A nickel(II) complex of a naphthaldehyde-derived bis-imine ligand for sunlight-driven dye remediation: mechanistic, intermediate identification, and recyclability studies

Abstract

Dyes have been proven to act as persistent organic pollutants in both freshwater bodies and the marine environment. Thus, suitable dye remediation measures have become obligatory to save aquatic biosystems and maintain human health. This work presents a study on the photocatalytic remediation of cationic dyes using a nickel(II) complex (NiL₁Et). The NiL₁Et complex was synthesized using the ONS donor ligand (L₁) and confirmed using spectroscopic techniques and elemental analysis. The Ni(II) complex NiL₁Et adopts a square planar geometry, which was envisioned through a computational study. The experimentally estimated direct band gap of 2.19 eV is in agreement with the HOMO–LUMO energy obtained in the computational study. This establishes the semiconductor-like property of the NiL₁Et complex. Moreover, the slower electron–hole recombination rate of the excited complex, as inferred from the emission intensities, further demonstrates the potential of NiL₁Et as a photocatalyst. Methylene blue (MB) was used as the model dye, and a maximum degradation efficiency of 80.15% was achieved using NiL₁Et within 60 min under natural sunlight and without any artificial light source. The effects of various parameters, including the catalyst amount, dye concentration, reaction time, pH, and H₂O₂ dose, were analyzed. It was observed that the complex could be efficiently used as a photocatalyst for 5 catalytic cycles. Furthermore, insights into the mechanistic pathway of MB degradation were obtained. The path of MB degradation was predicted by determining the intermediate species generated during the degradation process using LC-MS.

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Sustainability spotlight

Organic dyes, when released untreated, can pollute water bodies. Although a number of Fe(II)/Fe(III) and Cu(II) Schiff base complexes have been efficient in the degradation of organic pollutants, only a few Ni(II) Schiff base complexes were found to be efficient. This is due to their low degradation efficiency and extended degradation period. The NiL₁Et complex was found to be capable of degrading a cationic dye with high efficiency in a short period under natural sunlight, without the requirement of additional light sources. Thus, it is a sustainable and more energy- and cost-effective option. Our work contributes to achieving the Sustainable Development Goals (SDGs) 6 (Clean Water and Sanitation) and 14 (Life Below Water).

1 Introduction

A major concern of humankind is tackling the rising global water scarcity. The indiscriminate pollution of water bodies further limits the availability of safe drinking water. Numerous industry-based goods, including commodities such as pharmaceutical products, cosmetics, paints, and dyes, have been proven to be persistent organic pollutants due to their non-biodegradability.¹

Organic dyes constitute a class of organic molecules that play a major role in the pollution of water bodies. A large number of dyes are used in substantial quantities across various printing, textile, and rubber industries. The effluents from these industries contain a number of organic dyes, which can be toxic.^2,3 Common wastewater treatment plants in the textile industry are inefficient at completely removing dyes.² Thus, the effluents containing these dyes, like methylene blue (MB), when released into water bodies, not only affect the turbidity, pH, temperature, and color of the water but also lead to water pollution, eutrophication, and the production of various perilous by-products, which are generated as a result of these dyes undergoing different chemical reactions.⁴ These dyes can persist in the environment for prolonged periods of time and contaminate the groundwater, thereby posing serious health hazards, as they can be highly toxic and carcinogenic.² This is why researchers worldwide are devoting significant research attention to the development of sustainable and effective methods for the remediation of dyes like MB from effluents and water sources.

Photocatalytic advanced oxidation processes (AOPs) have emerged as some of the most popular and effective protocols for the remediation of organic dye pollutants. Such processes involve the transfer of electrons from the conduction band into the valence band of the photocatalyst when irradiated with light. When the electrons and holes from the photocatalyst come in contact with H₂O₂, O_2, or H₂O, radical oxygen species (ROS) like ·OH and ·O²⁻ are produced. The ROS thus produced cause the photodegradation of organic pollutants like dyes.⁵

Various nanoparticles and polymers have been reported to catalyze the photocatalytic degradation of organic pollutants, such as dyes.^5–8 However, metal nanomaterials were found to have specific adverse effects. These mainly include the high cost of synthetic routes and the toxicity of certain metal nanomaterials. Some of them were also found to interact with other species in the reaction medium during the degradation process. Moreover, separating and recycling nanomaterials is a complicated and expensive process that often requires sophisticated instruments. Some serious health hazards have been linked to the usage of various metal-based nanomaterials.⁹ Thus, researchers have studied different catalyst species as alternatives to these metal nanomaterials for their potential in achieving higher efficiency with a low-cost synthetic route and substantially lower toxicity.

Various metal complexes have shown great potential as photocatalysts for the mineralization of organic pollutants in aquatic systems. However, these metal complexes have always been associated with low heterogeneity and lower stability in reaction media, which leads to a lower recyclability of the catalytic species. In this regard, certain Schiff base metal complexes have proven to be excellent photocatalysts for the degradation of organic pollutants, including dyes, pharmaceutical products, and other organic molecules. Some Schiff base complexes were found to exhibit high degradation efficiency with desired recyclability over consecutive catalytic cycles.¹⁰ However, a majority of Schiff base metal complexes reported as photocatalysts for the degradation of organic pollutants, such as dyes, are mostly Fe(II)/Fe(III) and Cu(II) complexes.^4,10,11 Very few Ni(II) Schiff base complexes have been reported to be effective photocatalysts for the degradation of organic dyes. Low degradation efficiency and a longer degradation time were major drawbacks for most of the few reported Ni(II) Schiff base complexes.^12–14 However, very few Ni(II) Schiff base complexes were also capable of achieving degradation efficiencies greater than 90%.^15,16 This inspired us to study the photocatalytic capability of Ni(II) complexes of highly conjugated Schiff base ligands. A lower HOMO–LUMO energy gap is expected in such complexes in the presence of a conjugated ligand, thereby facilitating charge separation when irradiated with light. This, in turn, will favour higher photocatalytic activity.

In this work, we present the development of a complex (NiL₁Et) consisting of a Ni(II) centre being ligated to a naphthaldehyde-derived unsymmetric Schiff base-based ONS donor ligand (L₁). The NiL₁Et complex was evaluated for its photocatalytic efficiency in the degradation of MB dye via an advanced oxidation process under sunlight.

2 Experimental

2.1. Chemicals and instruments

All the required chemicals and solvents were procured from BLDpharm, SRL, and Merck India Ltd.

The vibrational spectra were obtained using a Bruker 3000 Hyperion Microscope FT-IR spectrometer. A 400-MHz JEOL JNM ECS400 NMR spectrometer was employed to record the ¹H NMR spectra. ESI-mass and LC-MS spectra were obtained using a XEVO G2-XS QTOF mass spectrometer and a Waters ACQUITY UHPLC system. Elemental analysis was performed using a FLASH Smart Thermo Elemental analyzer. UV-visible spectra were obtained using a Mortas Scientific UV plus UV-visible spectrophotometer. TGA was performed using a PerkinElmer TGA 4000 instrument. Inductively coupled plasma optical emission spectrophotometry (ICP-OES) analysis was carried out using an AVIO 220 MAX PerkinElmer spectrophotometer. An Elementar Enviro TOC instrument was used to carry out total organic carbon (TOC) analysis. The electron spin resonance (ESR) spectrum was recorded using a Bruker EMX ESP spectrometer. The zeta potentials were recorded using a Nano ZS90 Zeta Sizer by Malvern Panalytical.

2.2. Synthesis of the bis-imine ligand (L₁)

The ONS donor Schiff base ligand (L₁) was synthesized following a reported synthetic route.¹⁷

2.3. Synthesis of the nickel complex [Ni(L₁)(OC₂H₅)]·2H₂O (NiL₁Et)

To a solution of 0.5 mmol L₁ in ethanol, a 0.5 mmol solution of nickel(II) chloride hexahydrate (NiCl₂·6H₂O) in ethanol was added and stirred. Two drops of triethylamine were added and then allowed to reflux for 8 hours. The NiL₁Et complex was obtained as an orange-coloured precipitate. Cooling, followed by filtration, yielded the orange precipitate, which was then rinsed with methanol and then with deionized water. Later, the precipitate was dried in an oven.

Yield: 61.22%. Melting point >300 °C. Observed FT-IR peaks (ν, cm⁻¹): 3398 (O–H stretching vibrations of uncoordinated H₂O), 3048 (sp² C–H stretching vibrations), 2919 (sp³ C–H stretching), 1596 (C=N stretching), 1188 (C–O stretching), 821 (C–S stretching of thiophene ring) (Fig. S1). ¹H NMR (400 MHz, CDCl₃), δ (ppm): 10.27, 8.66, 8.15, 7.92, 7.84, 7.56, 7.10, 6.60, 6.46, 6.12, 4.74, 1.83, 1.24 (Fig. S2). UV-visible (DMSO, λ_max, nm): 319, 376, 398, 446, 538 [Fig. S3]. ESI-MS (m/z): calculated for C₁₈H₁₉BrN₂NiO₄S [M]⁺: 495.9602, found: 495.6107 (Fig. S4). Elemental analysis (%) calculated (found) for C₁₈H₁₉BrN₂NiO₄S: C, 43.41 (43.38); H, 3.85 (3.87); N, 5.63 (5.60); S, 6.44 (6.41).

2.4. Theoretical analysis

NiL₁Et was computationally studied employing density functional theory (DFT) using the B3LYP level of theory in the Gaussian 16 program.¹⁸ The optimized geometry of the NiL₁Et complex was analyzed using a 6-311++G(d,p) basis set for the ligand and LANL2DZ effective core potential (ECP) for Ni.¹⁹

2.5. Photocatalytic degradation of dyes

NiL₁Et was screened for its potential photocatalytic activity in the degradation of organic dyes. The widely used cationic dye, Methylene blue (MB), was used to assess the photocatalytic performance of the Ni(II) complex. NiL₁Et was added to a 50 mL MB solution. Different volumes of H₂O₂ were then added to this solution. The initial absorbance of the MB dye solution was recorded. Then, the solution was kept away from light for 30 minutes, giving it enough time to reach adsorption–desorption equilibrium. After 30 minutes, the absorbance of the solution was noted again. Then, the MB solution in the presence of NiL₁Et was mechanically stirred under direct sunlight. An aliquot of 5 mL was drawn every 10 min, and its absorbance was recorded. The experiments were conducted under direct natural sunlight from 9 am to 12 noon on bright sunny days in the month of September in Silchar, India. The strength of the sunlight was found to vary between 690 W m⁻² and 770 W m⁻² when measured using a solar power meter. The catalyst doses varied from 0.2 to 1 g L⁻¹, while the concentration of MB varied over a range of 10 to 30 ppm. The photocatalytic degradation of another cationic dye, Rhodamine B, and an anionic methyl orange dye was performed using a similar procedure.

The degradation efficiency of the dyes was calculated using the following equation:

where C₀ is the concentration of MB before the beginning of the degradation process, and C is the MB concentration of the aliquots taken at fixed time intervals.

The reaction kinetics of the methylene blue degradation using NiL₁Et as the photocatalyst were studied using the following equation:

where C₀ is the initial concentration of MB at time t = 0, while C is the concentration of MB at time t, and k is the rate constant.

3 Results and discussion

3.1. Synthesis of NiL1Et

NiL₁Et was obtained from nickel chloride hexahydrate and L₁ in ethanol in the presence of triethylamine (Scheme 1). NiL₁Et was found to be soluble in DMSO and DMF.

Scheme 1 Synthesis of [Ni(L₁)(OC₂H₅)]·2H₂O.

3.2. FT-IR spectral study

The formation of NiL₁Et was ascertained by comparing the bond vibrations in the FT-IR spectra of NiL₁Et and ligand L₁. FT-IR also revealed the binding sites of the metal to the Schiff base. The O–H stretching vibration of L₁, which was visible at 3576 cm⁻¹, was missing in the spectrum of NiL₁Et, which indicated the binding of the phenolate to the Ni(II) ion (Fig. S1).¹⁷ A shift in the C–O stretching band to a lower wave number, from 1182 cm⁻¹ to 1188 cm⁻¹, was observed when comparing the spectra of the ligand (L₁) and NiL₁Et. This further validates the bond formation between the metal and the phenoxide moiety of L₁.²⁰ Moreover, the presence of uncoordinated water molecules is indicated by the broad band observed at 3398 cm⁻¹ in the FT-IR spectrum of NiL₁Et.^21,22 The bands at 3048 and 2919 cm⁻¹ in the FT-IR spectrum of NiL₁Et can be attributed to the aromatic C–H stretching and sp³ C–H stretching vibrations, respectively.²³ When the spectrum of L₁ was compared to that of NiL₁Et, a shift in the C=N stretching vibrations from 1599 cm⁻¹ to 1596 cm⁻¹ was observed. This reduction in the wave number of C=N stretching vibrations indicates the binding of Ni(II) ions to the imine group of L₁ in the NiL₁Et complex.¹⁴ Similarly, a shift in the C–S stretching vibration of the thiophene ring from 829 cm⁻¹ in L₁ to 821 cm⁻¹ in NiL₁Et was observed; this validates the binding between the Ni(II) ion and the S atom of the thiophene ring.²⁴

3.3. ¹H nuclear magnetic resonance analysis

Comparing the ¹H-NMR spectrum of L₁ to that of the NiL₁Et complex, it was observed that the proton signal for the phenolic OH group was missing in the ¹H-NMR spectrum of NiL₁Et. This observation further confirms the binding of Ni(II) to the OH moiety of ligand L₁ (Fig. S2). The proton signal of the imine group that was observed at δ 9.63 for L₁ shifted to δ 10.27 in the ¹H NMR spectrum of NiL₁Et, thereby confirming the binding of Ni(II) to the >C=N moiety.²⁵ Additionally, the ¹H NMR spectrum of NiL₁Et exhibits two proton signals at δ 4.74 and δ 1.24 corresponding to the –O–CH₂ and –CH₃ protons, respectively.

3.4. ESI-MS spectral study

The formation of NiL₁Et was further confirmed via mass spectrometry (Fig. S4). The spectrum exhibits the molecular ion [M]⁺ peak of [Ni(L₁)(OC₂H₅)]·2H₂O at an m/z value of 495.6107, which is in close agreement with the calculated value of 495.960. The base peak at an m/z value of 341.1244 corresponds to the [L₁–OH]⁺ peak of the Schiff base L₁.

3.5. UV-visible spectral study

Upon comparing the UV-visible spectra of L₁ and NiL₁Et using DMSO as the solvent, it was discovered that the ligand (L₁) had π → π* transitions at λ_max 336 and 399 nm,¹⁷ showing hypsochromic shifts to 319 and 376 nm, respectively, after binding to Ni(II) in NiL₁Et (Fig. S3). Another absorption peak at 398 nm was observed, corresponding to the n → π* transition of the imine moiety in NiL₁Et.²⁶ Moreover, two additional absorption bands at 446 and 538 nm corresponding to ¹A_1g → ¹B_1g and ¹A_1g → ¹A_2g (d–d) transitions were observed in the spectrum of NiL₁Et. These bands in the absorption spectrum of NiL₁Et are indicative of a square planar geometry.¹⁴

3.6. Thermal study

The thermal stability of the NiL₁Et complex was analyzed by thermogravimetric analysis (TGA) in an open atmosphere at a heating rate of 10 °C min⁻¹. A loss of 7.57% from 53 °C to 144.5 °C was noted in the TGA curve (Fig. S5), corresponding to the loss of two uncoordinated H₂O molecules from the complex.²⁷ A weight loss of 15.87% was recorded within the temperature range of 236 °C to 397 °C for the loss of Br.^28–31 This was followed by the breakdown of the ligand at 447 °C.

3.7. Tauc plot

The UV-DRS analysis of NiL₁Et was performed within the range of 200 to 800 nm (Fig. S6), and the Tauc plot was subsequently obtained (Fig. 1). The Tauc plot was crucial in the determination of the direct band gap of NiL₁Et (E_g), which was 2.19 eV.

Fig. 1 Tauc plot of the NiL₁Et complex.

3.8. Computational analysis

The most probable geometry of NiL₁Et was optimized using DFT, and the electronic properties, stability, and reactivity of the complex were studied theoretically. The optimization yielded a square planar geometry for the NiL₁Et complex, where the O17, N19, and S29 of the ligand (L₁) were bonded to the Ni(II) center (Fig. 2). The O31 of –OC₂H₅ completed the fourth coordination site of the square planar geometry. The O17–Ni–N19, N19–Ni–S29, S29–Ni–O31, and O31–Ni–O17 bond angles were found to be 95.50°, 92.41°, 83.40°, and 97.93°, respectively. The bond lengths of O17–Ni, N19–Ni, S29–Ni, and O31–Ni were found to be 1.858 Å, 1.999 Å, 2.326 Å, and 1.793 Å, respectively. The computational determination of the HOMO and LUMO energies revealed that NiL₁Et has a HOMO–LUMO energy gap of 2.4325 eV, which is in close agreement with the value of the direct band gap obtained using the Tauc plot (Fig. 3). The HUMO and LUMO energies of NiL₁Et were −5.1780 eV and −2.7455 eV, respectively. The HOMO and LUMO energies of the complex were crucial in determining the electronegativity (χ), ionization potential (IP), chemical potential (µ), electron affinity (EA), global hardness (η), global electrophilicity (ω), and global softness (σ) (Table 1). NBO analysis revealed that the formal charge on Ni(II) decreased to +0.889 after complexation, which indicated the bonding of Ni(II) to the ligand, resulting in the transfer of electrons from the ligand to the Ni(II) center in NiL₁Et.

Fig. 2 Optimised structure of the NiL₁Et complex.

Fig. 3 HOMO and LUMO energies of the NiL₁Et complex.

Table 1 Global reactivity descriptors of the NiL₁Et complex

Parameters	NiL1Et
EHOMO (eV)	−5.1780
ELUMO (eV)	−2.7455
ΔE (eV)	2.4325
IP (eV)	5.1780
EA (eV)	2.7455
χ (eV)	3.96175
µ (eV)	−3.96175
η (eV)	1.21625
σ (eV^−1)	0.41109
ω (eV)	6.4524

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3.9. Photoluminescence analysis to determine the electron–hole recombination rate

The change in the intensities of the emissions observed in the photoluminescence (PL) spectrum is crucial in estimating the recombination rate of charge carriers (e⁻–h⁺ pair).^32–34 Higher emission intensities indicate a higher recombination rate of the electron–hole pairs, while lower emission intensities indicate a slower recombination of the electron–hole pairs. A lower electron–hole recombination rate suggests better photocatalytic efficiency.^33,35 The PL spectra of L₁ and NiL₁Et reveal a comparatively higher emission intensity for the ligand (L₁), while the photoluminescence intensity of NiL₁Et was much lower (Fig. 4). Thus, NiL₁Et can be expected to have much slower charge carrier recombination; therefore, NiL₁Et is expected to exhibit desirable photocatalytic activity.

Fig. 4 Photoluminescence spectra of L₁ and the NiL₁Et complex. Inset: zoomed photoluminescence spectrum of L₁.

3.10. Photocatalytic Methylene blue (MB) degradation

3.10.1. Effect of catalyst dosage. The extent of degradation of methylene blue was analyzed with varying photocatalyst amounts ranging from 0.2 to 1 g L⁻¹. In this experiment, a 20 ppm solution of MB in 50 mL of deionized water in the presence of 20 µL of H₂O₂ was placed under direct sunlight while maintaining the pH at 7. The maximum photodegradation efficiency obtained using NiL₁Et was found to be 76.39% at a catalyst dose of 0.8 g L⁻¹ (Fig. 5(a)). The kinetics of the reaction using varying catalyst dosages were determined (Fig. 5(b)). The photocatalytic MB degradation was found to follow pseudo-first-order kinetics, as R² > 0.95. The rate constant had a maximum value of 0.0233 min⁻¹ for 0.8 g per L NiL₁Et.

Fig. 5 (a) Plot of C/C₀ vs. time. (b) Reaction kinetics of the photodegradation of MB using varying doses of the NiL₁Et complex.

3.10.2. Effect of methylene blue concentration. The concentrations of MB taken initially for the degradation experiments were optimized with different concentrations of MB, starting from 10 to 40 ppm. The dose of NiL₁Et was fixed at 0.8 g L⁻¹. The MB solution was added to 20 µL of H₂O₂ while maintaining the pH at 7. The maximum degradation was achieved with an MB concentration of 20 ppm (Fig. 6(a)). The photodegradation efficiency decreased when the initial concentration of MB exceeded 20 ppm, which can again be attributed to reduced light penetration, decreasing the path length of photons entering the MB solution at higher initial concentrations. Moreover, the percentage of degradation of MB falls with a rise in the MB concentration exceeding 20 ppm, as the degradation of a higher initial concentration of MB requires a higher catalyst surface, but in this study, the catalyst dose of NiL₁Et was fixed.³⁶ The kinetics study revealed that (Fig. 6(b)) the rate constant was maximum (0.0229 min⁻¹) at an initial MB concentration of 20 ppm for NiL₁Et.

Fig. 6 (a) Plot of C/C₀ vs. time. (b) Reaction kinetics of the photodegradation with varying initial concentrations of MB in the presence of the NiL₁Et complex.

3.10.3. Effect of H₂O₂ dosage. Volumes of H₂O₂ ranging from 0 to 80 µL were added to evaluate its effect on the degradation efficiency. A 50 mL solution of 20 ppm MB in deionized water was irradiated with sunlight in the presence of 0.8 g L⁻¹ NiL₁Et. The pH of 7 was maintained throughout the experiment (Fig. 7(a) and (b)). The degradation efficiency was enhanced with increasing H₂O₂ volumes; however, a gradual fall in the efficiency was observed when H₂O₂ volumes higher than 40 µL were added. The rise in the degradation efficiency in the presence of H₂O₂ is mainly due to the self-decomposition of H₂O₂ to produce hydroxy radicals (·OH) when irradiated with sunlight. Moreover, the fall in efficiency when the added volume of H₂O₂ exceeded 40 µL was due to the scavenging of the reactive hydroxy radicals (·OH) by excess H₂O₂ molecules to produce ·O₂H radicals, which are less active than ·OH radicals.³⁷

Fig. 7 (a) Plot of C/C₀ vs. time. (b) Reaction kinetics of the photodegradation of MB at different H₂O₂ dosages in the presence of the NiL₁Et complex.

3.10.4. Effect of change in pH. The effects of pH on the photodegradation of MB were evaluated by varying the pH from 5 to 11. A 20 ppm solution of MB was taken, and the catalyst dose of 0.8 g per L NiL₁Et was added, followed by the addition of 40 µL of H₂O₂. The degradation efficiency increased with an increase in pH from 5 to 11 (Fig. 8(a)). At a pH greater than 7, the surface of the photocatalyst is negatively charged, and as MB is a cationic dye, the electrostatic interaction between the cationic MB dye and the negatively charged catalyst surface is enhanced. This is what causes the increase in the photodegradation efficiency at a higher pH.³⁸ Moreover, at a pH greater than 7, an enhanced adsorption of the cationic MB dye on the catalyst surface is expected.³⁹ The increased adsorption of MB on the catalyst surface further enhances the photodegradation of MB at a higher pH.⁴⁰ The kinetics of the degradation were studied (Fig. 8(b)). The rate of degradation at a pH of 11 was found to be 0.02569 min⁻¹ using NiL₁Et. This shows that a higher pH is desirable for better degradation of the MB dye. However, the study also indicates that NiL₁Et can degrade the MB dye even under neutral conditions, although the efficiency is lower than that obtained at a pH of 11.

Fig. 8 (a) Plot of C/C₀ vs. time. (b) Reaction kinetics of the photodegradation of MB at different pH levels using the NiL₁Et complex.

The zeta potential analysis of the catalyst was performed to examine its surface charge at different pH levels. The pH of the point zero charge (pH_ZPC) of NiL₁Et was found to be 6.46. The zeta potentials at pH < pH_ZPC were found to be positive, indicating that the catalyst surface is positively charged at pH < pH_ZPC, thereby attracting negatively charged molecules. The zeta potentials at pH > pH_ZPC were found to be negative; this indicates that the catalyst surface is negatively charged at pH > pH_ZPC, which will attract cationic molecules (Fig. S7).^35,41

3.10.5. Effect of reaction time. The time required to achieve the maximum possible MB degradation efficiency was evaluated. The optimum catalyst dose was 0.8 g per L NiL₁Et. A 20 ppm solution of MB in 50 mL of deionized water, maintained at a pH of 11, was irradiated with sunlight in the presence of 40 µL of H₂O₂. After 60 minutes, a degradation efficiency of 80.15% was achieved using NiL₁Et as the catalyst. No notable enhancement in degradation efficiency was observed beyond 60 min (Fig. 8(a)). Fig. S8 presents the absorption spectra of the photocatalytic degradation of MB under optimized conditions in the presence of NiL₁Et as the photocatalyst. The effectiveness of this degradation procedure using NiL₁Et as the photocatalyst under sunlight was further analysed by performing the dye degradation experiment under specific controlled conditions, such as degradation in the presence of only H₂O₂ in the dark, NiL₁Et and H₂O₂ in the dark, only H₂O₂ under sunlight, and only NiL₁Et without H₂O₂ under sunlight (Fig. S9). Upon comparing the degradation efficiencies achieved under the different conditions, it was discovered that NiL₁Et in the presence of H₂O₂ under sunlight is the required condition to obtain the maximum possible degradation efficiency.

A comparison of the photocatalytic efficiencies of NiL₁Et in the degradation of MB and those of some reported Ni(II) Schiff complexes is presented in Table 2. It was observed that a fairly high degradation efficiency was achieved using NiL₁Et under direct sunlight when compared to other Ni(II) Schiff base complexes. The degradation of MB using NiL₁Et was also compared with some of the reported nanomaterials used to obtain high degradation efficiency. It was observed that the degradation efficiency achieved by NiL₁Et in a relatively shorter period is also in a comparable range to those of some Ni(II) Schiff base complexes, which achieved higher degradation efficiencies. Although using the nanomaterials as photocatalysts yielded higher degradation efficiencies, NiL₁Et was capable of degrading MB comparatively within a shorter period in most cases. Furthermore, the degradation of MB using NiL₁Et was carried out following a more sustainable approach, using direct natural sunlight. This approach is cost-efficient and user-friendly, as no additional instrumentation or artificial power source is required. In contrast to this, the degradation of MB using some of the reported Ni(II) Schiff base complexes and the majority of the nanomaterials compared in Table 2 was performed under irradiation from artificial light sources like a UV or Hg lamp, which will require additional electricity. This can, in turn, be challenging for the application of such catalytic species in large-scale studies or in real, practical applications.

Table 2 Comparison of the degradation efficiencies of the NiL₁Et complex and some reported Ni(II) Schiff base complexes

Complex/catalyst	Ligand	Degradation efficiency	Irradiation time	References
[Ni(HL)L]Cl·2CH3OH·4H2O		47%	60 min under visible light	12
[Ni(HL)L]NO3·3H2O		58%	60 min under visible light
[Ni(HL)L]Br·4H2O		55%	60 min under visible light
Ni-ABzC		62.50%	40 min under sunlight	14
[Ni(PL)2]		72.35%	12 hours	43
Ni(II)-Schiff base complex		74.60%	Under UV lamp	44
SBLNi		76.80%	200 min under UV lamp	45
[Ni^II(L)2](ClO4)2·2H2O		84.50%	120 min under UV light	46
Ni(C40H36N4O8) 3CH3OH		85%	100 min under Hg lamp	47
[Ni(HL)2]		91% at pH = 12	60 min under UV light	15
[Ni(C22H26N2O10S2)]·2CH3OH		95.49%	140 min under Hg lamp	16
NiL1Et		80.15%	60 min under sunlight	This work
Green-synthesized CoFe2O4/TiO2 nanocomposites	—	98.70%	20 min under UV irradiation	48
TiO2 nanotubes	—	87%	300 min under UV irradiation	49
ZnO/Fe3O4 heteronanostructures	—	99.70%	150 min under UV and visible light	50
BiFeO3 nanoparticles	—	99%	15 min under 450 W mercury vapour lamps	51

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However, while determining the degradation efficiency of a photocatalyst, estimating only the discoloration of the organic dye is not enough; for a degradation process to be effective, it is essential that it causes the mineralization of the dye too. The mineralization of methylene blue (MB) was assessed through TOC analysis. Methylene blue (MB), having the molecular formula C₁₆H₁₈N₃SCl, has a carbon fraction of 0.6007.⁴² Thus, the initial 20 ppm MB solution has a TOC value of 12.014 mg L⁻¹. The degraded MB solution using NiL₁Et after 60 min of irradiation under sunlight has a final TOC value of 4.19 mg L⁻¹. This indicated that NiL₁Et in the presence of H₂O₂ under sunlight could not only cause the discoloration of the MB solution, but it could also mineralize the MB dye with a 65.12% TOC removal.^43–51

3.11. Photocatalytic degradation of Rhodamine B (Rh B) and methyl orange (MO) dyes

To fully understand the broader potential of the Ni(II) complex, NiL₁Et, as a photocatalyst for the degradation of organic dyes in an aqueous environment, the photocatalytic degradation of other cationic and anionic dyes was also investigated. Rhodamine B (Rh B) was used as the other cationic dye, and methyl orange was used as an anionic dye.^52,53 20 ppm solutions of both dyes were treated with 0.8 g L⁻¹ NiL₁Et in the presence of 40 µL of H₂O₂ under sunlight under neutral conditions. Moreover, degradation experiments were carried out separately in the presence of only H₂O₂ in the dark, in the presence of both NiL₁Et and H₂O₂ in the dark, in the presence of only H₂O₂ under sunlight, and in the presence of only NiL₁Et under sunlight (Fig. S10(a) and S11(a)). Degradation efficiencies of 77.33% after 80 min of irradiation and 54.85% after 120 min of irradiation were achieved while degrading the RhB and MO dyes, respectively (Fig. S10(b) and S11(b)). Thus, it was observed that NiL₁Et as a photocatalyst under sunlight is capable of achieving higher efficiency against cationic dyes as compared to anionic dyes. This observation is in agreement with the results of the zeta potential experiment. As NiL₁Et has a negatively charged surface at a neutral pH, cationic dyes will be more attracted to the catalyst surface compared to anionic dyes, resulting in better degradation efficiency in the case of cationic dyes.³⁵

3.12. Recyclability, stability, and leaching studies of the catalyst

The ability to reuse and recycle the catalyst in consecutive catalytic cycles is essential for cost-effective and feasible practical applications. This is also indicative of the structural stability of the catalyst species under reaction conditions. The used catalyst was recovered by centrifugation, followed by thorough rinsing with deionised water. The recovered catalyst was then dried in a hot air oven before reuse in the next catalytic cycle. NiL₁Et was used consecutively for five catalytic cycles (Fig. 9). Even after the fifth catalytic cycle, only a minute fall in the degradation efficiency of MB was noted. This fall in the efficiency of the photocatalytic degradation of MB after five consecutive cycles using the recovered NiL₁Et catalyst can be attributed to the blocking of some of the active sites on the NiL₁Et surface by the produced degradation intermediates.³⁵

Fig. 9 Recyclability of the NiL₁Et complex as a catalyst in the photodegradation of MB.

The stability of NiL₁Et in the reaction medium was further analyzed by comparing the FT-IR spectrum and XRD pattern of NiL₁Et before using it in the catalytic process and after the fifth consecutive catalytic cycle. All the peaks of the FT-IR spectrum of NiL₁Et recorded after the fifth catalytic cycle (Fig. S12) matched the FT-IR spectrum of the freshly prepared NiL₁Et. This indicated the high structural stability of NiL₁Et as the photocatalyst during the MB degradation process. A similar conclusion can be drawn when the diffraction pattern of the freshly prepared NiL₁Et was compared to that of the recovered NiL₁Et after the fifth consecutive catalytic cycle (Fig. 10). Both the diffraction patterns were identical. This further established the fact that NiL₁Et is a stable photocatalytic species where the reaction conditions had no significant effect on its structural integrity.

Fig. 10 XRD patterns of the freshly prepared NiL₁Et complex and after the fifth catalytic cycle in the photodegradation of MB, collected at a scan rate of 0.071112° per second.

Furthermore, a leaching study was conducted using ICP-OES on the sample collected after the catalytic cycle. The mass of Ni leached after the catalytic cycle was 25.09 µg L⁻¹ (25.09 × 10⁻⁶ g L⁻¹). Thus, the percentage leaching of Ni was 0.02%, which was negligible. Moreover, the obtained value of leaching Ni mass was well below the WHO-recommended limit of 70 µg L⁻¹ in drinking water.⁵⁴

3.13. Photocatalytic mechanism

To determine the photocatalytic mechanism, it is crucial to identify the active species responsible for dye degradation. Thus, to identify the reactive oxygen species generated during the photocatalytic degradation of MB, a radical-scavenging experiment was performed. During this experiment, the degradation of MB in the presence of various radical scavengers was studied. Here, potassium persulphate (K₂S₂O₈), disodium EDTA (Na₂EDTA), benzoic acid, and ascorbic acid were employed as scavengers for e⁻, h⁺, ·OH, and ·O₂⁻, respectively.^35,55 It was observed that in the photocatalytic degradation of MB using NiL₁Et as the catalyst, in the presence of potassium persulphate (K₂S₂O₈), disodium EDTA (Na₂EDTA), benzoic acid, and ascorbic acid, the degradation efficiency falls to 65.05%, 60.09%, 22.11%, and 53.61%, respectively (Fig. S13). These observations suggested that hydroxyl radicals (·OH) were the dominant active species in the degradation of MB. Furthermore, the generation of hydroxy radicals (·OH) was confirmed via an ESR spin trapping experiment. α-Phenyl-N-tert-butylnitrone (PBN) was used as the spin trapping agent. The obtained ESR spectrum matched the characteristic spectrum of the reported PBN–·OH adduct (Fig. S14). The obtained hyperfine splitting constants (HFSCs) a_N = 15.39 G and a_H = 2.86 G are in agreement with the reported HFSCs for PBN–·OH adducts.^56,57 This confirms the generation of hydroxy radicals (·OH) as the reactive oxygen species during the degradation of the dyes using NiL₁Et as the photocatalyst.

Thus, correlating the experimental findings with the available literature on the advanced oxidation process, the mechanism of the photocatalytic degradation of MB using NiL₁Et is expected to involve the transfer of electrons from the HOMO to the LUMO of the catalyst under direct sunlight, as NiL₁Et has a semiconductor-like low HOMO–LUMO energy gap of 2.19 eV.⁵⁸ This excitation of electrons to the LUMO also generates an equal number of holes (h⁺) in the HOMO of NiL₁Et. The lower electron–hole recombination rate of NiL₁Et, as indicated by the low emission intensity in the photoluminescence spectrum, further facilitates this charge separation. Thus, the holes (h⁺) produced in the HOMO can interact with hydroxyl ions (OH⁻) or water molecules and oxidize them to produce hydroxy radicals (·OH). The electrons in the LUMO of the catalysts react with the O₂ molecules present in the solution and reduce them to ·O₂⁻ radicals. The ·O₂⁻ radicals can, in turn, react with the available water molecules to produce hydroxy radicals (·OH).^10,58,59 The produced hydroxy radicals (·OH) can then react with MB molecules and degrade them to afford degraded products along with H₂O.^59,60 An illustration of the photocatalytic degradation mechanism using NiL₁Et as a catalyst is provided in Fig. 11.

Fig. 11 Schematic of the mechanism of the photocatalytic degradation of MB using NiL₁Et as the catalyst.

Furthermore, the H₂O₂ molecules present in the reaction medium can further dissociate to produce ·OH radicals, leading to an increase in the concentration of ·OH radicals in the reaction medium.³⁷

3.14. Photocatalytic degradation pathway of methylene blue

The degradation path of methylene blue (MB) can be traced by identifying the degradation intermediates. A sample was collected during the photocatalytic degradation of MB using NiL₁Et, and its LC-MS analysis was conducted to identify the intermediates during the course of the degradation. The corresponding chromatogram and mass spectra are provided in Fig. S15 and S16, respectively. It was observed that the photocatalytic degradation of MB initially involved the demethylation of MB molecules. The demethylated MB molecules then undergo degradation, yielding 2,5-diaminobenzenesulfonic acid as an intermediate. A benzothiazole derivative is also among the degradation intermediates identified in the LC-MS study. A plausible degradation pathway of MB was thus drafted based on the chromatograms and mass spectra of the degradation intermediates of MB degradation, as shown in Fig. 12. The proposed mechanism is in agreement with reported works.^61–63

Fig. 12 Probable degradation pathway of MB.

4 Conclusion

A new Ni(II) complex, NiL₁Et, of an ONS donor ligand (L₁) was synthesized. The formation of the complex was confirmed using spectroscopic techniques and elemental analysis. The thermal and optical study of the complex was conducted by TGA and UV-DRS. The energy-optimised geometry of NiL₁Et was obtained using DFT calculations. NiL₁Et was found to have a distorted square planar geometry. The experimentally determined band gap of 2.19 eV for NiL₁Et, computed using Tauc plots, was in close agreement with the theoretically obtained value. The NiL₁Et complex was found to have a low electron–hole recombination rate, which can be predicted from its very low emission intensity. The photocatalytic activity of NiL₁Et was investigated. The H₂O₂-assisted photocatalytic degradation of methylene blue (MB) occurred at a pH of 11. The degradation efficiency of MB reached the maximum of 80.15% using NiL₁Et when 50 mL of a 20 ppm MB solution was irradiated under sunlight. It was also observed that upon irradiation with sunlight and in the presence of the photocatalysts, MB degradation was enhanced with an increase in the pH. The zeta potential analysis was carried out to study the surface potential of the catalyst at different pH levels. The catalyst was found to have a negative surface potential at all pH levels above 6.46. Upon studying the mechanism of the photocatalytic degradation of MB using NiL₁Et as the photocatalyst, it was discovered that the hydroxy radicals (·OH) generated during the degradation process were crucial in causing the photocatalytic degradation. Moreover, the mineralization of the MB dye was assessed through TOC analysis. The efficacy of NiL₁Et in the degradation of other cationic dyes, like Rhodamine B, and anionic dyes, like methyl orange, was evaluated. The recyclability and stability of the complex as a photocatalyst were evaluated until the fifth consecutive catalytic cycle. The possible leaching of Ni was screened via ICP-OES, and the amount of Ni leached was found to be 25.09 µg L⁻¹, which was well below the WHO-recommended limit for Ni in water. The degradation pathway of MB was traced through the identification of the degradation intermediates using LC-MS.

Author contributions

Nilotpal Goswami: conceptualization, data curation, formal analysis, investigation, methodology, writing – original draft, and visualization. Nabajit Barman: investigation. Pranjit Barman: validation, supervision, and writing – review and editing.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5su00958h.

Acknowledgements

The authors acknowledge CSIR NEIST-SAIF, SAIF-IIT Bombay, SAIC Tezpur University, SAIF-IIT Patna, and IIT Ropar for facilitating the use of the sophisticated instruments. The authors are also thankful to NIT Silchar for the required laboratory facilities.

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