1 Introduction

ZnO is a most versatile multifunctional n-type semiconductor because of its wide direct band gap (3.35 eV at 300 K), high exciton binding energy (60 meV), high melting point, excellent radiation hardness and piezoelectric properties [1]. ZnO nanostructures have promising applications in photocatalysis, solar cells, optical waveguides, sensors, and nano-generator [2–8] etc. Among these applications, the photocatalytic activity has received much attention of researchers and widely studied because of its characteristic features such as, easy tuning to various surface morphologies [3, 9–13], low temperature sample processing, high defects, high quantum yield for peroxide production and low cost [14–24]. Photocatalysis involves the generation of active species upon illumination of aqueous suspensions of semiconductor with the light of suitable wavelength. These active species are proficient in carrying out the destruction of organic pollutants [25, 26]. ZnO nanostructures with different morphologies can be synthesized by various synthesis routes such as sol–gel, precipitation, solvothermal, electrodeposition, mechanochemical and sonochemical [27–33] etc. Xie et al. [3] have reported that the sheet –like structures of ZnO exhibit better photocatalytic activities as compared to its spherical structure. Wang et al. [10] have prepared ZnO with flowers and rods like morphologies and concluded that ZnO nanoflowers show the higher photocatalytic activity than that of ZnO nanorods. Sun et al. [9] have obtained ZnO-3D hierarchical structures of ZnO assembled by nanosheet, nanoneedle and found that flower shaped sample exhibits an enhanced photocatalytic performance as compared with the nanosheet, nanoneedle structures. In the present investigation, we have reported the synthesis, characterization and photocatalytic activity of hierarchically assembled architectures of ZnO-NFs obtained by the wet chemical method without the use of any surfactant or capping agent. The photocatalytic activity is systematically studied by observing the degradation of Rhodamine B (RhB) dye.

2 Experimental

2.1 Materials used

Zinc acetate dihydrate [Zn(CH3COO)2·2H2O-Purity ≥ 98 %] is obtained from Merck-India. Methanol is purchased from Sigma-Aldrich (CH3OH-Purity ≥ 99 %) and Lithium hydroxide (LiOH-Purity = 99 %) is purchased from Otto. RhB dye is obtained from Fisher scientific (Purity ≥ 99 %). All the chemicals are used without further purification.

2.2 Preparation of ZnO-NFs

A 0.25 M solution of Zn(CH3COO)2·2H2O is prepared in 100 ml methanol and refluxed at about 70 °C for 30 min under magnetic stirring to get a well-mixed solution. For hydrolysis, 0.25 M solution of LiOH in 100 ml methanol is added dropwise to it. The resulting solution is refluxed at 70 °C under continuous stirring for 2 h to complete precipitation process. The reacted mixture is then kept for some time and the clear solution is decanted. Finally, the precipitates are washed, filtered and dried in vacuum oven for 24 h and used for further characterization.

2.3 Characterization

The morphology and elemental investigations are carried out using Field emission-scanning electron microscope (FE-SEM, SIGMA HV—Carl Zeiss) equipped with energy dispersive X-ray analysis (EDX) Bruker machine. The crystal structure of the synthesized sample is investigated by X-ray diffraction (XRD, X’Pert Pro, Philips) with Cu-Kα radiation (λ = 1.5406 Å). The characteristic functional groups are investigated using Fourier transform infrared (FTIR PerkinElmer Spectrum 65) spectrophotometer. Raman study is performed with an InVia Raman spectrophotometer (Renishaw) with Ar+ ion laser beam having a wavelength of 514 nm. The Optical properties are determined by Ultraviolet–visible (UV–Vis) spectrometer (PerkinElmer, LAMBDA 750).

The photocatalytic activity of the as-prepared ZnO-NFs is investigated by the degradation of RhB under UV irradiation at room temperature. Prior to irradiation, the aqueous suspension of RhB and ZnO-NFs powder is stirred in dark for 30 min to establish the adsorption–desorption equilibrium and then irradiated under the 30 W UV lamp. A series of samples are taken out after a regular time interval and centrifuged to remove the suspended ZnO-NFs and analyzed by a UV–Vis spectrophotometer to obtain the absorbance of the RhB solutions.

The degradation efficiency X (%) is calculated from absorption spectra using the following relation:

$$X\,(\% ) = \frac{{(A_{0} - A_{t} )}}{{A_{0} }} \times 100,$$
(1)

where A0 represents the initial absorbance and At is the absorbance at certain time t. The photocatalytic degradation reaction of RhB follows pseudo-first order kinetics and the rate constant is determined by the following relation:

$$\ln \,\left( {\frac{{C_{0} }}{{C_{t} }}} \right) = k \times t = \ln \,\left( {\frac{{A_{0} }}{{A_{t} }}} \right),\,\,\,\,\,\,$$
(2)

The rate constant k is calculated from the slope of straight line portion of ln(A0/At) versus t plot.

3 Results and discussion

Figure 1a–c shows the FE-SEM images of synthesized ZnO-NFs assembled flowers taken at the different resolution. The images clearly reveal that these nanoflakes have the thickness of below 100 nm range and are grown upward to form flower-like morphology.

Fig. 1
figure 1

a–c FE-SEM images and d EDX spectrum of wet chemical route synthesized ZnO-NFs

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The Fig. 1d shows the EDX spectrum of ZnO-NFs. It clearly confirms the presence of zinc and oxygen elements in the sample. No peaks belonging to any other element is found in the spectrum, which confirms that the prepared sample is free from impurity.

Figure 2 shows XRD pattern of the wet chemical route synthesized ZnO-NFs. All the diffraction peaks are well matched and can be indexed as the hexagonal wurtzite ZnO as reported in standard diffraction data (JCPDS card No.: 36-1451).

Fig. 2
figure 2

XRD pattern of wet chemical route synthesized ZnO-NFs

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Figure 3 shows the Raman spectra of the as-prepared ZnO sample. For wurtzite ZnO (space group P63mc) near the center of the Brillouin zone (k = 0), group theory predicts the existence of the following optical modes: Γopt = A1 + E1 + 2E2 + 2B1. The B1 modes are silent, the E2 modes are non-polar and Raman active, while A1 and E1 are polar modes and Raman as well as infrared active [34].

Fig. 3
figure 3

Raman spectra of the as-synthesized ZnO-NFs

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The two dominant peaks at around 101 and 439 cm−1 are characteristic peaks of the wurtzite ZnO and are assigned to the low- and high-E2 mode. The peaks around 385 and 413 cm−1 are attributed to the A1(TO) mode and E1(TO) modes, respectively. A smaller peak around 334 cm−1 is attributed to a [E2(high) − E2(low)] mode. The Raman peak located around 583 cm−1 corresponds to A1 + E1 symmetry with LO modes. The presence of high intensity of E2 modes represents the good quality of ZnO nanomaterials, which is consistent with XRD result. Figure 4 shows the FTIR spectrum of prepared ZnO-NFs. It illustrates a series of absorption bands in the range of 450–4000 cm−1. The broad absorption band observed at ~3300 cm−1 is assigned to the O–H stretching mode for –COOH group in zinc acetate and H2O molecule. Two absorption bands at ~2880 and ~2940 cm−1 correspond to the C–H stretching vibrations of acetate. The bands appearing between ~1400 and 1600 cm−1 are due to the stretching modes (symmetric and asymmetric) of the acetate group (–COOH). The absorption band at ~1000 cm−1 is due to the C=O deformation mode. These groups are believed to be adsorbed on the surface of the nanoparticles during the synthesis process. The band positioned at ~460 cm−1 is associated with the characteristic wurtzite lattice vibrations (Zn–O) [35, 36].

Fig. 4
figure 4

FTIR spectrum of wet chemical route synthesized ZnO-NFs

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The optical band gap of the ZnO-NFs sample is estimated using the Tauc’s equation which demonstrates a relationship between absorption coefficient (α) and the incident photon energy (hν) as follows [37]:

$$(\alpha h\nu ) = A(h\nu - E_{g} )^{n}$$
(3)

Figure 5 shows the (αhν)2 versus hν plot using the data from optical absorbance spectra. An extrapolation of the linear portion of the plot onto the x-axis [(αhν)2 = 0] gives the value of Eg. The calculated value of optical band gap is 3.22 eV.

Fig. 5
figure 5

Tauc’s plot of wet chemical route synthesized ZnO-NFs. The inset shows UV–Vis absorption spectrum of ZnO-NFs

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3.1 Photocatalytic activity

The underlying mechanism of the photocatalytic degradation of RhB by the ZnO-NFs under irradiation can be explained on the basis of photogeneration of electron–hole pairs (excitons) in ZnO-NFs. The photo-generated charge carriers move freely on the surface of the ZnO-NFs and initiate redox reactions with water (H2O) and oxygen (O2) molecules. The holes in the valence band can act as the site for strong oxidation. They oxidize H2O molecules and hydroxyl (OH−) groups to generate highly reactive hydroxyl (OH·) radical and H+ ion. O2 molecules adsorbed on the surface is reduced by the electrons in the conduction band to form superoxide anion (O ·−2 ) radicals, which react with H+ to generate hydrogen peroxide (HO ·2 ) radicals. These HO ·2 radicals react with electrons and H+ ions to produce molecules of H2O2. In addition, the intrinsic crystal defects (O-vacancies, Zn-interstitial) in the ZnO crystals act as electron donors. These additional electrons from the intrinsic crystal defects further promote the reactions to yield OH· radicals. These highly reactive species OH· radicals and H2O2 molecules have the strong oxidative potential for partial or complete decomposition of various organic, inorganic and microbial contaminants adsorbed on the photocatalyst surface [38–40]. The temporal degradation and kinetics of photocatalytic activity of batches of solutions with different ZnO, dye concentration and pH of the solution are presented in Figs. 6, 7 and 8, respectively. From Fig. 6 it is observed that the degradation of dye increases with the increase in the ZnO loading from 0.25 to 2.0 g/L for a fixed value of RhB concentration (5 ppm). This is attributed to the increase in the availability of reaction sites for the photocatalytic reactions with the increase in the ZnO loading. The increase in the reaction sites creates more hydroxyl radicals with UV irradiation which results in the degradation of more dye molecules.

Fig. 6
figure 6

a The photodegradation time profile and b the first-order kinetic model fit for the photodegradation of RhB solution (RhB concentration: 5 ppm) in the presence of different ZnO-NFs loading (0.25–2.0 g/L)

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Fig. 7
figure 7

a The photodegradation time profile and b the first-order kinetic model fit for the photodegradation of RhB solution (RhB concentration: 5–30 ppm) in the presence of ZnO-NFs (1 g/L) photocatalyst

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Fig. 8
figure 8

a The photodegradation time profile and b the first-order kinetic model fit for the photodegradation of RhB solution (RhB concentration: 5 ppm and ZnO-NFs concentration: 1 g/L) with different solution pH

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Figure 7 represents the effect of the initial RhB concentration ranging from 5 to 30 ppm, on the photodegradation efficiency of ZnO-NFs. This shows that for a fixed load of ZnO (1.0 g/L), the rate of dye degradation decreases with the increase in the RhB initial concentration. It is due to the decrease in the both the availability of reaction sites and the path length of the photons entering into the solution. As the path length of photons entering into the solution decreases fewer photons reach the catalyst surface, which results in the decrease in the generation reactive hydroxyl and superoxide radicals.

Figure 8 shows the effect of solution pH on photocatalytic degradation of RhB. The pH of the solution is varied from 3.0 to 9.0 for fixed concentration of ZnO (1 g/L) and RhB (5 ppm). It is observed that the RhB photodegradation enhances with the increase in the pH of the solution. This is because, at high pH value, the presence of large quantities OH ions favours the formation of more hydroxyl radical and, therefore, increases the photodegradation of RhB.

Table 1 summarizes the calculated degradation efficiency and the rate constant with different catalyst loading, initial RhB concentration and solution pH. It represents that the degradation efficiency and the rate of degradation of RhB are more with the increase in the ZnO-NFs loading and the pH of the solution, whereas lowering with the increase in the concentration of RhB in the solution. These variations are attributed to the change in the in the availability of highly reactive species (OH· radicals and H2O2 molecules) with the change in the photocatalyst loading, dye concentration and pH of the solution values.

Table 1 Effect of photocatalyst loading, initial RhB concentration and solution pH on the photocatalytic activities of ZnO-NFs
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4 Conclusion

Flower-like ZnO-NFs structures have been synthesized successfully by simple wet chemical route without using any surfactant or capping agent. The formation of ZnO is confirmed by XRD, Raman, FT-IR and UV–Vis spectroscopy measurements. The photocatalytic study demonstrates an effective photodegradation of RhB dye by ZnO-NFs. These results suggest that as-synthesized ZnO flower-like structures can be employed as potential semiconductor photocatalyst for applications in wastewater treatment. Moreover, these ZnO flowers for their larger accessible surface area are also expected to be useful for other practical applications such as solar cells, sensors, and optical devices etc.