Background

Over the past few decades, semiconductor photocatalysis, as a type of “green technology,” has attracted much attention owing to its potential applications in environmental protection and energy production [1,2,3]. Typical semiconductors that have been investigated are TiO2 [4], ZnO [17], loading noble metals [4b and c, corresponding to the SEM in Fig. 4a of ZN2 sample, clearly reveal the uniform spatial distribution of zinc (Zn), nickel (Ni), and oxygen (O) elements, indicating that NiO nanosheets uniformly distributed on the surfaces of ZnO nanorods. The abovementioned elements in hierarchical ZnO@NiO are also confirmed by EDX measurement in Fig. 4, which is consistent with the observations of the SEM.

Fig. 4
figure 4

Typical energy dispersive X-ray spectroscopy (EDS) elemental map** images of ZN2. a The corresponding SEM image of the map** area. b O map**. c Zn map**. d Ni map**. e EDS images

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As observed in a representative TEM image in Fig. 5, the ZnO@NiO heterostructures (ZN2) have a core–shell structure that consists of ZnO nanorods as the core and NiO nanosheets as the shell. The diameter of the rod-like morphologies was ~ 200–300 nm. The high-resolution TEM image in Fig. 5b shows the interfaces of the crystalline ZnO and NiO crystal lattices. The interplanar distance of 0.26 nm coincides with the lattice spacing of the (002) plane of hexagonal wurtzite ZnO, while the lattice spacing of 0.241 nm corresponds to the interplanar spacing of the (111) plane of cubic NiO. Moreover, the distinct interface and continuity of lattice fringes observed between the NiO and ZnO nanostructures in Fig. 5b indicate the formation of p–n heterojunction between NiO and ZnO in the ZN2 nanostructure.

Fig. 5
figure 5

a TEM image of ZnO@NiO heterostructure (ZN2). b High-resolution TEM of the image of ZN2

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X-ray photoelectron spectroscopy (XPS) patterns of ZN2 are shown in Fig. 6. Peaks corresponding to four elements, Zn, O, Ni, and C, were observed in the XPS survey spectra (Fig. 6a). The C1s peak with a binding energy of 284.6 eV was used as the standard reference for calibration and is mainly attributed to hydrocarbon contaminants, typically present in XPS spectra [15]. In Fig. 6b, the XPS peaks located at 529.5 eV were ascribed to the lattice oxygen, whereas the energy peak at 532.2 eV was assigned to non-adsorbed O2 or surface hydroxyl species [35]. In Fig. 6c, the two peaks centered at binding energies of 1022.3 and 1045.2 eV were attributed to the Zn 2p3/2 and Zn 2p1/2 states [36], suggesting that Zn existed in the form of Zn2+. Figure 6d shows the Ni 2p XPS signals of ZN2, which could be deconvoluted into five peaks. The peaks at 854.0, 856.1, and 861.1 eV, corresponding to the Ni 2p3/2 state, could be ascribed to Ni–O. The remaining two peaks at 873.1 and 879.6 eV were attributed to the Ni 2p1/2 state [32].

Fig. 6
figure 6

XPS spectra of ZN2. a Survey spectra. b O 1s. c Zn 2p. d Ni 2p spectra

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To investigate the potential applicability of the ZnO@NiO nanocomposites, the photocatalytic activities of the samples were examined by measuring the degradation of RhB dye under ultraviolet light irradiation. The characteristic absorption of RhB at 554 nm was used to monitor its concentration during the degradation process. After 180 min, 95% of RhB was degraded in the presence of ZN2. In comparison, the ZnO nanorods and NiO nanosheets respectively degraded 38% and 33% of RhB only (Fig. 7a). Moreover, the photodegradation activity of the ZnO@NiO nanocomposites was much higher than that of the ZnO nanorods and NiO nanosheets. To measure the photodegradation activity, a plot of the photodegradation rate constant of RhB versus degradation time was used. The reaction can be described as a pseudo-first-order kinetics model as follows [9]:

$$ \ln \left(\frac{C}{C_0}\right)=- kt, $$

where C0 represents the initial concentration of RhB, C refers to the concentration of RhB at different irradiation times t, and k is the reaction rate constant. The linear plots of ln(C/C0) versus time of the photodegradation of RhB over ZnO, NiO, ZN1, ZN2, and ZN3 are shown in Fig. 7b. The rate constant (k) corresponds to the slope of the linear fits. The calculated k for the degradation of RhB over ZN2 was 0.01656 min−1, which was higher than those calculated for the reactions over ZnO nanorods (0.00257 min−1) and NiO nanosheets (0.00208 min−1). Overall, the photocatalytic activity decreased in the order of ZN2 > ZN3 > ZN1 > ZnO nanorods > NiO nanosheets. The experimental results suggest that the deposition of the NiO layer on the ZnO nanorods facilitates charge transfer, thus significantly improving the photocatalytic activity. According to the BET results (shown in Additional file 1), the specific surface areas of ZnO@NiO composites initially increase as the increment of deposition time of NiO and then decrease as the deposition time further increases; thus, the ZN2 exhibits the highest photocatalytic activity.

Fig. 7
figure 7

a Plots of relative concentration (C/C0) of RhB versus time for the degradation of RhB over ZN1, ZN2, ZN3, and ZnO nanorods and NiO nanosheets under UV light irradiation. b Corresponding plots of − ln(Ct/C0) versus irradiation time. c Plots of relative concentration (C/C0) of MO versus time for the degradation of MO over ZnO, NiO, and ZnO@NiO heterostructure under UV irradiation. d Repeated photocatalytic degradation of RhB over ZN2

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The photocatalytic degradation of MO dye under ultraviolet light irradiation was also examined, and the results are displayed in Fig. 7c. Likewise, the photocatalytic activity decreased in the order of ZN2 > ZN3 > ZN1 > ZnO nanorods > NiO nanosheets. It could be concluded that the ZnO@NiO nanocomposites exhibit superior photocatalytic activity over the ZnO nanorods and NiO nanosheets. The photocatalytic stability of ZN2 was assessed by conducting repeated photocatalytic degradation of RhB under ultraviolet light illumination. As observed in Fig. 7d, the degradation yield remained high (~ 95%) across the repeated cycles, with a slight decrease to 90% observed after the third cycle. These results demonstrate the high photocatalytic efficiency and the reusability of the ZnO@NiO heterostructures, which are important attributes for their practical use in real-life applications in eliminating organic pollutants from wastewater.

The corresponding photocurrent responses are shown in Fig. 8a. Several on–off light cycles were used to study the separation efficiency of the charge carriers. The NiO nanosheets displayed no changes in the current under both dark conditions and light illumination, whereas the ZnO nanorods showed a small photocurrent response under ultraviolet irradiation. In contrast, the ZnO@NiO composites showed a higher photocurrent density. And the photocurrent density decreased in the order of ZN2 > ZN3 > ZN1 > ZnO nanorods > NiO nanosheets. The fast photocurrent responses implied that charge transport in the samples was very quick. The enhanced photocurrent response of the ZnO@NiO composites can be ascribed to the formation of intimate interfacial contacts between the ZnO nanorods and NiO nanosheets. It is deduced that the photoexcited electrons in the NiO nanosheets can be generated and efficiently transferred from the CB of NiO to the neighboring ZnO nanorods under ultraviolet irradiation, where ZnO serves as an efficient electron sink and transporter, thus inhibiting recombination of photogenerated electron−hole pairs. It is worthwhile to note that the photocurrent of the ZnO@NiO composites initially increased as the deposition time of NiO increased and then decreased as the deposition time was increased further. It is possible that the interfacial contact surface area between the ZnO nanorods and NiO nanosheets initially increases and then decreases as the deposition time of NiO increases, which is consistent with the result of the photocatalytic activity.

Fig. 8
figure 8

a Photocurrent response of NiO nanosheets, ZnO nanorods, and ZnO@NiO heterostructure under UV lamp irradiation (10 W). b PL spectra of pure ZnO nanorods, NiO nanosheets, and ZnO@NiO composite

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Figure 8b displays the typical PL spectra of pure ZnO nanorods, NiO nanosheets and ZnO@NiO heterostructures measured under the same condition at room temperature. For pure ZnO nanorods, a strong emission peak at 378 nm is observed, which is corresponding to the near band edge emission of ZnO. For NiO nanosheets, no emission peak is observed. Furthermore, the PL emission intensity of ZnO@NiO composite is obviously weakened compared with that of pure ZnO nanorods, indicating that the recombination of photogenerated electron–hole pairs is restrained. The results of photocurrent and PL indicate that the ZnO@NiO nanocomposite can remarkably enhance the separation efficiency and interfacial charge transfer efficiency of photogenerated electron–hole pairs.

The improved photocatalytic activity of the ZnO@NiO heterostructures was ascribed to the fast carrier separation and transport at the interface of the ZnO@NiO heterostructures owing to the type II band alignment between ZnO and NiO. This proposed mechanism is consistent with that in previous reports [8, 10, 22]. Figure 9 shows a proposed energy band structure diagram of the ZnO@NiO heterostructure. ZnO is an n-type semiconductor, whereas NiO is a p-type semiconductor. A p–n heterojunction is formed when ZnO and NiO combine, and an inner electric field is generated at the interface between NiO and ZnO because of electron and hole transfers. Under UV light irradiation, the electrons in the VB are excited to the CB, leaving holes in the VB. Band alignment of the p-type NiO and n-type ZnO heterojunctions is beneficial for transferring the photogenerated electrons from the CB of NiO to the CB of ZnO, then the electrons can combine with the dissolved oxygen molecules and produce the superoxide radical anions (•O2−), which play an important role in the overall photocatalytic reaction. Conversely, the photogenerated holes can transfer from the VB of ZnO to the VB of NiO, and the holes are easily trapped by OH− at the catalyst surface to further yield the hydroxyl radical species (•OH), which is an extremely strong oxidant for decomposing the organic dye. Therefore, the ZnO@NiO nanocomposites exhibited superior photocatalytic performance over the ZnO nanorods and NiO nanosheets.

Fig. 9
figure 9

Scheme of the energy band alignment between ZnO and NiO

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Conclusions

ZnO@NiO heterostructures were successfully fabricated by a simple electrochemical deposition method. The photocatalytic activity of the ZnO@NiO nanocomposites was superior to that of ZnO nanorods and NiO nanosheets toward the degradation of MO and RhB dyes under UV light irradiation. The high photocatalytic performance was ascribed to the high separation efficiency of the photogenerated electron–hole pairs from the p–n heterojunction, as confirmed by the photocurrent response measurements. The results showed that more free carriers could be generated and separated in the ZnO@NiO heterostructures, thus leading to higher separation efficiency when compared with that achieved in the ZnO nanorods and NiO nanosheets. Moreover, the ZnO@NiO heterostructures could be easily recycled with minimal decreases of the photocatalytic activity. The high photocatalytic efficiency and reusability of the ZnO@NiO heterostructures, which allows easy separation from the solution, have important applications in eliminating organic pollutants from wastewater.