Preparation and Optical Properties of Biomimic Hierarchical ZnO

Preparation and Optical Properties of Biomimic Hierarchical ZnO Column Arrays Mei Yang,† Guangfu Yin,† Zhongbing Huang,*,† Yunqing Kang,† Xiaoming Liao,† and Hui Wang‡

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 707–714

College of Materials Science and Engineering, and Analytical & Testing Center, Sichuan UniVersity, Chengdu 610064, P. R. China ReceiVed NoVember 4, 2007; ReVised Manuscript ReceiVed October 9, 2008

ABSTRACT: Hierarchical ZnO column arrays have been successfully prepared in aqueous solution through the appropriate dimercaptosuccinic acid (DMSA) addition time and concentration. The morphology of the ZnO columns and their photoluminescence (PL) are remarkably similar to biomineral structures in Ostrea riVularis shells. PL spectra of ZnO arrays exhibit near-band-edge emissions, and it is more obvious in ZnO columns obtained using 1-6 h addition time due to the high defects concentration. Furthermore, some carboxyl (COOH) groups of DMSA linked in ZnO columns could remain free to graft a bimolecule, which implies a great potential for optical emission and biosensors.

1. Introduction ZnO, a wide-bandgap semiconductor (Eg ) 3.37 eV at 300 K) with a large exciton binding energy (60 meV), is a versatile, multifunctional material.1 It has been extensively used in hightechnology applications ranging from photonic crystals to lightemitting diodes, transparent conductive coating,2 electrode for dye-sensitized solar cells,3 gas sensors,4 and electro- and photoluminescent material.5 A variety of ZnO nanostructures such as nanospring,6,7 nanoflowers,8 nanoring,9 nanopropeller,10 nanosisal,11 nanocomb,12,13 aligned nanowire/nanorod arrays,14 and nanocastles15 have been reported so far. Among these nanostructures, typically ZnO nanowire/nanorod arrays have attracted intensive research interest due to their promising applications in room-temperature UV lasing, electricity generation, and biofunctional materials.16-18 Various techniques such as metal-organic chemical vapor deposition,19 vapor-liquidsolid epitaxial growth,20 pulsed laser deposition,21 spray pyrolysis,22 epitaxial electrodeposition,23 and radiofrequency magnetron sputtering24 have been developed for the preparation of well-oriented ZnO nanorod arrays. Also some groups have researched the biomimetic array of ZnO nanorods or films through multistep or long reaction time.25,26 However, the direct fabrication of large-scale arrays of hierarchical ZnO columns with a controlled crystalline structure remains a significant challenge. A complex and oriented ZnO nanostructure has been obtained through the addition of DMSA in the reaction process,27 but the shape of the column-like ZnO hierarchical structures in the growth process still remains an open question. In this paper, a variety of large-scale arrays of hierarchical ZnO columns were prepared through the addition of DMSA at different times during the reaction process, which were remarkably similar in morphology to those from biogenic calcium carbonates, and a few reports have mentioned it up to now. In this work, DMSA was used to control the morphology and structures of ZnO columns based on the following reasons: First, the thiol (SH) groups of DMSA can form strong covalent complexes with ZnO through Zn-O and Zn-S bonds. Second, some carboxyl (COOH) groups of DMSA have remained free on the nanosheets, which can be used to graft a bimolecule. * To whom correspondence should be addressed. E-mail: zbhuang@ iccas.ac.cn. † College of Materials Science and Engineering. ‡ Analytical & Testing Center.

Third, DMSA is a “nontoxic” product, and it often can be used in the treatment of arsenic and mercury poisoning of humans.28,29 Furthermore, a possible growth process of hierarchical ZnO columns has been suggested.

2. Experimental Section In this process, two steps were used for the preparation of ZnO columns. The first step was the preparation of buffered layers on the common glass substrates. The properly cleaned glass slides were immersed in a solution containing equimolar zinc nitrate-6-hydrate [Zn(NO3)2 · 6H2O] and hexamethylene tetramine (HMT) (0.05 M) for 5-20 min, and then the glass slides immersed in Zn(NO3)2 · 6H2O and HMT were heated at 500 °C for 10 min in a muffle furnace. The above coating process was repeated three times to ensure complete and uniform coverage of ZnO seeds on the substrates. The prepared ZnO seed layers were used as the buffered layers for the growth of ZnO column array film, and there are many reports describing the seeded growth.25,30-32 The second step was growth of ZnO column arrays film on the buffered layers using an aqueous solution method. The glass slides with buffered layers were placed vertically in a solution containing equimolar Zn(NO3)2 · 6H2O and HMT, and reacted at 80 ( 5 °C. In order to understand how the DMSA concentrations and addition time affect the morphology of ZnO column arrays film, DMSA solutions with different concentrations (0.125-25 mM) were added into the above aqueous solution after reactions have begun for 5 h, or 2.5 mM DMSA was added into the reactive solution when the growth process had begun for 0, 0.5, 1, 2, 4, 5, and 6 h, respectively (they were designated as 0, 0.5, 1, 2, 4, 5 h, and 6 h addition time, respectively), and continued to react at 80 °C for 12 h. Then, the glass slides were removed from the aqueous solution and rinsed with deionized water three times. Finally, the glass slides were dried at room temperature for 24 h. As expected, different structures of ZnO column arrays were prepared. The morphology of the ZnO column arrays were characterized by scanning electron microscopy (SEM, JSM-5900LV, Japan). The crystalline structure of the samples was analyzed using X-ray diffraction (XRD, X’Pert, Holand) with Cu KR radiation. Optical properties were investigated by PL measurements (F-7000, Hitachi) at room temperature. The crystal lattice of a sample of ZnO column with 4 h DMSA addition time was characterized by high-resolution transmission electronic microscopy (HRTEM, Philips TECNAI 20 high resolution TEM at 400 kV).

3. Results and Discussion 3.1. Morphology of Hierarchical ZnO Column Arrays. Figure 1 shows SEM images of ZnO column arrays with different DMSA concentrations. The DMSA addition time was retained 5 h after the reaction began. Figure 1a is an SEM image

10.1021/cg701089a CCC: $40.75  2009 American Chemical Society Published on Web 12/15/2008

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Figure 1. Different DMSA concentrations as a function of ZnO column arrays film morphology: (a) 0.1 mM; (b) 0.5 mM; (c) 2.5 mM; (d) 10 mM.

of ZnO column array film with a minimum concentration of DMSA (0.1 mM), showing that the diameter of ZnO short columns is about 300-600 nm, and there were few ZnO columns with sheet-like nanostructures. Compared to Figure 1a, the diameter and length of ZnO columns in Figure 1b increased to ∼900 nm and ∼1.5 µm, respectively, and there was a clear and orderly column-like growth of ZnO nanosheets when the concentration of DMSA increased to 0.5 mM. However, when the DMSA concentration was increased to 2.5 mM, the diameter and length of columns decreased to about 400 nm and 1.0 µm, respectively, and there was also a clear column-like growth of ZnO nanosheets (Figure 1c). Figure 1d is an SEM image of ZnO column arrays film with 10 mM DMSA concentration. It shows that some columns began to dissolve; namely, no clear and orderly ZnO columns are obtained in high DMSA concentration. It indicated that only products with a single hierarchical structure morphology could form in a DMSA aqueous solution with not too low or too high a percentage. This is in accordance with what was reported by Lu et al.33 In a word, the proper concentration of DMSA for the uniformity of diameter or the denseness of ZnO column arrays film is 2.5 mM. Some groups, such as Lifen Xu et al.,34 have studied the effect of different acid addition time on the morphology of resulting samples. In order to study the effect of DMSA addition time on the morphology of ZnO columns, the DMSA concentration was kept at 2.5 mM in this work. The addition time of DMSA varied from 0 to 6 h after the reaction began. It was found that the morphology of ZnO columns changed with various DMSA addition times (Figure 2). Figure 2a is the SEM image of the ZnO column array with 0 h DMSA addition time, showing that the columns had a diameter of about 150 nm and a length of less than 1 µm. Figure 2b is an SEM image of ZnO column arrays film with 0.5 h DMSA addition time, and the diameter of columns increased to ∼400 nm and the length was about 800 nm. It also shows the density of column arrays decreased compared to 0 h DMSA addition time. The SEM image of ZnO array with 1 h DMSA addition time shown in Figure 2c shows crescent-shape ZnO sheets with a thickness of 100 nm, and there are no ZnO column-like structures. When the DMSA addition time is 2 h, Figure 2d shows that wafer-like ZnO particles were stacked on the substrate and there were also no ZnO columns in them. These wafers with a diameter of ∼1 µm consisted of

Figure 2. SEM images of the as-prepared ZnO products with different DMSA addition times: (a) 0 h; (b) 0.5 h; (c) 1 h; (d) 2 h; (e) 4 h; (f) 5 h; (g) 6 h. Insets are the corresponding cross-view of column arrays films of (e), (f), and (g), respectively; (h) with 2.5 mM citric acid addition of 5 h addition time; (i) with 2.5 mM acetic acid addition of 5 h addition time.

several hexagonal sheets. With an increase of DMSA addition time to 4 h, the ZnO sheets could stack into column-like structures with a diameter of 700-800 nm (Figure 2e). The cross-view image shown in Figure 2e inset shows that the length of columns was about 1.4 µm. It can be observed that the columns almost grew vertically to the substrate. The tropism of column-like structure of ZnO arrays with 4 h DMSA addition time is better than that with 1 or 2 h addition time. When the DMSA addition time is 5 h, the column-like growth of ZnO sheets becomes clear (Figure 2f). The insets of Figure 2f and Figure 2g are the cross-view of ZnO columns with 5 and 6 h addition time of DMSA, respectively. The corresponding lengths of these columns are ∼2.5 and 2 µm, and their diameters are

Biomimic Hierarchical ZnO Column Arrays

Figure 3. Aspect ratios the different prepared ZnO products as a function of DMSA addition time.

700-800 nm and 300-600 nm, respectively. The inset of Figure 2f clearly shows that the ZnO columns were obtained through the growth of ZnO sheets. The sheets grew on top of the oriented ZnO columns. However, when the addition time of DMSA was 6 h, the ZnO arrays consisted of integrated and smooth columnlike structures. These results implied that with the increase of DMSA addition time, both the thickness and the diameter of the sheets that formed the ZnO columns decreased. There are two functional groups of DMSA: SH groups and COOH groups. To confirm which one was the main reason for the formation of column-like structures of ZnO sheets, a comparative experiment with citric acid, acetic acid (only containing COOH groups) instead of DMSA was performed, and SEM images of ZnO film with citric acid and acetic acids are shown in Figure 2h,i. The reactive conditions were identical to 5 h DMSA addition time. Figure 2h reveals the sheet-like ZnO particles film; there was no column-like stack. Figure 2i is the morphology of the sample in which acetic acid was added during the reaction process. It shows the clear hierarchical ZnO column array (as the inset shown) with a diameter of ∼125 nm and a length of ∼1 µm in large-scale, in which the final morphology is similar to the DMSA addition. Although the morphology of sample with citric acid addition is not a columnlike stack, it could be due to the improper reaction conditions. These results suggest that a hierarchical ZnO column array could be prepared with a wide range of organic acids under proper reaction conditions. It can be also deduced that the formation of column-like structures of ZnO sheets is primarily relative to the function of the COOH group of the DMSA additive. Moreover, it can be also deduced from SEM images of ZnO columns that the aspect ratio of ZnO columns is directly related to the DMSA addition time as shown in Figure 3. At the beginning, when the DMSA addition time is 0, 0.5, and 1 h, the aspect ratio of ZnO columns decreased rapidly, and no column-like structure of ZnO sheets was produced. When the DMSA addition time is 2, 4, and 5 h, the aspect ratios of ZnO columns increased gradually. At the same time, the columns stacked with ZnO sheets became more and more obvious, and the thickness of sheets that formed ZnO columns became thinner. However, when the DMSA addition time exceeds 5 h, the aspect ratio always increases, and there were no columns stacked by ZnO sheets; the intact and smooth ZnO column array was produced. The change of ZnO columns morphology with DMSA addition time is shown in Figure 3. The change of DMSA addition time provided us with a simple approach to control the aspect ratio and diameters of the ZnO columns.

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Figure 4. XRD patterns of the as-prepared ZnO products at different DMSA addition times.

Figure 5. FTIR spectra of the ZnO columns with DMSA.

3.2. Crystal Structure and Composition of Hierarchical ZnO Column Array. Figure 4 is XRD patterns of the as-prepared ZnO products with different DMSA addition times. All diffraction peaks can be indexed to the wurtzite phase of ZnO (Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 89-1397). From Figure 4, it can be observed that the (002) peak is intense and sharp at 0 h DMSA addition time. At 0.5 h DMSA addition time, there was a lower (002) peak compared to 0 h addition time, and the (002) diffraction peak almost disappeared at 1 h DMSA addition time. Subsequently, the (002) diffraction peak become more and more intense and sharp compared to the other peaks with the increase of DMSA addition time from 2 to 6 h, indicating a preferential c axial orientation. This is in conformity with SEM results, and indicates that with the increase of DMSA addition time, the tropism of ZnO column array decreased at the beginning, and then increased. Some other peaks such as (100), (101), (103), (102), and (110) were also detected, but they were very weak compared to the (002) peak. In a word, prolonging the DMSA addition time will enhance the tropism of ZnO column array. Figure 5 illustrates the IR spectra of the prepared ZnO columns with DMSA. A peak at ∼2923 cm-1 is attributed to the stretching vibrations of CH2 in DMSA, and the peaks at ∼457 cm-1 are attributed to the stretching vibration of the Zn-O bond in ZnO columns. Similarly, two peaks at 1737 and 1197 cm-1 are assigned to the vibration of CdO and the C-O bond in DMSA, respectively. The peak at 1420 cm-1 in the IR spectra


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Figure 6. HRTEM results of the ZnO columns. (a) Image of single ZnO column was corresponding to the rectangle region of inset; (b, c) HRTEM images and the typical layered feature of the ultrasheet; (d) TEM image of an ultrathin sheet; two rectangle regions shown correspond to images in b and c.

of ZnO columns can be seen, which is attributed to the hydroxyl groups in DMSA. These peaks reveal that there are some free carboxyl groups of DMSA in ZnO columns. Two weak absorption peaks at 1041 and 2562 cm-1 are assigned to the stretching vibration of the C-S bond and S-H bond in DMAS, which results from the condensation of two SH groups on the surfaces of ZnO nanosheets and there are a few free mercapto groups on the surfaces of ZnO columns. In order to further understand the structure of the hierarchical columns, HRTEM analyses were performed to characterize their crystal structure. Figure 6 shows HRTEM images of a region within a main shaft of a rod and within its ultrathin sheet. The HRTEM image in Figure 6a shows the 2.6 Å {001} lattice fringe parallel to the basal plane, which was taken from the rectangle region of the shaft in Figure 6a inset. It provides further confirmation that the columns were growing along the [002] direction. HRTEM image in Figure 6b shows that the measured distance between the parallel lattice planes is 0.28 nm, corresponding to a d-spacing of the (101j0) planes. Some misalignments are observed between the plates shown by the ellipse circles in Figure 6b. The exact amount of dislocation coalescence seems to vary from area to area and is still being analyzed. Figure 6c obviously shows a typical layered feature, as the numbers show four layers of ultrathin nanosheets overlapping. Figure 6d is a face-on view of an ultrathin sheet within the shaft. The HRTEM results suggest that the ultrathin sheets stacked in the ZnO column are orderly single crystal structures with some misalignments and dislocation. 3.3. The Growth Process of Hierarchical ZnO Column Array. In accordance with the above characterization, a possible growth process of different structures of ZnO columns was proposed as illustrated in Figure 7. First, lots of triangle-like ultrafine pieces of ZnO nanocrystal were formed in the early

Figure 7. The formation schematics of different structures of ZnO columns.

reactive solution, and these ultrafine pieces will be the nucleus for ZnO sheets, as shown in the inset of Figure 8a. Consequently, the nanocrystals grew out along the edges, which were the most active area due to high percentage of dangling bonds.15 After a reaction, a diamond-like nanoparticle including two or more triangle pieces grew at the corner of the hexagon (shown in Figure 8a). In the same process, two other diamond-like nanoparticles grew at the other two corners of the hexagon. With a prolonged reaction time, a crescent-like of ZnO particle was formed (shown in 2c). With the reaction time was increased to 2 h, more and more triangle-like ultrafine pieces of ZnO nanoparticles formed and the size became much larger and many hexagonal sheets were observed. Furthermore, some triangle ultrafine pieces were still observed (inset of Figure 8b), suggesting that new crystal nucleus occurred all the time during



Figure 8. The morphologies of ZnO with different growth times. (a) 10 min; (b) 30 min; (c) 1 h; (d) 2 h.

ZnO particles for the growth of ZnO columns. When DMSA was added at an early reactive time (1-2 h), these nanocrystals could be coated by DMSA, and some deposited on the underlayered crescent-like ZnO particles that also were coated by DMSA which formed the layered structure finally, and some nanocrystals deposited on the substrate directly. The layered structure could be proven by Figure 6c. The crescent-like ZnO particles continued to grow, and finally formed a hexagon sheet. When DMSA was added at the reactive medium time (3-5 h), a column-like structure of ZnO sheets was finally prepared due to DMSA coating on the surface of ZnO sheets and the condensation between DMSA molecules on two ZnO sheets (Figure 7). It could also be proven by the SEM image of Figure 8a, which shows that some nanoparticles attached to the underlayer and formed the layered structure. When DMSA was added during the reactive later time (more than 5 h), ZnO columns with a smooth surface started to grow out on the buffered layers and DMSA did not impact the formation of ZnO columns. However, when DMSA was added at a very early time (less than 1 h), hardly any nanocrystals formed in the reaction solution, and most DMSA molecules coated on the surface of ZnO nanoparticles in the buffered layers. So the obtained columns in the ZnO array were more slender than those at the 6 h addition time of DMSA. To substantiate the above growth process of hierarchical ZnO column arrays, the experiment of preparing ZnO column arrays with different growth times was performed. The morphologies of samples were shown in Figure 8. Figure 8a is the SEM image of ZnO with 10 min of growth time. It shows some fine nanocrystals on the underlayer. Compared to Figure 8a, the density of nanocrystals increased and the size increased to ∼20 nm in large-scale as shown in Figure 8b. Compared with 30 min of growth time, the size of nanocrystals increased with 1 h of reaction time (Figure 8c). It also shows the formation of triangle-like ultrafine pieces of ZnO nanoparticles as shown in the inset of Figure 8c. Figure 8d is the morphology of the sample with 2 h of growth time. It is evident that the large grains with the nearly hexagon shape began to appear. The inset shows more and more triangle-like ultrafine pieces of ZnO nanoparticles appeared and the size increased compared with 1 h of reaction time. In conclusion, the SEM images of ZnO with different growth times is nearly identical with our given growth process. Developing synthetic approaches to prepare such complex oriented structure remains a significant challenge.27 Formation of this structure is directly related to the function of DMSA. As we known, dimercaptosuccinic acid is a chelator, which can be tightly bound to the metals. It contains two mercapto groups, and the metal ions are chelated through the mercapto sulfur atom.35 In our experiment, the connection between DMSA and

Figure 9. A connection schematic between two ZnO ultrathin sheets with DMSA.

ZnO was zinc ions and mercapto sulfur atom or carboxyl oxygen atom. Next it will be discussed in detail. Figure 9 shows the possible connection between DMSA and the ZnO sheet, suggesting that the bonds of S-Zn will form because of the bond of SH cleaves and the strong affinity between S and Zn. The thiol groups of DMSA connect with ZnO and may inhibit the further growth of ZnO along the (002) direction; the thiol groups of DMSA between ZnO sheets can connect through the condensation of two SH groups. Sequentially, the unusual oriented column-like structures of ZnO sheets can be formed through orderly stacking of ZnO sheets, which are directly related to the function of DMSA. With 0-1 h DMSA addition time, ZnO columns with a smooth surface were formed on the buffered layer, and no layered feature ZnO columns were obtained possibly due to fusing among ZnO sheets. Moreover, the DMSA molecule includes two functional groups: mercapto and carboxyl. Strategically, some free carboxyl groups (COOH) of DMSA can be used for the attachment of target molecules.29 Therefore, DMSA on ZnO columns could be conjugated with specific biomolecules to identify target biomolecules, and hierarchical column arrays can be potentially used as biological detection and disease diagnosis. The conjugation of DMSA with biomolecules will be discussed in our future work.


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Figure 10. Oriented biomimetic ZnO structures. (a) Layered features in Ostrea riVularis shells; (b) oriented column-like structure of ZnO sheets after the secondary growth.

Figure 11. (a) PL spectra of the as-prepared ZnO column arrays for different DMSA addition times; (b) DMSA addition time as a function of PL intensity and wavelength; (c) PL spectra for primary growth and secondary growth; (d) PL spectra of O. riVularis shells.

To prepare ZnO columns with morphology much more similar to Ostrea riVularis shells, oriented ZnO column array (Figure 3f) was used as the starting material and secondary growth was performed in the presence of DMSA. Figure 10a is SEM image of O. riVularis shells, showing the clear layered structure, and every layer with a thickness of ∼300 nm, which consists of some sheets with a thickness of 100-150 nm (as shown by the arrow). Figure 10b shows the column-like structure of oriented ZnO columns, and their thickness that formed the layered features is 100-150 nm. Here we are not implying that biominerals and ZnO structures are formed by the same mechanism, but the similarities in morphology and structure are very obvious due to O. riVularis shells have 90% sheetlike calcite crystal and 10% organic compositions among calcite sheets. The similar molecular structure between the DMSA and cysteine, which is one of amino acids for biomineralization of

metal oxide,37 imply that the growth of these oriented sheets with DMSA additive results in the fabrication of biomimic structures. 3.4. Photoluminescence of Hierarchical ZnO Column Array. The optical emission studies on ZnO column array films were made by PL spectroscopy using 280 nm excitation wavelength of Xe laser at room temperature. Figure 11a shows PL detecting of ZnO arrays prepared with different DMSA addition times, and Figure 11b is the luminescence intensity and wavelength of prepared ZnO column arrays with DMSA at different addition times. The result reveals that, with the increase of DMSA addition time, PL intensities of different ZnO arrays are not always increased. The intensity reached the maximum at 1 h DMSA addition time, and the wavelength reached the bottom at the same addition time. When the DMSA addition time was changed to more than 6 h, the intensity was


hardly changed. However, the wavelength of the obtained array displayed a slight blue shift when the addition time was less than 1 h, and then a red shift with an increase in the DMSA time, revealing that the size of the ZnO crystalline was small in less than 1 h addition time, and subsequently increased gradually with an increase of the addition time. Several peaks, which are located at ∼385, ∼435, and ∼480 nm, respectively, could be observed in Figure 11a. The sharp excitonic emission and weak deep level emission peaks indicate that the ZnO columns have a low defects concentration and high optical property. The band gap of ZnO with defects increased up to the range of 2.4-3.3 eV.35 The UV emission around 385 nm (3.2 eV) is usually attributed to the radiative recombination of free excitons, that is, near band-edge emission. Weak peaks at 421 nm (3.0 eV) and 485 nm (2.6 eV) probably originate from effect state luminescence. It is known that visible luminescence mainly originates from defect states such as Zn interstitials and oxygen vacancies. In addition to the UV emission, the green bands (480-580 nm) are negligible in the PL spectrum of ZnO column array grown by this process. It has been suggested that the green band emission corresponds to the singly ionized oxygen vacancy in ZnO.37 The absent green band in these PL spectra indicates a very low concentration oxygen vacancy in the ZnO columns grown by Zn2+ solution. Previous studies were mostly concentrated on the relationship between the annealing conditions with the defects concerning the oxygen by monitoring the PL spectrum in the visible range. However, our attention in this experiment should be paid to DMSA of the solution having a predominant effect on the defect of the crystallized ZnO hierarchical structure. The reason we call the effect “predominant” is that control conditions with DMSA addition time from 1 to 6 h implied the obtained ZnO columns with low defects concentration in columns with more DMSA molecules. PL spectra of the samples showed strong band-edge emissions at a similar position (Figure 11a). Comparatively, when control conditions of DMSA addition time performed at less than 1 h or more than 6 h, their PL patterns are weak band-edge emissions due to high defects concentration in columns with few DMSA molecular. These results suggested that DMSA addition time has a predominant effect in the formation of hierarchical structure of ZnO columns, corresponding to the XRD and SEM results. Accordingly, emission from exciton recombination became stronger with the change of DMSA addition time (from 1 to 5 h). Further investigation is needed to better understand the detailed mechanism about the novel strengthening luminescence reaction, especially the role of DMSA. ´Curves 1 and 2 in Figure 11c are PL patterns of the ZnO columns from primary growth and secondary growth, respectively. It is observed that a red-shift of PL spectrum from secondary growth ZnO columns occurred, and it is believed to result from the increase of the size of crystalline. At the same time, the PL intensity in curve 2 is lower than that of curve 1 and reveals again that ZnO column array in primary growth possess lower defects concentration than in secondary growth, corresponding to the SEM results. In general, luminescence colors including the blue range can be emitted by organic devices.38 In many cases conjugated polymers have been utilized that were deposited directly into the inorganic materials, or were prepared in situ by conversion of a prepolymer. The PL spectrum of O. riVularis shells shown in Figure 10d reveals that the prepared ZnO column array also possesses a similar bio-optical property (Figure 11c,d). These show that PL emission of ZnO column array similar to the emission of O. riVularis shell


resulted from their similar hierarchical structure, namely, ZnO column array possess DMSA organic materials between most sheet-like crystals, consistent with SEM results.

4. Conclusions In summary, hierarchical ZnO column arrays were successfully prepared through a low-temperature, environmentally friendly, solution-based approach of DMSA addition. Moreover, this simple, low-temperature strategy has been developed for the low-cost and large-area fabrication of ZnO column arrays with biomimic structures. This two-step, wet-chemical approach exhibits well-controlled growth of highly oriented and densely packed ZnO arrays, displaying large-area hierarchical columns or columns with predictable morphologies, such as tunable aspects. It is noted that these aspect-tunable ZnO hierarchical columns or columns, ranging from tens of nanometers to a micrometers, have been achieved by systematically adjusting the DMSA addition time and its concentration in the reaction solution; the morphology of hierarchical ZnO arrays prepared is remarkably similar to biomineral structures in O. riVularis shells. Room-temperature PL spectra of ZnO arrays exhibit nearband-edge emission and also a PL pattern similar to that of O. riVularis shells. PL spectra of ZnO columns obtained with 1-6 h addition time of DMSA have stronger band-edge emissions than the samples from other addition times of DMSA due to their different defects concentration. PL intensity of ZnO columns from secondary growth is lower than that from primary growth. At the same time, a red-shift of the PL intensity of ZnO columns from secondary growth occurred due to the increase of the size of the crystal, revealing that the ZnO arrays in primary growth have lower defects than those in secondary growth, corresponding to the SEM results. On the basis of this experiment, some carboxyl groups of DMSA remained free on the hierarchical columns, and it may be possible to graft a biomolecule on the ZnO columns for great potential applications of optical emission, piezoelectric transduction, and biosensors. Acknowledgment. This work has been supported by the National Natural Science Foundation of China (Project No. 60871062 & 50873066). The support of Sichuan Province through a Science Fund for Distinguished Young Scholars of Sichuan Province (08ZQ026-007) and Key Technologies Research and Development Program of Sichuan Province (2008SZ0021 & 2006Z08-001-1) are also acknowledged with gratitude. This work was also supported by the Research Fund for the Doctoral Program of Higher Education from Ministry of Education of China (No. 20070610131). We thank Analytical & Testing Center, Sichuan University, for the assistance with the microscopy work.

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