Synthesis of TiO₂ nanoparticles using red spinach leaf extract (Amaranthus Tricolor L.) for photocatalytic of methylene blue degradation

ABSTRACT

Textile industry waste such as methylene blue can pose a serious threat to health and ecosystems. One solution to overcome this problem is to utilize photocatalytic technology using TiO₂ semiconductors. This research aims to make green synthetic TiO₂ nanoparticles (NPs) using red spinach (Amaranthus Tricolor L.) leaf extract and used for photodegradation of methylene blue. Extraction of red spinach leaves showed that 30 min and a temperature of 95°C showed the highest phenolic content. In this research, several phenolic compounds were also identified in red spinach extract. For TiO₂ NPs, the X-ray diffraction results show that TiO₂ has a pure anatase phase. Electron microscope results show that TiO₂ has the shape of agglomerated spherical clusters. The particle size distribution shows that TiO₂ has an average size range of 77–90 nm. The methylene blue photodegradation showed that TiO₂ had an efficiency of 90.36% for 180 min, higher than commercial TiO₂ Evonic. This result can be attributed to the small particle size and broadband spectra so that photocatalysis is more effective.

GRAPHICAL ABSTRACT

KEYWORDS:

• TiO₂ nanoparticles
• green synthesis
• red spinach
• photocatalysis

1. Introduction

Industrial production and use of synthetic dyes for textile coloring has become a giant industry today (1). In Indonesia, textile industry is one of the sectors with rapid growth, reaching 15.6% in 2022 according to data from the China Research and Intelligence (CRI) (2). Dye waste generated from the textile industry poses environmental problems. Its toxic nature has become a serious concern for the environment. The use of synthetic dyes harms all forms of life which are usually not biodegradable (3). Therefore, further treatment of textile industry waste is needed before it reaches final disposal. Dyes are among the organic compounds with complex aromatic structures, making them stable and resistant to degradation (4). Methylene blue is one of the azo dyes commonly used by industries, and it can be harmful to the environment (5). Hence, there is a need for environmentally friendly and effective methods to treat wastewater from the textile industry.

Numerous wastewater treatment methods have been developed, but they often raise new environmental issues (6). An alternative method that can be used is the combination of efficient photocatalysis principles for wastewater purification. In photocatalysis, dyes are broken down into simpler components, making them environmentally safe (7). Nanomaterials semiconductor such as ZnO, MnO₂, and TiO₂ are efficient for various reactions due to their large surface area and high surface activity (8). Nanotechnology can be applied in photocatalysis methods by synthesizing TiO₂ NPs. TiO₂ NPs-based photocatalysis are favored because nano-sized TiO₂ (ranging from 1–100 nm) exhibits superior chemical and physical properties compared to larger materials (9). TiO₂ also have better UV light activity, better stability, lower reaction time and wider band gap (3.2 eV). Wider band gap in TiO₂ accelerate photo charge recombination process making it stand-out among other metal oxide NPs (10).

Synthesis of TiO₂ nanoparticles (NPs) can be carried out in various ways, such as the sol–gel, hydrothermal, polyol synthesis, and precipitation (11). The sol–gel method uses a low-temperature process. However, if organic reagents are used during the experiment, the final product will contain a lot of carbon. To synthesize metal oxides directly from the solution, hydrothermal methods have been used. However, most previous studies on the synthesis of TiO₂ NPs via the hydrothermal method required high temperatures, which resulted in the formation of polydisperse powders (12). Until now, not much research has been carried out regarding the synthesis of TiO₂ NPs through a precipitation process using environmentally friendly chemical methods.

Green synthesis is environmentally friendly, safe, and cost-effective. It involves using plant extracts as capping agents in nanoparticle synthesis (13,14). Green synthesis with plant extracts utilizes organic compounds like secondary metabolites such as flavonoids, triterpenoids, polyphenols, and quercetin found in plants to reduce metal ions (15–17). In the presence of plant extract and suitable conditions, such as concentration and temperature, metal nanoparticles can be synthesized and the quality can even surpass those that use chemical method (18). Previous research used extracts from various plants, including white snakeroot (Ageratina altissima), ginger (Curcuma longa), Calotropis gigantea, and Aloe vera as capping agents for synthesis TiO₂ NPs (19–22). In other research, TiO2 NPs were synthesized using Averrhoa carambola and Nervilia aragoana leaf extracts which were used as a photocatalyst for the degradation of textile wastewater and as an antibacterial (23,24). TiO₂ NPs using the help of the extract have good properties such as smaller size compared to chemical methods, thereby having a better ability of photocatalytic activity (10).

In this research, TiO₂ NPs is synthesizes using red spinach (Amaranthus Tricolor L.) leaf extract for photocatalytic degradation of methylene blue. Red spinach leaves were chosen because they contain higher levels of secondary metabolites such as flavonoids and polyphenols than green spinach leaves (25). TiO₂ NPs were synthesized using titanium tetraisopropoxide as a titanium source, water as a solvent, and red spinach extract as a template. The effect of varying extract concentration and precursor concentration on TiO₂ NPs and their potential in photocatalytic applications for methylene blue degradation compare to commercial TiO₂ were also explored.

2. Materials and methods

2.1 Materials

Red spinach (Amaranthus Tricolor L.) was obtained from a local market in Bandung, Indonesia. Red spinach leaves were separated from their stems, finely chopped, and then dried at 45°C for 48 h before being pulverized. The materials used in this study include distilled water, gallic acid (C₇H₆O₅, Merck), absolute ethanol (C₂H₅OH, 99%, Merck), Folin–Ciocalteu reagent (Merck), methylene blue (Merck), sodium hydroxide (NaOH, Merck), hydrochloric acid (HCl, 37%, Merch), sodium carbonate (Na₂CO₃, Merck), and titanium tetraisopropoxide (TTIP 97%, Sigma-Aldrich). All the materials used were of analytical grade.

2.2 Preparation of gallic acid standard

Gallic acid (0.05 grams) was dissolved in 50 mL of methanol and homogenized. Gallic acid was prepared in several standard variations, namely 30, 50, 100, 300, 500, and 750 mg L⁻¹. A 50 µL gallic acid 300 mg L⁻¹ was added to 2.5 mL of 10% Folin–Ciocalteu reagent and 2 mL of 7.5% Na₂CO₃ solution. The solution was incubated at 45°C for 15 min, and the absorbance was measured at maximum wavelength.

2.3 Determination of polyphenol content in red spinach leaf extract

Red spinach leaves (3.6 g) pulverized were added to 100 mL of distilled water. They were then heated at 50°C for 30, 60, 90, and 120 min, filtered, and concentrated. Several grams of concentrated extract were then dissolved in 1 mL of methanol and centrifuged. Next, 50 µL of the solution was added to 2.5 mL of 10% Folin–Ciocalteu reagent, and 2 mL of 7.5% Na₂CO₃ solution was added. The solution was incubated at 45°C for 15 min, and the absorbance was measured at the maximum wavelength. The polyphenol content was then calculated. Once the time yielding the highest polyphenol content was determined, the temperature that produces the highest polyphenol content was sought using the same method at temperatures of 20, 40, 60, 80, and 95°C. The selected extract was then characterized using gas chromatography-mass spectrometry (GC-MS, Asquisition 10.0.368). The total polyphenol content was calculated using Equation (1), where TPC is the total phenolic content (mg GAE g⁻¹), C is the concentration of tea leaves (mg L⁻¹), V represents the volume of solvent (L), and m is the weight of the tea extract used (g). (1) $\mathrm{TPC}=(C\times V)/m$(1)

2.4 Synthesis and characterization of TiO₂ NPs

A total of 2 mL of TTIP was taken and added to 30 mL of ethanol, which was stirred for 1 min. Subsequently, red spinach leaf extract was added in volume ratios TTIP:red spinach leaf extract of 1:3, 1:5, 1:7, 1:10, and 1:15, and the mixture was stirred for 3 h. The solution was then centrifuged for 10 min, and the precipitate was collected. The precipitate was washed three times with distilled water and ethanol, followed by centrifugation for 10 min. Next, the precipitate was dried in an oven for 18 h. The dried precipitate was then calcined at a temperature of 500°C for 3 h. Synthesized TiO₂ NPs for a volume ratio of TTIP:red spinach leaf extract of 1:3, 1:5, 1:7, 1:10, and 1:15 is named as TiO₂-3, TiO₂-5, TiO₂-7, TiO₂-10, and TiO₂-15 respectively.

To determine the crystal structure, phase composition, and crystallite size, X-ray diffraction (XRD, Rigaku/miniflex 600, Tokyo, Japan) was used at room temperature using Cu Kα emission (wavelength λ = 0.15418 nm) and scanning was carried out in the range 20–70° (2θ Bragg) with a scanning speed of 10° min⁻¹. The resulting data was analyzed using the Rietveld method with HighScore Plus software (PANalytical) to determine the crystal structure and phase composition (26). The crystal size of TiO₂ NPs was calculated using the Scherrer equation. To determine the light absorption and band gap energy properties, UV-vis diffuse reflectance spectroscopy (UV-Vis-DRS, Jasco V-550, Tokyo, Japan) was used from the range 220–800 nm. To determine the functional groups contained in the TiO₂ NPs, Fourier-transform infrared (FTIR, Perkin Elmer Spectrum 100) characterization was used in the range 500–4000 cm⁻¹ from the average of 64 times scanning. To determine the thermal properties thermal gravimetric-differential thermal analysis (TG-DTA, Shimadzu, Japan) was used at a temperature of 20–900°C with a heating rate of 10°C/min and heated in air. To determine the morphology, size, and elements contained in the compound, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS, Tabletop Microscope-1000, Hitachi, Tokyo, Japan) and transmission electron microscope (Talos F200X G2 TEM, Thermo Fisher Scientific, Tokyo, Japan) was used.

2.5 Photocatalytic activity test

The photocatalysis activity was analyzed using a methylene blue dye solution. A 100 mL of methylene blue 10 mg L⁻¹ was prepared, and 0.1 g of synthesized TiO₂ nanoparticles was added. The pH was adjusted using 0.1 M HCl and NaOH. The solution was stirred at room temperature for 60 min for reach adsorption–desorption and 180 min for UV-B irradiation (Exo Terra Reptile UVB150), with samples taken every 30 min. The precipitate and solution were separated by centrifugation. The absorbance at the maximum wavelength was measured using UV-vis spectroscopy.

3. Result and discussion

3.1 Analysis of secondary metabolite content in red spinach leaf extract

3.1.1 Polyphenol content

In this experiment, the part of red spinach used was the leaf. In the extraction process, distilled water was used with the aim of reducing organic solvents in the extraction process and having the same polarity as phenolic compounds (27). In the determination of polyphenols, the principle of the Folin–Ciocalteu method is that when Folin–Ciocalteu reacts with phenolic compounds originating from polyphenols (Fig. S1). The reaction between the Folin–Ciocalteu reagent and polyphenols in the solution can be seen in Fig. S2 (28).

The maximum polyphenol content was obtained by heating the red spinach leaf powder at various time intervals at a temperature of 50°C. Figure 1(a) shows that the maximum total polyphenol content (TPC) was 2.37 mg GAE g⁻¹ for 30 min. After 30 min, there was a decrease in polyphenols. This can happen because the polyphenols in red spinach will decompose, thereby reducing the polyphenol content obtained. After determining the optimal time, heating was performed at various temperatures for 30 min to find the best temperature. Figure 1(b) shows that the highest polyphenol content in the red spinach leaf extract was at 95°C, which was 1.59 mg GAE g⁻¹. At this temperature the polyphenol content is highest because at high temperatures the polyphenols released are high because the solubility of phenolic compounds is greater. Therefore, the optimum time and temperature extraction was carried out at 95°C for 30 min.

Figure 1. Total polyphenol content (TPC) for (a) various of time at temperature of 50°C and (b) various of temperature for 30 min.

3.1.2 Secondary metabolites compounds

To determine the secondary metabolites compounds in the red spinach leaf extract, GC/MS analysis was performed. The analysis results using GC/MS can be seen in Table 1 and Fig. S3, which shows the identification of 8 compounds in the red spinach leaf extract. Based on the results, the compounds that play a role in the synthesis of TiO₂ nanoparticles are identified. 2-amino-5-methyl benzoic acid is a derivative of anthranilic acid (29). 3-buten-2-ol is an organic compound containing hydroxyl groups and a double bond as an alkene. 2-(3,4-dimethoxyphenyl)−5-hydroxy-6,7,8-trimethoxy-4H-chromen-4-one is polymethoxyflavone, and 3’,4’,5,6,7,8-hexamethoxyflavone is flavonoid group (30). Polymethoxyflavones and flavonoids are polyphenol category (31). As for 4,4'-bi-4H-pyran, 2,2’,6,6'-tetrakis(1,1-dimethylethyl)−4,4'-dimethyl is an organic compound with 15 carbons, which is a cyclic sesquiterpene alcohol (32). β-Amyrin is a chemical compound belonging to the triterpene class (33). Based on previous research, red spinach leaves contain secondary metabolites including anthocyanins, flavonoids, tannins, saponins, and squalene (34). This result is also similar to Fatimah and Aftrid’s research which shows that in spinach leaves there are at least 13 types of flavonoids and groups of phenolic compounds in spinach, namely gallic acid, caffeic acid, routine, ferulic acid, and quercetin (35).

Table 1. Compounds detected in the red spinach leaf extract.

Compounds	Chemical formulas	Structures	Retention times
2-amino-5-methyl benzoic acid	C8H9NO2		4.594
3-buten-2-ol	C4H8O		29.864
3’,4’,5,6,7,8-hexamethoxyflavone	C21H22O8		31.664
2,4,4-trimetyl-3-hydroxymetyl-5a-(3-metyl-but-2-enyl)-cyclohexene	C15H26O		32.284
2-(3,4-Dimethoxyphenyl)-5-hydroxy-6,7,8-trimethoxy-4H-chromen-4-one	C20H20O8		33.507
2,6,10-Dodecatrien-1-ol, 3,7,11-trimetil-9-(fenilsulfonil)-, (E,E)-	C21H30O3S		34.631
4,4'-bi-4H-pyran, 2,2’,6,6'-tetrakis(1,1-dimethylethyl)-4,4'-dimethyl	C28H46O2		35.706
β-Amyrin	C30H50O		38.794

3.2 Properties of TiO₂ NPs

The XRD pattern of the TiO₂ NPs are presented in Figure 2(a). The XRD pattern of all synthesized TiO₂ NPs closely matches the anatase Inorganic Crystal Structure Database (ICSD) reference code 98-002-4276, indicating that all samples are pure anatase and adopt a tetragonal structure. The peaks obtained at 2θ = 25.3°, 37.9°, 48.4°, 53.9°, 55.3°, and 62.7° are equivalent to the (011), (004), (020), (015), (121), and (024) planes (36). The lattice parameters of TiO₂ NPs were determined, representing the physical dimensions of the unit cell in the crystal lattice. Table 2 shows the lattice parameters of the synthesized TiO₂ NPs. Differences in extract volume variations did not significantly change the lattice parameters. However, there was a slight change in the a and b-axis which slightly increased with the amount of extract in the synthesis stage. On the other hand, the c-axis slightly decreases with the amount of extract, indicating the distortion of the crystal of green synthesis. To confirm the accuracy of the Rietveld refinement calculations, the goodness of fit (GoF) value is calculated. GoF is a statistical measure that assesses how well the experimental results match a set of observations (37). The GoF values from all samples are below 4 and indicate the accuracy of the calculations. The crystallite size of TiO₂ NPs after calcination is shown in Table 2, which indicates that the larger the amount of extract used, the larger the crystal size obtained. These results are smaller than in the TiO₂ synthesis using a commercial capping agent polyethylene glycol (PEG), ranging from 22–31 nm (38), which concluded that green synthesis produces smaller crystallite sizes.

Figure 2. (a) XRD pattern, (b) UV-vis spectra, (c) FTIR spectra, and (d) TGA graph of TiO₂ NPs.

Table 2. Crystal properties of TiO₂ NPs.

Sample	Lattice parameters (Å)		Crystallite size (nm)^1	GoF
	a = b	c
TiO2-3	3.7882(4)	9.4971(11)	10.67	2.43
TiO2-5	3.7878(4)	9.4880(12)	11.04	2.91
TiO2-7	3.7910(4)	9.4894(12)	12.20	2.78
TiO2-10	3.7906(3)	9.4917(9)	16.67	3.33
TiO2-15	3.7915(3)	9.4889(10)	16.84	3.54

¹ Calculated using the Scherrer equation from (011).

The optical properties of the samples were investigated by diffuse reflectance UV – vis spectroscopy. The absorption spectra in the range of 220–800 nm of the synthesized TiO₂ NPs is shown in Figure 2(b). In general, all TiO₂ absorbs light in the UV range (220–400 nm). However, there is a slight difference in absorption intensity at 325 nm. This difference in absorption is likely due to differences in crystal lattice parameters (39). Band gap energy from TiO₂ NPs is calculated using the Davis – Mott plot for indirect band gap (Fig. S4). All samples’ band gap energy values are around 2.98–3.00 eV. These results show that all variations of TiO₂ NPs do not show significant differences in band-gap values.

FTIR analysis was carried out to identify the functional groups of the synthesized TiO₂ NPs shown in Figure 2(c). The FTIR spectra of TiO₂ NPs shows a broad and strong absorption band in the wavenumber range 505–530 cm⁻¹, where this absorption band indicates the vibration of the O−Ti−O bond, thus confirming that the synthesized nanoparticles are TiO₂ compounds (40). In addition, there is an absorption band at the wavenumber 2350 cm⁻¹ which can be caused by saturated carbon dioxide bonds adsorbed on the catalyst (41). These results indicate that the TiO₂ NPs synthesized are pure and no other bonds are formed.

In this FTIR data, secondary metabolite compounds from red spinach leaf extract are no longer present in TiO₂ NPs. This is because secondary metabolite compounds disappear during the calcination process at a temperature of 500°C. This result is proven by TGA data on TiO₂ NPs before calcination (Fig. S5). The mass of TiO₂ NPs decreases drastically at a temperature of 50–500°C which is probably due to the disappearance of secondary metabolite compounds from red spinach leaf extract. Mass loss in TiO₂-3 before calcination to a temperature of 500°C was 25.6%, while in TiO₂-15 it was 28.7%. This shows that the greater the volume ratio of red spinach leaf extract used during the synthesis process, the more metabolites are contained in TiO₂ NPs.

Figure 2(d) shows the TGA graph of TiO₂ NPs after calcination. TiO₂ experiences surface water loss up to 200°C. After that, weight loss occurred at a temperature of 200–400°C mainly due to evaporation, partial dehydration of structural water, or due to the presence of some impurities from red spinach extract (42). There are slight differences between TiO₂ variations. TiO₂-3 appears to have more water contained in the surface and structural water than TiO₂-15. This may also be related to particle size (see next section particle size distribution). TiO₂-3 has a smaller particle size distribution so it is possible to absorb more water. After that, there was an increase in mass caused by the oxidation of TiO₂. The result of this oxidation is usually a TiO₂ phase transformation from anatase to rutile which has a different density (43). Fig. S6 shows the TG-DTA curve of TiO₂-3 and TiO₂-15. Both show exothermic reactions during the heating of TiO₂ to 900°C.

Figure 3(a) shows the SEM image of TiO₂-3. TiO₂ NPs show an agglomerated spherical particle shape. In addition, other variations of TiO₂ NPs are shown in Fig. S7a-e. All variations of TiO₂ NPs have a homogeneous round particle shape. Additionally, EDS mapping confirmed the high purity of the TiO₂ sample, which contains only titanium and oxygen atoms (Fig. S7f). The percentage of Ti and O atoms from the EDS results is shown in Table S1. Further characterization was carried out using TEM (Figure 3b) to observe TiO₂-3 more closely which showed the presence of a spherical structure. It is known that the particle size ranges from 90–100 nm.

Figure 3. (a) SEM and (b) TEM image of TiO₂-3.

The particle size distribution of TiO₂ NPs is presented in Figure 4. The smallest average particle size distribution of TiO₂ NPs is TiO₂-3, namely 77 ± 21 nm. The particle size distribution increased with the amount of extract added. This shows that if too much extract is added, it will make making nanoparticles ineffective. Apart from that, TiO₂-3 also has relatively high homogeneity compared to other variations, which shows that excess extract results in the growth of nanoparticles not being evenly distributed.

Figure 4. Particle size distribution of (a) TiO₂-3, (b) TiO₂-5, (c) TiO₂-7, (d) TiO₂-10, and (e) TiO₂-15.

In the process of forming TiO₂ NPs, red spinach leaf extract is used as a capping agent or stabilizing agent to stabilize the nanostructure (44). Commercially, commonly used capping agents include surfactants (cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS)), polymers (polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and poly-α, γ, L-glutamic acid (PGA)), and thiols (dodecanethiol and thioglycerol) (45,46). In red spinach leaf extract, secondary metabolites act as capping agents or stabilizers by coating the TiO₂ NPs formed, preventing agglomeration, reducing particle size, and forming a stable and homogeneous structure. The mechanism of the synthesis process that occurs in TiO₂ NPs can be seen in Figure 5. Functional groups contained in secondary metabolites are attached to the outer surface of TiO₂ nanoparticles through covalent or hydrogen bond interactions. As a result, the hydrophilic structure of secondary metabolites sticks to the TiO₂ NPs, while the hydrophobic structure remains outside (47,48).

Figure 5. Mechanism of formation green synthesis TiO₂ NPs with red spinach leaf extract.

3.3 Photocatalytic activity of TiO₂ NPs

In this research, methylene blue was used as a photocatalysis target because it is a common compound that can be found in industrial waste (49). Fig. S8 shows the maximum wavelength and standard curve of methylene blue. Before testing, photolysis was carried out in the absence of a photocatalyst. As shown in Figure 6(a), no activity was observed for photolysis for methylene blue degradation. When TiO₂ NPs is added, degradation occurs which indicates photocatalytic activity. TiO₂-3 has the highest activity with a degradation efficiency of 90.36% after 180 min. The Langmuir – Hinshelwood first-order kinetic model is shown in Figure 6(b). Green synthesized TiO₂ NPs showed better reaction rate kinetics for all variations compared to P25 Evonic (commercial), with the highest being TiO₂-3. This is due to the properties described previously, that TiO₂-3 has a smaller average particle size, allowing it to have a larger surface area. This large area causes photocatalysis reactions on the surface to occur efficiently. Table 3 shows several previous reports of the photocatalytic activities of TiO₂ synthesized using plant extracts, clearly showing that our data on photoactivity are compared with those published in the literature.

Figure 6. (a) Photocatalytic activity, (b) kinetics value for all TiO₂ NPs, and (c) photocatalytic activity of TiO₂-3 on different pH for methylene blue degradation.

Table 3. Comparison of the photocatalytic activity with several previous reports of TiO₂ synthesized using plant extracts.

Extract	Pollutant	Time	Constant rate (min^–1)	Efficiency of Degradation (%)	Ref.
Ageratina altissima (L.) King	MB	360 min	–	87	(19)
Carica papaya	RO–4	180 min	0.0088	91	(50)
Aloe vera	RhB	180 min	0.0023	41	(51)
Pulicaria undulata	MO	120 min	–	95	(52)
Mulberry	MB	120 min	0.0240	96	(53)
Lemon	RhB	120 min	–	70	(54)
Amaranthus Tricolor L.	MB	180 min	0.0131	90	This work

*MB: methylene blue, RO–4: reactive orange 4, RhB: rhodamine B, MO: methyl orange.

When TiO₂ is exposed to UV light, electrons (e^– ) are excited from the valence band (VB) to the conduction band (CB) and leave holes (h⁺) in the VB. Electron accumulation in CB reduces O₂ to ⋅O₂^– (36). In addition, h⁺ accumulation can develop ⋅OH because the VB position is more positive than the OH/⋅OH potential (+2.38 eV vs. NHE) (55). These ⋅OH and ⋅O₂^– species can degrade MB in solution. Meanwhile, photogenerated h⁺ due to electron transitions from the valence band (VB) has strong oxidation capabilities and can directly degrade MB to produce less harmful products such as water and carbon dioxide (56).

Photocatalytic activity can be influenced by several factors including band gap energy, size, morphology, recombination rate, pH, and surface area of the material (57). The difference between pH 7 and pH 10 is shown in Figure 6(c). An initial pH of 10 showed better methylene blue degradation efficiency than pH 7. The properties of methylene blue and TiO₂ may explain this. The TiO₂ photocatalyst has a point of zero charge (pH_PZC) of 6.25. As a result, the surface is mostly negatively charged over pH_PZC (TiOH + OH^– ↔ TiO^–  + H₂O) (36). In addition, methylene blue, a cationic dye, is almost completely cationic (58). Thus, methylene blue and TiO₂ show an attractive effect in alkaline pH, favoring the adsorption ability. The occurrence of electrostatic interactions is useful for improving adsorption properties thereby increasing the efficiency of concentration reduction. Meanwhile, at low pH, the efficiency of concentration reduction tends to be low due to electrostatic repulsion between TiO₂ NPs and methylene blue dye (59).

4. Conclusions

TiO₂ NPs were successfully made by green synthesis using red spinach leaf extract. Red spinach extract contains many different polyphenolic compounds, which can help the formation of nanoparticles. The extraction process of red spinach leaves shows that the highest amount of polyphenol is within thirty min and at a temperature of 95°C. The crystallite size of the resulting TiO₂ NPs ranges from 10–16 nm and has a pure anatase structure. The presence of O – Ti – O groups and CO₂ bonds that can be adsorbed on TiO₂ NPs was discovered through FTIR analysis. The agglomerated and spherical shape of the TiO₂ synthesis results is shown in the SEM image. Methylene blue (MB) as a dye was used to check the photocatalytic ability of the prepared TiO₂ NPs. The results showed that the sample synthesized using a volume ratio of TTIP and extract of 1:3 had a smaller particle size (77 nm), contributing to a degradation efficiency of 90.36% after two hours of ultraviolet irradiation. The method developed in this research is fast and environmentally friendly for producing TiO₂ NPs with good photocatalytic activity.

Disclosure statement

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

Additional information

Funding

This work was supported by [Universitas Padjadjaran] Grant Hibah Riset Unpad (HRU) by Riset Kompetensi Dosen Unpad (RKDU) the Academic Leadership Grant (ALG) under Grant the Academic Leadership Grant (ALG), Prof. Iman Rahayu [number 1764/UN6.3.1/PT.00/2024] and [Universitas Padjadjaran] under Grant Hibah Riset Unpad (HRU) by Riset Kompetensi Dosen Unpad (RKDU), Diana Rakhmawaty Eddy [number 1764/ UN6.3.1/PT.00/2024]. The authors extend their appreciation to the Deanship of Scientific Research at Northern Border University, Arar, KSA for funding this research “work through the project number “NBU-FFR-2024-540-04”.