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. 2026 Sep 30;16(46):59445–59461. doi: 10.1039/d6ra07567c

Green synthesis of transition metal-based mixed-phase materials from Convolvulus arvensis leaves: structural, cyclic voltammetry and photocatalytic studies

Ehsan Ullah Mughal a,✉, Nafeesa Naeem a, Ayza Jabeen a, Ayesha Javaid b, Muhammad Imran c,✉, Rehan Ullah d, Amina Sadiq e, Israa J Hakeem f, Hatem M Altass g, Rabab S Jassas h, Saleh A Ahmed g,✉
PMCID: PMC13625288  PMID: 42819912

Abstract

In this study, Convolvulus arvensis leaf extract was employed as a source of phytochemical constituents that facilitate metal-ion complexation and stabilization during material formation, yielding manganese oxide, Mn2O3 (1), copper hydroxide carbonate, Cu2(OH)2CO3 (2), iron oxide, Fe2O3 (3), and nickel carbonate hydroxide hydrate, Ni2CO3(OH)2·H2O (4). The synthesized materials were systematically characterized to elucidate their structural, morphological and compositional properties using advanced analytical techniques. Cyclic voltammetric (CV) studies showed enhanced charge transfer properties of the synthesized materials (1–4), with appreciable anodic and cathodic peaks in the range ∼0.22 V–∼0.30 V, ∼0.12 V–∼0.14 V, respectively. The photocatalytic performance of the synthesized materials (1–4) was evaluated for the removal of diclofenac potassium (DCF) under visible light irradiation. These materials exhibited excellent photocatalytic efficiency, i.e., 79.4% (1), 70.8% (2), 75.6% (3), and 83.2% (4) in terms of the removal of diclofenac potassium from aqueous solution. Among all, nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O (4) demonstrated the highest photocatalytic removal efficiency against DCF, which reached 90.2% under optimized conditions (irradiation time = 80 min, initial DCF concentration = 15 mg L−1, catalyst amount = 5 mg, pH = 5, and temperature = 50 °C). The sustainability of (4) was checked in up to seven consecutive photocatalytic cycles, revealing its good stability. Trapping experiments indicated the potential role of active species (OH˙ > ˙O2− > h+) during the photocatalytic process to propose a suitable reaction mechanism for DCF removal. Hence, this study highlights the potential of Convolvulus arvensis-mediated green synthesis as an eco-friendly and cost-effective route for synthesizing transition metal-based oxide/carbonate materials with promising electrochemical and photocatalytic applications.

Convolvulus arvensis leaf extract, rich in phytochemical constituents, was employed as a green complexing and stabilizing agent to facilitate the synthesis of Mn2O3 (1), Cu2(OH)2CO3 (2), Fe2O3 (3), and Ni2CO3(OH)2·H2O (4).graphic file with name d6ra07567c-ga.webp

1. Introduction

The occurrence of emerging contaminants in aquatic environments has become a growing global concern, primarily due to the continuous discharge of pharmaceutical compounds and industrial effluents. Rapid population growth, coupled with expanding healthcare access, has significantly increased the consumption of pharmaceuticals, many of which are only partially metabolized in the human body.1 As a result, unmetabolized residues are released into wastewater streams, where conventional treatment systems are often ineffective in achieving complete removal.2 Among these contaminants, non-steroidal anti-inflammatory drugs (NSAIDs), particularly diclofenac-based compounds, are frequently detected and are known for their persistence and potential ecotoxicological effects.3,4 These pollutants are typically resistant to natural degradation processes and can disrupt aquatic ecosystems by reducing light penetration and impairing photosynthetic activity.5

Conventional wastewater treatment technologies, including biological degradation and physicochemical methods, have shown limited efficiency in addressing such recalcitrant contaminants.6 Alternative approaches such as adsorption, electrochemical oxidation, and advanced oxidation processes have been explored; however, these methods often involve high operational costs, complex setups, or the formation of secondary pollutants.7,8 In recent years, photocatalysis has emerged as a promising and environmentally sustainable strategy for the degradation of persistent organic pollutants. This process relies on semiconductor materials that, upon light irradiation, generate electron–hole pairs capable of initiating redox reactions. These reactions produce highly reactive species, such as hydroxyl radicals and superoxide ions, which can effectively break down complex organic molecules into less harmful or fully mineralized products.9

Transition metal-based oxides and carbonates have attracted considerable attention as photocatalysts due to their unique optical and electronic properties.10–12 These materials have demonstrated significant potential in photocatalytic applications.13–15 However, conventional synthesis routes for these materials often involve toxic chemicals, high energy consumption, and limited control over particle morphology and surface characteristics.16–18 Additionally, particle aggregation and rapid recombination of photogenerated charge carriers can adversely affect their catalytic performance.

To overcome these limitations, green synthesis approaches utilizing plant raw extracts have gained increasing interest. Plant extracts are rich in polyphenolic compounds, including flavonoids, phenolic acids, and tannins, which can act as natural reducing, stabilizing, and capping agents during material synthesis.19,20 These compounds contain functional groups such as hydroxyl and carbonyl moieties that facilitate metal ion reduction and influence material growth, ultimately leading to improved structural and functional properties.21 Among various plant species, Convolvulus arvensis, a widely available medicinal plant, represents a promising yet underexplored candidate for material synthesis. Its phytochemical composition, particularly its flavonoid content, offers potential advantages for controlled particle formation and enhanced photocatalytic activity.

Keeping in view, the indigenous availability and richness of phytochemicals in Convolvulus arvensis, its raw extract has been employed to prepare transition metal-based photocatalysts, including manganese oxide Mn2O3 (1), copper hydroxide carbonate Cu2(OH)2CO3 (2), iron oxide Fe2O3 (3), and nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O (4). The specific objectives of the present study are (i) to evaluate the structural, morphological, and functional properties of the synthesized materials using spectroscopic and cyclic voltammetry techniques, (ii) to investigate the photocatalytic efficiency of the prepared materials for the removal of diclofenac potassium under light irradiation, (iii) to examine the effects of reaction parameters on DCF removal performance and reusability of the photocatalysts over multiple cycles.

2. Materials and methods

2.1. Materials

Manganese(ii) chloride hexahydrate (MnCl2·6H2O), copper(ii) sulfate pentahydrate (CuSO4·5H2O), iron(ii) sulfate heptahydrate (FeSO4·7H2O), nickel(ii) sulfate hexahydrate (NiSO4·6H2O), and sodium hydroxide (NaOH) obtained from Sigma-Aldrich were used during synthesis. Deionized water was employed throughout the synthesis and photocatalytic experiments.

2.2. Preparation of plant extract

Fresh leaves of Convolvulus arvensis were collected, washed thoroughly with tap water, followed by deionized water to remove dust and impurities. The cleaned leaves were shade-dried at room temperature and finely ground into powder. About 5 g of powdered leaves were transferred into a 500 mL beaker containing 100 mL of deionized water and heated at 50 °C for 10–15 minutes to obtain raw bioactive extract. The mixture was allowed to cool to room temperature and then filtered using Whatman filter paper no. 1. The clear pale-yellow extract obtained was stored at 4 °C for further use.22

2.3. Green synthesis of transition metal-based photocatalysts (1–4)

0.1 M solutions of each salt (MnCl2·6H2O, CuSO4·5H2O, FeSO4·7H2O, and NiSO4·6H2O) were separately prepared in 100 mL of distilled water. The above-prepared plant extract (20 mL) was added to each M(ii) solution (M = Mn(ii), Cu(ii), Fe(ii), Ni(ii)), and the reaction mixtures were stirred thoroughly at 90 °C, followed by the drop-wise addition of sodium hydroxide (1 M) until a pH of 9 was attained. The obtained precipitates were filtered and washed thrice with deionized water to remove impurities. Later, the precipitates were dried in an oven at 120 °C for 6 hours and finally calcined in a furnace at 400 °C for 2 hours (Fig. 1(a)). The underlying synthesis mechanism is proposed in Fig. 1(b), wherein phytochemicals present in the leaf extract of C. arvensis may interact with metal ions through their oxygen-containing functional groups, facilitating precursor complexation, nucleation, and particle growth. These constituents may also provide surface capping/stabilization, while NaOH promotes hydroxide/carbonate precipitation and subsequent calcination at 400 °C provides precursor decomposition and formation of the crystalline phases.

Fig. 1. (a) Schematic illustration for the green synthesis of transition metal-based mixed phase materials; manganese oxide Mn2O3 (1), copper hydroxide carbonate Cu2(OH)2CO3 (2), iron oxide Fe2O3 (3), nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O (4) and (b) underlying proposed mechanism for their synthesis using the raw extract of Convolvulus arvensis.

Fig. 1

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2.4. Characterization

X-ray diffraction (XRD, XRD-300 diffractometer, Unisantis, equipped with a graphite monochromator using Cu-Kα radiation, λ = 0.15406 nm, scan range = 20°–70°), X-ray photoelectron spectroscopy (XPS; JPS-9030 spectrometer, Al-Kα X-ray source, photon energy = 1486.6 eV), UV-visible spectrophotometry (Shimadzu UV-1800, Shimadzu Corporation, Japan), Fourier-transform infrared spectroscopy (FTIR; Agilent Cary 630, USA; spectral range = 500–4000 cm−1), and high-resolution scanning electron microscopy (SEM; Axia ChemiSEM, Thermo Fisher Scientific, USA) were used for the characterization of synthesized materials.

2.5. Cyclic voltammetric studies

Cyclic voltammetry was performed with a computer-controlled potentiostat/galvanostat using a three-electrode assembly. For each synthesized transition metal-based material (1–4), the working electrode was prepared by mixing 80 wt% of the respective material, 15 wt% carbon black, and 5 wt% poly(tetrafluoroethylene) (PTFE) as the binder. Tetrahydrofuran (THF) was used as the solvent to prepare a homogeneous slurry, which was uniformly coated onto a 1 × 1 cm2 nickel foam substrate. The nickel foam was weighed before and after slurry deposition to determine the active mass loading (∼2.0 mg cm−2). The coated electrode was pressed at 200 psi and dried at 70 °C before electrochemical testing. Electrochemical measurements were performed using the prepared electrode as the working electrode, Ag/AgCl as the reference electrode, and a platinum wire as the counter electrode. Cyclic voltammograms were recorded over a potential window of 0.0–0.40 V (vs. Ag/AgCl) at a scan rate of 50 mV s−1.

2.6. Photocatalytic removal of DCF

The synthesized transition metal-based photocatalysts (1–4) were investigated for their photocatalytic removal efficiency against diclofenac potassium. Briefly, the aqueous solution of diclofenac potassium (10 mg L−1, 50 mL) was treated with 10 mg of each synthesized material employed as a photocatalyst. The mixture was stirred in the dark for an initial 40 min to ensure proper equilibrium between the catalyst surface and DCF molecules before being irradiated with light. Afterwards, the suspension was continuously exposed to a 250 W tungsten–halogen lamp equipped with a UV cut-off filter (λ > 420 nm) mounted 10 cm away from the drug solution. The aliquots (3 mL) were withdrawn at regular time intervals of 20 min. The suspended particles of the photocatalyst were separated via centrifugation at 2500 rpm for 15–20 min. The absorbance of supernatants was measured at λmax = 276 nm, followed by the calculation of photocatalytic removal efficiency (%). For photocatalytic kinetic analysis, the decrease in DCF concentration was assumed to follow pseudo-first-order kinetics within the investigated concentration range, and the apparent rate constant was obtained from the linear relationship between ln(Co/Ct) and irradiation time (eqn (1)).23

2.6. 1

where k is the rate constant, Co is the initial DCF concentration, and Ct is the DCF concentration at a given time interval (t). Photocatalytic conditions were optimized for the most efficient photocatalyst (4) by varying key factors such as catalyst amount (5–25 mg), DCF concentration (5–30 mg L−1), temperature (25–75 °C), and pH (3–11) while keeping other parameters constant. The experimental conditions were selected based on preliminary investigations, and the optimized conditions were assumed to provide a sufficiently dispersed catalyst suspension and effective interaction among the photocatalyst, DCF molecules, and incident light. The point of zero charge (pHpzc) of the photocatalyst (4) was investigated using the pH drift method. Briefly, the initial pH (pHi) of NaCl (0.01 M) was varied between 2 and 12 by the addition of HCl and NaOH, 0.1 M each. Subsequently, 50 mg of the catalyst was added to each solution, and the obtained suspensions were shaken for 24 h. After equilibration, the suspensions were allowed to settle, and the final pH (pHf) of each solution was measured. The difference between pHf and pHi provided change in pH (ΔpH) and was plotted against the pHi. The intersection of the curve with the x-axis provided the pHpzc of the catalyst at which the catalyst surface is assumed to have zero charge. The reusability of the optimal photocatalyst (4) was assessed by recovering it from the DCF solution after a catalytic run, washing it with distilled water, followed by drying it at 60 °C, and subsequently reusing it in another photocatalytic cycle. The active species responsible for DCF removal were investigated using scavenging experiments. Isopropyl alcohol (IPA), triethanolamine (TEOA), and benzoquinone (BQ) were employed as scavengers for hydroxyl radicals, photogenerated holes, and superoxide radicals, respectively. Each scavenger was introduced separately in individual experiments at a concentration of 2 mM to 50 mL of DCF solution containing the photocatalyst (4). These photocatalytic runs were subsequently performed under identical optimized conditions. The changes in removal efficiency observed in the presence of each scavenger were used to evaluate the role of the corresponding reactive species in the photocatalytic removal process of DCF.

3. Results and discussion

3.1. XRD

The crystalline structure of the synthesized materials was analyzed by XRD patterns obtained in the range of 20°–70°. In Fig. 2(a) the appearance of prominent high-intensity peaks at 2θ = 33.5°, 45.1°, and 65.6° corresponds to the monoclinic phase of manganese oxide according to the JCPDS Card No. 00-002-0909. Besides, other less intense peaks exhibited at 2θ = 26.2° [JCPDS: 00-041-1379], 30.7° [JCPDS 00-018-0801], 36.8° [JCPDS 00-018-0802], and 51.1° [JCPDS 00-018-0801] matched well with respective JCPDS cards indicating monoclinic and hexagonal crystal structures. Overall, a multiphase polycrystalline manganese oxide photocatalyst was synthesized, which could provide enhanced activity by hampering the electron–hole recombination, as reported by Ferreira et al.24 The XRD pattern of 2 is shown in Fig. 2(b), where prominent diffraction peaks appeared at 30.6° [JCPDS: 00-033-0480], 33.7° [JCPDS: 96-900-8311], 36.7° [JCPDS: 00-033-0480], 41.5°, 44.9°, and 51.1° [JCPDS: 01-072-0075]. These diffraction peaks indicate the presence of mixed phases (cubic, monoclinic, and tetragonal) instead of a single copper oxide phase as expected. This is consistent with the findings of Lupan et al., who similarly reported the coexistence of mixed copper oxide phases in their synthesized heterostructures.25 According to the XRD pattern of 3 (Fig. 2(c)), the reflection peaks at 26.6°, 29.3°, 31.9°, 34.6°, 39.8°, 41.5°, 44.5°, 46.1°, 58.6°, 61.8°, 63.5°, and 66.4° can be assigned to the characteristic reflections of iron oxide [JCPDS No. 00-016-0653] with a monoclinic crystal system. A few peaks that appeared at 24.6°, 33.2°, 49.7°, 54.1°, and 57.3° are in agreement with rhombohedral iron oxide (JCPDS Card No. 00-024-0072).26 This pattern reveals that the synthesized material (3) contains a combination of monoclinic (major phase) and rhombohedral (minor phase) iron oxide. In the case of 4, the major peaks at 27.4°, 29.3°, 33.4°, 36.1°, 37.8°, 41.4°, 49.2°, 57.0°, and 62.3° are attributed to nickel carbonate hydroxide hydrate [JCPDS Card No. 00-029-0868].27 Two minor peaks appeared at 42.6° and 52.8° and were indexed to JCPDS Card No. 00-035-0501. Furthermore, a prominent peak in Fig. 2(d) observed at 22.2° may be attributed to carbonaceous residues from biomass used in the green synthetic process, as commonly reported in bio-mediated material synthesis.28Table 1 further elaborates on the chemical formula, crystal structure, and standard card number for each synthesized material. The crystallite sizes of all the synthesized materials were estimated using the classical Debye–Scherrer's formula (eqn (2)).

D = kλ/β cos θ 2

where, D is the average crystallite size, k denotes the Scherrer constant equal to 0.94, λ is the wavelength of the X-ray source (1.5405 Å), β is the full width at half maximum (FWHM) of the diffraction peak, and θ is the Bragg diffraction angle. The FWHM values of all prominent diffraction peaks were measured using OriginPro 9 software, and their average was used to estimate the average crystallite size of each material using the Debye–Scherrer equation. Furthermore, no correction for instrumental broadening was applied; therefore, the reported crystallite sizes should be regarded as approximate values. The estimated average crystallite sizes for synthesized materials were 225 nm, 355 nm, 146 nm, and 160 nm for 1, 2, 3, and 4, respectively. Besides crystallite size, the dislocation density (δ) was obtained using eqn (3).

δ = 1/D2 3

where δ represents dislocation density, and D is the average crystallite size. Table 2 presents the dislocation densities of synthesized materials, with the highest dislocation density observed for 3, revealing a great number of distortions and crystal defects as compared to the other materials.

Fig. 2. XRD patterns of (a) manganese oxide Mn2O3 (1), (b) copper hydroxide carbonate Cu2(OH)2CO3 (2), (c) iron oxide Fe2O3 (3), and (d) nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O (4) obtained by using the raw extract of Convolvulus arvensis (# represents the presence of carbonaceous residues from biomass in 2).

Fig. 2

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Table 1. Chemical composition and phase designation of synthesized materials (1–4).

Material Major phase Minor phase
Chemical formula JCPDS card no. Crystal structure Chemical formula JCPDS card no. Crystal structure
1 Manganese oxide Mn2O3 00-002-0909 Monoclinic Manganese oxide hydroxide (HMnO2) 00-041-1379 Monoclinic
Manganese oxide (Mn5O8) 00-018-0801 Monoclinic
Manganese oxide (MnO2) 00-018-0802 Hexagonal
2 Copper hydroxide carbonate Cu2(OH)2CO3 01-072-0075 Monoclinic Copper oxide (Cu4O3) 00-033-0480 Tetragonal
Rouaite (Cu4N2O12H6) 96-900-8311 Monoclinic
3 Iron oxide Fe2O3 00-016-0653 Monoclinic Iron oxide Fe2O3 00-024-0072 Rhombohedral
4 Nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O 00-029-0868 Orthorhombic Nickel carbonate hydroxide Ni2(CO3)(OH2) 00-35-0501 Monoclinic
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Table 2. Structural parameters of synthesized materials (1–4) obtained from XRD.

Material Crystallite size (D) (nm) Dislocation density (δ) (lines per m2)
1 225 1.97 × 1013
2 355 7.93 × 1012
3 146 4.69 × 1013
4 160 3.90 × 1013
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3.2. XPS analysis

The elemental composition of synthesized materials was determined using XPS, as described in Fig. 3 (survey spectrum). The survey spectrum noticeably shows the presence of Mn 2p, Cu 2p, Fe 2p, Ni 2p, and their corresponding C 1s and O 1s states. In addition, these peaks are consistent with the corresponding XRD results, demonstrating that all constituent elements were successfully incorporated into the synthesized catalyst.

Fig. 3. The XPS survey spectra of manganese oxide Mn2O3 (1), copper hydroxide carbonate Cu2(OH)2CO3 (2), iron oxide Fe2O3 (3), and nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O (4) obtained by using the raw extract of Convolvulus arvensis.

Fig. 3

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The C 1s signals detected in the XPS spectra of Mn2O3, Cu2(OH)2CO3, Fe2O3, and Ni2CO3(OH)2·H2O can be ascribed to carbonaceous compounds on the surface derived from the Convolvulus arvensis leaf extract utilized in the green synthesis, along with a potential influence from incidental carbon introduced during sample manipulation and atmospheric exposure. The leaf extract's phytochemicals, including polyphenols, flavonoids, carbohydrates, proteins, and various oxygenated organic compounds, can interact with metal ions via functional groups such as –OH, C Created by potrace 1.16, written by Peter Selinger 2001-2019 O, and –COOH, promoting particle synthesis and surface stabilization. During drying and heating, residual phytochemicals may partially degrade and carbonize, forming carbonaceous compounds on the oxide surfaces. Thus, the C 1s components linked to C–C/C–H, C–O, and C Created by potrace 1.16, written by Peter Selinger 2001-2019 O bonds may be attributed to these remaining plant-derived organic entities, validating the surface interaction of phytochemical elements with the produced metal oxides.

The high-resolution Mn 2p spectrum, shown in Fig. 4(a), exhibits two distinct peaks at 641.55 eV and 653.98 eV, corresponding to Mn 2p3/2 and Mn 2p1/2 spin–orbit states, respectively. The production of manganese oxide is indicated by these binding energies, which are typical of Mn3+ and Mn2+ at 641.57 eV and 642.17 eV, respectively, attributed to the major phase Mn2O3 and the minor phase MnO2.29,30 In addition, the spin-split Mn 2p3/2 and Mn 2p1/2 state energies having a difference of 12.43 eV confirm the formation of the major phase in manganese oxide.29 Furthermore, the C 1s spectrum in Fig. 4(b) displays a strong signal at 284.52 eV, indicating sp2-hybridized carbon.31 Likewise, the high resolution of the O 1s peak (Fig. 4(c)) at 530.74 eV is due to lattice oxygen bonded with C Created by potrace 1.16, written by Peter Selinger 2001-2019 O bonds and –OH moiety or carbonyl group at 532.25 eV.32,33 The oxygen component is ascribed to oxygen species in low-coordination, defect-rich environments, which are generally found in crystalline materials with smaller particle sizes.34,35 In the meantime, the large surface area of the manganese oxide is compatible with the oxygen contribution being attributed to physically or chemically adsorbed water molecules on or near the surface.36,37 The XPS spectrum of 2 shows distinctive binding energy peaks corresponding to various Cu oxidation states, as seen in Fig. 4(d). The Cu 2p spectrum reveals the presence of Cu(ii), mainly in the form of the major phase for Cu2(OH)2CO3, with peaks of binding energies for Cu 2p3/2 at 934.82 eV and 954.61 eV for Cu 2p1/2, along with a satellite peak at 943.61 eV, which are consistent with recently reported results.38 In addition, the minor tetragonal phase Cu4O3 is also responsible for a peak at ∼934.59 eV.39 Similarly, the spectrum indicated in Fig. 4(e) belongs to the C 1s at the binding energies of 284.77 eV and 286.38 eV, confirming the existence of C–C/C Created by potrace 1.16, written by Peter Selinger 2001-2019 C and C Created by potrace 1.16, written by Peter Selinger 2001-2019 O species in the copper oxide mixed phases.40 Moreover, the peaks in Fig. 4(f) at the binding energies of 532.42 eV correspond to C Created by potrace 1.16, written by Peter Selinger 2001-2019 O, and the binding energy at 533.20 eV corresponds to the hydroxyl groups (–OH) of water retained molecules, respectively.41–43 Thus, these XPS spectra reveal that the copper oxide material is mainly composed of Cu2(OH)2CO3 and a minor tetragonal phase, Cu4O3, as an additional phase.44

Fig. 4. The XPS analysis of (a–c) manganese oxide Mn2O3 (1), (d–f) copper hydroxide carbonate Cu2(OH)2CO3 (2), (g–i) iron oxide Fe2O3 (3), and (j–l) nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O (4) obtained by using the raw extract of Convolvulus arvensis.

Fig. 4

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The high-resolution spectrum of Fe 2p displays spin–orbit characteristic peaks, as shown in Fig. 4(g). Additionally, the binding energies detected at 712.49 eV and 728.60 eV are attributed to Fe 2p3/2 and Fe 2p½, respectively.45,46 In addition, the shifting of oxidation states, Fe2+ for the monoclinic phase at the binding energy of 712.49 eV and Fe3+ for the rhombohedral state at 714.95 eV, confirms the Fe2O3 state as ascribed in the literature.47 These surface-related states of Fe2O3 are responsible for the characteristic peak at ∼712 eV.47,48 Likewise, the spectrum features of Fe2O3 indicate the oxidation states of Fe2+ and Fe3+.49 The peak ascribed to ∼284 eV (Fig. 4(h)) belongs to the C 1s, where the C–C/C Created by potrace 1.16, written by Peter Selinger 2001-2019 C and C Created by potrace 1.16, written by Peter Selinger 2001-2019 O bonds at the binding energies of 284.92 eV and 286.68 eV are referred to as carbon and oxygen bonding at the surface of the molecules.50 Similarly, the O 1s spectrum in Fig. 4(i) is asymmetric, indicating the presence of oxygen species at the binding energies of (C Created by potrace 1.16, written by Peter Selinger 2001-2019 O, 531.94 eV) and 532.95 eV of the –OH moiety from water molecules. Subsequently, the major peak at ∼532 eV confirms the interaction of Fe with lattice oxygen in Fe2O3 material.51

The surface composition of 4 is shown in Fig. 4(j). The two distinctive spin–orbit components of Ni 2p3/2 and Ni 2p½ at the binding energies of 854.61 eV and 873.61 eV, along with a satellite peak at the binding energy of 859.63 eV, were identified from the deconvolution of the XPS spectrum.52 The Ni 2p3/2 peak in the binding energy range of 852.7 to 855.9 eV is assigned to Ni2+ species, indicating the existence of defects or oxygen vacancies in the lattice.53 Likewise, the prominent peaks at the binding energies of 284.82 eV and 286.14 eV are attributed to carbon-based residues interacting with oxygen, having bonds C–C/C Created by potrace 1.16, written by Peter Selinger 2001-2019 O and C Created by potrace 1.16, written by Peter Selinger 2001-2019 O, which confirms the organic species from the biosynthesis route, as shown in Fig. 4(k). The asymmetric profile of the high-resolution O 1s spectrum is deconvoluted as shown in Fig. 4(l), where the Ni2+ species connected to the (CO32−) and –OH moiety in nickel carbonate hydroxide are responsible for the binding energy peaks, which are situated at 531.90 eV and 533.10 eV.52 Similarly, the Ni–O–H bond and the –OH moiety at the surface are responsible for the higher binding energy around ∼532 eV.54

3.3. UV-Vis spectroscopy

The UV-Vis absorption spectra of synthesized materials (1–4) were recorded in the wavelength range of 200–800 nm (Fig. 5). All materials exhibit high initial absorbance at the beginning of the spectra, followed by a sharp decrease in absorbance with increasing wavelength. No distinct absorption peak is observed below 300 nm because the spectra start with high absorbance and continuously decrease. Among all materials, the observed absorbance pattern is as follows. Material 4 shows the highest absorbance throughout the entire wavelength range, whereas 2 exhibits the lowest absorbance. Materials 1 and 3 demonstrate intermediate absorbance values. The maximum absorbance observed for 4 illustrates its strongest light-harvesting capability, while 2 shows the comparatively weaker optical absorption. Due to the relatively superior absorbance of 4, it is expected to exhibit the highest photocatalytic performance. This observation is consistent with the previously reported nickel carbonate hydroxide, where appreciable UV-visible absorption and an optical band gap of approximately 2.4 eV were associated with its photocatalytic response under visible-light irradiation.52

Fig. 5. UV-Vis spectra of manganese oxide Mn2O3 (1), copper hydroxide carbonate Cu2(OH)2CO3 (2), iron oxide Fe2O3 (3), and nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O (4) obtained using the raw extract of Convolvulus arvensis.

Fig. 5

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3.4. FTIR

The FTIR spectra of the synthesized materials (1–4) were recorded in the range of 500–4000 cm−1 as shown in Fig. 6. A distinct absorption band in the 500–600 cm−1 region is observed for Mn2O3 and Cu2(OH)2CO3, which is attributed to the characteristic metal–oxygen (M–O) stretching vibrations consistent with previous reports.55,56 In contrast, well-resolved absorption bands in the fingerprint region cannot be clearly observed for 3 and 4 under the present instrumental conditions. This may be due to the limited spectral range, low band intensity, or overlap with the instrument cut-off region. Therefore, the formation of these materials is primarily confirmed by the complementary XRD and XPS analyses rather than FTIR alone. The absorption band observed in the 1050–1150 cm−1 region is assigned to C–O stretching vibrations of residual surface-bound organic species.57 Bands between 1400 and 1650 cm−1 are attributed to surface carbonate species and/or H–O–H bending vibrations of adsorbed water molecules.58,59 The weak band near 2350 cm−1, particularly noticeable for 4, is associated with adsorbed atmospheric CO2,60 while the broad band in the 3200–3500 cm−1 region corresponds to O–H stretching vibrations of surface hydroxyl groups and physically adsorbed water,61 which are commonly observed in metal-based materials. The comparatively noticeable shift and broadening of this band in 3 may reflect changes in the surface hydroxyl environment and hydrogen-bonding interactions.

Fig. 6. FTIR spectra of manganese oxide Mn2O3 (1), copper hydroxide carbonate Cu2(OH)2CO3 (2), iron oxide Fe2O3 (3), and nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O (4) obtained using the raw extract of Convolvulus arvensis.

Fig. 6

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3.5. Scanning electron microscopy (SEM)

The morphology of green-synthesized metal-based materials (1–4) was studied using SEM images, as described in Fig. 7. The SEM images of 1 are presented at different magnifications (50, 10, and 5 µm) in Fig. 7(a–c). These showed a highly porous and rough surface with a large void in the center (Fig. 7(a)) and irregular plate-like and flaky particles producing lamellar morphology (Fig. 7(b and c)) contains fine and compact granular particles, indicating homogeneous morphology. The SEM image of 2 exhibits large, irregular crystalline chunks with sharp edges, indicating good crystallinity (Fig. 7(d)). According to Fig. 7(e), an irregular and rough surface has been observed for 3, with some agglomeration of particles. The SEM image of 4 is depicted in Fig. 7(f) and shows several small and loosely agglomerated particles. Overall, the SEM images of 1–4 demonstrate a porous to compact crystalline and agglomerated morphology.

Fig. 7. SEM micrographs of (a–c) manganese oxide Mn2O3 (1), (d) copper hydroxide carbonate Cu2(OH)2CO3 (2), (e) iron oxide Fe2O3 (3), and (f) nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O (4) obtained by using the raw extract of Convolvulus arvensis.

Fig. 7

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3.6. Cyclic voltammetric studies

The electrochemical behavior of the synthesized materials (1–4) was evaluated using cyclic voltammetry over a potential range of 0.0–0.40 V (Fig. 8). Based on the shape and current response, these curves can be interpreted in terms of charge transfer kinetics and their likely correlation with photocatalytic performance. The observed anodic peaks were in the range of 0.22 V to 0.30 V. In contrast, cathodic peaks appeared in the range of 0.12 V to 0.14 V. This indicates that the peak separation is moderate and may facilitate the electron transfer kinetics. Among all synthesized materials, 4 demonstrates the largest current response over the entire potential range. It has been further observed that the current increases more sharply at higher potential (∼0.32 V), thereby producing the largest anodic current. It also encloses the largest CV area among all materials; this indicates that material 4 is best for the highest electrochemical activity and fastest charge transport. This fastest charge transport makes it an efficient photocatalyst among all synthesized materials, as evidenced by the complementary photocatalytic studies given in the section below (Section 3.7). This is also in agreement with well-established literature that materials having high charge transfer characteristics usually exhibit high photocatalytic activity due to less chance for recombination of photogenerated electrons and holes before participating in the surface redox reactions.62

Fig. 8. Cyclic voltammograms of manganese oxide Mn2O3 (1), copper hydroxide carbonate Cu2(OH)2CO3 (2), iron oxide Fe2O3 (3), and nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O (4) obtained by using the raw extract of Convolvulus arvensis.

Fig. 8

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3.7. Photocatalytic removal of DCF

The catalytic behavior of synthesized materials was investigated against diclofenac potassium as shown in Fig. 9. It was noticed that there was no removal of DCF in the photolysis process (only 0.25%) and similarly, catalyst performance was inefficient without light (dark). These control experiments highlighted that catalytic activity is obligatory to the mutual role of light and catalyst. It was also assumed that changes in DCF concentration during irradiation primarily resulted from photocatalytic degradation rather than adsorption or other non-photocatalytic processes, as accounted for by the appropriate control experiments. Therefore, when the reaction mixture containing the catalyst was exposed to light, a remarkable increase in removal efficiency was observed for all catalysts. It was also noticed that photocatalytic activity abruptly increased at the initial stages of the irradiation time, likely because a greater number of active sites were available for the photocatalytic removal of DCF. As the reaction proceeds, saturation of active sites leads to a slow increase in drug removal, and finally equilibrium is reached, where the rate is nearly constant. The removal efficiencies of 1, 2, 3, and 4 were found to be 79.4%, 70.8%, 75.6%, and 83.2%, respectively. Among these transition metal-based materials, 4 exhibited profound activity, which can be correlated to the following: (a) improved charge transfer kinetics as evidenced by CV and in agreement with the related literature,62 (b) high absorbance trend indicating better light harvesting capability as evidenced by UV-Vis studies.

Fig. 9. (a–c) Photocatalytic removal of DCF using manganese oxide Mn2O3 (1), copper hydroxide carbonate Cu2(OH)2CO3 (2), iron oxide Fe2O3 (3), and nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O (4) obtained by employing the raw extract of Convolvulus arvensis.

Fig. 9

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The relative removal efficiencies were also studied in terms of reaction kinetics (Fig. 10). For photocatalytic kinetic analysis, the decrease in DCF concentration was assumed to follow pseudo-first-order kinetics within the investigated concentration range, and the apparent rate constant was obtained from the linear relationship between ln(Co/Ct) and irradiation time. The photocatalytic reaction followed a pseudo-first-order kinetic model, and the obtained rate constant values agreed with the calculated removal efficiencies trend. The observed rate constants were 0.0187 min−1, 0.0143 min−1, 0.0167 min−1, and 0.0208 min−1 for 1, 2, 3, and 4, respectively. The highest rate-constant observed for 4 (0.0208 min−1) revealed its excellent performance towards DCF removal.

Fig. 10. (a and b) Linear fit of the pseudo-first-order kinetic model and corresponding rate constants of the green synthesized manganese oxide Mn2O3 (1), copper hydroxide carbonate Cu2(OH)2CO3 (2), iron oxide Fe2O3 (3), and nickel carbonate hydroxide hydrate Ni2CO3(OH)2·H2O (4) obtained by using the raw extract of Convolvulus arvensis for the photocatalytic removal of DCF.

Fig. 10

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3.7.1. Optimization of experimental parameters

Various experimental parameters influencing the photocatalytic reaction were optimized using the most efficient photocatalyst, i.e., nickel carbonate hydroxide hydrate (4). Each parameter varied in a certain range, keeping other parameters constant.

3.7.1.1. Effect of catalyst amount

The quantity of catalysts is known to influence the photocatalytic degradation process, as it affects the preliminary adsorption of pollutants onto the catalyst surface and the penetration of light into the reaction medium. It also minimizes the use of excess amounts of the catalyst, contributing to budget friendliness. Fig. 11 shows a removal efficiency of 4 for the removal of DCF employing its different dosages in each photocatalytic experiment. The catalyst amount varied in the range of 5 mg to 25 mg. All of the reactions were carried out at neutral pH, with a constant DCF concentration of 10 mg L−1. When no catalyst was used, there was a negligible removal of DCF. However, the addition of only 5 mg of the catalyst dramatically increased the removal efficiency to 83.2%. Whereas further increase in catalyst amount reduced the removal efficiency, reaching 64.6% at 25 mg of the catalyst. Actually, an excessive amount of catalyst may cause turbidity of the drug solution, which can impede suitable light penetration.63 Moreover, high catalyst loading may also promote particle agglomeration, reducing the effective surface area and availability of active sites for catalytic action.64 Based on the photocatalytic removal efficiency relative to the catalyst amount, 5 mg was chosen as the optimal photocatalyst amount for further experiments.

Fig. 11. (a–c) DCF removal under variable catalyst amount (5–25 mg) using 4. Reaction conditions: [DCF] = 10 mg L−1, pH = 7, temperature = 25 °C.

Fig. 11

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3.7.1.2. Effect of DCF concentration

Suitable pollutant concentration is another crucial parameter to achieve maximum removal efficiency of a catalyst. The optimal drug concentration for the efficient catalytic activity of 4 was explored by varying the initial concentration of DCF in the range of 5 mg L−1 to 30 mg L−1. In each experiment, 5 mg of the catalyst was used, and pH was maintained at 7. According to Fig. 12, it can be noticed that removal efficiency (86.6%) is maximum at 15 mg L−1 of DCF. The probability of interactions between DCF molecules and the catalyst surface is quite greater at lower DCF concentrations due to the availability of sufficient catalytic sites. These catalytic sites facilitate the generation of reactive species necessary for the sustainable catalytic process. This trend is consistent with the findings of Zafar et al., who reported a similar influence of the initial concentration of the pollutant on the photocatalytic reaction.65 However, a further increase in DCF concentration caused a decline in photocatalytic removal efficiency, with values decreasing to 74.1%, 68.4%, and 64.6% at 20 mg L−1, 25 mg L−1, and 30 mg L−1 of DCF, respectively. This drop is attributed to the excessive concentration of DCF molecules relative to the fixed concentration of catalyst molecules, which caused saturation of active sites on the catalyst surface. This condition cannot support appropriate photocatalytic action owing to the reduced pollutant-catalyst adsorption phenomenon and also due to the hindrance in photoabsorption by the catalyst surface.66 Consequently, the probability of reactive species generation is suppressed, ultimately decreasing efficiency.

Fig. 12. (a–c) DCF removal at variable drug concentrations (5–30 mg L−1) employing 4 as photocatalyst. Reaction conditions: catalyst amount = 5 mg, pH = 7, temperature = 25 °C.

Fig. 12

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3.7.1.3. Effect of temperature

The effect of reaction temperature on the photocatalytic removal of DCF was studied at three different temperatures, i.e., 25 °C, 50 °C, and 75 °C. According to Fig. 13, we observed that removal efficiency was higher at 50 °C as compared to 25 °C. It has been reported that an increase in temperature favors the fast movement of drug molecules; in this way, the probability of their penetration into the pores of the catalyst also increases.67 When the reaction temperature reached 75 °C, an opposite trend was observed; here, removal efficiency sharply decreased instead of rising. This could be supported by the findings of Waseem et al. who reported an increased recombination of electron–hole pairs at high reaction temperature due to an increased haphazardness.68

Fig. 13. (a–c) DCF removal at variable temperature (25–75 °C) employing 4 as a photocatalyst. Reaction conditions: [DCF] = 15 mg L−1, catalyst amount = 5 mg, pH = 7.

Fig. 13

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3.7.1.4. Effect of pH

The real wastewater exhibits variable pH due to the presence of a diverse variety of ions originating from different sources. This makes it crucial to study the behavior of the catalyst under different pH conditions. A pH range of 3 to 11 was investigated to study its influence on the removal efficiency (Fig. 14). At neutral pH, photocatalytic activity was 86.6%. Upon acidifying the solution by the addition of HCl, the catalytic activity increased to 90.2% at pH = 5. However, further decrease in pH (pH = 3) caused a sharp decline in catalytic efficiency, reaching 78.3%. To understand this trend of catalytic activity, the point of zero charge (pHpzc) of the catalyst (4) was determined using the pH drift method, and it was found to be 6.25. It is known that the catalyst surface is neutral when pH = pHpzc. Below this value, the catalyst's surface is positively charged, while at pH values above the pHpzc, the surface becomes negatively charged. DCF, the target pollutant, predominantly exists in protonated form and is electrically neutral at pH levels below its pKa value (4.2).69 Therefore, at pH = 3, there is no specific interaction between the positively charged catalyst and uncharged DCF interfaces. However, when pH approaches 5, the catalyst surface remains positively charged, but DCF becomes anionic. This results in electrostatic attraction between the two interfaces, enhancing pollutant adsorption at the catalyst surface and consequently improving catalytic activity. In basic conditions, where NaOH was added to raise the pH, the photocatalytic efficiency began to decline, dropping to 72.8% at pH = 9 and further to 70.5% at pH = 11. In this case, electrostatic repulsion dominates due to the negative charge on both the catalyst surface and the anionic form of DCF, reducing adsorption and catalytic efficiency. Our findings are well supported by the literature, where mild acidic conditions have been reported optimal for DCF removal. For instance, Bateni et al. reported maximum degradation of DCF at pH = 5 using a pectin–graphene oxide–magnesium ferrite–zinc oxide nanocomposite.70 Similarly, another study on the removal of diclofenac sodium reported an optimal pH of 4.4 for its efficient removal using a Z-scheme photocatalyst.71

Fig. 14. (a–c) DCF removal at variable pH (3–11) employing 4 as a photocatalyst, and (d) pHpzc of the photocatalyst (4). Reaction conditions: [DCF] = 15 mg L−1, catalyst amount = 5 mg, temperature = 50 °C.

Fig. 14

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3.7.2. Recyclability

The recyclability of the catalyst helps to determine its stability and hence commercial viability. Furthermore, estimation of catalyst stability is also important to ensure that the catalyst does not leach metal ions into the aqueous body, which could cause secondary pollution. Therefore, cyclic experiments were conducted for the best photocatalyst (4), up to seven consecutive cycles under optimal photocatalytic conditions (photocatalyst amount = 5 mg, DCF concentration = 15 mg L−1, pH = 5, temperature = 50 °C) (Fig. 15). The photocatalytic extents of 90.2%, 89.4%, 88.1%, 87.5%, 86.9%, 86.0%, and 84.8% were measured for the first to the seventh cycle, respectively. The photocatalyst retained ∼94% of its initial activity, indicating reasonably high stability under the applied operating conditions and demonstrating its potential for repeated use in successive photocatalytic cycles.

Fig. 15. (a and b) DCF removal after light irradiation for seven consecutive cycles using 4 as the photocatalyst under optimized reaction conditions.

Fig. 15

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3.7.3. Role of trapping agents and photocatalytic mechanism

The production of different reactive species and their respective contribution to the photocatalytic process was investigated by scavenging experiments. The addition of different trapping agents influences the overall removal efficiency because these agents could capture relevant active radicals. It is evident from Fig. 16(a and b) that the addition of isopropyl alcohol (IPA) caused maximum inhibition of catalytic activity, indicating the prime contribution of hydroxyl radicals in the DCF removal process. The addition of benzoquinone (BQ) also showed a relatively similar inhibitory effect, revealing a significant contribution of superoxide anions as well during the catalytic action. In the case of triethanolamine (TEOA), added to capture holes showed a slight inhibitory effect. This could be attributed to the consumption of holes during the redox reaction; however, the observed inhibitory effect confirmed its production. In this perspective, the possible DCF removal mechanism (Fig. 16(c)) can be proposed involving a typical charge transfer process when the catalyst is subjected to light irradiation (Step 1). The light energy causes the transfer of electrons from the valence band of the catalyst to the conduction band (CB), this, in turn, produces photoinduced holes in the valence band (VB) of the catalyst (Step 2). The photoelectrons present in the CB are captured by surrounding oxygen and converted to superoxide radicals by the reduction of these oxygen molecules. The photoinduced holes, on the other hand, participate in the oxidation process to make up the electron deficiency and, in this way, convert water molecules to hydroxyl radicals (Step 3).72 The equations shown in Fig. 16(c) represent the consequent photocatalytic reactions involved in the DCF removal employing 4 as a photocatalyst under visible light illumination (Step 4). All these active radicals play their role together to break down drug molecules into simpler ones (Step 5).

Fig. 16. (a and b) DCF removal using different scavengers under optimized conditions employing 4 as photocatalyst and (c) proposed reaction mechanism for drug removal.

Fig. 16

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3.7.4. Comparison with reported literature

A comprehensive literature survey demonstrates extensive interest in the scientific community for the removal of DCF from aqueous bodies. For example, a recent study showed 100% degradation of 20 mg per L DCF within 180 min using a carbon-nanotube/Ni@TiO2:W photocatalyst.73 Despite these advances, the development of cost-effective, environmentally friendly, and highly efficient photocatalysts remains an active area of research. The photocatalytic performance of the plant's raw extract-mediated Ni2CO3(OH)2·H2O (4) synthesized in the present study is compared with previously reported Ni-based photocatalysts for DCF removal (Table 3). The comparison reveals that the synthesized 4 exhibited enhanced DCF removal efficiency within a shorter irradiation period, demonstrating its high photocatalytic activity. Moreover, the present synthesis strategy eliminates the use of hazardous chemicals, providing an environmentally benign approach for the fabrication of a photocatalyst. Nickel-based photocatalysts prepared through conventional chemical routes have been previously investigated for DCF removal. For example, ultrasonically synthesized NiCo2O4/NiS/rGO has shown considerably lower DCF degradation of 65.98%.74 In comparison, the present study achieved 90.2% DCF removal within 80 min using a predominantly nickel carbonate hydroxide-based mixed-phase material synthesized via Convolvulus arvensis leaf extract. Notably, nickel carbonate hydroxide has recently been demonstrated as a photocatalytically active material.52 Furthermore, leaf extract of C. arvensis has previously been employed to produce catalytically active Ag nanoparticles capable of degrading organic dyes, with phenolic constituents implicated in nanoparticle formation and stabilization.75 However, its application in the plant-mediated synthesis of transition metal-based photocatalysts for pharmaceutical contaminants such as DCF remains scarcely explored, highlighting the significance of the present study.

Table 3. Comparative removal efficiencies of various nickel-based materials for the removal of DCF.
Catalyst Synthesis method Catalyst amount Drug conc. Light source Time Degradation Rate constant Reference
NiFe2O4 Green sonochemical synthesis using rosemary leaf extract 0.1 g L−1 25 mg L−1 UV-A 24 W 140 min 95% 0.038 min−1 76
TiO2-chitosan-bocalanine-Ni hybrid material Hydrothermal 50 ppm 25 ppm UV 254 nm 365 nm 250 min 94.3% N/A 77
NiFe-PANI composites Electrodeposition 0.25 cm2 Nanocomposite electrode 7 mg L−1 Philips 100 W incandescent lamp 60 min 97.89% N/A 78
NixZn1−xFe2O4 (x = 0.3) Co-precipitation 50 mg/100 mL 50 mg L−1 200 W xenon lamp 50 min ∼99% 0.1657 min−1 62
Ni2CO3(OH)2·H2O (4) Green cum co-precipitation 5 mg 15 mg L−1, 50 mL 250 W tungsten–halogen lamp 80 min 90.2% 0.0208 min−1 This work
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4. Conclusions

This study demonstrates a sustainable approach for the green synthesis of transition metal-based materials using raw extract derived from Convolvulus arvensis leaves. The plant-mediated route successfully synthesized materials with distinct morphological and functional characteristics. CV analysis revealed improved electron transfer kinetics for green-synthesized metal-based materials. Overall, these materials exhibited appreciable to good efficiency (70.8–83.2%) as a photocatalyst for the removal of DCF. Furthermore, upon optimization, 4 achieved excellent photocatalytic efficiency, i.e., 90.2%. Based on these findings, it can be concluded that the said green-synthesized materials can safely be employed/tuned for the removal of pharmaceutical contaminants from domestic/industrial wastewater. Future studies should systematically evaluate the performance of these photocatalysts in industrial-scale reactors using real wastewater effluents to establish their techno-economic feasibility, energy consumption, and environmental feasibility for practical wastewater treatment.

Author contributions

Ehsan Ullah Mughal: conceptualization, supervision, validation, investigation, resources, data curation, writing – original draft, writing – review & editing, visualization, project administration, funding acquisition. Nafeesa Naeem: data collection, visualization, validation, formal analysis, investigation, writing – original draft, writing – review & editing. Ayza Jabeen: experimental work performance and CV analysis. Ayesha Javaid: photocatalytic studies and writing. Muhammad Imran: photocatalytic studies and writing – review & editing. Rehan Ullah: formal analysis, visualization, validation, investigation and interpretation. Amina Sadiq: formal analysis, investigation. Israa J. Hakeem: validation, investigation, resources. Hatem M. Altass: characterization, validation, investigation, resources. Rabab S. Jassas: validation, investigation, resources. Saleh A. Ahmed: characterization, validation, investigation, resources.

Conflicts of interest

The authors have no conflicts of interest to declare.

Funding

This research work was funded by Umm Al-Qura University, Saudi Arabia under grant number: 26UQU4320545GSSR08.

Acknowledgments

The authors extend their appreciation to Umm Al-Qura University, Saudi Arabia for funding this research work through grant number: 26UQU4320545GSSR08.

Data availability

All the data is provided in the manuscript.

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