Morphological evolution of ZnO nanostructures and their aspect ratio-induced enhancement in photocatalytic properties

Abstract

This work presented controllable growth of ZnO nanostructures with different aspect ratios by the microwave irradiation method and investigated the photocatalytic degradation of methyl red (MR). X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) measurements showed that all ZnO nanostructures were of a hexagonal phase structure. It was revealed by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images that the morphology of ZnO can be effectively controlled as sheet-like, rod-like, brush-like, flower-like, prism-like, and pyramid-like only by changing the molar ratio (zinc acetate: KOH) and reaction time. With the increase of molar ratio and reaction time, modification in the E₂(high) and E₁(LO) Raman modes was observed. The energy band gap was found to be tuned by the aspect ratio of ZnO nanostructures. Photoluminescence spectroscopy revealed the low-intensity NBE emission and high and broad defect-related emission for high aspect ratio (14) nanorods. BET surface area porosity analysis confirmed the presence of a mesoporous network in all the nanostructures, showed high surface area and a uniform pore-size distribution for high aspect ratio nanorods. A terephthalic acid assay study confirmed the formation of hydroxyl radicals (OH) in MR dye solution treated with a ZnO nanostructures photocatalyst. The photodegradation of MR under UV light irradiation showed that ZnO nanorods with a high aspect ratio of ∼14 showed superior photodegradation (∼98% degradation of MR within 60 min) than that of the lower aspect ratio nanostructures. The apparent reaction rate constant for high aspect ratio (14) nanorods was higher than that of the lower aspect ratio nanostructures. The enhancement in photocatalytic performance could be due to the high surface area and enhanced charge separation and transfer efficiency of photoinduced charge carriers in the high aspect ratio nanorods.

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Introduction

With a wide and direct band gap of 3.37 eV with a large exciton binding energy of 60 meV, ZnO, which has been extensively used in field emission,¹ short-wavelength optoelectronics,² electroacoustic transducers,³ gas sensors,⁴ transparent conducting coating materials,⁵ piezoelectric devices,⁶ and photocatalysis,⁷ has proven to be a unique functional material during the past several years. Recently, the room temperature ferromagnetism (RTFM) behaviour with a Curie temperature (T_c) value above room temperature in pure ZnO has been reported by various groups,^8,9 which has attracted enormous interest from researchers for possible application in spintronic devices.

Nowadays, increased industrialization poses a threat to human health in the form of environmental pollution. Environmental remediation of organic pollutants by photocatalytic degradation using wide bandgap semiconductors has attracted considerable attention in recent times.^10–13 Furthermore, these materials in the nanostructured form exhibit enhanced properties, and therefore have potential applications such as in the fabrication of nanodevices. Various semiconductors, including ZnO and TiO₂, are being used in photocatalytic application because of their low cost, high photosensitivity, non-toxicity and environmentally friendly nature.^14–16 ZnO has been established to be a more efficient photocatalyst than TiO₂ because of its high surface reactivity owing to its large number of active surface defect states. It has also been shown that ZnO has high reaction and mineralization rates¹⁷ because of its more efficient hydroxyl-ion production.¹⁸ When ZnO is irradiated with UV light, electrons are excited from the valence band to the conduction band and electron–hole pairs are created. These electron–hole pairs activate the surrounding chemical species, and then promote the chemical reactions.^19,20 The efficiency of ZnO as a photocatalyst has been limited due to the recombination of photogenerated charge carriers, which is typically faster than the rate of production of reactive oxidation species. Thus, improving the charge separation efficiency needs a rational design of the photocatalyst structure. As photocatalytic activity of metal-oxide nanostructures depends on structures, including the morphology, surface area and surface defects, one-dimensional (1D) nanostructures such as nanorods, nanowires, and nanotubes are ideal candidates for the application in photocatalysis since they offer a larger surface-to-volume ratio than nanoparticulate thin films.²¹ 1D nanostructures with high aspect ratios have been shown to promote the separation of photogenerated charge carriers due to an increased delocalization of electrons in the 1D nanostructures, which is important for improving the efficiency of the photocatalysts. Therefore, it is very important to develop a shape and size-controlled synthesis method for ZnO nanostructures. In recent years, rapid progress has been made in the preparation of ZnO nanostructures, including nanowires,²² nanosheets,²³ nanotubes²⁴ and nanoflowers.²⁵ Numerous approaches were employed to fabricate ZnO nanostructures, which can be usually classified into two categories, including physical methods and the solution phase route. Physical methods include thermal evaporation, chemical vapour deposition and metal organic vapour phase epitaxy.^26–28 However, the physical process requires high temperature, a complex procedure, and sophisticated equipment, which make the procedure expensive, and thereby restricts possibilities of applications. In contrast, the solution phase approaches, including microemulsion, solvothermal, hydrothermal, precipitation, self-assembly and template-assisted sol–gel processes, can allow for the growth of ZnO nanostructures at much lower temperatures (<200 °C) and large-scale production.^29–34 Among them, the hydrothermal technique is a relatively simple, low temperature process, but it requires longer reaction time (from a few hours to several days). Thus, a simple and fast route for the synthesis of ZnO materials under ambient conditions to fulfil economic and industrial requirements is still required. Recently, the synthesis of nanostructured materials via microwave irradiation has been introduced. In comparison with conventional heating, microwave heating has unique effects such as rapid and homogeneous volumetric heating, high reaction rate, short reaction time, enhanced reaction selectivity, energy savings and low cost.⁹ The effect of some synthesis parameters on the morphology of ZnO nanostructures has been studied. Vayssieres et al. reported that the size of the ZnO nanostructures was decreased, and the aspect ratio was increased with a decrease in the solution concentration.²⁹ The reaction time in the solution reaction is another important parameter reflecting the thermodynamic and/or kinetics processes because of its close relation to the change of concentration and reaction rate in the reaction process.³⁵ Recently, work has been reported on the growth of ZnO nanorods using a hydrothermal method and their photocatalytic activity dependent on aspect ratio.³⁶ However, the growth of ZnO nanorods required hydrothermal treatment for 6–48 h in order to obtain nanorods with different aspect ratios. Similarly, in another report,³⁷ photocatalytic properties of ZnO nanorods were studied. The nanorods were prepared by autoclave conditions at 120 °C for 2–6 h. These reports showed that the preparation method used for ZnO nanostructures is both time- and energy-consuming and does not fulfil the economic and industrial requirements of ZnO nanostructures-based photocatalysts. Nevertheless, the morphological evolution of ZnO nanostructures from nanosheets to nanorods, nanobrushes, nanoflowers, nanoprisms, and nanopyramids with high aspect ratios and the enhancement of photocatalytic properties by aspect ratio have not been reported to the best of our knowledge.

In this paper, we described the growth of ZnO with a diversity of well-defined morphologies with different aspect ratios via the simple microwave-assisted solution route within a few minutes, and the photocatalytic activity of these nanostructures were investigated by measuring the degradation of MR. The experimental results showed that the as-obtained ZnO nanorods with high aspect ratios exhibited excellent photocatalytic activity compared with the low aspect ratio nanostructures. This study will provide the platform to tune the morphology of ZnO nanostructures using a microwave irradiation method, in which various dimensional nanostructures such as 1D, 2D, and 3D can be prepared simultaneously for future applications. Moreover, the detailed structural, morphological, and optical characterization of ZnO nanostructures will provide helpful information on the structure–property relationship. We envisage that this work will help to develop new photocatalysts based on high aspect ratio ZnO nanorods for practical application due to the excellent photocatalytic behaviour, and this simple and one-step method can suitably be scaled up for large-scale synthesis.

Experimental section

All the reagents involved in the experiments were of analytical grade and utilized as-received without further purification. Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O; 99.999%), and potassium hydroxide (KOH; 99.99%) were purchased from Sigma Aldrich. The synthesis was carried out in a domestic microwave oven (Samsung, 750 W). In a typical experiment, different molar ratios (Zn_M : KOH_M; 1 : 10, 1 : 15, 1 : 20 and 1 : 25) of Zn(CH₃COO)₂·2H₂O to KOH were dissolved in 100 mL distilled water in a round-bottomed flask, and then placed in a domestic microwave oven. Microwave irradiation proceeded at 300 W (irradiation 12 s, stop 10 s) for different time intervals (10, 20, 30, 40 and 60 min). After microwave processing, the solution was cooled to room temperature. The resultant precipitate was separated by centrifugation, and then washed several times with deionized water and absolute ethanol, and then dried in an oven at 80 °C for 24 h.

Methyl red (MR) was used as the test contaminant for the photocatalysis experiments. A 10 μM aqueous solution of MR was prepared in which 10 mg of ZnO catalysts were suspended. Before illumination, the suspension was magnetically stirred in the dark for 30 min to attain an adsorption–desorption equilibrium. A high-pressure mercury lamp was used as a light source. The beaker containing MR and ZnO catalysts was then placed under the UV light for the photodegradation of MR. At certain time intervals, 5 mL of aliquot was sampled and centrifuged immediately to remove the photocatalyst particles. Then, the supernatant solution was analyzed by monitoring the maximum absorption peak at 428 nm (λ_max for MR) using a UV-vis spectrophotometer (Agilent 8453). In order to estimate the photocatalytic stability of the ZnO nanostructures, the time courses of the photocatalytic degradation of MR using photocatalyst were conducted.

The phase purity of the as-obtained product was characterized by X-ray diffraction using a Phillips X'pert (MPD-3040) X-ray diffractometer with Cu Kα radiations (λ = 1.5406 Å) operated at a voltage of 40 kV and a current of 30 mA. Field-emission scanning electron microscopy (FESEM) images were obtained using a MIRA II LMH microscope. The elemental composition of ZnO was determined by energy dispersive X-ray spectroscopy (EDX, Inca Oxford, attached to the FESEM). Transmission electron microscopy (TEM) micrographs, selected area electron diffraction (SAED) pattern and high-resolution transmission electron microscopy (HRTEM) images were obtained using a FE-TEM (JEOL/JEM-2100F version) operated at 200 kV. To prepare samples for TEM examination, ZnO nanostructures were dispersed in an ethanol solution, followed by an ultrasonic treatment for 10 min. A minute drop of ZnO suspension was cast onto a carbon-coated copper grid with subsequent drying in air before transferring it to the microscope. In order to perform the phonon vibrational study of the ZnO nanostructures, micro-Raman spectrometer (NRS-3100) was used with a 532 nm solid-state primary laser as an excitation source in the backscattering configuration at room temperature. Room temperature optical absorption spectra were recorded in the range of 200–800 nm using a UV-vis spectrophotometer (Agilent-8453). The Brunauer–Emmett–Teller (BET) specific surface area measurements were carried out by nitrogen adsorption using an Autosorb®-1 (Quantachrome Instruments, Boynton Beach, FL, USA). The photoluminescence measurements were carried out using a luminescence spectrometer (JASCO, FP-6500) with a xenon lamp as the excitation source at room temperature. The excitation wavelength used was 325 nm. Electrochemical and photoelectrochemical measurements were performed on standard three-electrode quartz cells containing 0.2 M sodium sulphate (Na₂SO₄) of electrolyte solution and measured by an electrochemical system (Versa STAT 3, Princeton Research, America). The as-fabricated ZnO nanostructures with various aspect ratios served as the working electrode. Platinum wire was used as the counter electrode, and Ag/AgCl (saturated with KCl) was used as the reference electrode. The photoresponses of the photocatalysts as UV light on and off were measured at 0.0 V. Electrochemical impedance spectra (EIS) were measured at 0.0 V over the frequency range of 1–5000 Hz.

The generation of hydroxyl radicals (˙OH) on the surface of photo-illuminated ZnO nanostructures were detected by the PL technique using terephthalic acid (TA) as a probe molecule. Terephthalic acid readily reacts with ˙OH to produce a highly fluorescent product, 2-hydroxyterephthalic acid, which exhibits a PL signal at 425 nm. Experimental procedures were similar to the measurement of photocatalytic activity, except that the MR aqueous solution was replaced by the 5 × 10⁻⁴ M terephthalic acid with a concentration of 2 × 10⁻³ M NaOH solution. The generated 2-hydroxy terephthalic acid (TAOH) was measured by a JASCO FP-6500 fluorescence spectrophotometer using an excitation wavelength of 315 nm.

Results and discussion

Fig. 1 shows typical XRD patterns of ZnO nanostructures prepared at different molar ratios of zinc acetate and KOH after 20 min of microwave irradiation. All the diffraction peaks can be readily indexed using POWDER-X software to the hexagonal wurtzite ZnO phase in the standard data (JCPDS 89-1397) with calculated lattice parameters a and c of 0.324 and 0.520 nm, respectively, and it can be clearly seen from the patterns that all of the samples showed a single-phase nature with a wurtzite structure except for the sample prepared at the 1 : 10 molar ratio. Zn(OH)₂ phases are present in the sample for the 1 : 10 molar ratio, which is in good agreement with the standard data JCPDS no. 38-0385 of Zn(OH)₂. With the increase in molar ratio to 1 : 15 and 1 : 20, the diffraction peaks of Zn(OH)₂ disappeared completely, and only the peaks of ZnO were materialized. In addition, the intensity of the peaks increased with an increase in the molar ratio. It can be clearly seen from Table 1 that with the increase in molar ratio, the FWHM corresponds to decrease in (101) peak, which revealed the increase in the crystallinity. The crystallite sizes of the prepared samples estimated from X-ray line broadening of (101) peak using Scherrer's equation³⁸ were found to be 25 nm, 28 nm, and 34 nm for samples prepared at molar ratios of 1 : 10, 1 : 15, and 1 : 20, respectively.

Fig. 1 XRD pattern of ZnO nanostructures for different molar ratios of zinc acetate and KOH.

Table 1 Change in FWHM ((101) peak) with the molar ratio

Molar ratio	FWHM (degree)
1 : 10	0.33
1 : 15	0.29
1 : 20	0.24

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Fig. 2 shows the XRD patterns of ZnO nanostructures prepared at the 1 : 20 molar ratio of zinc acetate and KOH for different reaction times. All the samples showed a single-phase nature with a hexagonal wurtzite structure. No characteristic peaks of any impurities are detected, which demonstrates that all of the samples have high phase purity, and the sharpness of the peaks indicates the high crystallinity of the as-prepared ZnO. The intensity and FWHM (see Table 2) of the diffraction peaks were found to increase and decrease, respectively with an increase in reaction time, which indicates that the increase in the reaction time improves the crystalline nature of prepared ZnO nanostructures. The crystallite sizes of these nanostructures estimated from X-ray line broadening of (101) peak using Scherrer's equation³⁸ were observed 26 nm, 29 nm, 32 nm, and 39 nm for the samples prepared at 10 min, 20 min, 30 min, and 60 min, respectively. Moreover, no remarkable shift of the diffraction peaks among all of the samples reveals that lattice expansion and/or shrinkage should be neglected.

Fig. 2 XRD pattern of ZnO nanostructures for different reaction times.

Table 2 Change in FWHM (101 peak) with the reaction time

Reaction time (min)	FWHM (degree)
10	0.31
20	0.28
30	0.26
60	0.21

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Fig. 3 depicts the room-temperature Raman spectra of ZnO nanostructures at different molar ratios of zinc acetate and KOH for 20 min. The Raman spectrum ZnO nanostructures show conventional vibration modes³⁹ of E₂(high) − E₂(low), A₁(TO), E₁(TO), E₂(high) and longitudinal optical E₁(LO), centered at 332 cm⁻¹, 381 cm⁻¹, 408 cm⁻¹, 439 cm⁻¹, and 580 cm⁻¹ respectively, as shown in Fig. 3. However, for the sample prepared at the 1 : 10 molar ratio, an extra mode appeared at 522 cm⁻¹, which may be due to the Zn(OH)₂ intermediate phase. With the increase in molar ratio from 1 : 10 to 1 : 15 and then 1 : 20, this mode disappeared, indicating the complete conversion of Zn(OH)₂ into ZnO. These results are in good agreement with the XRD analysis. It can be clearly seen from Fig. 3 that the intensity of E₂(high) mode increases with the increase in molar ratio, which is due to the fact that an increase in supersaturation resulted in the increase in the intensity of Raman modes.⁴⁰ However, the intensity of E₁(LO) mode remained the same for all samples.

Fig. 3 Raman spectra of ZnO nanostructures for different molar ratios of zinc acetate and KOH.

The Raman spectrum of ZnO nanostructures prepared at the 1 : 20 molar ratio for different reaction times is shown in Fig. 4. The peak intensity related to the E₂(high) mode and E₁(LO) mode increases. The E₁(LO) mode is well known, and it is related to defects such as oxygen vacancies and zinc interstitials in ZnO;⁴¹ thus, it may be expected that the change in the E₁(LO) mode for ZnO nanostructures may be due to defects introduced by increasing the reaction time with minimum defects exhibited in 20 min irradiated sample. The large surface area and high surface roughness indicate pronounced enhancement of the surface activity compared with that of the bulk crystals, and may activate the normally forbidden E₁(LO) mode. Another noteworthy feature is that the E₂(high) mode is shifted to a lower wavenumber, which may be mainly due to the distortion of the lattice and the defects.⁴²

Fig. 4 Raman spectra of ZnO nanostructures for different reaction times.

The morphological evolution of ZnO nanostructures in our microwave-irradiation method is achieved by changing the reaction time and molar ratio of zinc acetate and KOH.

Fig. 5 shows FESEM images of the basic ZnO nanostructures with different morphologies synthesized from different molar ratios of zinc acetate and KOH. The summarized morphologies and reaction conditions are illustrated in Table 3. Fig. 5(a) depicts morphology of ZnO without microwave process, agglomerated particles with diameter ranging from 90–150 nm can be seen. It is obvious that the size of the nanostructures increases with an increase in the molar ratio. When the molar ratio of the solution is low (1 : 10), the ZnO nanosheets with lateral dimensions of ∼1 μm and thicknesses of ∼30 nm (aspect ratio ∼33) were formed (Fig. 5(b)). With an increase in the molar ratio to 1 : 15 (Fig. 5(c)), ZnO NRs (length ∼1 μm and diameter ∼90–100 nm; aspect ratio ∼11) connected with nanosheets (lateral dimension ∼1.5 μm, thickness ∼40–50 nm; aspect ratio ∼37) were obtained. These nanosheets grow in different directions, and each nanosheet consists of numerous ZnO nanorods. When the molar ratio is increased to 1 : 20, the nanorods grow larger and exhibit improved crystal perfection, which is shown in Fig. 5(d). These nanorods have a regular hexagonal shape and a flat end with a diameter of ∼150 nm and a length of ∼2 μm (aspect ratio ∼14). Brush-like morphology consisting of nanorods with a diameter of ∼90 nm and a length of ∼4 μm (aspect ratio ∼20) was formed in the molar ratio of 1 : 25 (Fig. 5(e)). These results demonstrate that the molar ratio plays a crucial role in the fabrication of ZnO morphology.

Fig. 5 FESEM images of ZnO (a) without microwave process (b) 1 : 10, (c) 1 : 15, (d) 1 : 20 and (e) 1 : 25 molar ratio of zinc acetate and KOH. (f) EDX spectrum of ZnO nanostructures.

Table 3 Summarized morphologies and reaction conditions

KOH concentration (M)	Molar ratio Zn^2+ : OH^−	Reaction time (min)	Morphology	Aspect ratio
0.5	1 : 10	20	Nanosheets	33
0.75	1 : 15	20	Nanosheets; nanorods	37; 11
1	1 : 20	20	Nanorods	14
1.25	1 : 25	20	Nanorods (brush-like)	20
1	1 : 20	10	Nanorods (flower-like)	10
1	1 : 20	20	Nanorods	14
1	1 : 20	30	Nanorods (flower-like)	3.75
1	1 : 20	40	Nanorods (prism-like)	4.5
1	1 : 20	60	Nanorods (pyramid-like)	5

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The elemental composition of the ZnO nanostructures was investigated using energy dispersive X-ray spectroscopy (EDX). The EDX plot as shown in Fig. 5(f) depicts peaks of Zn and O for all of the ZnO nanostructures, which indicates that the ZnO structures are composed of only Zn and O. No evidence of other impurities was found, which also confirms the high purity of the ZnO nanostructures.

In order to further investigate the morphological evolution of ZnO, by keeping the molar ratio fixed at 1 : 20, reaction time-dependent experiments were carried out. The ZnO sample reacted for 10 min exhibited flower-like morphology with several symmetric petals consisting of a number of aggregative nanorods with lengths ∼2.5 μm and diameters of ∼250 nm (aspect ratio ∼10) (Fig. 6(a)). It can be clearly seen that the branches of the single product grow in different directions and are composed of symmetric nanorods extending radially from the centre. With the prolongation of the reaction time to 20 min, the shape of the ZnO nanorods changed from sword-like sharp tips to flat ends having lengths of ∼2 μm and diameters of ∼150 nm (aspect ratio ∼14) (Fig. 6(b)). Interestingly, the shape of the ZnO nanorods (flat end) again changed from hexagonal to tapered with lengths and diameters of ∼1.5 μm, ∼400 nm (aspect ratio ∼3.75), respectively, and started to separate from the bunch of flowers as the reaction time was increased to 30 min (Fig. 6(c)). When the reaction time is further increased to 40 min, the crystal shape developed into a prism-like morphology having a length and diameter of ∼2.2 μm, ∼500 nm (aspect ratio ∼4.5), respectively (Fig. 6(d)). Finally, after increasing the reaction time to 60 min, well-dispersed flat-tip semi-porous nanorods (pyramid-like) with lengths of ∼3 μm and diameters of ∼600 nm (aspect ratio ∼5) were formed as shown in Fig. 6(e). The semi-porous nature of the nanorods can be clearly seen from the high-magnification image in Fig. 6(f). This shape transformation can be explained in terms of the differences in the growth rates of various crystal faces. In general, the crystal plane with the highest growth rate disappears quickly, thus making the relative growth rate crucial to determine the morphology of the crystal.

Fig. 6 FESEM images of ZnO nanorods of (a) 10 min (b) 20 min, (c) 30 min, (d) 40 min, and (e) 60 min and (f) high-magnification image of 60 min microwave irradiation.

Additional morphological characterization is achieved through the TEM, as shown in Fig. 7. Fig. 7(a) shows a typical TEM image of a single ZnO nanorod prepared in 20 min in which a well-developed hexagonal phase with a flat end can be seen clearly. The TEM image of single nanorod prepared for 30 min is shown in Fig. 7(c). From this TEM image, it can be seen that the end of the ZnO nanorod is a sharp sword-like tip. Fig. 7(e) shows the TEM image of a single ZnO semi-hollow nanorod (pyramid-like). The atomic structures of ZnO nanostructures were characterized by high-resolution transmission electron microscopy (HRTEM). The HRTEM images in the insets of Fig. 7(a), (c) and (e) show that all of the ZnO nanorods are highly crystalline with a lattice spacing of about 0.26 nm, corresponding to the distance between the (002) planes in the ZnO crystal lattice. Moreover, the selected area electron diffraction (SAED) patterns of the three ZnO structures (Fig. 7(b), (d) and (f)) are indexed to hexagonal ZnO, indicating that ZnO nanorods are single crystalline and have growth along the [001] direction.

Fig. 7 TEM images of ZnO nanorods for (a) 20 min, (c) 30 min and (e) 60 min, respectively. Corresponding SAED patterns (b), (d) and (f), and HRTEM images are shown in the insets of (a), (c) and (e), respectively.

A schematic representation of the possible growth mechanism of ZnO nanostructures is shown in Fig. 8. It is well known that ZnO is a positively Zn²⁺-terminated (001) and negatively O₂²⁻-terminated (00−1) polar surfaces, which induces a net dipole moment along the c axis.⁴³ The growth habit of ZnO under solution conditions has been widely investigated, and it is generally accepted that the final morphology of ZnO crystals is related to both their intrinsic crystal structure and external factors.^44,45 The crystal formation process is divided into two stages of nucleation and growth. The overall reaction for the growth of ZnO nanocrystals may be expressed as follows:

Zn(CH₃COO)₂ + 2KOH → Zn(OH)₂ + 2CH₃COOK (1)

Zn²⁺ + 2OH⁻ → Zn(OH)₂ (2)

Zn(OH)₂ + 2OH⁻ → [Zn(OH)₄]²⁻ (3)

[Zn(OH)₄]²⁻ → ZnO + H₂O + 2OH⁻ (4)

when the KOH is first introduced into the Zn²⁺ aqueous solution, the solution is observed to become turbid due to the formation of white Zn(OH)₂ colloids (reactions (1) and (2)). In the solution environment, a part of the Zn(OH)₂ colloids dissolves into Zn²⁺ and OH⁻. Large quantities of ZnO nuclei form when the concentrations of Zn²⁺ and OH⁻ ions exceed the critical value,⁴⁶ and a subsequent crystal growth process develops (reactions (3) and (4)). Based on the above discussion, for the sample without the microwave process, large quantities of ZnO nuclei first aggregate instead of exhibiting anisotropic growth due to a lack of active sites around the circumference of ZnO nuclei at low temperatures. Moreover, the number of growth units Zn(OH)₄)²⁻ is still not enough; consequently, only dispersive ZnO nanoparticles are obtained (see Fig. 8(a)). Under the microwave process, at a lower molar ratio of 1 : 10, the OH⁻ ligands are fewer, and the energy of the ions is high enough to make the high-energy surface smoother, and thus the ions will diffuse to low-energy surfaces.⁴⁷ Under these conditions, the growth is determined by dynamics instead of thermodynamics. The 2D growth mode is more preferred under this nonequilibrium process.⁴⁸ This is because of the fact that the growth velocities are in the order V{010} > V{001}, and the nanosheets (Fig. 8(b)) were formed at the 1 : 10 molar ratio of zinc acetate and KOH. The formation of the nanosheets is attributed to the close surface energy of 1D/2D crystals under proper experimental conditions.⁴⁹ When the molar ratio is increased to 1 : 15, the quantity of Zn(OH)₄)²⁻ increases, and the anisotropic growth is favourable, resulting in the ZnO nanorods growing along the c-axis connected with the nanosheets (Fig. 8(c)). In the proper molar ratio of 1 : 20, only (001)-oriented ZnO nuclei grow preferentially along the c-axis, leading to the growth of (001)-oriented hexagonal ZnO nanorods (Fig. 8(d)). For a higher molar ratio of 1 : 25, the OH⁻ ligands are large enough, and they promote crystal growth along the c-axis direction (Fig. 8(e)). This anisotropy growth is supported by the abovementioned crystallographic habit of ZnO; hence, the 1D growth of the nanorods in the (001) direction is preferred.

Fig. 8 A schematic representation of the possible growth mechanism of ZnO nanostructures.

With an increase in the reaction time from 10 min to 60 min, as shown in Fig. 8, both the diameters and lengths of the ZnO nanostructures increase. This indicates the transversal and longitudinal growth of the single nanostructure with time. Therefore, the different growth pattern should be considered. Note that the uneven width change along the length for the single nanostructure may be related to the anisotropy of the ZnO material. The crystal planes, which have higher surface energy, possess faster growth velocity. Because of the different surface energy, the growth velocity of the (0001) planes is higher than that of the (1000) planes. As a result, when different crystal planes grow at different velocities, those surfaces with the faster growth velocities ((001) planes) will continually decrease their area and the surfaces with slower growth velocities ((100) planes) will gradually dominate the morphology of crystal, which results in the formation of 1D structure with uneven width along its length. During the longer reaction time in our experiments, the zinc species in the solution can be largely consumed. In this case, the Ostwald ripening will become appreciable and the thermodynamics will dominate the reaction process.³⁵ The morphology of the ZnO nanostructures will be directed to lower the system energy. The hollow structure may have a lower energy than that of the solid structure due to the higher energy at the polar surfaces.²⁴ This may facilitate the formation of the ZnO semi-hollow nanorods (pyramid-like).

The optical absorption spectra were employed to study the effects of shape and aspect ratio on the optical properties of ZnO nanostructures. The inset of Fig. 9 shows the absorption spectra of the ZnO nanostructures with various aspect ratios. The absorption edge of the high aspect ratio (14) ZnO nanorods shifts to shorter wavelengths; however, for low aspect ratio nanostructures, the absorption band is shifted toward higher wavelengths.

Fig. 9 The plot of (αhν)² versus hν for ZnO nanostructures. The inset shows the corresponding UV-vis absorption spectra.

Because the interband transition in ZnO is the allowed direct transition, the optical-absorption coefficient a of ZnO can be expressed by⁵⁰

(αhv) = A(hν − E_g)^1/2, (5)

where A is a constant, hν is the photon energy, and E_g is the optical bandgap energy. Fig. 9 shows the plots of (αhν)² versus (hν) for the ZnO nanostructures with various aspect ratios. The band gap (E_g) values of the nanostructures were determined from the intercept of (αhν)² versus (hν) curves and found to be 3.47 eV for ZnO nanoflower (A.R ∼ 10). Interestingly, the increased bandgap of 3.48 eV was observed for high aspect ratio (14) nanorods. In contrast, the band gap obtained decreased to 3.45 eV and 3.43 eV for nanoprisms (A.R ∼ 3.7) and nanopyramids (A.R ∼ 5), respectively (see Fig. 9). These results indicated that the aspect ratio of ZnO nanostructures could give rise to a negative/positive correction to the conduction and the valence-band edges, leading to a bandgap tuning.

Photocatalytic properties of the ZnO nanostructures (1 : 20 molar ratio, 10–60 min reaction time) with different morphologies and aspect ratios (A.R) were examined by the decomposition of MR. The characteristic absorption of MR at ∼428 nm was chosen to monitor the photocatalytic degradation process. Fig. 10 illustrates the time-dependent absorption spectra of MR aqueous solutions during UV light irradiation in the presence of ZnO nanorods (A.R = 14). As a control, the absorbance peak of the MR solution was monitored under two different conditions: (i) with a photocatalyst in the dark and (ii) without photocatalysts under UV light illumination (ESI* 1†). The change in the absorbance peak of MR under these conditions is found to be negligible, indicating that there is no loss of MR without an irradiated photocatalyst. For ZnO nanorods (A.R = 14) placed in an MR solution, the maximum absorption of the MR solution was found to decrease with illumination time and disappeared almost completely after irradiation for about 60 min. In particular, the photocatalytic performance of ∼50% was rapidly achieved within 10 min of photoirradiation for ZnO nanorods, whereas after 60 min illumination of UV light, MR was almost completely (∼98%) removed over ZnO nanorods. In order to evaluate the relationship between photocatalysis and aspect ratio, nanoflowers, nanoprisms, and nanopyramids were used as the contrast examples. Fig. 11 shows the relative concentration (C/C₀) of MR as a function of time for various nanostructures, where C is the concentration of MR at the irradiation time (t) and C₀ is the concentration of the dye before irradiation. When the suspensions were magnetically stirred in the dark for 30 min to ensure the establishment of an adsorption/desorption equilibrium of MR on the sample surface, only a slight decrease in the MR solution concentration was observed. This demonstrates that the adsorption of MR on the samples is limited after the adsorption–desorption equilibrium is reached. A control experiment revealed negligible decolorization of the dye solution treated with photocatalysts in the dark. The extent of decolorization was similar to the blank sample comprising a dye solution illuminated with UV light without a photocatalyst (ESI* 1†). It is clear from Fig. 11 that for all ZnO nanostructures placed in the MR solution, the concentration of the MR solution is observed to decrease with irradiation time, indicating that all of the nanostructures show UV-light photocatalytic properties in the degradation of MR. The catalytic activity of the these nanostructures possesses a sequence of nanorods > nanoflowers > nanopyramids > nanoprisms for 60 min of irradiation.

Fig. 10 UV-visible absorbance spectra of photodegradation of MR in the presence of ZnO nanostructures.

Fig. 11 Temporal evolution of MR absorption spectra with various ZnO nanostructure photocatalysts.

The kinetic behavior of these photocatalysts was further studied, and the results are shown in Fig. 12. There is an obvious linear relationship between the value of ln(C₀/C) and the irradiation time. The photocatalytic process can be regarded as pseudo first-order reaction, and the rate equation is expressed as ln(C₀/C) = Kt, where t is the reaction time, K is the apparent reaction rate constant, and C₀ and C are the concentrations of MR at the time of 0 and t, respectively. The apparent reaction rate constant K for the degradation of MR was calculated to be 6.32 × 10⁻² min⁻¹, 2.91 × 10⁻² min⁻¹, 1.53 × 10⁻² min⁻¹ and 1.05 × 10⁻² min⁻¹, respectively, for ZnO nanorods (A.R ∼ 14), nanoflowers (A.R ∼ 10), nanopyramids (A.R ∼ 5) and nanoprisms (A.R ∼ 3.7), respectively. The inset of Fig. 12 shows the relationship between k and different aspect ratios nanostructures. It is clear from inset of Fig. 12 that the reaction rate constant for high aspect ratio (14) nanorods is higher than that of lower aspect ratio nanostructures, which reveals the higher photocatalytic activities of higher aspect ratio (14) ZnO nanorods.

Fig. 12 Kinetic relationship of ln(C₀/C) versus irradiation time for various ZnO nanostructure photocatalysts. The inset shows the plot of rate constant versus the aspect ratio of nanostructures.

Fig. 13 shows the percentage decolorization (degradation) of MR dye as a function of aspect ratio. It can be clearly seen from Fig. 13 that the percentage decolorization increases with the increase in aspect ratio of nanostructures. The higher photocatalytic performances of ∼98% was achieved within 60 min of photo-irradiation for high aspect ratio (14) nanorods, whereas only 80%, 58%, and 44% degradation efficiency of MR was obtained with nanoflowers (A.R = 10), nanopyramids (A.R = 5), and nanoprisms (A.R = 3.7), respectively. This indicates that high aspect ratio ZnO nanorod photocatalysts a more superior to than that of lower aspect ratio nanostructures. In addition, by comparing the photocatalytic performance of the high aspect ratio ZnO nanorods obtained in this study with other reported nanostructures, ZnO nanorods showed better photocatalytic properties. For example, Comparelli et al.⁵¹ showed the degradation of MR under UV irradiation in the presence of nanosized ZnO and TiO₂. Their results revealed that ZnO degraded 50% of MR for 140 min irradiation time, whereas about 90% of MR was degraded with TiO₂. In another report, Kanjwal et al.⁵² presented hierarchical nanostructures consisting of ZnO and TiO₂ prepared by an electro-spinning process, followed by a hydrothermal technique for use as a photocatalyst for dye degradation. The results showed that the introduced ZnO–TiO₂ hierarchical nanostructure can eliminate almost all the MR dye within 90 min. They also showed that less than 30% of the MR dye was removed using pure ZnO nanoflowers, even after 180 min. However, in the case of pristine TiO₂ nanofibers, up to 50% of dye was removed after 180 min. The ZnO-doped TiO₂ nanofibers showed better results than the first two catalysts and removed more than 80% of dye after 3 h.

Fig. 13 Percentage decolorization (degradation) of MR dye as a function of aspect ratio on various ZnO nanostructure photocatalysts.

In the present work, ZnO nanorods degraded more than 50% of MR within 10 min and for 60 min of UV light irradiation, MR was almost completely degraded. Therefore, the ZnO nanorods obtained in the present work are photocatalytically more superior than the others.

Photocatalytic stability is the main limitation in the development of photocatalysts for organic dye degradation. To estimate the photocatalytic stability of the ZnO nanostructure photocatalysts, the time courses of photocatalytic degradation of MR using high aspect ratio (14) nanorods were conducted and are shown in Fig. 14. No noticeable degradation of MR was observed in repeated runs for the photocatalytic reaction of 60 min, which reveals that ZnO nanorods have good stability and reusability performance and can be a potential candidate for practical photocatalysis applications.

Fig. 14 The stability of ZnO nanorods (A.R = 14) for photodegradation of MR.

The activation energy has been calculated from the Arrhenius equation:⁵³

k = Ae^−E_a/RT, (6)

where E_a is the activation energy, k is the rate constant, A is the pre-exponential factor, T is the temperature and R is the gas constant, which equals to 8.314 J mol⁻¹ K⁻¹, respectively. In the photocatalytic reaction, the activation energy is the energy required to promote photoelectrons from the photo catalyst to be trapped at surface by adsorbed oxygen molecules.⁵⁴

The photocatalytic decolorization of MR dye using ZnO nanostructures with different reaction temperatures ranging from 300 K to 325 K was represented by a pseudo first order, and the activation energy (E_a) and pre-exponential factor (A) were calculated by plotting ln(k) versus 1/T and found by using the following equation:

Slope = −E_a/R, Intercept = ln(A). The activation energy and pre-exponential factor of various ZnO nanostructures were obtained to be approximately 6.88 kJ mol⁻¹ and 1.8 × 10², 8.81 kJ mol⁻¹ and 5.5 × 10², 10.41 kJ mol⁻¹ and 2.4 × 10³, and 11.35 kJ mol⁻¹ and 4.9 × 10³ for nanostructures with aspect ratios of 14, 10, 5 and 3.7, respectively. The activation energy for high aspect ratio (14) ZnO nanorods is lowest (6.88 kJ mol⁻¹); therefore, the photocatalytic reaction is fast and ends at 60 min. These results are consistent with the trend observed in the photocatalytic activities of these nanostructures, showing the maximum photocatalytic activity for high aspect ratio (14) ZnO nanorods.

The surface composition and chemical states of ZnO nanostructures with various aspect ratios were investigated by XPS analysis, and the results are shown in Fig. 15(a) and (b). Fig. 15(a) shows the high-resolution XPS spectra of the ZnO nanostructures recorded for the Zn 2p regions. The binding energy of Zn 2p_3/2 and Zn 2p_1/2 of all ZnO nanostructures are 1021.47 eV and 1044.25 eV (Fig. 15 (a)), which are very well matched with the standard values of ZnO.⁵⁴ The binding energies of Zn 2p_3/2 and Zn2p_1/2 at 1021.47 eV and 1044.25 eV show an obvious shift to high energy with an increase in the aspect ratio of the ZnO nanostructures.

Fig. 15 XPS spectra of ZnO nanostructures with various aspect ratios: (a) Zn-2p and (b) O-1s.

The O 1s spectra from the nanorods are shown in Fig. 15(b). An asymmetric profile of the O 1s region reveals the existence of three different O species in the sample. The O 1s profile can be deconvoluted using a Gaussian fitting technique into three different peaks located at binding energy values of 530.2 eV, 531.4 eV and 532.6 eV, respectively, (Fig. 15(b)) in all samples. The peak at 530.2 eV can be indexed to the O²⁻ species in ZnO. The higher energy peak at 531.4 eV can be assigned to oxygen vacancies or defects (O_V), and the peak at 532.6 eV can be indexed as chemically absorbed oxygen (O_C) species.^55,56

Because the physically absorbed hydroxyl groups on ZnO can be easily removed under the ultra-high vacuum condition of the XPS system, ZnO nanorods with fine surface structures will not give significant signals in the XPS.^57,58 Therefore, the distinct signals of the hydroxyl groups observed should be due to hydroxyl groups, i.e. Zn–OH and H₂O, strongly bound to surface defects on ZnO; i.e., the hydroxyl groups in XPS are associated with surface defects, and the visible hydroxyl groups should indicate the existence of surface defects on ZnO samples. According to the XPS results, the O_V (531.4 eV) and O_C (532.6 eV) values of O 1s are higher for the high aspect ratio nanorods (Fig. 15(b)), which indicates that higher aspect ratios of ZnO nanorods have higher amounts of surface defects and visible hydroxyl groups. Furthermore, oxygen species around 532.6 eV are caused by the surface hydroxyl group. The presence of surface hydroxyl groups facilitates the trapping of photoinduced electrons and holes, thus enhancing the photocatalytic degradation process.

Fig. 16 shows the room-temperature PL spectra of the ZnO nanostructures with various aspect ratios. A near-band-edge (NBE) emission at ∼369 nm and four visible light emission bands: two weak blue bands at ∼407 and ∼431 nm, a weak blue-green band at ∼465 nm and a strong and broad green band at ∼587 nm have been observed. The NBE emission is attributed to a well-known recombination of free excitons through an exciton–exciton collision process of the wide bandgap ZnO⁵⁹ and the weak blue, blue-green and broad green light emissions are resulted from the recombination of a photo-generated hole with a singly ionized charge state of specific defect.⁶⁰ It is generally complicated in experiments to distinguish PL bands caused by zinc interstitials (Zn_i) and oxygen vacancies (V_O). Theoretically, Kohan et al.⁶¹ and Van de Walle⁶² calculated the formation energies and electronic structure of native point defects in ZnO. Based on their results, oxygen and zinc vacancies are the two most common defects in ZnO. In zinc-rich conditions, the oxygen vacancies (V_O) have lower formation energy (1.2 eV) than the zinc interstitials (Zn_i) and will dominate in the defect; however, in oxygen-rich conditions, zinc vacancies (V_Zn) ought to dominate. In our microwave aqueous growth condition, Zn source supplies from zinc salts and the O comes from the OH⁻. This aqueous growth method can be classified as Zn-rich conditions due to the high solubility of the zinc salts. Therefore, PL spectra indicate that the oxygen vacancies (V_O) may possibly be responsible for the green light emission, whereas zinc interstitials (Zn_i) and zinc vacancies (V_Zn) may be excluded in the synthesized ZnO rods. These results are in good agreement with the earlier report.⁶³ We predict that the rapid microwave heating might enhance the defects.

Fig. 16 Room-temperature PL spectra of ZnO nanostructures with various aspect ratios.

Generally, the NBE peaks directly reflect the separation situation of the photogenerated charge carriers; i.e., the stronger the NBE peaks, the higher the recombination rate of photogenerated charge carriers, which probably results in a slower photocatalytic activity.⁶⁴ High aspect ratio (14) ZnO nanorods showed a decrease in the NBE intensity and hence, a higher separation rate and lower recombination rate of photoinduced charge carriers, which leads to higher photocatalytic activity.

Two impurity levels, which can enhance the electron–hole pair separation rate in ZnO nanorods, are generated in the presence of oxygen vacancies and interstitial oxygen defects.⁶⁵ As seen in Fig. 16, the green light emission intensity is higher for high aspect ratio (14) nanorods; hence, the photocatalytic activity of the ZnO nanorods can be enhanced with aspect ratio. It can be concluded that abundant surface oxygen vacancies or defects exist in ZnO nanorods, which may play a vital role in photocatalytic activity.

To study the textural properties and nature of porosity of the samples, BET analysis was carried out, and the parameters of surface area obtained. BET analysis of the samples showed a surface area of 28.2 m² g⁻¹, 19.4 m² g⁻¹, 9.6 m² g⁻¹ and 6.1 m² g⁻¹ for the ZnO nanostructures with aspect ratio of 14, 10, 5 and 3.7, respectively. The isotherm of the ZnO nanostructures with various aspect ratios exhibit type IV isotherms, according to IUPAC classifications, characteristic of the porous materials, which confirms the presence of mesoporous materials with a narrow range of uniform pores. The pore volume distribution determined by the BJH method was found to be 0.082 cm³ g⁻¹, 0.075 cm³ g⁻¹, 0.058 cm³ g⁻¹ and 0.046 cm³ g⁻¹ with an average uniform pore-size distribution of ∼9.4 nm, ∼7.8 nm, ∼4.6 nm and ∼3.7 nm for ZnO nanostructures with aspect ratio of 14, 10, 5 and 3.7, respectively.

The parameters that affect the photocatalytic activity include the catalyst, band gap, surface area, porosity, crystal structure, crystallinity, purity, density of surface, hydroxyl groups, e and h⁺ migration characteristics, surface acidity and size distribution.

The maximum photocatalytic degradation of MR was achieved for high aspect ratio (14) ZnO nanorods, in which the surface area and pore volume were maximum. The amount of dye adsorbed on the surface of the catalysts is directly related to the surface area, which is available for adsorption. Therefore, the large specific surface area of high aspect ratio (14) ZnO nanorods can improve the contact opportunity between the dye and photocatalyst, which can accelerate the rate of the photocatalytic reaction. The advantage of nanopores in the high aspect ratio (14) ZnO nanorods might be helpful to understand the rapid photocatalysis performance. The nanoholes in ZnO nanorods provide ideal channels for easy and fast diffusion of the dye molecules to contact the different interplanar surfaces of the nanoparticles, which greatly increases the chances and velocities of the encounter of the produced electron–hole pair with the dye molecules and thus enhances the photocatalytic activity.

The transient photocurrent responses of a photocatalysis may directly correlate with the recombination efficiency of the photogenerated carriers.^66–68 Fig. 17 shows the transient photocurrent density responses of ZnO nanostructure electrodes under UV light irradiation. A generation of photocurrent with good reproducibility for all samples is observed via various on–off cycles of intermittent illumination. This indicates that the electrode is stable, and the photocurrent is quite reversible. The photocurrent of the ZnO nanostructures electrode is found to decrease with a decrease in aspect ratio, showing maximum current for high aspect ratio (14) nanorods. The enhancement in the photocurrent of the ZnO nanorod (14) photocatalyst indicates an enhanced photoinduced electrons and holes separation.

Fig. 17 The transient photocurrent density responses of ZnO nanostructure photocatalyst electrodes with light on–off cycles under UV light irradiation.

Complementary information regarding the interface charge separation efficiency of the ZnO nanostructure photocatalysts was investigated by the electrochemical impedance measurements. Fig. 18 shows the representative electrochemical impedance spectra (EIS), presented as a sum of real impedance (Z′) and imaginary impedance (Z′′) in the form of a Nyquist plot for ZnO nanostructures. The radius of the arc on the EIS spectra reflects the interface layer resistance occurring at the surface of electrode. The smaller arc radius indicates the higher efficiency of charge transfer.⁶⁹ Fig. 18 shows that the diameters of the arc radius for the high aspect ratio (14) ZnO nanorod electrodes is smaller than that of the lower aspect ratio nanostructure electrodes. These observations indicated that both the solid-state interface layer resistance and the charge-transfer resistance on the surface of ZnO have significantly decreased for high aspect ratio (14) nanorods. This suggests that a high aspect ratio (14) ZnO nanorods acts as nanoscale electrodes and accelerates charge transfer, decreases the intragranular resistance, inhibits recombination between charge carriers, and enhances photocatalytic efficiency.⁷⁰ This result is in good agreement with the transient photocurrent responses of these photocatalysts.

Fig. 18 Nyquist plots for ZnO nanostructures with various aspect ratios in light on/off cycles under UV light irradiation.

Fig. 19 presents the PL spectral changes observed during the UV illumination of the different photocatalysts in the aqueous basic solution of terephthalic acid. Usually, PL intensity at about 425 nm is proportional to the amount of produced hydroxyl radicals.⁷¹ In general, hydroxyl radicals (˙OH) has been regarded as the dominant active species responsible for the photocatalytic oxidation reactions. Obviously, it can be seen that the intensity of the generated 2-hydroxy-terephthalic acid (TAOH) of high aspect ratio (14) nanorods was stronger than that of lower aspect ratio nanostructures, indicating that the formation rate of ˙OH radicals on high aspect ratio (14) nanorods was larger than that of the others.

Fig. 19 Detection of ˙OH radical by fluorescence method using terephthalic acid as trapping reagent.

Thus, it can be concluded that high aspect ratio (14) nanorods could produce more ˙OH content than that of lower aspect ratio nanostructures, thereby possessing higher photocatalytic performance. Therefore, ˙OH experiments further confirm that ˙OH is actually produced and indeed participates in photodegradation reactions.

The photocatalytic activity of a catalytic material is influenced by several factors such as carrier recombination, size of the particles, surface area, surface acidity, and presence of a higher number of hydroxyl groups. In this work, ZnO nanorods with high aspect ratios show a higher percentage of degradation as compared to the low aspect ratio nanostructures as well as the previously reported work. The enhancement of photocatalytic activity was most likely due to the relative increase of active morphological surfaces because of the increased surface-to-volume ratio, as well as the low recombination rate of the electron–hole pairs generated during optical exposure, owing to largely available surface states. This is mainly due to changes in space-charge regions because this is well-defined along the longitudinal direction of the nanorods.^72,73 The enhanced oxygen adsorption in the high aspect ratio ZnO nanorods is expected to be very effective in accepting photogenerated holes. This leads to a reduction in the recombination probability of photogenerated carriers, which in turn forms oxygen species⁷⁴ and thus results in high photocatalytic activity. The superior photocatalytic performance of ZnO nanorods can be extended to remove other organic pollutants from wastewater.

Fig. 20 shows the schematic diagram of a UV light photocatalytic mechanism with ZnO nanorods. Photocatalytic degradation was generally operated by the action of hydroxyl radicals formed during the reaction.⁷⁴ The photocatalytic mechanism is proposed in the following manner: when the ZnO nanostructures were illuminated with light, electrons were excited from the valence band of ZnO to the conduction band, leaving a hole in the valence band. The hydroxyl groups present on the surface of the ZnO react with the photogenerated hole to produce hydroxyl radicals. Similarly, peroxide (O₂⁻) is formed when the dissolved oxygen interacts with photogenerated electrons. This peroxide takes one proton to yield a superoxide (HO₂⁻) followed by the formation of hydrogen peroxide (H₂O₂). A hydroxyl radical was also produced by the attack of a photogenerated electron to the hydrogen peroxide. These reactive radicals and intermediate species react with dye and degrade them into non-toxic organic compounds.

ZnO + hν → ZnO + e⁻ + h⁺ (7)

OH⁻ + h⁺ → OH⁻ (8)

O₂ + e⁻ → O₂⁻ (9)

O₂⁻ + H⁺ → HO₂ (10)

2HO₂ → H₂O₂ + O₂ (11)

H₂O₂ + e⁻ → OH⁻ + OH⁻ (12)

Fig. 20 Schematic representation of UV light photocatalytic process in the presence of ZnO nanostructures.

Mashkour et al.⁷⁵ reported the effect of H₂O₂ addition on the reaction rate constant of photocatalytic degradation. Their results showed that the apparent rate constant increased with increasing the concentration of H₂O₂ due to the increase in the formation of ˙OH radicals, in addition to inhibiting the recombination of electron–hole pairs.^76,77

In the present work, the reaction rate is higher for high aspect ratio nanorods, whereas the reaction rate is lower for the low aspect ratio. The reaction rate is dependent on the generation of hydroxyl radicals. In general, a hydroxyl radical (˙OH) has been regarded as the dominant active species responsible for the photocatalytic oxidation reactions. Obviously, it can be seen that the intensity of the generated 2-hydroxy-terephthalic acid (TAOH) of high aspect ratio (14) nanorods was stronger than that of the lower aspect ratio nanostructures, indicating that the formation rate of ˙OH radicals on high aspect ratio (14) nanorods was larger than that of the others. Thus, it can be concluded that high aspect ratio (14) nanorods could produce more ˙OH content than that of lower aspect ratio nanostructures, thereby possessing higher photocatalytic performance. Therefore, it is logical to state that, in the present work, H₂O₂ formed in the photocatalytic reaction, which in turn formed ˙OH radicals and degraded the organic dye, which is in good agreement with the earlier report.⁷⁵

It is well known that the photocatalytic process is determined by the separation of electron–hole pairs,⁷⁸ and the separation of photogenerated carriers could be due to the crystalline phase, the aspect ratio of the nanorods, and the electronic structure of the facets and defects.⁷⁹ Among these factors, surface defects have to be considered for most cases, except for perfect single crystals. Because the current ZnO nanorods were synthesized via a microwave irradiation method at relatively low temperatures, the diffusion lengths of atoms during the decomposition reaction of the precursor is relatively short. It is therefore reasonable to believe that a large number of surface defects exist in the final products, which result in visible emissions (400–700 nm).

Generally, the recombination probability of the photogenerated carriers would be high, and the separation of the electron–hole is the rate-limiting step for the photocatalytic process.⁷⁸ The aspect ratio of nanostructures is expected to enhance the separation of photogenerated carriers. Therefore, decreased recombination centers with a lesser number of interparticle junctions and high electron delocalization in high aspect ratio nanorods lead to higher efficiency.⁸⁰ For 1-D nanostructured crystals, the space-charge region is well constructed along the longitudinal direction of the ZnO nanocrystal, which indicates that photogenerated electrons can flow in the direction of the crystal length. Increased delocalization of electrons at 1-D nanostructured crystals can lead to a remarkable decrease in charge carrier recombination probability. Consequently, larger numbers of electrons and holes exist on the active sites of the nanocrystal surface, resulting in higher activity.⁷⁵ Therefore, in the present work, the higher aspect ratio (14) ZnO nanorods exhibit higher photocatalytic activity.

Conclusions

In summary, highly crystalline ZnO with different morphologies have been successfully prepared by a convenient microwave-assisted solution route. XRD, HRTEM and SAED measurements showed that all of the ZnO nanostructures are of a hexagonal phase structure. The exception of secondary phase formation (Zn(OH)₂) was observed for the sample prepared using the 1 : 10 molar ratio, which finally converted to ZnO upon increasing the molar ratio. PL and XPS studies revealed the existence of defects in the ZnO nanorods. FESEM and TEM images revealed that the morphology of ZnO can be effectively controlled as sheet-like, rod-like, brush-like, flower-like, prism-like, and pyramid-like, which was ascribed to the process parameter-dependent tuning of the nanostructures. The aspect ratio continued to change as a consequence of changes in the reaction time and molar ratio, showing maximum (14) for the nanorods prepared at the 1 : 20 molar ratio for 20 min. It is suggested that the zinc species and environment, including OH⁻ and K⁺ as well as the reaction time, are the crucial factors for the morphologies of obtained ZnO. Such knowledge would allow for quick synthesis of ZnO with anticipated morphology simply by selecting the appropriate molar ratio and reaction time. The photocatalytic properties of ZnO nanostructures with different aspect ratios toward MR under UV light irradiation were found to be attractive. The observed rates of degradation were found to be proportional to the rates of formation of active hydroxyl radicals studied by the emission spectra of 2-hydroxy-terephthalic acid. EIS study shows that the high aspect ratio (14) ZnO nanorods minimized charge recombination and effectively enhanced the photocatalytic activity. The enhanced percentage degradation of MR with ZnO nanorods was found to be ∼98% within 60 min of UV light illumination, which was due to the higher aspect ratio. This enhancement in photocatalytic activity may be attributed to the easy separation of photogenerated charge carriers in the higher aspect ratio nanorods, which resulted to the enhanced oxygen chemisorptions. For bridging between the laboratory and industry, considering the excellent photocatalytic performance and facile preparation method, the prepared ZnO nanorods are believed to have potential applications in the field of environmental remediation and photocatalysis.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIP) (no. 2011-0030058). This work was also supported by the MSIP (Ministry of Science, ICT & Future Planning), Korea. Under the ITRC (Information Technology Research Centre) support program supervised by the NIPA (National IT Industry Promotion Agency) (NIPA-2014-H0301-14-1016).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra02470b

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