Introduction

Over the years, scientists have been concerned about the high water contamination level and are therefore committed to getting rid of these contaminants (Rouse 2015). The synthesis of various analogues of nanomaterials and semiconductors has effectively tackled wastewater problems (Kuang et al. 2015). Several nanomaterials have been used as a photocatalyst in the oxidation of various organic pollutants. Among the nanomaterials, titanium dioxide (TiO2) nanoparticles have played influential roles in decomposing various pollutants. However, this material exhibits a wider band gap (3.2 eV), making it difficult for the material to absorb light in the visible region. Also, since the solar spectrum comprises only 5% of the ultraviolet light, its photocatalytic potential cannot be appropriately harnessed (Dargahi et al. 2018). This has stimulated scientists in testing various visible-light active semiconductor photocatalysts for the oxidation of organic pollutants. Some of the most commonly applied visible-light active semiconductors include Cu2O (Paula et al. 2019), Fe2O3 (Tang et al. 2018), Bi2O3 (Wang et al. 2019b), g-C3N4 (Kuang et al. 2015), MnO2 (Ma et al. 2017), Ag3VO4 (Li et al. 2017), Ta3N5 (Wang et al. 2019a), and NiO (Muhambihai et al. 2020).

Nickel oxide (NiO) has attracted major attention due to its fascinating 3d electronic structure, that is localised in space but spread out over different energy range because of strong Coulomb repulsion between them (Liang et al. 2019). NiO is a p-type semiconductor with a wide bandgap (3.5 eV). The striking features of NiO include photosensitivity, low cost, abundant resources, huge light absorption, photoactive, stable under various pH, and ease of different morphological preparation (Liang et al. 2019). These qualities have stimulated their application in various fields such as batteries, supercapacitors, water splitting, and gas sensors (Liang et al. 2019). However, the pitfall of the application of NiO is due to the strong quantum confinement effects that increase the recombination of photogenerated electron–hole pair. Unfortunately, most semiconductors’ major drawbacks in their application for photocatalysis are poor transport of charge carriers and fast recombination of photo-excited electron–hole pairs (Peleyeju and Arotiba 2018). These challenges are resolved by controlling the material’s morphology, doping with noble metals, using catalysts, and constructing heterojunction using intelligent semiconductors (Peleyeju and Arotiba 2018). It is essential to highlight that the formation of heterojunction is the best method for resolving the drawbacks of the recombination of electron-holes (Peleyeju and Arotiba 2018). Moreover, the construction of p-type and n-type semiconductor heterojunction has been used to enhance the photocatalytic activity of semiconductors by elongating the charge carriers’ lifespan through an efficient separation of the photogenerated electron–hole pair (Orimolade et al. 2019). Therefore, NiO has been coupled with various semiconductors such as BiVO4 (Fu et al. 2019), SnO2 (Alshehri et al. 2018), CdS (Chen et al. 2013), and WO3 (Wu et al. 2018), among others to form the heterojunction that will discourage high rate of electron–hole pairs recombination.

In this work, manganese dioxide (MnO2), a famous n-type semiconductor with a narrow bandgap of (2.33 eV), has been synthesised to construct heterojunction with NiO. The MnO2 has been preferred as an excellent material of choice owing to its novel analytical features including high thermal stability, affordability, high surface area, environment-friendliness, catalytic properties, resource-rich biocompatible, and good compatibility with various analogues of carbon and other materials to form composites (Ma et al. 2017; Su et al. 2019). These qualities have attracted the use of MnO2 in various fields such as biosensors (Mahajan et al. 2015; Wang et al. 2019d), lithium batteries (Kong et al. 2019; Liu et al. 2019), supercapacitors (Wang et al. 2019c; Zhang et al. 2019b), sensor (Dai et al. 2019; Wang et al. 2019c), and photocatalysis (Chen et al. 2016; Kumar et al. 2017). Different synthetic routes are used to prepare various morphologies of MnO2 such as hydrothermal, co-precipitation, sol–gel, electrodeposition, and thermal decomposition (Zhao et al. 2016; Bai et al. 2018; Guan et al. 2018). Thus far, few reports on the application of MnO2 for the degradation of organic pollutants have been documented in the literature. For instance, Humaira et al. employed MnO2 and choline intercalated graphitic carbon nitride to mineralise 2,4-dichlorophenol and phenol. The nanocomposite material could degrade 60 and 47% of the 2,4-dichlorophenol and phenol in 3600 s (Yasmeen et al. 2019). In another report, reduced graphene oxide was decorated on MnO2 nanorods as adsorptive photocatalyst for the removal of coloured dye and ciprofloxacin in water (Chhabra et al. 2019). Quoling et al. constructed MnO2/TiO2 nano-tube arrays photoelectrodes for the completer removal of methylene orange (MO) in water (Ma et al. 2017). Similarly, the collaboration of TiO2 and MnO2 nanostructure was used in the degradation of toluene. The heterostructure assisted in suppressing the recombination of photogenerated electron–hole pairs (Zhang et al. 2019a). Furthermore, various heterostructures of MnO2 such as MnO2/TiO2 (Ma et al. 2017), BiVO4/MnO2 (Trzci et al. 2016), and MnO2/WO3 (Ouyang et al. 2019) have been reported to display better-quality catalytic activities towards the decomposition of various organic pollutants under light irradiation.

In this study, we report on a simple method for synthesising nickel oxide nanoparticles@manganese (IV) oxide nanorods (NiONPs@MnO2NRs), and it was interrogated for photocatalytic degradation of bromophenol blue dye for the first time based on our knowledge. Our environment is facing unprecedented challenges of pollution from different organic pollutants such as dyes, and it is imperative to find ways to mitigate the impact of this problem. Hence, this study adds to the knowledge of photocatalysis that has been established, and the composite material is employed in such studies for the first time.

Materials and methods

Chemicals and sample preparation

Potassium permanganate (KMnO4), manganese sulphate monohydrate (MnSO4∙H2O), sodium hydroxide (NaOH), potassium hexacyanoferrate (II) [K4Fe(CN)6], nickel nitrate hexahydrate (NiNO3)2∙6H2O, sodium sulphate (Na2SO4), potassium chloride (KCl), isopropanol (C3H8O), ethanol (C2H5OH), potassium hexacyanoferrate (III) [K3Fe(CN)6], and bromophenol blue (C19H10Br4O5S). The reagents were purchased from Sigma-Aldrich, Germany.

Synthesis of NiONPs

NiONPs were synthesised by a simple and cost-effective co-precipitation method. Briefly, a mixture of 2 M (NiNO3)2∙6H2O and 1 M aqueous KOH solution was stirred for 3 h at ambient temperature, and the resulting precipitate was washed with distilled water to remove K+ and dried in the oven at 70 °C. The dried sample was grounded and calcinated at 750 °C for 150 min. The proposed equations for the synthesis are presented in Eqs. 1 and 2:

$${Ni\left({NO}_{3}\right)}_{2(s)}+{KOH}_{(s)}\to {Ni\left(OH\right)}_{2(s)}+{KNO}_{3(aq)}$$
(1)
$${Ni\left(OH\right)}_{2(s)}\to {NiO}_{(s)}+{H}_{2}{O}_{(l)}$$
(2)

Preparation of MnO2 nanorod

The MnO2 nanorod was synthesised by adding a mixture of 0.5 g manganese sulphate (VI) monohydrate and 1.26 g of potassium permanganate to 35 mL deionised water. The resulting mixture was transferred into a Teflon-lined autoclave and heated at 180 °C for half a day. The precipitate formed was washed copiously with a mixture of ethanol and water, and the final product was oven-dried at 80 °C to form a dark brown manganese nanorod (Idris and Arotiba 2018). The proposed equation for the synthesis is presented in Eq. 3:

$${3MnSO}_{4(s)}+{2KMnO}_{4(s)}+{2H}_{2}{O}_{(l)}\to {K}_{2}{SO}_{4(aq)}+{2H}_{2}{SO}_{4(aq)}+{5MnO}_{2(s)}$$
(3)

Synthesis of NiONPs@MnO2NRs heterostructure

Equal masses of MnO2 and NiONPs were separately dispersed in 30 mL ethanol by ultrasonication for half an hour. The two suspensions were mixed and ultrasonicated for 30 min. Subsequently, the mixture was vigorously stirred for 180 min, dried in an oven overnight at 60 °C.

Characterisation

The FESEM images were obtained on a JEOL JSM-7800F field emission scanning electron microscope (FESEM) coupled with Thermo Scientific Ultradry EDS detector (Thermo Fisher Scientific, Waltham, MA, USA). The TEM and SAED results were recorded using a Tecnai G2F2O X-Twin MAT (FEI Company, Eindhoven, Netherlands) operating at 200 kV. Rigaku SmartLab X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) with CuKα (\(\lambda\)= 0.154059 nm) radiation was used for XRD analysis. PerkinElmer FTIR spectrometer Frontier™ (spectrum 100 spectrometer) was employed for functional group analysis using the KBr pellet method. Thermogravimetric analysis (TGA) of the materials was done under nitrogen at a flow rate of 50 mL min−1. It was heated from ambient temperature to 1000 °C at a heating rate 10 °C min−1, using a thermogravimetric TGA 5500. The UV–vis DRS spectra were recorded using PerkinElmer Lambda™ 6505 UV–vis spectrometer from 200 to 800 nm, and BaSO4 was used as the reference material.

Preparation of NiONPs, MnO2NRs, and NiONPs@MnO2NRs electrodes

The electrodes were prepared by adding 1 mg of NiONPs, MnO2NRs, and NiONPs@MnO2NRs into the mixture (20 µL Nafion and 20 µL of isopropanol). Consequently, the mixture was sonicated for 1 h and drop coated onto the FTO surface (50 mm × 13 mm × 3 mm) with surface resistivity of − 8 Ω/sq. These modified electrodes were placed in the oven at 35 °C for few hours. The resulting products were meticulously rinsed with deionised water and labelled as FTO/NiONPs, FTO/MnO2NRs, and FTO/NiONPs@MnO2NRs electrodes. Finally, electrochemical characterisation was done on a potentiostat/galvanostat (Autolab, PGSTAT 302 N model, Metrohm, Swiss instruments), using a three-electrode configuration. The working, counter, and reference electrodes were FTO, platinum wire, and Ag/AgCl (3 M KCl), respectively. The electrochemical studies, including charge transfer features of the fabricated electrodes, were investigated using 5 mM [Fe(CN)6]3−/4− probe prepared in 0.1 M KCl solution using CV, LSV, and EIS.

Photocatalytic performance

The photocatalytic performance of the materials was evaluated using bromophenol (BPB) (50 mL of 10 mg L−1) blue dye as a model organic pollutant. The experiment started with an establishment of adsorption–desorption equilibrium in which 30 mg of catalyst was stirred in 50 mL of BPB solution for 30 min. The simulated light from ASAHI HAL-320 solar simulator with irradiation of 300 W from Xenon lamp was used for the photodegradation experiment while continuously stirring the suspension. At a pre-determined time interval, the 3 mL aliquots were samples and analysed with Lambda 6505 PerkinElmer UV–vis spectrometer after filtered through 0.45-µm GHP Acrodisc 13 (Pall) filters (Pall Corporation, Port Washington, NY, USA). The removal efficiency (RE (%)) was calculated as follows:

$$RE\left(\%\right)=\left(1-\frac{{C}_{t}}{{C}_{0}}\right)\times 100\%$$
(4)

Results and discussion

FESEM and EDX analysis

The FESEM images of the semiconductors were taken to confirm their morphologies. As seen in (Fig. 1a), the MnO2NRs exhibited a rod-shaped morphology while the NiONPs obtained spherically shaped particles (Fig. 1b). In contrast, the nanocomposite of NiONPs@MnO2NRs exhibited the synergy of both rod and spherical shape-like morphologies as illustrated in (Fig. 1c). This observation was further confirmed by EDX analysis (Fig. 1d), which revealed nickel, manganese, and oxygen in the NiONPs@MnO2NRs nanocomposite material.

Fig. 1
figure 1

FESEM images of a MnO2NRs, b NiONPs, and c NiONPs@MnO2NRs at 100 nm scale view. d EDS spectrum of NiONPs@MnO2NRs

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TEM analysis

The morphology of the prepared nanomaterials was investigated using TEM. Figure 2 presents the images of the nanomaterials and their corresponding selected area electron diffraction (SAED) patterns. The MnO2NRs revealed a rod shape-like morphology with the diameter range of 17.3–35.4 nm (Fig. 2a). On the HRTEM image of MnO2, fringes were observed with inter-planar distance of 0.316 nm indexed to (310) crystal plane of MnO2. This agrees with XRD results. The bulk defects were also observed on HRTEM image of MnO2. It is reported that bulk defects immerge due to lack of oxygen in the materials (Zhu et al. 2017). On the other hand, the TEM images of NiONPs were displaying the sphere and cubic-like morphologies. The diameters of the particles were determined to be 19.5 nm for sphere and 25.4–33.7 nm for cubic ones (Fig. 2b). The nanocomposite of NiONPs@MnO2NRs revealed a hybrid of rod and spherical shape morphology (Fig. 2c) and the morphology proved that the two photocatalysts were blended together to form a composite catalyst that could be ideal for photocatalytic applications. The inter-planar distance of 0.341 nm corresponding to (310) plane of MnO2 was observed while there were no fringes observed got NiONPs. The interfacial contact of MnO2NRs and NiONPs could be a reason for effective catalytic degradation of BPB pollutant. The TEM-SAED analysis was used as another method for an in-depth understanding of the crystallinity of the materials. The well-defined ring formation in the SAED patterns illustrates the materials’ high-crystallinity nature as shown in Fig. 2a–c.

Fig. 2
figure 2

TEM/HRTEM images and corresponding SAED pattern of a MnO2NRs, b NiONPs, and c NiONPs@MnO2NRs photocatalysts

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XRD analysis

The crystallinity of the as-prepared materials was assessed by X-ray diffraction analysis, as shown in Fig. 3. The peaks observed in NiONPs diffractogram located at 37.3°, 43.4°, 63.0°, 75.5°, and 79.5° could be indexed to (111), (200), (220), (311), and (222) of the cubic phase structure of NiO as per the standard spectrum card (JCPDS No.: 04–0835) (Sankar et al. 2016). For pure MnO2NRs, the peaks centred at 17.0°, 24.1°, 28.8°, 37.7°, 49.9°, and 60.1° were ascribed to (200), (220), (310), (221), (411), and (521) of α-MnO2 according to JCPDS No.: 44–0141 (Guo et al. 2020). Furthermore, the peaks corresponding to both NiONPs and MnO2NRs were observed in the XRD pattern of the composite material, confirming the co-existence of the two pristine materials in the composite prepared. The interfacial contact developed here would be beneficial for the performance of the composite for photodegradation purposes. The crystallite sizes (D) were also determined using Debye–Scherrer equation (Sithole et al. 2021):

$$D=\frac{0.9\lambda }{\beta Cos\theta }$$
(5)

where λ is the CuKα radiation wavelength (0.15418 nm), β is the full width at half maximum (FWHM) in radians, and θ is the Bragg angle. The values of D were determined to be 13.9, 32.6, and 16.2 nm for MnO2, NiO, and NiO@MnO2, respectively. The crystallite sizes for these catalysts are very close to particle sizes determined by TEM results.

Fig. 3
figure 3

XRD diffractograms of the three synthesised catalysts

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FTIR analysis

The functional groups present in the nanomaterials were interrogated using FTIR (Fig. 4). The peak around 492 and 679 cm−1 is due to the Ni–O bond stretching vibrations. The broadness of the peak revealed that the NiONPs are crystalline. The absorption band at 2078 cm−1 is because of the symmetric and the asymmetric stretching mode of vibrations of the carbon dioxide molecule absorbed from the air (Jayakumar et al. 2017). More so, the broad peak at 3431, 1387, and 1638 cm−1 can be accredited to the stretching and bending vibration of –OH group absorbed on the nanomaterial surface (Jayakumar et al. 2017). For the MnO2NRs, the broadband at 3430 cm−1 is attributed to the stretching vibrations of hydrogen-bonded surface water molecules and hydroxyl groups. In comparison, the bands at 1637 and 1384 cm−1 represent quantum numbers of residual –OH groups, which involve the O–H vibration mode of traces of adsorbed water. The band at 519 cm−1 can be attributed to the Mn–O vibrations of MnO2 nanorod (Pradeep Kumar et al. 2014). Finally, the bands at 531 and 695 cm−1 revealed that Ni–O stretching vibration of NiONPs and Mn–O vibration of MnO2NRs are present in the nanocomposite. The presence of more O–H groups on the surface of catalysts could help in production of more hydroxyl radicals.

Fig. 4
figure 4

FTIR spectra of the nanomaterials

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Thermal analysis

Thermogravimetric analysis is essential in showing the thermal stability of any material. Figure 5 shows the TGA curves for the different materials and revealed that they are relatively stable. However, MnO2NR is the most stable (weight loss ≈ 3%) material, while NiONPs@MnO2NRs is the least stable material (weight loss ≈ 11%). It has been reported that weight loss under 250 °C as seen in this study is due to the evaporation of surface adsorbed and interlayer water (Liu et al. 2016). Further weight loss after 500 °C could be due to the successive dehydroxylation to form a metal oxide, as reported by Bandar et al. (2021). The unique TGA curve for NiONPs@MnO2NRs is an indication that the composite material with properties different from NiONPs and MnO2NRs is formed.

Fig. 5
figure 5

Thermograms of the prepared materials

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Optical analysis

The optical properties of the synthesised NiONPs, MnO2NRs, and NiONPs@MnO2NRs were examined using UV–visible reflectance spectroscopy, as shown in Fig. 6a. The absorption edges are attributed to 325 nm, 628 nm, and 642 nm for NiONPs, MnO2NRs, and NiONPs@MnO2NRs. The Tauc formula (Eq. 6) was employed in calculating the band energy of the semiconductor, as depicted in Fig. 6b.

$$\alpha h\nu =A{\left(h\nu -{E}_{g}\right)}^{n/2}$$
(6)

where α, h, A, Eg, and v are the absorption coefficient, Planck’s constant, constant, bandgap incident light frequency and n is constant, which relies mainly on the optical transition features of the materials such as MnO2, the value of n is 1 (Umukoro et al. 2018). The plot of (αhv)2 versus hv was plotted, and the Eg values of the semiconductors were 3.08 eV, 1.51 eV, and 1.79 eV, respectively. This reduction in the energy bandgap can be ascribed to the heterojunction form between the p-NiONPs and the n-MnO2NRs. The narrow bandgap energy obtained from the nanocomposite revealed that it absorbs light at the visible region and can play pivotal roles for various environmental and photocatalytic applications.

Fig. 6
figure 6

a UV–visible diffuse reflectance spectra and b bandgap edges of the semiconductors

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Electrochemical characterisation of the nanomaterials

The FTO/NiONPs, FTO/MnO2NRs, and FTO/NiONPs@MnO2NRs electrodes were characterised using CV, LSV, and EIS in 5 mM [Fe(CN)6]3−/4− redox probe prepared in 0.1 M KCl as supporting electrolyte. The CV results (Fig. 7a) revealed that FTO/NiONPs@MnO2NRs electrode gave the highest faradaic current signal, followed by FTO/NiONPs and FTO/MnO2NRs. More so, the electroactive surface area was calculated using Randles–Sevcik equation (Eq. 7):

$${I}_{p}=\left({2.69\times 10}^{5}\right){n}^{3/2}AC{\left(D\upsilon \right)}^{1/2}$$
(7)

where Ip = peak current, n = number of electrons exchanged in the reaction, A = area of electrode active surface (cm2), C = concentration (mol cm−3), D = diffusion coefficient of ferrocyanide is 7.6 × 10−6 (cm2 s−1), and υ = scan rate (V s−1). The electroactive surface areas of the electrode at a scan rate of 50 mV s−1 were 0.863, 0.809 cm2, and 0.202 cm2 for FTO/NiONPs@MnO2NRs, FTO/NiONPs, and FTO/MnO2NRs, respectively. The highest electroactive surface area was obtained from the electrode fabricated from the nanocomposite. In addition to this, the LSV was done in the same redox probe as the CV experiment. The current vs. potential plot is shown in Fig. 7b using a potential range from − 0.2–0.5 V and a scan rate of 50 mV s−1. The linear voltammograms revealed that peak currents of 0.00376 A, 0.00284 A, and 0.000678 A were obtained for FTO/NiONPs@MnO2NRs, FTO/NiONPs, and FTO/MnO2NRs, respectively. The NiONPs@MnO2NRs gave the highest peak current response, which agrees with the CV result. The improvement in the peak current response is ascribed to the synergistic effect of the NiONPs and MnO2NRs.

Fig. 7
figure 7

a Cyclic voltammetry, b linear sweep voltammetry, and c EIS of FTO/MnO2NRs, FTO/NiONPs, and FTO/NiONPs@MnO2NRs electrodes

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Furthermore, the electrochemical impedance spectroscopy technique was used to interrogate the prepared electrodes’ charge-transfer properties using a bias potential of + 0.5 V. The Nyquist plots for the fabricated electrodes (Fig. 7c) were used to predict the electrodes’ charge transfer resistance (Rct). The semi-circular arc (Fig. 7c) gave crucial evidence on the Rct of the semiconductors. It is essential to highlight that the lower the arc radius, the greater the charge transfer efficiency and better electron transfer performance. A fast and efficient charge transfer at the electrode–electrolyte interface can hinder the recombination of charges, which results in the improvement in the effective separation of photogenerated electrons and holes. Thus, NiONPs@MnO2NRs gave the highest charge transfer efficiency followed by NiONPs and MnO2NRs, respectively, as depicted in Table 1. The EIS results corroborated the results obtained from CV and LSV, respectively. Besides, the electron transfer apparent rate constant (kapp) associated with the charge was determined using Eq. 8 (Ntsendwana et al. 2013):

Table 1 Electrical information obtained from the Randles equivalent circuit fitting inset in Fig. 7c
Full size table
$${k}_{app}=\frac{RT}{{F}^{2}{R}_{ct}C}$$
(8)

R is the ideal gas constant, T is the absolute temperature, C is the concentration of [Fe(CN)6]3−/4−, and Rct is the charge transfer resistance. The Kapp values of 11.3 × 10−5, 6.18 × 10−5, and 3.81 × 10−5 cm s−1 were obtained for FTO/NiONPs@MnO2NRs, FTO/NiONPs, and FTO/MnO2NRs, respectively. These Kapp results further confirmed that the high electrochemical kinetics were in FTO/NiONPs@MnO2NRs, followed by FTO/NiONPs, and lastly, FTO/MnO2NRs. The results agree well with CV, LSV, and EIS Nyquist results.

Photocatalytic performance

The photocatalytic performance of the materials was evaluated using bromophenol (BPB) (50 mL of 10 mg L−1) blue dye as a model organic pollutant. The experiment started with an establishment of adsorption–desorption equilibrium for 30 min in the dark. After that, the simulated solar light was illuminated onto the suspension. Figure 8a displays UV–vis spectra of BPB at 591 nm (wavelength of maximum absorption) as a function of reaction time. It was decreasing with an increase in irradiation time to 180 min. This showed that the chromophore responsible for 591 nm was indeed getting reduced in the presence of NiONPs@MnO2NRs catalyst. As shown in Fig. 8b, all the catalysts were tested for the degradation of BPB under simulated light. The removal efficiencies of 18.9, 29.0, and 81.5% were obtained in the presence of MnO2NRs, NiONPs, and NiONPs@MnO2NRs composite catalysts, respectively. The enhanced photocatalytic performance displayed by the NiONPs@MnO2NRs composite catalyst is attributed to the ability of the catalyst to harvest visible light to due to its narrow band gap energy, synergistic interaction between the interface of the two materials, high oxidation and reduction power of the catalyst as shown in the CV, high electron transfer mobility shown in the LSV, and high charge transfer efficiency and low resistance as illustrated by Nyquist plots (Mafa et al. 2020b). The composite catalyst showed high electron–hole separation, which contributed to the degradation of BPB dye.

Fig. 8
figure 8

a UV–vis absorbance spectra of BPB dye versus time, b ratio of concentrations as a function of time, c pseudo-first-order kinetics of different catalysts, and d bar chart of rate constants for different catalysts

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The photocatalytic degradation of BPB dye on MnO2NRs, NiONPs, and NiONPs@MnO2NRs composite catalysts was fitted with a Langmuir–Hinshelwood first-order model displayed by Eq. 9 (Mafa et al. 2020a) (Fig. 8c):

$$ln\left(\frac{{C}_{0}}{{C}_{t}}\right)={k}_{app}t$$
(9)

where the slope of ln(C0/Ct) versus time (t) gives the apparent rate constant (kapp). The obtained apparent rate constants for MnO2NRs, NiONPs, and NiONPs@MnO2NRs composite catalysts were 0.00113, 0.00202, and 0.00935 min−1 as indicated by a bar chart in Fig. 8d. Malefane reported that the beneficial effect of pristine catalysts in the composite could be evaluated by synergy factor (SF) deduced from the calculated apparent rate constants as given by Eq. 10 (Malefane 2020):

$$SF=\frac{{K}_{NiONPs@MnO2NRs}}{{K}_{NiONPs}+{K}_{MnO2NRs}}$$
(10)

The synergy factor (SF) for NiONPs@MnO2NRs composite catalyst was 2.97 for BPB dye degradation. This confirmed that the synergistic interaction between the pristine materials was feasible and occurred to improve photocatalytic activity.

Photocatalytic performance

In photocatalytic degradation of organic pollutants, reactive oxygen species such as superoxide (˙O2−) and hydroxyl (˙OH) are remarkably responsible for the destruction of organics on the surface of semiconductor materials (Mafa et al. 2021). As shown in Fig. 9, the degradation of BPB dye on the surface of NiONPs@MnO2NRs catalyst was proposed as depicted. NiONPs are p-type, while the MnO2NRs are n-type semiconductors. In p-n heterojunction formed, both NiONPs and MnO2NRs absorb light of energy equal or bigger than their band gaps. The electrons are excited from valence bands (VBs) of both materials to their conduction bands (CBs), leaving holes behind. The photogenerated electrons would migrate from the CB of p-NiONPs to the CB of n-MnO2NRs due to the CB offset. Likewise, the holes in the VB of MnO2NRs would migrate to the VB of NiONPs driven by offset difference. The photogenerated electrons and holes are stabilised by the redistribution of electrons on the CB of n-MnO2NRs and holes on the VB of p-NiONPs, respectively (Nikokavoura and Trapalis 2018). This phenomenon inhibits the recombination of photogenerated electrons and holes and promotes their long lifetime, and significantly improved photocatalytic process. The electrons on the CB of n-MnO2NRs participate in reducing reactions with molecular oxygen to produce superoxide radicals. The holes in the VB of p-NiONPs partake in the oxidation of hydroxyl ions into hydroxyl radicals and in direct degradation of BPB pollutants. The reactive oxygen radicals generated participated in photocatalytic degradation of BPB dye, as shown in Fig. 9. The p-n heterojunction formed is very vital for longer separation of electron–hole pairs.

Fig. 9
figure 9

Proposed electron–hole separation and degradation mechanism of BPB on NiONPs@MnO2NRs catalyst

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Conclusion

This work demonstrates the synthesis of nickel oxide nanoparticles, manganese oxide nanorods, and composite material. The composite materials’ structural integrity was interrogated using thermal, crystallographic, different microscopic, and spectroscopic techniques. These techniques revealed the successful preparation of the pristine and nanocomposite material. The FESEM and TEM results showed the formation of NiONPs on the surface of MnO2NRs with nano-ranged sizes. The electrochemical analysis displayed that the NiONPs@MnO2NRs had improved electrochemical activities than the pristine materials and reduced the electron–hole recombination rate. This was associated with the high removal efficiency of 81.5% obtained with composite catalysts over 18.9 and 29.0% obtained with MnO2NRs and NiONPs. The NiONPs@MnO2NRs material’s applicability clearly shows that this material can be explored further for various organic pollutants degradation, electrochemical energy production, and drug delivery research.