Green synthesis of Syzygium cumini fruit extract mediated Tb³⁺doped ZnO nanoparticles for enhanced photocatalysis and electrochemical sensing

Abstract

This study presents a green and convenient approach for synthesising pure and Tb³⁺doped ZnO nanoparticles using naturally available Syzygium cumini fruit extract as fuel. The nanoparticles were thoroughly characterised for size, morphology, optical properties, photocatalytic, and cyclic voltammetric studies using various characterisation techniques. Powder X-ray diffraction (PXRD) analysis confirmed their high purity and crystallinity, with average sizes around 25 nm. Effective incorporation of Tb³⁺ ions into the ZnO lattice was verified. Scanning electron microscopy (SEM) images demonstrated the porous nature of the material. The prepared Tb^3+-doped ZnO nanoparticles exhibited promising photocatalytic degradation performance of Titan yellow (TY) dye under UV light. Electrochemical studies using a modified electrode showed enhanced performance for sensing paracetamol. These results highlight the material’s potential for applications in sensors, catalysis, supercapacitors, batteries, and other fields.

Keywords:

• ZnO: Tb³⁺nanocatalyst
• diffused reflectance spectroscopy
• photocatalysis
• Electrochemical sensor
• CV

1. Introduction

The escalating field of nanotechnology has sparked significant interest among researchers and industries, driving exploration into the unique characteristics of both emerging and traditional materials on the nanoscale. In particular, oxide semiconductors like ZnO, TiO₂, SnO₂, In₂O₃, and others have gathered considerable attention due to their promising potential for diverse future applications. In this context, we have chosen to employ zinc oxide nanoparticles for our study, as they exhibit semiconducting properties at the nanoscale (Happy, Kumar, and Rajeshkumar 2017).

Zinc oxide nanoparticles are one of the great potential semiconductors owing to their advanced properties, low cost, abundance, high surface activity, and environmental friendliness. ZnO is a wide-band gap semiconductor (3.37 eV). There are three unit structures, hexagonal wurtzite, zinc blende, and rock salt in ZnO, among the various structures, hexagonal wurtzite is the most recognised owing to its superior properties (Abderrahim et al. 2022). It has various applications such as gas sensors, biosensors, solar cells, organic LEDs, batteries, optoelectronic, spintronic, photocatalytic and antibacterial, nanomedicine, photochemical devices, technological and industrial domains, etc. However, undoped ZnO nanoparticles are affected by Photo-induced electron-hole pairs recombination and low light translation efficiency. The semiconducting properties of metal oxides can be altered by providing defects with the help of atomic/molecular manipulations through nanotechnology (Daksh and Agrawal 2016).

Moreover, the incorporation of dopants into ZnO nanomaterials has become a crucial focus of research. Currently, extensive investigations are underway employing rare earth ion dopants to enhance the photocatalytic degradation capabilities of ZnO. Notably, Tb³⁺ doping has been found to exert a substantial influence on crystal defects, surface morphology, as well as electrical and optical properties. These dopants effectively reduce the recombination rate of electron-hole pairs and introduce energy levels within the band gap, thereby extending the material’s light absorption capacity (Zhang et al. 2015).

Exploring the realm of energy storage and catalytic studies for future applications, one encounters two pivotal areas of interest: batteries and supercapacitors. Batteries are essential in our modern world, powering an array of devices from smartphones to electric vehicles. The research into batteries continues to evolve, with a strong emphasis on improving energy density, cycle life, and safety. Advancements in battery technology are pivotal for the widespread adoption of renewable energy sources, such as solar and wind power, as well as for the development of electric transportation (Manjunatha 2020: Vinod et al. 2017).

Catalytic studies play a pivotal role in addressing environmental and industrial challenges. Catalysts are substances that facilitate chemical reactions, enabling them to occur at lower temperatures and with higher efficiency. They are vital in fields ranging from energy production, and conversion to pollution control and pharmaceuticals. Ongoing research endeavours are aimed at discovering novel catalysts, understanding their mechanisms, and optimising their performance for various applications (Nalla Muthuchamy et al. 2018: Ashokkumar and Muthukumaran 2014).

Electrochemical sensors have garnered significant interest in the scientific community due to their rapid response, simple design, portability, high sensitivity, and selectivity. In complex sample matrices, various electrochemical sensors based on nanomaterials like graphene, conducting polymers (CPs), and metal nanoparticles have been developed in recent years (Amrutha et al. 2021; Manjunatha, Deraman, and Basri 2015; Chandrashekar et al. 2010). Paracetamol is a widely used analgesic and antipyretic drug, that contains an active phenolic hydroxyl group that can be electrochemically oxidised (Deepa et al. 2024). Creatinine is generally considered safe, but at high doses, it can cause serious side effects, such as kidney damage, and may inhibit the body’s natural creatinine production. Developing a commercially affordable sensor with superior sensitivity and selectivity would benefit both social and economic sectors. Consequently, there is considerable interest in developing various electrochemical techniques for detecting acetaminophen(paracetamol) (Chu et al. 2020; Charithra and Manjunatha 2019).

The synergy between advancements in batteries, sensors, supercapacitors, and catalytic studies offers tremendous potential for tackling global energy and environmental challenges. This synergy opens avenues for cleaner energy sources, improved energy storage efficiency, and sustainable chemical processes. These developments are crucial for realising a greener, more technologically advanced future. (Taghavi et al. 2023, Farzaneh et al. 2020, Taghavi et al. 2023). Numerous techniques, both chemical and physical, are employed in the production of nanoparticles (Chu et al. 2008). Many of these methods are associated with high energy consumption and present environmental challenges. In comparison to other methodologies, the combustion method has very good advantages over other preparative routes. This method involves the exothermic reaction between fuel and oxidiser sources, typically involving a metal or metal-containing compound as the precursor. The combustion process generates high temperatures and can lead to the rapid formation of desired products. rapid synthesis, homogeneity, relative simplicity, high purity, energy efficiency scalability, etc. are the advantages of the combustion method over other preparative routes (Bomila et al. 2017).

Here we presented a simple synthesis technique to fabricate pure and Tb³⁺ ions doped ZnO nanoparticles and the prepared materials were tested for photocatalytic degradation, battery, supercapacitor, and other applications.

2. Experimental

2.1. Materials

Terbium nitrate hexahydrate (Tb (NO₃)₃.6H₂O, 99.99%) and zinc nitrate hexahydrate (Zn(NO₃)₂.6H₂O, 99.99%) of analytical grade.

2.2. Extraction of fuel

Fresh Jamun fruit was collected from a botanical garden. These fruits were washed several times thoroughly using running tap water and then again washed with distilled water, dried in air. Washed Syzygium cumini portions were chopped into fine pieces, pulp of the fruit was separated from the seeds. 100 gm of chopped pulp was taken and crushed using a pestle and mortar, juice was extracted by filtering, and the same was used for the synthesis.

2.3. Synthesis of ZnO: Tb³⁺ (0-5mol %) nanoparticles

The stochiometric ratio of Tb(NO₃)₃.6H₂O, Syzygium cumini fruit extract, and Zn(NO₃)₂.6H₂O was put into a beaker containing 10 ml of distilled water kept at 450 °C in a muffle furnace for 30 min. the sample is calcined at the temperature 600 °C for 2 hr. The white solid was cooled down, pulverised, and then stored in a sample holder for further experiments and the corresponding procedure illustrated in Figure 1. Undoped ZnO NPs were also prepared in the mentioned above procedure. Tb doped ZnO with different Mol % of Tb³⁺ concentrations ranging from 1 to 5 mol % were denoted by ZnO: 1 mol % Tb³⁺, ZnO: 3 mol % Tb³⁺, Zno:5 mol % Tb³⁺ respectively.

Figure 1. Schematic diagram of synthesis of ZnO:Tb³⁺ doped nanoparticles.

2.4. Photo catalytic experimental procedure

20 ppm of 250 ml aqueous solution of Titan Yellow(TY) and 60 mg of synthesised photocatalysts were taken in a circular glass reactor with an area of about 176.6 cm². The aqueous (20 ppm of 250 ml) solution of the Titan Yellow dye with the synthesised photocatalyst (60 mg) was added into a circular glass reactor with an area of about 176.6 cm²and allowed to reach an adsorption–desorption equilibrium before UV light irradiation and the reaction mixture was stirred using a magnetic stirrer for the entire period of an experiment to ensure the establishment of adsorption/desorption.

2.5. Preparation of electrode

In typical Cyclic Voltammetry (CV), the carbon paste electrode was used as a working electrode. During the preparation of the working electrode, 75%, 15%, and 10% of prepared nanomaterial, graphite powder, and polytetrafluoroethylene respectively were mixed thoroughly for about 30 min after which the mixture was inserted into nickel mesh and crushed well at 20 MPa for 5 min to get an electrode. The current produced was measured using Ag/AgCl as the reference electrode and platinum wire as the counter electrode.

3. Results and discussions

To confirm the structure and crystal phase of pristine and doped ZnO nanomaterials, the samples were analysed by Powder X-ray diffractometer (PXRD). Figure 2 displays the PXRD patterns of pristine and doped ZnO. All the obtained peaks and their relative intensities of the prepared samples are well-matched with JCPDS No. 89-1397. The sharp peaks in PXRD indicate the high crystalline nature of prepared materials (Bomila et al. 2017). The average crystallite size of all the pristine and Tb³⁺ doped ZnO nanomaterials is estimated using the Scherrer equation and W-H methods (Figure 3(a) & (b)) (Girish, Prashantha, et al. 2018, Kaini, Ali, and Saeid 2023). (1) $$\mathbf{D}=\frac{\mathbf{K}\mathbf{\lambda }}{\mathbf{\beta }\mathbf{\text{cos}}\mathbf{\theta }}$$(1)

Where, D = Crystallite size, $\lambda$ = wavelength of X-rays K = Scherrer constant, $\beta$ = Full width half maximum (Fardood et al. 2022). Also, the strain in the Tb³⁺ doped ZnO nanomaterials was evaluated using the W-H plots. (2) $$\mathbf{\beta }\mathbf{\text{cos}}\mathbf{\theta }=\frac{0.9\mathbf{\lambda }}{\mathbf{D}}+4\mathbf{\epsilon }\mathbf{\text{sin}}\mathbf{\theta }$$(2)

Figure 2. PXRD patterns of the prepared nanomaterials.

Figure 3. (a&b) WH Plots and scherr plots of prepared nanomaterials.

Dislocation density and strain associated with the material were estimated using the following relations and all the estimated values for the synthesised nanomaterials are given in the Table 1. (3) $$\mathbf{\epsilon }=\frac{\mathit{\beta }\mathit{\text{COS}}\mathit{\theta }}{4}$$(3) (4) $$\mathit{\delta }=\frac{1}{{\mathit{D}}^2}$$(4)

Table 1. Estimated crystallite parameters of prepared materials.

Nano Material	Crystallite size			Dislocation Density (δ)	Strain (Ɛ)
	Scherrer Equation (nm)	Scherrer Plot (nm)	WH Plots (nm)
ZnO	25	28	30	1.6 × 10^-3	0.017
1 % Tb^3+: ZnO	26	29	31	1.4 × 10^-3	0.018
3 % Tb^3+: ZnO	24	26	25	1.7 × 10^-3	0.016
5 % Tb^3+: ZnO	27	30	29	1.3 × 10^-3	0.019

The morphology of the prepared nanomaterials was studied using Scanning Electron Microscopy(SEM) analysis, and the resulting SEM images are illustrated in Figure 4. The observations from Figure 4 indicate the porous and agglomerated nature of the prepared material, also, revealed an interconnected network structure with a high rate of agglomeration and this kind of morphology is attributed to the rapid evolution of gases during the combustion process (Girish, Prashantha, and Nagabhushana 2017; Girish et al. 2016).

Figure 4. SEM images of of prepared nanomaterials.

Diffused Reflectance Spectroscopic (DRS) studies were performed to analyse the optical properties of the synthesised Pristine and Tb³⁺ doped ZnO, the recorded diffused reflectance spectrum shown in Figure 5 (inset). The band gap energy of the pristine and Tb³⁺ doped ZnO was calculated from the DRS using the Kubelka-Munk equation (5)(5) $$\mathit{F}\left({\mathit{R}}_{\mathrm{\infty }}\right)=\frac{\left(1-{\mathit{R}}_{\mathrm{\infty }}\right)}{2{\mathit{R}}_{\mathrm{\infty }}}$$(5) . (5) $$\mathit{F}\left({\mathit{R}}_{\mathrm{\infty }}\right)=\frac{\left(1-{\mathit{R}}_{\mathrm{\infty }}\right)}{2{\mathit{R}}_{\mathrm{\infty }}}$$(5)

Here R_∞ is the absolute reflectance and F(R_∞) is the Kubelka-Munk function. The optical band gap values are estimated by plotting the variation of Kubelka-Munk function with photon energy through the following equation (Kubelka and Munk 1931). (6) $$\mathbf{F}\left(\mathbf{R}\right)\mathbf{h}\mathbf{\nu }=\mathbf{A}\mathrm{ }{\left(\mathbf{h}\mathbf{\nu }–{\mathbf{E}}_{\mathbf{g}}\right)}^{\mathbf{n}}$$(6)

Here ‘Eg’ is the optical band gap energy, hν: incident photon energy and n: the nature of the sample transition depends on the type of optical transition triggered by photon absorption. Obtained characteristic Kubelka-Munk plots of prepared nanomaterial shown in Figure 5 and the energy gap values for the prepared ZnO samples was found to be in range of 3.14 -3.20 eV, it was lesser than the reported value (∼3.3 eV). The reduction in the band gap energy might be owing to ZnO lattice crystal defects and also the presence of excess Zn in interstitial wurtzite lattice (Girish et al. 2016).

Figure 5. Diffused reflectance spectrum (inset), and energy gap spectrum of the prepared materials.

To investigate the effect of Tb³⁺ in ZnO structure, photocatalytic studies were performed on TY dye for a total duration of 60 mins with a time interval of 15mins and the corresponding absorption spectra of Tb³⁺ (3 mol %): ZnO catalyst presented in the Figure 6 and the efficiency of degradation was evaluated by equation. (7) $$\text{Degradation\hspace{0.17em}\hspace{0.17em} efficiency}=\frac{\mathrm{C}0-C}{C0} X 100\%$$(7)

Figure 6. Absorbance spectra of TY dye.

The degradation of TY dye tracks the Langmuir-Hinshel wood first order kinetics model and can be expressed as ln (C/Co) = −k t, where k: constant of apparent reaction rate, C₀: initial concentration of aqueous dye, t: reaction time and C: is the concentration of aqueous FOR dye at the reaction time of t (Yen et al. 2021).

The percentage of TY dye degradation with Tb³⁺ (1-5 mol %) activated ZnO catalyst illustrated in the Figure 7 and Tb³⁺ (3 mol %) activated ZnO catalyst reveals that highest % of TY dye was degraded and the detailed mechanism of photocatalytic degradation process (Figure 8) under UV light irradiation are presented below. $$\text{ZnO}+\text{h}\upsilon \to {\text{e}}^{-}+{\text{h}}^{+}$$ $${\mathrm{O}}_2+{\text{e}}^{-}\to {\text{O}}_2{}^{.-}$$ $$\text{OH}+{\text{h}}^{+}\to {\text{OH}}^{.}$$ $${\mathrm{O}}_2{}^{.-}+{\text{OH}}^{.}+\text{dye}\to \text{degradation\hspace{0.17em}\hspace{0.17em} product}$$

Figure 7. Degradation spectra of prepared material.

Figure 8. Detailed degradation mechanism of prepared material.

During UV irradiation, the electrons present in the valence band excited to conduction band and interacts with Tb³⁺ ions present in the ZnO, the Tb³⁺acts as an electron scavenger reduces the recombination of the charge carriers that increase the degradation activity by generating the O₂^-. and ^.OH. The Tb²⁺ ions produced from Tb³⁺ are less stable. The Tb²⁺ ions act on O₂ and promotes the generation of O₂^-. The degradation mechanism for Tb³⁺ -doped ZnO is shown in the following equations (Girish et al. 2023). $${\text{Tb}}^{3+}+{\text{e}}^{-}\to {\text{Tb}}^{2+}$$ $${\mathrm{O}}_2+{\text{e}}^{-}\to {\text{O}}_2{}^{-.}$$ $${\mathrm{O}}_2{{}^{-.}}_{+}{\text{Tb}}^{3+}\to {\text{Tb}}^{2+}$$ $${\text{Tb}}^{2+}+{\text{O}}_2\to {\text{Tb}}^{3+}+{\text{O}}_2{}^{-.}$$

The synthesised catalyst underwent a reusability test to assess its photostability, durability, and reusability. Even after five successive runs, there was only a slight decrease in degradation efficiency, with the decomposition efficiency remaining high at 73.30% by the end of the 5th cycle as shown in Figure 9. The physical properties of the new and used photocatalyst were found to be nearly identical, indicating the catalyst’s stability over multiple uses. This established stability enhances its practical utility as a catalyst for dye photodecomposition. Therefore, the prepared material demonstrated both stability and reusability, making it a promising candidate for catalytic applications.

Figure 9. Reusability of the prepare material.

Electrochemical studies were performed to examine electrochemical properties of Tb³⁺ doped ZnO for battery and supercapacitor applications, the obtained CV curve of Tb³⁺ (3 mol%): ZnO electrodes with various scan rates are shown in Figure 10. It was noticed the pseudo-capacitive nature of Tb³⁺: ZnO electrode (Sulciute et al. 2021).

Figure 10. Cyclic voltammetry (CV) curve of prepared materials.

To understand the charge transfer resistance, electrochemical impedance studies (EIS) were performed, and the obtained spectra of Tb³⁺ (0-5 mol%): ZnO electrode are seen in Figure 11. Z′ is the real component, discloses the ohmic properties, and Zʺ is the virtual component reveals the capacitive properties. EIS spectra show an elevated arc (semicircle), generally, arc with larger radii indicates the higher charge transfer resistance (Raghupathy, Ramgopal, and Ravikumar 2022; Kumar, Ranjith, and Kumar 2018; Karthik et al. 2018). the results indicates semicircle at the higher frequency region and a straight line at the lower frequency region, representing that the electrodes has a superior capacitive behaviour, pristine, 1 mol % and 5 mol % doped ZnO electrodes show semicircle with larger radii compared to Tb³⁺ (3 mol %) represents the higher charge transfer resistance and 3 mol % of Tb³⁺ doped ZnO electrode shows semicircle with smaller radii denotes the smaller charge transfer resistance and thus Tb³⁺ (3 mol %) doped ZnO electrode possess superior capacitive properties than the pristine and other doped electrodes (Girish, Prashantha, et al. 2018; Kumar et al. 2021; Kusuma et al. 2023).

Figure 11. Nyquist plots of the prepared materials.

Furthermore, to evaluate the suitability of the suggested method as an electrochemical sensor, the electrodes that were prepared were utilised to detect the paracetamol content in the available Dolo-650 tablets. Figure 12 presents the CV plots of the pure and Tb³⁺ doped ZnO electrodes. The noticeable change in the CV profile after the introduction of paracetamol clearly indicates that, this observed and modified CV profile is indeed a result of the presence of paracetamol. This demonstration confirms that the proposed method (Kumar et al. 2023), can effectively detect the presence of paracetamol in pharmaceutical medications. All the obtained results highlights that the prepared material is potential for multifunctional applications, including wastewater purification, decoloration, supercapacitors, batteries, and sensors (Lavanya, Ramakrishnappa, Girish, Suresh Kumar, Basavarajur, et al. 2024).

Figure 12. Cyclic voltammetry (CV) curve with paracetamol.

The enhanced performance of photocatalytic and sensing properties of prepared materials is attributed to the introduction of Tb³⁺ ions, which create defects that serve as photo-electronic traps, enhancing the separation of charge carriers and reducing recombination and also modify the redox chemistry of the materials and altering the charge transfer kinetics at the electrode-electrolyte interface. However, it is crucial to note that the optimal doping concentration of Tb³⁺ ions is critical, as excessive doping can lead to the formation of recombination sites, reducing catalytic efficiency (Lavanya, Ramakrishnappa, Girish, Suresh Kumar, Basavarajur, et al. 2024). As can be seen from the Nyquist plot, the diameter of the semicircle of the Tb³⁺ (3 mol %) doped ZnO electrode is comparatively smaller than that of the other samples, thereby evidencing the increase in the separation of photogenerated electrons and holes. Remarkably, this shows that Tb³⁺ (3 mol %): ZnO has very low charge transfer resistance as it promotes migration of electrons and interfacial charge separation together with an efficient reduction of exciting quenching and energy dissipation. Also from Table 1 the crystallite size for the Tb³⁺ (3 mol %): ZnO is comparatively less. As a result of this process, the Tb³⁺ (3 mol %): ZnO material sample provides good degradation efficiency and sensor properties compared to other prepared materials. As a result, prepared material is a prominent candidate for applications requiring enhanced photocatalysis and electrochemical sensing capabilities and other applications.

4. Conclusion

We successfully synthesised pure and Tb³⁺activated ZnO nanomaterials with various dopant concentrations using Syzygium cumini fruit extract as fuel via combustion technique. PXRD analysis confirmed the effective integration of Tb³⁺ ions into the ZnO crystal lattice. Scanning electron microscopy images revealed the porous nature of the material. The prepared catalysts demonstrated excellent catalytic activity for the degradation of TY dye under UV light, attributed to their small particle size and effective separation of charge carriers. Among the prepared catalysts, the 3 mol% Tb³⁺ doped ZnO catalyst exhibited the highest catalytic activity during the degradation process. The optimised Tb³⁺ concentration on the ZnO electrode significantly influenced its electrochemical activities and showed better sensitivity for the detection of paracetamol. These results highlight the potential of the synthesised material for multifunctional applications, including wastewater purification, decoloration, supercapacitors, batteries, and sensors.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are available on request from the corresponding author.