Effective Regulation of ZnO Surface Facets for Enhanced Photoluminescence Properties Assisted by Zinc Quaternary Ammonium Salts.

Novel ZnO twined-mushroom structures highly exposed in (001̅) planes were fabricated via a facile solvothermal synthesis with assistance of a zinc quaternary ammonium salt in the methanol-water solvent to show enhanced photoluminescence properties. A series of ZnO morphologies regulated with different surface facets were obtained in both MeOH-H2O and EtOH MeOH-H2O solvents respectively, tuning the proportion of alcohol. The self-aggregation mechanism was proposed based on the time-controlled experiment to evaluate the formation of twined-mushroom structures. The selective adsorptions of anions from zinc salt precursors determine the shape of subunits and direct the subunits, which act as building blocks to form the order aggregations.

Introduction

In past decades, ZnO has attracted immense attention of the researchers due to its unique physical, chemical, and optical properties ascribed to its variable morphologies from zero-dimensional (0D) to three-dimensional (3D) structures.[1−4] Consequently, ZnO is widely applied in many fields, such as chemical sensors,[5] lithium ion batteries,[6] solar cells,[7] and optical devices.[8−10] All of these promising usages attract great interest among people to develop various synthetic strategies, including solution methods,[11] chemical vapor deposition,[12] aqueous pathway,[13] and so on. The solution methods are simple, powerful, and convenient due to easy tuning of the experimental conditions to control the crystal growth process and selectively synthesize various morphologies for better use. Especially, the solvothermal method is currently an effective strategy because it allows the formation of a variety of ZnO morphologies assisted by different organic reagents.[14]

The ZnO crystal is often considered as a sequence of hexagonal ZnO units composed of alternating staking of 4-fold tetrahedrally coordinated Zn2+ and O2– ions connected alternatively along the c-axis.[15−17] Thus, the ZnO hexagonal prism is made up of the Zn2+-terminated (001) and O2–-terminated (001̅) polar faces as well as six nonpolar Zn2+ and O2– co-terminated prismatic faces. The growth rates of each face determine the final morphology of ZnO crystals. However, crystallization is usually interfered by the species dissolved in the reaction solution as the nucleation and growth processes are sensitive. Therefore, additives are introduced into the ZnO crystal growth to either impede or promote further growth of the different faces. The additive-assisted synthesis of mediating ZnO growth is a very promising strategy to vary the ZnO structure, resulting in a wide variety of curious morphologies of ZnO.[18,19]

ZnO crystals highly exposed in ±(001) facets are desired for enhancing their performance in many applications in view of various properties of ZnO with different crystal faces.[20−23] Mostly, single ZnO crystals exposed in ±(001) faces could be synthesized by using additives to inhibit the growth rate along the [001] direction for forming a two-dimensional (2D) structure. The ZnO mesocrystals are stacked with lamellar structures exposed in ±(001) planes as building blocks.[24,25] More complex ZnO twin spheres exposed in ±(001) facets were prepared by introducing organic additives.[8] Herein, ZnO twined mushrooms exposed in (001̅) planes were fabricated via a facile solvothermal synthesis in the methanol–water solvent using zinc N-dodecyl-N,N-dimethylammonioacetic bromide (Zn(DDAB)2)[26] as a zinc salt precursor.

Experimental Section

Synthesis of ZnO Twined Mushrooms in Methanol (MeOH)–H2O System

A high concentration of 0.25 M Zn(DDAB)2 solution was prepared with reference to our previous work.[26] Then, this solution was diluted to 0.0125 M with mixed solvents of MeOH and deionized water. The volume fractions of MeOH in the reaction mixtures were controlled to vary from 80 to 95%. Hexamethylenetetramine (HMTA) with an equal molar ratio of Zn2+ was dissolved in the above mixtures under stirring to form transparent solutions. Subsequently, these solutions were transferred to Teflon-lined steel autoclaves where clear fluorine-doped tin oxide (FTO) glass substrates were put beforehand for deposition and heated at 105 °C for 12 h. Besides the substrates, the white precipitates deposited at the bottom were also collected by centrifugal separation, washed with MeOH several times, and then kept at 60 °C in vacuum for 12 h.

Synthesis of ZnO Twined Spheres in Ethanol (EtOH)–H2O System

The 0.25 M Zn(DDAB)2 solution was diluted to 0.0125 M with mixed solvents of EtOH and deionized water. The volume fractions of EtOH in the reaction mixtures varied from 80 to 95%. The subsequent procedure for preparation of ZnO was the same as in the MeOH–H2O system.

Characterization Studies

The purities and crystal structures of the collected products were detected by powder X-ray diffraction (XRD) on a Rigaku Smartlab 3 with monochromatized Cu Kα radiation. Scanning electron microscopy (SEM, FEI), transmission electron microscopy (TEM, JEM-2100), and high-resolution transmission electron microscopy (HRTEM) were used to investigate the morphologies and microscopic features of the samples. Room-temperature photoluminescence (PL) spectra of different shapes of ZnO were conducted on a HORIBA FluoroMax-4 fluorescence spectrophotometer at an excitation wavelength of 345 nm. Electrochemical impedance spectroscopy (EIS) was performed on the electrochemical workstation (CHI 660E) at a bias voltage of 5 mV in a frequency range from 10–1 to 105 Hz.

Results and Discussion

The XRD patterns in Figure indicate that all samples obtained from different concentrations of MeOH solvents are well-crystallized hexagonal ZnO (JCPDS cards, No. 36-1451) without observation of characteristic peaks derived from any impurities, demonstrating the high phase purity of all samples. The relative intensities of the reflection peaks corresponding to the (002)/(100) facets increase significantly with increasing MeOH concentration (Table S1), indicating an incremental exposure of polar ±(001) faces of ZnO.[27] The crystallite sizes in the c- and a-axis (Table S1), namely, Dc and Da, are deduced from the full width at half-maximum (FWHM) of (002) and (100) peaks by the Scherrer formula. With increasing MeOH concentration, Dc changes slightly accompanied by a gradual decrease of Da from 45.3 to 24.7 nm, leading to a significant increase of dimension ratios for the c- and a-axis. This phenomenon is similar to the observation of ZnO synthesized in ethylene glycol/water.[20] Therefore, it could be deduced that the increasing MeOH concentration interrupts the growth of each individual ZnO crystallite especially in the a-axis direction.

Figure 1

XRD patterns of the products obtained in the MeOH–H2O solvents with varied volume fractions of MeOH.

The SEM and TEM observations in Figure further display the evolution of the typical morphologies and microstructures of ZnO generated from varied MeOH concentrations. In the 80 vol % MeOH solvent, the product is ZnO twined rods with slightly thinner waists and rounded tips ( Figure a1,a2). Distinguished from smooth prisms, the rods exhibit rough surfaces on which palpable grooves and tiny bulges are scattered. As the MeOH concentration increased to 83.3 vol %, the bulges on the surfaces grow into wedges of different sizes, which are aligned approximately parallel and wrapped successively in generations along the ends of each rod core (Figure b1,b2), forming a twined-bundle-like structure. With the increase of MeOH concentration, the ZnO surfaces are composed of incremental generations of the nonuniform wedges, forming the twined-pinecone-like structures (Figure c1,c2). On further increasing the MeOH concentration (Figure d1,d2), the drops of each generation became relatively uniform from the side view; in other words, the surfaces are covered with basically uniform wedges, resulting in pronounced arc outlines for the formation of twined-custard-apple structures. In the 93.3 vol % MeOH solvent (Figure e1,e2), ZnO appears similar to twined-custard-apple-like structures but with smaller sizes. The difference is thinning down of the wedges and changing into small facets, which link with each other via tilting step-by-step to form the scaly and curved surfaces of the twined-custard-apple structures. The sizes of the small facets are further reduced and packed much tightly to form twined-mushroom-like structures after further increasing the MeOH concentration to 95 vol % (Figure f1,f2). It is worth noting that the morphologies generated in different solvents in Figure are representative and dominant in large scale (Figure S1), although tiny amounts of unexpectedly impure shapes are observed. All products consist of two halves joined together at their concave waists. TEM images (Figure a3–f3) clearly display the general trends of morphological evolutions with the increase of MeOH concentration from 80 to 95 vol %. A general downward trend is exhibited on the individual sizes including the maximum width, waist width, and longitudinal length. Meanwhile, the curve profiles of the individual surfaces increase gradually, leading to a variety of morphologies.

Figure 2

SEM images of side and top views and TEM images of ZnO prepared in different vol % of MeOH in MeOH–H2O solvents. (a1–a3) 80 vol % MeOH, (b1–b3) 83.3 vol % MeOH, (c1–c3) 86.6 vol % MeOH, (d1–d3) 90 vol % MeOH, (e1–e3) 93.3 vol % MeOH, and (f1–f3) 95 vol % MeOH.

ZnO obtained in the 95 vol % MeOH solvent shows a uniform twined-mushroom-like structure with maximum exposure of polar ±(001) faces. Figure A exhibits the TEM image of a typical twined-mushroom particle, consisting of two mushrooms, which are composed of half-sphere and truncated cone united symmetrically at a common base as marked with a red arrow. for an indepth understanding of the microscopic surface structure, HRTEM observations (Figure a–g) are taken clockwise around the spherical surface of the twined mushroom at different locations, marked with blue rectangles in Figure A. The lattice fringe spacings in these HRTEM images are about 0.26 nm, calculated from the fast Fourier transform (FFT) analyses, corresponding to the interspacing of the (002) crystal facets of ZnO. Additionally, the lattice spacing of 0.532 nm indicated in Figure d is consistent with the d-spacing of (001) planes. In view of this, the spherical surfaces of ZnO twined mushroom are enclosed in polar ±(001) faces; that is, the ZnO twined mushroom should display either negatively or positively charged surfaces because of the polar oxygen (001̅) or polar zinc (001) basal planes. This case had been substantiated via electrostatic staining in previous reports.[28,29] In our staining experiments, a negatively charged dye (Acid Green 25) and a positively charged dye (methylene blue) were introduced and their adsorption behaviors were investigated by visual inspection and SEM observation (Figure S2). Only the positively charged dye adsorbs on the spherical surfaces of the ZnO twined-mushroom structures, suggesting the feature of electronegativity on their surfaces caused by the oxygen-terminated (001̅) planes. That is, ZnO twined-mushroom structures are exposed in (001̅) planes, and the percentage of exposed polar facets is estimated to be about 90%. Unexpectedly, the use of the negatively charged dye causes the structural damage of the twined mushrooms, which break down at the waist into mushroom structures, indicating that the twined-mushroom structures are composed of two mushrooms growing at a base plane in between. This twining phenomenon commonly occurs during the ZnO growth with the (001) or (001̅) plane as the juncture.[29,30] In line with the results reported previously, it could be deduced that the twined-mushroom structures grow along the [001̅] direction in both parts from the juncture, which is presumably formed via an initial alignment of (001) to (001) facets (probably mediated by the anion of Zn(DDAB)2, namely, DDAB–).

Figure 3

TEM and HRTEM images of ZnO synthesized in 95 vol % MeOH solvent. (A) TEM image of a typical ZnO twined mushroom, and (a–g) HRTEM images of the marked parts in (A) with indicated d-spacings calculated from the fast Fourier transform (FFT) analyses.

To further clearly observe the surface morphology of the ZnO twined mushroom, the high-magnification SEM image is recorded as shown in Figure a, which displays the top view of one twined mushroom and the side view of a defective ZnO twined mushroom. The top view reveals that the spherical surfaces are covered by the tiny facets with recognizable edges and corners. As highlighted in red lines, the measured angles of the two corners of an individual facet are 119 and 119.3°, respectively. The above XRD pattern (lilac pattern in Figure ) and HRTEM images (Figure ) indicate that the twined mushrooms are indexed to hexagonal ZnO and exposed in polar (001̅) planes; hence, it could be concluded that the facets are partial hexagons with O2–-terminated planes. The side view of a defective twined mushroom reveals the growth habit of the ZnO structure. It is clearly seen that the twined-mushroom structure is composed of ZnO nanowedges of 20–100 nm size. The wedge-shaped morphology tapering toward the inside is relatively visible in a fragment (Figure S3a). These nanowedges attach to each other tightly and radiate outward on both sides from the basal plane in between. Obviously, this formation of the twined-mushroom structure proceeds via an oriented aggregation of initially formed subunits proposed by Cölfen,[31] rather than the classical ion attachment. We assume that the DDAB– anions are incorporated in the formation of the twined-mushroom structure. This conjecture is evidenced by the thermogravimetric analysis, which indicates that the twined-mushroom-like ZnO contains approximately 4% of organic material (Figure S3b).

Figure 4

(a) SEM image of top and side view of ZnO twined mushroom, (b) TEM image of a defective ZnO twined mushroom, (c, e) magnified TEM images of the parts in (b), and (d, f) HRTEM images with indicated d-spacings calculated from the FFT analyses of the parts in (c) and (e).

Further TEM observation is to investigate the arrangement of the subunits. Figure b shows a typically defective ZnO twined mushroom with a visible breach in itself. Two parts, respectively, marked with blue (I) and red (II) dashed lines on the breach are magnified as displayed in Figure c,e, indicating fairly rough brim, presumably caused by the aggregation of ZnO wedges on the breach and the conical surface. Figure d shows the HRTEM image of the side part on the conical surface as marked in Figure c. Figure f exhibits the HRTEM image of the top part on the breach as marked in Figure e. The lattice fringe spacing of about 0.28 nm calculated from the FFT analyses (Figure d,f) corresponds to the interspacing of (100) plane of ZnO. The sizes of the nanowedges are estimated in a range of 20–30 nm, which is in agreement with the calculated result from the XRD analysis (Table S1). In addition, the degrees of the orientational alignment of the nanowedges consist of the (100) planes located at the same part. Hence, it could be deduced that the orientational aggregates of the nanowedges radiate outward with regard to their c-axis.

To further clarify the formation of the ZnO twinned-mushroom structure, a time-dependent experiment was conducted in the 95 vol % MeOH solvent and the products were collected after defined time intervals for further XRD and SEM investigations (Figure ). The XRD patterns (Figure a) display the fast crystallization of ZnO. Even if the sample was collected after a short reaction time of 1 h, pure ZnO with high crystallinity is observed. Prolonging the reaction time, there is scarcely any change in the purity and crystallinity of ZnO. Focusing on the individual crystallites, their crystallite sizes Dc and Da are estimated from the FWHM of (002) and (100) diffraction peaks using the Scherrer formula. As provided in Table S2, the subunits of the sample collected at 1 h show the thinnest rod/wedge-like habit with Dc of 42.2 nm and Da of 20.5 nm. A slow growth of the subunits in both directions proceeds with the increase in the reaction time, and the growth in a-axis dominates relatively, leading to a gradual decrease of dimension ratio for the c- and a-axis. That is, the subunits become thicker with time and their growth appears to be completed after 8 h of reaction.

Figure 5

(a) XRD patterns and (b–d) SEM images of the products obtained at different reaction times in 95 vol % MeOH solvent. (b) 1 h, (c) 4 h, and (d) 8 h.

The morphology evolution of these samples is monitored with SEM images (Figure b–d). The formation of the twined-mushroom-like structure proceeds within a short period of time. This typical morphology appears at the early stage of the reaction with relatively small dimensions of ∼0.54 μm width and 1.07 μm longitudinal length (Figures b and S4a). With increasing reaction time, the sizes of the subunits are increased as mentioned above, resulting in further growth of the twined-mushroom structure whose size is up to about 0.87 μm in width and 1.6 μm in longitudinal length (Figures d and S4c). With the increase in the size of the twined-mushroom structure, however, the amount of the aggregated subunits is relatively constant in appearance. Consequently, the growth of the structure presumably proceeds with the growth of individual subunits rather than further adjoining of crystallites.

In consideration of the observation and analysis mentioned above, a formation mechanism is postulated for the ZnO twined mushroom as depicted in Scheme . Commonly, HMTA could be hydrolyzed in water to provide a continuous release of OH– with increasing temperature.[32−34] Although the solvent used in the experiment was mainly composed of methanol, there was still a little water in the solvent for reaction with HMTA of which self-hydrolysis might be restrained and slowed to an extent. In addition, the EIS measurement indicates that the ionization equilibrium existed between the Zn(DDAB)2 precursors in the MeOH–H2O solvent, forming Zn2+ and DDAB– ions with coexistence of partial zinc salt molecules (Figure S5).

Scheme 1

Schematic Illustration of the Formation of the ZnO Twined-Mushroom Structure in 95 vol % MeOH Solvent

With OH– ions being slowly generated from HMTA at an initial stage, ZnO nuclei are formed and modified by DDAB ions, preventing themselves from aggregation. Analogously, ZnO nanocrystals have been reported previously to be stabilized by the anions composed of ionic liquid salts containing Zn2+ cations.[35,36] And then, the defined crystal faces are developed to incubate the hexagonal ZnO units as the basic growth layers when OH– ions are gradually released from HMTA. As Zn2+ ions are consumed for forming ZnO, the ionization of zinc salt could be promoted to release dissociative DDAB– ions, which act as in situ capping agents to coordinate in steps to the tetrahedral Zn2+ ions exposed on the unit surfaces.[26] As a result, DDAB– ions anchor onto Zn2+-terminated (001) faces as well as Zn2+ and O2– co-terminated prismatic faces, most likely with their polar head-groups bound to the surfaces and alkyl chain toward the outside,[37,38] causing the crystal growth inhibition along the [001] and [100] directions. That is, the preferential growth of O2–-terminated (001̅) faces could be realized to have the highest growth rate.

The crystal growth along the [001̅] direction could be considered as an orientational stacking of hexagonal ZnO units layer by layer along the c-axis, like the graphite structure.[16] When new ZnO hexagonal layers are generated on the basic growth layers, it is time for DDAB– ions to bind to the nonpolar prismatic faces of the new layers, thus forming larger dimension of the new layers than that of basic layers. As each new layer generated with bigger dimension, the wedge-shaped ZnO subunits as building blocks for further aggregation would be formed predictably after tens of generation of the new layers. Meanwhile, the twinning structure occurs at an early stage by attaching the wedge-shaped ZnO subunits to themselves via the (001) faces. DDAB– ions binding to the (001) faces could possibly overcompensate and counterbalance the like charges on their facets,[28] allowing the appearance of twinning. Besides, the subunits are arranged side by side with each nonpolar surface, resulting in the formation of radiating aggregates due to the wedge shape of individual subunit (Figure S3a). Further aggregation of ZnO wedges would be carried through the stacking of the (001) faces on (001̅) faces exposed on the radiating aggregates, forming the twined-mushroom-like structures (Figure b). The aggregating process seems to be completed at an early stage; that is, the number of subunits in the aggregates nearly remain constant for a long period (Figure b,c). Hence, further growth of the aggregates is considered to be the individual growth of each subunit. With slow coarsening of the subunits (Table S2), the radian of the profile of aggregates becomes more round, forming the typical twined mushrooms exposed in the (001̅) facets in maximum (Figure ).

It is well known that the property of the dissolvent would change the nucleation and growth rates of crystallites to determine the final morphologies. In the series of MeOH–H2O solvents, the increasing proportion of water could accelerate the self-hydrolysis of HMTA, promoting the nucleation and incubation of the hexagonal ZnO basic growth layers at an initial stage. Meanwhile, less inhibition of ionization of the Zn(DDAB)2 precursor allows local supersaturation of Zn2+ ions for ZnO nucleation and more DDAB– anions to bind fast to the nonpolar prismatic faces of the newly ZnO generated layers, leading to the new layers being similar in diameter to the basal layers. As a result, the subunits formed after tens of generation of the new layers change their shape gradually from wedge shape to rod shape, accompanied by the increase of the crystallite sizes in a-axis (Table S1) with increasing proportion of water in the MeOH–H2O solvents. Predictably, the morphological evolution of the final structures aggregated by these subunits as building blocks proceeds from twined mushrooms to the twined-rod structure, which is less exposed in polar faces (Figures and 2).

To confirm the validity and universality of the mechanism presented above, parallel experiments were conducted by replacing MeOH with EtOH and keeping the other reaction conditions unchanged (Figure ). The XRD patterns shown in Figure a indicate good crystallinity and purity of all of the artefacts despite the impure diffraction peaks (marked with ★) arising from the FTO glass (Figure S6). With increase in the EtOH concentration, there is an obviously increasing tendency of the relative intensities of the diffraction peaks corresponding to the (002)/(100) planes (Table S3), indicating an increased exposure of polar facets. In addition, significant decreases of Da and Dc are apparent with the increase of EtOH concentration; that is, the growth of each individual ZnO crystallite in both a- and c-axis direction is inhibited by the increasing concentrations of EtOH. At low EtOH concentration of 80 vol %, the twined-column structure of ZnO is observed with distinct and smooth prism surfaces (Figures b and S7a). Several wedges appear symmetrically on the prism facets around the twined columns when EtOH concentration is increased to 83.3 vol % (Figures c and S7b). With further increase in the MeOH concentration (Figures d and S7c), a stem is observed after tens of generation of wedges growing around the side facets of the twined columns to connect two rough hemispheres for forming the dumbbell structure. The surface of the hemisphere is composed of a hexagonal plane at the top of the column and fragmentarily hexagonal facets at the top of the wedges. At the 90 vol % EtOH solvent (Figures e and S7d), twined-hemisphere structures are obtained with two hemispheres fused at the contracted central plane. The surface of the hemisphere is constructed by connecting each fragment through tilting step-by-step. When EtOH concentration is as high as 93.3 vol % (Figures f and S7e), the twined-hemisphere structure is maintained with much smaller fragments packed tightly to form relatively rounded surfaces. Some separated hemispheres are observed. Unexpectedly, the twined structures would be totally destroyed to form mushroom structures at higher EtOH concentrations (Figure S7f). Based on these observations, the evolution of crystal structures and morphologies of ZnO by regulating the content of alcohol in EtOH–H2O solvents is almost the same as that in MeOH–H2O solvents. Consequently, the orientational self-aggregation mechanism of forming twined-mushroom structures in MeOH–H2O system is also suitable for the growth of the twined-hemisphere structures in EtOH–H2O system.

Figure 6

SEM images (a–e) and XRD patterns (f) of the samples synthesized in different vol % of EtOH in EtOH–H2O solvents: (a) 80 vol % EtOH, (b) 83.3 vol % EtOH, (c) 86.6 vol % EtOH, (d) 90 vol % EtOH, and (e) 93.3 vol % EtOH.

PL spectra of the selected ZnO samples with varied morphologies are shown in Figure to explore the influence of ZnO morphological regulation on its optical property. All samples exhibit obvious blue emission peaks at a wavelength of around 420 nm. The blue emission was also observed in other shapes of ZnO, such as hollow nanoparticles,[39] nanoshells,[40] flower-like rods,[41] and dumbbell-shaped microstructures,[42] suggesting that the PL property is influenced by the shape of ZnO.[43] Although the mechanism of excitation emission is controversial, the generation of the blue emission is commonly ascribed to the electron transition from the zinc interstitials (Zni) to the valence band and oxygen defects.[44−46] It is worth pointing out that the ZnO twined-mushroom structure exhibits almost 5-fold enhancement of PL intensities than the blue emission of the ZnO twined-bundle-like structure. That is, the morphological evolution for regulating exposed polar facets greatly influences the emission from the ZnO material. Two plausible reasons could be suggested to explain this emission enhancement. One is the obvious increase of the percentage of exposed (001̅) facets via morphological evolution. The previous work on principal studies of ZnO indicated that the blue light could be emitted by ±(001) facets of ZnO, but the O2–-terminated (001̅) facets emit much stronger blue light than the Zn2+-terminated (001) facets.[47] Another one is that the ZnO subunits, which aggregated radially for the formation of the superstructures, are modified by the organic ions on their surfaces to lead to much more structural defects. In addition, a common green emission of ZnO at around 560 nm is negligible. Consequently, the strong blue emission confirms the twined-mushroom ZnO being a promising candidate in light-emitting devices, which require a monochromatic emission.

Figure 7

PL spectra of ZnO samples obtained in the MeOH–H2O solvents with different MeOH concentrations.

Conclusions

In summary, a series of well-controlled ZnO superstructures have been prepared via the MeOH–H2O solvothermal method. ZnO morphologies evolving from twined rods to twined mushrooms were realized only by regulating the MeOH concentration from 80 to 95 vol %. Careful time-controlled experiments indicate that the Zn(DDAB)2 precursor plays a crucial role in the formation of the twined-mushroom morphology. The cations of the precursor supply a constant zinc source for the ZnO crystal growth. The anions of the precursor, DDAB–, act as capping agents to selectively adsorb on the primary ZnO crystallites to build subunits and synchronously aggregate orderly. Owing to the different ionizations of the Zn(DDAB)2 precursor in these solvents, the concentration of DDAB– anions and their adsorption rates on primary crystals are influenced, resulting in different shapes of the subunits. Further growth is attributed to the slow growth of individual subunits. The morphological evolution, in particular, the increase of the exposed (001̅) facets, greatly enhances the PL property of ZnO. A similar morphological evolution from twined columns to twined hemispheres was also observed in EtOH–H2O solvents by varying the EtOH concentration. This work provides a simple method for regulating the morphologies of metal oxides to obtain superstructures by the organic metal salt precursors, which have a similar structure of Zn(DDAB)2.