Introduction

Zinc oxide nanoparticles (ZnONPs) in nanotechnology have dominated nanomaterials as an area of interest for researchers due to their wide bandgap, high exciton binding energy, given their importance in diverse applications, including electronics, biomedical engineering, and photocatalysis [1–4]. Improving the functional properties of ZnONPs often relies on modifying their structure through the infusion of rare-earth elements. Among which lanthanum (La³⁺) doping is an ideal candidate to change the electronic states, lower the bandgap energy, and enhance the surface reactivity. Recent studies indicated that La-doped ZnONPs (La-ZnONPs) exhibit better crystallinity, increased defect density, and improved light-harvesting properties [5–8]. These properties make them promising candidates for photocatalytic and semiconductor applications.

Concurrently, green synthesis is considered as sustainable replacement for conventional chemical routes, which rely on toxic chemicals, high temperatures, or energy-intensive processes. Plant extracts are most appealing because they are rich in bioactive compounds such as polyphenols, flavonoids, and organic acids that act as reducing, capping, and stabilizing agents [9–12]. This phytochemical-mediated approach helps to avoid hazardous chemicals and supports the circular economy by using waste valorization principles. Numerous biological sources such as aloe vera, banana peel, and citrus extracts have been used to modify metal oxide nanoparticles; however, their efficiency depends on specific phytochemical composition and reducing ability [13, 14].

Pineapple peel extract (PPE) is abundant in phytochemical compounds, which are particularly valuable for extracting bioactive functional groups that enable strong chelation between metal ions and efficient nanoparticle stabilization. These phytochemicals serve as capping agents, preventing nanoparticle agglomeration and enhancing their stability [11, 15, 16]. Exploring sustainable ways to utilize pineapple waste, such as peels, crown, stem, in diverse ways not only reduces its negative environmental footprint but also unlocks its potential in diverse applications. One innovative method involves utilizing this biodegradable biomass as a precursor for synthesizing nanoparticles through eco-friendly processes [2, 17, 18]. Despite containing a high phytochemical content and being easily available as an agricultural waste globally, its potential remains underexplored for the green synthesis of La-ZnONPs. Many studies focus on undoped ZnONPs, but there is no published work that addresses the incorporation of La³⁺ into ZnONPs using PPE as a green reducing and capping agent. Current work on La-ZnONPs almost exclusively uses chemical precipitation or high-temperature calcination methods that lack an environmentally friendly green synthesis route.

We hypothesize that a higher concentration of La dopant into ZnONPs will systematically modulate its key physicochemical properties, such as crystallite size, bandgap energy, morphology, and surface area. It is done due to two factors, such as lattice distortion and surface modification induced by La³⁺ incorporation and interactions with the phytochemicals present in PPE [5, 19]. This study aims to fabricate La-ZnONPs using PPE as a phytochemical reducing agent by varying the La dopant concentrations. By integrating rare-earth doping with a green synthesis approach, this work introduces a sustainable pathway for producing modified ZnO nanomaterials and provides new insight into dopant-induced property evolution under phytochemical-assisted synthesis conditions. To the best of our knowledge, many researchers utilize La as dopant to increase the efficiency of nanoparticles, but along with green sources such as pineapple extract, as in this study, is a novel aspect added to the field of nanotechnology to increase the efficiency and potential of photocatalysts in future.

Materials and methods

Fabrication of ZnONPs and La-ZnONPs

A straightforward co-precipitation procedure outlined in reference [5, 10] was used to fabricate ZnONPs. Ten milliliters of distilled water and 50 mL of PPE were mixed in a 500 mL beaker with 4 g of Zn(CH₃COO)₂·2H₂O in this mixture at constant stirring. The pH of the solution was gradually raised to 11–12 by adding 0.1 M NaOH. The white precipitation of Zn(OH)₂ is shown at the moment of the addition of NaOH. The solution was stirred continuously for 1 h at 40°C to 50°C and then centrifuged for 1 min at 1,800 g once it had cooled to room temperature. After 6 h of drying at 120°C, a white precipitate was formed from the resultant material, which was subsequently crushed into a fine powder using a mortar and pestle (Chembio, Malaysia) and followed by calcination at 300°C. The undoped ZnONPs were collected accordingly.

For La doping, La(NO₃)₃·6H₂O was introduced at a concentration of 1%, 2%, and 3% by weight during the precipitation step. Each suspension was stirred for 1 h at 45–50°C. After 1 h reaction, each suspension was centrifuged for 1 min at 1,800 g. This is sufficient because the intermediate Zn(OH)₂ precipitate forms an aggregate before calcination. The precipitate was washed three times with DI water and twice with ethanol to remove residual NaOH and hydroxide ions. It was repeated before with undoped ZnONPs also. Then, after drying at 120°C for 6 h performed to achieve a white precipitate as the resultant material, which was subsequently crushed into a fine powder using a mortar and pestle, and followed by calcination at 300°C. This temperature helps to convert Zn(OH)₂ into ZnO and removes organic residues. It aligns with prior green synthesis protocols (250–400°C). Separate syntheses were performed for each La concentration. The schematic representation for fabrication is depicted in Figure 1.

Display full size

Figure 1. Representation for the fabrication of La-ZnONPs. Zn(CH₃COO)₂·2H₂O: zinc acetate dihydrate; La(NO₃)₃·6H₂O: lanthanum nitrate hexahydrate; NaOH: sodium hydroxide; La: lanthanum; La-ZnONPs: La-doped zinc oxide nanoparticles.

Results

XRD patterns ranging between 20° to 80° depicted in Figure 2A for undoped ZnONPs and 1%, 2%, and 3% La-ZnONPs reveal peaks characteristic of the hexagonal wurtzite ZnO structure (JCPDS No. 36-1451) with the highest reflections between 30° to 40° corresponding to the (001), (002), and (101) planes of ZnONPs, respectively [6, 22]. The dominant (101) peak appears at 2θ angles of 36.296, 36.307, 36.298, and 36.351 with minor variation in peak position as La concentration increases (Figure 2B).

Display full size

Figure 2. XRD pattern for (a) ZnONPs, (b) 1% La-ZnONPs, (c) 2% La-ZnONPs, and (d) 3% La-ZnONPs (left side, A) and XRD highest peak in zoom in to distinguish level (right side, B). Intensity is expressed in arbitrary units (a.u). XRD: X-ray diffraction; ZnONPs: zinc oxide nanoparticles; La-ZnONPs: lanthanum-doped ZnONPs.

FTIR analysis of the extract and its interaction with fabricated ZnONPs and dopant concentration at 1%, 2%, and 3% on ZnONPs at room temperature reveals distinctive bands in the range of 400 to 4,000 cm^–1 shown in Figure 3. The pineapple extract consisted of various chemical constituents, primarily carbohydrates and phenolic compounds, with minimal quantities of flavonoids. The bioorganic compounds present hydroxyl functional groups (O–H) at 3,312 cm^–1, carbonyl moieties (C=O) at 1,632 cm^–1, aromatic linkages (C=C) at 1,389 cm^–1, and C–O stretching at 1,039 cm^–1 (Figure 3a), which are vital for enhancing the biogenic generation of ZnONPs. The pineapple extract confirms the presence of carbonyl-rich biomolecules, which played a role in reducing and stabilizing the nanoparticles.

Display full size

Figure 3. FTIR spectra of pineapple extract (a), ZnONPs (b), 1% La-ZnONPs (c), 2% La-ZnONPs (d), and 3% La-ZnONPs (e). La: lanthanum; FTIR: fourier transform infrared spectroscopy; ZnONPs: zinc oxide nanoparticles; La-ZnONPs: La-doped ZnONPs.

The fabricated doped and undoped ZnONPs are analyzed morphologically by SEM-EDX data. The SEM images are displayed below in Figure 4. Morphologically images showed significantly nanoflower petals’ shape as the dopant concentration increased due to the increment of dopant inside the matrix of ZnONPs to provide a larger surface area. The elemental composition of Zn in the ZnONPs was analyzed through EDX to determine the elements in the sample of the powder. Figure 4 displays the EDX spectra for various synthesized ZnONPs.

Display full size

Figure 4. SEM depictions of (a) undoped ZnONPs at 300°C at 20,000× magnification, (b) 1% La-ZnONPs for 300°C at 20,000× magnification, (c) 2% La-ZnONPs at 300°C at 20,000× magnification, and (d) 3% La-ZnONPs at 300°C at 20,000× magnification, with respective EDX data besides each SEM images (scale bar = 1 μm). La: lanthanum; EDX: energy dispersive X-ray; SEM: scanning electron microscopy; ZnONPs: zinc oxide nanoparticles; La-ZnONPs: La-doped ZnONPs.

The optical properties of ZnONPs and La-ZnONPs materials with three doping concentrations were analyzed by UV-Vis absorption spectra, represented in Figure 5. The absorption spectra show that 1%, 2%, and 3% La-ZnONPs have peaks at 410 nm, 420 nm, and 429 nm, while undoped ZnONPs have peaks at 397 nm. By analyzing the variation of (αhν)² and energy as a function of hν (Figure 6), the calculated bandgap values were determined to be 3.21, 3.18, 3.13, and 3.11 eV for ZnONPs and La-ZnONPs with 1%, 2%, and 3% La³⁺ ions, respectively.

Display full size

Figure 5. UV-Vis plot for (a) undoped ZnONPs, (b) 1% La-ZnONPs, (c) 2% La-ZnONPs, and (d) 3% La-ZnONPs. UV-Vis: ultra-violet visible; ZnONPs: zinc oxide nanoparticles; La-ZnONPs: lanthanum-doped ZnONPs.

Display full size

Figure 6. Tauc plots of (a) undoped ZnONPs, (b) 1% La-ZnONPs, (c) 2% La-ZnONPs, and (d) 3% La-ZnONPs. ZnONPs: zinc oxide nanoparticles; La-ZnONPs: lanthanum-doped ZnONPs.

The assessment of surface area utilizing the BET methodology was undertaken to elucidate the surface characteristics of undoped ZnONPs and La-ZnONPs. Upon the deposition of La-ZnONPs, the La-ZnONPs exhibited a noteworthy enhancement in specific surface area, escalating from 37.1206 to 65.4184 m²/g. BJH pore-size distribution analyses show that undoped ZnONPs have a narrow distribution (12–18 nm), while La-doped samples exhibit broader distributions and increased pore volume.

Discussion

The combined characterization techniques demonstrate that La incorporation and the presence of PPE significantly influence the structural, morphological, optical, and surface properties of ZnONPs synthesized through green co-precipitation.

XRD results indicate a reduction in crystallite size with increasing La content. A slight broadening of the (101) peak is observed with increasing La content, which corresponds to a reduction in crystallite size, as shown in Figure 2B. To enable clearer comparison, peak parameters of the main (101) peak, including 2θ value, FWHM, and crystallite size, were summarized in Table 1. Equation 1 is used to calculate the crystallite size (D) to analyze the effect of increasing dopant concentration on the fabrication of La-ZnONPs. On the basis of calculations derived from XRD analysis, the increased dopant content decreases the crystallite size from 27.25 nm to 21.27 nm, as shown by the decreasing trend in Table 2. Although doping can sometimes cause measurable peak shifting, the variations in 2θ in this study fall within the typical range reported for low-level rare-earth doping and do not indicate a clear systematic shift.

The observed reduction in crystallite size can be attributed to the emergence of La-O-Zn entities on the surfaces of the doped materials, which impede the process of crystallite growth, as elucidated by several researchers [6, 23, 24]. Similarly, this reduction may also stem from the dispersion of Zn-O-Zn matrix induced by La³⁺ ions, resultantly diminishes the accumulation rate of La-ZnONPs crystallization. However, XRD alone cannot conclusively confirm La³⁺ substitution into Zn²⁺ lattice sites because of the low dopant concentrations, but the observed variations are consistent with partial surface association, as supported by literature on similarly doped ZnO synthesis.

FTIR spectra of both undoped ZnONPs and La-ZnONPs (Figure 3b–e) preserve several functional groups originating from PPE, confirming surface adsorption at 3,360 cm^–1, 1,557 cm^–1, 1,403 cm^–1, indicated O–H stretching, C=O stretching of carbonyls, and COO bending vibrations, respectively. Upon La incorporation, new or intensified bands emerge at 782–855 cm^–1, assigned to Zn-O-La and La-O stretching, indicating lattice modification caused by La³⁺ substitution. The doped samples also retain weak O–H and carbonyl bands, suggesting stronger binding of oxygenated organic residues due to increased surface coordination sites. The combined spectral evidence loss of extract peaks and emergence of Zn-O/La-O vibrations that confirms the effective role of plant biomolecules during green synthesis with embedding of La³⁺ into ZnO, and improved surface stabilization through residual organic ligands [25].

From 20,000× magnification of Figure 4, it can be seen that more pores exist and crystallization can obviously be seen. The discernible “pores” depicted in Figure 4b–d do not signify intrinsic porosity present within individual nanoparticles. On the contrary, these aspects are voids between particles formed by the aggregation process and the elimination of surplus organic compounds in the calcination method. Such voids are frequently observed in green-synthesized ZnO, attributable to phytochemical-mediated nucleation followed by subsequent thermal decomposition. These structural voids enhance the accessible surface area; however, they do not denote genuine microporosity at the level of individual nanoparticles.

The EDX spectra substantiate the presence of Zn and O as predominant elements across all samples, alongside detectable La signals in the doped specimens. The recorded atomic percentages depict the relative elemental composition. For instance, the La-doped sample containing 3% La exhibits a more substantial relative La signal in comparison to the samples containing 1% and 2% La, indicating an augmented incorporation of the dopant. From Figure 4a, the main constituents of the sample are Zn (78.97%) and O (21.03%). Figure 4d exhibits the highest elemental percentage of Zn (78.41%), while Figure 4c shows the highest elemental percentage of La (7.52%). We noticed that these materials preserved their nanoparticle structure even after calcination. Overall, the SEM-EDX results support the successful synthesis of ZnO nanoparticles modified with La dopant and demonstrate morphological changes that correlate with increased dopant concentration.

UV-Vis results reveal a slight red shift in the absorption edge and a reduction in bandgap energy from 3.21 to 3.11 eV. This difference in peak locations is likely caused by the photo-excitation of electrons from the valence band (VB) to the conduction band (CB). With the addition of La dopant, the absorption edge shows a small red shift and a marginal broadening toward longer wavelengths. Although the individual peak maxima in the spectra cannot be distinguished as separate values for each dopant concentration, the general pattern suggests that the optical response gradually changes as the La content increases. Such variations are frequently attributed to lattice strain, defect states, or localized energy levels introduced by rare-earth dopants, which can slightly decrease the effective bandgap and alter electron-hole dynamics. This steady bandgap narrowing is consistent with previous results on La-doped ZnO systems and can be attributed to the introduction of defect states and modifications to the electrical structure caused by La incorporation.

Tauc plots are derived from the data of UV-Vis spectra that give evidence of the bandgap decreasing trend, which is the effective point for any fabrication of photocatalyst. Tauc plots of undoped ZnONPs and 1%, 2%, and 3% La-ZnONPs are shown in Figure 6. According to the literature, ZnO exhibits bandgap energies ranging from 3.00 to 3.37 eV, depending on factors such as the synthesis method, morphology, and structure of the material [1, 26]. In this study, the findings indicated that the Eg energy for ZnONPs is lower than the values typically reported for bulk ZnONPs. Literature [19, 27, 28] shows that the value of Eg decreases due to the addition of dopants and increases the stability of nanoparticles.

This increase is attributed to suppression of crystallite growth, leading to smaller particle domains and more loosely packed aggregates, consistent with SEM and XRD observations. Thus, N₂ adsorption-desorption techniques were employed to examine further the BET-specific area and BJH pore size distribution of La-ZnONPs (Figure 7).

Display full size

Figure 7. N₂ adsorption-desorption isotherms for fabricated (a) 1% La-ZnONPs, (b) 2% La-ZnONPs, (c) 3% La-ZnONPs, and (d) undoped ZnONPs, with insets of pore structure depicted as (i) 1% La-ZnONPs, (ii) 2% La-ZnONPs, (iii) 3% La-ZnONPs, and (iv) undoped ZnONPs. P/P₀ represents the relative pressure of nitrogen adsorption. La-ZnONPs: lanthanum-doped zinc oxide nanoparticles.

The nitrogen adsorption-desorption isotherms of undoped and La-ZnONPs exhibit type IV isotherms with H3-type hysteresis loops, confirming their mesoporous nature. The undoped ZnO shows the highest adsorption capacity, indicating a well-developed mesoporous network. Doping with 1% La reduces the adsorbed quantity and narrows the hysteresis loop, suggesting pore blocking. At 2% La, there is a significant decrease in adsorption volume and a broader hysteresis loop, indicating structural distortion. Increasing the La concentration to 3% partially restores the adsorption volume and leads to a more uniform pore-size distribution. Overall, La doping alters the textural properties of ZnO, diminishing mesoporosity at low to moderate levels before stabilizing at higher concentrations. The values of undoped and La-ZnONPs (1%, 2%, and 3%) specific surface area are mentioned in Table 3. As a photocatalytic activity, the surface area played a major role in enhancing the performance of the photocatalyst. By this analysis, it is clearly evident that more surface area provides more active sites for photocatalytic activity, and even the polyphenols from pineapple included in the fabrication of La-ZnONPs assisted in increasing this surface area as compared to previously studies fabricated La-ZnONPs [26, 29].

Hypothesized photocatalytic mechanism by La-ZnONPs based on the literature

It is important to point out that no photocatalytic degradation activity was conducted in this study. However, the observed features after analysis, such as decrease in bandgap energy, increase in specific surface area, and variation in morphology of La-ZnONPs synthesized in the presence of PPE, are considered favorable characteristics for photocatalytic applications. In this context, we provide here a brief literature-based mechanism as a hypothesis for how such green La-doped ZnO systems may operate under light irradiation, to guide future work.

When La-ZnONPs absorbed photons from visible light sources, the semiconductor’s CB was activated by the electrons from the VB. Producing the electron-hole pairs (e^– and h⁺) that are displayed in Equation 2.

(2)$La - ZnO + hv \to La - ZnO(e^{-} + h^{+})$

The photogenerated holes (h⁺) can oxidize surface-adsorbed water or hydroxide ions to form hydroxyl radicals, while the electrons (e^–) reduce dissolved oxygen to superoxide radicals, shown in Equations 3 and 4.

(3)${h^{+} + H}_{2}O \to \cdot OH + H^{+}$

(4)$e^{-} + O_2 \to O_2{.}^{-}$

Reactive oxygen species participate in the oxidation and degradation of organic pollutants (R), as commonly reported for ZnO-based photocatalysts, shown in Equation 5.

(5)$\cdot OH, O_2{.}^{-} + R \to intermediate \to {CO}_2\uparrow + H_{2}O + mineralized products$

In the case of La-ZnONPs, La³⁺ acts as an electron trapper, resisting the recombination rate of e^– and h⁺ and enhancing the life span of electrons and radicals. This eventually gives more time to electrons and holes to react with water and generate abundant free radicals ($O_2{.}^{-}$ and $\cdot OH$), which subsequently degrade wastewater and different organic dyes. The mechanism of hypothesized photocatalytic activity of La-ZnONPs is illustrated in Figure 8.

Display full size

Figure 8. Literature-based hypothetical photocatalytic mechanism of La-ZnONPs. La denotes lanthanum dopant; hν represents photon energy. ZnONPs: zinc oxide nanoparticles; La-ZnONPs: La-doped ZnONPs.

This concept is consistent with the bandgap narrowing and increased surface area observed in this work, although it has not been experimentally verified here. The reaction scheme in Equations 1–4 should therefore be interpreted purely as a conceptual mechanism based on previously reported La-ZnO systems, serving as a starting point for future photocatalytic studies on the present materials.