1 Introduction

Polyphenols are aromatic compounds containing hydroxyl groups directly attached to benzene rings, and they are found in many plants, such as vegetables, fruits [1,2,3], flowers [4, 5], and some parts of plants [6,7,8]. Depending on the number and interaction position of OH groups with the chemical framework, the physicochemical properties or biological activity change. Polyphenols play a vital role in plant life, such as creating characteristic colors and protecting plants from the harmful effects of insects, oxidation, and ultraviolet rays [9]. In medicine, polyphenols are natural compounds with many effects, such as potent antioxidant [10, 11], anti-inflammatory [12,13,14], antibacterial [15,16,17], anti-aging [18, 19], and anti-cancer [20,21,22,23]. More than 10,000 polyphenol compounds have been found in various plant species. Polyphenols may be classified into different groups based on the number of phenol rings they contain and the structural elements that bind these rings to one another. The main classes include phenolic acids, flavonoids, stilbenes, and lignans.

Research on the extraction, enrichment, and purification of biologically active natural compounds is essential in modern pharmaceuticals and medicine. Recently, scientists have expanded the application range of natural compounds in many fields, such as environmental treatment and control, oil additives, and metal anti-corrosion additives. Natural compounds can be extracted from various sources, including plants, animals, bacteria, and fungi. Scientists separate substances, enrich compounds, or purify useful active substances from plant sources to effectively make the most of available natural sources. Many extraction methods have been used to obtain polyphenols from different plant sources [3, 4, 12, 24]. The efficiency of the extraction process depends on many factors, such as the nature of the plant, the solvent, the ratio of solvent and raw material, temperature, extraction time, etc. In addition, additional steps must be taken to remove other compounds and impurities. Polyphenol extraction techniques are diverse and can be combined, including thermal extraction [25, 26], microwaves [26, 27], ultrasound [28, 29] with ionic liquid [30, 31], supercritical CO2 [32, 33], deep eutectic solvents [28, 34, 35], etc.

Copper(I) oxide is an inorganic compound with the formula Cu2O and one of the principal oxides of copper. Cu2O is very stable with heat, insoluble in water, slowly soluble in concentrated alkali or ammoniac, and well soluble in acidic solutions. Depending on the particle size, cuprous oxide can have colors ranging from yellow to orange to red. In addition, particle size also affects the photochemical properties of Cu2O. Cu2O has a band gap energy of Eg = 2.14 eV at the micrometer scale. However, as the particle size decreases to the nanometer region, the size effect changes the band gap energy. Along with the development of nanotechnology, Cu2O nanoparticles are applied in many different fields, such as catalysis [36,37,38], sensors [39,40,41], semiconductors [42, 43], solar cells [44,45,46], etc. Methods Diverse methods of synthesizing Cu2O nanoparticle include pyrolysis [47, 48], hydrothermal [37, 49], thermal oxidation [50,51,52], electrochemical [53, 54], etc. In addition, the chemical reduction method with reducing agents is used to manufacture Cu2O nanoparticle [55,56,57,58].

Over the past 10 years, nanotechnology has been regarded as one of science's most important frontiers. Numerous industrial and technological sectors, including information technology, homeland security, healthcare, energy, transportation, food safety, and environmental research, are significantly improved by nanotechnology. Its many uses and rapidly increasing market demand have opened the door for creative approaches to producing higher-quality nanomaterials. Many research projects related to the synthesis, characterization, and applications of nanoparticles have been published [59,60,61,62,63,64]. Initially, nanomaterials were synthesized using conventional synthesis techniques that required high-energy input and carcinogenic chemicals. There is a demand for environmentally safer synthesis techniques because of the pollution that traditional synthesis methods produce. Natural agents are used in green nanomaterial synthesis methods to produce nanomaterials [65]. Green synthesis uses low-energy techniques and naturally available starting materials to produce nanomaterials with the same effectiveness as classic synthesis but more sustainably. Therefore, including green synthesis offers a possible way around the drawbacks of conventional synthesis techniques. Recently, scientists have been interested in using natural compounds as reactants in chemical reactions in green chemistry processes. For example, Cu2O-based nanocomposites have been synthesized using Commelina benghalensis leaf extract and oolong tea leaf extract [66,67,68]. These are some of the many raw material sources for extracting natural polyphenols as reducing agents in synthesizing nanomaterials [69,70,71]. To date, plant species containing many polyphenols have been exploited. However, the extraction of polyphenols from Terminalia catappa leaves as a synthetic agent for Cu2O nanoparticles has yet to be evaluated. In this study, polyphenols from Terminalia catappa leaf extract are considered green reducing agents to synthesize Cu2O nanoparticles for the photocatalytic decomposition of toxic organic compounds in water.

2 Experiments and methods

2.1 Chemical

Ethanol (C2H5OH, 95%), Chloroform (CHCl3, 98%), Sodium carbonate (Na2CO3, 99.5%), Copper (II) sulfate pentahydrate (CuSO4·5H2O, 98.0%), Potassium sodium tartrate tetrahydrate (KNaC4H4O6·4H2O, 99.0%), Ethylene Glycol (C2H6O2, 99.5%) and Sodium chloride (NaCl, 99.5%) were purchased from Macklin Company (China). Sodium hydroxide (NaOH, 97.0%), Gallic acid (C7H6O5, 98.5%) and Folin-Ciocalteu’s phenol reagent (2N) were purchased from Sigma-Aldrich (USA).

2.2 Preparation of the Terminalia catappa extract

The Terminalia catappa shoots were harvested in the summer. Leaves were cut to about 1 cm, dried under dry air for 72 h, and ground to 100 μm. Disperse the dry leaf powder in a water/ethanol mixture (v/v: 1/1) at a ratio of 50 g/L under ultrasound for 30 min. The suspension was heated at 60 °C for 2 h and filtered to remove the residue. Chloroform is used to remove chlorophyll in the extract. The infrared spectrum of polyphenols extracted from Terminalia catappa in the range of 4000–400 cm−1 demonstrates the presence of characteristic functional groups. Polyphenol content was determined using the Folin-Ciocalteu method by UV–Vis spectroscopy at 713 nm. The equation for determining polyphenol concentration is shown as follows: C = 25,302 * Abs − 0.3903 (R2 = 0.9916).

2.3 Synthesis of Cu2O nanoparticles

100 ml of the solution mixture containing 7.055 g KNaC4H4O6·4H2O and 2 g NaOH was slowly added (5 ml per minute) into 50 ml of 0.5 M CuSO4 solution under circular stirring conditions. Then, 50 ml of ethylene glycol was added to the mixture and stirred for 10 min. 50 ml of purified extract is gradually dropped into the stirring suspension, raising the temperature to 60 °C. After 60 min, the precipitate was collected and washed with distilled water three times (250 ml/time). The purified Cu2O nanoparticle was dried in Ar gas at 80 °C for 24 h.

2.4 Characterization of Cu2O nanoparticles

The material's morphology was observed using scanning electron microscopy (SEM) techniques with magnification capabilities up to 100,000 times. The crystal structure of the Cu2O nanoparticles was analyzed through an X-ray diffraction (XRD). At 1.54 Å in a copper X-ray tube (Cu-Kα), the device is operated at 44 mA and 40 kV in the range (2θ) from 10° to 80°. The optical absorption characteristics of the Cu2O nanoparticle were determined by ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) in the range of 200–800 nm.

2.5 Removing methylene blue with Cu2O photocatalyst

The photocatalytic activity of the prepared Cu2O nanoparticle: In each typical experiment, 25 mg Cu2O nanoparticle was exposed to 50 mL of 10 ppm MB solution in a transparent glass tube. The adsorption process took place in the dark after 30 min. Then, the glass tubes are put into the lighting cabinet, using the light source as a Xenon lamp under air circulation conditions. The MB solution was removed from the mixture after a predetermined amount of time, and the concentration was measured using UV–Vis photometry on UV–Vis DV-8200 equipment (Drawell). The equation to determine MB concentration was built as follows: C = 6.3746 * Abs − 0.2229 (R2 = 0.9938) where C is the concentration of MB solution, ppm; Abs is the light absorption intensity at λ = 662 nm.

The degradation efficiency of MB in the presence of a catalyst is calculated according to the formula:

$$H\left( {\text{\% }} \right) = \frac{{\left( {C_{0} - C_{t} } \right)}}{{C_{0} }} \times 100{\text{\% }}$$

The MB removal data by photocatalysis were fed into a pseudo-first-order kinetic model for kinetic analysis:

$$- ln\frac{{C_{t} }}{{C_{0} }} = k_{1} .t$$

where Co and Ct are concentrations of MB (ppm) initially and at time t, and k1 is the constant reaction rate.

3 Results and discussion

3.1 Characterization of the Terminalia catappa extract

The polyphenols isolated from Terminalia catappa are shown in Fig. 1a, illustrating the existence of distinctive functional groups. A prominent obtuse peak is detected at 3219 cm−1 in the wavenumber range. The symmetric and asymmetric relaxation vibrations of the polymer hydroxyl group (O–H), or H-bond relaxation, are represented by the range 3400–3200 cm−1 and are typical of polyphenolic compounds [72]. A vibration that may be detected at 1713 cm−1 suggests that the six-carbon aromatic ring has a relationship with the –C=O group [73]. Further evidence for the presence of the –CH– group of methylene on aromatic rings stretching comes from deformation fluctuations at wave numbers 1440 cm−1 in the regions 1430–1470 cm−1, which is attributed to C–H linkage [74]. The area spanning from 1000 to 1100 cm−1 is commonly called the fingerprint zone due to the abundance of distinctive low-intensity single bands linked to particular functional groups within this range. A vibration at 1052 cm−1 is associated with the –C–O–C– stretching [75]. Ultimately, the phenyl radical's C–H bond can be attributed to the wavenumber at 745 cm−1 [76].

Fig. 1
figure 1

FTIR spectroscopy (a) and UV–Vis spectroscopy (b) of polyphenol from Terminalia catappa

The UV–Vis photometric spectrum of Terminalia catappa leaf extract is shown in Fig. 1b. On the spectrum, two adjacent peaks at 220 nm and 259 nm are believed to be characteristic of polyphenols in Terminalia catappa leaves. In addition, a lower peak at 379 nm was also observed. Polyphenol content in Terminalia catappa extract was calculated through the equation to determine polyphenol concentration by UV–Vis photometric method. The result of polyphenol concentration in Terminalia catappa extract was 24,291 mg/L.

3.2 Characterization of Cu2O nanoparticle

Post-visual inspection and UV–Vis spectral analysis were utilized to confirm the synthesis of Cu2O nanoparticles. The creation of Cu2O nanoparticles is tentatively confirmed by the constant color shift to brick red after heating the Terminalia catappa leaf extract and copper sulfate solution. It also shows that during the manufacturing process, the phenolic compounds change the valency of copper from 2 to 1. Figure 2 shows the spectra of the leaf extract from Terminalia catappa and the Cu2O nanoparticles between 190 and 590 nm. The extract's inherent phenolic moieties underwent an electronic transition, either from π to π* or from n to π*, as shown by the absorption double peaks seen in both spectra at 234 nm/264 nm and 256/283 nm. The formation of Cu2O nanoparticles was recorded when a broad peak appeared at 414 nm. At the same time, the narrow peak at 370 nm in the spectrum of Terminalia catappa leaf extract disappeared. This confirms that the production of Cu2O nanoparticles is aided by reducing agents found in Terminalia catappa leaf extract. Monitoring the phenomenon during the formation of Cu2O is recorded in Fig. 2. The hypothesis about the formation process of Cu2O is explained [77, 78]. The predicted mechanism for the formation of Cu2O nanoparticles can be given as follows:

$${\text{Cu}}^{2 + } \mathop{\longrightarrow}\limits^{{TC\;{\text{leaf}}\;{\text{extract}}}}{\text{Cu}}^{ + } \mathop{\longrightarrow}\limits^{{OH^{ - } }}{\text{CuOH}}\mathop{\longrightarrow}\limits^{{}}{\text{Cu}}_{{2}} {\text{O}}$$
Fig. 2
figure 2

UV–Vis spectra of Terminalia catappa leaf extract and Cu2O nanoparticle

The particle size of Cu2O nanoparticles depends mainly on the synthesis mode, including initial concentration, surfactant content, stirring speed, etc. Meanwhile, the material particle shape depends on the above factors and the selected reducing agent. The SEM image of Cu2O nanoparticle obtained by the reduction method with Terminalia catappa leaf extract is shown in Fig. 3. The morphology of the created Cu2O at 60 °C has a uniform round sphere shape. Particle size distribution ranges from 50 to 150 nm. Cu2O nanoparticles synthesized at 40 °C have a uniform morphology, but their size ranges from a few dozen nanometers to more than 150 nm. Meanwhile, Cu2O nanoparticles obtained at higher temperatures (80 and 95 °C) tended to be heterogeneous in morphology and size. The sample synthesized at 95 °C also shows cubic and obtuse cubic crystals.

Fig. 3
figure 3

The SEM images of Cu2O nanoparticle with different synthesis temperatures: 40 °C (a), 60 °C (b), 80 °C (c), and 95 °C (d)

Most studies have used ethylene glycol as a stabilizer in the synthesis of nanoparticles. Some other studies use EG as a size control agent for nanoparticles [79,80,81,82]. The results of investigating the influence of EG on the state of Cu2O nanoparticles are shown on the SEM image in Fig. 4. For the sample without EG, the obtained Cu2O nanoparticles were uneven; the nanoparticle surface was rough and tended to mechanical instability. The presence of EG in the reaction mixture helps the formed Cu2O nanoparticles have a uniform size and smooth surface, which can create higher mechanical strength.

Fig. 4
figure 4

The SEM images of Cu2O nanoparticle without (a, b) and with ethylene glycol (c, d)

The Cu2O’s crystal structure was observed by examining the XRD pattern (Fig. 5a). The 2θ diffraction peaks at 29.6°, 36.5°, 42.4°, 61.5°, 73.7°, and 77.6°, respectively, are in excellent accord with the standard cards (JCPDS file no. 05-0667) of Cu2O [37, 47]. These correspond to the [110], [111], [200], [220], [311] and [222] crystal planes of Cu2O [37, 50, 53]. The absence of Cu, CuO, and Cu(OH)2 diffraction peaks suggests that the produced Cu2O nanoparticle are in their pure crystal phase. Furthermore, the sharp crystallinity of the products was demonstrated. The Cu2O nanoparticle’s chemical composition was determined using the energy-dispersive X-ray method. The results of determining the elemental content shown on the EDX spectrum (Fig. 5b) show the existence of two elements, Cu and O, with respective masses of 80.31% and 9.67% (elemental ratio is approximately 2:1), similar to the phase composition in the XRD data.

Fig. 5
figure 5

The XRD pattern (a) and EDX spectrum (b) of Cu2O nanoparticle

The optical properties of the material were also evaluated through the UV–Vis DRS spectrum. The results presented in Fig. 6 show that the band gap energy Eg of Cu2O nanoparticle, calculated according to the formula of Tauc, is 1.945 eV, respectively, with the highest adsorption intensity at wavelength 477 nm. The results on the band gap energy of Cu2O nanoparticle obtained are equivalent to previous publications [83,84,85]. Cu is a transition metal element. Although the outermost electron is distributed in the 4s1 layer, it belongs to the d subshell, so this single electron can easily change its energy level to achieve photocatalytic activity and is well represented in the visible region.

Fig. 6
figure 6

The UV–Vis DRS spectrum (a) and band gap energy diagram (b) of Cu2O nanoparticle

3.3 Removing methylene blue with nano Cu2O photocatalyst

The photocatalytic ability of Cu2O nanoparticle to oxidize dye (methylene blue) was evaluated under simulated sunlight conditions. Experimental conditions were set up, including an initial MB concentration of 10 ppm, catalyst content of 0.5 g/L, and Xenon lamp power of 300 W.

The results showed that MB decomposed quickly after 10 min of irradiation and the photodegradation efficiency reached ~ 72% after 120 min (Fig. 7a). It can be said that the photocatalytic efficiency of MB decomposition of biologically synthesized Cu2O nanoparticles using Terminalia catappa leaf extract is comparable to researched and published works as shown in Table 1.

Fig. 7
figure 7

The UV–Vis spectrum (a) and efficiency of photodegraded MB solution under simulated sunlight with Cu2O nanoparticle as catalyst

Table 1 Comparative photodegradation of MB by Cu2O photocatalyst synthesized from different methods

Theoretically, adsorbed O2 and focused electrons on the Cu2O surface may react to form ·O2 under simulated sunlight. Following that, hydrogen peroxide (H2O2) and hydroxyl radical (·OH) are produced when ·O2 reacts with H2O. This explains why Cu2O nanoparticle has a high activity in breaking down MB. O2 and OH are the main reaction types of MB degradation, and the following reactions illustrate the photocatalytic process of MB by Cu2O nanoparticle:

$$hv + {\text{Cu}}_{2} {\text{O}} \to h^{ + } + e^{ - } { }$$
$${\text{O}}_{2} + {\text{ e}}^{ - } \to { }^{ \cdot } {\text{O}}_{2}^{ - } { }$$
$$^{ \cdot } {\text{O}}_{2}^{ - } + 2{\text{H}}_{2} {\text{O}} \to 2^{ \cdot } {\text{OH}} + {\text{H}}_{2} {\text{O}}_{2}$$
$$^{ \cdot } {\text{O}}_{2}^{ - } + {\text{ H}}_{2} {\text{O}}_{2} { } \to^{ \cdot } {\text{OH}} + {\text{O}}_{2} + {\text{OH}}^{ - }$$
$${\text{OH}}^{ - } + h^{ + } \to^{ \cdot } {\text{OH}}$$
$${\text{MB }} + { }^{ \cdot } {\text{O}}_{2}^{ - } \left( {^{ \cdot } {\text{OH}}} \right) \to {\text{CO}}_{2} + {\text{H}}_{2} {\text{O}}$$

Methylene blue (MB) degradation was investigated for up to 4 h under visible light irradiation to determine the photocatalytic activity of Cu2O nanoparticle. The MB decomposition efficiency of Cu2O nanoparticle after 10, 30, and 120 min was 26.82, 44.31, and 71.99%, respectively, as shown in the graph in Fig. 7b. As illustrated in Fig. 8, the photocatalytic degradation data were computed using a pseudo-first-order reaction kinetic model. The MB decomposition rate constant (k1) of Cu2O nanoparticle is 0.0084 min−1.

Fig. 8
figure 8

The pseudo-first-order kinetic of MB degradation using Cu2O nanoparticle photocatalyst under visible light irradiation

4 Conclusions

To sum up, polyphenols from Terminalia catappa leaves were extracted using a green chemical method. With the use of indirect ultrasound and heat treatment at 60 °C, the ethanol/water solvent mixture was extracted. In Terminalia catappa leaf extract, the polyphenol level exceeded 24 g/L, or 48% by weight. The reduction technique was utilized to produce copper(I) oxide nanoparticles using polyphenol from Terminalia catappa leaf extract. The generated Cu2O nanoparticle was appropriately characterized using physicochemical techniques. The obtained Cu2O nanoparticle showed a spherical shape morphology with particle sizes between 50 and 150 nm. In the visible spectrum, the produced Cu2O nanoparticle exhibits strong photocatalytic activity. In 2 h, the Cu2O nanoparticle's MB breakdown efficiency at a concentration of 10 ppm in simulated sunlight was 71.99%. The earth's environment is affected when natural components are used as chemical agents in the synthesis of products. Thus, using Terminalia catappa leaf extract as a reducing agent shows promise for a green approach in real-world applications.