Growth Mechanism of Sea Urchin ZnO Nanostructures in Aqueous Solutions and Their Photocatalytic Activity for the Degradation of Organic Dyes.

This work reports the development of a fast and simple route for the synthesis of ZnO sea urchin (SU) nanostructures by the formation and assembly of ZnO nanorods under favorable growth conditions in an aqueous solution. The thermal treatment of a basic zinc acetate solution in ethanol results in the formation of aggregated seed clusters consisting of small ZnO nanorods, which were then grown in a precursor solution containing Zn(NO3)2·6H2O and hexamethylenetetramine to assemble the SU structures from the anisotropic ZnO nanorods on the surface of the seed clusters. Each ZnO nanoparticle in the aggregated seed clusters grew sequentially into a ZnO nanorod, and the nanorods were concentric to the core of the clusters yielding the unique SU-like shape. In the presence of a capping agent such as cetyl trimethyl ammonium bromide (CTAB), the aggregated seed clusters were not formed, and the growth of the CTAB-capped ZnO nanorods resulted in separated rods with average aspect ratios of ∼10. The SU ZnO nanostructures exhibit a hexagonal wurtzite crystal structure and higher specific surface area (26.9 m2/g) than the CTAB-capped nanorods (17.7 m2/g). The SU ZnO nanostructures show superior photocatalytic efficiency for the degradation of three common organic dyes compared to the ZnO nanorods. The removal efficiencies of indigo carmine, methylene blue, and rhodamine B by the SU nanostructures were 99, 86, and 96%, respectively, after 1 h of UV irradiation. Therefore, the ZnO SU structures have the potential to be a versatile photocatalyst for the photodegradation of organic dyes in industrial wastewater.

Introduction

In recent years, 3-dimensional (3D) semiconductor metal oxide nanostructures have gained considerable attention due to their wide range of applications in numerous fields.[1−4] However, very few semiconductors possess the attention that ZnO has received mainly due to its unique chemical and physical properties.[1−5] ZnO exhibits a wide bandgap (3.37 eV), high intrinsic electron mobility (∼300 cm2 V–1 s–1), and high exciton binding energy (60 meV).[5] ZnO is an n-type semiconductor, which can be converted to a p-type by doping elements such as N and Na.[6−8] ZnO is also well renowned for its long-term stability under extreme conditions in addition to its biocompatibility and biosafety.[9,10] Due to these unique properties, inexpensive material cost, and high abundance with properly manipulated synthetic strategies and device engineering, ZnO has been successfully used as a cost-effective material in a number of diverse applications including photocatalysis, photovoltaics, sensors, active antibacterial surfaces, optoelectronics, and batteries.[11−20]

It has always been a challenging task to obtain well-defined 3D architectures of ZnO with a proper understanding of the crystal growth. Over the years, a significant amount of research efforts has been dedicated to developing facile synthetic routes to obtain different 3D nanostructures of ZnO.[1−5,21−24] Compared to one-dimensional structures such as rods and wires and two-dimensional structures such as thin films and disks, 3D nanostructures exhibit unique features such as high surface area, low aggregation, and superior carrier mobility, which can serve well in photocatalysis and sensing applications. Common 3D nanostructures studied in the literature include tetrapods,[25,26] hollow[27,28] and nonhollow spheres,[29] and flower-like structures,[14,23,30−32] which are synthesized using different strategies such as hydrothermal, template-assisted aging, and chemical vapor deposition.[25−32]

General approaches to obtain 3D architectures include the utilization of template directional growth[29] or selective manipulation of growth rates of specific crystal facets using surfactants.[33−35] For example, Sun et al. developed a hydrothermal route where a zinc amino complex is autoclaved at 180 °C for 12 h to obtain a sea urchin-like 3D architecture for gas sensing applications.[36] However, a detailed mechanistic understanding of the growth of these nanostructures can be limited by the growth technique. For instance, it can be laborious to track the sequential growth of a nanostructure if the hydrothermal technique is used.

Based on the World Bank estimation, 17–20% of contributors for the water pollution is related to the textile industry.[37] Annual production of textile dyes is almost a million tons, and around 15% of it is generally released to natural water bodies with minimal or no treatment.[37,38] Photodegradation of organic dyes is one of the most widely studied and important areas of photocatalysis, which has the advantages of the usage of costless, inexhaustible, and clean solar energy.[39] Semiconductor metal oxides, such as ZnO, an efficient photocatalyst, can be used as a cost-effective tool for pollution control due to the unique chemical and physical properties mentioned earlier. In this work, we present a simple two-step aqueous phase synthetic route to obtain 3D sea urchin (SU)-shaped ZnO nanostructures. Compared to the reported techniques, this synthetic strategy does not involve harsh growth conditions or expensive instruments and only requires 2 h to be completed.[40] More importantly, a mechanistic study of the sequential formation and growth processes of the SU nanostructures is reported. Furthermore, the photocatalytic activity of the SU ZnO nanostructures is evaluated based on the photodegradation of three organic dyes, indigo carmine (IC), rhodamine B (Rh B), and methylene blue (MB). The results provide evidence for the superior photocatalytic efficiency of the SU ZnO nanostructures.

Results and Discussion

Catalyst Characterization

Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of the synthesized ZnO seeds are shown in Figure . The TEM images (Figure a,b) suggest that the ZnO seeds are clusters of small rod-shaped ZnO nanoparticles. The small ZnO nanoparticles in the size range of 4–6 nm are aggregated to form spherical seed clusters, which are around 80 nm in size, as shown in Figure c,d.

Figure 1

TEM (a, b) and SEM (c, d) images of ZnO seeds.

To gain insight on the mechanism of the formation of the ZnO seeds, TEM images of the growing seeds were recorded at different time intervals (5, 15, 30, 60 min), as shown in Figure a–d. After adding the base to zinc acetate, the solution became cloudy in about 5 min indicating the formation of the ZnO seeds, as shown in the TEM image in Figure a. In the absence of a capping agent in the solution, the ZnO nanoparticles started to aggregate under the constant heating at 60 °C, as shown in the TEM image in Figure b recorded after 15 min from adding the base. The aggregation of the individual nanoparticles continues, and a large number of small individual nanoparticles started to be replaced by a smaller number of larger aggregated clusters, as shown in TEM image in Figure c recorded after 30 min from adding the base. Finally, the growth process of the seed aggregates appeared to be terminated after 60 min as shown in the TEM image in Figure d where the small individual ZnO nanoparticles were totally replaced by a few aggregated clusters in the size range of 80–100 nm. The disappearance of the initially formed small ZnO nanocrystals and the formation of the larger aggregated clusters are a direct consequence of the Ostwald ripening mechanism where the small nanocrystals attain a lower-energy state by transformation into large aggregated clusters with reduced surface to volume ratios.[41]

Figure 2

TEM images (a–d) showing the sequential growth of ZnO nanoparticles to form seed clusters at different time intervals of 5, 15, 30, and 60 min, respectively, from adding the base to the zinc acetate solution at 60 °C. (e) UV–vis spectra corresponding to the solutions used for images (a–d).

The growth of the seed clusters was also studied by recording the UV–vis absorption spectra at different times, as shown in Figure e. The absorption edge at 334 nm due to electronic transitions between the valence and conduction bands in the initially formed nanocrystals (5 min) is significantly blue-shifted from the absorption of bulk ZnO, which absorbs at approximately 385 nm (3.2 eV).[5] The absorption edge is red-shifted from 334 nm (3.71 eV) to 350 nm (3.54 eV) after the complete growth of the aggregated clusters (60 min) indicating a decrease in bandgap due to increasing particle size. The increase in the bandgap of the small seed clusters is attributed to the quantum confinement mechanism and the seed clusters grow to larger sizes, the bandgap decreases toward the bulk bandgap. These aggregated seed clusters are stable as a suspension in ethanol as confirmed by the TEM images (similar to the image shown in Figure d) taken after being suspended in ethanol for 1 week.

The main hypothesis in the present synthesis approach is that the formation of ZnO-aggregated seed clusters (Figure d) is the key requirement for the secondary growth process to form the unique “sea urchin”-shaped ZnO nanostructures. To test this hypothesis, a capping agent cetyl trimethyl ammonium bromide (CTAB) was added to the zinc acetate solution to prevent the aggregation of the initially formed ZnO nanocrystals into the aggregated seed clusters. As expected, the very small ZnO nanocrystals remained separated and only exhibited a small degree of growth after 1 h of heat treatment at 60 °C (Figure S1). Without CTAB, the absorption edge of the ZnO-aggregated clusters increased to 350 nm after 1 h of heating at 60 °C, as shown in Figure e. However, in the presence of CTAB, the absorption edge of the ZnO seeds increased only to 336 nm from the original value of 321 nm (Figure S1e) indicating a minimal aggregation of the nanocrystal seeds and a small increase in particles’ size.

To compare the growth patterns of the ZnO seeds in the absence and presence of CTAB, both types of seeds were incubated at 90 °C in a solution containing Zn(NO3)2·6H2O and HMTA. After the thermal hydrolysis of HMTA, NH3 is slowly released to the solution which then hydrolyzes in the presence of water to form NH4(OH).[42] As the solution becomes basic, Zn2+ ions react with OH– to form ZnO, as shown in eqs –3.In the case of uncapped ZnO seeds (no CTAB), the growth of the aggregated clusters results in the formation of ZnO rods, which grow along the [0001] direction in a pattern where all of the rods are concentric to the core of the seed cluster yielding the sea urchin-like shape, as shown in the TEM images in Figure recorded at different time intervals of the growth process at 90 °C. Due to the polar nature of the ZnO lattice structure, the surface can be either negatively or positively charged depending on the crystal structure of the terminated plane. Zn-terminated planes will be positively charged, whereas O-terminated planes will be negatively charged. Depending on the charge, the surface will attract counterions in the medium (OH– or Zn2+) and form a layer of alternating charges on the surface resulting in an ordered ZnO lattice.[42] This sequence will continue to form an ordered ZnO lattice in the [0001] direction until the concentration of the precursor ions in the solution is depleted. Each nanoparticle on the surface of the seed clusters shown in Figure a has the potential to grow into a rod, and this is shown clearly in Figure b,c where the rods are grown on the surface of the seed cluster. At a longer growth time, the grown rods become longer and sharper before the growth is terminated by the depletion of the ions in the growth solution as shown in the TEM image in Figure d recorded after 60 min during the growth process. The absorption edge of the developed SU ZnO nanostructures exhibits a small red shift to 370 nm compared to the original value of 355 nm of the seed clusters, as shown in Figure e.

Figure 3

TEM images showing the sequential growth of SU ZnO at different stages, 0 min (a), 15 min (b), 30 min (c), 60 min (d), and UV/vis spectra of the SU ZnO at different growth time intervals, as shown in (e).

Figure displays TEM and SEM images of the full-grown SU ZnO nanostructures. The size distribution of the SU nanostructures ranges from 0.5 to 1.5 μm most likely due to the stagnant growth in the absence of stirring. Interestingly, the growth of the CTAB-capped ZnO seeds did not result in the formation of SU structures but only separated rods with average aspects ratios of ∼10, as shown in Figure S2 (Supporting Information). This result supports the hypothesis that the formation of the aggregated seed clusters shown in Figure a is necessary for the growth of the SU structures. It appears that these aggregated seed clusters act as efficient heterogeneous nuclei for the addition of ZnO units to develop the 3D sea urchin structures.

Figure 4

TEM images (a, b) and SEM images (c, d) of the SU ZnO nanostructures.

Figure a,b displays the X-ray diffraction (XRD) patterns of the grown SU nanostructures and the seed clusters of ZnO, respectively. In both cases, the crystal structure belongs to the hexagonal wurtzite structure of bulk ZnO with the space group P63mc. However, the ZnO seed clusters (Figure b) show broader XRD peaks compared to the SU ZnO (Figure a) due to the smaller particle size. Also, the peak intensity representing the (002) plane is relatively small in the seeds, and it increases significantly in the SU structures, which confirms the suggested mechanism for the growth of the SU structures.

Figure 5

XRD patterns of SU ZnO (a), ZnO seeds (b) and TEM images showing how the seeds transformed into SU-shaped ZnO.

To further confirm the hexagonal wurtzite structure of the SU ZnO nanostructures, the Raman spectra of the SU ZnO and the ZnO rods (formed in the presence of CTAB) are shown in Figure . In both cases, the prominent peak at 437 cm–1 corresponds to the nonpolar E2 (high) optical phonon mode, and the 378 and 580 cm–1 are attributed to the polar transverse optical (TO) A1 and longitudinal optical (LO) E1 phonon modes, respectively.[7] The peak at 332 cm–1 corresponds to the 2E2 mode. According to the selection rules, Raman active phonon resonance modes for the ZnO wurtzite structure are A1 + 2E2 + E2.[43] Therefore, the Raman data shown in Figure confirm that the SU ZnO nanostructures have the hexagonal wurtzite crystal structures.

Figure 6

Raman spectra for SU ZnO and ZnO rods.

Photocatalytic Activity

The photocatalytic activity of the SU ZnO nanostructures was evaluated based on the photodegradation efficiency of three organic dyes IC, MB, and Rh B under UV irradiation. The results are shown in Figure , which displays the UV–visible spectra of Rh B, IC, and MB recorded at different time intervals during the photodegradation experiments. The degradation percentages of the Rh B, IC, and MB dyes after UV irradiation for 1 h were determined as 96, 99, and 86%, respectively. The photodegradation reactions for all of the three dyes follow pseudo-first-order kinetics (Figure d), and the degradation of IC shows the fastest kinetics with a rate constant of 0.076 min–1. According to the data, MB exhibits a slight resistance to photodegradation by the SU ZnO nanostructures compared to the other two dyes. MB is the only anionic dye among the three dyes, and SU ZnO surfaces are expected to possess a slightly negative charge in aqueous solutions due to the surface hydrolysis.[44] On account of the electrostatic repulsion, MB showed a little drop in the absorption peak after the initial adsorption–desorption equilibrium for 1 h, which is evident at the 0 min mark of Figure c. Weak electrostatic adsorption of MB could shorten its retention time on the surface of the SU ZnO catalyst, which would decrease its photodegradation efficiency. On the other hand, a significant drop in the absorption peak intensity was observed for the Rh B and IC dyes after the initial dark reaction as shown in Figure a,b indicating strong electrostatic adsorption on the catalyst surface, and, therefore, a rapid photodegradation was observed within 1 h irradiation in both cases.

Figure 7

Photocatalytic activity of SU ZnO in degrading organic dyes (a) Rh B, (b) IC, (c) MB and (d) the apparent rate constants for the photodegradation of Rh B, IC, and MB.

Finally, the photocatalytic activities of the SU ZnO nanostructures and the ZnO rods in the photodegradation of the Rh B dye were compared. As shown in Figure a,b, the photocatalytic activity of ZnO rods is significantly lower than that of the SU nanostructures. As shown in Figure c,d, after 1 h of UV irradiation, ZnO rods only degraded 34% of the dye (10 ppm), whereas the SU nanostructures degraded 96% of the dye. The increased activity of the SU nanostructures could be attributed to the BET specific surface area of 26.9 m2/g, which is significantly larger than that of ZnO rods (17.7 m2/g).[45,46] The relevant N2 adsorption/desorption isotherms of the SU ZnO nanostructures and the ZnO rods are shown in Figure S3 (Supporting Information). The enhanced photocatalytic activity could also be attributed to the porous structure of the SU ZnO nanostructures, which would enhance the adsorption and probably the trapping of the organic dyes within the assembly of the short rods for efficient photodegradation.

Figure 8

Comparison of photocatalytic activities of SU ZnO and ZnO rods prepared using CTAB-capped ZnO seeds in photodegrading Rh B. (a, b) UV–visible spectra of Rh B showing photodegradation by SU ZnO and ZnO rods, respectively, and (c, d) kinetic plots of the photodegradation reaction by SU ZnO and ZnO rods, respectively.

Conclusions

In summary, a simple approach has been developed for the synthesis of sea urchin nanostructures by the formation and assembly of ZnO nanorods under favorable growth conditions in an aqueous solution. The thermal treatment of a basic zinc acetate solution in ethanol results in the formation of aggregated seed clusters consisting of small ZnO nanorods, which were then grown in a precursor solution containing Zn(NO3)2·6H2O and hexamethylenetetramine to assemble the SU structures from the anisotropic ZnO nanorods on the surface of the seed clusters. Each ZnO nanoparticle in the aggregated seed clusters grew sequentially into a ZnO nanorod, and the nanorods were concentric to the core of the clusters yielding the unique SU-like shape. In the presence of a capping agent such as CTAB, the aggregated seed clusters were not formed, and the growth of the CTAB-capped ZnO nanorods resulted in separated rods with average aspect ratios of ∼10. The SU ZnO nanostructures exhibit a hexagonal wurtzite crystal structure and higher specific surface area (26.9 m2/g) than the CTAB-capped nanorods (17.7 m2/g). The SU ZnO nanostructures show superior photocatalytic efficiency for the degradation of three common organic dyes compared to the ZnO nanorods. The removal efficiencies of indigo carmine, methylene blue, and rhodamine B by the SU nanostructures were 99, 86, and 96%, respectively, after 1 h of UV irradiation. Therefore, the ZnO SU structures have the potential to be a versatile photocatalyst for the photodegradation of organic dyes in industrial wastewater.

Experimental Section

Preparation and Characterization of Sea Urchin-Shaped ZnO Nanorod Clusters

Zinc oxide seeds were prepared in a colloidal solution using ethanol as the solvent according to the method described in ref (39). In a typical synthesis, 2 mM zinc acetate [Zn(Ac)2, Alfa Aesar] in ethanol was heat-treated for 30 min at 70 °C under stirring in a water bath. Then, 4 mM potassium hydroxide [KOH, Sigma] in ethanol was added dropwise. The molar ratio between Zn(Ac)2 and KOH was maintained at 1:1. Then, the temperature was reduced to 60 °C with continued heating for 1 h under constant stirring. The reaction solution started to become cloudy with time indicating the formation of the ZnO seeds.

To synthesize sea urchin-shaped ZnO nanorod clusters, an equimolar aqueous solution of zinc nitrate hexahydrate [Zn(NO3)2·6H2O, Sigma-Aldrich] and hexamethylenetetramine [HMTA, Sigma] (20 mM) was used as the growth solution. First, 25 mL of the ZnO seed solution and 100 mL of the growth solution were mixed together and sonicated for 15 min to disperse the seeds well in the solution. Then, the mixture was incubated in a water bath at 90 °C for 1 h without stirring. Finally, the nanorod clusters were formed and separated from the growth solution and washed with deionized water and ethanol separately to remove the growth solution. The white powder of the sea urchin-shaped ZnO was then oven-dried at 70 °C overnight.

The prepared zinc oxide samples were characterized using the following techniques. Transmission electron microscope (TEM) images were obtained using a Jeol JEM-1230 TEM with the Gatan Orius SC1000 side mount CCD camera at 120 kV. Scanning electron microscope (SEM) images were obtained with a Hitachi SU-70 FE-SEM. UV–visible absorbance spectra were obtained using an HP-8453 spectrophotometer. Powder X-ray diffraction (XRD) patterns were acquired with the PANalytical MPD X’Pert Pro with a copper filter (Cu Kα, λ = 1.5405 Å) at 45 kV and 40 mA with a scan speed of 0.5 2θ/min. Raman spectra were obtained from Thermo Scientific DXR SmartRaman (532 nm). A micromeritics 3Flex surface characterization analyzer was used for the surface area measurements. Samples were activated overnight at 100 °C, and the nitrogen adsorption–desorption isotherms were obtained at 77 K to determine the BET surface area.

Photocatalytic Experiments

Photocatalytic activities of the prepared ZnO samples were evaluated based on the photodegradation of three organic dyes, MB, Rh B, and IC, and the photocatalytic experiment is briefly described as follows. A 4 W, 365 nm UV lamp was used as the source of irradiation, and a Petri dish (diameter of 3.5 in.) was used as the reaction container. The UV lamp was placed above the Petri dish irradiated with the UV light at 365 nm. Before the irradiation experiment, 50 mL of the aqueous solution containing dispersed ZnO nanostructures (0.25 mg/mL) and the dye (10 ppm) was added into the container. Then, the solution was kept in the dark for 1 h to achieve adsorption–desorption equilibrium. Finally, the lamp was turned on, and the photocatalytic reaction was started. Samples were taken at different time intervals to analyze the concentration variation of the dyes. The photodegradation efficiencies were calculated based on the UV–vis absorption of the dyes at different irradiation times.

Analytical Methods

The concentrations of the dyes were determined by absorbance measurements due to their strong absorption bands in the visible region and the linear correlation between the dye concentration and the absorbance. The absorption peaks of the samples were recorded by a UV–vis spectrophotometer and each measurement was repeated at least three times, and the averaged absorbance values are used to calculate the dye concentrations.