1. Introduction

The control over assembly of nano-building blocks into one-dimensional (1D), two-dimensional (2D), and three dimensional (3D) hierarchical nanostructures is fascinating because the variation of the arrangements of the building blocks provides a choice to tune the property of the material [1]. The development of structured semiconductors in search for novel optical and electronic properties has generated a number of strategies including the thermal reduction, thermal oxidation, oriented aggregation and hydrophobic interactions [2–5]. Recent studies confirm that the mild hydrothermal processes show extraordinary ability in the direct preparation of advanced nanostructures [6]. This method could generate highly crystalline products with high purity. Moreover, the morphology and crystal form of the products can also be controlled by adjusting the hydrothermal reaction conditions, such as solvent used, temperature of preparation, concentration of the reactants, and the use of surfactant templates, etc. Template-direct synthesis is an important synthetic strategy, which is widely applied to control inorganic crystal morphology. Although proton beam writing [7], nanoimprint lithography [8], electron beam lithography techniques [9,10] and even scanning probe microscopy [11,12] have yielded some promising results, there is a lack of simple and cost-effective techniques for producing high-quality and controllable nanostructure arrays on a large scale. In addition, many of the commonly used etching techniques result in surface damage as well as contamination. Laser direct writing is a very attractive tool for micro- and nanofabrication; the remarkably sharp threshold for laser-induced material damage enables the structures to be modified with nanometer precision [13–17]. Laser direct writing has some advantages over abovementioned competitive techniques, such as, wide range from macro- to nanoscale fabrication, neither ambient gas nor vacuum are required, processing speed is very high, cost-effective, etc.

As well known, the wetting behavior of a solid surface is one of the most important aspects of surface physical chemistry, which affects the degree of interaction a surface with the surrounding species. According to Cassie’s law [18], wettability is determined by both the chemical composition and the geometrical structure of solid surfaces. It has been reported that the CF₃ terminated surfaces possess the lowest free energy and thus the highest hydrophobicity. Furthermore, research has been focused on tunable wettability by controlling surface topographic structures in order to enhance long-life functional durability. Recent studies have revealed that micro- and nanostructures exhibiting hierarchical roughness cause an amplification of photoresponsive contact angle (CA) changing [19].

On the other hand, nanoscale materials are also believed to perform much better than their bulk counterparts in photocatalytic properties due to the higher surface-to-volume ratio and higher redox potentials. 3D micro/nanocomposite structures would possess high specific area on the surface of the particles, and likely be an ideal host material for catalysis, water treatments, and other highly functional and effective devices [20–22]. In the photocatalysis processes, the illuminated semiconductors catalyst absorb light and generate electronic species which lead to complete oxidation of organic components in wastewaters. Among the wide band gap semiconductors catalyst employed, ZnO and TiO₂ are the most extensively used photocatalyst due to their high photocatalytic activity. Although TiO₂ is considered as the most active photocatalyst, ZnO also can be a suitable alternative to TiO₂ because its cost is lower and it has a similar band gap energy. In addition, the potential of electron derived from ZnO is more negative than that generated by TiO_2, thus ZnO has a stronger electron affinity.

In this paper, we report the selective growth 3D flowerlike ZnO nanostructures by combining laser direct writing and hydrothermal method. The wettabilities and photocatalytic activities of the flowerlike ZnO nanostructures are studied. This work provides a new way to design desirable micro/nanoarchitectures. It is also worth to mention that our technique is a very potential route in the preparation of metal and semiconductor nanostructures on various material substrate. This controllable technique opens the possibility of creating novel nanostructures for future functional devices.

2. Experimental section

As shown in the left of Fig. 1 , 3D flowerlike ZnO arrays were fabricated in three steps: substrate preparation, laser processing, and selective growth of nanostructures on the substrate. In a typical procedure, a 1 µm thick layer of PMMA was spun on the GaN/LiAlO₂ substrate at a speed of 4000 rpm. After that, the substrate was baked on a hot plate at 100°C for 10 min. The second step was laser processing. A commercial regenerative amplified Ti:Sapphire laser (Spitfire, Spectra-Physics) with a pulse duration of 120 fs and a repetition rate of 1 kHz was used in this experiment. The sample was mounted on a computer-controlled x-y-z translation stage. The surface of the sample was positioned perpendicular to the propagation direction of the incident laser beam in the focal plane of a 100 × objective lens (NA = 0.8). The last step was selective growth ZnO nanostructures. The aqueous precursor was prepared by mixing zinc nitrate and hexamethyltetramine (HMT) at a molar ratio of 1:1 and a zinc concentration of 0.025 mol/L. After the pretreated substrates were immersed in the bottle filled with the above precursor solution, the whole system was heated from room temperature and maintained at the constant temperature of 90°C for 120 minutes with continuous stirring. After the system cooled down to room temperature, the substrate was washed with distilled water, and dried at room temperature.

 

Fig. 1 Fabrication process of 3D flowerlike nanostructures: deposition of a PMMA layer on a GaN/LiALO₂ substrate, laser processing and hydrothermal growth of ZnO nanostructure. The right side shows the schematic diagram of photoreaction apparatus.

Download Full Size | PDF

 

Fig. 3 CCD images of CA measurements of ZnO grid surfaces at different time: (a) 0 min, (b) 10 min, (c) 20 min, (d) 30 min, (e) 40 min, respectively (Media 1). (f) Water CA spectra of ZnO grid as a function of time, the time interval is 2 min.

Download Full Size | PDF

The wettability of the ZnO nanostructure films was measured by means of the water contact angle using a contact angle system (SL200B) at ambient temperature of 10°C. After dropping a 2 μL water drops onto the film surface, images were recorded with a CCD camera after adjusting contrast and focus. Subsequently, the contact angles can be determined by the analysis system. The photocatalytic activity measurement was conducted by putting the substrate into 100 mL of Rhodamine B (2.5 × 10⁻⁷M) solution, as shown in the right of Fig. 1. The suspension was irradiated by using a 8 W fluorescent UV lamp (365 nm) under continuous stirring at room temperature. The lamp was positioned 10 cm away from the suspension, and parallel to the sample. Prior to the irradiation, the reaction mixture was kept in the dark for 10 min to ensure sufficient adsorption of the dye. Analytical samples for absorption measurement were taken out from the reaction suspension at different time intervals. A spectrophotometer (Jasco V570) was used to record the absorption spectra of the solutions.

3. Results and discussion

Figure 2 shows the general morphologies of the product, the location at which the ZnO nanostructures is defined by the site of the irradiation dot. In Fig. 2a, a single flower has a diameter of 5 μm which is made up of many thin nanopetals. The nanopetals with smooth surfaces are about 100 nm in thickness; these edge-standing nanopetals provide necessary surface roughness as well as a highly porous structure to our later wettability engineering and film functionalization. In this dot, the substrate was irradiated by 20 femtosecond laser pulses with an energy of 1 μJ. Larger dots template the growth of larger flowers, and finally, the nanopetals were connected to each other to build the 3D flowerlike structures. Figure 2b clearly demonstrates that by tuning the laser irradiation conditions, the ablated dot can be size adjusted. The control of flower density is another important aspect in spatial organization. Figure 2c shows a matrix of flowers, the distance between the flowers can be mediated through the pre-determined dots. We further studied the relations between the laser parameters and structure. Interestingly, while the substrate was irradiated in line-scan mode at a speed of 10 μm/s and a pulse energy of 1 μJ, the flower grows out of the opening, and could form a correspondingly long range architecture along the irradiated lines. Figure 2d shows that the nanostructures extend along the laser scanning line, forming a continuous line of uniform thickness. Similarly, we successfully obtained a ZnO micro-grid by laser cross scanning (Fig. 2e). It is also worth to mention that no nanostructures are observed in the un-irradiated area. Clearly, judicious choice of pattern size offers control and flexibility in designing nanomaterial architectures. As shown in Fig. 1f, energy dispersive X-ray spectroscopy (EDX) analysis proves that only Zn and O exist in the product (the element C originates from the thin C layer sputtering on the sample in the test). During the hydrothermal growth process, the negative nature of the growth unit [Zn(OH)₄ ^2-] will lead to different growth rates of planes. When there is no organic additive in solution, spherical ZnO particles will be easily developed because of the Ostwald ripening process [23]. In our experiments, hexamethyltetramine (HMT) is expected to serve as the organic template during the heating process, thus dynamically modifying the nucleation process. The substrate/crystal surface has a boundary layer of charged ions, with increasing concentration of Zn²⁺ and OH^-, ZnO nuclei will be developed under the low precursor concentration and the action of HMT. The self-assembly of a number of active sites that trigger the nucleation at the interface, promoting the formation of petal crystals extending from the interface.

 

Fig. 2 FE-SEM images of ZnO flowerlike nanostructures (a) single flower, (b) flower with different size, (c) flower matrix, (d) continuous line-mode structures, (e) ZnO grid, (f) EDX analysis took from the flower petals, and the substrate (the inset), respectively.

Download Full Size | PDF

It is well known that the wettability of a solid surface is closely related to its micro/nanostructure, because structure or texture could influence the spreading of liquids [24]. There are two models that describe the water CA on rough surfaces. One is Wenzel’s model [25], with cosθ_f = rcosθ_w, where r is the surface roughness, θ_f and θ_f are water CA on rough and smooth surfaces, respectively. Another one is the Cassie-Baxter model [18], which is generally valid for heterogeneous surfaces, cosθ_f = f_s cosθ_w-f_v, where f_s and f_v are the area fractions of the projecting solid and air on the surface, respectively. This equation indicates that a high area fraction of air contributes greatly to the enhancement of hydrophobicity. Sun et al. reported the water CA of continuous ZnO nanosheet films on Si substrates is 15.8° [26]. In our experiment, the time-dependent water CA measurements were conducted on the ZnO grid surfaces (Fig. 2e). The CA of the bare surface is 59°, as can be seen from Fig. 3, the water CA of ZnO grid in our experiments changed from 92° at the beginning to almost 0° after 40 minutes, the droplet evaporation time is about 60 min. During the measurement, the out most edge of the droplet is fixed. The CA varies as a function of time is expressed as θ = θ_i (1− t / t_f), where θ_i is the initial contact angle and t_f is the total evaporation time [27]. In our experiments, we can see that some changes occur on the surrounding region of the water droplet from the CCD images (Media 1). It is reasonable to conclude that the liquid penetrated the interstitial spaces between the features, leading to the rapid decrease of contact angle.

The study of photocatalytic activities is important for searching strategies to design functional nanostructures. The as-produced ZnO nanoflowers were investigated for the applicability in photodegrading organic dyes of Rhodamine B (RB). The use of this aromatic compound as a model dye is mainly due to their recurrent occurrence in the industrial field. The reaction time was scheduled to be 0, 1, 2, 3, 4 and 5 h, respectively. The degradation is monitored by studying the decrease in absorbance of RB and quantified by plotting a first order decay plot of the absorbance. As shown in Fig. 4a , the absorption spectra corresponding to the RB molecules at 552 nm decreases in intensity rapidly with the extension of exposure time. The decomposition rate is high within the first 2 h. With time evolution, the concentration of RB decreases, the probability of RB reacting with ZnO nanostructures decreases, and consequently, the decomposition rate decreases.

 

Fig. 4 (a) UV-vis adsorption spectra of RB solution catalyzed by the ZnO nanostructure films under UV irradiation. (b) Degradation of the RB under UV irradiation as a function of time.

Download Full Size | PDF

The photodegradation of RB solution accords with the exponential decay formula C(t) = C(0) exp (-kt) [28], where C(t)is the relative concentration of RB at time t, C(0) is the initial relative concentration, and k is the photodegradation rate constant, the result is plotted and shown in Fig. 3b. The solid squares are the experimental data, and the solid line is the fitting curve. The rate constant of the degradation reaction for RB is 0.58 ± 0.03 h⁻¹. It is worth to mention that, due to the very small thickness and low density of the nanostructures on the substrate, the total weight of the nanostructures that evolved in the photodegradation is less than 0.1 mg, which cannot be determined by our analytical balance. Further experiments are required to clarify the relationship between the weight of the nanostructures and the degradation rate, and to compare it with normal photocatalysts in degradation of organic dyes.

4. Conclusion

To summarize, a novel method was proposed for the selective growth of 3D ZnO flowerlike nanostructures. The flowerlike structure could be varied by adjusting hydrothermal reaction conditions and laser irradiated parameters. The wettability is studied by measurement of time-dependent contact angles. In addition, superior photocatalytic activity of the flowerlike ZnO nanostructures in the degradation of Rhodamine B has been demonstrated; the rate constant of the degradation reaction for RB is 0.58 ± 0.03 h⁻¹. Our results will be expected to find vast applications in the synthesis of other similar oxide nanocrystals, and creating novel nanostructures for future functional devices.