Ionic liquid-based solvent-induced shape-tunable small-sized ZnO nanostructures with interesting optical properties and photocatalytic activities

Abstract

We report a simple and one-pot size- and shape-controlled synthesis of small crystalline ZnO nanostructures through the hydrolysis of zinc acetylacetonate precursor using an ionic liquid, tetrabutylphosphonium hydroxide in different organic solvents and solvent mixtures of different polarities. Transmission electron microscopy (TEM) results indeed show the formation of very small (D ∼ 5–7 nm) ZnO spheres and ZnO nanorods of very low diameters (D ∼ 8–11 nm) depending on the solvents. The size and shape of ZnO nanostructures can be easily controlled by simple variation of the solvent and reaction temperature. Protic solvents favor the growth of spherical nanoparticles, whereas the rod-shaped ZnO is formed in aprotic solvents. The spherical ZnO nanostructures exhibit excitonic absorption at lower wavelengths compared to that of rod-shaped ZnO nanostructures. Through adjustment and control of the size and shape of ZnO nanostructures, we can tune the fluorescence emission from blue to green to yellow, when dispersed in the aqueous medium. The obtained ZnO nanostructures show very high photocatalytic activities towards the degradation of different cationic organic dyes (such as rhodamine 6G, crystal violet and methylene blue) as they contain a large number of defect states, as evident from the photoluminescence study. The as-synthesized ZnO nanostructures also show very high stability towards the photocatalytic degradation of organic dyes.

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1. Introduction

Nanostructured zinc oxides (ZnO) are interesting materials to study as they possess very interesting physical and chemical properties such as acoustic,¹ electronic,² optical,³ catalytic⁴ and photocatalytic⁵ properties. Because of such technologically important properties, ZnO nanostructures find applications in room temperature UV-lasers,⁶ light emitting diodes (LED),⁷ field-effect transistors,⁸ solar cells,⁹ sensors and optoelectronics.¹⁰ Usually, II–VI semiconductor quantum dots, such as CdS (Se, Te) and ZnS (Se, Te), exhibit emission wavelengths that can be tuned from blue to red by controlling the particle size.^11,12 But most of the II–VI semiconductor quantum dots are toxic to the biological systems.¹³ In this regard, ZnO is a good semiconductor material that can be used as a substitution for the II–VI quantum dots for biomedical and imaging applications. Thus, a lot of study had been done to explore the various properties of the ZnO nanostructures, but only very few were reported on the synthesis of ZnO nanostructures of tunable photoluminescence.^3,13–15 The visible photoluminescence of ZnO nanoparticles (NPs) is mostly related to surface defects/vacancies, in which the trapped electrons and holes recombine randomly at different energy levels and thus visible emission has a rather broad peak and quite low quantum yields.^3,14 Thus, it is also important to synthesize visible light emitting ZnO NPs for different applications, such as cell imaging.^13,16

Several different synthetic approaches have also come into effect to tune the size, shape and hence the properties of the ZnO materials.¹⁷ Among these techniques, colloidal chemical routes are the most popular as they are easy to perform, do not require any sophisticated instrumentation, are facile to scale-up and it is easy to control the particle sizes and morphologies.^18,19 In general, metal oxide NPs are mostly synthesized through hydrolytic methods.¹⁷ The hydrolytic methods include firstly the solubilization of an inorganic or organometallic salt of Zn in an aqueous or alcoholic solvent, followed by hydrolysis using NH₄OH or NaOH.^17,20,21 In this process, templates or structure directing agents are commonly used to tune the shape and to stabilize the formed ZnO.^4,22–30 Previously, we also reported the preparation of differently shaped hierarchical ZnO nanostructures in water using poly(vinyl methyl ether)²⁰ and sodium ascorbate³¹ as the structure directing agents. It was noticed that the reaction media also play an important role in controlling the shape of the obtained ZnO nanostructures.^21,32 The nature of the reaction medium controls the hydrolysis rate and thereby controls the growth process, which eventually determines the shape of the nanoparticles.^19,33 There have only been a few reports that try to understand the effects of the reaction medium on the synthesis of the ZnO nanomaterials;^19,21,32,34 and there still needs to be more study to gain a better understanding of the effect of the medium on the growth of the ZnO materials. Thus, it would be interesting to use a variety of different water miscible organic solvents and solvent mixtures as the reaction medium for better understanding of the effect of the medium on the synthesis of ZnO nanomaterials.

Not only the reaction medium but also the nature of the hydrolyzing agent controls the growth of the particles during the formation of the nanostructures.³³ In this context, room temperature ionic liquids (RTILs) become more and more popular to the researcher as these RTILs can also act as soft templates for the generation of shaped oxide NPs.^33,35,36 It is well known that ionic liquids (ILs) exhibit very high polarity in the medium and also form different pre-organized solvent structures,³⁶ which assist in the anisotropic growth of metal oxides in solution.^35,36 Therefore, it would be interesting to develop a method in which hydrolysis of the zinc salt can be performed in different reaction media (protic and aprotic solvents) to synthesize high quality size- and shape-tunable ZnO NPs using ILs as the hydrolyzing agents, as there is no study on the effect of tetrabutylphosphonium hydroxide (TBPH) ILs as the hydrolyzing agent on the synthesis of shaped ZnO nanostructures.

It is also known that ZnO is an important photocatalyst, which makes the material more interesting to study.^37–41 The photocatalytic reactions that have been studied so far include the photolysis of water to generate hydrogen,^42–44 photodecomposition of organic pollutants^45–47 etc. The activity of the nanostructured ZnO photocatalysts largely depends on its size and shape.⁴⁷ The surface defects of the ZnO nanostructures are also important criteria in determining its photocatalytic activity.^47–49 Thus, the optical properties of ZnO nanostructures have a very close relationship to their photocatalytic activity.⁴⁸ However, there has not been much study to resolve the relationship between the optical properties and photocatalytic activities of ZnO nanomaterials.

Here, we report the controllable synthesis of size- and shape-tunable ZnO nanostructures using tetrabutylphosphonium hydroxide (TBPH) IL as a hydrolyzing agent in different water miscible organic solvents without the addition of external additives at relatively low temperature. It is observed that there is a pronounced effect of the reaction medium on the shape of the obtained ZnO nanostructures, which is studied using pure solvents like methanol, DMF, 1,4-dioxane, THF and DMSO, and also mixtures of these solvents in different proportions. The hydrolysis of zinc acetylacetonate using TBPH is also carried out at different temperatures to obtain ZnO nanostructures of various shapes. The formation of spherical and/or rod-shaped ZnO nanostructures in the cases of protic and aprotic solvent–solvent mixtures, is shown by TEM. It is noticed that the as-synthesized ZnO NPs exhibit interesting and tunable blue, green and yellow emissions. Finally, it is observed that the as-synthesized ZnO NPs exhibit very high photocatalytic activities towards the degradation of various cationic organic dyes. such as rhodamine 6G (R6G), crystal violet (CV) and methylene blue (MB) at neutral pH. It is also shown that the photocatalytic activity of the ZnO depended largely on the number of surface defects and the size and shape of the materials. The rod-shaped ZnO NPs show higher photocatalytic activities compared to that of spherical ZnO NPs.

2. Experimental section

2.1 Materials

Zinc acetylacetonate [Zn(acac)₂], tetrabutylphosphonium hydroxide (TBPH) (40 wt% aqueous solution), methanol (HPLC grade), rhodamine 6G (R6G) and crystal violet (CV) were obtained from Sigma-Aldrich. 1,4-Dioxane (GR), dimethyl formamide (GR), tetrahydrofuran (GR) and dimethyl sulphoxide (GR) were obtained from Merck, India. Methylene blue (MB) was obtained from Merck, Germany. All the chemicals were used as received. Triple distilled water was used as the dispersion medium for the ZnO NPs.

2.2 Preparation of shape-tunable ZnO nanostructures

In a typical synthesis, 46.5 mL methanolic solution of Zn(acac)₂ was taken in a reaction vessel (and the reaction vessel was heated according to the temperature given in Table 1) and then 3.5 mL aqueous TBPH solution (40 wt%) was added to the reaction mixture with constant stirring. The concentration of Zn(acac)₂ was maintained in such a way that the overall concentration in the medium would be 0.02 M. The overall concentration of TBPH in the medium was 0.1 M. After 2 h of reaction, the mixture was centrifuged at 4000 rpm and the residue was collected and redispersed in pure methanol. The process was repeated three times. After purification, the solid mass was dried at 60 °C under vacuum for 12 h. This solid mass was termed as “ZnO-1”. Similarly, other samples were prepared by dissolving the Zn(acac)₂ in different solvents, as given in Table 1. In each case, 3.5 mL TBPH (40 wt%) solution was added dropwise to 46.5 mL Zn(acac)₂ solution at the different reaction temperatures (Table 1). The solid mass was then isolated by centrifugation, followed by redispersion in pure methanol. The purified and dried solid samples were then used for diffraction study. The purified and dried solid ZnO samples were then dispersed in triple distilled water for the optical and photocatalytic studies.

Table 1 Reaction recipe for the preparation of size- and shape-tunable ZnO nanostructures and their characterization data. Conditions: [Zn(acac)₂] = 0.02 (M); [TBPH] = 0.1 (M); time = 120 min

Sample name	Solvent	Temperature (°C)	Shape	Size (nm)	R(002)/(101)^d	Specific surface area (m^2 gm^−1)
a Length of the rod.b Diameter of the rod.c Unable to determine.d Measured intensity ratio of the peaks corresponding to planes (002) and (101), obtained from the XRD patterns in Fig. 1.e Not determined.
ZnO-1	MeOH	30	Spherical	5	0.48	98.3
ZnO-2	MeOH	65	Spherical	7	0.55	100
ZnO-3	DMF	80	Rod	33^a & 8^b	1.03	125
ZnO-4	Dioxane	80	Rod	30^a & 9^b	0.69	110
ZnO-5	THF	70	Rod	21^a & 9^b	0.67	96.7
ZnO-6	DMSO	80	Rod	28^a & 10^b	0.77	—^e
ZnO-7	Dioxane–MeOH = 1 : 1	80	Spherical	4.5	0.58	93
ZnO-8	Dioxane–DMF = 1 : 1	80	Rod & Spherical	—^c	0.87	68
ZnO-9	Dioxane–DMF–DMSO = 45 : 45 : 10	80	Rod	62^a & 11^b	0.89	—^e

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2.3 Photocatalytic degradation of different organic dyes using different shaped ZnO NPs

In a typical photocatalytic reaction, ∼1 mg of different ZnO NP samples was taken in 3 mL of 3 mg L⁻¹ aqueous solution of different organic dyes, such as R6G, CV and MB in a quartz test tube. The suspension was then sonicated for 10 min to disperse the ZnO NPs uniformly. The solution was then stirred for about 10 min for homogenization in the dark. The photocatalytic degradations of organic dyes were performed using these solutions under UV light irradiation (350 nm) at room temperature in the photoreactor. The progress of the reactions was monitored by monitoring the disappearance of the peaks at λ_max = 526, 590 and 663 nm of organic dyes, R6G, CV and MB, respectively, with time using a spectrophotometer. We also varied the concentration of R6G from 2 to 4 to 5 mg L⁻¹, keeping the concentration of ZnO-3 constant (∼1 mg) in each case.

2.4 Characterization

The crystallinity and microstructure of the purified and dried ZnO NPs powder was studied using a Bruker D8 X-ray diffractometer operated at an accelerating voltage of 40 kV with a current intensity of 40 mA.

For high-resolution TEM measurements, one drop of the purified and re-dispersed ethanolic suspension of ZnO nanostructures were placed on a carbon-coated copper grid and allowed to dry in air. The grids were then observed on a JEOL JEM-2010 electron microscope operated at an accelerating voltage of 200 kV.

UV-visible absorption spectra of an aqueous suspension of ZnO NPs were recorded using a Hewlett Packard 8453 spectrophotometer by transferring an appropriate volume of the ZnO suspension in a quartz cuvette. The photocatalytic activities of the ZnO NPs were also measured using the same spectrophotometer.

The photoluminescence spectra of an aqueous suspension of ZnO nanostructures were recorded using Jobin-Yvon Fluoromax-3 Spectrophotometer. For photoluminescence measurement, the samples' water suspensions were excited at a wavelength of 325 nm having excitation/emission slits of 5/5.

The photocatalytic degradation of the aqueous solution of dyes was performed in the Luzchem-ICH2 (Canada) photoreactor using a 350 nm UV lamp with a light intensity of 13 mW cm⁻². The light intensity reading was obtained using a Digital Lux Meter (SMART SENSOR, AR823).

Zeta potentials of the aqueous suspension of samples ZnO-2 and ZnO-3 at different pH were measured using Malvern Zetasizer NANO ZS 90 (Model no. 3690) using a HeNe gas laser of 632.8 nm.

The specific surface areas of the as-prepared ZnO samples were determined by the Brunauer–Emmett–Teller (BET) method using Quantachrome Instrument (NOVA 4000 E series), USA. About 20–60 mg of the ZnO sample was taken in both cases and the sample was degassed for about 8 h before starting the surface area measurement. The pore size distribution (PSD) was calculated by means of the Barrett–Joyner–Halenda (BJH) method.

High performance liquid chromatography (HPLC) study for the photodegradation of R6G with ZnO-3 (as a representative case) was performed using pure acetonitrile by Waters 1525 binary HPLC pumps and Waters 2487 UV detector. The solvent flow was 0.8 mL min⁻¹. The absorbance was monitored at 254 nm.

¹H spectra of R6G and the photodegraded R6G in CDCl₃ were acquired using a Bruker DPX 300 MHz spectrometer.

3. Results and discussion

3.1 Synthesis of size- and shape-tunable ZnO nanostructures

Size- and shape-tunable ZnO nanostructures were prepared by a wet chemical hydrolytic method from Zn(acac)₂ using an IL, TBPH in different water-miscible organic solvents as the reaction medium. Table 1 summarized the entire reaction conditions for the synthesis of the differently sized and shaped ZnO nanostructures. We chose Zn(acac)₂ salt as the precursor of zinc, as it is soluble in different organic solvents.^19,50 In this case, TBPH acted as the hydrolytic agent for Zn(acac)₂. Furthermore, we chose different organic solvents with varying polarity that miscible with water. We used methanol, DMF and DMSO, which are highly polar solvents, dioxane with lower polarity, and THF with medium polarity. The temperature of the reaction was varied from set to set as it depended on the boiling point of the solvent used. Note that the TBPH was added to the reaction medium after the desired temperature was reached. We also synthesized ZnO nanostructures in different mixed solvents of intermediate polarities and to examine the effects of polarity on the size and shape of the obtained ZnO NPs (see the reaction conditions for samples ZnO-7, ZnO-8 and ZnO-9). In these three reaction sets, the temperature of the reactions was maintained at 80 °C.

3.2 X-ray diffraction study

The purity and the crystalline nature of the obtained ZnO nanostructures were examined via powder X-ray diffraction (XRD). The XRD patterns (Fig. 1) of different ZnO samples (Table 1) showed peaks at 2θ = 31.8, 34.5, 36.4, 47.5, 57.1, 63.2, 66.7, 67.8, 69, 72.6, and 76.8°, which matched well with that of bulk wurtzite hexagonal ZnO having lattice constants ‘a’ and ‘c’ equal to 3.25 and 5.21 Å, respectively (JCPDS file no. 36–1451).^20,31 No other characteristic peaks corresponding to impurities, such as zinc hydroxide, were observed in the XRD patterns, indicating the formation of pure ZnO.^20,31 The measured intensity ratio, R_(002)/(101), of the peaks belonging to the (002) and (101) planes for all the samples were also depicted in Table 1. The measured intensity ratios, R_(002)/(101), for samples ZnO-1, ZnO-2 and ZnO-7 prepared in methanol or methanol and dioxane mixture were very close to the standard value of 0.44 of bulk hexagonal wurtzite ZnO.²⁰ These results indicated the formation of spherical ZnO NPs for those samples.^20,27 But the values of R_(002)/(101) for samples ZnO-3, ZnO-4, ZnO-5, ZnO-6, ZnO-8 and ZnO-9 were much higher than the corresponding standard values for bulk hexagonal wurtzite ZnO. These results indicated that these nanostructures might be anisotropic in shape.^20,27,31

Fig. 1 XRD patterns of different ZnO nanostructure samples, as mentioned in Table 1: (a) ZnO-1; (b) ZnO-2; (c) ZnO-3; (d) ZnO-4; (e) ZnO-5; (f) ZnO-6; (g) ZnO-7; (h) ZnO-8; (i) ZnO-9.

3.3 Transmission electron microscopic study

TEM micrographs of ZnO-1, ZnO-2, ZnO-3, ZnO-4, ZnO-5 and ZnO-6 samples prepared in neat solvents were presented in Fig. 2. The samples ZnO-1 and ZnO-2 showed similar spherical morphologies, as in both cases methanol was used as the reaction medium (Fig. 2A and B). The average sizes of the ZnO-1 and ZnO-2 spheres were found to be around 5 and 7 nm, respectively (see Table 1). The samples, ZnO-3, ZnO-4, ZnO-5 and ZnO-6, showed rod-shaped particles that were prepared in DMF, dioxane, THF and DMSO solvent, respectively. The lengths of the rods for the samples ZnO-3, ZnO-4, ZnO-5 and ZnO-6 were measured to be around 35, 30, 21 and 28 nm, respectively (Table 1). The diameters of these nanorods for all four samples varied from 8–10 nm. We performed the high resolution TEM measurement for better understanding of the growth process of both the spherical and rod-shaped particles. The HRTEM images for the representative samples ZnO-2 and ZnO-3 are shown in Fig. 3. From Fig. 3A, it was clear that the spherical nanoparticles are formed due to growth along the (101) direction, whereas the rod-shaped particles were formed due to the preferential growth along the (002) direction, as can be seen from Fig. 3B.³¹ We can explain these differences in the resultant shape of the particles from the Hansen solubility parameters of the different solvents (see Table S1 in the ESI†).⁵¹ From these parameters, it is clear that methanol, a protic solvent, has a greater tendency to form the hydrogen bond but less tendency to disperse.⁵¹ On the other hand DMF, dioxane, THF and DMSO are aprotic solvents and have a very high tendency to disperse and a low tendency to form the H-bond.⁵¹ Due to this higher H-bond forming tendency, methanol stabilizes all the facets of the synthesized ZnO NPs, resulting in the formation of spherical NPs, as observed in the case of ZnO-1 and ZnO-2. Because of this higher dispersion tendency of other solvents except methanol, these solvents preferentially stabilize the (002) plane of the synthesized ZnO NPs, allowing growth along the (002) plane to form rod shaped particles (for samples ZnO-3, ZnO-4, ZnO-5 and ZnO-6 and see Fig. 3 and Table 1 & XRD section).³¹ TEM micrographs of ZnO-7, ZnO-8 and ZnO-9 samples (Table 1) prepared with different solvent mixtures were presented in Fig. 4. The sample ZnO-7, synthesized in methanol and dioxane (1 : 1) mixture, gave spherical particles with an average diameter of 4.5 nm (Fig. 4A). When the dioxin–DMF (1 : 1) mixture was used as the reaction medium, we observed mostly rod shaped particles for the ZnO-8 sample, along with a few spherical particles, as can be seen from Fig. 4B. Finally, sample ZnO-9 showed rod-shaped particles when prepared in DMF–dioxane–DMSO (45 : 45 : 10) mixture (Fig. 4C). The lengths of the nanorods were about 62 nm and an average diameter of about 11 nm.

Fig. 2 TEM images of different ZnO nanostructure samples prepared in different solvents, as mentioned in Table 1: (a) ZnO-1; (b) zno-2; (c) ZnO-3; (d) ZnO-4; (e) ZnO-5 and (f) ZnO-6.

Fig. 3 HRTEM images of (a) ZnO-2 and (b) ZnO-3 nanostructures.

Fig. 4 TEM images of different ZnO nanostructure samples prepared in different mixed solvents, as mentioned in Table 1: (a) ZnO-7; (b) ZnO-8 and (c) ZnO-9.

3.4 Optical properties study

The optical properties of the re-dispersed aqueous suspensions of different shaped ZnO nanostructures were studied via UV-vis absorption and photoluminescence (PL) spectroscopy. The UV-vis spectra (Fig. 5) of all the ZnO samples of varying morphologies exhibited an excitonic⁵² absorption band with maxima at 334, 332, 347, 357, 350, 360, 341, 357 and 360 nm for ZnO-1, ZnO-2, ZnO-3, ZnO-4, ZnO-5, ZnO-6, ZnO-7, ZnO-8 and ZnO-9, respectively, which are far below the bulk ZnO absorbance at 375 nm.^20,31,52,53 These values are tabulated in Table 2. These values revealed that ZnO-1, ZnO-2 and ZnO-7 showed a quantum confinement effect due to their smaller size.^52–54 A careful examination of the absorption maxima value revealed that the spherical ZnO NPs gave absorption maxima at lower wavelength, whereas the ZnO nanorods exhibited absorption maxima at higher wavelengths.³¹

Fig. 5 UV-vis absorption spectra of the aqueous suspension of size-and shape-tunable ZnO nanostructures as given in Table 1: (a) ZnO-1; (b) ZnO-2; (c) ZnO-3; (d) ZnO-4; (e) ZnO-5; (f) ZnO-6; (g) ZnO-7; (h) ZnO-8; (i) ZnO-9.

Table 2 Optical properties of aqueous suspensions of ZnO nanostructure samples

Sample	λmax^a (abs) (nm)	λmax^b (PL) (nm)	λmax^c (PL) (nm)
a Absorption maxima obtained from Fig. 5.b Band–band emission maxima obtained from Fig. 6.c Trap state emission maxima obtained from Fig. 6.
ZnO-1	334	352	549
ZnO-2	332	370	557
ZnO-3	347	373	570
ZnO-4	357	371	600
ZnO-5	350	370	610
ZnO-6	360	371	520
ZnO-7	341	358	551
ZnO-8	357	374	565
ZnO-9	360	382	—

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Fig. 6A and B show the PL spectra of aqueous suspensions of different shaped ZnO nanostructures samples synthesized in pure solvents and in different solvent mixtures, respectively. All the samples were excited at 325 nm. Most of the samples, such as ZnO-2, ZnO-3, ZnO-4, ZnO-5, ZnO-6, and ZnO-8, exhibited an excitonic emission band at around 370 nm (see Fig. 6).^{13–15,53,55,56} However, ZnO-1 (Fig. 6a) and ZnO-7 (Fig. 6g) showed excitonic emissions at much lower wavelengths (∼355 nm).^13–15 Interestingly, ZnO-1 and ZnO-7 gave very low excitonic emission whereas their trap state emissions were very intense at around 550 nm (Fig. 6 and Table 2).⁵³ These results clearly stated that the obtained ZnO NPs were capable of exhibiting green emission.⁵³ The sample ZnO-2 also exhibited green emission at around λ_max = 557 nm. But, ZnO-3 (570 nm), ZnO-4 (600 nm) and ZnO-5 (610 nm) mostly provide trap state emission in the yellow regimes.¹⁶ The ZnO-6 sample predominantly showed band–band emission at around 371 nm and a small green trap state emission (520 nm) at higher wavelengths.^{13–15,55,56} Interestingly, the samples synthesized in solvent mixtures (Fig. 6B), such as ZnO-7 (551 nm) and ZnO-8 (565 nm), exhibited strong green trap-state emission, whereas ZnO-9 did not show any trap state mission. The ZnO-9 sample predominantly exhibited band-to-band emission at 382 nm in the blue regimes.^18,55 These trap state green and/or yellow emissions were the manifestation of the recombination of the conduction band electrons with the deep traps in the singly ionized oxygen vacancy sites and/or the single negatively charged interstitial oxygen ions, respectively, in the synthesized ZnO NPs.⁵⁷

Fig. 6 Photoluminescence spectra of the aqueous suspension of size-and shape-tunable ZnO nanostructures as given in Table 1: (A) (a) ZnO-1; (b) ZnO-2; (c) ZnO-3; (d) ZnO-4; (e) ZnO-5; and (f) ZnO-6; (B) (g) ZnO-7 (h) ZnO-8 and (i) ZnO-9.

3.5 Dye adsorption study of the different shaped ZnO nanostructures

The adsorption of the organic dyes at the surface of the ZnO nanostructures was also an important factor in determining the activity, such as the photocatalytic activity of the materials.^46,48,58 Therefore, we quantified the adsorption of the organic dyes at the surface of different ZnO nanostructure samples. As a representative case, we added ∼1 mg of ZnO-3 sample in 3 mL solution of R6G at concentrations of 2, 3, 4 and 5 mg L⁻¹. The solutions were kept in the dark for an hour and the absorption spectra of R6G were acquired, as shown in Fig. 7. Fig. 7 clearly shows that the amounts of adsorption of R6G dye are almost the same for the ZnO-3 sample at different concentrations of R6G. We also determined the amount of R6G adsorption for different ZnO samples (∼1 mg) using 3 mL solution of R6G with a concentration of 3 mg L⁻¹ (see Fig. S1 in ESI†). It was clear that the amounts of dye adsorption for different kinds of ZnO samples were very small (Fig. S1†). To better understand these very small amounts of adsorption at the surface of ZnO, we further performed zeta potential measurements of the samples ZnO-2 and ZnO-3 as the representative cases at different pH values (see Fig. 8 and S2 in ESI†). From Fig. 8, it was clear that the surface of the ZnO nanostructures were positively charged in the neutral pH range. This positive charge may be attributed to the adsorption of a small amount of tetrabutylphosphonium cation during the growth of the nanomaterials.³¹ Because of this positive charge at the surface of the obtained ZnO samples, R6G, a cationic dye, had much less tendency to be adsorbed on its surface (see Fig. S3 in the ESI†). The small adsorption occurred because of the small amount of adsorbed OH⁻ ion present at the ZnO surface.⁵⁸ It was observed that both ZnO-2 and ZnO-3 samples adsorbed a very small amount of CV and MB dyes when kept for an hour in the dark (see Fig. S4 in the ESI†). These results are similar to that of adsorption of R6G dye on the surface of different ZnO samples (Fig. S1†).

Fig. 7 UV-vis absorbance spectra of R6G at different concentrations (a) 2 mg L⁻¹ (b) 3 mg L⁻¹ (c) 4 mg L⁻¹ and (d) 5 mg L⁻¹ in the presence of ∼1 mg ZnO-3.

Fig. 8 Zeta potentials of ZnO-2 and ZnO-3 samples at different pH values.

3.6 Photocatalytic studies of different shaped ZnO nanostructures

It is known that ZnO has been widely used as a low cost and environmentally friendly photocatalyst with a very high activity.^38–41 The photocatalytic activities of the different shaped ZnO NPs were measured by measuring the rate of photodegradation of different organic dyes, such as rhodamine (R6G), crystal violet (CV) and methylene blue (MB) at neutral pH. R6G showed absorbance maxima at λ_max = 526 nm. The rates of degradation of R6G were measured by monitoring the disappearance of the 526 nm peak with time, which causes fading and ultimate bleaching of the color of the reaction mixture. The representative spectra showing the decrease of the absorbance maxima for R6G are provided in Fig. S5 of the ESI.† The apparent rate constants of this reaction in the presence of different ZnO samples were obtained from linear fittings of ln A₅₂₆ (A = normalized absorbance at 526 nm) versus time (Fig. 9). It is well known that the reaction follows (pseudo) first order kinetics.⁵⁹ The values of the apparent rate constants (k_apps) for all the synthesized samples towards this reaction are given in Table 3. For comparison, the catalytic activities of different samples are expressed in term of normalized rate constant (k_nors), which were obtained by normalizing the k_app values with the total amount of sample used, and all the data are tabulated in Table 3. From these normalized values, it was clear that all the different ZnO nanostructures showed strong photocatalytic activity towards the degradation of R6G. Among all the samples, the normalized rate constant of ZnO-5 (k_nor = 3.4 × 10⁻² min⁻¹ mg⁻¹) showed a very high catalytic activity towards the degradation of R6G in water.^60,61

Fig. 9 The logarithmic plots of the absorbance at λ_max = 526 nm vs. time for calculating rate constants of photocatalytic degradation of R6G with different ZnO nanostructure samples, as given in Table 1.

Table 3 The apparent and normalized rate constants obtained from the photocatalytic degradation of three organic dyes using different sized and shaped ZnO nanostructures

Sample name	kapp^a (R6G) (min^−1) @ 526 nm	knor^b (R6G) (min^−1 mg^−1) @ 526 nm	kapp^a (CV) (min^−1) @5 90 nm	knor^b (CV) (min^−1 mg^−1) @ 590 nm	k^aapp (MB) (min^−1) @ 663 nm	knor^b (MB) (min^−1 mg^−1) @ 663 nm
a Apparent rate constant.b Normalized rate constants with respect to weight.
Blank	8.3 × 10^−4	8.3 × 10^−4	2.9 × 10^−4	2.9 × 10^−4	1 × 10^−4	1 × 10^−4
ZnO-1	1.9 × 10^−2	1.4 × 10^−2	1.3 × 10^−2	0.9 × 10^−2	2.3 × 10^−2	1.7 × 10^−2
ZnO-2	0.75 × 10^−2	0.65 × 10^−2	0.77 × 10^−2	0.63 × 10^−2	1.1 × 10^−2	0.8 × 10^−2
ZnO-3	1.1 × 10^−2	1.35 × 10^−2	1.0 × 10^−2	1.23 × 10^−2	0.5 × 10^−2	0.37 × 10^−2
ZnO-4	0.53 × 10^−2	0.46 × 10^−2	1.3 × 10^−2	1.21 × 10^−2	1.1 × 10^−2	2.3 × 10^−2
ZnO-5	3.5 × 10^−2	3.4 × 10^−2	8.4 × 10^−2	7.0 × 10^−2	3.8 × 10^−2	2.8 × 10^−2
ZnO-6	0.31 × 10^−2	0.28 × 10^−2	1.8 × 10^−2	1.21 × 10^−2	0.22 × 10^−2	0.14 × 10^−2
ZnO-7	0.33 × 10^−2	0.26 × 10^−2	0.38 × 10^−2	0.37 × 10^−2	0.54 × 10^−2	0.35 × 10^−2
ZnO-8	2.7 × 10^−2	2 × 10^−2	5.8 × 10^−2	6.1 × 10^−2	4.4 × 10^−2	4.8 × 10^−2
ZnO-9	2.0 × 10^−2	1.85 × 10^−2	2.0 × 10^−2	2.06 × 10^−2	2.3 × 10^−2	2 × 10^−2

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Similarly, CV and MB gave a strong absorbance having λ_max = 590 and 663 nm, respectively. The progress of the reaction was also observed by monitoring the disappearance of the λ_max peaks with time, which causes fading and ultimate bleaching of the colour of these reaction mixtures. The corresponding decay curve for a few representative cases was given in Fig. S6 and S7 (ESI†) for the organic dyes CV and MB, respectively. The apparent rate constants of these two reactions in the presence of different ZnO samples were obtained from linear fittings of ln A₅₉₀ and ln A₆₆₃ (A = normalized absorbance at 590 and 663 nm) versus time for the organic dyes CV and MB, respectively (Fig. 10 and 11). The values of the apparent rate constants (k_apps) for the ZnO samples towards these reactions are also given in Table 3. For comparison, the catalytic activities of different samples are also expressed in terms of the normalized rate constant (k_nors) for CV and MB degradation, as obtained for R6G degradation (see Table 3). For the degradations of CV, ZnO-5 and ZnO-8 samples showed a very high catalytic activity, having k_nor = 7 × 10⁻² min⁻¹ mg⁻¹ and 6.1 × 10⁻² min⁻¹ mg⁻¹, respectively, when compared with recently reported values.⁵⁸ Similarly, for MB degradations, the values of the normalized rate constants obtained for the samples ZnO-5 and ZnO-8 were k_nor = 2.8 × 10⁻² min⁻¹ mg⁻¹ and 4.8 × 10⁻² min⁻¹ mg⁻¹, respectively, which was also higher than the recently reported rate constants for the same reaction.⁶² The normalized rate constants (k_nors) obtained for the degradation of all the organic dyes are tabulated in Table 3. The k_nor value clearly shows that almost all of the as-synthesized ZnO NPs showed high activity towards the photodegradation of all three organic dyes. The very high photocatalytic activities of the ZnO-5 and ZnO-8 samples might be attributed to the anisotropic (rod) shaped ZnO particles and also the greater amount of reactive interstitial oxygen ions present at the surface of the materials, as evidenced from the photoluminescence spectra (Fig. 6).³⁸ These reactive oxygen ions reacted with the dye molecules very fast, causing the degradation of the dye, which eventually gave us very high photocatalytic degradation rate constants.³⁸ During the photocatalytic degradation of all the organic dyes, the only absorbance maxima decrease with time but there was no shift in the absorbance maxima, and no other new peak developed, which clearly indicates that the organic dyes were degraded during this photocatalytic reactions.⁵⁹

Fig. 10 The logarithmic plots of the absorbance at λ_max = 590 nm vs. time for calculating rate constants of the photocatalytic degradation of CV with different ZnO nanostructure samples, as given in Table 1.

Fig. 11 The logarithmic plots of the absorbance at λ_max = 663 nm vs. time for calculating rate constants of the photocatalytic degradation of MB with different ZnO nanostructure samples, as shown in Table 1.

To demonstrate further that the dye was really degraded in the presence of ZnO material under UV irradiation, we monitored the degradation of dye using high performance liquid chromatography (HPLC). As a representative case, we monitored the degradation of R6G by monitoring the residual R6G present in the aliquot after its degradation by ZnO under UV irradiation. Fig. S8(ESI†) showed the peak corresponding to R6G present in the aliquot, which decreased on increasing the UV irradiation time for degradation of the dye. It was clear that with increasing UV irradiation time, the amount of R6G present in the solution decreased and eventually R6G degraded almost completely. The decomposition of the R6G dye was also confirmed by NMR experiment (see Fig. S9, ESI†). Fig. S9A† showed the characteristic signals of neat R6G in CDCl₃, but after photodegradation it did not show any characteristic peak of R6G (see Fig. S9B, ESI†). These results confirmed the photocatalytic degradation of R6G in the presence of ZnO under UV irradiation.

In our case, almost all the synthesized ZnO nanostructures showed very high rate constants for the degradations of three organic dyes. Among these, the ZnO-1, ZnO-3, ZnO-4, ZnO-5 and ZnO-8 samples showed very high catalytic activity, which is also comparable to the recently reported rate constants for the degradation of R6G, CV and MB (see Table 3).^{38,39,60–63} ZnO-5 showed the highest photocatalytic activity for the degradation of all three dyes used. The obtained rate constant for ZnO-5 was one of the highest reported, when compared with the other reported values for R6G,^60,61 CV⁵⁸ and MB.^62,64 To better understand the photocatalytic activities of these spherical and rod-shaped ZnO samples, we measured the BET specific surface areas of seven samples (ZnO-1, ZnO-2, ZnO-3, ZnO-4, ZnO-5, ZnO-7, and ZnO-8) of varying shapes (Table 1) by N₂ adsorption/desorption analysis. The N₂ adsorption/desorption isotherms were provided in Fig. S10 (ESI†). The measured specific surface areas of the six samples were in the range 93–125 m² g⁻¹ (see Table 1). But the surface area (68 m² g⁻¹, see Table 1) of sample ZnO-8 was a little bit lower than that of the other samples. Overall, the surface area values of all these ZnO samples were quite close and were relatively small, because these ZnO materials are not porous, which is expected from their method of preparation. This is the reason why we did not observe any porous structure by TEM, as mentioned above. There was also no conclusive evidence of the presence of a porous structure from the BJH analysis of the isotherms of a few ZnO samples (see Fig. S11 in the ESI†). From the surface area measurement results, it was very clear that the specific surface area values of all these samples were not following any clear trend. Therefore, it is not logical to correlate the difference of photocatalytic activities of these nonporous ZnO particles with the help of specific surface areas (see Tables 1 and 3). As mentioned earlier, the trap state emission of the synthesized ZnO nanostructures played a vital role in photocatalytic degradation reactions.³⁸ A similar type of observation was also reported earlier by Zheng et al.³⁸ Since the trap states contain a large amount of oxygen defects, such as singly ionized oxygen vacancy sites and/or the single negatively charged interstitial oxygen ions, these oxygen defects took a predominant role during the photobleaching process, causing faster degradation of the organic dyes.³⁸ For this reason the as-synthesized ZnO-5 and ZnO-8 NPs showed very high photocatalytic activities towards the degradation of the organic dyes, as they contain a large number of oxygen defects, as evident from the green and yellow photoluminescence of the ZnO NPs described above.

We also checked the stability of the ZnO materials after photocatalysis. As a representative case, ZnO-3 was characterized after the photocatalytic degradation of R6G and the data is presented in Fig. 12. Fig. 12A reveals that ZnO-3 showed exactly the same absorbance spectra as that before catalysis, which showed that the structure of the material remained intact after catalysis. Fig. 12B shows the TEM image of the ZnO-3 after catalysis, which also matches well with the size and shape of the same samples before catalysis (see Fig. 2C).

Fig. 12 (A) The UV-vis absorbance of ZnO-3 sample before and after photocatalysis (B) TEM image of ZnO-3 sample after photocatalysis.

4. Conclusions

In conclusion, ZnO nanostructures of various sizes and shapes have been synthesized in different solvents and in solvent mixtures using zinc acetylacetonate as the zinc precursor and an ionic liquid, tetrabutylphosphonium hydroxide, as the hydrolyzing agent. The reaction medium played an important role during the growth of the particles. The obtained ZnO nanostructures were highly crystalline in nature, as characterized using X-ray diffraction. TEM study showed the formation of different sized and shaped ZnO nanostructures. Protic solvents favored the growth of spherical ZnO nanoparticles, whereas the aprotic solvents preferentially stabilize the (002) plane to form rod-shaped particles. The size and shape of ZnO NPs can be easily tuned only by varying the reaction solvent. The as-formed ZnO nanostructures showed tunable absorbance from 332 nm to 360 nm, depending on the size and shape of the materials. The spherical ZnO nanoparticles showed a quantum confinement effect and absorb in the lower wavelength regions compared to the rod-shaped ZnO nanostructures. The fluorescence emission can be tuned from blue to green to yellow through adjustment and control of the size and shape of ZnO nanostructures, when dispersed in aqueous suspension. The obtained ZnO nanostructures showed very high photocatalytic activity towards the degradation of the different cationic organic dyes (such as rhodamine 6G, crystal violet and methylene blue) as it contains a large number of defect states, as evident from the photoluminescence study. Along with the defect states, the rod-shaped ZnO nanoparticles showed higher photocatalytic activity when compared with the spherical ZnO nanoparticles. The as-synthesized ZnO nanomaterials were very stable under the photocatalytic conditions.

Acknowledgements

MB thanks CSIR, New Delhi for providing fellowship. This research is supported by grants from the CSIR, New Delhi, India. Thanks are also due to the partial financial support from BRNS, India. We also thank Dr Rajendra Nath Basu, CSIR-Central Glass and Ceramic Research Institute for helping us in measuring the surface areas of the ZnO samples.

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Footnote

† Electronic supplementary information (ESI) available: Successive UV-vis absorption spectra of photocatalytic degradation of different dyes, spectral evidence of dye adsorption on ZnO surface, N₂ adsorption/desorption isotherms. Chemical structure of the dyes, Zeta potential measurements, HPLC data for R6G and NMR spectra of R6G. This material is available free of charge via the Internet at E-mail: http://www.pubs.rsc.org; . See DOI: 10.1039/c3ra44859b

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