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Abstract

This study synthesized semiconducting cobalt-doped zirconium oxide–graphitic carbon nitride (Co-ZrO₂/g-C₃N₄) nanocomposites using a simple technique with low production cost and investigated the collective impact of Co and g-C₃N₄ on the efficiency of ZrO₂ photocatalyst in degrading eosin yellow (EY) dye in synthetic wastewater. The Co-ZrO₂/g-C₃N₄ composites were synthesized via the chemical co-precipitation technique. They were systematically analysed using X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible (UV-vis) spectrophotometry, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDX) and dynamic light scattering (DLS). Structural analysis not only confirmed the successful integration of Co and g-C₃N₄ into ZrO₂ but also showed the transformation of the monoclinic ZrO₂ into the cubic phase on the addition of the dopant. The UV-vis characterization showed a slight enhancement in light absorption capacity by the Co-ZrO₂/g-C₃N₄ composite system on increasing the concentration of cobalt with an associated decrease in the bandgap from 3.82 to 3.11 eV. The addition of Co and g-C₃N₄ into ZrO₂ nanoparticles improved the performance of the virgin ZrO₂ with degradation efficiencies from 30% up to 98% and rate constants (k) from $0.57 \times 10^{- 3}$ up to $20.3 \times 10^{- 3} m i n^{- 1}$ . The research emphasizes the importance of maintaining a balance in cobalt doping and underscores the potential of ZrO₂/g-C₃N₄ nanocomposites as effective catalysts, for water treatment applications.

Keywords

cobalt eosin yellow graphitic carbon nitride nanocomposites photocatalysis zirconium oxide

Introduction

As a consequence of rapid urbanization, industrialization, and population growth, about 2 million tonnes of waste pollutants are recklessly released daily into the public water supply rendering the water unsafe for humans and other lives in the ecosystem.¹ Given that 17%–20% of water pollution is attributable to dyeing industries and textile finishings,² the heavy dependence on dyes including Rhodamine B (RhB), Victoria blue, Rose Bengal, Indigo Red, Carmine, Red 120, Eriochrome, Eosin Y (EY), Methylene Blue (MB), Black-T (EBT) and Thymol blue, in the global clothing industries represents a significant environmental threat.³ Dye-polluted effluents are non-biodegradable as they exhibit thermal and photoresistant stability, highly poisonous and coloured pigments that threaten the aquatic environment and human health.⁴

Although research efforts aimed at industrial wastewater treatment, in general, dye-polluted effluents, in particular, such as flocculation, coagulation, ion exchange, photocatalysis and membrane filtration⁵ have not been spared. The intractable nature of the challenge and the recent increasing concentrations of these dyes in our water sources globally demand highly effective, economically sustainable, and environmentally benign techniques. Among these methods, photocatalysis is often considered the most favourable due to its cost-effectiveness, complete removal of various pollutants, and minimal or no generation of secondary waste.⁶ Metal-oxide-based semi-conductors including TiO₂, WO₃, SnO₂, CeO₂, ZrO₂, ZnO and Fe₂O₃^7–11 have widely been employed in the field of photocatalysis for organic dye decomposition.

ZrO₂ with a bandgap range of 5.0–5.85 eV^12,13 semiconductor with an n-type electrical conductivity has recently attracted the attention of researchers due to its exceptional properties, such as high chemical and thermal stability, low thermal conductivity, high dielectric constant, high radiation hardness and high melting point.^12,14,15 Moreover, pure zirconium oxide, commonly called zirconia, exists mainly in three different crystal structures: monoclinic (m-ZrO₂), tetragonal (t-ZrO₂) and cubic (c-ZrO₂). Each of these structures has distinct properties and applications and can be controlled by factors, such as temperature, pressure and the use a of stabilizing agent. The ability to control and manipulate these crystal structures of zirconium oxide allows engineers and scientists to tailor the material for specific applications.¹⁶ However, virgin ZrO₂ has a wide bandgap and a limited surface area of only 100 m²/g. Again, there is a slow rate of generating photo-induced charge carriers but a less electron-hole separation time or high recombination rate. Pristine ZrO₂ can only absorb UV light and therefore will decompose compounds in that region of light.^17,18

Therefore, ZrO₂-based nanocomposites that are visible light-active and possess advanced photocatalytic properties are more desirable for removing dyes from water. By doping pure ZrO₂ with metallic elements, including Co, Gd, Ni and La, researchers have modified the surface acidity and oxygen vacancy concentration of nanocomposites thereby enhancing their structural, electrical and photocatalytic properties.¹⁹ These dopants introduce additional energy levels within the energy gap of the host material which acts as trap centres to impede charge carrier recombination rate²⁰ leading to a long period of electron-hole separation.²¹

Despite its recognition as not being entirely ‘green’ due to environmental concerns, Co when used reasonably is considered an effective dopant and has therefore enjoyed a wide range of applications. Co has many unique properties, such as varying electronic states, capable of replacing Co²⁺ in the host material’s crystal lattice without causing significant changes (due to their comparable ionic radii, that is: Co²⁺ = 0.74 Å and Zr⁴⁺ = 0.80 Å)²² and also the capacity to increase the number of oxygen vacancies in ZrO₂’s structure. For instance, Ahmed and Iqbal²² confirmed the successful replacement of Co²⁺ in ZrO₂ lattice with a mean crystalline size of 18.0 ± 1.7 nm, significant bandgap tuning from 3.70 to 2.12 eV, improved the dielectric electric constant up to 2.8 × 10⁴ with minimal loss and enhanced the photodegradation of MB dye by nearly 97%. Again, Reddy et al.²³ also synthesized Co-doped ZrO₂ nanoparticles via a hydrothermal method to remove methyl orange (MO) under UV-vis light illumination within 100 min. They reported a bandgap decrease from 4.95 to 2.12 eV, which resulted in a remarkable decomposition efficiency of 93.7%. Co-doped nanocomposites have therefore received extensive photocatalytic applications in the environment and energy fields.^21,24,25

Besides, a few studies have shown that compositing ZrO₂ with g-C₃N₄ results in an excellent visible light absorption property, making it a good semiconductor for environmental remediation applications. g-C₃N₄ with a bandgap of 2.7 eV has peculiar properties, including efficient absorption of light and generation of electron-hole pairs, high physicochemical stability, cost-effectiveness and ecofriendliness which make it ideal for photocatalytic activity.^26,27 g-C₃N₄ can be modified structurally and chemically to adjust its reactivity and structure, control its semiconducting characteristics and increase its catalytic uses.²⁸ For example, Chand and Mondal²⁹ reported a remarkable photodegradation efficiency of 94.12% when they applied g-C₃N₄/ZrO₂ composite to degrade RhB dye.

The structural, optical and photocatalytic activities of metal oxides are believed to be influenced greatly by Co concentration. For instance, by varying Co concentration, Ahmed and Iqbal²² observed shifts in bandgap and photocatalytic activity against MB dye. In another study, Jeba et al.³⁰ also asserted that Cu dopant concentration significantly affected the crystallite size and magnetic properties of ZrO₂.

Nonetheless, as far as we are aware, a comprehensive study on the photoactivity of Co-ZrO₂/g-C₃N₄ nanocomposites to photodegrade eosin yellow (EY) dye in wastewater is yet to be done. In addition, the Co concentration on the chemical and surface characteristics of ZrO₂/g-C₃N₄ composites has not been explored in the literature.

This research therefore aimed primarily to synthesize and characterize Co-ZrO₂/g-C₃N₄ nanocomposites for photodegradation of EY. g-C₃N₄ was synthesized via thermal polymerization, whereas the nanocomposites were prepared by simple but cost-effective chemical co-precipitation technique. UV-vis, XRD, FTIR, dynamic light scattering (DLS), and scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX) analyses were used to characterize the synthesized nanomaterials. The collective impacts of Co and g-C₃N₄ on the crystal structure, optical absorption and catalytic activities of ZrO₂ photocatalyst have also been studied.

Experimental section

The following analytical-grade chemicals were procured from Sigma-Aldrich, (Germany): Urea (CH₄N₂O), cobalt (II) sulphate heptahydrate (Co(SO₄)·7H₂O), zirconium oxychloride octahydrate (ZrOCl₂·8H₂O), ammonium hydroxide (NH₄OH) and ethylene glycol (C₂H₆O₂). None of these chemicals were further processed before being used.

Synthesis of g-C₃N₄

G-C₃N₄ was produced by heating urea to a high temperature, following a method from Praseetha et al.³¹ An aluminium foil–covered crucible containing 35 g of urea was placed in a furnace and subjected to a temperature of 600°C for 180 min. After overnight cooling, the resultant g-C₃N₄ was ground into powder and used in the synthesis of the Co-ZrO₂/g-C₃N₄ heterostructure.

Synthesis of series of Co-ZrO₂/g-C₃N₄

The Co-doped ZrO₂/g-C₃N₄ nanomaterials were synthesized by adopting the co-precipitation method used in the work by Agorku et al.¹³ with slight modification. Typically, about 20 mg of g-C₃N₄ (0.2% relative to ZrO₂) and 10 g of zirconium oxychloride octahydrate were mixed and dissolved in 200 mL of distilled water. While being magnetically stirred, 2 mL of ethylene glycol was added. Furthermore, appropriate amounts of cobalt (II) sulphate heptahydrate were introduced to achieve Co-to-Zr ratios ranging from 0.3 to 1.0 wt%. Afterwards, 25% of NH₄OH was added dropwise to adjust the pH between 9 and 10. The reaction mixture was allowed to proceed for 30 min while being stirred. The resulting particles were centrifuged, washed severally using ethanol and deionized water and dried at 60°C for 24 h. The samples were then calcined at 500°C for 2 h. The synthesized Co-doped ZrO₂/g-C₃N₄ was crushed into powder and stored for further analysis and catalytic application. A similar procedure was employed to synthesize bare ZrO₂ and ZrO₂/g-C₃N₄ heterostructure without the addition of Co(SO₄)·7H₂O or g-C₃N₄. The synthesis method described is graphically presented in Figure 1.

Figure 1.

Schematic diagram showing the synthesis pathway for Co-ZrO₂/g-C₃N₄ nanocomposites.

Co-doped ZrO₂/g-C₃N₄ characterization

A variety of analytical methods were used to probe into the characteristics of the synthesized nanocomposites. X-ray powder diffraction (XRD) was used to examine the crystal structures of samples. Optical absorption properties of the as-prepared nanomaterials were also measured using ultraviolet-visible (US-vis) spectrophotometry. Other characteristics, such as functional groups and chemical bonding, morphology, elemental analysis, hydrodynamic particle size and zeta potential, were also studied using FTIR, SEM, EDX and DLS analyses. The Bruker D2 Phaser X-ray powder diffractometer (Bruker, Germany) was employed for the XRD analysis within 2θ values from 5° to 80°. The FTIR studies were carried out with a PerkinElmer Spectrum Two photometer (PerkinElmer, USA) for wave numbers between 400 and 4000 cm⁻¹. In addition, a UV-1800 Shimadzu UV spectrophotometer (Shimadzu, Japan) was used to examine the light absorption behaviour of the prepared nanomaterials between the wavelengths of 200 and 800 nm. A Zeiss EVO MA 15 SEM equipment (Oxford Instruments, United Kingdom) was used to analyse the morphology and elemental investigations of the nanomaterials. The hydrodynamic particle size and surface charge were ascertained using the Malvern Zetasizer Ultra Nano series (Malvern Panalytical Limited, UK).

Photodegradation studies

To conduct an investigation into the photocatalytic activity, 100 mg of the photocatalyst was added to 100 mL of a 20 mg/L EY aqueous solution and stirred vigorously at room temperature for 30 min in the dark. It was then exposed to simulated solar light created with port 9600 full-spectrum solar simulators fitted with a 150 W ozone-free Xenon lamp adjusted at 10 cm. However, 5 mL of the solutions were carefully taken from the reactor every 30 min for 3 h using a syringe fitted with 0.4 µm filters made of polyvinylidene fluoride (PVDF) membrane. Absorbance measurements were done on each of the 5 mL samples to estimate the amount of the EY dye left after each 30-min time interval. The photodegradation performance of the nanomaterials was evaluated through kinetic studies and degradation efficiencies. Using Equation 1, the decomposition efficiency (%E) catalysts were calculated:

% E = \frac{C_{0} - C_{t}}{C_{0}} \times 100

(1)

where C₀ and C_t are the before and after concentrations of the dye.

Equation 2, which represents the simplified first-order kinetic model of Langmuir–Hinshelwood, was also employed to fit the light-induced catalytic degradation of the dye:²⁶

k t = - \ln \frac{c}{c_{0}}

(2)

here k, c and c_o denote the rate constant, the solution of face concentration, and the concentration at t = 0, respectively.

Results and discussion

ZrO₂, ZrO₂/g-C₃N₄ and a number of Co and g-C₃N₄ co-doped ZrO₂ photocatalysts have been synthesized successfully through co-precipitation approach. These synthesized samples were applied for the removal process of EY dye from synthetic wastewater. The nanomaterials obtained were analysed with FTIR, XRD, UV-vis, DLS, SEM and EDX techniques prior to the dye removal process.

FTIR analysis

FTIR spectroscopy was employed in the investigation of the functional groups on the as-synthesized photocatalysts. The FTIR spectra of photocatalysts are given in Figure 2 and consist of g-C₃N₄, ZrO₂, ZrO₂/g-C₃N₄ and (0.3%–1.0%) Co-ZrO₂/g-C₃N₄.

Figure 2.

FTIR spectra of g-C₃N₄, ZrO₂, ZrO₂/g-C₃N₄ and Co/g-C₃N₄ co-doped ZrO₂ with different Co concentrations.

The bands between 3400 and 3500 cm⁻¹ represent the stretching and distortion OH (hydroxyl group) bonds by water molecules that may adhere to zirconia’s surfaces.^32,33 Aromatic heterocycles have stretching vibrations of C–N bonds that are very pronounced in g-C₃N₄ pattern. The frequency emitted by C–N bands of aromatic heterocycles occurs between 1185 and 1727 cm⁻¹.³⁴ The band found at 3108–3320 cm⁻¹, in which the N–H band is linked with the stretching vibration of NH₂ groups in urea. A peak seen at around 773 cm⁻¹ represents the breathing mode of triazine units found in carbon nitrides.³⁵

XRD analysis

The structural characteristics of the synthesized nanomaterials, such as crystal structure, crystallinity and crystallite size were obtained using XRD spectroscopy. The diffraction patterns of pure ZrO₂, ZrO₂/g-C₃N₄, (0.3%–1.0%) Co-ZrO₂/g-C₃N₄ and g-C₃N₄ nanoparticles are seen in Figure 3.

Figure 3.

XRD patterns of bare ZrO₂, ZrO₂/g-C₃N₄, Co/g-C₃N₄ co-doped ZrO₂ with different Co concentrations and g-C₃N_4.

The patterns showed that the synthesized nanomaterials were polycrystalline except for 0.8% Co-ZrO₂/g-C₃N₄ which appeared more amorphous-like material instead of a crystalline. Figure 3 showed the formation of g-C₃N₄ as two prominent peaks at around 2θ = 10.68° and 27.53°,³⁶ corresponding to (100) and (002) planes, respectively (JCPDS Card no. 87-1526).^35,37 These two peaks can be related to the interlayer stacking sequence and interplanar structural arrangement of g-C₃N₄.^38–40 Moreover, the diffraction patterns of bare ZrO₂ showed 10 peaks at 2θ = 17.61°, 24.13°, 28.22°, 31.41°, 34.42°, 40.93°, 44.93°, 50.27°, 55.5° and 59.87° which are associated with the crystallographic planes denoted as (100), (011), (11 $\bar{1}$ ), (111), (020), (21 $\bar{1}$ ), (112), (220), (013) and (131), respectively. The peaks correspond well with the monoclinic phase of zirconium oxide (JCPDS Card No. 37-1484).^41,42 It should be noted that the ZrO₂/g-C₃N₄ sample showed the presence of eight peaks related to the monoclinic and cubic phases of zirconium oxide. In particular, four sharp peaks for (111), (200), (220) and (311) planes were detected at 2θ = 30.27°, 35.14°, 50.41° and 60.15°, respectively. These four peaks align closely with the cubic phase of zirconium oxide (JCPDS Card No. 81-1550).^43,44 The remaining four peaks are linked to the monoclinic phase and were detected at 2θ = 17.30°, 24.25°, 28.19° and 40.91° for (100), (011), (11 $\bar{1}$ ) and (21 $\bar{1})$ , respectively. Examining ZrO₂/g-C₃N₄ in Figure 3 reveals that the intensity of peaks that are linked to the monoclinic phase is reduced when compared with bare ZrO₂. This reduction is due to the emergence of the cubic phase brought about by the addition of g-C₃N₄. When Co²⁺ ions were introduced into the ZrO₂/g-C₃N₄ matrix, the monoclinic phase kept on diminishing until at relatively higher Co²⁺ ions concentrations, it disappeared completely, leaving only the cubic phase. Thus, we report in this work that the incorporation of Co into ZrO₂/g-C₃N₄ has led to a complete transformation from a pure monoclinic phase of zirconium oxide to the cubic phase. This phase transition culminated in energy gap narrowing, as reported in the optical absorption analysis. The c-ZrO₂ is reported to possess a wider light absorption range (including visible), which is essential for photocomposition of dyes. Usually, the creation of reactive species needed degradation is greatly enhanced by an increased light absorption.⁴⁵ In addition, all the Co-ZrO₂/g-C₃N₄ nanocomposites showed no impurity or dopant-related peaks, indicating that Co²⁺ ions and g-C₃N₄ were successfully incorporated into the ZrO₂ matrix.⁴⁶ No additional peak in XRD data was observed, indicative of the purity of the synthesized nanomaterials that was confirmed by EDX spectroscopy.

The mean particle size, D, was calculated using the full width at half maximum (FWHM), β, of the most intense peak, the wavelength of light, λ (1.5406 Å), the constant, K (0.89) and the Bragg’s angle, θ, which are described in the Debye–Scherrer’s formula (Equation 3):⁴⁷

D = \frac{K λ}{β \cos θ}

(3)

The mean particle size was determined to be 27 nm.

SEM/EDX studies

The morphology and elemental distribution of the 0.8% Co-ZrO₂/g-C₃N₄ nanocomposites was studied by SEM and EDX methods. The SEM image of the synthesized nanomaterial represented by Figure 4(a) highlights the sheet-like morphology of g-C₃N₄⁴⁶ functionalized with Co-ZrO₂ crystal particles with irregular shapes and sizes.

Figure 4.

(a) SEM image of 0.8% Co-ZrO₂/g-C₃N₄, (b) EDX spectrum of 0.8% Co-ZrO₂/g-C₃N₄; images depicting the elemental distribution of (c) C, (d) N, (e) O, (f) Co and (g) Zr at the highlighted area of SEM image.

Elemental mapping that was done within the selected area from the SEM image identifies the distribution of C, N, O, Co and Zr elements (Figure 4(c)–(g)). The elemental mapping of the observed area of Figure 4(a) illustrated that all elements distributed in each part were well distributed in Co-ZrO₂/g-C₃N₄ nanocomposites. Also, Figure 4(b) represents the EDX spectra, where all five elements were confirmed, with their atomic and weight percentages. No impurity was observed in the EDX spectra, confirming the purity of the nanomaterials as corroborated by the XRD investigation.

Surface charge and particle size distribution studies

The Malvern Zetasizer was used to measure the electrophoretic and particle size movement of the particles suspended in deionized water at 25°C. From the spectrograph in Figure 4, the zeta potential (ZP) value was found to be −6.38 mV, indicating that the particles in the suspension carry a negative charge. This ZP is consistent with our previous report on La-doped ZrO₂/g-C₃N₄.⁴⁸ The possibility of particle aggregation is suggested by the comparatively low value of ZP, which is consistent with the morphological studies reported in this work. This negative charge could emanate from the presence of ions or surface functional groups. One crucial metric that shows the stability of nanomaterials is the ZP (Figure 5).⁴⁹ The relatively low polydispersity index (PDI) of 0.311 suggests a narrow or good uniformity in particle size distribution.⁵⁰ The hydrodynamic particle size was found to be 230.1 nm. This value is greater than the average crystallite size obtained from the XRD analysis. The higher hydrodynamic particle size is indicative of the tendency of the nanoparticles to aggregate and form larger clusters in a solvent.⁴⁹

Figure 5.

Size distribution by (a) intensity, (b) volume and (c) apparent zeta potential for the 0.8% Co-doped ZrO₂/g-C₃N_4.

Optical absorption analysis

UV-visible spectrophotometric analysis was carried out to ascertain the optical behaviour of the synthesized nanomaterials. The wavelength range of 200–800 nm was employed in this study. The results are as presented in Figure 6.

Figure 6.

Absorbance spectra of ZrO₂, ZrO₂/g-C₃N₄ and Co-ZrO₂/g-C₃N₄ with varied Co concentrations.

Figure 6 shows that the fundamental absorption edge increased slightly towards a longer wavelength with increasing dopant concentration up to 0.8% Co, after which it decreased, suggesting a reduction in the optical bandgap. Hence, 0.8% Co is the optimal percentage concentration of dopant for the synthesis of Co-ZrO₂/g-C₃N₄ in this study under the experimental conditions outlined earlier. This improvement in the optical absorption of ZrO₂ resulting from the introduction of dopants into ZrO₂ matrix may be attributed to the creation of dopant-induced defect states within the bandgap of the host material. These defect states are believed to enhance the creation of e–h pairs thereby impacting photocatalytic activity positively.²²

Using Stern’s relation provided in Equation 4, the bandgaps of the synthesized nanomaterials were calculated from the absorbance data:⁵¹

A = \frac{{[k (h υ - E_{g})]}^{n / 2}}{h υ}

(4)

where A represents the absorbance, k is a constant, hv signifies the photon energy, E_g denotes the energy gap and n takes values of 1 for direct-allowed transitions and 4 for indirect-allowed transitions. In this report, the value of n was taken as 1 because ZrO₂ is a direct bandgap semiconductor.⁵² Plotting ${(A h v)}^{2}$ against hv and extrapolating the linear section of the ${(A h v)}^{2}$ versus hv plot to join the hv axis at ${(A h v)}^{2} = 0$ gives the bandgap. The Stern’s plots for the bare ZrO₂, ZrO₂/g-C₃N₄ and Co-ZrO₂/g-C₃N₄ nanocomposites with varied Co concentrations are given in Figure 7 and the estimated bandgaps are presented in Table 1. The optical bandgap values decreased from 3.82 to 3.11 eV as the dopant concentration was increased from 0.0% to 1.0%. It is worth mentioning that all the nanocomposites showed enhanced optical absorption properties than the bare ZrO₂ which may be linked to the combined influence of Co and g-C₃N₄ on ZrO₂. This is also an indication of the assumed chemical interaction including electrostatic, H-bonding and π–π interactions between the components of the nanocomposites, which may have reduced the bandgap, as displayed in Figure 6.⁵³ In addition, the reduction in the energy gap is a confirmation of the successful incorporation of Co and g-C₃N₄ into ZrO₂.⁵⁴ Reddy et al.⁵⁵ reported a similar bandgap decrease from 5.02 to 2.1 eV when they doped ZrO₂ with copper and attributed it to d–d orbital transitions and charge transfer by the Cu ions into the ionic states of ZrO₂.

Figure 7.

Stern’s plot of ZrO₂, ZrO₂/g-C₃N₄ and Co/g-C₃N₄ co-doped ZrO₂ with different Co concentrations.

Table 1.

The bandgaps of the various synthesized photocatalysts with their degradation efficiencies, rate constants and R² values.

Photocatalyst	Bandgap (eV)	Degradation efficiency (%)	Rate constant $\times 10^{- 3}$ (min⁻¹)	R ²
ZrO₂	3.82	30.4	0.57	0.972
ZrO₂/g-C₃N₄	3.67	45.2	3.21	0.990
0.3%Co-ZrO₂/g-C₃N₄	3.34	72.4	7.48	0.994
0.6%Co-ZrO₂/g-C₃N₄	3.20	96.6	17.2	0.908
0.8%Co-ZrO₂/g-C₃N₄	3.11	98.2	20.3	0.936
1.0%Co-ZrO₂/g-C₃N₄	3.45	87.3	11.4	0.992

Photocatalysis studies

The photoactivities of the synthesized nanoparticles were evaluated via EY dye degradation under UV-vis light illumination over 3 h. To analyse the decomposition processes, the maximum absorption peak of 510 nm⁵⁶ chosen for the EY was chosen. The photocatalytic degradation results for the various photocatalysts against EY dye are shown in Figure 8(a)–(c).

Figure 8.

(a) Degradation efficiency, (b) degradation profile and (c) kinetic studies of ZrO₂, ZrO₂/g-C₃N₄ and Co/g-C₃N₄ co-doped ZrO₂ with different Co concentrations.

The findings of the research that the photoactivity of the ZrO₂ nanomaterial was enhanced by the incorporation of the Co and g-C₃N₄, as the degradation efficiency of the bare ZrO₂ was increased by about threefolds. The increasing order of photocatalytic performance of the nanomaterials was ZrO₂, ZrO₂/g-C₃N₄, 0.3% Co-ZrO₂/g-C₃N₄, 0.6% Co-ZrO₂/g-C₃N₄, 1.0% Co-ZrO₂/g-C₃N₄ and 0.8% Co-ZrO₂/g-C₃N₄, respectively. Furthermore, as dopant concentration increased, the degradation efficiency also increased up to a maximum (98.2%) before decreasing slightly to 87.3%. The relatively poor performance of the bare ZrO₂ (30.4%) may be attributed to its wide bandgap of 3.82 eV and high rate of charge carrier recombination. Besides, when the ZrO₂/g-C₃N₄ was used, the decomposition efficiency increased to 45.2%. The degradation efficiencies for all the photocatalysts synthesized in this work are given in Table 1. These results compare favourably with up to the 93% in 120 min observed in the work by Wang et al.⁴⁶ who applied g-C₃N₄/Ni-doped ZrO₂ for the decomposition of RhB. Vattikuti et al.⁵⁷ also reported up to 97% efficiency within 140 min using g-C₃N₄/ZrO₂ against a mixture of RhB and CV. Detailed comparative studies between the current work and similar others in the literature have been given in Table 2. The performance of the heterostructure in this paper compares well with the reported values, suggesting it is a feasible agent for removing dyes. The observed degradation patterns indicated that the photodegradation of EY conforms to a pseudo-first-order kinetic model. Therefore, the rate constant (k) was estimated using Equation 2 and plotting ln(C/C_o) versus irradiation time, t. That is, the slope of ln(C/C_o) versus illumination time, t graph gives the value of k. The results of this plot are given in Figure 8(c) and the k values, with their respective R² values can be found in Table 1. From the table, k-values for bare ZrO₂, ZrO₂/g-C₃N₄, 0.3% Co-ZrO₂/g-C₃N₄, 0.6% Co-ZrO₂/g-C₃N₄, 0.8% Co-ZrO₂/g-C₃N₄% and 1.0% Co-ZrO₂/g-C₃N₄ are, respectively, $0.57 \times 10^{- 3} m i n^{- 1}$ , $3.21 \times 10^{- 3} m i n^{- 1}$ , $7.48 \times 10^{- 3} m i n^{- 1}$ , $17.2 \times 10^{- 3} m i n^{- 1}$ , $20.3 \times 10^{- 3} m i n^{- 1}$ and $11.4 \times 10^{- 3} m i n^{- 1}$ . From these data, the addition of g-C₃N₄ to ZrO₂ to form ZrO₂/g-C₃N₄ nanocomposite increases the degradation rate from $0.57 \times 10^{- 3}$ to $3.21 \times 10^{- 3} m i n^{- 1}$ which is 5.6 times greater. In addition, when Co was introduced into the composites, the degradation rate further increased to a maximum value of $20.3 \times 10^{- 3} m i n^{- 1}$ for 0.8% Co which is 35.6 times higher than that of bare ZrO₂. This indicates that the incorporation of Co and g-C₃N₄ into ZrO₂ could improve the photoactivity of bare ZrO₂.

Table 2.

Comparison between the current study and previous work.

Photocatalyst	Pollutant	Light source	Time (min)	Degradation efficiency (%)	Reference
g-C₃N_4@ZrO₂	MBEY	UVc	40	96.595.6	Mosleh et al.²⁶
Α-ZrP/g-C₃N₄	CV	Solar	90	97.8	Gupta et al.⁵⁸
ZrO₂-doped LiTa_0.5Nb_0.5O₃	RhB	UV solar	60	98.2	Lazar et al.⁵⁹
ZnO@ZrO₂	BF	Visible	30	99.3	Deepika and Veerakumar⁶⁰
Bi/Co co-doped ZnO-GCN	XO		180	99.0	Pardeshi et al.⁶¹
C₃N₄/ZrO₂	Mixture of RhB + CV	Simulated solar	140	97.0	Vattikuti et al.⁵⁷
g-C₃N₄/MXene	EY	50 W LED light	120	98.0	Bijari et al.⁶²
g-C₃N_4@ZnO	MB	Visible	120	96.4	Jang et al.⁶³
Al-doped ZnO	MB	UV	90	97.1	Amari et al.⁶⁴
Cu@ZnO	MG	Visible	50	80.0	Chandekar et al.⁶⁵
g-C₃N₄/Ni-doped ZrO₂	RhB	Visible	120	92.5	Wang et al.⁴⁶
Mn/TiO₂	Orange II	UV	180	97.0	Reddam et al.¹¹
Co-ZrO₂-g-C₃N₄	EY	UV-visible	180	98.2	Current study

Proposed mechanism of photocatalytic degradation

A catalyst’s conduction band (CB) and valence band (VB) edge positions influence its photocatalytic activity greatly in photocatalysis.⁶⁶ Consequently, by assessing the CB and VB energies of ZrO₂ and g-C₃N₄ catalysts in the Co-ZrO₂/g-C₃N₄ system, one will be able to provide an explanation for the EY dye degradation mechanism. Butler and Ginley model (Equations 5 and 6) were employed to determine VB energy (E_VB) and CB energy (E_CB) of ZrO₂ and g-C₃N₄ in Co-ZrO₂/g-C₃N₄ heterostructure.⁶⁷ The model strongly depends on semi-conductors’ free electron energy (Ee = 4.5 eV), absolute electronegativity (χ) and their bandgaps (Eg):

E_{C B} = χ - E^{e} - \frac{1}{2} E_{g}

(5)

E_{V B} = χ - E^{e} + \frac{1}{2} E_{g}

(6)

From literature, 4.72 and 5.91 eV were the electronegativity values of g-C₃N₄ and ZrO₂, respectively.^68,69 After substituting the forbidden band energies of ZrO₂ (3.82 eV) and g-C₃N₄ (2.72 eV), their respective electronegativity values and the free-electron energy into Equations 5 and 6, the CB and VB edge positions were estimated to be −0.5 eV and +3.32 eV, and −1.14 eV and +1.58 eV, respectively, for of ZrO₂ and g-C₃N₄.

Usually, lower reduction and oxidation potentials favour photogenerated charge carriers. Under these current experimental conditions, e–h separation primarily occurs at g-C₄N₄ when the composites under consideration are exposed to simulated solar light. This is because g-C₃N₄ has a lower bandgap of 2.72 eV as compared with 3.82 for ZrO₂.⁶⁷ It was important to highlight that since the CB of g-C₃N₄ is more negative than the CB of ZrO₂ (−0.5 eV), the light-induced electrons are transported straight into the CB of ZrO₂. However, because the oxidation potential of O₂/*O₂⁻ (−0.33 eV) is less negative compared with the CB edge potential of ZrO₂ (−0.5 eV), the electrons in the CB of ZrO₂ readily react with the surrounding O₂ molecules to form *O^2- which in turn attack the dye molecules to cause degradation.

Furthermore, the light-induced h⁺ is kept in the VB of g-C₃N₄ because the VB of ZrO₂ is more positive than that of g-C₃N₄. The degradation of EY molecules into CO₂ and H₂O is directly facilitated by these holes in the VB of g-C₃N₄. The holes are not able to cause the formation of hydroxide radicals because the reduction potential of –OH (+2.38 eV) and H₂O (2.71 eV) are more positive than the VB edge potential of g-C₃N₄ (1.58 eV).^29,67,70,71

More importantly, the addition of metal ions (such as Co) into a semiconducting material is believed to result in the creation of a Schottky barrier at the metal–semiconductor interface,⁷² which is said to enhance the degradation activities by serving as electron traps to capture the electrons at the surface of the semiconductor.⁷⁰ When the concentration of Co ions in ZrO₂/g-C₃N₄ is carefully controlled, the e–h pair recombination rate is hindered by capturing the electrons into their empty d-orbitals and hence increasing e–h separation time.⁷³ As a result, the enhanced photocatalytic activity in the visible light region by the Co-ZrO₂/g-C₃N₄ photocatalyst might be attributed to the mutually beneficial interaction occurring between Co and g-C₃N₄ within the ZrO₂ framework. Figure 9 is an illustrative depiction of the photodegradation process elaborated in this work.

Figure 9.

Proposed photocatalytic degradation mechanism of Co/g-C₃N₄ co-doped ZrO₂ over EY dye.

Conclusion

Co-ZrO₂/g-C₃N₄ nanocomposites were studied for their optical, structural and photocatalytic performance against EY in synthetic water. A simple chemical co-precipitation technique was used to synthesize the nanomaterials. A range of analytical methods such as XRD, SEM and FTIR were employed to confirm the successful synthesis of ZrO₂, ZrO₂/g-C₃N₄ and (0.3%–1.0%) Co-ZrO₂/g-C₃N₄ photocatalysts. The structural examination revealed a full transformation from monoclinic to cubic structures in zirconium oxide due to inclusion of Co into ZrO₂/g-C₃N₄ heterostructure. The relatively low PDI of 0.311 observed suggests a narrow or good uniformity in particle size distribution, while the ZP value of −6.38 mV confirms that the particles in the suspension carry a negative charge. The reduced bandgap from 3.82 to 3.11 eV also depicted enhanced optical absorption properties of the Co-doped ZrO₂/g-C₃N₄ resulting in an improved photocatalytic efficiency of the virgin ZrO₂ by over threefolds (30.4%–98.2%). The degradation rate constant, k also changed significantly by over 35.6 times; from $0.57 \times 10^{- 3}$ to $20.3 \times 10^{- 3} m i n^{- 1}$ . The findings of this study suggest that Co-doped ZrO₂/g-C₃N₄ promises to be good. The high efficiency observed for the Co-doped ZrO₂/g-C₃N₄ makes a stronger case for its application in wastewater treatment against organic pollutants. Future work on the same material will focus the effect of g-C₃N₄ concentration on the properties and functionality of the nanocomposites. Recovery and reusability tests, and evaluation of the stability and activity of the same nanomaterials will also be explored. In addition, the photoactivity of the nanomaterials would also be tested against wastewater.

Limitations of the study

This work is limited to the synthesis and impact of Co-dopant concentration on the optical, structural and photocatalytic properties of Co-doped ZrO₂/g-C₃N₄ against EY dye in artificial wastewater.

Footnotes

Acknowledgements

The authors thank the Department of Physics, Department of Chemistry, all at the Kwame Nkrumah University of Science and Technology (KNUST), Kumasi-Ghana and the Department of Chemical Sciences, University of Energy and Natural Resources, Sunyani-Ghana.

Author contributions

C.K.B. contributed to formal analysis, visualization and writing – original draft; C.K.B., P.O., M.H. and D.A.P. involved in investigation; M.H., D.A.P., B.Y.D., E.S.A., F.K.A. and R.K.N. participated in writing – review & editing; E.S.A. contributed to conceptualization; A.T. participated in data curation; D.A.P., E.S.A., F.K.A. and R.K.N. performed supervision.

Consent to participate

This paper has no human participants and informed consent is not required.

Consent for publication

The authors declare that they have no known competing financial interests or personal relationships that may affect the work reported in this paper.

Data availability

Data will be made available on request from the corresponding author.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this paper.

Ethical considerations

This paper does not contain any studies with human or animal participants.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this paper.

ORCID iDs

Charles Kwame Bandoh

David Adu-Poku

Moro Haruna

Ophelia Pinto

Bernice Yram Danu

Agnes Tutuwaa

Eric Selorm Agorku

Francis Kofi Ampong

Robert Kwame Nkum

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The influence of cobalt and graphitic carbon nitride on the efficiency of zirconium oxide for photodegradation of an organic dye in wastewater

Abstract

Keywords

Introduction

Experimental section

Synthesis of g-C3N4

Synthesis of series of Co-ZrO2/g-C3N4

Co-doped ZrO2/g-C3N4 characterization

Photodegradation studies

Results and discussion

FTIR analysis

XRD analysis

SEM/EDX studies

Surface charge and particle size distribution studies

Optical absorption analysis

Photocatalysis studies

Proposed mechanism of photocatalytic degradation

Conclusion

Limitations of the study

Footnotes

Acknowledgements

Author contributions

Consent to participate

Consent for publication

Data availability

Declaration of conflicting interests

Ethical considerations

Funding

ORCID iDs

References

Synthesis of g-C₃N₄

Synthesis of series of Co-ZrO₂/g-C₃N₄

Co-doped ZrO₂/g-C₃N₄ characterization