Introduction

Nitroaromatic compounds and/or organic dyes are substances that have toxic properties for humans, animals, and plants but are widely used in industry (Muhammad et al. 2019; Najafabadi et al. 2022; Nava et al. 2022). Their removal is essential for the protection of the health of living organisms and can be achieved through adsorption, advanced oxidation processes, chemical reduction, and aerobic biodegradation (Fast et al. 2017). Chemical reduction is an important and inexpensive method for the extraction of nitroaromatics and azo dyes by converting hydrogen into relatively low-toxicity products that can be easily degraded in nature (Li et al. 2021a, b; Rahman and Jonnalagadda 2008). High surface area activated carbon, and microalgae have been used as catalysts by many researchers to achieve high degradation performance towards organic pollutants (Mohd Hanafi et al. 2022; Jasri et al. 2023; Abdulhameed et al. 2022; Nadhirah Long Tamjid Farki NNA 2023). Razali et al. synthesized high surface area activated carbon (MSMPAC) using mixed fruit waste from mango (Mangifera indica) seeds (MS) and peels (MP), microwave-induced ZnCl2 activation and evaluated it for the removal of methylene blue (MB) from an aqueous medium (Razali et al. 2022). On the other hand, most of the metal-based catalysts for this hydrogenation reaction of nitroaromatic compounds heavily depend on noble metals (Li et al. 2021a, b; Zaera 2017; Kim et al. 2022). Most of the catalytic reactions in the noble metal nanoparticles (NP) take place only on the surface of the nitroaromatic compounds, and most of the atoms in the nucleus are catalytically inactive (Seitkalieva et al. 2021; Mohanty et al. 2010). However, the process is not financially friendly, and this reduces its areas of use. For this reason, to generate a large percentage of the noble metal atoms accessible for catalysis and to reduce their use, the internal noble metal atoms must be replaced by non-noble metals such as iron (Fe), cobalt (Co), and nickel (Ni) (Badruzzaman et al. 2020; Karimi et al. 2021; Ryabchuk et al. 2018). As an alternative, heterogeneous catalysts produced using non-precious metals, hydroxides, and oxides have gained importance due to their superior attributes with substantially more feasible costs compared to noble metals (Singh et al. 2017; Kurnaz Yetim et al. 2022; Wang et al. 2015; Ozkan 2023; Wen et al. 2018). Making a comparison with non-precious metals with higher oxidizing properties, and with challenging production procedures, oxide metals show similar reaction properties, superior chemical stability, and easier production aspects (Zhang et al. 2021). For this reason, non-precious metal oxides are one of the most commonly used functional materials for various catalytic implementations (Naseem et al. 2021; Danish et al. 2020; Gebre and Sendeku 2019).

The transition of cobalt oxide is of significant importance thanks to its electrical, optical, and magnetic properties (Prakash et al. 2022; Anuma et al. 2021; Ambika et al. 2019). Cobalt possesses Co4+, Co3+, and Co2+ oxidation steps. For this reason, it exists in the forms of cobalt (II) oxide (CoO), cobalt (III) oxide (Co2O3), and cobalt (II, III) oxide (Co3O4). The Co3O4 phase is the most commonly seen of these forms. Co3O4 is highly stable in terms of chemical activity and possesses rich redox reactivity in numerous reactions (Liu et al. 2022; Xu et al. 2022; Cheng et al. 2021). Size, shape, surface area, crystallinity, defects, and surface oxidation state are the important parameters that affect the catalytic activity of Co3O4. In previous studies, various approaches were adopted including size and pore modulation, ion doping, surface defect generation, and support-induced interactions to modify mass transfer and electron transfer that increase the catalytic activities of Co3O4 nanoparticles in chemical reduction reactions (Zhang et al. 2017; Mogudi et al. 2016).

In the last 10 years, inorganic nanoparticles (NPs) with unique physical, chemical, and biological properties have become of particular importance against bacterial infections (Khan et al. 2019; Jeevanandam et al. 2018). In general, organic antimicrobial agents have lower stability, especially at high temperatures or pressures, and can be seriously harmful and/or toxic. On the other hand, inorganic materials with antibacterial properties including inorganic metal oxides are rigid and ductile. Their superior properties over organic antimicrobial agents include stability, rigidness, and chemical stability over a longer time (Pugazhendhi et al. 2021). In addition, metal oxide NPs replace the most frequently used silver oxides, which due to their toxicity have adverse effects on humans and the surrounding environment (Kavitha et al. 2017).

Various Co3O4 with different morphologies have been reported in the literature. These have shown various catalytic performances based on their surface area, surfactant species, reducibility, and morphology (Chiu et al. 2020; Din et al. 2021; Xu et al. 2022). Therefore, it will be necessary to investigate Co3O4 with various morphologies, especially nanostructures, to offer insights towards optimizing Co3O4 design to investigate the morphology-based catalytic reactivity and antimicrobial effect of Co3O4 catalysts. Therefore, the aim of this work is to investigate Co3O4 catalysts with various nanostructured morphologies for p-NP reduction.

In this study, Co3O4 structures with three different morphologies were obtained by using the hydrothermal synthesis method (Kurnaz Yetim 2021). The effect of the morphology of the Co3O4 NPs produced on the catalytic and antimicrobial properties against pathogenic strains (Gram ( −) and Gram ( +) bacteria and yeast) were examined (see Fig. 1).

Fig. 1
figure 1

Schematic representation of the catalysis reaction mechanism and antimicrobial properties of Co3O4 nanoparticles

Full size image

Materials and methods

Spectral data measurements

A Rigaku MiniFlex 600 X-ray diffractometer equipped with a Ni-filtered Cu Kα source was utilized to determine the X-ray diffraction (XRD) patterns over a scan range of 10° < 2θ < 90°. The infrared spectrum was recorded using a Jasco FT-IR-6700 spectrometer, and the wavelength range was between 400 and 4000 cm−1. In addition, scanning electron microscopy (SEM) was utilized to examine the surface morphology of the Co3O4 structures. Energy dispersive x-ray spectroscopy (EDX) was adopted for the determination of the elemental composition of the Co3O4 structures. A FEI Quanta 400F model device was utilized for the SEM–EDX analyses. Brunauer–Emmett–Teller (BET) analysis was performed to examine the surface area of the nanostructures. Quantachrome-Nova Touch LX4 instrument was used for this purpose.

Synthesis of the Co 3 O 4 structures

The synthesis of Co3O4-urea, Co3O4-ed, and Co3O4-NaOH structures was carried out following the procedure in the previous research (Kurnaz Yetim 2021). In Fig. 2, the synthesis scheme of Co3O4 nanoparticles is presented.

Fig. 2
figure 2

Synthesis scheme of Co3O4 nanoparticles obtained using different surfactants

Full size image

Co3O4-urea was prepared by dissolving 1.45 g of Co(NO3)2·6H2O and 1.5 g CO(NH2)2 in 40 mL of water under stirring for 30 min, and a homogeneous solution was obtained. The resulting mixture was placed into a Teflon-lined stainless steel autoclave with a capacity of 50 mL, autoclaved in an oven at 150 °C for 4 h. The precipitate was rinsed with distilled water and ethanol and dried at 80 °C for 24 h. Finally, the product was left to anneal at 450 °C in the air for 2 h under ambient conditions at a rate of 10 °C/min.

To obtain Co3O4-ed, 1.45 g of Co(NO3)2·6H2O was dissolved in 25 mL of water and then 0.5 mL of ethylenediamine was added. The pH was adjusted to 12 using 2 M of NaOH. The solution was stirred for 30 min, and the mixture was transferred to a 50-mL capacity Teflon-lined stainless steel autoclave. The solution in the autoclave was then placed in an oven and autoclaved 150 °C for 12 h. The solution was cooled to room temperature, and the precipitate was rinsed with distilled water and ethanol, and then left to dry at 60 °C for 12 h. Finally, the product was left to anneal at 350 °C in the air for 2 h at a rate of 10 °C/min.

To obtain Co3O4-NaOH, 5.82 g of Co(NO3)2·6H2O and 0.2 g of sodium hydroxide were dissolved in deionized water (10 mL) under vigorous stirring for 10 min. The solution was then transferred in a Teflon-lined stainless steel autoclave of 50 mL capacity and autoclaved at 150 °C for 6 h. The solution was cooled to room temperature, and the precipitate was rinsed with distilled water and ethanol, and then left to dry at 60 °C for 10 h. Finally, the product was left to anneal at 500 °C in the air for 3 h at a rate of 10 °C/min.

Reduction of p-NP and MB

Catalysis studies were conducted by observing the conversion of p-NP molecules into p-AP molecules by Co3O4 NM-based catalysis. In this procedure, NaBH4 was utilized as the hydrogen source. Accordingly, approximately 3-mg Co3O4 flower-like particles were placed into a 3-mL solution containing 0.1 mM of p-NP and 0.3 mL of 0.2 M NaBH4. The concentration of the p-NP and p-AP was examined utilizing a spectrophotometric method (Kurnaz Yetim and Hasanoğlu Ozkan 2021).

To realize a reduction study, approximately 3 mg of Co3O4 NPs was placed in 4 mL of a 7.5 mg/L MB aqueous solution, then 0.3 mL of fresh NaBH4 aqueous solution was added. The resulting mixture was then examined by measuring the absorbance of the solution at 664-nm wavelength at different periods to examine the concentration of the remaining MB solution (Erdogan 2020).

Analysis of the antimicrobial potential of Co 3 O 4 NPs

Detection of antimicrobial activity

The antibacterial activity of Co3O4 NPs was tested against the six Gram-negative bacteria (Salmonella typhi, Escherichia coli, Enterobacter aerogenes sp., Klebsiella pneumoniae, Proteus vulgaris, and Pseudomonas aeruginosa), five Gram-positive bacteria (Staphylococcus aureus, Staphylococcus epidermis, Micrococcus luteus, Bacillus cereus, and Listeria monocytogenes), and one yeast (Candida albicans) by the Agar well diffusion assay method. The NPs were kept dry at room temperature and dissolved (100 µg/mL and 200 µg/mL) in DMSO. DMSO was utilized as the solvent for the compound and the control. It was determined that DMSO had no antimicrobial activity against any of the pathogenic microorganisms. A 1% (v/v) 24-h broth culture (pathogenic bacteria and yeast) containing 106 cfu/mL was placed on a sterile plate. Mueller–Hinton Agar (MHA) (15 mL) at 45 °C was poured into Petri dishes and left to cool and solidify. Then, 6-mm-diameter wells were carefully drilled utilizing a sterile cork drill and filled with the synthesized NPs and incubated for 24 h at 37 °C (Ogutcu et al. 2017). At the end of incubation, the average of the two wells was utilized to calculate the growth inhibition zone of each pathogenic bacteria and yeast (to compare the degree of inhibition, bacteria and yeast were tested for resistance to four antibiotics (kanamycin, ampicillin, amoxicillin, and sulfamethoxazole) and one anticandidal (nystatin) (Anar et al. 2016).

Results and discussion

Characterization of Co 3 O 4 NPs

FT-IR, XRD, and XPS analyses of Co3O4 NPs are given in the Supporting Information (Kurnaz Yetim 2021). The SEM images of the Co3O4 samples prepared using different ligands are given in Fig. 3. The figure shows the morphological and structural properties of the Co3O4 structures.

Figure 3(a) presents the Co3O4-urea in the nanosheet form. The expanded figure shown in Fig. 3(d) was prepared to identify the well-assembled multi-layered microplates in porous form. Figure 3 (b) shows the Co3O4-ed sample. This sample was in a clover leaf-like form; the accumulation of small clover-like formations can be seen. The size of the clover-like formations was in the range of 500–1000 nm. SEM images of Co3O4-NaOH sample are shown in Fig. 3(e) and (f). SEM images of Co3O4-NaOH show that the structure is in the form of nanospheres. The size distribution of the nanospheres was narrow, and the average size was approximately 700 nm.

Fig. 3
figure 3

The SEM images of (a) Co3O4–urea, (b) Co3O4-ed, (c) Co3O4-NaOH and corresponding magnified SEM images (d), (e), and (f)

Full size image

The surface properties of Co3O4 catalysts were investigated by N2 gas adsorption–desorption method at 77 K. Surface areas were calculated according to Brunauer–Emmett–Teller (BET) method, and pore volume distribution was calculated according to Barrett-Joyner-Halenda (BJH) method using adsorption analysis, and isotherms are presented in Fig. 4. When the N2 adsorption–desorption isotherms of metal oxides were examined, the characteristic of mesoporous materials containing hysteresis loop suggested that the isotherms classified as type IV according to IUPAC. The specific surface area of the samples was calculated by BET method and found to be 146.185 m2/g, 106.506 m2/g, and 31.0885 m2/g for Co3O4-urea, Co3O4-ed, and Co3O4-NaOH, respectively. The average pore diameter of the produced Co3O4 NPs was found to be 3.48978 nm, 3.6477 nm, and 2.5318 nm for Co3O4-urea, Co3O4-ed, and Co3O4-NaOH, respectively. The measured surface areas of Co3O4 nanoflowers were in line with the results reported in previous studies, which were 34.61 m2/g and 51.2 m2/g (Zhang et al. 2008; Sun et al. 2013). When compared with the literature data, it is seen that Co3O4 structures have a very large surface area. When the rate constants for the reduction reaction of p-NP are examined, it can be said that the surface areas of the catalysts used are parallel to the reaction rate.

Fig. 4
figure 4

BET analysis and pore size distribution of Co3O4 nanostructures

Full size image

Catalytic activity

Catalytic degradation of p-NP

The reduction process of p-NP to p-AP involves both electron transfer and hydrogen transport. It is widely known that negative hydride species (H−) obtained from BH4− anions present electrons and hydrogen atoms. This study investigated the catalytic activities of synthesized Co3O4 NPs with different morphologies partaking in the process of reducing p-NP to p-AP in the presence of NaBH4. The catalytic activities of noble metals and metal oxides in this reaction have been frequently studied (Najafi and Azizian 2020). However, there are very few studies on the effect of morphology on catalytic activity in this reduction process (Ye et al. 2021; Liu et al. 2021). For all experiments, p-NP and NaBH4 were reacted together with initial concentrations of 0.1 mM and 0.2 M, respectively. The p-NP bound peak observed at a wavelength of 317 nm in the UV–Vis spectra shifted immediately to 400 nm after the addition of the freshly prepared NaBH4 solution. This peak is due to the formation of the p-nitrophenolate ion in the alkaline state caused by the addition of NaBH4. The simultaneous appearance of a new peak around 295 to 300 nm, with the addition of Co3O4-urea, Co3O4-ed, and Co3O4-NaOH, resulted in reduced absorption of the characteristic peak at a wavelength of 400 nm that confirmed the formation of p-AP. The time taken to complete the conversion varied depending on the morphology of the catalyst.

In the absence of the catalyst, conversion of the p-NP solution to p-AP takes up to 4 to 5 h. When 3 mg of Co3O4 catalyst was added to the medium, it was observed that this conversion took place in 4 to 5 min. Therefore, it appears that the less efficient electron and hydrogen transfer from the BH4− species to the aromatic nitro compound without a catalyst increases significantly in the presence of metal oxides. Table 1 summarizes the activity of metal oxides and the variation of the reduction reaction according to the amount of catalyst. The reaction rate constants (Kapp) for Co3O4-urea, Co3O4-ed, and Co3O4-NaOH in the reduction of p-NP were found to be 1.86 × 10−2 s−1, 1.83 × 10−2 s−1, and 2.4 × 10−3 s−1, respectively. These results indicate that all three catalysts can successfully catalyze the reduction reaction (see Fig. 5).

Table 1 Reduction results of p-NP in the presence of Co3O4 nanoparticles and nanocomposites in literature
Full size table
Fig. 5
figure 5

UV–Vis spectra obtained from the p-NP reduction in the presence of a Co3O4-urea, b Co3O4-ed, and c Co3O4-NaOH nanostructures and d the rate constants of the reaction

Full size image

Catalytic degradation of MB

The catalytic degradation of MB was carried out in the presence of Co3O4 NPs. MB absorbs strongly at a wavelength of 664 nm in the visible region and gives a deep blue color upon the addition of aqueous NaBH4. Time-dependent UV–Vis spectra of MB reduction are presented in Fig. 6a–c. Absorption spectra were recorded every 5 min. In the presence of Co3O4-urea catalyst, 98.06% degradation of MB was observed within 60 min. Time-dependent UV–Vis absorption spectra exhibited that the intensity of the absorption peak of the dyes gradually decreased in the presence of Co3O4, disappearing over time. Also, the position of the absorption peak did not noticeably vary throughout the reduction. Furthermore, the degradation kinetics of MB by NaBH4 in the presence of Co3O4 NPs was examined by pseudo-first-order kinetics. Figure 6 d shows the linear relationship between ln (Ct/C0) and reaction time. Also, the reaction rate constants were calculated from the slopes.

Fig. 6
figure 6

UV–Vis spectra obtained in the catalytic degradation of MB in the presence of Co3O4-urea (a), Co3O4-ed (b), and Co3O4-NaOH (c), and the rate constants for the reaction (d)

Full size image

The reaction rate constants (Kapp) for Co3O4-urea, Co3O4-ed, and Co3O4-NaOH in the reduction of MB were found to be 6.0 × 10−4 s−1, 2.0 × 10−4 s−1, and 3.0 × 10−4 s−1, respectively (see Table 2).

Table 2 Reduction results of MB in the presence of Co3O4 nanoparticles and different nanocomposites in literature
Full size table

Antibacterial activity

The NPs considered showed variable growth activity (11 to 22 mm) for the pathogenic microorganisms used, and the activity mainly differed between moderate to high in Fig. 7 which shows images of the antimicrobial effectivity of Co3O4 NPs. Furthermore, NPs were more effective on Gram-negative bacteria than Gram-positive bacteria. Antimicrobial activity data shown in Table 3 are as follows.

Fig. 7
figure 7

Antimicrobial activity (inhibition zone [mm]) of Co3O4 NPs in Gram( −) and Gram( +) bacteria and yeast

Full size image
Table 3 Antimicrobial activity of NPs and standard reagents (diameter of zone of inhibition in mm)
Full size table

Co3O4-urea showed high activity against B. cereus, E. coli, and C. albicans. In addition, this compound showed the same inhibitory effect as AMC30 (20 mm) for B. cereus (Fig. 8). This bacterium is known as an opportunist pathogen and is associated with food-borne illness (Nartop et al. 2019; Nartop et al. 2020a, b). Co3O4-ed showed high inhibitory activity against B. cereus, K. pneumoniae, and C. albicans (Fig. 8). Co3O4-NaOH exhibited high antimicrobial activity against B. cereus, E. coli, and C. albicans. All three NPs showed a greater inhibitory effect than AMP10 (11 mm) against Gram-negative S. typhi (Co3O4-urea, Co3O4-ed, and Co3O4-NaOH, respectively: 13 mm, 13 mm, and 14 mm) (Fig. 8). Salmonella serovars lead to many different clinical symptoms including those related to asymptomatic infections, severe typhoid-like syndromes in infants, or some high-sensitivity animals (Koçoğlu et al. 2021; Nartop et al. 2020a, b). In addition, all three NPs showed higher inhibitory activity against E. coli than AMP10 (10 mm) and AMC30 (14 mm) (Co3O4-urea, Co3O4-ed, Co3O4-NaOH, respectively: 17 mm, 16 mm, and 17 mm). Co3O4-urea and Co3O4-NaOH exhibited high activity against the Gram-negative E. aerogenes (Fig. 9). All three NPs showed higher activity in C. albicans than the antifungal. Examining Table 1, it was observed that the cobalt (II, III) oxide (Co3O4) NPs prepared in this study recorded high antimicrobial activity similar to the reference drugs used and could be helpful as antimicrobial agents. From the result obtained, it was concluded that these NPs were more effective in Gram( −) than in Gram( +) bacteria. The possible reason for this might be the presence of an external impermeable membrane, a fine peptidoglycan monolayer, and the presence of periplasmic cavity and cell wall composition in Gram( −) bacteria (Graham et al. 2021).

Fig. 8
figure 8

Graphical illustration of Gram ( +) pathogens bacteria (M. luteus, S. epidermis, S. aureus, B. cereus, L. monocytogenes) and standard reagents

Full size image
Fig. 9
figure 9

Graphical illustration of Gram ( −) pathogenic bacteria (P. aeruginosa, K. pneumonia, E. aerogenes, S. typhi, E. coli, and P. vulgaris) and standard reagents

Full size image

Conclusions

In this study, three Co3O4 catalysts with different nanostructured morphologies were produced and their catalytic activities on the reduction of p-NP to p-AP, and on the degradation of MB, were compared. In addition, the effect of morphology on antibacterial properties was investigated. As the Co3O4 structures exhibited quite different morphologies, their physical and chemical properties varied greatly, thus exhibiting different catalytic activities and antimicrobial properties. In general, Co3O4 structures showed much higher catalytic activities than many metal oxides (such as NiO, Fe3O4, ZnO) for p-NP reduction as they have a high surface area and porous nanostructures. Co3O4-urea appeared to be the most advantageous for p-NP reduction and completed the reduction of p-NP with k = 1.86 × 10−2 s−1 in 270 s. Co3O4-urea had a larger surface area, resulting in superior catalytic activity. Likewise, Co3O4 structures showed superior performance in the catalytic degradation of MB. In the presence of Co3O4-urea catalyst, 98.06% degradation of MB was observed within 60 min. Noble metals are frequently used in such reduction reactions. It shows that Co3O4 structures have great potential as non-noble catalysts for practical applications, and they are certainly promising for the reduction of p-NP and MB.

It was determined that Co3O4 NPs showed antibacterial and antifungal activities at moderate to good levels against both Gram ( +) bacteria, Gram ( −) bacteria, and yeast. Co3O4-urea showed high activity against B. cereus, E. coli, and C. albicans. In addition, this compound showed the same inhibitory effect as AMC30 (20 mm) for B. cereus. It was concluded that these NPs could definitely compete with or even yield better results from commercial antibiotics used in the treatment of microbial infections. For this reason, it is thought that these nanoparticles can be used as a good antimicrobial agent against pathogenic microorganisms or as an additive in antimicrobial products.