuclei in the early period of the precipitation, grew quickly by swallowing the precursors of precipitate monomers provided by the continuous feeding. The growth of the well-dispersed particles suggests that the secondary nucleation has been inhibited or avoided effectively under the as-designed experimental conditions. It could be confirmed by that fact the finer particles did not appear due to the secondary nucleation in the experimental observations. By statistical estimation of the particles size, it was surprisingly found in Fig. 8e that the average radial size of the precipitate increases linearly with the reaction time, and the average size were 8.62 lm (3 h), 10.96 lm (5 h), 19.34 lm (10 h) and 25.66 lm (15 h) respectively. On the basis of these data, linear equation was functioned as d = 1.454 t + 4.14 lm (d denotes the radial size of the flaky particles, lm; and t is the feeding time, h), demonstrating the positive correlation between the particle size and the growth time. The slope of the linear equation is calculated to be 1.4545 lm/h, which gave an apparent growth rate of the radial size of the particles in the investigated precipitation system and conditions. The strong linear dependence of the radial growth of precipitated crystals on the reaction time provided a chance for the particle size controlling in the precipitation process. It can be deduced that different pre-
cipitating systems should have their own characteristic growth rates, which provides the more chances of preparing the various inorganic ultrafine particles with controlled size. 3.7. Effect of pH Based on the same precipitation conditions, effect of the pH values of the feeding solutions were investigated by increasing to much higher as described in the caption of Fig. 9, and it can be found that at higher pH the induction period time will be prolonged drastically that even after 10 h of feeding the solution still kept transparent evenly. And till 12 h, a light turbidity comes out, and so the precipitate sample was collected at 15 h. The high pH of the solution suggests that the precipitate occurs under the condition of much stronger chelating effect, suggesting that much slower precipitating process that may be good for the formation of well-dispersed particles due to the controllable release of zinc ions for the slow precipitation and inhibition of secondary nucleation. And the quantitative explain of chelating effect of citrate on the precipitation process could be performed in the following based on the thermodynamic equilibrium equations and basic calculations.
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Fig. 9. SEM photos of the dihydrate zinc oxalate particles (a, 200; b, 1000) obtained from the suspension obtained at the different initial pH for solution A (pH 6.0), B (pH 6.5) and C (pH 6.5) respectively at 15 h reaction, and the other experimental conditions were the same as that of the Fig. 2c.
In order to learn more about the effect of pH on the precipitation process, the thermodynamic diagrams of Zn-oxalate-H2O and Zn-citrate-oxalate-H2O were calculated based on the simultaneous equilibrating principles [49,50] (as shown in the supplementary information in details), as illustrated in Fig. 10, in which the equilibrium concentrations of zinc ions and oxalate were set at
Fig. 10. Species distribution of zinc in the solution Zn-oxalate-H2O (total zinc and oxalate concentration is 0.1 mmol/L and 0.12 mmol/L respectively (a) and Zncitrate-oxalate-H2O (total zinc and oxalate concentration is 0.1 mmol/L and 0.12 mmol/L respectively with the total concentration of citrate is 1 mmol/L, b, and 5 mmol/L c). Notes: Zn(ox), soluble complex species of zinc ions and oxalate anions; Zn(ox)(s), insoluble precipitate of zinc oxalate; ZnO(cr), crystalline precipitate of zinc hydroxide; Zn(cit) and Zn(cit)4 2 , complex species of zinc ions and citrate; Zn(Hcit), complex species of zinc ions and protonated citrate; 4 Zn2(cit)2(OH)2 , complex species of zinc ions, citrate and hydroxyl; Zn(OH)+, 2 Zn(OH)2, Zn(OH) 3 , and Zn(OH)4 , complex species of zinc ions, and hydroxyl.
extremely low to simulate the precipitation reaction conditions in the bottom solution. The typical three diagrams respectively correspond to the zinc oxalate precipitation system with zero, 1 mmol/L and 5 mmol/L citrate contained in the solutions. In these solution systems, the zinc ions may react with oxalate anions to 4 form the complexes like ZnC2O4, Zn(C2O4)2 2 and Zn(C2O4)3 or Zn 4 (ox), Zn(ox)2 2 and Zn(ox)3 ; or coordinate with hydroxyl ligands 2 to form Zn(OH)+, Zn(OH)2, Zn(OH) 3 , and Zn(OH)4 under the alkaline conditions; or react with citrate to produce the chelating com0 4 pounds like Zn(cit) and Zn(cit)4 2 , Zn(Hcit) and Zn2(cit)2(OH)2 . In above systems, two solid phases of zinc oxalate or zinc hydroxide may occur to precipitate under a certain condition. Obviously, the competition among these species would occur usually depending on the concentration of zinc ions, oxalate, citrate and pH, and finally a thermodynamic equilibrium state would appear, i.e., all the species attained a simultaneous equilibrium. Based on these calculation results, combining with the concrete experimental conditions, it is good to learn the solution details for the whole precipitation process, that will be much better to help tailor the size and shape of the precipitated particles in a scrupulously controllable way. As shown in Fig. 10(a), it clearly shows that zinc ions and oxalate will form the precipitate solid in the pH range of 3–8, as well as their complex, and small part of zinc will form its hydrate species of Zn(OH)+ from pH 6.0. Experiment was designed under the conditions of double-jet feeding of 0.05 mol/L Zn2+ and 0.06 mol/L C2O2 4 at a rate of 1 ml/min into 500 ml water at pH 5, it was found that the precipitate occurred after around 30 min feeding that could be regarded as the nucleation induction period. While in the presence of 1 mmol/L citrate, the nucleation induction period would become longer to 60 min, and 5 mmol/L citrate would make it to 120 min which could further prolong to more 10 h in the case of pH 6.5. Obviously, the presence of citrate plays a key role for all these results due to its strong affinity to zinc ions. Fig. 10(b) shows that even a very amount of citrate can cause the great variation of zinc species in the pH below 8, the predominance zone of zinc ions and oxalate via precipitation and coordination both shrank due to the formation of complex compounds of zinc and citrate in the species of Zn(Hcit)0 and Zn(cit), and it would become more remarkable in the presence of 5 mmol/L citrate as illustrated in Fig. 10(c). Therefore, it is the strong chelating affinity of citrate to zinc ions makes the zinc release much slower and precipitate by oxalate in a controllable way, which leads to the growth of the nuclei have a chance to follow its own crystallization habit, probably according to the growth mode of Vsurface deposition < Vsurface diffusion. 3.8. Formation mechanism of different morphologies Based on above results and discussion, it is confirmed that the presence of citrate is the key to influencing the dispersity and
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precipitation process by coordinating the nanoparticles via binding zinc ions on the surface of particles, or via H-bonding with structural water in the zinc-oxalate nanoparticles. It is easy to understand that the COO groups from citrate molecules could coordinate with Zn2+ in the Zn(ox)(s), and H-bonding interactions could be formed between OH ligands in citrate and carboxylic ions in oxalate groups of Zn(ox)(s). These tiny Zn(ox)(s) particles will aggregate into larger crystals, and on this occasion, the surface charging state will strongly affect the aggregation direction via the selective deposition on the specific facets of the crystals. Under the experimental conditions, at pH near 6.0, the citrate is negatively charged as shown in the Fig. 11, thus its binding to the surface of Zn(ox)(s) will be more preferential to the specific facets under the electrostatic interactions. Combined with the XRD analysis, it could be deduced that the selective absorption of citrate 0 2i, h0 0 2i and ligand onto the certain crystalline facets of h2 0 2i, inhibiting drastically and causing the weakening of their h4 XRD peaks, and no absorption onto the other facets such as o 1 2i, h1 3 0i and h1 1 3i with enhanced peaks. It h0 2 0i, h0 2 1i, h1 is easy to find that the tiny particles will of course finally grow into the flake-like shape via such a 2-dimensional growth mode as described above. Fig. 11. Schematic illustration of the formation mechanism of the dihydrate zinc oxalate with different morphologies (Remarks: the whiskers surrounding the subunits denoted the citrate polymers adhered on the subunits.)
morphology, which may play a multiple role as the chelating reagent to control the release rate of zinc ions, dispersing for the precipitated particles due to the steric hindrance, and tuning of crystalline directional growth via selective absorption on the specific facets. Therefore, the morphological evolution of the zinc oxalate hydrate particles could be illustrated in Fig. 11, in which the feeding rate and concentration of citrate in the precipitation system were regarded as the main factors to affect the final particle morphologies. In the precipitation system described in the Fig. 1, when the feeding rate is controlled to be quite slow, e.g., 1 ml/ min, and the citrate concentration is 5 mM, the dispersed hydrate zinc oxalate particles with flaky and rectangle shape were obtained, while if increasing the feeding rate or decreasing the citrate concentration, then the aggregation would occur seriously. So the slow feeding rate and higher amount of citrate in the precipitation system is the key parameters to prepare the well-dispersed hydrate zinc oxalate particles. Obviously, it is easy to understand that the fast feeding will lead to the greater supersaturation and the number of the precursors for nuclei growth will be much larger than that of the surface growth, causing the aggregation. While at the slower feeding rate and higher citrate concentration, the release rate of the free zinc ions will be controlled quite fit to the nuclei growth and produced the well-dispersed particles finally. As for the formation mechanism of the rectangular and prismatic shaped particles obtained in the slow and fast feeding rates respectively, we thought it might be due to the different supersaturation that leaded to their respective growth habits. Anyway, both particles were flake-like. As for the formation of well dispersed flakelike shape for the precipitated zinc oxalate particles, the role of citrate has been proven indispensable. By referring to the related study on the calcium oxalate precipitate under the effect of citrate [42], it can be assumed that the Zn2+ and C2O2 4 formed the polynuclear stable complexes like Zn(ox) that aggregate into larger assemblies (clusters), from which the zinc oxalate nucleates. When the supersaturation of the Zn(ox) is high enough that causes the nucleation, and a lot of tiny particles will be produced. In the early period of nucleation, citrate shows its inhibitory effect on the formation of zinc oxalate complexes, as shown in Fig. 10a and b. While after nucleation, citrate will still participate in the
4. Conclusion The experimental results presented above offer the example of the advantages of the controlled double-jet precipitation (CDJP) technique for the preparation of uniform dihydrate zinc oxalate particles. The presence of citrate in the precipitation system including the feeding reagents and the bottom solutions at the same levels was to enhance the controllability of the reaction process to form the dispsered particles. Citrate ligand plays the role of not only the controllable release of zinc ions from their coordinated complex as well as the morphological tunning agent for the crystal orientated growth, but also the dispersing surfacant during the whole precipitation process. The controllable aggregation of many microsized subunits into a larger particle was proposed as the main formation mechanism of well-dispersed flaky ZnC2O42H2O particles. It can be deduced that by choosing different chelating reagents the zinc oxalate particles with various morphologies and shapes will be produced, and this synthesis idea also can find more extensive application in the preparation of any other dispersed particles according to the similar setups and operational procedures, which will expand and enrich the current dispersed particles gallery. Acknowledgement The authors want to show their thanks to the Dr. Li for his help in SEM observation from Institute of Automation, Chinese Academy of Sciences. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2019.06.013. References [1] Q. Nie, L. Yang, C. Cao, Y. Zeng, G. Wang, C. Wang, S. Lin, Interface optimization of ZnO nanorod/CdS quantum dots heterostructure by a facile two-step lowtemperature thermal treatment for improved photoelectrochemical water splitting, Chem. Eng. J. 325 (2017) 151–159. [2] R. Kumar, A. Umar, G. Kumar, H.S. Nalwa, Antimicrobial properties of ZnO nanomaterials: a review, Ceram. Int. 43 (2017) 3940–3961. [3] D.T. Nguyen, K.-S. Kim, Structural evolution of highly porous/hollow ZnO nanoparticles in sonochemical process, Chem. Eng. J. 276 (2015) 11–19.
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