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Controllable Growth of Unique Three-Dimensional Layered Basic Zinc Salt/ZnO Binary Structure Bo Song, Xia Cui, Yuqiao Wang, Lifang Si, Zhaoxia Kou, Wenwen Tian, Chang Yi, and Yueming Sun Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00340 • Publication Date (Web): 10 Aug 2016 Downloaded from http://pubs.acs.org on August 16, 2016

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Crystal Growth & Design

Controllable Growth of Unique Three-Dimensional Layered Basic Zinc Salt/ZnO Binary Structure Bo Songa, Xia Cuia, Yuqiao Wanga*, Lifang Sib, Zhaoxia Koub, Wenwen Tiana, Chang Yia, Yueming Suna* a

Jiangsu Province Hi-Tech Key Laboratory for Bio-Medical Research, School of Chemistry and

Chemical Engineering, Southeast University, Nanjing 211189, China b

Department of Physics, Southeast University, Nanjing 211189, China

*

Corresponding author. Tel: +86 25 52090619; Fax: +86 25 52090621

*

Email: [email protected] (Y Wang), [email protected] (Y Sun)

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ABSTRACT

A novel three-dimensional (3D) binary structure was successfully synthesized with assist of soluble long-chain tetraalkylammonium carboxylate zinc salt by hydrothermal treatment. The unique binary structure consists of two different crystal structures in one individual, with the head being layered basic zinc salt (LBZS) microsphere and the body being ZnO rod. During the synthesis process, careful concentration and time-controlled experiments were needed to achieve a key intermediate of stable self-assembled LBZS, which was subsequently converted into LBZS/ZnO binary structure. The presence of the binary structure demonstrate a possible growth mechanism involving stepwise evolution as: zinc salt → LBZS → LBZS/ZnO → ZnO, giving an improvement of understanding of the growth of ZnO in solution. And thus, this study provides the rationality for the existence of the solid-phase transformation as well as the competition relationship between the solid-phase transformation and the dissolution-renucleation process during the transformation process of LBZS to ZnO.

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INTRODUCTION Layered basic zinc salts (LBZSs) are generally thought as brucite-type compounds composed of positively charged zinc hydroxide layers and the intercalated anions that balance the whole charge. Owing to the characteristic lamellar structure, the LBZSs have potential applications in anion exchange, additives in polymers, catalysis, and electrochemical supercapacitors, etc.1-3 They also could be used as precursor for the formation of novel porous ZnO with high specific surface,4, 5 which might not be formed in traditional crystal growth. Although a variety of synthetic technologies have been developed to obtain LBZSs,6-9 to synthesize LBZSs with designed structure is still difficult. And most reports concentrate on low temperature (i.e., lower than 60 oC), long reaction time (i.e., over 24 h) or surfactant-assisted procedures.10-12. ZnO with controllable morphology, such as nanoflower13 and nanocup14, have been attractive materials for potential application including solar cell,15 photocatalysts,16-18 and gas sensors.19 The solution-phase synthesis of ZnO is one of the most popular approaches, a process that can be easily controlled by adjusting parameters such as temperature, time, concentration and surfactants.16, 20-24 In crystalline process, the morphology of ZnO is generally affected by the chemical of transition state and intermediates which are subsequently transformed into final crystalline structure. So far, the LBZSs have been considered as important intermediates formed in solution synthesis of ZnO.25-27 In classical

nucleation

theory,

the

phase

transformation

usually

takes

place

through

dissolution-renucleation process referred to Ostwald’s Law.23, 28-30 The pyrolytic transformation of LBZS into ZnO occurs via a solid-phase transformation.4, 5, 31 Recently, a detailed investigation on ZnO growth was reported to indicate that growth conditions influence the progression of phase transformation of intermediates to ZnO.32 In that work, two common hydrothermal synthesis routes,

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ZnAc2-NH3 system and Zn(NO3)2•6H2O-hexamethylenetetramine (HMTA) system, were used to compare ZnO formation where progress of conversion occurred through dissolution-renucleation process and solid-phase transformation, respectively. However, it still lacks visual evidence for occurrence of solid-phase transformation in solution synthesis. In this work, we synthesized novel LBZS/ZnO binary structure in hydrothermal synthesis by introducing

a

soluble

alkyl

carbocylate

zinc

salt

precursor,

zinc

N-dodecyl-N,N-dimethylammonioacetic bromide (Zn(DDAB)2) as reported in our previous work.33 The obtained LBZS/ZnO binary structure is in itself an obvious certification for occurrence of solid-phase transformation, which is competitive with the dissolution-renucleation process in our solution. Additionally, uniform 3D morphologies of LBZS were also successfully synthesized under high temperature, short time and surfactant-free conditions. EXPERIMENTAL SECTION Synthesis of LBZS/ZnO binary structure: All the reagents were purchased from Sinopharm Chemical Reagent Company and used without any further purification. 125 mM Zn(DDAB)2 aqueous solution (Scheme S1) was prepared according to our previous work33 and then diluted with deionized water into appropriate concentration. And then, a solution with 1:1 molar ratio of HMTA and Zn2+ was prepared by dissolving proper amount of HMTA in 12.5 mM Zn(DDAB)2 aqueous solution with stirring (solution pH = 6.2). In contrast, the low and high concentrations of 5 mM and 50 mM Zn(DDAB)2 solution with equimolar HMTA were also prepared, respectively. Clear glass substrates were placed in Teflon-lined steel autoclaves, which were filled up with 50 mL as-prepared solution and then treated at 105 oC for constantly extended time. After natural cooling down to room temperature, the substrates were taken out carefully and white precipitates were

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collected by centrifugation and washing with deionized water and ethanol for several times and then dried in vacuum oven at 60 oC overnight. Selected samples were calcined at 600 oC for 2 h in air with a heating rate of 10 oC min-1. Characterizations: The morphologies of synthesized samples were characterized by scanning electron microscopy (SEM, FEI) and transmission electron microscopy (TEM, JEM-2100). The composition analysis was measured by energy dispersive spectrometer (EDS, INCA X-act). The crystal structure was investigated by X-ray diffraction (XRD) using a Rigaku Smartlab 3 with CuKα radiation (λ = 1.54 Å). The thermal decomposition behavior of the sample was identified by thermogravimetry-differential thermal analysis (TG-DTA) conducted on a SDT Q600 at a heating rate of 20 oC min−1 in flowing air. RESULTS AND DISCUSSION Characterization of LBZS The crystal structures of initial products are monitored by XRD in Figure 1a, which are typical of layered compounds. The diffraction peaks at low 2θ range (2θ ˂ 20°) exhibit equally spaced peaks with d values (d = the interplanar spacing) of 15.28 (2θ = 5.78°), 10.16 (8.69°), 7.60 (11.63°), and 6.06 Å (14.60°), indexing as (00l) planes of the layered structure composed of complex positively charged zinc hydroxide layers and interlayer anionic species. The peaks at d = 2.71 (2θ = 33.00°), 1.57 (58.87°) and 1.36 Å (69.10°) could be attributed to the (100), (110) and (200) planes, which are in agreement with most of LBZSs based on the brucite-type zinc hydroxide structure.34 The sharp basal diffraction peaks of (00l) planes at low 2θ range (2θ ˂ 20°) indicate an ordered stacking of the hydroxide layers along the perpendicular direction to the layers. However, an 'starred' peak as a shoulder peak of (100) plane appears unexpectedly, which might be arisen from the intermediate layer presented between the ordered zinc hydroxide layers. With

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prolonged reaction time, the 'starred' peak disappears, indicating that the intermediate layers might be shifted along the direction with the continuous release of OH− by hydrolysis of HMTA and continuous transport of the small amount of intercalated anions to promote crystallization.35 It is worth noting that no other impurity peaks are observed. The positions of the basal diffraction peaks depend on the size and arrangement of the intercalated anions. In our Zn(DDAB)2-HMTA system, the most possible intercalated anions are DDAB− groups and Br−, of which the molecular length are ~18 Å and ~4 Å, respectively. Considering the basal spacing of the LBZS determined from the (00l) planes (30.56 Å) and the thickness of the zinc hydroxide layer (4.8 Å),36 the DDAB− groups should be intercalated in a tilt and tail-to-tail bilayer arrangement10 to fill the space between two contiguous zinc hydroxide layers and keep neutral charge. Therefore, the chemical formula could be described as Zn(OH)2-x(DDAB)x•nH2O. The approximate chemical composition of the selected LBZS is determined by TG-DTA analysis presented in Figure 1b. A first weight loss of 7.98% appearing around 145 oC concomitant with a sharp endothermic peak is presumably due to the departure of water intercalated into layers and dehydroxylation of the zinc hydroxide layers.34, 37 The total second and third gradual weight loss of 52.57 % is accompanied by exothermic peaks observed around 334 and 476 oC, and is attributed to the decomposition and oxidation of the DDAB− groups in the flowing air.38 The LBZS demonstrates higher decomposition temperatures of the incorporated DDAB− groups than those of the corresponding carboxylic acid (233 and 308 oC as seen in Figure. S1a), suggesting that DDAB− groups are well stabilized by being intercalated into the zinc hydroxide layers.37 As a result, approximate chemical composition of the LBZS is calculated to be Zn(OH)1.65(DDAB)0.35•0.35H2O, giving a 2.86 : 1 :1 stoichiometric ratio of Zn to N to Br. And the complete pyrolysis of LBZS gives

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a much higher temperature (700 oC) than that of layered basic zinc acetate (LBZA) (Fig. S1b)4, indicating that the intercalated DDAB− groups are more stable than the acetate anions, a result of good stability of LBZS. The composition of LBZS is further detected by EDS. The peaks of Zn, N and Br are clearly visible in the spectrum. The element analysis data displayed in the inserted table yield the atomic percentage of Zn, N and Br as 14.29%, 4.91% and 5.3%, respectively, which give a 2.70 : 0.93 : 1 stoichiometric ratio of Zn to N to Br and correspond to the chemical composition calculated from TG-DTA analysis above. On the basis of the XRD, TG-DTA and EDS analysis, the chemical formula of LBZS is approximated to be Zn5(OH)8(DDAB)2•2H2O, which is intercalated mainly by DDAB− anions. From the inset, the LBZS exhibits uniformly flowerlike microspheres of several micrometers in diameter. Figure 1d depicts the TEM image of single LBZS microsphere constructed of crumpled nanosheets which are self-assembled out of the microsphere in a divergent way. Figure 1e presents a set of concentric rings, indicating a polycrystalline electron-diffraction feature. The lattice spacing calculated by this diffraction rings are well indexed into the (100), (110) and (200) plane of LBZS crystal structure, in agreement with the above XRD result. However, the diffraction rings assigned to (00l) planes should be located very closely to the circle center and could not be distinguished clearly by observation because of the much larger d values as calculated from the strong diffraction peaks at low 2θ range in XRD pattern. Thus, the circular spot observed in the circle center might be arisen therefrom. As shown in Figure 1f, the individual LBZS nanosheet with a thickness of about 30 nm displays a clearly laminar structure.

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Figure 1. (a) XRD patterns of LBZS (40 and 80 min), (b) TG-DTA curves, (c) EDS spectrum, (d) TEM image, (e) corresponding selected-area electron diffraction (SAED) pattern, and (f) HRTEM image of LBZS (80 min) obtained in 12.5 mM Zn(DDAB)2 solution. The inset in (c) is corresponding SEM image and table of data for EDS spectrum. 8


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Characterization of LBZS/ZnO binary structure As depicted in Figure 2a, the (00l) planes of LBZS almost disappear. The dominant peaks indexed to hexagonal wurtzite ZnO (JCPDS card No. 36−1451) appear instead, demonstrating that LBZS/ZnO binary structure is formed via phase transformation. It is worth noting that the enhanced peak intensity of the (002) plane of ZnO is distinct from the standard value, indicating that ZnO crystals may preferentially grow along orientation in the early nucleation stagy.39 The inset SEM image displays a unique morphology of LBZS/ZnO binary system with the head showing as microsphere and the body as hexagonal rod. The LBZS/ZnO binary system is further identified by TEM, giving an umbrella-shaped morphology (Figure 2b). The SEAD patters in Figure 2c, d and e correspond to the parts in Figure 2b, respectively, confirming the crystal nature for every part of the binary system. During the phase transformation process of LBZS to ZnO, the crystallinity and structure stability of the LBZS on the binary structure is weakened, especially under the electron bombardment. The appreciable collapse of microsphere structure is presented in Figure 2b and Figure S2. Thereby, the diffraction rings in Figure 2c is fuzzy, but it still could be indexed to the (100), (110) and (200) planes in agreement with the above XRD and TEM analysis (Figure 1), indicating the polycrystalline feature of LBZS. As seen in Figure 2d, the diffraction spots are assigned to the (001) and (210) planes of hexagonal ZnO of the side surface, indicating the monocrystalline nature and growth direction along orientation.40 As depicted in Figure 2e, the diffraction patterns of juncture area exhibited monocrystalline ZnO spots accompanied by dimly polycrystalline LBZS rings, suggesting phase transformation from LBZS to ZnO. Additionally, the (002) plane of ZnO and the (100) plane of LBZS possess similar d values (2.6 Å and 2.7 Å). ZnO growth has preferential orientation along direction in hydrothermal process.41 Therefore, it might be proposed that the (002) plane of

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ZnO is transformed directly from (100) plane of LBZS which provides zinc source for ZnO nucleation and crystal growth. And it explains why the (00l) planes of LBZS almost disappear while the peaks of ZnO prevail. For this, the binary system with the head being LBZS microsphere and the body being ZnO rod provides an evidence of continuous phase transformation process, suggesting the nucleation and growth of ZnO crystallized from LBZS via a solid-phase transformation process.

Figure 2. (a) XRD pattern and (b) TEM image of LBZS/ZnO binary structure obtained at 90 min in 12.5 mM Zn(DDAB)2 solution, (c), (d) and (e) SAED patterns of marked parts in (b). The inset in (a) is SEM image of single LBZS/ZnO binary structure. 10


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Influence of reaction time The evolution process of forming LBZS/ZnO binary system is monitored by SEM as shown in Figure 3. At the early stage (up to 80 min), only LBZS is observed at different intervals with 3D morphologies such as fluffy nest, flower and flowerlike microsphere (Figure 3a1−d2). The diameter of these morphologies is reduced from tens of micrometer to several micrometer, accompanied by self-assembling crumpled nanosheets into ordered 3D structures. And these morphologies can well be maintained even by calcination at 600 oC for 2 h and evolved into unusual ZnO structures without morphological deformation (Figure S3). At reaction time of 90 min, ZnO microrods with rough surfaces are elongated from flowerlike microsphere LBZS, displaying the umbrella-shaped LBZS/ZnO binary structure produced. The dimension of the half LBZS microsphere on the binary structure is a little smaller than the whole microsphere, owing to the phase transformation of LBZS. This binary structure is the major product as seen in Figure 3e1 and e2, corresponding to above XRD and TEM analysis. Subsequently, the crystal growth of ZnO microrods with longer length is observed distinctly accompanied by gradual decomposition of microsphere LBZS (Figure 3f1 and f2). Some twinned hexagonal ZnO nuclei with apparently smooth surfaces are scattered across the substrate sketched out by the blue dashed lines in Figure 3f1, originated possibly from dissolution of LBZS and renucleation.28 And among them, a few of ZnO crystals consist of the asymmetrical twinned hexagonal rods, which has been reported in forming ZnO by varying additives.39, 42, 43 With increasing hydrothermal time, LBZS completely disappear and only hexagonal ZnO microrods with more smooth surfaces and uniform dimension are obtained after Ostwald ripening44 (Figure 3g1−h2). SEM clearly shows the dynamic evolution process of the growth of LBZS and its transformation to ZnO, in which the LBZS/ZnO binary structure could be obtained by carefully controlling reaction time, suggesting that the ZnO

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formation proceeds via a stepwise reaction as Zn(DDAB)2 → LBZS → LBZS/ZnO → ZnO. Additionally, the LBZS/ZnO binary system provides a powerful evidence of solid-phase transformation.

Figure 3. SEM images of the growth of LBZS and its evolution to ZnO in 12.5 mM Zn(DDAB)2 solution: (a1−a2) 40 min, (b1−b2) 60 min, (c1−c2) 70 min, (d1−d2) 80min, (e1−e2) 90 min, (f1−f2) 100 min, (g1−g2) 180 min and (h1−h2) 240 min. Growth mechanism of LBZS/ZnO binary structure Having taken into account the above results and interpretation, the relevant chemical reactions experienced in the Zn(DDAB)2-HMTA system is tentatively proposed. HMTA acts as a pH buffer via a gradual self-decomposition in water to

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continuously supply a slow release of NH3 which subsequently hydrolyzes to generate OH− [Eq. (1) and (2)].45

It is know that zinc ions are in octahedral coordination with water molecules in the neutral aqueous solution.46 With the gradually increasing OH− released from the hydrolysis of HMTA, the coordinated water molecules are replaced by OH− to form octahedral structure with subsequent polymerization by edge-sharing or corner-sharing to form brucite-type zinc hydroxide units with positive charge,12 with a composition of [Zn5(OH)8⋅2H2O]2+. These species are intercalated by DDAB− groups to balance the charge [Eq. (3)].47 As a consequent, the initial product of LBZS nanosheets are incubated in solution.

With elevated reaction temperature, the decomposition rate of HMTA would be increased and thus more OH− ions are generated for forming ZnO. At this stage, the as-formed LBZS undergoes loss of the layered molecules (DDAB−) by anion exchange with OH− and then would be dehydrated to form ZnO nuclei [Eq. (4)].48

After that, the continuous hydrolysis of HMTA at elevated temperature increases the concentration of NH3 that is enough as a coordination agent for LBZS to form soluble zinc ammonium complex ions [Eq. (5)]. And these zinc ammonium complex ions subsequently coordinate with OH− to generate ZnO seeds [Eq. (6) and (7)],49 suggesting the partial dissolution of LBZS and renucleation of ZnO (Fig. 3f1).

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The proposed growth mechanism for the formation of LBZS/ZnO binary system is illustrated in Scheme 1. At initial stage, the LBZS nanosheets are formed by combining the DDAB− anions and neighboring zinc hydroxide layers. The DDAB− anions that coordinated with tetrahedral zinc atoms on the surface of the nanosheets not only keep charge neutrality, but also act as capping agents to induce LBZS growth. These anions most likely cap on the surface of nanosheets with their polar headgroups (−CH2(CH3)2N+Ac−Br−) bound to surface and hydrophobic alkyl chains towards the outside,50, 51 giving high steric hindrance to prevent the nanosheets from disordered stacking and keep their uniformity.52 The high steric hindrance of long-chain alkyl and enriched electronics of headgroups might form a protecting layer to impede the progress of dissolution of LBZS at low NH3•H2O concentration as described in Equation (6). Thus, the LBZS nanosheets could be stable for a certain time in hydrothermal treatment and preliminarily prepare for the subsequent self-assembly. During this self-assembly process, the hydrophilic ions of OH−, DDAB−, and Zn2+ preferably attack from the hydrophilic plane that is vertical to the zinc hydroxide layers due to the polar hydroxyl groups comprised in the plane.53,

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Consequently, the crystal growth of LBZS

preferentially occurs at the edge of each nanosheet, leading to the formation of uniform 3D morphologies of LBZS (Figure 3b1-d2). The relatively stable and self-assembled LBZS provides the possibility for visible observation of LBZS transformed to ZnO. Afterwards, the LBZS transformation occurs by decomposition to induce ZnO nuclei formation accompanied by a slight shift of the remaining zinc and oxide atoms and sustain crystal growth along the lowest surface

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energy plane at a certain concentration of OH− ions as shown in Figure 3e1 and e2. Considering the similar d-spacing (Figure 2), the above LBZS → ZnO transformation is topotactic with the relationship [100]LBZS//[002]ZnO,55 suggesting that the ZnO nuclei are directly generated from LBZS. The (001) plane of ZnO terminated with zinc atoms has a slightly positive charge, which could be grown preferentially on the intercalated DDAB− anions by the electrostatic charge compensation,56 resulting the ZnO growth preferred along orientation (Figure 2d and e), which is often believed as the highest growth rate in hydrothermal process. As a result, the binary system with head being LBZS microshpere and body being ZnO rod are formed, demonstrating that the transformation of LBZS to ZnO could be via a solid-phase transformation in solution. Subsequently, under high NH3 concentration released from elevated hydrolysis of HMTA at high temperature, some ZnO clusters are formed to be twinned, which is common in ZnO structure, thought to reduce the surface energy and lower the ΔG of the system.57 Therefore, the extant LBZS supplies zinc source for crystal growth of ZnO via solid-phase transformation and dissolution-renucleation process simultaneously, giving some side LBZS/ZnO binary structures and twinned hexagonal ZnO clusters.

Scheme 1. Schematic of forming LBZS/ZnO binary structure.

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Influence of reactants concentration In order to support the formation mechanism of the binary system assumed above, concentration-dependent experiments would be carried out carefully. At low Zn(DDAB)2 concentration (5 mM) in Figure 4a, the pattern exhibits the coexistence of LBZS and wurtzite ZnO clusters (dot-marked peaks) at initial stage (40 min). Compared with pattern of 12.5 mM (Figure 1a), the (00l) planes of LBZS are weakened due to the formation of ZnO nuclei, corresponding to the pattern shown in Figure 2a. As observed in Figure 4b1 and b2, the urchin-like microspheres of LBZS are stacked on the substrate, accompanied by the scattering twinned hexagonal ZnO nuclei. When the reaction is prolonged to 60 min, LBZS microspheres are decomposed into hemispherical structure and gradually disappear. Meanwhile, the hexagonal ZnO rods became the prominent structure (Figure 4c1) as evidenced by the dominant diffraction peaks of ZnO (Figure 4a). It is worth noting that only a small fraction of nonuniform head or side LBZS/ZnO binary structure could be observed (Figure 4c2 and inset).

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Figure 4. XRD patterns (a) and SEM images of the samples collected at different stages of hydrothermal treatment in 5 mM Zn(DDAB)2 solution: (b1−b2) 40 min, and (c1−c2) 60 min. The inset in (c2) is TEM characterization of the side LBZS/ZnO. In this condition, the evolution of crystals is a little different to that in medium concentration of Zn(DDAB)2 solution. At early stage in low concentration solution, the protonated species (CH2)6N4Hnn+ formed from initial hydrolysis of HMTA [Eq. (8)]47, 58 would consume local DDAB− anions, resulting in more diluted concentration of DDAB− anions. With the lack of DDAB− anions intercalated into zinc hydroxide layers in the diluted concentration of Zn(DDAB)2 solution, the octahedral structure unit in association with high energy is unstable and will favor for the Zn(OH)42− for its low energy state.12 Zn(OH)42− takes tetrahedral structure and will be further 17



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dehydrated to generate ZnO nuclei [Eq. (9) and (10)]. Consequently, the initial product is a mixture of LBZS microspheres and twinned hexagonal ZnO clusters (Figure 4b1 and b2). The subsequent process of phase transformation and crystal growth is similar to that in the medium concentration solution, in which the ZnO rods become the dominant product in association with a small fraction of head or side LBZS/ZnO binary structure. (Figure 4c1 and c2)

At high Zn(DDAB)2 concentration (50 mM in Figure 5a), the 80-min product is assigned to be LBZS, although the (002) and (003) planes disappear. Asterisk-marked peaks might be attributed to the influence of intercalated carbonate anions. A by-product of HMTA hydrolysis, formaldehyde, can be oxidized into carbonic acid under hydrothermal conditions, which supplies carbonate anions into solution.59, 60 A large fraction of incomplete flowers self-assembled by LBZS nanoflakes are accumulate densely on the substrate (Figure 5b1 and b2). Subsequently, the LBZS nanoflakes shrunk gradually due to dissolution, and newly imperfect dumbbell ZnO blocks are formed (Figure 5c1 and c2). In this phase transformation process, no binary structure is formed without any clear evidence of LBZS.

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Figure 5. XRD patterns (a) and SEM images of the samples collected at different stages of hydrothermal treatment in 50 mM Zn(DDAB)2 solution: (b1−b2) 60 min, and (c1−c2) 120 min. The illustration in (c2) is TEM image of imperfect dumbbell ZnO morphology. In this condition, the initial reactions are similar to that at medium concentration. Owing to time needed for buffering pH of high Zn(DDAB)2 solution by HMTA, no precipitate is observed at reaction time of 40 min (Figure S4a). Although the protonated species would consume local DDAB− anions, the concentration of DDAB− anions is still high enough for intercalation into zinc hydroxides to form abundant amount of LBZS nanoflakes without any ZnO observed (Figure 5b1 and b2). Subsequently, the progress of phase transformation has been preliminarily presented through dissolution-renucleation process rather than solid-phase transformation due to the high concentration of NH3 in solution released by rapid decomposition of concentrated HMTA at high

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temperature. The comparatively unstable surface of LBZS is once dissolved to form zinc ammonium complex ions and then the stable surface of ZnO nuclei is created to reduce the surface energy and increase the thermodynamic stability of the crystal.61-63 This interpretation could be well demonstrated in Figure 5c1 and c2, according to the higher rate of LBZS dissolution than that of new ZnO crystal growth. During the renucleation process, the excess HMTA could also coordinate to the ZnO crystal and be a bidedentate ligand capable of bridging active Zn2+ ions, participating as a catalyst in the formation of dumbbell ZnO (Figure S4b).56, 64 For these reasons, no LBZS/ZnO binary structure would be observed in phase transformation process. Based on the above discussion and analysis, it could be concluded that the formation process of LBZS/ZnO binary system consists of two key stages: (1) the pure product of uniform 3D LBZS formation and (2) the transformation of LBZS to ZnO via the solid-phase transformation preceding the dissolution-renucleation process, which could be realized by controlling the experimental conditions involved. CONCLUSIONS In this Zn(DDAB)2-HMTA system, uniform LBZSs with diversified 3D morphologies such as nest-like morphology, flower structure and flower-like microsphere were expected successfully in the synthesis of ZnO by hydrothermal reaction. During the process, evolution of crystal growth proceeded through a multistep pathway following: Zn(DDAB)2 → LBZS → ZnO. By carefully controlling the solution concentration, a unique LBZS/ZnO binary structure could be obtained in low (5 mmol) and medium (12.5 mmol) concentration of Zn(DDAB)2 solution, indicating that the transformation of LBZS to ZnO via solid-phase transformation. In contrast, none of LBZS/ZnO binary structure was synthesized in high (50 mmol) concentration of Zn(DDAB)2 solution,

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suggesting that ZnO crystals grew via dissolution-renucleation process. Consquently, a competitive procedure between solid-phase transformation and dissolution-renucleation process might be proposed during the phase transformation in this synthetic system. Therefore, our work supports previous studies and further makes contribution to the visual observation on understanding of the complex growth mechanisms of ZnO in solution. Additionally, this method is expected to be potential for producing layered basic metallic salts (LBMSs) with stable unique structures and for further understanding on mechanisms of transformation of LBMSs to their derivatives. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. Synthetic procedure of ZnO crystals, TGA of DDAAB, TEM images of LBZS, and SEM images of ZnO.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y Wang), [email protected] (Y Sun) ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21173042), National Basic Research Program of China (2013CB932902), Natural Science Foundation of Jiangsu (BK20141338), Fundamental Research Funds for the Central Universities (2242014K10025, 3207045419), and Seminar Funds of Southeast University (1107041501). REFERENCES (1) Carbajal-Arizaga, G. G.; Satyanarayana, K. G.; Wypych, F. Solid State Ionics 2007, 178, 1143−1162. 21



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7535−7543.

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For Table of Contents Use Only

Controllable Growth of Unique Three-Dimensional Layered Basic Zinc Salt/ZnO Binary Structure Bo Songa, Xia Cuia, Yuqiao Wanga*, Lifang Sib, Zhaoxia Koub, Wenwen Tiana, Chang Yia, Yueming Suna*

Table of Contents Graphic

Synopsis A novel binary structure bearing two different crystal structures combined into one individual is successfully prepared, with the head showing as layered basic zinc salt microsphere and the body as ZnO rod.

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Controllable Growth of Unique Three-Dimensional ... - ACS Publications

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