Synthesis TiO2/ZnO Nanoparticles Based on Plant Extracts: Comparison
Please note this is a comparison between Version 1 by Predrag Putnik and Version 2 by Jason Zhu.

Possible eco-friendly synthesis of TiO2, ZnO, as well as a few special nanomaterials, using different plant extracts instead of harmful organic solutions. The main role of the biomolecules in the synthesis of different NPs is the reduction of metal salts, as well as capping and stabilizing them. Hence, the plant-mediated nanomaterials have a variety of different shapes and sizes comparing to the general, chemical-based synthesis processes. Furthermore, these bio-compounds not only reduce the metal salts, but also functionalize the surface of the newly synthesized NPs, which includes synergistic effects for various applications. Furthermore, the pure plant extracts can also act as catalysts. Even though, that these phytochemicals are mainly used as reduction agents, they can also possess photoactivity and can be used as photocatalysts. 

  • TiO2/ZnO
  • plant extract

1. Introduction

Undoubtedly, there are high hopes for the application of nanomaterials not only as photocatalysts in heterogeneous photodegradation but in other scientific fields too. However, the conventional techniques have their limitations, such as expensive equipment, toxic and non-biodegradable precursors, the need for expertise, low yield of product, as well as long reaction times [1][2][29,30].
To this end, green and sustainable methods should be developed, in order to eliminate or at least decrease the application of harmful chemicals, which negatively affect the environment and living organisms. Fortunately, the scientific society noticed the importance and advantages of green techniques. The number of publications about the topic of green synthesis of different nanocomposites with photocatalytic activity has been increasing continuously since 2018 (Figure 15).
Figure 15.
Number of publications on the “green synthesis” topic for photocatalytic purposes (Scopus, September 2021).

2. Synthesis TiO2/ZnO Nanoparticles Based on Plant Extracts 

One example for the green approaches in the synthesis of TiO2 nanoparticles is explained in the study by authors Dash et al. [1][29]. In their work the leaf extract of Azadirachta indica was applied. For the preparation of the plant extract, firstly, the fresh, green leaves were collected and washed with tap water. After that, they were also washed using cetyltrimethylammonium bromide (CTAB) solution as well as with distilled water and with 2 M NaOH solution. After washing the leaves, they were dried at ambient temperature. When the drying process was finished, 3 g of the finely cut leaves were mixed with 200 mL distilled water and boiled until the amount of water decreased to 60 mL. Finally, the mixture was filtered and the extract was stored for further use. In order to prepare a mesoporous form of TiO2, different amounts of plant extract (8, 12, 16 and 20 mL) were mixed with 0.4 mL of titanium tetraisopropoxide. The reaction mixtures were continuously stirred for 12 h at 35 °C. Then, the temperature was step-by-step increased to 70 °C to eliminate the water from the solution. Finally, after evaporating the water, the obtained product was calcined at 400 °C for 3 h. The formation of white powder proved the successful synthesis of TiO2. The newly synthesized NPs, depending on the amount of the plant extract (8, 12, 16 and 20 mL) were named as MTO-8, MTO-12, MTO-16 and MTO-20, respectively (Figure 26).

Figure 26. Schematic preparation of TiO2 NPs (size range of 240−410 nm) using leaf extract of Azadirachta indica. Reprinted with permission from Ref. [1][29], Copyright 2021 Elsevier.
For the characterization of the green synthesized NPs, various techniques were applied, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis spectrometry, Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) analysis. The FTIR analysis proved the TiO2 structure. All the investigated samples showed characteristic broad peaks in the region of 480–900 cm−1, which were corresponding to the stretching vibrations of Ti-O-Ti linkage. Additionally, the findings showed that in the samples with higher amount of leaf extract, the stretching vibration decreased. The SEM images determined that the particle size of the newly synthesized NPs were in the range of 240−410 nm. They also confirmed that the initial amount of the plant extract has an effect on the size of the NPs. Namely, increasing the amount of the extract resulted in decreased particle size, which can be explained with the higher amount of the biomolecules present in the reaction system. The XRD analysis confirmed the anatase phase of TiO2 and also showed that the crystallinity of TiO2 increased with increasing amount of leaf extract. The average crystal sizes determined by XRD for MTO-8, MTO-12, MTO-16 and MTO-20 were 16.8, 14.5, 13.3 and 12.7 nm, respectively. The UV-Vis results showed absorbance at 335, 322, 304, and 292 nm for the MTO-8, MTO-12, MTO-16 and MTO-20 NPs, respectively. A significant blue shift in the absorbance was observed which is resulted by the higher crystallinity in the samples with higher amount of leaf extract. The bandgaps were also calculated for the newly synthesized NPs, and were 3.08, 2.91, 2.81 and 2.66 eV for MTO-20, MTO-16, MTO-12 and MTO-8, respectively. BET findings showed the surface of MTO-20, MTO-16, MTO-12 and MTO-8 which was found to be 157.35, 91.87, 39.12 and 8.55 m2/g. In addition, the pore volumes were also determined (i.e., they were found to be 0.31, 0.26, 0.17 and 0.14 cm3/g for MTO-20, MTO-16, MTO-12 and MTO-8, respectively) [1][29].
The green synthesis of a triple nanocomposite was also investigated. Namely, Jiang et al. [3][34] applied Cinnamomum camphora leaf extract in the synthesis of Au-Ag/TiO2 catalyst for photocatalytic purposes. The leaf extract was prepared by grinding and washing the leaves, which were followed by their drying at ambient temperature. Two grams of this powder were dissolved in 100 mL of distilled water and stirred for 4 h. Finally, the mixture was filtered and stored at 4 °C for further use. The leaf extract acted as both reducing and capping agent. Whereas the Au-Ag bimetallic and Au-Ag/TiO2 NPs were synthesized using the following procedure. First, 0.1 mL of 100 mM AgNO3 solution was mixed with 20 mL of leaf extract and stirred at 30 °C for one day. After that, 0.2 mL of 50 mM HAuCl4 was added to this solution and the reaction was carried out at 30 °C for 2 h which ended up with the formation of bimetallic NPs. Afterwards, for the synthesis of Au-Ag/TiO2 catalyst the previously prepared bimetallic NPs were used. Firstly, 0.5 g TiO2 was added to the NPs sol at ambient temperature and stirred for 4 h. This was followed by filtration and drying at 60 °C. Following that, 0.5 g of [BMIM]PF4 was mixed with 0.5 g of dried catalyst and 20 mL of deionized water, stirred for 2 h and dried at 60 °C. Finally, the catalysts were calcined at 350 °C as well as at 400 °C for 4 h. For the characterization, different techniques were used: Transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), XRD, FTIR, inductively coupled plasma with mass spectrometry [3][34]. The TEM and STEM results confirmed the successful synthesis of the NPs and the presence of Ag and Au alloyed metals. Additionally, the TEM analysis determined a spherical shape and a uniform size of 12.6 ± 1.7 nm. The FTIR spectra proved the successful green synthesis of these NPs, as well as the oxidants and capping agent ability of the Cinnamomum camphora leaf extract. The XRD analysis proved the anatase phase of TiO2 without any obvious changes when Au-Ag metals were added. The photocatalytic activity of the green synthesized NPs was also investigated in the photodegradation of methyl orange (MO) under UV-Vis irradiation. Findings showed a high activity of the newly synthesized NPs. Specifically, with the fresh Au-Ag/TiO2 89.4% of MO was degraded after 60 min of irradiation. In addition, the photocatalytic activity was also examined in the case of mix dyes (MO; methylene blue, MB; and rhodamine B). The results showed that all of the absorption peaks in the UV-Vis spectra disappeared after 60 min of irradiation, which confirmed the high photodegradation efficiency. In addition, the stability of the NPs was also questioned. According to the results it was concluded that after 5 cycles of photodegradation, there were no changes in the activity of the NPs, which confirmed a possible industrial utilization usage of these NPs [3][34].
Udayabhanu et al. [4][36] investigated the possible use of Euphorbia hirta leaf extract in the synthesis of TiO2 nanoparticles for photocatalytic application. For the extract preparation the leaves were firstly washed from impurities and the extract was prepared using microwave irradiation. Mass of 20 g of the leaves were cut into small pieces, homogenized and mixed with distilled water. The mixture was boiled under microwave irradiation for 10 min. The extract was cooled down to room temperature and filtrated. The obtained Euphorbia hirta extract acted as both reducing and capping agent for TiO2 synthesis. For the green synthesis, a mixture of 100 mL of leaf extract and 900 mL of 5 mM TiO4 solution was prepared and incubated under sunlight for 24 h. To complete the formation of NPs, the reaction mixture was centrifuged at 8000 rpm for 10 min and the formed pellet was dissolved using several drops of hydrofluoric acid (40%) and washed several times. Finally, it was dried at ambient temperature for two days to turn it into powder form. For the characterization the following techniques were used: UV-Vis spectrometry, FTIR, SEM, EDX and XRD [4][36]. The UV-Vis spectrophotometry was used to confirm the reduction of titanium ions. Peak has appeared in the spectra at 420 nm, which was originated from the TiO2 in the examined samples. The FTIR analysis determined the presence of biologically active compounds that were adsorbed to the surface of green synthesized NPs. It is believed that the bioactive substances may have involved in the bio-reduction of TiO2 NPs. The XRD patterns confirmed the cubic phase of titanium.
The possibilities of the banana peels extract in the synthesis of ZnO were also investigated by Fernanda et al. [5][46]. For the preparation of the extract, 150 g of banana peel was washed and boiled in 150 mL deionized water. Then, the boiled peels were pounded until smooth and mixed with the earlier used water. After that, the mixture was filtered using a cloth. For the synthesis of ZnO, 500 mL of ZnSO4 (0.0783 M) was added to 129.47 mL of banana extract. The pH was set to 12 by adding NaOH solution and the mixture was washed using a Buchner funnel until the filtrate showed neutral pH. The product after the filtration was dried in an oven at 60 °C until its weight was constant. For the characterization, XRD analysis was used. The findings showed that the size of crystals was 18.86–20.72 nm which depends on the used type of banana (i.e., on the concentration of secondary metabolites contained in the extracts). The synthesized particles are believed to be useful photocatalysts in water purification [5][46].
The characteristics and efficiency of the green synthesized photoactive nanoparticles are presented in the Table 13.
Table 13.
Basic information about the discussed methods of green synthesis and the main characteristics of the synthesized NPs.
Type of Catalyst Applied Plant Extract in Experiments Method of Synthesis Size of the Newly Synthesized Particles Structure of the Newly Synthesized Particles Type of Pollutant in the Photocatalytic Experiments Applied Irradiation Efficiency of the Photocatalytic Degradation (%) Reaction Rate Constant Study
TiO2 Leaf extract of Azadirachta indica Plant-mediated synthesis Average crystal size in the range of 12.7–16.8 nm Mesoporous structure of TiO2 Rhodamine 6 G UV irradiation 64% after 57 min of irradiation 0.0321 min−1 [1][29]
TiO2 Aloe Vera gel from the plant leaf Hydrothermal synthesis Size of pure TiO2 57 nm, while the Ag@TiO2 38 nm Combination of anatase and rutile phase Picric acid Visible irradiation After 50 min of irradiation a decent amount of PA was removed Not mentioned [6][33]
TiO2 Leaf extract of Cinnamomum camphora Synthesis under ambient conditions 12.6 ± 1.7 nm Spherical shape and anatase phase of the Au-Ag/TiO2 methyl orange (MO), rhodamine B and methylene blue UV-Vis irradiation (Xe lamp) 89.4% of MO after 60 min of irradiation; Complete degradation in the case of mixture dyes after 60 min of irradiation 0.0356 min−1 in the case of MO degradation; For the mixture the constant was not mentioned [3][34]
TiO2 Leaf extract of Deinbollia pinnata Sol–gel method Average crystal size in the range of 19–21 nm Aggregated, semi-spherical shape with anatase phase Methyl orange UV irradiation 97.53% after 150 min of irradiation Not mentioned [7][35]
TiO2 Leaf extract of Euphorbia hirta Plant-mediated synthesis Avarage crystal size in the range of 20–50 nm Spherical shape and cubic phase of TiO2 Methylene blue (MB), MO, alizarin red (AR) and crystal violet (CV) Direct sunlight 86.8% (CV); 81.3% (AR); 77.5% (MO) after 6 h of irradiation Not mentioned [4][36].
ZnO Leaf extract of Syzygium Cumini Not mentioned 11.35 nm Agglomerated, well-crystallized hexagonal wurtzite structure Methylene blue Sunlight irradiation 91.4% after 180 min of irradiation Not mentioned [8][37]
ZnO Pullulan, product of Aureobasidium pullulans fungus Precipitation method Average particle size 110.86 nm Flower-like strucutre Methyl orange UV irradiation 97% after 300 min of irradiation Not mentioned [9][10][38,39]
ZnO Leaf extract of Cinnamomum tamala Plant-mediated synthesis Average particle size 35 nm Hexagonal wurtzite crystallite structure Methylene blue Direct sunlight 98.07% after 90 min of irradiation Not mentioned [11][40]
ZnO Plant extract of Gynostemma pentaphyllum Co-precipitation method 35.41 nm Hexagonal structure of crystalline nanoparticles Malachite green UV irradiation 89% after 180 min of irradiation Not mentioned [12][41]
ZnO Peel extract of Cavendish bananas Plant-mediated synthesis 15.3 nm Triangular and spherical shaped particles with hexagonal wurtzite structure BB9 organic dye; Crystal violet (CV) and Congo red (CR) UV-Vis irradiation (xenon lamp) 100% of BB9 after 90 min of irradiation; 97.79% of CV and 81.70% of CR after 420 min of irradiation 0.5254 h−1 for CV and 0.2837 h−1 for CR [13][42]
ZnO Leaf extract of Alchornea laxiflora Plant-mediated synthesis 29–38 nm, depending on the volume of leaf extract Quasi-hexagonal shape with hexagonal crystallographic phase Congo red Direct sunlight 87% after 60 min of irradiation 0.0401 min−1 [14][43]
ZnO Peel extract of banana Plant-mediated synthesis 18.86–20.72 depending on the type of banana Nanocrystalline ZnO Not mentioned Not mentioned Believed to be effective in the photodegradation Not mentioned [5][46]
ZnO Jujube fruit extract Plant-mediated synthesis 19 nm Highly spherical shape with hexagonal wurtzite structure Methylene blue (MB) and Eriochrome black-T (ECBT) Direct sunlight 85% of both dyes after 300 min of irradiation 0.0087 min−1 for MB and 0.0067 min−1 for ECBT [15][47]
ZnO Leaf extract of Prunus cerasifera Plant-mediated synthesis Average crystal size 12 nm Aggregated spheroidal shape with wurtzite hexagonal phase Bromocresol green (BG), Bromophenol Blue (BB), Methyl red (MR) and Methyl blue (MB) Direct sunlight 93.12% of BG; 90.54% of BB; 88.49% of MR and 76.76% of MB after 10 min of irradiation Not mentioned [16][48]
ZnO Leaf extract of Becium grandiflorum Biological approach Average crystal size of 20 nm Hexagonal wurtzite structure Methylene blue UV irradiation 69% after 200 min of irradiation 0.0019 min−1 [17][49]
ZnO Root extract of Codonopsis lanceolata Modified co-precipitation method 500 nm Spherical, flower-like shape with hexagonal wurtzite structure of ZnO Methylene blue UV irradiation 90.3% after 40 min of irradiation 0.057 min−1 [18][50]
ZnO Leef extract of Peltophorum pterocarpum Plant-mediated synthesis 11.64 nm Flowershaped particles with hexagonal wurtzite phase of ZnO Methylene blue Sunlight irradiation 95% after 120 min of irradiation 0.021 min−1 [19][51]
ZnO Husk extract of Zea mays (Z-ZnO) and peel extract of Artocarpus heterophyllus (A-ZnO) and Punica granatum (P-ZnO) Co-precipitation method under low temperature 28 (Z-ZnO), 55 (A-ZnO) and 25 (P-ZnO) nm Z-ZnO flower-like; A-ZnO cauliflower-like and P-ZnO small nanoflower structure with hexagonal ZnO wurtzite phase Antibacterial activity Visible light irradiation 93.2% (Z-ZnO), 85.7% (A-ZnO) and 99.2% (P-ZnO) after 180 min of irradiation 0.0130 (Z-ZnO), 0.0091 (A-ZnO) and 0.0280 (p-ZnO) min−1 [20][52]
ZnO Leaf extract of Sapindus mukorossi Plant-mediated synthesis 10–1000 nm Spherical-spiral shape Methylene blue Sunlight irradiation 99% (ZnO-PMMA); 98% (Ni2O3-PMMA); 93% (CuO-PMMA); 90% (Fe3O4-PMMA) after 130 min of irradiation 0.1349 (ZnO-PMMA); 0.1321 (Ni2O3-PMMA); 0.1263 (CuO-PMMA); 0.1231 (Fe3O4-PMMA) min−1 [21][53]
ZnO Leaf extract of curry with coconut water Plant-mediated synthesis 1.80, 1.62 and 1.88 nm with respect to 10-, 15- and 20-mL concentration of extract Agglomerated, irregular spherical shape Methylene blue Sunlight irradiation 98.45% after 60 min of irradiation 0.0579 min−1 [22][54]
ZnO Leaf extract of Stevia rebaudiana Co-precipitation method Average crystallite size 4.71 nm Agglomerated flower-like shape with hexagonal wurtzite structure of the ZnO Methylene blue UV irradiation 76% after 30 min of irradiation Not mentioned [23][55]
ZnO Root extract of Saponaria officinalis Precipitation method 42–5500 nm Sowrd-like shapes with hexagonal wurtzite phase of ZnO Methylene blue Visible light irradiation 15–42% depending on the applied catalyst, after 40 min of irradiation Lower than the used reference value (i.e. lower than 0.0344 min−1) [24][56]
ZnO Leaf extract of Amaranthus dubius Plant-mediated synthesis 82–250 nm for ZnO and 71–280 nm for 1% Fe-ZnO Spherical cubic phase Naphthalene UV irradiation 63.5% (ZnO) and 71.7% (Fe-ZnO) after 240 min of irradiation 0.0045 (ZnO) and 0.0054 (Fe-ZnO) min−1 [25][57]
ZnO Leaf extract of Rosemary Plant-mediated synthesis Average crystalline size 28.946 ± 0.002 nm Quasi-hexagonal structure with high degree of agglomeration Textile effluent Visible light irradiation 63% after 100 min of irradiation 0.0111 s−1 [26][59]
ZnO Leaf extract of Solanum lycopersicum Plant-mediated synthesis Average crystalline size 33 nm Agglomerated spherical shape with hexagonal wurtzite structure of ZnO Congo red Sunlight irradiation 80% after 300 min of irradiation Not mentioned [27][60]
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