Construction of micro- and nanoscale semiconductor materials with special size, morphology, and hierarchy has attracted considerable attention for potential application due to their distinctive functions, novel properties, and potential applications in advanced devices and biotechnologies 1 2 . Rational control over the experimental condition has become a hot topic in recent material research fields. ZnO is currently one of the most attractive semiconducting materials for optical and electronic applications because of its direct wide band gap (3.37 eV) and high exciton binding energy (60 meV) 3 . Since Yang observed the room temperature UV lasing from ZnO nanorod arrays 4 , much effort has been devoted to tailor the morphology and size to optimize the optical properties. As a result, various ZnO nanostructures, including nanowires 5 6 7 , nanotubes 8 9 , nanobelts 10 , nanoflowers 11 , nanospheres 12 , nanobowls 13 , dandelions 14 , cages 15 , and shells 16 17 have been obtained by solid-vapor phase growth 18 , microemulation 19 , and hydrothermal methods 20 21 . Hereunto, nanobowls, nanocups, or nanodishes have attracted much interest because they have been envisaged to further contain nanoparticles 22 and immobilize biomolecules 23 24 . Although conventional methods can produce various ZnO micro-/nanostructures, these different synthesis methods often greatly suffer from problems of high temperature, need for high vacuum, lack of control, and high cost. To develop a simple, fast, and controllable synthetic route that can not only control the ZnO micro-/nanostructures with series of shapes under ambient condition, but also produce the hierarchical structure, remains an important topic of investigation.

The electrochemical deposition technique has been recently developed as a promising alternative means for the fabrication of nanomaterials under ambient condition due to the low cost, mild condition, and accurate process control. Recently, Yang and co-workers 25 reported the synthesis of ultrathin ZnO nanorods/nanobelts arrays on Zn substrates by electrochemical deposition. Our group 26 reported an electrochemical route for the fabrication of highly dispersed composites of ZnO/carbon nanotubes. Herein, we report a tunable self-assemble strategy to selectively fabricate a series of ZnO with unique, pure, and larger quantity morphologies including petal-, flower-, sphere-, nest- and clew-shaped structures by electrochemical deposition. The size and morphology of the ZnO are systematically controlled by judiciously adjusting the concentration of the sodium citrate and the electrodepositing time in the self-assembly process. Significantly, the nestlike structure dominates the further formation of hierarchical superstructure. The ZnO nestlike structure can be used as a container not only to hold several interlaced ZnO laminas, but also to fabricate Ag-ZnO heterostructures by growing silver nanoparticles or clusters in the center of nests by electrochemical deposition method. The multiphonon Raman scattering of as-fabricated Ag-ZnO nestlike heterostructures is also largely enhanced by the strongly localized electromagnetic field of the Ag surface plasmon.

Different ZnO morphologies can be selectively obtained by simply varying the concentration of sodium citrate and the electrodeposition time within the certain pH range and supplying current (shown in Figure  1). The image of the small petals intersected by some laminas in one another is shown in Figure  1a,b by controlling the concentration of sodium citrate of 0.05 mmol for deposition time of 1 min at room temperature. The average size of these small petals is about 800 nm. In 0.1 mmol of sodium citrate at deposition time of 3 min, the compact ZnO flowers with average diameter of 1 to 2 μm are formed (Figure  1c,d). The microstructure is actually composed of a random growth of seemingly flexible nanolaminas that can be bent and connected with each other. The nanolaminas extend from the center of the microflowers outward. The ZnO nestlike structures with concave centers are obtained in good yield with a diameter from 2 to 5 μm (Figure  1e,f) for the electrochemical deposition of 1 min in the presence of 0.01 mmol sodium citrate aqueous solution. The shape of the hatch of these nests is square, and the length of the sides is about 4 μm. Figure  1f shows that the nestlike structure is composed of densely packed layers from the bottom to the top. Every layer consists of four well-edged square nanolaminas with the side length of about 2 μm. At the base of the nestlike structure in Figure  1e, if the concentration of sodium citrate is changed to 0.05 mmol with the deposition time of 5 min, ZnO nests holding the interlaced nanolaminas of ZnO are obtained (Figure  1g,h). The ZnO nanolaminas located in the center of ZnO nests are analogy to the flower pistil. Many of these flower pistils show secondary laminas, which have started to grow on the concave of the nests with a slightly different orientation: the secondary laminas form an angle with the basal plane of the main structure and trend to self-assemble in the center of the nests. With the electrochemical deposition going on, the central cavity of the nest is gradually filled by the nanolaminas to form clew-like structure (Figure  1i,j). However, the different growth directions for the nest and its pistil are easily recognized from their gap (Figure  1j). Using 0.1 mmol sodium citrate at deposition time of 5 min, the flower-like microstructure of Figure  1d gradually disappeared and transformed into microsphere structure with an average diameter of 5 μm (Figure  1k,l). These ZnO microspheres are in fact built from small one-dimensional nanolaminas in a highly close-packed assembly. These nanolaminas are aligned with one another perpendicularly to the more compact ZnO spherical surface. The nanolaminas also served as new nucleation sites for more nanolaminas growth and the eventual development into a well-defined three-dimensional spherical structure. But when further increasing the reaction time to 10 min and keeping the concentration of sodium citrate certain, nearly all of the ZnO microspheres show large cracks along the equatorial circumference in Figure  1m,n, which may be due to the slightly increased tension of the inner spheres.

The TEM image of the two typical broken laminas of ZnO from any structure in Figure  1 obtained by ultrasonic treatment for several minutes is shown in Figure  2a. The electron diffraction (ED) pattern (Figure  2b) of these nanolaminas suggests that they have a polycrystalline structure 8 .

A serials of experiments showed that the existence of citrate ions played a key role in the formation of the ZnO complex microstructures. For the control experiment in the absence of citrate as we previously reported, the products were mainly nanoflowers which were composed of nanorods 26 . When citrate was introduced in the solution, the period for the formation of ZnO nuclei (induction and latent periods in the crystal growth process) 27 was remarkably prolonged, which suggested that citrate could slow down the nucleation and subsequent crystal growth of ZnO. On the basis of the previous analysis, we proposed a reasonable mechanism for the formation of ZnO structures. It is believed that sodium citrate is extensively used as the stabilizer and structure-directing agent because of its excellent adsorption ability 28 29 . The additive citrate can form strong complexes [Zn(C₆H₅O₇)₄]¹⁰⁻ with Zn²⁺ and owing to the stability of [Zn(C₆H₅O₇)₄]¹⁰⁻ which is larger than [Zn(OH)₄]²⁻ in the present situation, there exists a large quantity of [Zn(C₆H₅O₇)₄]¹⁰⁻ with negative charge and a small quantity of [Zn(OH) ₄]²⁻ in the precursor solution. It has been previously reported that citrate anions have been known to act as a capping agent of the (0001) surface of the ZnO crystal by adsorbing on the positive polar face of the (0001) surface 30 31 . Thus, these [Zn(C₆H₅O₇)₄]¹⁰⁻ ions are preferred to absorb positive polar plane (0001) surface through the -COO⁻ and -OH functions, and decrease the growth rate of (0001) ZnO crystal surface by competing with growth units [Zn(OH)₄]²⁻, which limits the anisotropy growth of ZnO at experimental pH value and leads to the formation of lamina-like ZnO nanostructures, as shown in Figure  1a,b. The stacking of the laminas is not completely ordered, and the laminas’ self-assembly at a later time is progressively more tilted leading to the formation of petal-like, flower-like, nestlike, clew-like, and spherical aggregates for adjusting the electrodeposition time and the concentration of sodium citrate.

It is worth mentioning that the morphologies of the products varied remarkably with the concentration of citrate. On the basis of the experiment results, we found that when the concentration of citrate was lower than 0.05 mmol (0.01 mmol in Figure  1e,f), the nascent square nanolaminas would self-assemble from bottom to top to form nestlike structures. On the other way around, when the concentration of citrate was higher than 0.05 mmol (0.1 mmol in Figure  1d,l,n), the nascent nanolaminas would self-assemble from center outwards to generate flower-like or microsphere structures. It has been reported that high citrate concentration (higher than 0.05 mmol) will attain [Zn(C₆H₅O₇)₄]¹⁰⁻ supersaturated solution and Ostwald ripening controls structure growth by the diffusion of [Zn(C₆H₅O₇)₄]¹⁰⁻ ions along the matrix-particle boundary tending to form spherical/hemispherical shapes from the center 32 33 . In contrary to this, the lower citrate concentrations will not form [Zn(C₆H₅O₇)₄]¹⁰⁻ supersaturated solution, which tend to self-assemble from bottom to top.

Through the above morphological controls, we have succeeded in preparing a series of ZnO with unique morphologies including petal-, flower-, sphere-, nest- and clew-shaped structures by electrochemical deposition self-assembly. The distinctive nestlike ZnO structures have provided opportunities for creating more sophisticated structures. Figure  1h,g has clearly demonstrated that it can hold ZnO laminas as a pistil. Then we further place silver nanoparticles or nanoclusters in the center of ZnO nests by electrochemical deposition. Figure  3a shows the SEM image of blank ZnO nests. Figure  3b,c,d show the typical results of the ZnO nests after the silver deposition at −0.6 V for 1 min. It can be clearly seen that the nanosized silver particles or silver clusters are apt to form in the center of each ZnO nests. Nearly no silver clusters structures or particles were found outside of the nestlike structures. This indicates that the formation of the silver nanostructures exhibits a location-selective property. Namely, the center of ZnO nests is the place where the Ag nanostructures formed facilely, likely because it is close to the surface of the electrode.

The XRD pattern of Ag-ZnO nestlike heterostructures is shown in Figure  4. The Zn(101) and (102) peaks can be observed due to the used Zn foil substrate (JCPDS card number 040831). These (100), (002), (101), and (102) peaks can be indexed to hexagonal wurtzite ZnO (JCPDS card number 361451). The appearance of the Ag(111), (200), and (220) peaks provides evidence that crystalline Ag is formed in the nestlike ZnO, with the (111) peak being especially strong. The three reflection peaks can be indexed to the Ag face-centered cubic crystal structure compared with the standard JCPDS card (040783). In addition no diffraction peaks from the other crystalline forms are detected.

The photoluminescence (PL) spectra of the as-synthesized Ag-ZnO nestlike heterostructures together with blank nestlike ZnO as a comparison were investigated. As shown in Figure  5, a broad green emission peak centering at around 505 nm is observed in the visible region when the samples are excited at 325 nm. Despite the intensive studies on the green emission of ZnO crystals, its nature remains controversial, and a number of hypotheses have been proposed to explain this emission, such as a singly ionized oxygen vacancy 34 , an oxygen antisite defect 35 , and a zinc vacancy 36 . We ascribe the green emission at about 505 nm to the singly ionized oxygen vacancy on the surface of ZnO structures. It is obvious that the green emission intensity of the as-synthesized Ag-ZnO nestlike heterostructures decreases when compared with the blank nestlike ZnO. This phenomenon reveals that the decrease of the ionized oxygen defect density on the surface of ZnO nests in the Ag-ZnO nestlike heterostructures is due to the holding Ag nanoparticles in the center of the nestlike ZnO. In comparison with the blank nestlike ZnO nanostructures, the Ag-ZnO nestlike heterostructures show no shift in the UV region, and a large (approximately 20 nm) blue shift in the visible emission which could be attributed to the interaction between Ag and ZnO resulting in the Fermi level of ZnO is changed. The quenching of the trapped emission is expected via the new nonradiative pathways created by the proximity of the metal, possibly resulting from electron transfer from ZnO to Ag 37 .

In order to further detect the interface between ZnO and Ag, surface-enhanced Raman scattering (SERS) spectrum was measured for Ag-ZnO nestlike heterostructures with blank nestlike ZnO as comparison (Figure  6). As is evident from the curve b, blank nestlike ZnO has weaker Raman signal. However, for the Ag-ZnO nestlike heterostructures (curve a), a strong Raman scattering line is observed at 578, 1,153, and 1,726 cm⁻¹ which is assigned to the ZnO 1LO, 2LO, and 3LO modes 38 . The 1LO photo mode of the Ag-ZnO nestlike heterostructures shows threefold enhancement compared to that of blank nestlike ZnO. In addition the 4LO (2,318 cm⁻¹), 5LO (2,932 cm⁻¹), and 6LO (3,506 cm⁻¹) 39 can be observed distinctly when Ag nanoparticles were deposited in the center of ZnO nests. In the range of larger wavelength, the baseline of the Raman intensity has declined. This phenomenon might be associated with the quenching fluorescence of ZnO in the Ag-ZnO nestlike heterostructures. Theoretical and experimental studies on SERS mechanisms have revealed that the SERS signals are primarily attributed to the electromagnetic excitation of strongly localized surface plasmon of noble metals 40 . In the Ag-ZnO nestlike heterostructures, we also count the localized electromagnetic effect of the Ag surface plasmon as mostly responsible for the enhancement of multiphonon Raman scattering. In addition, based on the fact that surface plasmon energy of metal Ag matches well with the emitted visible photon energy from the ZnO, the surface plasmon of the Ag nanoparticles might be resonantly excited through energy transfer in the near field and create a stronger local electromagnetic field 41 . The incident light field coupling to the local surface plasmon field might induce stronger localized electromagnetic field in the interface between ZnO and Ag, which further enhances the multiphonon Raman scattering of ZnO, demonstrating the formation of Ag-ZnO heterostructures.

In summary, a convenient approach based on sodium citrate as capping reagent has been developed for the shape-selective synthesis of ZnO with controllable morphologies at room temperature by electrochemical deposition. Four important results were highlighted in this work:

(1) The room-temperature synthesis of ZnO with controllable morphologies, such as petal-, flower-, sphere-, nest-, and clew-shaped structures with adjusting the concentration of sodium citrate, and the deposition time by electrochemical route, has been realized for the first time. Only one or a few kinds of shapes within a narrow size range can be achieved from one of the previous methods 42 . This result should facilitate the development of an effective and low-cost fabrication process for high-quality ZnO.

(2) The product morphologies and sizes were highly controllable and modifiable and evolved from several micro-compressed laminas to flowerlike structures assembled by laminas and to the nestlike microstructure and microsphere in last.

(3) The nest-shaped ZnO microstructures consisting of nanolaminas have been successfully synthesized by using sodium citrate. Our experimental results indicate that the ZnO nestlike structures can be used as a container not only to hold lamina-like ZnO, but also to be used to grow silver nanoparticles in the center of ZnO nests by electrochemical method.

(4) The optical properties (PL and SERS) of the ZnO nests holding nanoparticles of Ag exhibit strong coupling between the metal and semiconductor.

ED: electron diffraction;PL: photoluminescence;SEM: scanning electron microscope;SERS: surface-enhanced Raman scattering;TEM: transmission electron microscope;XRD: X-ray powder diffraction

The authors declare that they have no competing interests.

LD performed the experiment and drafted the manuscript, RZ proposed the idea and participated in the experiment. LF supervised the work and finalized the manuscript. All authors read and approved the final manuscript.