Effects of swift heavy ion irradiation on structural, optical and photocatalytic properties of ZnO–CuO nanocomposites prepared by carbothermal evaporation method

ZnO–CuO nanocomposite thin films were prepared by carbothermal evaporation of ZnO and Cu, combined with annealing. The effects of 90 MeV Ni7+ ion irradiation on the structural and optical properties of ZnO–CuO nanocomposites were studied by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV–visible absorption spectroscopy and Raman spectroscopy. XRD studies showed the presence of ZnO and CuO nanostructures in the nanocomposites. FESEM images revealed the presence of nanosheets and nanorods in the nanocomposites. The photocatalytic activity of ZnO–CuO nanocomposites was evaluated on the basis of degradation of methylene blue (MB) and methyl orange (MO) dyes under sun light irradiation and it was observed that swift heavy ion irradiation results in significant enhancement in the photocatalytic efficiency of ZnO–CuO nanocomposites towards degradation of MB and MO dyes. The possible mechanism for the enhanced photocatalytic activity of ZnO–CuO nanocomposites is proposed. We attribute the observed enhanced photocatalytic activity of ZnO–CuO nanocomposites to the combined effects of improved sun light utilization and suppression of the recombination of photogenerated charge carriers in ZnO–CuO nanocomposites.

Several methods have been used to modify the optical, electrical and structural properties of nanostructured materials and nanocomposite thin films. Swift heavy ion (SHI) irradiation using high electronic excitation is one of the promising techniques used to controllably engineer the size, shape, crystallinity and hence the physicochemical properties of nanostructured materials and nanocomposites. Energetic ions moving in a solid lose energy in elastic and inelastic scattering with the target nuclei and electrons, respectively. The localized deposition of high energy results in the formation of defects and induces structural transformations in solids. SHI irradiation depositing high energy in electronic excitations can lead to formation of latent tracks along ion path and results in material modifications including growth [33] and elongation [34] of nanoparticles embedded in different insulating matrices. Several attempts have been made to study SHI-irradiation-induced changes in the optical, electrical and structural properties of ZnO. Kumar et al. [35] have irradiated Co doped ZnO thin films, prepared by sol-gel route, with 100 MeV Ni 7+ ions and studied the modifications in their structural and optical properties. Kumar et al. [36] studied 130 MeV Ni 7+ irradiation induced morphological and optical changes of zinc aluminum oxide coated over porous silicon substrates. The changes in the structural and optical properties of ZnO thin films due to 100 MeV Au 8+ irradiation were investigated by Agarwal et al. [37]. Even though there have been several studies on the ioninduced evolution of the structural and optical properties of ZnO nanostructures, not much work has been done on the SHIinduced modifications in ZnO-CuO nanocomposites.
In this paper, we report on the effects of 90 MeV Ni 7+ ion irradiation on the structural, optical and photocatalytic properties of ZnO-CuO nanocomposites, which were prepared by a simple carbothermal reduction-based vapor deposition method. We have demonstrated that swift heavy ion irradiation can be employed to significantly enhance the sun light driven photocatalytic activity of ZnO-CuO nanocomposites toward the degradation of methylene blue (MB) and methyl orange (MO) dyes in water.

Results and Discussion
The FESEM images of pristine and irradiated nanocomposite samples are shown in Figure 1. The FESEM image of the pristine sample clearly illustrates the presence of a large number of nanosheets in addition to few nanorod-like structures, as shown in Figure 1a. It can be clearly seen that these nanosheets and nanorod like structures consist of smaller nanoparticles. Figure 1b shows the FESEM image revealing the surface morphology of nanocomposite following irradiation with 90 MeV Ni ions at a fluence of 3 × 10 13 ions/cm 2 . It can be clearly seen that swift heavy ion irradiation at a fluence of 3 × 10 13 ions/cm 2 resulted in the formation of a high density of nanosheets with reduced thickness. The FESEM images showing the surface morphology of nanocomposite following irradiation with 90 MeV Ni ions at a fluence of 1 × 10 14 ions/cm 2 are shown in Figure 1c and Figure 1d. The presence of large nanorods with distinct facets and increased width can be clearly seen. However, the density of nanorod-like structures formed is small and the average aspect ratio of such nanostructures was found to be 2.7, which is much smaller than that of the nanostructures in the pristine sample and the samples irradiated at lower fluences. The average thickness of nanosheets decreased from 52 nm (for pristine) to 45 nm at a fluence of 3 × 10 13 ions/cm 2 , whereas the average thickness of nanostructures increased to 91 nm for the nanocomposite irradiated at a fluence of 1 × 10 14 ions/cm 2 . The observed change in morphology from nanosheets to nanorods upon swift heavy ion irradiation at higher fluence is really interesting considering the appreciable radiation stability of ZnO.    The UV-visible absorption spectra of the pristine and irradiated nanocomposites are shown in Figure 3a. It can be clearly seen that swift heavy ion irradiation leads to enhanced absorption of the nanocomposite in the visible region. The band gap of pristine nanocomposite and sample irradiated at fluences of 3 × 10 12 , 1 × 10 13 , 3 × 10 13 and 1 × 10 14 ions/cm 2 are estimated from Tauc plots (shown in Figure 3b) to be 3.23, 3.22, 3.19, 3.19 and 3.18 eV, respectively. Swift heavy ion irradiation has been found to result in a decrease in the band gap with an increase in the ion fluence. This can be attributed to doping of Cu into ZnO nanostructures upon swift heavy ion irradiation, which in turn leads to the introduction of defect levels within the band gap. It must be pointed out here that a decrease in the band gap facilitates an easy passage of electrons from the conduction band and therefore leads to an increase in the electron flow in the irradiated samples as compared to the pristine sample. This property can be employed in various optoelectronic devices as well as for achieving improved photocatalytic efficiency.   Yuan et al. [39] and Thandavan et al. [40] have earlier reported the mechanism of formation of ZnO nanowires mediated by Cu-Zn alloys. The mechanism underlying the growth of ZnO nanosheets and nanorods prepared by carbothermal evaporation of ZnO and Cu followed by annealing can be understood as follows: The carbothermal evaporation of ZnO and Cu mixture led to the deposition of ZnO-CuO nanocomposite film with excess Zn onto the substrate. When this as-deposited film is annealed at 600 °C for 1 h in oxygen atmosphere, it led to the formation of Cu-Zn eutectic nanodroplets at the film surface.
Since the melting point of Zn is low (419 °C), during annealing at 600 °C a fraction of the excess Zn atoms evaporates by forming Zn vapor, which dissolves into the Cu-Zn eutectic nanodroplets and oxidizes forming ZnO nanoparticles. These ZnO nanoparticles formed by the catalytic action of Cu-Zn eutectic nanodroplets on the film surface combine through an oriented attachment mechanism, leading to the formation of ZnO nanorods and nanosheets on the surface of the nanocomposites. The schematic diagram depicting the growth mechanism is shown in Figure 5. Irradiation of the ZnO-CuO nanocomposite with 90 MeV Ni 7+ ions results in localized melting and lateral mass flow leading to the formation of larger nanorod like structures with increased width and distinct facets, as can be seen in Figure 1c and Figure 1d.
The photocatalysis studies were carried out by taking MB and MO as model organic dyes to demonstrate the capability of ion beam engineering to optimize the photocatalytic activity of ZnO-CuO nanocomposites. Figure 6 and Figure 7 show the UV-visible absorption spectra of 3.7 μM MB and MO dyes Our results show that swift heavy ion irradiation leads to significant enhancement in the photocatalytic efficiency of ZnO-CuO nanocomposites toward sun light driven degradation of MB and MO dyes in water. The photocatalytic efficiency increases with increase in ion fluence, reaching the maximum efficiency at the highest fluence of 1 × 10 14 ions/cm 2 . The enhanced photocatalytic efficiency for the sample irradiated with highest fluence is due to the reduced band gap energy which facilitates the easy transfer of electrons from the valence band to the conduction band. In addition, the improved suppression of recombination of photogenerated charge carriers in ZnO-CuO nanocomposites contributes to the enhanced photocatalytic efficiency. Earlier studies have shown that swift heavy ion irradiation inducing high electronic excitations can be used to control the defect concentration and engineer the shape and size of nanostructured materials [44,45]. Fabricating nanocom-   posites consisting of 1D and 2D metal-oxide semiconductor nanostructures with higher surface area for efficient adsorption of dye molecules and optimal defect concentration, crystallinity and band gap are important for developing advanced photocatalytic coatings. In this work, we demonstrate that swift heavy ion irradiation can be used to controllably engineer the shape of ZnO nanostructures (nanorods and nanosheets) and enhance the photocatalytic activity of ZnO-CuO nanocomposites, improving their applicability as reusable photocatalysts.

Conclusion
ZnO-CuO nanocomposite thin films were prepared by carbothermal evaporation of ZnO and Cu, combined with annealing. FESEM studies showed the presence of ZnO nanosheets and nanorods, which are formed by Cu-Zn alloy nanodroplets assisted oriented attachment of ZnO nanoparticles. The effects of swift heavy ion irradiation on the structural, optical, and photocatalytic properties of the nanocomposite were studied. Swift heavy ion irradiation has been found to result in significant enhancement in the photocatalytic efficiency of ZnO-CuO nanocomposites, towards sun light driven degradation of methylene blue and methyl orange dyes in water. The possible mechanism for the enhanced photocatalytic activity of ZnO-CuO nanocomposites is tentatively proposed. We have demonstrated that the combined effects of reduced band gap energy which facilitates easy transfer of electrons from valence band to conduction band, improved sun light utilization and reduced recombination of photogenerated electrons and holes result in the observed enhancement in photocatalytic activity of swift heavy ion irradiated ZnO-CuO nanocomposites.

Experimental Materials
ZnO, Cu and graphite powders were used as the starting materials for the synthesis of ZnO-CuO nanocomposite thin films. ZnO and graphite powders were purchased from Merck, India, while Cu powder was purchased from Loba Chemie. Methylene blue (MB) and methyl orange (MO) were procured from SRL, India. All chemicals used were of analytical grade and were used without any further purification.

Synthesis and ion beam engineering of ZnO-CuO nanocomposites
ZnO-CuO nanocomposite thin films were synthesized by a simple carbothermal reduction-based vapor deposition process. In this process graphite powder was thoroughly mixed with ZnO powder in a ratio of 1:10 along with 30% of Cu metal powder and pelletized. Thin films on thoroughly cleaned silica glass substrates were deposited by thermal evaporation under a vacuum of 1.4 × 10 −5 Torr. The as-prepared samples were then annealed at 600 °C for 1 h in oxygen flow. These samples were then irradiated with 90 MeV Ni 7+ ions to fluences varying from 3 × 10 12 to 1 × 10 14 ions/cm 2 .

Characterization of ZnO-CuO nanocomposites
The structural and optical properties of the as-prepared and irradiated samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Raman spectroscopy and UV-visible absorption spectroscopy. Grazing incidence XRD patterns were recorded by Bruker D8 Advance diffractometer at grazing incidence of 2° using Cu Kα (λ = 1.5406 Å) source operating at 40 kV and 40 mA. Raman spectra were recorded by using a Horiba Jobin Yvon LabRam with a spot size of 1 µm at a wavelength of 488 nm.

Photocatalytic measurements
The photocatalytic activity of the pristine and irradiated samples was evaluated by monitoring the degradation of methylene blue (MB) and methyl orange (MO) dyes in water under sun light irradiation in a similar manner, as described previously in [41].
For the photocatalytic studies, aqueous solutions of 3.7 μM MB and 3.7 μM MO with the pristine and irradiated samples dipped in them were irradiated with sun light for different durations of time (10, 30 and 44 min). These experiments were carried out at mid day during peak summer at the same time spanning up to 44 min to ensure exposure with sun light of maximum luminosity. Following sun light irradiation, the photocatalysts were removed from the aqueous dye solutions. The concentrations of the dye in the resultant solutions were monitored by UV-visible absorption spectroscopy with double distilled water as the reference medium, in the wavelength range of 300-800 nm.