Green-synthesized LaOCl/ZnO heterojunction for efficient photocatalytic degradation of methyl orange

Abstract

A novel LaOCl/ZnO photocatalyst was synthesized via a Terminalia catappa-assisted coprecipitation route for methyl orange (MO) degradation under simulated sunlight. The leaf extract acted as a green structure-directing medium that promoted nucleation, stabilization, particle dispersion, and composite formation. XRD, EDS, TEM mapping, and XPS confirmed the coexistence of crystalline ZnO and LaOCl with Zn, O, La, and Cl distributed in the composite. The optimal material was obtained at a La/Zn molar ratio of 20% and 500 °C calcination. SEM/TEM observations showed aggregated nanoscale particles with interparticle voids, whereas the extract-free control displayed compact aggregation and weaker activity. UV-Vis/PL analyses revealed a 3.16 eV band gap and reduced electron–hole recombination. The optimized composite degraded 97.38% of 10 mg L⁻¹ MO within 180 min at 1.0 g L⁻¹ catalyst dosage, outperforming ZnO, LaOCl, and the extract-free control. Langmuir–Hinshelwood kinetics gave a maximum k_app of 0.02056 min⁻¹. Scavenger tests identified ·OH and ·O₂⁻ as dominant species, while the catalyst retained 84.68% efficiency after five cycles with acceptable operational stability.

A novel LaOCl/ZnO photocatalyst was synthesized via a Terminalia catappa-assisted coprecipitation route for methyl orange (MO) degradation under simulated sunlight.

Introduction

The discharge of dye-containing effluents from textile, printing, and chemical industries remains a persistent environmental challenge due to their high color intensity, chemical stability, and resistance to biodegradation.¹ Among various azo dyes, methyl orange is frequently used as a model pollutant because of its complex aromatic structure and potential toxicity to aquatic ecosystems and human health.² Conventional treatment ways like as adsorption, coagulation, and biological processes often suffer from incomplete removal or secondary pollution.^3–6 Methyl orange adsorption shows highly variable capacities depending on adsorbent type, typically ranging from tens to >1000 mg g⁻¹, with dominant monolayer chemisorption.⁷ The electrocoagulation with Al–Fe electrodes treating 500 mg L⁻¹ methyl orange achieved ∼99.6% color removal and ∼83% COD removal within 60 min (optimal 20 V).⁸ The acid-modified bio-adsorbents for methyl orange exhibit adsorption capacities ranging widely from ∼3 to 945.6 mg g⁻¹ and removal efficiencies from ∼5.9% to 99% depending on biomass type and conditions.⁹ In this context, semiconductor-based photocatalysis has emerged as a promising advanced oxidation technology capable of mineralizing organic pollutants into harmless end products under light irradiation.^10–14

Among photocatalysts, zinc oxide has attracted considerable attention owing to its high photosensitivity, strong oxidation ability, low cost, and environmental compatibility. However, its practical application is limited by a wide band gap (3.2–3.3 eV), rapid recombination of photogenerated electron–hole pairs, and restricted visible-light absorption.¹⁵ To overcome these drawbacks, the modification of ZnO with rare-earth oxides has been widely explored. The ZnO and RE-modified ZnO (2 at% Ce, Y, Eu) nanosheets with reduced crystallite sizes (from 45 to 29 nm), achieving significantly enhanced photocatalytic performance, where ZnO : Ce reached ∼95% methylene blue degradation, nearly 20 times higher than TiO₂–P25.¹⁶ Rare earth-doped ZnO micro/nanospheres with an optimal doping level of ∼2.0 Nd at% exhibited enhanced visible-light absorption and superior photocatalytic activity for phenol degradation compared to pure ZnO and TiO₂, attributed to improved charge separation and increased ·OH radical generation.¹⁷ The optimal ZnO–Ce photocatalyst (500 mg L⁻¹) achieved ∼80% removal of contaminants of emerging concern within 45 min (≈4.4 kJ L⁻¹ UV dose) under natural solar irradiation, demonstrating superior performance compared to other modified ZnO systems.¹⁸ In particular, lanthanum oxide is an effective dopant or coupling component due to its unique electronic configuration, which can enhance charge separation, increase surface basicity, and introduce defect sites that promote photocatalytic activity.^19,20

In parallel, the development of green and sustainable synthesis strategies has become increasingly important to reduce the environmental footprint of nanomaterial fabrication. Recently, plant-mediated synthesis has emerged as an eco-friendly alternative to conventional chemical routes, utilizing bioactive compounds such as polyphenols, flavonoids, and organic acids as reducing, stabilizing, and capping agents.^21,22 Leaves of Terminalia catappa, which are abundant in tropical regions such as Vietnam, are rich in these phytochemicals and have been widely used in green synthesis of metal and metal oxide nanomaterials.^23,24 The incorporation of such natural extracts in co-precipitation processes not only reduces the use of hazardous chemicals but also influences nucleation, particle growth, and surface functionalization, potentially leading to improved catalytic properties. Despite these advances, the integration of plant extract-assisted synthesis with rare-earth-modified ZnO systems remains insufficiently explored, particularly in terms of understanding their synergistic effects on photocatalytic performance. In particular, lanthanum-based compounds have attracted increasing attention as effective modifiers for ZnO due to their unique electronic structures and strong ability to regulate charge transfer processes. While lanthanum oxide (La₂O₃) has been widely investigated, recent studies suggest that lanthanum oxychloride (LaOCl) exhibits distinct physicochemical properties, including layered crystal structure, abundant surface defects, and enhanced electron mobility, which are beneficial for photocatalytic applications. Recent studies have shown that LaOCl-based heterojunctions, especially rare-earth-doped LaOCl³⁺, Er³⁺ systems coupled with BiOBr or Bi₂MoO₆/Ag, can broaden light absorption through upconversion luminescence, promote charge separation through S-scheme heterojunction formation, and improve photocatalytic degradation of organic pollutants, confirming the strong potential of LaOCl as a functional component in advanced photocatalysts.^25,26 The presence of Cl⁻ ions in LaOCl can induce oxygen vacancies and modify the local electronic environment, thereby facilitating charge separation and suppressing electron–hole recombination. Therefore, constructing LaOCl/ZnO heterostructures is expected to provide improved interfacial charge transfer, enhanced light absorption, and superior photocatalytic performance compared to ZnO.

In this study, a LaOCl/ZnO photocatalyst was successfully synthesized via a Terminalia catappa-assisted co-precipitation method. The as-prepared materials were systematically characterized to elucidate their structural, morphological, and optical properties. Their photocatalytic activity was evaluated through the degradation of methyl orange under light irradiation. The study aims to demonstrate a sustainable and efficient approach for designing advanced photocatalysts by combining rare-earth modification with bio-assisted synthesis, contributing to the development of environmentally benign technologies for wastewater treatment.

Experimental section

Materials

Zinc chloride (ZnCl₂, ≥98%) and lanthanum chloride heptahydrate (LaCl₃·7H₂O, ≥99%) were used as precursor salts for Zn²⁺ and La³⁺, respectively. Sodium hydroxide (NaOH, ≥98%) was used as the precipitating agent. Methyl orange (MO, analytical grade) was selected as the model organic dye pollutant. Fresh leaves of Terminalia catappa were collected in Hanoi, Vietnam, and used for green synthesis. All substances were of analytical quality and utilized without additional purification. Deionized water was used in all preparations.

Preparation of Terminalia catappa leaf extract

Fresh Terminalia catappa leaves were washed thoroughly with pure water, air-dried, and cut into small pieces. Approximately 10 g of dried leaves were boiled in 200 mL of deionized water/ethanol (v/v: 1/1) at 70–80 °C for 60 min (after ultrasound for 30 min). The mixture was cooled to room temperature and filtered to obtain a clear extract, which was used immediately as a bio-reducing and stabilizing agent.

Synthesis of LaOCl/ZnO photocatalyst

The LaOCl/ZnO composite was synthesized via a Terminalia catappa-assisted co-precipitation method. Aqueous solutions of ZnCl₂ and LaCl₃·7H₂O (0.05 M) were prepared separately and then mixed under magnetic stirring to achieve a La³⁺ : Zn²⁺ molar ratio of x : y (optimal condition: 1 : 4).

The mixed solution (200 mL) was heated to ∼80 °C under continuous stirring. A mixture of Terminalia catappa extract (50 mL) and 10 M NaOH solution (2 mL) was then added dropwise over 30 min to induce co-precipitation. The formation of a homogeneous precipitate was maintained under constant stirring and temperature.

After reaction, the suspension was cooled to room temperature. The precipitate was separated and washed several times with deionized water until a neutral pH (∼7) was achieved. The product was dried at 105 °C, ground into fine powder, and calcined at 500 °C for 3 h to obtain the final LaOCl/ZnO photocatalyst.

Characterization of LaOCl/ZnO composite

Utilizing X-ray diffraction (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation (λ = 1.5406 Å), the crystalline structure of the samples was examined. Transmission electron microscopy (TEM, JEM-2100, JEOL, Japan) and scanning electron microscopy (SEM, S-4800, Hitachi, Japan) were used to analyze the surface structure and morphology. Over the range of 400–4000 cm⁻¹, functional groups were detected using Fourier transform infrared spectroscopy (FTIR; Tensor II, Bruker, Germany). The bandgap energy and optical characteristics were measured with an integrating sphere attachment using UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS, V-750, JASCO, Japan). A combination of energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM) was used to examine the distribution and composition of the elements (UK). We used X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA) to study the surface's chemical states.

Photocatalytic degradation of methyl orange

Photocatalytic activity was evaluated using methyl orange (MO) as the target pollutant. MO aqueous solutions with initial concentrations ranging from 5 to 25 mg L⁻¹ were prepared. Typically, 50 mL of MO solution was transferred into a reaction vessel, and the photocatalyst was added at a dosage of 0.25–1.0 g L⁻¹.

Prior to irradiation, the suspension was magnetically agitated in the absence of light for 60 min to establish adsorption–desorption equilibrium. Photocatalytic experiments were conducted under a light source (e.g., 350 W xenon lamp as simulated sunlight) with continuous stirring to ensure uniform dispersion.

At predetermined time intervals, aliquots were withdrawn, centrifuged to remove catalyst particles, and analyzed using a UV-Vis spectrophotometer at the maximum absorption wavelength of MO (464 nm). The degradation efficiency was calculated using:

1

where C₀ and C_t (mg L⁻¹) represent the initial and instantaneous concentrations of MO, respectively.

All experiments were conducted in triplicate, and the results are presented as mean ± standard deviation to ensure reproducibility and reliability.

Effect of operational parameters

The effects of key experimental parameters on photocatalytic performance were systematically evaluated to elucidate their roles in catalyst activity and reaction efficiency. Specifically, the La³⁺ : Zn²⁺ molar ratio and calcination temperature were varied to optimize the photodegradation efficiency. In addition, the initial methyl orange (MO) concentration (5–25 mg L⁻¹) and catalyst dosage were adjusted to assess the impact of pollutant degradation efficiency.

Photodegradation kinetics of MO by LaOCl/ZnO composite

The photocatalytic degradation kinetics of methyl orange (MO) over the LaOCl/ZnO composite were analyzed using the Langmuir–Hinshelwood kinetic model. At low initial pollutant concentrations, the Langmuir–Hinshelwood model can be simplified to a pseudo-first-order kinetic equation:

2

where C₀ and C_t (mg L⁻¹) are the initial concentration and the concentration of MO at irradiation time t, respectively, and k (min⁻¹) is the apparent pseudo-first-order rate constant. The value of k was obtained from the slope of the linear plot of ln(C₀/C_t) versus irradiation time. The goodness of fit was evaluated using the correlation coefficient (R²), and the obtained rate constants were used to compare the photocatalytic performance of the synthesized samples under different experimental conditions.

Recyclability and reusability test

The recyclability and reusability of the LaOCl/ZnO photocatalyst were evaluated through repeated photocatalytic degradation cycles of methyl orange (MO) under identical conditions. After each reaction cycle, the photocatalyst was separated by centrifugation, washed several times with deionized water and ethanol to remove any adsorbed intermediates or residual dye molecules, and then dried at 105 °C for 4 hours before reuse. The recovered catalyst was subsequently applied in the next degradation cycle under the same experimental conditions, including identical MO concentration, catalyst dosage, and irradiation time. This procedure was repeated for multiple consecutive cycles (5 cycles) to assess the stability and durability of the photocatalyst. The degradation efficiency of MO was monitored after each cycle using UV-Vis spectroscopy at its characteristic absorption wavelength (464 nm). The retained photocatalytic activity after successive cycles was used as an indicator of catalyst stability and reusability.

Results and discussion

Characterization of LaOCl/ZnO composite

Fig. 1a shows that the synthesis yield of LaOCl/ZnO strongly depends on the La content during the Terminalia catappa-assisted preparation. The yield initially increases as La content rises from 5% to 20%, reaching a maximum of 43.41% at 20% La. This suggests that an appropriate amount of La precursor favors the co-precipitation and formation of the La-containing phase in the ZnO-based matrix. At low La contents, the amount of La species may be insufficient to effectively form LaOCl domains. However, when the La content exceeds 20%, the yield gradually decreases, falling to 12.4% at 100% La. This decline may result from excessive La species disturbing the precipitation balance, reducing ZnO formation, or increasing the loss of soluble/unreacted lanthanum-containing species during washing. Therefore, 20% La appears to be the most suitable composition for obtaining LaOCl/ZnO with a relatively high synthesis yield, rather than simply adding more La and hoping chemistry will politely obey.

Fig. 1. Effect of La content on LaOCl/ZnO synthesis yield (a) and effect of calcination temperature on LaOCl phase formation (b) via Terminalia catappa-assisted route.

Fig. 1b presents the XRD patterns of samples calcined at 400–700 °C, confirming the key role of calcination temperature in phase development. At 400 °C, the diffraction peaks are relatively weak, indicating poor crystallinity and incomplete phase formation. Increasing the temperature to 500 °C and 600 °C enhances the crystallinity of ZnO and promotes the appearance of LaOCl-related reflections. At 700 °C, the LaOCl phase becomes more evident, as indicated by the characteristic peaks marked in the pattern, while ZnO peaks remain visible. This demonstrates that a higher calcination temperature facilitates the transformation of La-containing intermediates into crystalline LaOCl and enhances the coexistence of LaOCl and ZnO. However, excessively high calcination temperatures may also promote particle growth and aggregation, thereby reducing surface area and photocatalytic activity. Thus, phase formation and photocatalytic optimization should be balanced rather than worshipping crystallinity, which fixes everything.

Fig. 2 compares the SEM morphology of pristine ZnO, Terminalia catappa-assisted LaOCl/ZnO, and LaOCl/ZnO synthesized without plant extract. Pristine ZnO (Fig. 2a and b) consists of relatively uniform and densely packed particles with short rod-like or rounded grains, mostly in the range of approximately 40–100 nm, forming compact aggregates larger than 500 nm. After LaOCl incorporation via the Terminalia catappa-assisted route (Fig. 2c and d), the composite exhibits a more heterogeneous, porous morphology, with smaller nanoparticles (20–80 nm) distributed among larger, irregular La-containing domains (100–200 nm). The higher-magnification image confirms that the extract-assisted LaOCl/ZnO composite possesses looser aggregation and more visible interparticle voids than pristine ZnO, which may improve surface accessibility, dye diffusion, and exposure of photocatalytic active sites. In contrast, the LaOCl/ZnO control sample prepared without Terminalia catappa extract (Fig. 2e and f) shows more compact aggregation, with irregular particles and densely packed clusters. Although nanosized particles are still observed, the control sample appears less open and less uniformly dispersed than the extract-assisted composite. This comparison indicates that bioactive compounds in Terminalia catappa extract likely act as soft stabilizing and structure-directing agents during precipitation, regulating nucleation and particle growth. Therefore, the green extract-assisted route contributes not only to composite formation but also to a more favorable surface texture for MO photodegradation.

Fig. 2. SEM morphology of ZnO and LaOCl/ZnO photocatalysts: pristine ZnO (a and b), Terminalia catappa-assisted LaOCl/ZnO composite (c and d), and LaOCl/ZnO prepared without plant extract as the control sample (e and f).

Fig. 3a shows that pristine ZnO exhibits the typical diffraction peaks of crystalline hexagonal wurtzite ZnO, with reflections at approximately 31.9°, 34.6°, 36.4°, 47.7°, 56.8°, 63.0°, 68.1°, and 69.2° (ref. 27) (PDF 01-071-6424-Zincite, Hexagonal). After the Terminalia catappa-assisted synthesis with La precursor, the LaOCl/ZnO composite retains all major ZnO diffraction peaks, indicating that the ZnO framework is preserved rather than destroyed, because apparently the lattice decided to behave for once. In addition, new diffraction peaks appear at 25.4°, 30.8°, 40.7°, 44.1°, 51.4°, and 66.7°, which are assigned to the LaOCl phase²⁸ (PDF 00-008-0477-Lanthanum Oxide Chloride, Tetragonal). The coexistence of ZnO and LaOCl reflections confirms the successful formation of a biphasic LaOCl/ZnO composite. The absence of obvious impurity peaks suggests that the chloride-containing La precursor preferentially forms LaOCl during calcination instead of separate La₂O₃, which is chemically reasonable given the use of LaCl₃·7H₂O as the La source.

Fig. 3. XRD patterns (a) and EDS spectroscopy (b) of ZnO and LaOCl/ZnO composite via Terminalia catappa-assisted route.

The EDS spectrum in Fig. 3b further confirms the elemental composition of the synthesized composite. Pristine ZnO contains only O and Zn, with 41.55 wt% O and 58.45 wt% Zn, corresponding to 52.57 at% O and 47.43 at% Zn (Table 1). In contrast, the LaOCl/ZnO composite contains O, Cl, Zn, and La, with 16.25 wt% O, 4.36 wt% Cl, 66.82 wt% Zn, and 12.57 wt% La. The corresponding atomic percentages are 48.43 at% O, 5.22 at% Cl, 42.58 at% Zn, and 3.77 at% La. The simultaneous detection of La and Cl, together with the XRD-identified LaOCl peaks, strongly supports the formation of LaOCl within the ZnO matrix. Meanwhile, Zn remains the dominant metallic element, confirming that ZnO is the major phase, while LaOCl serves as a secondary modifying phase that may provide interfacial charge-transfer sites to enhance photocatalytic activity.

Table 1. The elemental content of the LaOCl/ZnO composite by the EDS method.

Element	ZnO		LaOCl/ZnO
	Weight%	Atomic%	Weight%	Atomic%
O	41.55	52.57	16.25	48.43
Cl	0	0	4.36	5.22
Zn	58.45	47.43	66.82	42.58
La	0	0	12.57	3.77
Total	100.00	100.00	100.00	100.00

Fig. 4a and b shows that the LaOCl/ZnO composite synthesized via the Terminalia catappa-assisted route consists of strongly aggregated nanoscale particles. At low magnification (Fig. 4a, scale bar 200 nm), the particles form interconnected clusters with irregular open networks, suggesting that the bio-assisted co-precipitation process favors agglomeration of primary nanoparticles rather than formation of isolated particles. The aggregate size is roughly in the range of 100–500 nm, while the higher-magnification TEM image (Fig. 4b, scale bar 50 nm) reveals smaller primary particles of approximately 10–40 nm. Some larger, darker domains of about 50–100 nm can also be observed, which may correspond to overlapped particles or LaOCl-rich regions with higher electron density. The coexistence of fine nanoparticles and larger irregular domains is consistent with the formation of a heterogeneous LaOCl/ZnO composite, providing multiple interfacial contact points that are favorable for charge separation during photocatalysis.

Fig. 4. TEM images (a and b) and EDX mapping (c and d) of LaOCl/ZnO composite via Terminalia catappa-assisted route.

The SEM image and EDX elemental mapping in Fig. 4c and d further confirm the spatial distribution of the main elements in the LaOCl/ZnO composite. The mapped region, with a scale length of approximately 60 µm, shows granular aggregates distributed over a broad area. Elemental mapping demonstrates that Zn and O are widely distributed throughout the examined region, confirming that ZnO is the dominant matrix phase. Meanwhile, La and Cl signals are also clearly detected and overlap with the Zn/O-rich areas, indicating that LaOCl is not isolated as a completely separate bulk phase but is dispersed within or on the ZnO matrix. The relatively homogeneous distribution of La and Cl supports the successful formation of LaOCl domains through the chloride-containing lanthanum precursor. This intimate contact between LaOCl and ZnO is important because it can create interfacial heterojunctions, promote electron transfer, reduce electron–hole recombination, and increase the number of accessible reactive sites for methyl orange degradation.

Fig. 5a shows the UV-Vis absorption spectra of ZnO and the LaOCl/ZnO composite synthesized via the Terminalia catappa-assisted route. Pristine ZnO exhibits a strong UV absorption edge at approximately 349 nm, which is consistent with the intrinsic band-to-band transition of ZnO. After incorporation of LaOCl, the absorption edge shifts slightly to longer wavelengths, about 358 nm, indicating a modest red shift. This shift suggests that the coupling between LaOCl and ZnO slightly modifies the electronic structure of ZnO, probably through interfacial interactions, defect formation, and oxygen-vacancy-related states. Both samples show weak absorption tails beyond 400 nm, but the LaOCl/ZnO composite displays a slightly enhanced visible-region response, which is beneficial for photocatalytic activity under simulated sunlight. The corresponding Tauc plots inert in Fig. 5a further confirm the variation in the optical band gap. Pristine ZnO shows an estimated band gap of approximately 3.17 eV, close to the typical value of ZnO. In comparison, the LaOCl/ZnO composite exhibits a slightly narrower band gap of 3.16 eV. Although the difference is small, this narrowing suggests that LaOCl incorporation can subtly enhance light harvesting without compromising the ZnO semiconductor framework. More importantly, the LaOCl/ZnO heterointerface may promote charge separation and suppress electron–hole recombination, which contributes more significantly to photocatalytic enhancement than band-gap reduction alone. Therefore, the improved MO degradation performance of LaOCl/ZnO is likely attributed to the combined effects of slight red-shifted absorption, heterojunction formation, and more efficient interfacial charge transfer.²⁹ This is consistent with the manuscript's interpretation that LaOCl/ZnO heterostructures improve interfacial charge transfer, light absorption, and photocatalytic performance compared with ZnO. Fig. 5b presents the photoluminescence (PL) spectra of pristine ZnO and the LaOCl/ZnO composite synthesized via the Terminalia catappa-assisted route. Pristine ZnO shows a strong broad visible emission centered at approximately 580 nm, which is generally associated with defect-related radiative recombination, such as oxygen vacancies, zinc interstitials, and surface defect states. After coupling with LaOCl, the PL intensity of LaOCl/ZnO is clearly reduced compared with ZnO, while the emission band becomes broader and shifts toward longer wavelengths, with noticeable features around 600 and 640 nm. The lower PL intensity indicates that the radiative recombination of photogenerated electron–hole pairs is suppressed in the LaOCl/ZnO composite. This suggests more efficient interfacial charge separation after formation of the LaOCl/ZnO heterostructure. The red-shifted and broadened emission may arise from defect states and interfacial electronic interactions introduced by LaOCl incorporation.³⁰ Therefore, the PL result supports the enhanced photocatalytic activity of LaOCl/ZnO, as more photogenerated carriers can participate in reactive oxygen species formation instead of recombining rapidly.

Fig. 5. UV-Vis diffuse reflectance spectra with Tauc plot inset (a) and photoluminescence spectra (b) of ZnO and LaOCl/ZnO composite synthesized via the Terminalia catappa-assisted route.

Fig. 6a presents the XPS survey spectrum of the LaOCl/ZnO composite, confirming the coexistence of Zn, O, La, Cl, and C elements on the material surface. The intense Zn 2p signal indicates that ZnO is the major surface component,³¹ while the detectable La and Cl signals verify the successful formation of LaOCl within the composite. The high-resolution Zn 2p spectrum (Fig. 6b) shows a main Zn 2p_3/2 peak at 1021.6 eV, characteristic of Zn²⁺ in ZnO.³¹ The O 1s peak at 530.1 eV (Fig. 6c) corresponds mainly to lattice oxygen in metal–oxygen bonds,³² such as Zn–O and La–O. The La 3p_5/2 peak at 835.3 eV (Fig. 6d) confirms the presence of La³⁺ species,³³ while the Cl 2p peak at 198.9 eV (Fig. 6e) is consistent with chloride species bonded in LaOCl.³⁴ The C 1s peak at 284.8 eV (Fig. 6f) is attributed to adventitious carbon or residual organic species from the plant-assisted synthesis.³⁵ The XPS results confirm the surface chemical composition and support the successful construction of the LaOCl/ZnO heterostructure.

Fig. 6. Sum XPS spectroscopy (a) and atom XPS spectroscopy (b–f) of LaOCl/ZnO composite via Terminalia catappa-assisted route.

Fig. 7a compares the FTIR spectra of ZnO and LaOCl/ZnO prepared via the Terminalia catappa-assisted route. For pristine ZnO, the broad band at approximately 3395 cm⁻¹ is attributed to the stretching vibration of surface –OH groups or adsorbed water, while the band at 1611 cm⁻¹ corresponds to H–O–H bending vibration. The absorption bands in the low-wavenumber region at around 460 and 398 cm⁻¹ are assigned to Zn–O lattice vibrations, confirming ZnO formation. After LaOCl incorporation, the LaOCl/ZnO spectrum shows characteristic bands at approximately 640, 492, and 441 cm⁻¹, which can be associated with La–O/La–O–Cl and Zn–O vibrations, supporting the coexistence of LaOCl and ZnO phases.^28,36,37Fig. 7b presents the N₂ adsorption–desorption isotherms of ZnO and LaOCl/ZnO. Both samples show a sharp uptake at high relative pressure, indicating the presence of interparticle pores formed by aggregated nanoparticles. According to Table 2, pristine ZnO exhibits a higher BET surface area of 36.93 m² g⁻¹ and a much larger pore volume of 1.16 × 10⁻¹ cm³ g⁻¹ STP, with a small pore diameter of 1.69 nm. In contrast, LaOCl/ZnO shows a lower surface area of 13.30 ± 0.13 m² g⁻¹ and a pore volume of 2.17 × 10⁻³ cm³ g⁻¹ STP, but a larger pore size of 39.7–58.1 nm. The decrease in surface area and pore volume after LaOCl loading may result from partial pore blockage, particle aggregation, or coverage of ZnO surfaces by LaOCl domains. However, the larger interparticle pores in LaOCl/ZnO may still facilitate MO diffusion, while photocatalytic enhancement is mainly governed by heterojunction formation and improved charge separation rather than surface area alone.³⁸

Fig. 7. FTIR spectroscopy (a) and nitrogen adsorption isotherm (b) of ZnO and LaOCl/ZnO composite via Terminalia catappa-assisted route.

Table 2. BET surface area, pore volume, and pore size of ZnO and LaOCl/ZnO composite synthesized via the Terminalia catappa-assisted route.

Material	S BET, m2 g−1	Vpore, cm3 g−1 STP	d pore, nm
ZnO	36.93	1.16 × 10−1	1.69
LaOCl/ZnO	13.30 ± 0.13	2.17 × 10−3	39.7–58.1

Photocatalytic degradation of methyl orange on LaOCl/ZnO composite

Fig. 8a shows that the La/Zn molar ratio strongly affects the photocatalytic degradation of MO over LaOCl/ZnO under simulated sunlight. The degradation efficiency increases with irradiation time for all samples; however, the activity varies markedly with La content. The sample containing 20% La exhibits the highest activity, reaching 35.35% after 30 min, 59.17% after 60 min, 79.45% after 90 min, and 91.88% after 120 min. Lower La contents, such as 5% and 10%, give final efficiencies of 72.09% and 80.15%, respectively, indicating that insufficient LaOCl formation may limit heterojunction development. In contrast, excessive La content reduces photocatalytic performance, with the 100% La sample reaching only 43.40% after 120 min. These results suggest that an optimal LaOCl/ZnO ratio is necessary to maximize interfacial charge transfer, whereas excess LaOCl may decrease the number of ZnO active sites and hinder light absorption. The catalyst dosage also significantly affects MO degradation efficiency, as shown in Fig. 8b. Increasing the LaOCl/ZnO dosage from 0.25 to 1.0 g L⁻¹ improves the degradation efficiency from 73.29% to 91.88%, mainly due to the increased number of photocatalytic active sites and enhanced generation of reactive oxygen species. A further increase to 1.25 g L⁻¹ gives a similar efficiency of 91.85%, indicating that the reaction has approached an optimum. However, at 1.5 g L⁻¹, the efficiency slightly decreases to 90.43%, which may be attributed to excessive catalyst loading, light scattering, particle aggregation, and shielding effects. Therefore, 1.0 g L⁻¹ can be considered the optimal catalyst dosage for MO photodegradation under the present conditions. Fig. 8c illustrates the influence of the initial MO concentration on photocatalytic performance. The highest degradation efficiency is obtained at 10 mg L⁻¹, reaching 91.88% after 120 min, while 5 mg L⁻¹ gives a slightly lower efficiency of about 90%. As the MO concentration increases to 15, 20, and 25 mg L⁻¹, the final degradation efficiencies decrease to 83.38%, 72.15%, and 61.51%, respectively. This decline is mainly attributed to the increased number of MO molecules competing for the limited active sites and reactive oxygen species. In addition, higher dye concentrations reduce light penetration through the solution because of stronger absorption by MO molecules, thereby decreasing photon availability at the catalyst surface. The solution pH strongly influences the photocatalytic performance of LaOCl/ZnO, as shown in Fig. 8d. High degradation efficiencies are observed under acidic to near-neutral conditions, reaching 95.55%, 92.94%, and 91.88% at pH 3.07, 4.83, and 6.71, respectively. In contrast, the degradation efficiency decreases sharply under alkaline conditions, falling to 64.05% at pH 8.87 and 43.06% at pH 10.63. This behavior can be explained by the surface charge of LaOCl/ZnO and the anionic nature of MO. Under acidic conditions, protonation of the catalyst surface enhances electrostatic attraction to MO molecules, thereby improving surface adsorption before photodegradation. Under alkaline conditions, electrostatic repulsion between the negatively charged catalyst surface and MO anions reduces adsorption and suppresses degradation efficiency. Thus, mildly acidic to near-neutral conditions are favorable for MO photodegradation over LaOCl/ZnO.

Fig. 8. Effects of La/Zn molar ratio (a), catalyst dosage (b), initial MO concentration (c), and solution pH (d) on the photocatalytic degradation efficiency of methyl orange over LaOCl/ZnO under simulated sunlight.

Fig. 9a compares the photocatalytic degradation of MO over Terminalia catappa-assisted LaOCl/ZnO, pristine ZnO, LaOCl, and the LaOCl/ZnO control sample synthesized without Terminalia catappa extract. During the dark adsorption stage from −60 to 0 min, only a slight decrease in MO concentration is observed for all samples, indicating limited adsorption before irradiation. After light exposure, the Terminalia catappa-assisted LaOCl/ZnO composite exhibits the fastest decline in MO concentration, decreasing to approximately 6.98, 4.41, 2.22, and 0.88 mg L⁻¹ after 30, 60, 90, and 120 min, respectively. In comparison, pristine ZnO shows a slower degradation rate, with a residual MO concentration of about 3.23 mg L⁻¹ after 120 min, while LaOCl alone retains approximately 6.11 mg L⁻¹ MO. Notably, the LaOCl/ZnO control sample prepared without Terminalia catappa extract exhibits the weakest activity among the composite samples, with a residual MO concentration of approximately 7.9 mg L⁻¹ after 120 min. This result confirms that the superior activity of LaOCl/ZnO is not only related to the coexistence of LaOCl and ZnO phases, but also to the extract-assisted synthesis route, which likely improves particle dispersion, interfacial contact, and accessibility of photocatalytic active sites. Fig. 9b presents the time-dependent MO degradation efficiency and the corresponding theoretical SO₄²⁻ concentration formed during MO decomposition over the optimized Terminalia catappa-assisted LaOCl/ZnO composite. The degradation efficiency increases rapidly from 35.32% at 30 min to 59.14% at 60 min and 79.41% at 90 min. After 120 min, the efficiency reaches 91.88%, then gradually increases to 94.57% and 97.38% after 150 and 180 min, respectively. The rapid initial degradation indicates efficient generation of reactive oxygen species and favorable interaction between MO molecules and active sites. The slower increase at longer irradiation times may be attributed to the reduced MO concentration, competition from degradation intermediates, and lower probability of contact between pollutant molecules and reactive species. Based on the stoichiometric conversion of sulfur in MO to sulfate, the theoretical SO₄²⁻ concentration increases progressively with irradiation time and reaches approximately 3.08 mg L⁻¹ after 180 min. This trend further supports the gradual oxidative decomposition of MO. However, because sulfate formation was estimated from decomposed MO concentration rather than directly measured by ion chromatography, it should be interpreted as theoretical sulfate generation under the assumption of complete sulfur conversion to SO₄²⁻.

Fig. 9. MO photodegradation performance of LaOCl/ZnO compared with ZnO, LaOCl, and the Terminalia catappa-free control sample (a), and time-dependent degradation efficiency with corresponding SO₄²⁻ formation over the optimized LaOCl/ZnO composite (b).

Table 3 compares the photocatalytic degradation of MO using different materials under varied experimental conditions. CuO/Al₂O₃ nanocomposite achieved 94.2% MO removal at an initial concentration of 20 mg L⁻¹ with a relatively high catalyst dosage of 2.0 g L⁻¹ under visible light for 3 h. Silica–titania fibers showed the highest reported efficiency (98%) for 30 mg L⁻¹ MO under UV irradiation for 8 h, although the reaction time was relatively long. Fe₃O₄/TiO₂ magnetic nanocatalysts removed 90.3% of MO within 1 h under UV irradiation at 10 mg L⁻¹. In contrast, TiO₂ thin films showed only 22–23% removal after 180 min, indicating limited activity. The LaOCl/ZnO composite in this study achieved 97.38% degradation of 10 mg L⁻¹ MO using 1 g L⁻¹ catalyst under simulated sunlight for 180 min, demonstrating highly competitive photocatalytic performance.

Table 3. Several studies of removal MO by photodegradation.

Material	Conditions	Efficiency, %	References
CuO/Al2O3 nanocomposite	Initial concentration: 20 mg L−1, dose of 2.0 g L−1, visible light, 3 hours	94.2%	39
Silica-titania fibers	Initial concentration: 30 mg L−1, catalyst dosage: 0.1 g L−1, UV irradiation, 8 h	98%	40
Fe3O4/TiO2 magnetic nanocatalysts	Initial concentration: 10 mg L−1, UV irradiation, 1 h	90.3%	41
TiO2 thin films	Initial concentration: 10 mg L−1, UV irradiation, 180 min	22–23%	42
LaOCl/ZnO composite	Initial concentration: 10 mg L−1, dose of 1 g L−1, simulated sunlight, 180 min	97.38%	This study

The degradation kinetics of MO over the LaOCl/ZnO composite were evaluated using four kinetic models, as shown in Fig. S1 and Table S1. The pseudo-first-order model gives a rate constant k₁ of 0.025 min⁻¹ and a calculated q_e of 125.874 mg g⁻¹, with a correlation coefficient of R² = 0.973. This high R² value indicates that MO photodegradation is reasonably consistent with first-order behavior, where the reaction rate depends mainly on the remaining MO concentration. The pseudo-second-order model also fits the experimental data well, with k₂ = 7 × 10⁻⁵ g mg⁻¹ min⁻¹, q_e = 152.439 mg g⁻¹, and R² = 0.975. The slightly higher R² compared with the pseudo-first-order model suggests that surface-mediated interactions between MO molecules and active sites on LaOCl/ZnO also contribute to the degradation process. Among the tested models, the Elovich model provides the best fit, with a correlation coefficient (R²) of 0.980. The Elovich parameters a = 3.274 mg g⁻¹ and b = 0.027 g mg⁻¹ indicate that MO degradation occurs on a heterogeneous catalyst surface with variable activation energies. This is consistent with the composite nature of LaOCl/ZnO, in which ZnO, LaOCl, oxygen vacancies, and interfacial sites may all participate in photocatalytic reactions. In comparison, the Bangham model provides a lower but still acceptable fit, with R² = 0.958, k_B = 0.054, and α = 0.583, indicating that pore diffusion or intraparticle transport contributes to the process but is not the dominant rate-controlling step. Overall, the kinetic analysis suggests that MO photodegradation over LaOCl/ZnO is mainly governed by heterogeneous surface reactions, accompanied by concentration-dependent degradation and limited diffusion effects.

Fig. 10 presents the Langmuir–Hinshelwood kinetic analysis of MO photodegradation over the LaOCl/ZnO composite at different initial dye concentrations. The time-dependent (C₀/C_t) profiles in Fig. 11a show a continuous increase with irradiation time for all MO concentrations, confirming progressive photocatalytic degradation under simulated sunlight. Among the tested concentrations, the 10 mg L⁻¹ MO solution exhibits the steepest increase in (C₀/C_t), indicating the most favorable degradation behavior. At lower concentration, 5 mg L⁻¹, the reaction proceeds efficiently but with a slightly lower apparent rate, probably due to the lower probability of interaction between MO molecules and reactive species. When the initial MO concentration increases from 15 to 25 mg L⁻¹, the (C₀/C_t) values decrease progressively, suggesting that excess dye molecules compete for limited photocatalytic active sites and reactive oxygen species. In addition, higher MO concentrations may reduce light penetration through the solution due to stronger dye absorption, thereby decreasing photon availability at the LaOCl/ZnO interface. The linear plots of ln(C₀/C_t) versus irradiation time in Fig. 11b–f further confirm that MO photodegradation over LaOCl/ZnO follows the simplified Langmuir–Hinshelwood apparent first-order kinetic model. As summarized in Table 4, the apparent rate constant k_app increases from 0.01791 min⁻¹ at 5 mg L⁻¹ to a maximum of 0.02056 min⁻¹ at 10 mg L⁻¹, then decreases to 0.01486, 0.01062, and 0.00808 min⁻¹ at 15, 20, and 25 mg L⁻¹, respectively. The high correlation coefficients, ranging from 0.97183 to 0.99646, indicate good linear fitting and validate the applicability of the apparent first-order Langmuir–Hinshelwood model under the studied concentration range. The highest k_app at 10 mg L⁻¹ demonstrates that this concentration provides an optimal balance between sufficient pollutant availability and effective generation/utilization of reactive oxygen species. In contrast, the reduced k_app values at higher MO concentrations reflect active-site saturation, light-shielding effects, and increased competition between parent dye molecules and degradation intermediates. These results are consistent with the trend in degradation efficiency and confirm that the photocatalytic process is strongly dependent on the initial MO concentration.

Fig. 10. Langmuir–Hinshelwood kinetic analysis of MO photodegradation over LaOCl/ZnO at different initial MO concentrations: time-dependent C₀/C_t profiles (a) and linear plots of ln(C₀/C_t) versus irradiation time for 5 mg L⁻¹ (b), 10 mg L⁻¹ (c), 15 mg L⁻¹ (d), 20 mg L⁻¹ (e), and 25 mg L⁻¹ (f).

Fig. 11. The photodegradation mechanism for MO by LaOCl/ZnO composite.

Table 4. Apparent rate constants for MO photodegradation over LaOCl/ZnO derived from the simplified Langmuir–Hinshelwood model.

Initial MO concentration	k app (min−1)	Linear equation	R 2
5 mg L−1	0.01791 ± 0.00154	(y = 0.01791t − 0.10208)	0.97823
10 mg L−1	0.02056 ± 0.00202	(y = 0.02056t − 0.14859)	0.97183
15 mg L−1	0.01486 ± 0.00060	(y = 0.01486t − 0.04472)	0.99521
20 mg L−1	0.01062 ± 0.00037	(y = 0.01062t + 0.02083)	0.99646
25 mg L−1	0.00808 ± 0.00076	(y = 0.00808t + 0.05403)	0.97384

Proposed mechanism for MO photodegradation by LaOCl/ZnO catalyst

Fig. S2 shows the effect of radical scavengers on MO photodegradation over the LaOCl/ZnO composite under simulated sunlight. In the absence of a scavenger, the degradation efficiency increases rapidly with irradiation time, reaching approximately 91% after 120 min, confirming the high photocatalytic activity of LaOCl/ZnO. However, the addition of IPA, a hydroxyl radical (·OH) scavenger, strongly suppresses MO degradation, with the efficiency decreasing to only about 10% after 120 min. This indicates that ·OH radicals play a dominant role in the oxidation of MO molecules. Similarly, the presence of BQ, a superoxide radical (·O₂⁻) scavenger, markedly inhibits the degradation process, giving only about 15–16% efficiency after 120 min, demonstrating that ·O₂⁻ is also a major reactive species. In contrast, EDTA, used as a hole (h⁺) scavenger, causes a moderate decrease in efficiency, with MO degradation reaching about 56% after 120 min. This suggests that photogenerated holes also participate in the reaction, but their direct contribution is less dominant than that of ·OH and ·O₂⁻. Therefore, the scavenger experiments confirm that MO photodegradation over LaOCl/ZnO is mainly governed by radical-mediated oxidation pathways involving ·OH and ·O₂⁻, while h⁺ contributes as an auxiliary oxidative species.⁴³ Under simulated sunlight irradiation, the LaOCl/ZnO composite absorbs photons with energy equal to or higher than its band gap, generating electron–hole pairs. ZnO acts as the main photoactive semiconductor, while LaOCl serves as an interfacial modifier that promotes charge separation. The coupling between LaOCl and ZnO facilitates migration of photogenerated electrons and holes across the heterointerface, thereby suppressing rapid electron–hole recombination. In addition, LaOCl may introduce defect sites and oxygen vacancies, which can further trap charge carriers and enhance the formation of reactive oxygen species. These reactive species, mainly ·O₂⁻, ·OH, and h⁺, subsequently attack methyl orange molecules, leading to azo bond cleavage, aromatic ring opening, and gradual mineralization into smaller molecules, CO₂, H₂O, and inorganic ions.⁴⁴ The possible photocatalytic reaction steps are proposed as follows:

LaOCl/ZnO + hν → LaOCl/ZnO (e⁻ + h⁺) 3

e_CB⁻ + O₂ → ·O⁻₂ 4

·O₂⁻ + H⁺ → ·HO₂ 5

·2HO₂ → H₂O₂ + O₂ 6

H₂O₂ + e⁻ → ·OH + OH⁻ 7

h_VB+ + H₂O/OH⁻ → ·OH + H⁺ 8

MO + h⁺/·OH/·O₂⁻ → degradation intermediates 9

intermediates + ·OH/·O₂⁻ → CO₂ + H₂O + NO³⁻ + SO₄²⁻ + other mineralized products 10

Fig. 11 illustrates the proposed photocatalytic degradation mechanism of MO over the LaOCl/ZnO composite under simulated sunlight. Upon irradiation, ZnO absorbs photons with energy equal to or greater than its band gap (∼3.16 eV), generating photogenerated electrons (e⁻) in the conduction band and holes (h⁺) in the valence band. The coupled LaOCl phase acts as an interfacial mediator, helping separate charge carriers and suppress electron–hole recombination, because apparently even electrons need traffic control. The excited electrons reduce dissolved O₂ to ·O₂⁻, which can further form ·HO₂ and H₂O₂, subsequently producing ·OH radicals. Meanwhile, photogenerated holes oxidize H₂O or OH⁻ to ·OH.⁴⁵ These reactive species, together with h⁺, attack MO molecules, inducing azo bond cleavage, aromatic ring opening, and stepwise oxidation into intermediates, finally mineralizing into CO₂, H₂O, NO₃⁻, SO₄²⁻, and other small products.

Fig. 12 shows that the LaOCl/ZnO composite maintains good photocatalytic stability during five consecutive MO degradation cycles. The degradation efficiency decreases gradually from 91.88% in the first cycle to 88.67%, 86.97%, 85.58%, and 84.68% in the second to fifth cycles, respectively. Correspondingly, the reduction in efficiency increases from nearly 0% after the first cycle to 3.46%, 5.30%, 6.82%, and 7.80% after subsequent cycles. This moderate decline may be attributed to partial catalyst loss during recovery, blockage of active sites by adsorbed intermediates, or slight aggregation after repeated irradiation and washing. Nevertheless, the catalyst still retains more than 84% degradation efficiency after five cycles, confirming its acceptable reusability and operational stability for photocatalytic MO removal.

Fig. 12. MO photodegradation efficiency and reduction degradation efficiency by LaOCl/ZnO composite.

Conclusion

A LaOCl/ZnO composite photocatalyst was successfully synthesized through a Terminalia catappa-assisted co-precipitation route using ZnCl₂ and LaCl₃·7H₂O precursors. The plant extract functioned as a bio-assisted stabilizing and structure-directing medium, improving particle dispersion and interfacial contact compared with the extract-free control. The optimal material was obtained at 20% La and 500 °C calcination. XRD, EDS mapping, and XPS confirmed the coexistence of ZnO and LaOCl phases with Zn, O, La, and Cl elements, while SEM/TEM showed aggregated nanoscale particles with visible interparticle voids. UV-Vis DRS indicated an absorption edge near 358 nm and a band gap of 3.16 eV, and PL spectra confirmed suppressed electron–hole recombination after LaOCl coupling. The optimized composite achieved 97.38% MO degradation under simulated sunlight within 180 min using 10 mg L⁻¹ MO and 1.0 g L⁻¹ catalyst, outperforming ZnO, LaOCl, and the extract-free sample. Langmuir–Hinshelwood analysis gave the highest k_app of 0.02056 min⁻¹. Scavenger tests identified ·OH and ·O₂⁻ as the main reactive species, while recyclability remained 84.68% after five cycles. These results confirm the effectiveness of green LaOCl/ZnO for the treatment of dye wastewater.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data for this article, including SEM, XRD, BET, FTIR, …, and electrochemical properties measurements are available at Open Science Framework at https://osf.io/u4hp3/.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra04025j.