Electrochemical and photocatalysis applications of green synthesized zirconia ceramics modified by cucumber seeds

Tabinda Afzal; Mahwish Bashir; Waqar Ahmad; Qaiser Ali Sultan; Sana Saeed; Farzana Majid; Adnan Saeed; Mohamed E. Assal; Mufsir Kuniyil; Baji Shaik

doi:10.25259/AJC_857_2025

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Original Article

:19;

8572025

doi:

10.25259/AJC_857_2025

Electrochemical and photocatalysis applications of green synthesized zirconia ceramics modified by cucumber seeds

Tabinda Afzal^a, Mahwish Bashir^a,*, Waqar Ahmad^b, Qaiser Ali Sultan^c, Sana Saeed^d, Farzana Majid^e, Adnan Saeed^a, Mohamed E. Assal^f, Mufsir Kuniyil^f, Baji Shaik^g

aDepartment of Physics, Government College Women University Sialkot, Sialkot, Punjab, Pakistan

bDepartment of Chemistry, University of Science and Technology Bannu, KPK, Pakistan.

cConformity Assessment/Standards Development Centre, Pakistan Standards and Quality Control Authority, Haripur, KPK, Pakistan

dDepartment of Chemical Engineering, NFC Institute of Engineering and Technology, Multan, Punjab, Pakistan

eDepartment of Physics, University of the Punjab, Quid e Azam Campus Lahore, Punjab, Pakistan

fDepartment of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia

gSchool of Chemical Engineering, Yeungnam University, Gyeongsan, Republic of Korea

*Corresponding author: E-mail address: mahwish.bashir@gcwus.edu.pk (M. Bashir)

Received: 2025-07-23, Accepted: 2025-10-01, Epub ahead of print: 2026-01-28, Published: 2026-03-02

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Abstract

Dye pollution in water sources presents a considerable risk to environmental and public health, requiring efficient cleanup solutions. Nanomaterial-based technologies have become viable ways to get rid of dye pollution because they are more selective, efficient, and long-lasting than older methods. This research investigates the synthesis of tetragonal zirconia (t-ZrO₂) nano ellipses reinforced with cucumber seeds via a rapid and economical sol-gel technique. Cucumber seeds are used to stabilize, reduce, and capping material. The effect of cucumber seed powder (ranging from 0.1 g to 0.5 g) on the phase stabilization and morphological characteristics of the resulting ZrO₂ nanostructures was systematically investigated using various analytical techniques (X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), UV-Vis, Raman). Notably, the use of cucumber seed powder facilitated the stabilization of the t-ZrO₂ phase without any subsequent heat treatment confirmed by XRD and RAMAN. Synthesized t-ZrO₂ employed for photocatalytic and electrochemical capabilities against crystal violet degradation and supercapacitor electrode applications. Field emission (FE)-SEM shows the formation of ZrO₂ nano ellipses (100-150 nm diameter, 200 nm length). The nano ellipses synthesized at 0.5 g demonstrated superior performance, achieving 92% degradation of Crystal Violet under UV irradiation and exhibiting a high specific capacitance of 21.17 F/g (at 3 mVs^-1), highlighting the potential of this sustainable synthesis route for high-performance applications.

Keywords

Cucumber seeds

Crystal violet

Photocatalytic activitySol-gel method

Super-capacitor

Zirconia

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1. Introduction

Zirconia, also known as zirconium dioxide (ZrO₂), exhibits several properties, such as heavy metal adsorption, chemical stability, durability, and suitability for membrane filtration, making it an excellent material for water purification [1]. Photocatalysis, a cutting-edge technology, is intended to harness light energy to trigger chemical reactions that break down pollutants in water, offering a promising solution for purification [2]. Among monoclinic, tetragonal, and cubic zirconia, the tetragonal phase provides more strength and efficient electrochemical properties [3]. The monoclinic phase is usually more stable and active at lower temperatures. The tetragonal phase is more active, but it needs stabilizers. An important area of research is figuring out how to control the synthesis to get a tetragonal, stable, and highly active phase, as well as a shape (such as nano ellipses or nanoflakes) that has a large surface area for reactions.

Another problem with zirconia is a wider bandgap, and it exhibits absorption only in the VB region. The overall efficiency is low since UV light makes up just a small part of the solar spectrum. Current research is centered on altering zirconia to enhance its capacity to collect and utilize a greater segment of the solar spectrum, particularly visible light [4].

Zirconia also has a rapid rate of recombination for electron-hole pairs that are created by light. This means that the excited electrons and holes quickly go back to their ground state instead of taking part in the redox reactions that are needed for photocatalysis. This makes it far less effective. Different dopants and organics have been introduced into the zirconia lattice to vary its band gap [5]. Here, we use cucumber seeds as additives to tailor the band gap, as organic additives create interstitial sites in the host atoms. Further, these organic additives enhance the dielectric properties by creating hinders sites and the capacitance of zirconia

Zirconia possesses a significantly negative conduction band potential, allowing it to produce intense oxidizing agents, such as hydroxyl radicals (•OH), which are particularly efficient in dissolving organic contaminants [6]. The carbon-based compounds (organics) have a bandgap that is substantially less than that of many common photocatalysts. This lets them absorb light in the wide spectrum, which makes them much more efficient than wide-bandgap materials for solar-powered uses. Carbon derived from cucumber seeds serves as an electron mediator when included in composites/with other semiconductors, such as ZrO₂. It can accept photogenerated electrons from the semiconductor, inhibiting the quick recombination of electron-hole pairs and thereby enhancing the overall photocatalytic efficacy. Flavonoids and tannins are two examples of biomolecules found in cucumber seeds that can act as natural doping agents [7]. They add heteroatoms (such as nitrogen and oxygen) to the carbon structure during synthesis. This doping is very important because it can make surface flaws and active sites, which are both important for improving photocatalytic activity.

Cucumber seeds possess a rich biomolecular profile that enhances their utility as green additives for nanomaterial synthesis. They contain high levels of polyphenols and flavonoids (2.14 ± 0.56 mg/g; quercetin, kaempferol, apigenin, orientin, vitexin), which act as strong reducing agents for Zr⁴⁺ ions. The substantial protein content (26.68%) provides amino acid residues serving as natural capping agents and morphological templates, favoring nano-elliptical structures. Additionally, cellulose and polysaccharides (32.27% crude fiber) contribute to hydrophobic surface coating and structural stabilization, while tannins and glycosides act as chelating agents that promote metal ion coordination and defect formation within the lattice [8]. In cucumber seed-mediated synthesis, polyphenols play a key role by reducing Zr⁴⁺ ions to lower oxidation states, thereby initiating nanoparticle formation. Proteins function as natural capping agents, preventing agglomeration and regulating particle size. Cellulose and related polysaccharides act as stabilizing agents by forming hydrophobic barriers that limit water absorption and enhance particle stability. Moreover, the organic matrix of biomolecules serves as a morphological template, directing the controlled growth of nano-elliptical structures.

Zirconia nanostructures have been synthesized using many techniques, e.g., the combustion method [9], co-precipitation [10], sol-gel [11], etc. This study utilized the sol-gel method due to its simplicity, versatility, and ability to control particle size. In the sol-gel method, first, a stable sol is formed by dissolving zirconium salt in a solvent. Aging and drying of this sol form the gel, and further drying removes OH ions to crystallize the zirconia [12]. Different researchers have employed many dopants and additives for the stabilization of tetragonal zirconia and reported their efficacy in water purification and energy storage. Huang et al. fabricated cauliflower-like Er:YAG/(Fe₃O₄@Y-ZrO₂) nanostructures and reported their photocatalytic degradation against methyl blue up to 91.4% [13]. Recently, Ghazal et al. synthesized eggshell-reinforced zirconia nanorods and studied their electrochemical properties. They reported specific capacitance around 10.7881 F/g at a constant scan rate of 3 mV/s [14]. Vinayagam et al. studied the effect of solar irradiation on the breakdown of methylene blue using zirconia nanoparticles and reported 89.11% degradation after 5 h [15]. Joseph et al. studied the effect of zirconia phases on supercapacitor applications. They reported that the amorphous phase provides specific capacitance around 160 F/g at 3 A/g [16]. Further literature study found that Jalil et al. achieved 83.6% decolorization of methylene blue using UV-stabilized t-ZrO₂, while others synthesized pure t-ZrO₂ with various agents (e.g., honey, gelatin) using microwave and sol-gel methods, yielding crystallite sizes from 5 to 26 nm under varying conditions [17-23].

However, there is no study that has used cucumber seeds as a carbon source for photocatalysis and supercapacitor applications. Our study prepared novel cucumber seed-additives-based zirconia nanostructures and utilized them for water purification and energy storage applications. To explore the influence of the cucumber seed content on morphology and crystal structure, varied weights of the seeds (0.1 g, 0.2 g, 0.3 g, 0.4 g, and 0.5 g) were used. Different structural and morphological analyses like scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman, and photoluminescence (PL), etc., demonstrated the variations with cucumber seed content. XRD, Fourier transform infrared (FTIR), and Raman analysis showed the presence of pure phase t-ZrO₂. The band gap calculated using the Tauc plot showed the distinct effect of organic additives with a decreasing trend. By exploring their morphology, nano ellipses were observed, which give excellent photocatalytic (∼92% degradation of Crystal Violet) and electrochemical performance (Specific capacitance 21.17 F/g).

2. Materials and Methods

2.1. Experimental details

Zirconia nanostructures were synthesized by Zirconium oxychloride octa-hydrate (ZrOCl₂.8H₂O) (Sigma Aldrich) and de-ionized (DI). The organic additives used here are cucumber seeds.

2.1.1. Cucumber seed powder

Cucumber seeds were washed with DI water and roasted over a low flame. After roasting, they were dried in an oven at 50-60 ˚C and ground using a mortar and pestle. The powder of cucumber seeds was sieved twice to obtain a fine powder. Cucumber seeds were used as organic additives as they contain cellulose. Cellulose is a good source of carbon. Carbon obtained from this cellulose shows excellent electrochemical properties.

2.1.2. Synthesis of zirconia

For this, a 0.1 M zirconia oxychloride stock solution was prepared in DI water. To obtain a transparent and homogenous sol stock solution was stirred for 30 min at room temperature. When zirconium precursor dissolves in DI water following reaction occurs: (reaction 1) [18].

(1)

Z r O C l_{2} .8 H_{2} O \to Z r {(O H)}_{4} + 2 H C l + 5 H_{2} O

As reported earlier [24], formation of tetragonal zirconia is followed by an amorphous phase (metastable phase). Zr(OH)₄ is known as a metastable phase. HCl is formed as by byproduct and evaporates at room temperature. This evaporation is also very important, as described by early researchers [25]. Then, 0.1, 0.2, 0.3, 0.4, and 0.5 g of cucumber seed powder were added per 100 mL of sol. During synthesis, cucumber seeds attached to zirconium.

The biogenic synthesis of zirconia utilizing cucumber seeds can be delineated as a three-phase process:

Cucumber seeds are included in the precursor solution. The extract’s phytochemicals, especially the polyphenols, promptly interact with the zirconium ions (Zr⁴⁺) in the solution.

This constitutes the fundamental interaction mechanism. The biomolecules in the solution decrease Zr⁴⁺ ions, resulting in the creation of elemental zirconium. This state is exceedingly unstable, and upon exposure to water, zirconium promptly oxidizes to produce zirconia (ZrO₂) nuclei.

As nuclei develop, additional biomolecules (proteins, carbs, etc.) adhere to their surface. This capping inhibits additional growth and agglomeration, leading to the production of stable, monodisperse nanostructures. The ultimate size and morphology of the nanostructures are directly affected by the number and types of biomolecules present, together with reaction factors such as temperature and pH.

The utilization of cucumber seeds powder, rather than a specific isolated phytochemical, engenders a synergistic impact. The intricate combination of biomolecules in the seed collaborates to establish a highly efficient and effective mechanism for nanoparticle formation. The fibrous and cellular architecture of the seed may function as a physical template, directing the production of nanoparticles into certain, intended shapes. This intricate interaction renders this strategy innovative and deserving of additional exploration, as it may facilitate the production of zirconia nanoparticles with distinctive features unattainable using traditional chemical procedures [26].

The crystal structure is distorted by this metallic interaction, changing from monoclinic to tetragonal. There is reduced possibility of hydrostatic stresses when organic materials are present on the particle surface. Particle surfaces and rejected water molecules may be coated by them. Water molecule adsorption or absorption mostly results in phase purity instability or the conversion of tetragonal zirconia to monoclinic zirconia. No further heating was required for the synthesis of these nanoparticles. The interaction between water and zirconia may be expressed using the Kroger-Vink equation because high temperatures cause water to be absorbed by zirconia when the cucumber seed layer is absent, which fills oxygen vacancies [27]. A larger fraction of monoclinic zirconia results from this occupation of oxygen vacancy states, as demonstrated by Chevlier’s model [28] (reaction 2);

(2)

H_{2} O + V_{0}^{\cdot \cdot} + O_{0}^{x} \overset{Z r O_{2}}{\leftrightarrow} 2 O H_{O}^{\cdot}

At room temperature, the above solution was stirred. In the meantime, to obtain pH 9, 2M NaOH was added dropwise. Tetragonal zirconia is formed by optimizing the pH of the sol as described earlier [29]. Denkewicz et al. [30] designed a model to produce crystalline zirconia at various pH ranges. According to this, Zr⁺ and OH^- ions are primarily responsible for zirconia crystallization. Higher concentrations of OH^- ions often produce tetragonal zirconia. Phase composition and particle size are significantly affected by the addition of mineralizers, such as NaOH and NH₃. Through the crystallization process, the effects of pH and mineralization may be understood. Tetramer complex [Zr₄(OH)₈(H₂O)₁₆]⁸⁺ is produced by dissolving zirconium salt in DI water. Eight hydroxo bridges (OH) and 16 neighboring water molecules make up this tetramer [31]. According to the following reaction (3), the sol would release specific numbers of H⁺ ions from the nearby water when its pH shifted to a basic tetramer.

(3)

{(Z r_{4} {(O H)}_{8} {(H_{2} O)}_{16})}^{8 +} \to {(Z r_{4} {(O H)}_{8 + X} {(H_{2} O)}_{16 - X})}^{(8 - X)} + X H^{+}

According to the above equation, zirconia sol has shortage of H⁺ or a greater concentration of OH^- ions. According to Liu et al. [32], Na⁺ (derived from NaOH) also aids in the development of tetragonal zirconia by integrating into surface voids.

Synthesized sols were dried at 50-60°C. These synthesized powders were washed thoroughly with DI to remove byproducts. Reactions (4-6) explain the growth mechanism and formation of tetragonal zirconia using cucumber seeds.

(4)

Z r {(O H)}_{4} + 7 H_{2} O + 2 H C l {[Z r {(O H)}_{4}]}_{s o l} + 7 H_{2} O

(5)

{(Z r {(O H)}_{4})}_{s o l} + 7 H_{2} O \overset{S t i r r i n g + C u c u m b e r s e e d s}{\to} Z r^{4 +} + (4 O H^{-}) + 7 H_{2} O

When dried

(6)

Z r^{4 +} + O^{2 -} \to Z r O_{2}

2.3. Characterizations

The XRD pattern was obtained by a Bruker D8 Advance diffractometer CuKα, with wavelength 1.540 Ǻ. The scanning step was 1°/min in a 2θ range from 20 to 70°. Field emission scanning electron microscopy (FE-SEM, Nova Nano SEM) was utilized to study the microstructure of cucumber seeds added with zirconia powder. Using a JASCO V-750 spectrophotometer, UV-vis DRS were measured with BaSO₄ as the reference material. The JASCO FP-8300 spectrofluorimeter was utilized to analyze the PL properties using an excitation source. The apparatus was run at a pressure of 6×10^-9 Torr in the analytical chamber. The Raman spectra of cucumber-added zirconia were recorded at room temperature using a Horiba Mini Raman Spectrometer in the range of 100 to 800 cm^-1.

2.3.1. Photocatalytic activity

The degradation of crystal violet was done by the photo-catalytic activity of the ZrO₂ nanostructures. The initial dye dose was used as 20 ppm (20 mg crystal violet in 1000 ml water) while zirconia nanostructures were taken at different doses, 0.75 g/L, 0.60 g/L, and 0.45 g/. These different concentrations of photocatalyst dose were dissolved in dye solution and irradiated for different time intervals (10 min) up to 90 min. Similarly, in the second step, the photocatalyst concentration was fixed at 0.75 mg/L and the dye concentration was varied as 20 mg/L, 40/mg/L, and 60 mg/L. The lamp emitted mainly at 365 nm with an intensity of 4.24 × 10⁷ Einstein L⁻¹s⁻¹ and 7.2 W m⁻². The outer jacket of the reactor was constantly cooled with water at 25°C to keep the reaction temperature constant during the photocatalytic process.

Following Eq. (7) was used to measure the degradation percentage.

(7)

Degradation rate (%) = \frac{C_{0} - C_{t}}{C_{t}} \times 100

2.3.2. Electrochemical study

The CV of zirconia powders was done by the CHI660E electrochemical analyzer. The frequency range used for CV analysis was 1 MHz to 500 Hz, with a starting voltage of 0.2V. CV is a three-electrode setup, where zirconia nanostructures are used as an active material; 0.05 mg are dissolved in 1 M KOH solution. Hg/HgO was used reference electrode with a platinum wire as the counter electrode.

3. Results and Discussion

3.1. Structural analysis

XRD graphs of cucumber seeds added ZrO₂ have been shown in Figure 1. The scanning range for 2θ for cucumber seeds added zirconia across concentrations (0.1 g, 0.2 g, 0.3 g, 0.4 g, and 0.5 g) was 20° to 70°. All the samples show a crystalline nature.

XRD cucumber-seed-added zirconia synthesized at (a) 0.1 g, (b) 0.2 g, (c) 0.3 g, (d) 0.4 g, and (e) 0.5 g.

The peaks at 30.34°, 35.73°, 44.3°, 53.9°, and 62.59° correspond to the (101), (110), (102), (201), and (202) planes. The presence of a peak at 30.35° shows the phase purity of tetragonal zirconia (t-ZrO₂). All observed peaks are consistent with the JCPDS card no. 79-1771 and literature [19]. The formation of t-ZrO₂ even at room temperature is attributed to the fact that the organic additive plays a vital role. The organic additive coats the surface of zirconia nanostructures and stops agglomeration, which hinders the transformation of tetragonal zirconia to monoclinic zirconia. At lower cucumber seed content (0.1 g), traces of monoclinic zirconia m(012) were observed. As the cucumber seed content increases from 0.2 g to 0.5 g, monoclinic content completely vanished, and the strengthening of tetragonal zirconia was observed. Further, the broadness of the peaks was observed with cucumber seed content, which is proof of the t-ZrO₂ phase.

Organic components from cucumber seeds typically assist in keeping the t-ZrO₂ phase stable at lower temperatures or even at room temperature. The surface energy effects caused by the capping agent stop the change to the stable tetragonal phase.

The organic additives can influence the kinetics of the solid-state transformations, potentially allowing the desired crystalline phase to form at a lower temperature than is required by traditional chemical methods.

Lattice parameters for zirconia nanostructures were calculated and have been tabulated in Table 1.

Table 1. XRD parameters of cucumber seeds added zirconia.

Cucumber seeds (g)	Lattice constant (A^o)		Crystallite size (D²) Nm	Dislocation density × 10¹⁵	Unit cell volume a² × c (A^o)³	X-ray density (g/cm³)	Porosity (%)
Cucumber seeds (g)	a	c	Crystallite size (D²) Nm	Dislocation density × 10¹⁵	Unit cell volume a² × c (A^o)³	X-ray density (g/cm³)	Porosity (%)
0.1	3.612	5.241	18	3.09	68.36	5.79	5.08
0.2	3.608	5.232	17.4	3.30	68.1	5.82	4.59
0.3	3.601	5.212	16.2	3.81	67.58	5.85	4.09
0.4	3.591	5.210	15.1	4.39	67.18	5.91	3.11
0.5	3.586	5.220	14	5.10	67.12	5.94	2.62

The following Eqs. (8-12) [33] was used to calculate crystallite size, dislocation density, unit cell volume, X-ray density, and porosity of the synthesized samples.

(8)

D = \frac{K λ}{B Cos θ}

The crystallite size was observed to vary from 14-18 nm with the cucumber seed content, as shown in Table 1. The value of crystallite size is consistent with literature for the existence of pure phase t-ZrO₂ [34]. Smaller crystallites were observed due to the presence of an organic additive, which tends to coat the surface of the zirconia nanostructure and prevent Ostwald ripening [35]. A slight decrement in crystallite size has been observed with cucumber seeds, which is the transformation to t-ZrO₂ phase, as observed in Figure 1. In XRD data broader characteristic peak has been observed, which is attributed to the t-ZrO₂ as reported in literature. Higher content of cucumber seeds stops the emergence of nuclei and, therefore, slightly decreases the crystallite size. This smaller crystallite size is beneficial for photocatalytic activity by two ways (1) A smaller crystallite size means that a given amount of material has a bigger specific surface area. The zirconia nanostructures have a larger surface area, which provides more active/surface sites. More surface sites mean more chances for contaminants to break down through photocatalysis. (2) When light hits a semiconductor photocatalyst like zirconia, it moves an electron from the valence band to the conduction band, making a “hole” in the valence band that is positively charged. This combination of electrons and holes is a strong oxidizing and reducing agent. But if the electron and hole come together too rapidly, the photocatalytic efficiency drops. A smaller crystallite size makes it easier for charge carriers to get to the particle surface by shortening the distance they have to travel. This lowers the risk of recombination and makes the photocatalyst more efficient [36].

Following Eq. (9) is used to calculate the dislocation density. Very few dislocation lines were observed with cumber seeds, which is important for energy storage applications [33].

(9)

D i s l o c a t i o n d e n s i t y = \frac{1}{D^{2}}

The tetragonal phase of zirconia has a smaller unit cell volume as compared to other phases. Unit cell volume of zirconia nanostructures was calculated by using Eq. (10) [33] and presented in Table 1.

(10)

V_{T e t r a g o n a l} = a^{2} c

Where,

a, c are lattice parameters

Table 1 represents a slight decrease in unit cell volume with cucumber seeds. Again, this decrement is attributed to the tetragonal phase strengthening. Here, cucumber seeds work as a capping agent and water repellent, preventing the absorption of OH ions on the surface of the zirconia nanostructures. This controlled synthesis and absence of OH ions lead to shrinkage in unit cell volume. The presence of water molecules may stabilize the t-ZrO₂. X-ray density of zirconia nanostructures was calculated by using the following Eq. (11) [33].

(11)

ρ = \frac{1.66042 Σ A}{V}

Where

A = Sum of all atoms’ atomic weights in a unit cell.

The value of X-ray density ranges from 5.79 to 5.94 g/cm³ has been observed in cucumber seeds with added zirconia nanostructures. X-ray density was also used to calculate the porosity of the samples using Eq. (12) [33].

(12)

P o r o s i t y (%) = 1 - \frac{ρ_{o b s e r v e d}}{ρ_{T h e o r e t i c a l}} \times 100

3.2. Morphology and EDX spectra

Figures 2(a-f) represent the FE-SEM images of cucumber seeds added zirconia powders synthesized at 0.2 g of cucumber seeds content, Figures 2(a-c) and 0.5 g Figures 2(e, f). Figures revealed the formation of nano ellipses covering a large surface area. At a relatively low content of cucumber seeds slight agglomeration of nano ellipses has been observed. The diameter of nano ellipses is around 150-180 nm, Figure 2(g), and the length is in the range of 200-250 nm. The diameter and length of nano ellipses decrease with the cucumber seed content. It can be seen from Figures 2(e, f) significant decrease in agglomeration has been observed. Nano ellipses with well-defined boundaries are observed. The diameter of nano ellipses at 0.5 g content of cucumber seeds is ∼100-150 nm, Figure 2(h), and the length is around 200 nm. Further FE-SEM images confirm the role of cucumber seeds in defining and stabilizing zirconia structures, as for the formation of tetragonal zirconia, well-defined and smaller-sized nanostructures are mandatory. The shape of the nanostructures is also very important. Unique shapes, like those of flowers or ellipses, can make the facets and pore structures more reactive, which increases the surface area and active sites even more. The elongated, elliptical shape possesses a greater specific surface area than spherical particles of equivalent volume. An expanded surface area offers increased active sites for the adsorption of contaminant compounds. The augmented interaction between the catalyst and the contaminants results in an accelerated degradation rate. The proteins present in the cucumber seed act as templates for the formation of different nanostructures. Figure 2(d) represents the large area growth and dense formation of nano ellipses as observed in XRD.

FE-SEM images of cucumber seeds added zirconia at (a) 0.2 g, (b-c) Magnified view at 0.2 g, (d) 0.5 g, (e-f) Magnified view at 0.5 g. Circled regions show the specifically formation of nano ellipses, and (g-h) Diameter of nano ellipses at 0.2 g and 0.5 g, respectively.

The proteins and amino acids in the seed extract function as reducing agents and capping agents [37]. Amino acids, especially those possessing sulfur-containing groups (such as cysteine) or carboxyl and hydroxyl groups, can interact with zirconium ions. The protein’s backbone can then encase the newly synthesized nanostructures, inhibiting their agglomeration. The “capping” effect is essential for regulating the dimensions and morphology of the nano ellipses, resulting a consistent distribution [38]. Elliptical forms can scatter or absorb light more efficiently than spherical particles, especially in the UV spectrum, which is crucial for the photocatalytic properties of zirconia. The enhanced light usage results in increased overall photocatalytic efficiency.

Figures 3(a,b) represent the EDX spectra at 0.2 g and 0.5 g, respectively. These nano ellipses are formed using cucumber seeds as an additive, a rich source of cellulose. Therefore, the chemical composition at 0.2 g and 0.5 g contains carbon. The spectra of the composition of the zirconia nano ellipses confirm the presence of zirconium and oxygen >90%, which is attributed to the purity of the synthesized nano ellipses.

EDX spectra of cucumber seeds added zirconia at (a) 0.2 g and (b) 0.5 g.

Polysaccharides and simple carbohydrates, including glucose and fructose, may facilitate the reduction of zirconium ions [18]. Their aldehyde groups (in the linear form) can be converted to carboxyl groups while concurrently decreasing the metal ions. Moreover, these molecules function as stabilizing agents by enveloping the nano ellipses’ surfaces via electrostatic or steric interactions, thereby inhibiting aggregation.

3.3. Zeta potential and XPS

A major hurdle for using metal oxide nanostructures in electrochemical, catalytic, and toxicological studies is their tendency to become unstable in water. This instability often leads to agglomeration, reducing their effective surface area and performance. Toxicological studies and applications like water purification and supercapacitors demand highly dispersible nanomaterials to ensure accurate results and efficient operation. Stable dispersions create better interfaces, which are essential for high efficiency. Zeta potential is used to assess the stability of the nanostructures in water. A zeta potential range of +30 mV to -30 mV is typically considered a threshold for reasonable stability. The study reports a very high zeta potential value (∼103.69 mV) for the ZrO₂ nano ellipses in distilled water, when illuminated by 1,064 nm laser light Figures 4(a, b). The high zeta potential suggests that the synthesized ZrO₂ nano ellipses possess excellent stability in water, which is a significant advantage. This stability is likely to translate into improved electrochemical and catalytic performance, as well as enhanced efficiency in applications like water purification and supercapacitors. The use of cucumber seeds points towards a more environmentally friendly approach to nanomaterial synthesis. This high value indicates a highly stable dispersion with minimal agglomeration.

Zeta potential of cucumber seeds added zirconia at (a) 0.2 g and (b) 0.5 g. XPS spectrum of zirconia for 0.5 g of cucumber seed content (c) Zr3d and (d) O1s.

X-ray photoelectron spectroscopy (XPS) was utilized to examine the compositional constituents and chemical states of the ZrO₂ nanostructure in the sample, which was prepared with 0.5 g of cucumber seed content. Figures 4(c, d) illustrate the fitting of the XPS curve for the ZrO₂ surface. The spectrum illustrated in Figure 4(c) has significant peaks at 184.18 and 184.03, corresponding to Zr3d_5/2 and Zr3d_3/2 linked to the Zr-O bond [39]. Furthermore, the O 1s spectra depicted in Figure 4(d) exhibit a large range from 528 to 537 eV. Upon deconvolution, it further separates into two distinct peaks at 530.10 eV and 535.32 eV. Figure 4(d) indicates that the binding energy peak at 530.10 eV was ascribed in literature to the high binding energy component resulting from the progressive loss of oxygen or the formation of oxygen vacancies. The moderate and elevated energy peak at 535.03 eV is ascribed to chemically adsorbed oxygen species on the surface, OOH [40]. The intensity signifies a substantial increase in the number of oxygen vacancies on the sample’s surface.

3.4. Raman, PL UV-Vis and FTIR analysis

Figure 5(a) depicts the Raman spectra of cucumber seeds added with zirconia nano ellipses. Generally, RAMAN spectra are associated with the lattice vibrations. Normally, zirconia vibrations exist in the range of 100 to 800 cm^-1. Although in Figure 5(a), a distinct band at 148 cm^-1 has been observed is characteristic of tetragonal zirconia [17]. Another wide and weak band at 333 cm^-1 also corresponds to the tetragonal phase of zirconia [41]. Tetragonal zirconia contains six basic modes (1A1g + 3Eg + 2B1g). Here, the E mode has two dimensions, whereas, other has one dimension. Figure 5(a) confirms the phase purity of tetragonal zirconia as observed in Figure 2. The intensity of the characteristic band is notably increasing with the cucumber seed content, which further confirms the strengthening of t-ZrO₂ and is consistent with XRD data

(a) Raman, (b) PL, (c) PL at various excitation wavelengths (d) UV-Vis and (e) FTIR spectra of cucumber seeds added zirconia synthesized at various content (1) 0.1 g, (2) 0.2 g, (3) 0.3 g, (4) 0.4 g, (5) 0.5 g and (6) Cucumber seeds.

Figure 5(b) shows the PL properties of zirconia. The wider and weak peak at 608 nm is caused by stimulation in the UV band (353 nm). Interestingly, the excitation source (353 nm) does not affect the emission spectra (608 nm). Oxygen vacancies in the structure may be the cause of the variation in the emission from ultraviolet to optical, which results in non-radiative transitions [42]. Oxygen vacancies act as trapped electrons and influence ZrO₂ luminescence characteristics. Peak appears at 608 nm, and becomes more intense as the cucumber seed content increases from 0.1 g to 0.5 g, as seen in Figure 5(b). Tetragonal zirconia is characterized by magnetic dipole (⁵D_o→ ⁷F₁) and electric dipole (⁵D_o→ ⁷F₂) transitions, which are represented by the peaks at 594 and 608 nm. The Raman and XRD spectra confirm that the tetragonal phase is stabilized and strengthened by the increase in cucumber seed content. A few defects in ZrO₂ are indicated by the peak centered at 710 nm, which is less noticeable at 0.5 g content. The minor errors show that electronic radiative transitions account for the majority of the contribution. Adding an organic additive to the zirconia may settle down the tetragonal phase, but it can also cause defects, change the lattice properties, and create oxygen vacancies and strain. Furthermore, organic additives have the potential to change the strain, size, and lattice parameters of the nanostructure, which might change the luminescence characteristics of cucumber seeds added with zirconia.

The emission spectra of zirconia were obtained at several excitation wavelengths ranging from 230 to 353 nm (Figure 5c). The synthesized zirconia nano ellipses display a singular emission maximum when stimulated within the range of 200-345 nm. The XPS results indicate that nanostructures exhibited reduced oxygen content, potentially linked to defect states, particularly oxygen vacancies arising from oxygen deficiency in zirconia.

The absorption spectra of zirconia nano ellipses produced at various cucumber seed contents have been displayed in Figure 5(d). The charts show the sharp edge progressively as the cucumber seed content increases from 0.1 g to 0.5 g. There is just a single tetragonal phase in the synthesized sample, as indicated by this sharp edge, especially at 0.5 g. The Tauc relation is used to measure bandgap [17-23] (reaction 13).

(13)

{(α h ν)}^{n} = A (h ν - E g)

Here, hv is the energy of the photon, E_g is the bandgap energy, and α is the absorption coefficient. For nano ellipses synthesized at 0.1 g, absorbance peaks have been seen at 249 nm. The band gaps calculated by using the Tauc relation are 4.97 eV. A slight blue shift in the absorbance peak has been observed with cucumber seed content. When the cucumber seed content increases to 0.5 g, the band gap energy tends to decrease. The lowest band gap energy is achieved at 0.5 g.

The decrease in the value of Eg in t-ZrO₂ samples based on chemical additions could be due to the interstitials or oxygen vacancies introducing more defects in the ZrO₂ lattice. Researchers have shown that oxygen vacancies are the main reason why ZrO₂ nanostructures have more photocatalytic activity. It is generally known that defects and vacancies in semiconductors and insulators can create localized states around the edge of the conduction band. The band gap has distinct energy levels that characterize the disturbances caused by faults and impurities.

Figure 5(e) displays the functional group analysis of zirconia nano ellipses produced using cucumber seeds as an organic additive. The distinct peaks in the FTIR spectrum show that the produced nano ellipses have different functional groups and linkages. A broad peak at 3630 cm-¹ is due to O–H stretching vibrations, which means that hydroxyl groups and water molecules are stuck to the surface of the nano ellipses. The signal at 1630 cm⁻¹ corresponds to the H–O–H bending vibrations, which confirms that molecular water is present [43]. The two strong bands at 2920 and 2856 correspond to primary alcohol and amine salt, respectively. These bands are present in cucumber seeds spectrum Figure 5(e), and therefore get stronger as cucumber seeds content increases in the zirconia matrix. The peak at 1400 cm⁻¹ is caused by C–H stretching vibrations from aromatic amines, which generally show up between 1300 cm⁻¹ and 1400 cm⁻¹. This peak can also be explained by the C–N bending vibrations of methyl groups, which are usually seen at about 1260 cm⁻¹. This means that the organic functional groups from cucumber stabilize the zirconia. Adding an organic ingredient during synthesis will show bands in the carbon area at 1000–1200 cm^-1. The peak at 1050 cm⁻¹ is due to the stretching vibrations of C–C and C–O bonds in the organic compounds and carbohydrates found in the cucumber seeds. The peak at 670 cm⁻¹ is attributed to Zr–O vibrations, which shows that t-ZrO₂ phase has formed [44]. Organic functional groups confirm that cucumber seeds are vital for changing and stabilizing the zirconia nanostructures.

3.5. Photo-catalytic activity (PCA)

The photo-catalytic activity (PCA) of cucumber seeds added zirconia was evaluated for the aqueous solution of crystal violet under UV-light irradiation. For this, 0.75 g/L, 0.60 g/L, and 0.45 g/L of the photocatalyst were dissolved in the dye solution (20 ppm), and the mixture was subjected to agitation for 1 h to ensure homogeneity of the solution prior to the exposure to UV. The solution was irradiated by a UV light lamp for 90 min.

3.5.1. Blank test

In order to study the effect of UV light, and blank test (without zirconia dose) was studied. For this, 20 mg/L crystal violet was prepared and irradiated with a UV lamp for different time intervals. Only 3.25% degradation was observed, which was not accounted for. Blank test confirms the importance and necessity of the catalyst.

3.5.2. Effect of zirconia dose

Various zirconia doses (0.75, 0.6, and 0.45 g/L) were utilized to study the effect of a catalyst on the photo-induced degradation of crystal violet Figures 6(a-c). A gradual decrease in photocatalytic activity was observed with decreasing catalyst dose. This decrease in photo degradation is attributed to the presence of more active sites with a higher catalyst dose [45]. At 0.75 g/L, an optimum dose of catalyst is present to degrade the dye up to ∼92% [46].

Photocatalytic activity of cucumber seeds added zirconia at catalyst doses (a) 0.75 g/L, (b) 0.6 g/L, and (c) 0.45 g/L, respectively, (d) % degradation of crystal violet dark absorption and desorption equilibrium at various catalyst doses.

Figure 6(d) presents the graphical comparison of degradation for various catalyst doses.

3.5.3. Effect of dye concentration

This study also discusses the effect of dye concentration (20, 40, 60 mg/L) with a fixed amount of catalyst doses (0.8 g/L), as presented in Figures 7(a-c). The results presented the maximum degradation at 20 mg/L of dye concentration. At higher dye concentrations, a distinct decrease in photo degradation was observed, which is due to the fact that at higher dye concentrations, most of the light is absorbed by the dye instead of the catalyst, thus reducing efficiency [47].

Photocatalytic activity of cucumber seeds added zirconia at dye concentration (a) 20 mg/L, (b) 40 mg/L, and (c) 60 mg/L, respectively, (d) % degradation of crystal violet and dark absorption and desorption equilibrium at various dye concentrations.

Figure 7(d) shows the graphical comparison of degradation and decrement at various dye concentrations.

3.5.4. Kinetic study and stability

Kinetic study of cucumber seeds added zirconia nano ellipses at different catalyst and dye concentrations was also carried out.

The Langmuir-Hinshelwood Kinetic model presented in Eq. (14) was used to evaluate the kinetic study and plotted in Figures 8(a, b) [17].

(14)

l n (C_{o} / C) = k t

Kinetic study of cucumber seeds additive zirconia nano ellipses at (a) catalyst dose and (b) dye concentration.

Figures 8(a,b) represent the linear behavior of $l n (C_{o} / C)$ vs irradiation time for different catalyst doses and dye concentrations. It is clear from the figures that crystal violet follows the pseudo-first-order rate constant. Table 2 summarizes the value of R² and the rate constant “K.” The lowest value of R² ∼ 0.9181 at a higher dye concentration has been achieved. As all values greater than 0.9181 confirm the fitness of the kinetic model.

Table 2. The extracted parameters from the plots.

Variables	Conc.	R²	K (min^-1)
Catalyst (g/L)	0.75	0.997	0.0281
	0.60	0.976	0.0195
	0.45	0.978	0.0237
Dye (mg/L)	20	0.997	0.0281
	40	0.968	0.0174
	60	0.918	0.0156

Table 3 presents the comparative study of zirconia with previous studies and shows the efficiency of these synthesized nano ellipses.

Table 3. Comparison of zirconia nanostructures with the literature against dye degradation.

Catalyst	Concentration of dye	Degradation %	Degradation time (min)	Specific capacitance (F/g)	Ref.
Gadolinium zirconium oxide	Crystal violet	90	60		[58]
Zirconia	Methylene blue	83.6	240		[21]
Zirconia	Acid blue 25	65	75		[59]
t-zirconia	Methylene blue	35	60		[50]
ZrO₂			95		[60]
Zirconia	5			120	[61]
ZrO₂ –carbon black				43.2	[62]
CNFs-Sn-ZrO₂				102.37	[63]
CNFs-ZrO₂				53.69	[63]
Ultrafine nano ZrO₂				95	[60]
t-ZO₂	Crystal violet	92	90	21.7	Present study

3.5.5. TOC study

The total organic carbon (TOC) was measured in terms of the percentage degradation of crystal violet and plotted in Figure 9. A gradual increase in TOC was observed directly proportional to irradiation time, and almost 80% of degradation after 90 min of irradiation has been observed. It can be depicted from Figure 9 that during photo degradation, first intermediate degradation products are formed and then mineralization of these products occurs.

Total organic carbon analysis of cucumber seeds added zirconia nano ellipses.

3.5.6. Photocatalytic activity mechanism

The mechanism involved in the degradation of crystal violet has been explained below. Compared to zirconia nano ellipses samples without light irradiation, the active sites during excitation are much higher when zirconia nano ellipses are present. Electrons from the valence band were excited to the conduction band during the photocatalysis reaction on the photocatalyst material. Additionally, the valence band generated an equal number of holes. Subsequently, the trapped and conduction-band electrons moved together to the photocatalyst’s surface, where they were trapped by the oxygen vacancies [48-50]. Superoxide radicals (O2^−•) were created as a result of the oxygen vacancies trapping the O₂ molecules. The produced superoxide and hydroxyl radicals are extremely reactive and non-selective oxidants. They assault the organic Crystal Violet molecules, resulting in their degradation. In the meantime, the oxygen molecules in the crystal violet dye solution reacted with the oxygen vacancies on the surface and transformed into superoxide radicals. Similarly, hydroxyl radicals (OH^•−) formed when holes were trapped by water (H₂O) molecules or OH⁻ groups. They target the intricate organic framework of Crystal Violet, cleaving C-C and C-N bonds and triggering a sequence of chain events that ultimately result in the mineralization of the dye into benign inorganic substances such as carbon dioxide (CO₂), water (H₂O), and basic inorganic ions. Finally, the target dye is efficiently degraded by the produced radicals’ interaction with the pollutants.

3.6. Electrochemical analysis for the application of supercapacitors

A cyclic voltammogram of each sample’s electrochemical performance as measured by a potentiostat electrochemical testing station has been displayed in Figures 10(a-e).

Cyclic voltammogram of cucumber seeds added zirconia nano ellipses at(a) 0.1 g, (b) 0.2 g, (c) 0.3 g, (d) 0.4 g, and (e) 0.5 g, (f) specific capacitance of zirconia nano ellipses, and (g) Relation between crystallite size and specific capacitance.

At varying scan rates, rectangular curves were seen, which is normal behavior. An ideal capacitor should have a perfect rectangular CV curve because the current (I) is directly proportional to the capacitance (C) and the constant scan rate. The zirconia with cucumber seeds CV curve had clear rectangular shapes even at high scan rates (20 mV/s), which meant it was very efficient at high rates. The charging and discharging curves that were almost symmetrical showed better capacitive performance. Both an oxidation and a reduction peak were seen in the cucumber seeds content at 0.1 g, with oxidation being a bit lower than reduction. Significant intensities of both oxidation and reduction were seen at 0.2 g. Under 0.3 g content, oxidation decreased, but reduction intensified. Reduction and oxidation were both significantly enhanced at 0.4 g. Ultimately, reduction increased and oxidation declined at 0.5 g. Carbonization and activation of cucumber seeds transform the organic components, including cellulose, into a porous carbon material. This ultimate carbon material is exceptionally advantageous for electrochemical applications such as supercapacitors and batteries.

Carbon functions as an electrical conductor. The extensive surface area enables rapid electron transit across the electrode material. The carbon framework offers mechanical and structural reinforcement to the active material (such as ZrO₂ or other storage substances), alleviating volumetric fluctuations during successive charge/discharge cycles and enhancing cyclability. The study showed that the loop’s expanded area showed that there were more active sites for charge storage, which improved its electrochemical performance [51-53].

The following formulas (Eqs. 15,16) were used to calculate the electrode’s specific capacitance (Cp, F/g) [54]:

(15)

C p = A / (2 \times K \times m \times Δ V)

(16)

C p = (I Δ t) / (m Δ V)

In this case, the CV curve’s area is represented by A, the scan rate by K, the active or loaded mass by m, the potential window by ΔV, the discharge current by I, and the time difference during discharge by Δt. The specific capacitance values obtained were 13.55, 18.79, 20.55, 20.7, and 21.17 F/g at a constant scan rate of 3 mV/s, as illustrated in Figure 10(f). The data indicate an increase in specific capacitance with increasing cucumber seed content at a scan rate of 3 mV/s.

The electrochemical properties of zirconia are also affected by its surface-to-volume ratio and the size of its crystallites, as shown in Figure 10(g). As discussed earlier, smaller crystallites have greater surface area, which means there are more places for electrochemical reactions to happen [55,56]. A nanostructure with a larger surface area and porosity (to some extent) makes it easier for ions and molecules to move to and from the electrode surface [36,57]. This is very important for batteries and supercapacitors, which need ions to travel quickly to charge and discharge quickly. The smaller particles make pores that are related to each other. These pores can act as conduits for this transfer.

4. Conclusions

The formation of tetragonal zirconia nano-ellipses was achieved through a facile synthesis method employing cucumber seed powder as a stabilizing agent. The effect of varying the precursor concentration (0.1-0.5 g of seed powder) on the resulting ZrO₂ nanostructures was systematically investigated in terms of crystallinity, morphology, and electrochemical performance. Structural analysis revealed the presence of phase-pure t-ZrO2 up to 0.5 g of cucumber seed content. Crystallite size showed a decreasing trend, which is beneficial for photocatalytic and supercapacitor applications due to the large surface area. Morphological characterization revealed the formation of nano ellipses with diameters ranging from 100 to 150 nm and a length of 200 nm. Raman, FTIR, and PL spectra further confirm the formation of this phase purity. The presence of tetragonal zirconia is attributed to the presence of oxygen vacancies, as evidenced by XPS spectra. These synthesized zirconia nano ellipses were evaluated as an anode material for supercapacitors, exhibiting a maximum specific capacitance of 21.17 F/g at a scan rate of 3 mVs^-1, and as a photocatalyst for the degradation of crystal violet, achieving 92% degradation under 90 min of UV irradiation. This study presents a bio-inspired, straightforward approach for the synthesis of stabilized tetragonal zirconia nanostructures, highlighting their versatility and suitability for applications in energy storage and photocatalytic water purification. In the Future, these green-synthesized, cucumber seed-modified zirconia could be used to create highly efficient and stable solid electrolytes for solid-state batteries, as supercapacitors, solid oxide fuel cells, water and air purifiers. The organic components from the cucumber seeds introduce functional groups that enhance ion transport, leading to higher power density and longer cycle life.

Acknowledgment

The authors would like to thank Ongoing Research Funding Program, (ORFFT-2025-076-1), King Saud University, Riyadh, Saudi Arabia for financial support.

CRediT authorship contribution statement

Conceptualization, Tabinda Afzal and Mahwish Bashir; methodology, Tabinda Afzal; software., Waqar Ahmad, Mohamed R. Assal, Muf.K., M.R.H.S. and Mahwish Bashir; validation, Tabinda Afzal, and Mahwish Bashir; formal analysis, Qaiser Ali Sultan, Adnan Saeed, Mohamed R. Assal, Farzana Majid, Muf.K., Sana Saeed, Baji Shaik, and Mahwish Bashir; investigation, Tabinda Afzal, Mahwish Bashir and Adnan Saeed; resources, data curation, Farzana Majid, Waqar Ahmad, Tabinda Afzal, Mohamed R. Assal, and Mahwish Bashir; writing—original draft preparation, Tabinda Afzal, and Mahwish Bashir; writing—review and editing, Baji Shaik, Muf.K., and Mahwish Bashir; visualization; All authors have read and agreed to the published version of the manuscript.

Declaration of competing interest

All the authors declare that they have no established conflicting financial interests or personal relationships that may have influenced the research presented in this paper.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that they have used artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript or image creations.

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1. Introduction

2. Materials and Methods

2.1. Experimental details
2.3. Characterizations
2.3.2. Electrochemical study

3. Results and Discussion

3.1. Structural analysis
3.2. Morphology and EDX spectra
3.3. Zeta potential and XPS
3.4. Raman, PL UV-Vis and FTIR analysis
3.5. Photo-catalytic activity (PCA)
3.6. Electrochemical analysis for the application of supercapacitors

4. Conclusions

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Electrochemical and photocatalysis applications of green synthesized zirconia ceramics modified by cucumber seeds

Abstract

Keywords

Cucumber seeds

Crystal violet

Photocatalytic activitySol-gel method

Super-capacitor

Zirconia

1. Introduction

2. Materials and Methods

2.1. Experimental details

2.1.1. Cucumber seed powder

2.1.2. Synthesis of zirconia

2.3. Characterizations

2.3.1. Photocatalytic activity

2.3.2. Electrochemical study

3. Results and Discussion

3.1. Structural analysis

3.2. Morphology and EDX spectra

3.3. Zeta potential and XPS

3.4. Raman, PL UV-Vis and FTIR analysis

3.5. Photo-catalytic activity (PCA)

3.5.1. Blank test

3.5.2. Effect of zirconia dose

3.5.3. Effect of dye concentration

3.5.4. Kinetic study and stability

3.5.5. TOC study

3.5.6. Photocatalytic activity mechanism

3.6. Electrochemical analysis for the application of supercapacitors

4. Conclusions

Acknowledgment

CRediT authorship contribution statement

Declaration of competing interest

Declaration of generative AI and AI-assisted technologies in the writing process

References

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