Introduction

Zinc oxide (ZnO) is a wide band-gap II–VI semiconductor, low cost and low toxicity that has attracted resurgent interest for numerous applications. It attracted the researchers’ interest for their exceptional optical, luminescent, electronic, optoelectronic and photocatalytic properties (Ocakoglu et al. 2015; Mansour et al. 2017). Also, ZnO nanocrystals show novel physical and chemical properties in comparison with their bulk forms (Mansour et al. 2017). Recently, several methods were used to synthesize ZnO nanocrystals in one-dimensional (1D) structures with various morphologies such as nanowires, nanorods, nanoneedles and nanorings (Ocakoglu et al. 2015; Cia et al. 2023; Liu et al. 2023). The distinctive properties resulting from the distinctive morphologies would open up new opportunities for ZnO nanomaterials to be qualified for several functional devices (Ocakoglu et al. 2015; Ozgür et al. 2005). The use of ZnO nanostructured material as a photocatalyst for removal of organic pollutants is considered one of the promised applications of such material. Although of that, pristine ZnO usually shows some limitations in field of photocatalysis due to the high probability of the recombination process between photoinduced electrons and holes. So, there are lot of research works that have been introduced to enhance the separation rate of the photoinduced charge carriers doping the pristine ZnO with metallic ions (Yu et al. 2002). The doping with small amount of such metallic ions could be enabled to adjust the electronic energy band structure by increasing density of trap levels with keeping the original vital properties of the photocatalysts (Wang et al. 2023). Cobalt (Co)-doped ZnO is considered one of the most important ZnO-based compounds for the abundant electron states of Co and similarity between Co ionic radius and that of Zn which increase the solubility of Co in ZnO matrix (Farha et al. 2016; Chen et al. 2023). In view of the benefits and limitations of such photocatalytic materials, there are attempted to optimize their photocatalytic activity by various methods. In this respect, altered strategies were used, such as surface and interface adjustment by controlling morphology, particle size, composite or coupling materials such as conducting polymers (CPs), transition-metal doping, nonmetal doping, co-doping with different configurations, noble-metal deposition and surface sensitization by organic dye and metal complexes, to improve the photocatalytic feature. Recently, conductive polymers such as polyaniline (PANI) has much interest in the photocatalysis applications due to its remarkable chemical stability compared to other CPs, facile synthetic route, relatively high electrical conductivity and high ability of dispersion. The coupling of PANI with various semiconductor materials, such as ZnO, CoFe2O4, TiO2 and Bi3NbO7, has been reported for enhancing the photocatalytic activity (Sharma et al. 2016).

Worldwide, synthetic dyes are produced and used in numerous industrial applications, such as textile, leather tanning, food processing, cosmetics, electroplating, paper and pharmaceutical industries; otherwise, about 5–10% of such dyes are released into the ecosystem (Epolito et al. 2005; Yahya et al. 2008; Bazin et al. 2012). However, those dyes possess a hazard effect to the environment since they significantly compromise the aesthetic quality of water bodies, increase biological and chemical oxygen demand (BOD and COD), impair photosynthesis, inhibit plant growth, enter the food chain and may promote toxicity and cause carcinogenic effects to the human bodies (Lellis et al. 2019). Hence, treating and mineralizing such dyes from wastewater streams is a must. Various conventional chemical or may be biological treatment technologies are introduced to eliminate dyes from aqueous effluents. But, those conventional technologies are unfavorable since their high treatment cost or they may be transferred the pollutants from one phase to another without mineralizing them (Thabet et al. 2021). To add up, a concentrated sludge is resulting from treatment that requires further treatment. Also, such techniques could not reach to complete pollutants elimination from wastewater. Moreover, those methodologies are insufficient in removing small concentrations nor low reaction rates (Tony et al. 2018). Photocatalytic oxidation technology, is signified as one AOT, has been emerged as superior efficient reactions (Tony 2020). Photocatalytic oxidation is based on the formation of high reactive groups, such as ·OH radicals, which are responsible of oxidizing and mineralizing such pollutants to harmless end products, such as CO2, H2O and inorganic matter (Tony and Mansour 2019). Several catalysts have been introduced as a source of catalytic oxidants, i.e., ZnO (Tony and Mansour 2020a, b, c), TiO2 (Kositzi et al. 2004; Tony and Mansour 2020a) and iron oxides (Pirsaheb et al. 2019; Tony and Ali 2022), are subjected as photocatalyst for wastewater oxidation. However, searching for novel coupled materials with sufficient treatment capabilities and innovative characteristics is attaining the scientists’ attention. In this respect, PANI/ZnO and PANI/AL-doped ZnO have been emerged quite good results through their usage in AOT under visible light as reported in degradation of methylene blue and methylene orange, respectively (Mitra et al. 2017; Olad and Nosrati 2012). The present work aims to study some physical properties for PANI nanocomposites loaded by Co-doped ZnO nanorods to investigate the photocatalytic performance of such nanocomposites. The usage of Co as doping element to ZnO matrix could be enabled from the shift of the optical absorption edge toward visible-light region that is leading to narrowing in band-gap energy. So, doping of ZnO with Co extends the absorbing region to the Vis spectrum (Muralikrishna and Manickam 2017). Moreover, the Co-doped ZnO nanorods as 1D nanostructured material offered good interaction with PANI matrix, especially with high aspect ratio of the investigated nanorods. Accordingly, the PANI nanocomposites loaded by Co-doped ZnO nanorods is considered a novel and a promise catalyst. The fabricated PANI/ Co-doped ZnO nanocomposites are introduced to be a photocatalyst to oxidize Procion Blue dye wastewater effluent which simulated the textile stream aqueous discharge.

Materials and methods

Materials

The used reagents in the hydrothermal synthetization and the polymerization processes were of analytical grade and no further purification was done. Zinc acetate dihydrate (Zn (CH3COO)2⋅2H2O; Winlab), cobalt acetate hydrate (Co (CH3COO)2⋅4H2O, Aldrich), sodium hydroxide (NaOH, ADWIC) and ethanol (Aldrich) were used in synthetization of Co-doped ZnO nanorods. However, aniline provided from Aldrich, ammonium persulfate (APS, bioWORLD) and hydrochloric acid (HCL 37%, spectrum) were utilized in PANI polymerization process.

Synthetization of Co-doped ZnO nanorods

Co-doped ZnO nanorods were synthesized via hydrothermal route as seen in Fig. 1a. In the typical route, Co and Zn precursors were dissolved in ethanol at room temperature (RT) to form 0.1 M solution. The used molar percentage of Co(NO3)2.6H2O is 0, 1, 3, 5 and 7% from the total concentration of precursors. Separately a solution of 0.5 M from NaOH in ethanol was prepared. For each sample, 10 mL of Co and Zn precursors was gradually added to 20 mL of NaOH solution under stirring to get clear solution. Then, the obtained clear solution was introduced to Teflon-lined stainless-steel autoclave and heated at 140 °C for 24 h. After slow cooling, the precipitate was collected and washed with water and ethanol for three times using Whatman filter paper. The collected precipitate was dried in air at RT for 48 h. The obtained sample powders are fine with color varied from moderate green to light green for the doped samples with the decrease in Co concentration, whereas the pristine ZnO sample is in off-white color. The Co-doped ZnO nanorod samples are labeled as CZ0, CZ1, CZ3 CZ5 and CZ7, corresponding to the utilized molarity concentration of Co (NO3)2 0.6H2O; 0, 3, 5 and 7%, respectively.

Fig. 1
figure 1

Schematic diagram for a hydrothermal process of synthetization of Co-doped ZnO nanorods, b PANI nanocomposites polymerization process in the presence of Co-doped ZnO nanorods

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Fabrication of PANI/Co-doped ZnO nanocomposites

PANI/Co-doped ZnO nanocomposites were fabricated using the direct oxidation of aniline in acidic solution as seen in Fig. 1b. The acidic for all composites are constant by using the same amount of HCL. The weight ratio of Co-doped ZnO nanorods to aniline was 7:93%, accordingly the obtained nanocomposites labeled as PANI-CZ0, PANI-CZ1, PANI-CZ3, PANI-CZ5 and PANI-CZ7. The used full weight of nanorods and aniline was 1 g. The oxidant aqueous solution was prepared by dissolving 2.28 g from APS in 25 mL of distilled water. Separately, the desired amount of aniline and the synthesized nanorod samples was added from previously prepared acidic solution using 25 mL distilled water and 4.35 mL HCL. After 20 min of stirring, the aniline-nanorods solution was sonicated for 5 min to get good dispersion of nanorods in the solution. Thereafter, the prepared oxidant solution was added in a drop-wise fashion into the aniline-nanorods solution during ~ 10 min under exposing to sonication. The gotten solution kept under stirring for one day at RT to complete the process of polymerization. The nanocomposites were obtained from the suspended solution by filtration and washing three times with distilled water and ethanol. Finally, PANI/ Co-doped ZnO nanocomposites were dried vacuum overnight at 50 °C.

Characterizations and measurements

The phase structure of the synthesized Co-doped ZnO nanorods as well as the fabricated PANI nanocomposites was obtained by using XRPhillips X'pert-MPD3040 diffractometer using monochromatic CuK-α with 1.5406 Å wavelength. The step-scan mode was used to register the intensities of the diffracted X-rays with 0.02° step over 2ϴ ranging from 10° to 80°. The morphology of both synthesized Co-doped ZnO nanorods and the investigated PANI/NC nanocomposites were examined via JEM-2100 high-resolution transmission electron microscope (HR-TEM). The Fourier transform infrared (FTIR) measurements of the synthesized nanorods and nanocomposites were recorded in the wavenumber ranging from 400 to 4000 cm−1 via JASCO FT/IR-4100 spectrometer. The UV–Vis absorption spectra throughout the wavelength range of 200–800 nm for the alcoholic diluted solutions of the synthesized Co-doped ZnO samples were obtained by using UV–Vis spectrophotometer of type Unico UV-2100 that is to measure in scan mode via step motor to scan the wavelength range with step 1 nm.

For DC measurements, the fabricated PANI/ Co-doped ZnO NCs were pressed under five tons to form pellet samples in the shape of disk with diameter equal to 1 cm and thickness around 0.15 cm and. The DC conductivity (\(\sigma \)) of the nanocomposite samples was measured using four-probe method and carried out using KEITHLEY 2635A System SourceMeter. A LabVIEW-based software was utilized in transferring and controlling the obtained data from the sourcemeter using IEEE-488-GPIB. The value of \(\sigma \) was determined by the reciprocal of the resistivity (\(\rho \)) which recorded via the used LabVIEW. The values of \(\rho \) were obtained from the linear slope of current (I)–voltage (V) plot, where the used values of I and V are the average of that collected for 16 times at each measuring step. The value of \(\rho \) was calculated using the following equation (Schroder and Corporation 2006):

$$ \rho = 2\pi S F\frac{V}{I} $$
(1)

where S is the probe spacing, here is 0.2 cm, and F is correction factor depending on the sample dimension and probe placement (Schroder and Corporation 2006).

Photo-oxidation experiments

To evaluate the photocatalytic activity of the all-synthesized samples, Co-doped/PANI, on Procion Blue MX-7RX dye oxidation, a simple photochemical reactor is used which consists of ultraviolet illumination source using UV lamp (model UV-12 W, 230 V and 50 Hz with 253.7 nm wavelength) enclosed in silica glass jacket immersed in a glass container. The dye solution is subjected to the glass container after the pH is adjusted, if required (Adwa AD1030 pH/mV and temperature meter, thereafter the prepared photocatalyst at certain doses is added to initiate the reaction). The solution is subjected to a magnetic stirring to ensure mixing and dispersion. Then, samples were withdrawn periodically for spectrophotometric analysis at a maximum wavelength of 570 nm using Unico UV-2100 Spectrometer. The schematic representation of the experimental setup is given in Fig. 2.

Fig. 2
figure 2

Schematic representation of the experimental setup of the photocatalytic experiment

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Results and discussion

Structure and morphological characterization of Co:ZnO nanostructured samples

Figure 3 displays XRD patterns of the synthesized Co:ZnO samples. The XRD patterns of all the investigated samples reveal the formation of a wurtzite ZnO structure, in agreement with standard JCPDS Card: 36-1451 (Cheng and Samulski 2004). Moreover, there are any other peaks assigned to other ZnO phases and/or phases for Co and cobalt oxides. Such result about the absence of other additional phases has been reported in many research works for Co:ZnO samples using various preparation techniques even with high concentration of Co, see, for example, (Farha et al. 2016; Mesaros et al. 2014a; Hammad et al. 2013; Kumbhakar et al. 2008; Kaphle et al. 2019). The formation of only single-phase structure, wurtzite ZnO, has been attributed to substitution of Co ions in zinc sites in the lattices of ZnO (Farha et al. 2016). Moreover, Fig. 4 shows the shift in the positions of the first three diffraction peaks of the investigated Co:ZnO samples to higher 2\(\theta \) values with increasing in Co contents. This shift is commonly obtained for Co-doped ZnO compositions and confirming to the occupation of Co ions at the Zn sites in the host structure (Ozgür et al. 2005; Farha et al. 2016; Shi et al. 2013).

Fig. 3
figure 3

The XRD patterns of a ZnO and Co-doped ZnO nanorods

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Fig. 4
figure 4

The XRD patterns of a ZnO and Co-doped ZnO nanorods (1st three peaks)

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The crystalline size (D) of Co:ZnO samples could be estimated using the value of full-width at half maximum (FWHM) for the obtained diffraction peaks according the following Debye–Scherrer relation (Scherrer 1918; Patterson 1939; Langford and Wilson 1978).

$$D= \frac{0.9 \lambda }{\beta \mathrm{cos} \theta }$$
(2)

where λ, β and θ are the wavelength of the used X-ray, FWHM and the Bragg diffraction angle, respectively. The calculated values of D via Eq. 2 for all the investigated Co:ZnO samples showed the nanostructure formation using all the recorded diffraction lines of each sample, where the obtained D values are between 32.4 and 39 nm as illustrated in Table 1 with mean value of 35.8 ± 2.8 nm, while the values of D for each sample using the most preferred orientation at (101), D101, are between 34.1 and 42.7 nm with mean value of 38.2 ± 3.1 nm. Although the Debye–Scherrer method is a well-known method that uses XRD patterns to calculate the crystallite size based on FWHM of the diffraction peak, it neglects the peak the probable broadening that obtained from the inhomogeneous strain and instrumental effects (Varghese and Pradeep 2003). Accordingly, Williamson–Hall (W–H) plot could be used to provide information about lattice strain, \(\varepsilon \), and effective particle size, \({D}_{W-H}\), for the investigated samples. The W–H equation has the following form (Williamson and Hall 1953):

Table 1 Particle size and lattice parameter of Co-doped ZnO samples
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$$\frac{\beta \mathrm{cos} \theta }{\lambda }=\frac{1}{{D}_{W-H}}+\frac{\varepsilon \mathrm{sin}\theta }{\lambda }$$
(3)

Figure 5 represents the plot between \(\frac{\beta \mathrm{cos} \theta }{\lambda }\) versus \(\frac{\mathrm{sin} \theta }{\lambda }\). As shown in Fig. 5, the obtained linear relation has a positive slope for all the investigated nanoparticles which ascribed to tensile strain in crystals of the studied Co:ZnO samples (Senthilkumar et al. 2010). The calculated values of the tensile strain and the effective particle size are listed in Table 1.

Fig. 5
figure 5

Williamson–Hall plot of Co-doped ZnO nanorods

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Lattice constants (c and a) for the wurtzite ZnO structure and their ratio were estimated from the XRD data for investigated Co:ZnO nanoparticle samples using the relation; \(\frac{1}{{d}_{hkl}^{2}}=\frac{4}{3}\left(\frac{{h}^{2}+ {k}^{2}+hk}{{a}^{2}}\right)+\frac{{l}^{2}}{{c}^{2}}\) (Chatterjee 2008). The obtained lattice constant and the volume of unit cell of the studied nanoparticles are listed in Table 1. The calculated values of a and c of Co-doped ZnO samples are very close to these obtained for pure ZnO sample that confirms the substitution of Zn ions by Co ions without great change in the volume of the unit cell (Farha et al. 2016; Neto et al. 2019; Nirmala and Anukaliani 2011). Figure 6 shows comparison between particle size obtained from Scherrer, average particle size of all peaks, and Williamson–Hall methods. The variation in both methods takes approximately the same trend.

Fig. 6
figure 6

Comparison between the particle size calculated by Scherrer and Williamson–Hall (W–H) methods

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The HR-TEM micrographs in Fig. 7 reveal that the Co-doped ZnO in nanorods shape with reasonable uniformity in size. 1D nanostructure shape is expected for ZnO nanocrystals that is synthesized using hydrothermal route. The obtained micrographs for all the investigated samples showed agglomerated features of the nanorods. In fact, the nanoparticles be likely to agglomerate together due to their highly surface energy, intrinsic crystal structure and external factors such as temperature, solvents and impurities (Cheng and Samulski 2004). Moreover, the investigated Co–ZnO samples displayed commonly magnetic feature for their room temperature ferromagnetic behavior (Mansour et al. 2021).

Fig. 7
figure 7

HR-TEM micrographs of Co-doped ZnO samples; CZ0 (a), CZ1 (b), CZ3 (c), CZ5 (d), CZ7 (e)

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UV–Visible absorbance spectroscopy of Co:ZnO nanostructured samples

The UV–Visible absorption measurements were done by dispersing the synthesized nanoparticles in ethanol with 1 mg/mL concentration for each studied Co:ZnO sample. Figure 8 presents the UV–Visible absorption spectra of the pristine and Co-doped ZnO samples with various concentrations of Co. Such figure revealed strong well-defined absorbance peak for all the investigated samples at close range from 374 to 362 nm and the peak values showed blue shift with increase in the Co concentration. This shift could be attributed to sp-d exchange interactions between the localized 3d electrons of Co+2 ions and sp band electrons of the host ZnO nanocrystals (Nirmala and Anukaliani 2011; Hammad et al. 2012). Due to this interaction, the Fermi level merges into the conduction band with an increase in the carrier concentration. Thus, the low energy transitions are blocked. The obtained results are in good agreement with the results reported by Sakai et al. (2006). The insertion of the cobalt Co ions into the ZnO structure leads to the appearance of an additional absorption peak (Mesaros et al. 2014b) at around 408 nm for all the doped samples.

Fig. 8
figure 8

UV–Vis spectra of pure and Co-doped ZnO nanorods

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Structure and morphological characterization of the fabricated PANI/Co-doped ZnO nanocomposites

Figure 9 shows the obtained XRD patterns of PANI/ Co-doped ZnO nanocomposites. The obtained diffraction peaks for pure PANI sample at 2θ = 15.2°, 20.5° and 25.2° with a d spacing of 5.8 Å, 4.3 Å and 3.5 Å, respectively, which shows low crystallinity of the CPs due to repetition benzenoid and quinoid rings in PANI chains (Shi et al. 2009). The recorded diffraction peaks for all the investigated nanocomposites confirm the formation of PANI with such diffraction angle range due to the parallel and perpendicular periodicity of the PANI chains (Mostafaei and Zolriasatein 2012). The peak at 2θ = 25.2° is stronger than that obtained at 2θ = 20.5°, the same results usually obtained for highly doped emeraldine PANI (Pouget et al. 1991; Xia and Wang 2002). Such results suggest the successful high doping level of the synthesized PANI nanocomposites.

Fig. 9
figure 9

XRD patterns of PANI/Co-doped ZnO nanocomposites

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FTIR analyses were carried out to investigate the chemical functional interaction of Co-doped ZnO nanoparticles with the PANI matrix. In this respect, Fig. 10 shows the FTIR spectra of PANI/Co-doped ZnO nanocomposites. As can be seen from Fig. 10, the absorption peaks at 615, 801, 1106, 1297, 1484, 1563, 2359 and 3213 cm−1 are consequent to the characteristic absorption and vibration modes of PANI (Mansour et al. 2021). Table 2 briefly describes the peaks and their corresponding bonds. The absorption peaks at 1563 cm−1 and around to 1484 cm−1 which correspond to quinone- and benzene-ring stretching vibrations, respectively, showed slight shift to higher values of wavenumbers that is referring to strong interaction between PANI and Co-doped ZnO nanoparticles (Zamani et al. 2019). The same trend was obtained for the absorption peak at 1106 cm−1 that is assigned to the aromatic C–H in-plane bending modes.

Fig. 10
figure 10

FTIR for PANI/Co-doped ZnO nanocomposites

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Table 2 Characteristic peaks obtained from FTIR spectrum of PANI/co-doped ZnO nanocomposites
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Figure 11 shows the HR-TEM micrographs of PANI/CZ1 and PANI/CZ5 nanocomposites. The gotten PANI polymer shows ribbon-like nanofiber shapes, as manifested by white arrows, with high degree of agglomerations especially for PANI/CZ1 sample. Moreover, Fig. 11 shows the combination between PANI and Co-doped ZnO nanoparticles. Figure 11 shows the combination between PANI and the Co-doped ZnO nanorods (as manifested by black arrows) which refers to that the quite dispersion of the Co-doped ZnO nanorods in the polymer matrix due to ultrasonic radiation in situ of polymerization process of aniline. The PANI ribbon-like structure eased the dispersion of the Co-doped ZnO nanorods in the polymer matrix that confirms the interaction between PANI and the nanorods.

Fig. 11
figure 11

HR-TEM micrographs of PANI nanocomposites loaded with Co-doped ZnO samples; a, b PANI-CZ1, c, d PANI-CZ5

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DC Electrical conductivity of the fabricated PANI/ Co-doped ZnO nanocomposites

Figure 12 shows the obtained values of σ at RT for PANI/ Co-doped ZnO nanocomposites using the four-point probe measurement technique. Each obtained value of conductivity (σ) represents the average of measurements for three times. The obtained values of σ for the investigated samples are close to each other with nonmonotonic trend. Such result could be attributed to the usage of the same weight percentage of Co:ZnO nanoparticles. Although of that, the highest values of σ are obtained for PANI/CZ1 sample and are found to be 3.918 × 10–5 Ω−1 cm−1. The nonmonotonic trend of σ with Co/Zn molarity ratio could be attributed to the features of the filler such as size, shape and agglomeration (Cho et al. 2001). Here, one can notice that, the used filler, Co-doped ZnO, usually has ferromagnetic behavior that is could be reflected on the degree of agglomeration of the used filler (Farha et al. 2016; Mansour et al. 2021). Indeed, there are many considerations which could be affected on the DC conductivity of the CP nanocomposites. Some of these considerations lead to the decrease in σ value such as: (i) the increase in the electronic scattering due to the good dispersion of nanoparticles throughout polymer matrix (Mansour et al. 2021) and (ii) the probable interruption of the chains ordering of polymer with the introduction of Co-doped ZnO (Mostafaei and Zolriasatein 2012; Kim and O'Shaughnessy 2003; Sharma et al. 2009). However, other considerations lead to the increase in σ value as a result of the improvement in the molecular ordering of the polymer due to the grown of polymer in the nanoscopically confined sites of the nanofiller (Cho et al. 2001).

Fig. 12
figure 12

Electrical conductivity (DC) for synthesized nanocomposites using 4-probe method at room temperature

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Photocatalytic oxidation study

Effect of different photocatalysts

To investigate the best cobalt doping percentage added to maximize the catalytic activity of the prepared samples (PANI/CZ), various catalysts with various percentages were checked on Procion Blue (MX-7RX) oxidation. Figure 13 illustrates the plots of different catalyst behavior on the dye oxidation using 10 ppm Procion Blue at RT, 20 mg/L of the catalyst and keeping the pH of the wastewater at its original value. As seen from Fig. 12, PANI/CZ1 recorded the highest oxidation percentage that is reached to 93% within 90 min of irradiation time. This could be due to various reasons including: the small particle size, charge separation and large content of oxygen vacancies or defects which is produced by doping with Co+2 that reduces the recombination process (Xu et al. 2010). The main oxidizing species in the reaction media are the OH radicals. Such radicals generated in the reaction medium are non-selective. ˙OH radicals attack the dye molecules and strongly oxidize them. Furthermore, the increase in the electrical conductivity since the increase in the electron–hole pairs helps in the generation of more radicals that helps in oxidizing more pollutant (Zhang et al. 2014). However, increasing the cobalt percentage covered the active surface of ZnO and thus reduces the dye oxidation (Adeel et al. 2021). Primarily, the PANI-CZ catalyst can promote the photocatalytic reaction across acyclic reaction. The displacement of a water molecule from the first solvation layer of the catalyst is suggested as the favorable process. OH radicals are signified as the main horsepower of oxidation reaction. When the initiation reaction is preceded under ultraviolet illumination, extra hydroxyl radicals are generated. The overall reaction is the produced radicals attacking the dye molecules to further intermediate and harmless end products (Thabet et al. 2022a).

Fig. 13
figure 13

Effect of different photocatalysts on photocatalytic oxidation for Procion Blue under UV light (photocatalyst dose = 20 mg L−1, pH = 7, dye concentration = 10 ppm and room temperature)

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Moreover, to attain the optimum operating conditions, PANI/CZ1 was tested under different conditions to further maximize the oxidation performances. Hence, in this concept the catalyst concentration, operating temperature, pH and dye concentration are then checked.

Effect of photocatalyst dose

Various catalyst (PANI/CZ1) concentrations are investigated and the results are displayed in Fig. 14 using 10 ppm dye at RT and original pH of the aqueous dye solution. It is revealed from the data in plot that reducing the catalyst concentration to 10 mg/L enhances the oxidation process and minimizing the oxidation time. Since the oxidation reaction takes place in only 60 min of illumination time with complete dye oxidation reached to 100% removal. However, 93% of dye oxidation is only attained in 90 min when 20 mg/L is applied to the oxidation reaction. To add up, as seen from the plot in Fig. 14, when 5 mg/L of PANI/CZ1 is subjected to the oxidation medium the reaction is hindered and only 91% of the dye is oxidized in 90 min of reaction time. This could be illustrated by the increase in the photocatalyst concentration over the optimal dose may lead to particles aggregation that takes place, which results in a significant reduction of electron–hole pairs and hence hinders the radicals’ production (Yimer et al. 2014; Tju et al. 2017). However, when less catalyst is used it means the catalyst is insufficient to complete the oxidation reaction in the aqueous medium. This in agreement with the previously cited in the literature by Tju et al. (2017) who reported that same catalytic dosage in methylene blue (MB) oxidation using ZnO/CeO2/nanographene platelets photocatalyst.

Fig. 14
figure 14

Effect of the photocatalyst dose on the photo-oxidation of Procion Blue for PANI/CZ1 photocatalyst under UV illumination (photocatalyst dose = 10 mg L−1, pH = 7 and dye concentration = 10 ppm)

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Effect of pH

Aqueous solutions’ pH has a significant role on the rate of reaction taking place on semiconductor surfaces since its influences on surface charge properties of the photocatalysts (Yimer et al. 2014). Figure 15 displays the effect of pH change on dye oxidation (suing 10 ppm dye; 10 mg/L PANI/CZ1 and at RT). As illustrated in Fig. 15, increasing the initial pH value greater than its original pH of the wastewater which recorded 7, nor decreasing such value, results in a reduction in the oxidation efficiency. It can be attributed to aggregation of charge on the outer of the synthesized nanocomposites which significantly effect on their dispersion in dye solution (Abbasi and Hasanpour 2017). Also, it could be because less hydroxyl radicals are generated which is related to the optimal pH value (Tony and Mansour 2019). Although the photo-oxidation of PANI/CZ1 in acidic solution (3.3 and 5.3) is better than that in the alkaline solution (8.7), the best oxidation results are attained at the neutral pH conditions. This could be related to the reaction between organic compounds and dissolved oxygen is better at the neutral pH conditions. Moreover, the reaction of radical species generation, that are the main responsible of the reaction, is related to the pH of the medium (Abbasi and Hasanpour 2017). Abbasi et al. (2017) reported that the optimal pH value for photo-degradation efficiency of methylene orange, applied at different photocatalysts such as ZnO, SnO2 and ZnO-SnO2, was pH = 7 rather than acidic (pH = 4) and alkaline (pH = 10) which confirm the obtained results in the present work.

Fig. 15
figure 15

effect of pH on the photo-oxidation of Procion Blue for PANI/CZ1 photocatalyst under UV illumination (photocatalyst dose = 10 mg L-1, pH = 7 and dye concentration = 10 ppm)

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Effect of Temperature

Changing the temperature of solution during oxidation process from RT to 60 °C was studied and illustrated in Fig. 16 (suing 10 ppm of dye, 10 mg/L of PANI/CZ1 and pH 7). It is found that increasing solution temperature reduces dye removal efficiency as lower OH radicals are produced at the higher temperature values. Such radicals are considered the chief responsible of the oxidation reaction; therefore, the oxidation yield is deduced. The removal percentage reduces from 100% at room temperature to 23.5% at 60 °C in 60 min of illumination time. This in agreement with the previously reported by Barakat et al. (2013) who applied Ag-doped TiO2 nanofibers for the photo-oxidation of Rhodamine B dye under UV illumination and they concluded that increasing temperature over 25 °C showed a negative effect on the dye oxidation. However, different results were reported for treating such dye using other catalyst since they reported that increasing the temperature to 60 °C slightly increases the MX-7RX dye removal rate using photo-Fenton system (Tony and Mansour 2019). Here, it is worth to mention that the obtained adverse effect of the temperature on the photo-degradation process of pollutants could be clarified in view of the surface energy feature of the used photocatalysts (Prousek 1996). More specifically, the surface to volume ratio of 1D nanostructure such as nanorods (as in case of the present study) is higher than that ration for 3D nanostructure as nanospheres. The high surface area of the particles enables to obtain a high surface energy of such particles that could be activated the photocatalysis reaction as expected at normal ambient condition for 1D shape. By increasing the temperature, the surface energy of the photocatalysts increases and may be reached to ultra-active surface which leads to escaping the pollutant molecules from the active zones surrounding the nanorods that lead to a decrease in the photocatalytic activity. Although temperature has an adverse effect on the 1D nanostructure shape, charge carrier transfer at low temperatures is more promoted in comparison with the 3D shape due to the large surface-to-volume ratio in the case of 1D, resulting in high freedom for charge carriers. That plays a crucial role in the oxidation process of organic dyes.

Fig. 16
figure 16

Effect of Temperature on the photo-oxidation of Procion Blue for PANI/CZ1 photocatalyst under UV illumination (photocatalyst dose = 10 mg L-1, pH = 7 and dye concentration = 10 ppm)

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Effect of initial dye loading

For the object of real applications, various dye concentrations including higher doses were checked to investigate the system reality. The effect of changing the dye concentration, from 5 to 40 ppm, on the oxidation activity at RT and 10 mg/L of catalyst concentration at normal pH of wastewater is displayed in Fig. 17. The highest oxidation rate was recorded at the low dye concentrations (5 and 10 ppm). This is because the small amount of dye concentration is oxidized at the same amount of the photocatalyst which is kept constant. However, while increasing the dye concentration, at the same catalyst dose, the radicals generated are insufficient to oxidize the amount of the dye molecules and the oxidation yield is reduced (Yimer et al. 2014). Also, increasing the dye concentration could cause shadowing effect which prevents the UV radiation from penetrating the solution and thus low radicals yield is generated in the reaction medium and the oxidation is hindered (Tony and Mansour 2019).

Fig. 17
figure 17

Effect of initial dye concentration on the photo-oxidation of Procion Blue for PANI/CZ1 photocatalyst under UV illumination (photocatalyst dose = 10 mg L-1, pH = 7 and room temperature)

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Oxidation kinetics

To fully understand the oxidation of PANI/CZ1 on the dye oxidation, the oxidation kinetics was studied at various initial dye concentrations ranging from 5 to 40 ppm. Zero-, first- and second-order kinetics were estimated, by calculating the slope of linear plot of concentration (C) vs. time (t), ln(C) vs. t and 1/(C) vs. t for zero-, first- and second-order reaction, respectively, and plotted according to the linearized equations form of the models (plots are not included). Then, slopes and intercepts of these plots are used to determine the reaction kinetic constants (K0, K1 and K2) and half-life time (t1/2), which are listed in Table 3 as well as the regression coefficients (R2) that were used to evaluate such models. Based on (R2) values listed on Table 3, first-order reaction system possesses the highest values for all dye concentrations 5, 10, 20 ppm systems compared to zero- or second-order systems. In only 40 ppm system, the second order had higher R2 value than first order with small value. Hence, the Procion Blue photocatalysis oxidation followed first-order kinetic reaction. Tony and Mansour (2019) reported that the Procion Blue followed second-order kinetic reaction using Fe2O3 photo-Fenton system while changing system temperature up to 60 °C.

Table 3 Zero-, first- and second-order kinetic rate constants (K0, K1 and K2) and reaction half-life \({(t}_\frac{1}{2})\) of Procion Blue photocatalytic oxidation at different concentrations
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Table 4 tabulates a comparison between current study and other previous studies reported in the literature for various organic dyes removals from aqueous solution by oxidation treatment. It is observed that current study, PANI/CZ1, could show a complete removal of Procion Blue commercial dye in 30 min, reaction time, using low photocatalyst concentration compared to other listed in Table 4. It can be concluded from the table that although a high removal efficiency is achieved, they have some disadvantage such as high photocatalyst dose (Mitra et al. 2017; Olad and Nosrati 2012; Ahmad et al. 2013; Gaim et al. 2019; Kumar 2018), pH adjustment (Tony and Mansour 2019, 2020b), presence of hydrogen peroxide (H2O2) (Tony and Mansour 2019, 2020b) and/or temperature treatment (Tony and Mansour 2019, 2020b) which gain the benefit of no further treatment that is required for current study. Moreover, comparing current dye removal process with photo-Fenton process (Tony and Mansour 2019, 2020b) listed in Table 4, Fenton process produces iron sludges that need to be removed after treatment, the pH required to be adjusted to the acidic region (pH ̴ 3) and after oxidation process the wastewater required neutralization of pH (Domingues et al. 2018).

Table 4 Comparison of various organic dyes removals in aqueous media using different oxidation treatment systems
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Conclusion

Co-doped ZnO nanorods have been synthesized via hydrothermal route. The microstructural and morphological results confirmed the formation of high quality of such nanorods with replacement of Zn ions by Co ions in the ZnO host matrix. Moreover, the obtained UV–Vis absorbance results definite the blue shift band gaps of the investigated nanorods with Co content increasing. PANI/Co-doped ZnO NCs were successfully fabricated using in situ oxidative polymerization. The obtained microstructural and morphological results showed quite good interaction between nanorod fillers and PANI matrix. Moreover, the values of DC conductivity show high slightly increase for the nanocomposites with Co-doped ZnO nanorods compared with the undoped PANI. Subsequently, the photocatalytic activity of PANI/Co-doped ZnO nanocomposites, as new photocatalyst, for Procion Blue dye remediation was studied and the optimum operating parameters were monitored. A complete dye removal was attained within 60 min of irradiance time using 10 mg/L, PANI-CZ1 photocatalyst at pH 7.0 and RT. The kinetics of the oxidation was studied, and the reaction is following the first-order reaction rate. To conclude, the obtained data confirmed the importance of nanostructured fillers in improving the conductive polymers’ performance that is appeared in the enhancement of the electrical conductivity and photocatalytic activity of the investigated nanocomposites. Also, the used facile fabrication route of PANI/Co-doped ZnO nanocomposites and the obtained results are encouraging the usage of polyaniline nanocomposites based on wide band-gap metal oxide nanoparticles in the field of wastewater treatment.