Morphogenesis of ZnO nanostructures: role of acetate (COOH⁻) and nitrate (NO₃⁻) ligand donors from zinc salt precursors in synthesis and morphology dependent photocatalytic properties

Abstract

We have studied the anion ratio effect of acetate and nitrate ions on the formation of different morphologies of ZnO crystals in the presence of NaOH and HMTA. We have varied the concentration of zinc salt precursors as well as adopted three different methods of synthesis (chemical, ultrasonic and hydrothermal) to study their effects on the size/shape of the formed ZnO nanostructures. We found that these anions which spontaneously get introduced along with zinc salt can modify the structure depending on their ratio. The photocatalytic properties of the as obtained ZnO nanostructures (nanoflower, nanospindle and nanorod) were tested towards anionic dye methyl orange. The results show that rod like ZnO nanostructures are catalytically more reactive than nanoflowers and nanospindles. Based on these results we propose that selection of appropriate zinc salt is important to rationally design experiments for a particular morphology. Furthermore, it is also suggested that the role of other precursor salts in synergy will lead to the formation of various nanostructures and this can be extended to other metal oxides of interest for modification.

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Introduction

ZnO represents a material whose properties such as band gap, exciton binding energy, optical and electrical properties are strongly sensitive to morphology, size, aspect ratio, size orientation and crystal density.^1–3 These structural-characteristics-dependent fundamental properties of ZnO has led to its applications in lasers,⁴ light emitting devices,⁵ piezoelectric transducers,⁶ chemical sensors,⁷ solar cells,⁸ transparent electronics,⁹ and photocatalysts.¹⁰ A variety of ZnO nanostructures (nanoflower,¹¹ nanobelt,¹² nanorod,^13,14,16 nanosheet,^14,16 nanotube,¹⁵ nanoribbon,¹⁶ nanowire¹⁷ nanopyramid¹⁰ and nanocrystal^10,11,16) have been successfully developed by physical and chemical routes for example, vapor phase method, hydrothermal synthesis, microwave assisted solution reaction, spray pyrolysis, magnetron sputtering, pulsed laser deposition, molecular beam epitaxy and wet chemical route.^2,18–20 However, compared to physical methods, solution phase methods have a clear superiority in terms of easy synthesis, flexibility, productivity and cost efficiency.

The formation of diverse shaped ZnO nanostructure predominantly rest on the relative surface energy of the crystal planes of the flourishing nanostructure.^18,21,22 ZnO crystals mainly have three crystal planes: a top polar zinc (0001) face, six symmetric nonpolar {1010} planes parallel to the [0001] direction, and a basal polar oxygen (0001) face. These different planes have different polarities (atomic arrangements) and are chemically distinct. Therefore, permutation and combination of different solvents, templates, surface capping agents and reaction conditions (such as pH, temperature etc.) generates various size and shape of ZnO nanostructures.^23–28 Moreover, it has been shown that introducing any growth modifiers (for example capping ligands) which have specific affinity towards any crystal face or surface can modify the relative growth rates and this modification reflects in the final crystal shape.²³ Though much attention and studies have been done in context of capping agents/different soft templates (surfactants,²⁶ polymers,^29,31 di-block copolymers,³⁰ citric acid,³¹ ascorbic acid¹¹ and amino acids³²) but the effect of anions (NO₃⁻, COOH⁻, Cl⁻ etc.) which get introduced when any zinc salt is used in the reactions is not consolidated.

Therefore, we sought to investigate the effect of two different salts of zinc (acetate and nitrate) on crystal formation when introduced together in the reaction at different ratio in aqueous media. We also involved two different alkaline sources (NaOH and HMTA) expanding the investigation further by adopting three different solution phase methods (chemical, ultrasonic and hydrothermal). Furthermore, the present article embarks on the following questions: what effect do different salt precursors of zinc have with changing ratio in defining the final morphology? How does changing the reaction conditions with changing different salts ratio affects the final morphology? After these investigations have been achieved, we embark on one more interesting objective, that is: with different morphology thus obtained which nanostructure is imperative for exploring the full potential as a photocatalyst? Previous studies showed that photocatalysis is an important application of ZnO nanostructures for environmental pollution remediation.^10,33 Low dimensional nanostructures have higher tendency to promote the photo-generated charge carriers due to an increased delocalization of electrons.^33–35 Therefore, the question of different nanostructures based photo-catalysis performance becomes interesting.

Experimental

Chemical and reagents

All of the analytical-grade reagents were purchased commercially and used as received. One molar aqueous solution of zinc acetate dihydrate (98% purity, Sigma Aldrich), zinc nitrate hexahydrate (98% purity, Sigma Aldrich), HMTA hexamethylenetetramine (99.99% purity, Sigma Aldrich) and NaOH (97% Sigma Aldrich) were prepared in stock, separately.

Synthesis of zinc oxide nanostructures

Three different methodologies were adopted to fabricate ZnO nanostructures viz. chemical method, hydrothermal method and ultrasonic method. For each mentioned methods we performed two sets of reactions, one in presence of NaOH and second in presence of HMTA. To avoid the confusion in discussion of the results these reactions were named as ChemR^NaOH and ChemR^HMTA (for chemical method), HydroR^NaOH and HydroR^HMTA (for hydrothermal method) and SonicR^NaOH and SonicR^HMTA (for ultrasonic method). Moreover, for all the above reactions (viz. ChemR^NaOH, ChemR^HMTA, HydroR^NaOH, HydroR^HMTA, SonicR^NaOH and SonicR^HMTA) were executed using zinc acetate and zinc nitrate salt with varying millimolar (mM) ratio of both the salt precursors to elucidate the morphological evolution of ZnO nanostructures. In brief, for ChemR^NaOH reaction different millimolar ratios of (as mentioned above) were dissolved in 100 ml deionized water. Further, pH of each solution was adjusted to 8.0 using 1 M solution of NaOH. Then, reaction mixture was stirred at 80 °C for 5 h. For ChemR^HMTA all reaction conditions were same as of ChemR^NaOH except that 10 mM HMTA was introduced in all the reactions instead of NaOH. Similarly, HydroR^NaOH and HydroR^HMTA reactions were performed with the difference that each prepared solution was transferred to a Teflon-lined autoclave and then maintained at 120 °C for 5 h. Similarly, for SonicR^NaOH and SonicR^HMTA reactions each prepared solution was transferred into an ultrasonic bath at room temperature for 5 h. For all the reactions mentioned above the resultant white precipitate was centrifuged and washed thrice with deionized water and finally with absolute ethanol. Samples were dried at 60 °C for overnight. All the reactions set-up and conditions has been summarized in Table 1.

Table 1 Reaction recipes for the synthesis of ZnO nanostructures and their summarized morphologies

Concentration ratio (mM) zinc acetate : zinc nitrate	Time (h)	Reaction methods and morphology
		Chemical route^b		Ultrasonication^c		Hydrothermal^d
		ChemR^NaOH (pH = 8)	ChemR^HMTA^a	SonicR^NaOH (pH = 8)	SonicR^HMTA	HydroR^NaOH (pH = 8)	HydroR^HMTA
a HMTA (hexamethylene tetramine).b Temperature = 80 °C.c Temperature = room temperature.d Temperature = 120 °C.
0 : 10	5	Nanoflower	Nanoneedle	Nanoflower	Nanorod	Nanosheet-flower	Nanorod
5 : 5	5	Nanoflower	Nanorod	Nanospindle	Nanorod	Irregular nanosheet	Distorted nanorod
10 : 0	5	Spherical aggregate	Nanoneedle	Nanoflower	Nanorod	Spherical aggregate	Nanorod

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Photocatalytic activity

All photocatalysis experiments were carried out under identical conditions using a representative of an anionic azo dye, methyl orange (MO) as test contaminant. A 12 ppm solution of MO was prepared in deionized water in which 40 mg of ZnO nanostructures as catalysts were suspended. The photocatalytic activities were evaluated by the photodegradation of MO under UV light irradiation with a light irradiation system containing an Hg-lamp in dark chamber at room temperature. The distance between dye solution and lamp was kept 5 cm. Prior to illumination of UV light, the solution was magnetically stirred in the dark for 30 min to attain an adsorption–desorption equilibrium. At regular intervals, optical absorption spectra were recorded using an UV-vis spectrophotometer in order to monitor the rate of photodegradation by analyzing the reduction in the absorption intensity of MO at a maximum adsorption peak at 463 nm (λ_max for MO). The time course of the photocatalytic degradation of MO was also determined to understand the photocatalytic stability of the ZnO nanostructures.

Results and discussion

We divided the anion-related (acetate and nitrate ions) ZnO nanostructure evolution into three groups according to the method adopted to synthesize ZnO. Group A contained chemical method, Group B contained ultrasonic method and Group C contained hydrothermal method. For all the three groups reactions has been performed in two sets, one in presence of NaOH and second in presence of HMTA. An alkaline solution is essential for the formation of ZnO nanostructures because normally divalent metal ions do not hydrolyze in acidic environments.³⁶ NaOH is the most widely used alkaline source for ZnO formation. Furthermore, it has been suggested that Na⁺ forms a virtual capping layer around the nanocrystal thus inhibiting the nanocrystal growth.³⁷ Moreover, the concentration of OH⁻ determines the size and growth rate of ZnO nanostructures. Also it is suggested that pH value of solution has a complex relationship with OH⁻ concentration and increasing pH values increases the growth rate however increasing beyond 10 tend to lower this rate.³⁸ Though, it occurs that researchers agree on the role of NaOH, as a matter of fact there is not a general consensus on the precise role of HMTA despite the fact that a number of papers have been published since its first utilization 30 years ago.³⁹ Among so many papers on the role of HMTA, study conducted by Sugunan et al. and McPeak et al. are very interesting as they contradict.^40,41 Sugunan et al. proposed that HMTA preferentially attaches to the nonpolar facets of ZnO acting as a nonpolar chelating agent. This induces anisotropic growth along the c-axis. However, McPeak et al. refuted this mechanism and proposed that the role of HMTA is only to control the saturation index of ZnO through the slow release of OH⁻ ions. Recently, Vincenzina et al. proposed that HMTA plays a dual role participating both as supplier of OH⁻ and as capping agent promoting anisotropic growth.⁴²

Interestingly a lot has been investigated on the role of alkaline source, capping ligand and physical parameters (pH, temperature etc.). However, there is a lack of consolidated and systematic information on the role of anions which are very much present and come along with zinc salts being used as precursors. We investigated how the presence of one anion effects the overall ZnO growth in the presence of another anion. We found that the presence of two anions do effect the ZnO growth and morphology and change with changing reaction conditions, as shown in Table 1. We have restrained ourselves from elucidating reaction equations as to precisely conclude the reaction equation a high-resolution solid-state NMR measurements are needed and seeing the number of reactions involved it is not feasible for the present article. However, we left this question for future investigation and focused on the dynamics of anions and ZnO morphology evolving out of this.

Effect of anions on ZnO crystal formation via simple chemical method (group A)

SEM images of ZnO nanostructures grown via simple chemical method are shown in Fig. 1. The images (a, c and e) represent morphologies grown under ChemR^NaOH and images (b, d and f) shows morphologies obtained under ChemR^HMTA. The images (a, b), (c, d) and (e, f) are corresponding to zinc acetate : zinc nitrate millimolar ratio 0 : 10, 5 : 5, and 10 : 0 respectively. Fig. 1a for zinc acetate : zinc nitrate millimolar ratio 0 : 10 clearly shows the formation of flower shaped ZnO nanostructures, whereas Fig. 1e shows spherical morphology for zinc acetate : zinc nitrate millimolar (mM) ratio of 10 : 0. Interestingly, Fig. 1b, d and f show formation of nanorod with varying aspect ratio.

Fig. 1 SEM images of ZnO nanostructures are grown via simple chemical route under ChemR^NaOH (a, c and e) and ChemR^HMTA (b, d and f). The images (a, b), (c, d) and (e, f) are corresponding to zinc acetate : zinc nitrate ratio 0 : 10, 5 : 5, and 10 : 0 respectively.

During ChemR^NaOH reaction in the presence of only nitrate ligand, as shown in Fig. 1a, ZnO structure evolved into flower shape. Because the initial ZnO seed has multiple polar (0001) surfaces and factually, nonpolar {1010} has lower surface energy and are more stable than polar planes the system tends to minimize the total surface energy, ZnO crystal spines grow along the [0001] direction forming flower shape. Furthermore, studies have shown that in the presence of ligands (for example ascorbate ligand or PEG) which has many oxygen atoms Zn(OH)₄²⁻ growth units were adsorbed by the O atom and the fast crystal growth rate of ZnO resulted in the formation of flower like shape.^11,43 In this case nitrate ion has three oxygen atoms available. It seems that presence of at least three oxygen atoms is a vital parameter for growth of ZnO crystal to flower shape because we observed spherical aggregates or particle growth in the presence of acetate ligand which has only two oxygen atoms (Fig. 1e). Fig. 1e results further suggests that the stabilization effect of the surface by acetate ligands on Zn(OH)₄²⁻ growth units probably depress the straightforward growth of ZnO, leading to spherical particles.⁴⁴ Fig. 1c for zinc acetate : zinc nitrate millimolar ratio 5 : 5 demonstrates the same results as Fig. 1a with only difference in the width of the spines increased. This is probably due to the synergistic behavior of nitrate and acetate ions. However, Fig. 1c demonstrates the dominance of nitrate ions over acetate ions in controlling the final shape of ZnO crystal to flower shape. This result further suggest that using acetate salts will tend to give much smaller particle of uniform diameter compared to nitrate salts due to low decomposition temperature.

Fig. 1b and f for zinc acetate : zinc nitrate millimolar ratio 0 : 10 and 10 : 0 respectively show that ZnO merges as nanoneedles form of varied aspect ratio in the presence of HMTA (ChemR^HMTA reaction). However, for zinc acetate : zinc nitrate millimolar ratio 5 : 5 we observed uniform nanorod. These results suggest that HMTA is the main controlling factor in determining the final shape of ZnO. However, with equal ratio of acetate and nitrate ligand a more controlled growth occurs. Therefore, the role of both anions in forming the homogeneous nanorod cannot be ruled out. These results further supports the observation of Vincenzina et al.⁴² that HMTA plays a double role, acting as pH regulator, and induces the vertical growth of ZnO nanorod along the c-axis.

Effect of anions on ZnO crystal formation via ultrasonication method (group B)

SEM images of ZnO nanostructures grown via ultrasonication method are shown in Fig. 2. The images (a, c and e) represent morphologies grown under SonicR^NaOH and images (b, d and f) shows morphologies obtained under SonicR^HMTA. The images (a, b), (c, d) and (e, f) are corresponding to zinc acetate : zinc nitrate millimolar ratio 0 : 10, 5 : 5, and 10 : 0 respectively. Fig. 2a for zinc acetate : zinc nitrate millimolar ratio 0 : 10 clearly show the formation of flower shaped ZnO nanostructures as were observed in Fig. 2a for ChemR^NaOH reaction. Interestingly for zinc acetate : zinc nitrate millimolar ratio 10 : 0 we still observed the flower shaped ZnO which is different from what we observed in Fig. 1e for ChemR^NaOH reaction. This change of morphology (from agglomerated spherical nanoparticles to flower shaped) can be attributed to the ultrasound radiation. It can be clearly noticed that there is first formation of spindle structure, and then it grows as emerging budding like shape as partially grown flower and finally convert into complete flower. These flowers actually form with a central spindle and a few half spindles pointing outward that appear as the petals of a flower initiating from center of the half spindle (shown in Fig. 2a). The sonication process comprises the formation, growth and collapse of bubbles that can break the chemical bonds of materials in an aqueous medium. Sonication process generates two possibilities: (1) this condition induces the quantum confinement effect (or formation of multi-polar surface) by rupturing the agglomeration and thus increasing the chemical activity. This results in the ZnO crystal spines growth along the [0001] direction forming flower shape. (2) The ultrasonic radiation accelerates the chemical reaction between the surface of the anions and cations.⁴⁵ In particular, the ultrasonic radiation causes cavitations around the surface of the ZnO seed and heating of the precursor solution. As the cavitations collapse near the surface of the ZnO seed, the shock waves cause effective mixing of the precursor solution, resulting in a fast diffusion of hydroxyl ions on the surface of the ZnO seed.⁴⁶ Thus limiting the stabilizing effect of acetate ions as was observed in ChemR^NaOH reaction for Fig. 1e. Though we observed flower shaped ZnO formation when acetate and nitrate ligands perform independently in the presence of NaOH under sonication we observed a totally different spindle like morphology for zinc acetate : zinc nitrate millimolar ratio 5 : 5. This again suggests that presence of two different anionic ligands in the reaction can alter the morphology and has controlling effect on the growth kinetics and direction. The role of two anions in creating different morphology presents a new perspective on formation of new morphology. Moreover, it will be interesting to see how morphology evolves when three or more anions ligands (or zinc salts) are introduced in the reactions adopting different methodology.

Fig. 2 SEM images of ZnO nanostructures are grown via ultrasonication under SonicR^NaOH (a, c and e), SonicR^HMTA (b, d and f). The images (a, b), (c, d) and (e, f) are corresponding to zinc acetate : zinc nitrate ratio 0 : 10, 5 : 5, and 10 : 0 respectively.

During the SonicR^HMTA reaction the role of HMTA is still the main controlling factor giving rod shape morphology as evident from Fig. 2b, d and e. Compared to ChemR^HMTA reaction sonication process has decreased the aspect ratio of the nanorod for all the anion ligand ratios. For zinc acetate : zinc nitrate millimolar ratio 5 : 5 (Fig. 2d) semi-porous nanorod was formed. This shape transformation can be explained in terms of the difference in the growth rates of various crystal faces in the presence of acetate and nitrate ligands.⁴⁷ This may expedite the formation of the ZnO semi hollow nanorod as thermodynamic will dominate the reaction under ultrasonic radiation. Furthermore, we also suspect the role of acetate ligand in making such semi hollow nanorod as we observed the similar structure in HydroR^HMTA reaction (Fig. 3f) for zinc acetate : zinc nitrate millimolar ratio 10 : 0. This might be due to the interplay of lower energy (of acetate ligand) on hollow side and high energy (of nitrate ligand) on polar surfaces.^15,48

Fig. 3 SEM images of ZnO nanostructures are grown via hydrothermal method HydroR^NaOH (a, c and e) and HydroR^HMTA (b, d and f). The images (a, b), (c, d) and (e, f) are corresponding to zinc acetate : zinc nitrate ratio 0 : 10, 5 : 5, and 10 : 0 respectively.

Effect of anions on ZnO crystal formation via hydrothermal method (group C)

SEM images of ZnO nanostructures grown via hydrothermal method are shown in Fig. 3. The images (a, c and e) represent morphologies grown under HydroR^NaOH and images (b, d and f) show morphologies obtained under HydroR^HMTA. The images (a, b), (c, d) and (e, f) are corresponding to zinc acetate : zinc nitrate millimolar ratio 0 : 10, 5 : 5, and 10 : 0 respectively. For molar ratio of zinc acetate : zinc nitrate 0 : 10 (Fig. 3a) nanosheets-assembled hierarchical flower-like ZnO nanostructures formed.⁴⁹

In the presence of both acetate and nitrate ions, nanosheets with some irregularities were obtained. Whereas for molar ratio of zinc acetate : zinc nitrate 10 : 0 (Fig. 3e) spherical aggregates of nanoparticles can be seen. There are reports where ZnO sheets have been fabricated using zinc acetate via hydrothermal or reflux methods.^16,50 However, all these experiments were performed in the presence of capping agent or additive. Liu et al.⁵⁰ has presented a probable mechanism for sheet formation with zinc acetate. They suggest that under alkaline conditions layered basic zinc acetate clusters are generated. These layered clusters induce ZnO nanoparticles to attach to the laminar surface through acetate anions thus forming sheets. However, we noticed that they performed this reaction in ethylene glycol solvent. The role of ethylene glycol cannot be ruled out. This is the reason that probably we didn't see sheet like morphology for molar ratio of zinc acetate : zinc nitrate 10 : 0 (Fig. 3e). Moreover, for zinc acetate : zinc nitrate millimolar ratio 5 : 5 we observed irregular nanosheets (Fig. 1c). We anticipate that under hydrothermal condition the dynamic of acetate and nitrate anions did not worked out well as compared to other cases.

During the HydroR^HMTA reaction for zinc acetate : zinc nitrate millimolar ratio 0 : 10 we observed well crystalline nanorod. The role of HMTA is well documented for fabricated nanorod and also been discussed in previous sections. Interestingly, for zinc acetate : zinc nitrate millimolar ratio 5 : 5 though nanorod was formed but they are not smooth compared to what we observed for Fig. 1d (ChemR^HMTA). Probably under hydrothermal condition the dynamic of both the anions did not worked out well as compared to above cases. However, for zinc acetate : zinc nitrate millimolar ratio 10 : 0 we observed semi hollow nanorod with high aspect ratio. Under HydroR^NaOH probably acetate anions interact with Zn²⁺ on ZnO nanoparticles causing lowering of the surface energy and attaining a metastable intermediate^15,48 similar to what we observed for SonicR^HMTA (Fig. 2d). The only difference is of high aspect ratio which can be interpreted by high thermodynamics under hydrothermal conditions.⁴⁷

Characterization

For XRD, Raman, UV absorption characterization and photocatalytic properties we selected flower shaped ZnO nanostructure from Group A (Fig. 1c), spindle shaped ZnO from Group B (Fig. 2c) for zinc acetate : zinc nitrate millimolar ratio 5 : 5.† As evident nanorod are formed in each group with varying aspect ratio. Moreover, for zinc acetate : zinc nitrate millimolar ratio 5 : 5 ratio we didn't observe sample homogeneity therefore we chose sample (Fig. 3b) as a representative for nanorod based on high aspect ratio for photocatalytic performance.

Fig. 4a represents XRD patterns of ZnO for each group. All the diffraction peaks are in good agreement with standard data JCPDS no. 89-1397. It can be clearly noticed that pattern of all peaks showed a single phase nature with a hexagonal wurtzite structure of ZnO.

Fig. 4 (a) and (b) XRD and Raman spectra of different ZnO nanostructures (nanoflower, nanorod, nanospindle), respectively; (c) the plot of (αhν)² versus hν for ZnO nanostructures and inset shows the corresponding UV-vis absorption spectra; (d) UV-visible absorbance spectra of photodegradation of MO in the presence of ZnO nanostructures.

Fig. 4b represents the Raman spectrum of nanoflower (Group A), nanospindle (Group B) and nanorod (Group c). The spectrum shows conventional vibrational modes of E₂(high)–E₂(low), A₁(TO), E₁(TO), E₂(high) and longitudinal optical mode E₁(LO). The nonpolar Raman active modes E₂(high) and E₂(low) have been assigned to oxygen atoms or vibrations and Zn sub-lattice.⁵¹ The Raman peaks centered at 381 cm⁻¹, 408 cm⁻¹, 437 cm⁻¹, and 577 cm⁻¹ were attributed to the A₁(TO), E₁(TO), E₂(high) and E₁(LO) Raman modes of the wurtzite phase of ZnO, respectively.³ It can be clearly observed from Fig. 4b that the intensity of E₂(high) becomes weaker on shifting from nanorod to nanoflower nanostructures, which shows the more intrinsic defects associated with O atoms. The high intensity of E₂(high) mode for nanorod nanostructures occurred due to increase in supersaturation.⁵² However, there was no change noticed in the intensity of E₁(LO) mode for all the samples. It is noteworthy that normally forbidden E₁(LO) mode has activate for these nanostructures, which indicate pronounced enhancement of the surface activity compared with that of the bulk crystals with large surface area and high surface roughness.

Fig. 4c represents the optical absorption spectra for nanoflower (Group A), nanospindle (Group B) and nanorod (Group C) respectively. The absorption spectrum for each group representative is shown in inset Fig. 4c. The absorption spectrum of nanoflower is shifted to shorter wavelength but for nanospindle and nanorod, the absorption band is shifted towards higher wavelength with high intensity. These results indicated that the aspect ratio of the ZnO nanostructures leads to the corrections in the energy bands causing the band gap tuning. The band gap energy of different nanostructures of ZnO was calculated using the following formula:⁵³

(αhν) = A(hν − E_g)^1/2 (1)

where α, h, ν, E_g and A are absorption coefficient, Planck constant, light frequency, band gap energy and a constant, respectively. The plot of (αhν)² versus (hν) for nanoflower, nanospindle and nanorod are shown in Fig. 4c. The band gap energy (E_g) values of different nanostructures were calculated from the intercept of (αhν)² versus (hν) curves by extra-plotting tangent to them. The band gap of nanorod found to be 3.43 eV. However, the band gap decreased to 3.42 eV and 3.39 eV for nanospindle and nanoflower respectively (see Fig. 4c).

Photocatalytic performance

Photocatalytic properties for nanoflower, nanospindle and nanorod were evaluated by the decomposition of an anionic dye methyl orange (MO). The characteristic adsorption of MO was chosen at ∼463 nm to monitor the photocatalytic degradation process.

The time dependent absorption spectra (Fig. 4d) of MO aqueous solution were recorded during UV light irradiation in the presence of ZnO nanorod. A very slight decrease in the MO solution concentration was noticed, when the solutions were stirred for 30 minutes in dark to establish adsorption–desorption equilibrium of MO on the sample surface. It indicates that limited adsorption of MO is occurred on the samples surface after the adsorption–desorption equilibrium is set up. As a control, the absorbance peak of the MO solution was monitored without photocatalyst under UV light illumination. Insignificant change in the absorbance peak of MO under these conditions was observed demonstrating that there is no noticeable loss of MO concentration (Fig. 5a). When ZnO nanorod placed in an MO solution, the maximum absorption of the solution was found to decrease with irradiation time and color disappeared almost completely in 180 minutes. The photocatalytic performance ∼50% was achieved within 60 minutes irradiation of UV light, whereas after 180 minutes irradiation of UV light, ∼89% photocatalytic degradation of MO was occurred over ZnO nanorod. Further, in order to evaluate the relationship between photocatalysis and morphology, nanoflower and nanospindle were used as the contrast examples. The Fig. 5a shows the relative concentration (C/C₀) of MO versus irradiation time plots for various nanostructures, where C represents concentration of MO at the irradiation time (t) and C₀ is the concentration of the MO before irradiation. It was observed that concentration of the MO solution decrease with irradiation time for all ZnO nanostructures, indicating that all of the nanostructures show UV-light photocatalytic properties in the degradation of MO. The higher photocatalytic performance of ∼89% was achieved for nanorod within 180 minutes of photo-irradiation, whereas only ∼80% and ∼69% degradation capacity of MO was noticed with nanospindle and nanoflower, respectively (Fig. 5a). Therefore, it was observed that ZnO nanorod exhibited significantly higher photocatalytic activity for the photodegradation of MO than that of nanospindle and nanoflower.

Fig. 5 (a) Photocatalytic activity for the degradation of MO solution by using different ZnO nanostructures photocatalyst under UV light irradiation; (b) the kinetic relationship of ln(C/C₀) vs. irradiation time curves; (c) the stability of ZnO nanorod for photodegradation of MO; (d) schematic representation of photocatalytic process in the presence of ZnO nanostructures.

To understand the kinetics behavior of these photocatalyst, the experimental data were fitted by following first-order reaction:

ln(C₀/C) = Kt (2)

where K is the apparent rate constant (min⁻¹), t is the reaction time, C₀ and C are the concentration of MO dye at 0 and t, respectively. There is a linear relationship between ln(C₀/C) and the irradiation time for MO degradation as shown in Fig. 5b. As evident from Fig. 5b, the photocatalytic degradation curves in all cases fit well with pseudo-first-order kinetics. The apparent reaction rate constant K for the photodegradation of MO was found to be 1.226 × 10⁻² min⁻¹, 0.894 × 10⁻² min⁻¹ and 0.6689 × 10⁻² min⁻¹ for nanorod, nanospindle and nanoflower, respectively. The photocatalytic activity is directly related to the value of reaction rate constant. We observed that the reaction rate constant for nanorod is higher compared to nanoflower and nanospindle. This demonstrates the higher photocatalytic activities of ZnO nanorod. We found that the photocatalytic performance of the ZnO nanorod obtained in this study is comparatively better (even at low concentration) than other published reports.^54–56 This enhanced performance even at low concentration can be attributed to the absence of any capping ligand and or template in our reactions.

Stability and reusability of photocatalyst is important to reduce overall cost of photocatalysis process and strengthens practical utility. To evaluate the photocatalytic stability of the ZnO nanostructure photocatalysts, stability tests were conducted by executing recycling reactions five times for the photodegradation of MO under UV-vis light irradiation for nanorod. As shown in Fig. 5c, no noticeable loss of the photocatalytic activity of ZnO nanorod was observed for MO degradation reaction after repeated cycles. It indicates that ZnO nanorod have good stability and reusability performance and might be potent material for practical photocatalysis applications.

The UV light photodegradation mechanism of ZnO nanostructure photocatalysis is schematically illustrated in Fig. 5d. In the present work, ZnO nanorod show higher photocatalytic activity as compared to nanospindle and nanoflower. According to the ref. 57–59 several factor determine the photocatalytic activity of ZnO such as, active morphological surfaces, delay in recombination of electron–hole pairs and enhancement in adsorption of molecules on space charge region along the longitudinal direction of the nanorod. The enhanced oxygen adsorption on the surface of ZnO nanorod decreased the recombination probability of carriers by accepting photogenerated holes, which in turns forms reactive oxygen species and finally enhance the photocatalytic activity.⁶⁰ Probably, the formation rate of hydroxyl radical along the longitudinal surface of ZnO nanorod is high compared to nanospindle and nanoflower, owing to higher photocatalytic efficiency of nanorod.

Conclusions

In summary, we have studied the anion ligand effect of acetate and nitrate ions on the formation of different morphology of ZnO crystals in presence of NaOH and HMTA. We have varied the concentration of zinc salt precursors as well as adopted three different methods of synthesis (chemical, ultrasonic and hydrothermal) to study their effects on the size/shape of the formed ZnO nanostructures. Results for each group where NaOH was used, suggests that morphology evolved to nanoflower, nanospindle, spherical aggregates and nanosheet-assembled flower-like ZnO nanostructures, which was ascribed to the process parameter and varying ratio of anions dependent tuning of the nanostructures. On the other hand, for each group where HMTA was used, morphology evolved in nanorod and semi-hollow nanorod of different aspect ratio, clearly demonstrating the dominance of HMTA. It is suggested that the presence of anions coming from zinc salts is a crucial factor for the morphologies of the obtained ZnO nanostructures. Such knowledge would allow laboratories and industries to rationally design experiments for anticipated morphology simply by selecting the appropriate zinc salt precursors. Furthermore, it is also suggested that role of other precursors salts in synergy will lead to formation of varied nanostructures and can be elaborated to other metal oxides of interest for modification. The photocatalytic properties of ZnO nanostructures (nanoflower, nanospindle and nanorod) towards MO under UV irradiation were found to be alluring. The enhanced percentage degradation of an anionic dye methyl orange with nanorod was found to be ∼89% within three hours at very low concentration of ZnO nanorod (40 mg). Results show that rod like ZnO nanostructures are catalytically more reactive than nanoflower and nanospindle. These results also demonstrate that ZnO nanorod has good stability and reusability performance as photocatalyst.

Acknowledgements

The authors would like to thanks the National Research Foundation (NRF), South Africa and University of Johannesburg for financial support. The authors thank Dr Ajay Mishra (Department of Applied Chemistry, University of Johannesburg) for useful discussions and suggestions.

Notes and references

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Footnote

† Morphologies of the ZnO nanostructures were studied using field emission scanning electron microscopy (TESCAN, VEGA SEM) under a 20 kV electron acceleration voltage coupled with an energy dispersive X-ray spectrum (EDS). The phase purity and crystal structure of the as prepared materials were investigated by X-ray powder diffraction (XRD) using Rigaku Ultima IV, X-ray diffractometer at 40 kV and 40 mA with Cu-Kα radiation of the 0.15418 nm. The diffraction data were recorded for 2θ angles between 5° and 80° (step size: 0.02°, step time: 1 min). All peak positions were indexed according to standard files to identify the crystalline phase. Raman spectrums were recorded in order to perform the phonon vibrational study of the ZnO nanostructures using micro-Raman spectrometer (NRS-3100) with a 532 nm solid-state primary laser as an excitation source in the backscattering configuration at room temperature. UV absorbance measurements were taken using Shimadzu UV-2450, UV-vis spectrophotometer in the range of 200–800 nm.

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