TiO2/Co3O4 nanocomposite: Synthesis via Catharanthus roseus (L.) G. Don leaf extract, characterization and its photocatalytic activity for malachite green degradation

TiO2/Co3O4 nanocomposite was successfully synthesized using leaf extract of Catharanthus roseus (L.) G. Don. (CRE), precursors of TTIP and Co(NO3)2. FT-IR characterization shows that TiO2/Co3O4 has been formed due to the presence of secondary metabolites in CRE which have a major role as a weak base source for nanocomposite synthesis. Particle size analysis was employed by PSA and TEM, which were found to be 38.04 nm and 90.80 nm, respectively. Besides, the UV-Vis DRS result shows that TiO2/Co3O4 has bandgap energy at 2.8 eV. According to the morphological analysis, TiO2/Co3O4 nanocomposite has a spherical shape confirmed by SEM. The structural analysis was confirmed by XRD that TiO2/Co3O4 nanocompositehas a diffraction pattern combination of TiO2 and Co3O4 nanoparticle. The photocatalytic activity of TiO2/Co3O4 nanocomposite was examined for malachite green degradation. The result shows that the degradation percentage was 82.61 % under visible light illumination for 120 min.


Introduction
The utilization of plant media in nanoparticle synthesis is one step to reduce the risk in the use of harmful substances to the environment. Catharanthus roseus (L.) G. Don. is one of the plants for metal oxide nanoparticle synthesis, containing 11.11 % of alkaloids in the leaves as a source of weak base and capping agents [1]. Datura metel L., Tinospora crispa, and Moringa oleifera have been reported as tropical plants for metal oxide nanoparticle synthesis [2][3][4].
On the other hand, TiO2 is a semiconductor that has a high photocatalytic activity in the form of anatase [5] and has a high melting point, photoactivity, and thermal stability. Also, TiO2 is an excellent catalyst to be applied in the environment since it is non-toxic, low cost and inert material [6]. TiO2 has been reported to have a bandgap energy (Eg) around 3.0-3.2 eV [7]. The value of Eg is related to the maximum absorption wavelength in the range of 365-400 nm, which is confirmed to be able to work in the ultraviolet (UV) area. This condition has become one of the significant obstacles in the use of TiO2based sunlight because only 4-5 % of the sun rays contain the UV light. There are several thoughts regarding the development of TiO2 as a catalyst material under sunlight illumination. One of them is to decrease their value of Eg to enhance the absorption of sunlight, especially in the visible area [8]. On the other hand, Co3O4 can be used for TiO2 doping due to its physical and chemical property [9]. Besides, Co3O4 has a bandgap value of 2.07 eV, which makes it efficiently absorbs the photons of visible light [10]. TiO2 was successfully prepared to modify the Au nanoparticles to decrease its bandgap value [11]. This modification has been performed for the degradation of methyl orange [12], methylene blue and crystal violet [13].
In this work, we reported the synthesis of TiO2/Co3O4 nanocomposite using Catharanthus roseus (L.) G. Don leaf extract for the degradation of malachite green under the visible light illumination.

Preparation of Catharanthus roseus (L.) G. Don leaf extract
Catharanthus roseus (L.) G. Don leaves were washed by distilled water, then dried at room temperature for 10 days. The dried-Catharanthus roseus (L.) G. Don was ground to obtain the powders. Fifty g of leave powders were macerated in methanol for 7 days. The extract was filtrated, and further partitioned by hexane to obtain the methanol fraction. The extract was evaporated to separate its methanol, then dissolved into distilled water to get an aqueous fraction of CRE for nanocomposite synthesis [14][15].

Synthesis of TiO2 nanoparticle
TiO2 nanoparticles were prepared by stirring 2.96 mL Ti(OCH(CH3)2)4 (TTIP) and 40 mL isopropanol for 20 min. The mixture was added with 10 mL CRE and stirred for 5 h, then calcined at 500 °C for 150 min to form the nanoparticle powders [16].
2.4. Synthesis of TiO2/Co3O4 nanocomposite 0.1 g of TiO2 powders were dispersed into 25 mL distilled water, then ultrasonicated for 30 min (Part A). Meanwhile, Co(NO3)2 0.01 M was added with 10 mL CRE and stirred for 20 min (Part B). Subsequently, part A and B were mixed and stirred for 5 h, then calcined at 800 °C for 150 min to form the nanocomposite powders [12].

FT-IR analysis of Catharanthus roseus (L.) G leaf extract
FT-IR analysis was performed to determine the primary functional group of CRE. There were some functional groups such as -OH groups (3373 cm -1 ), C-H (2945 cm -1 ), C = C (1589 cm -1 ), C-H (1459 cm -1 ), and C-N (1028 cm -1 ) [18]. The presence of alkaloids was signed by the appearance of C = C and C-N functional groups. The -OH functional group indicates the presence of flavonoids secondary metabolite compound. Figure 1 shows that there are -OH, C=C, and C-N functional group at the wavenumber of 3367, 1641 and 1028 cm -1 , respectively. The presence of these groups illustrate that CRE can be utilized in the synthesis of TiO2/Co3O4 nanocomposite.

Structural analysis of TiO2/Co3O4 nanocomposite
The structural of TiO2/Co3O4 nanocomposite was analyzed by XRD. Figure 2 shows the 2Ʌ value of TiO2/Co3O4, which was a combination of the

Particle size analysis of TiO2/Co3O4 nanocomposite
Particle size of TiO2/Co3O4 nanocomposite was analyzed with PSA and TEM. The average particle size distribution was 38.04 nm, confirmed by PSA as presented in figure 3a. For further characterization, TEM was utilized to confirm the particle size in more details. According to figure 3b, TiO2/Co3O4 nanocomposite exhibits the particle size around 90.80 nm.

Morphological analysis of TiO2/Co3O4 nanocomposite
TiO2/Co3O4 nanocomposite was characterized using SEM-EDX to determine its morphology and atomic composition. SEM image shows that TiO2/Co3O4 nanocomposite has a spherical shape, as shown in figure 4a. Based on the EDX result, TiO2/Co3O4 nanocomposite contained the atomic composition of Ti, Co and O, which were found to be 19.42; 3.79 and 66.21 %, respectively, as shown in figure 4b.

Photocatalytic degradation of malachite green using TiO2/Co3O4 nanocomposite
The photocatalytic activity of TiO2/Co3O4 nanocomposite was tested for the malachite green degradation under the visible light illumination. The degradation was observed at the wavelength of 617 nm as a typical peak of malachite green, as shown in figure 6. The results show that the degradation percentages of malachite green using TiO2/Co3O4 nanocomposite and TiO2 nanoparticle were 82.61 % and 46.03 % for 120 min, respectively.

Conclusion
TiO2/Co3O4 nanocomposite has been successfully synthesized using CRE. Alkaloids and flavonoids were the essential secondary metabolites contained in CRE for nanocomposite synthesis. The bandgap value of TiO2/Co3O4 nanocomposite was narrower than the bandgap value of TiO2 nanoparticle. Also, TiO2/Co3O4 nanocomposite has a good photocatalytic activity rather than TiO2 nanoparticle, which significantly works as photocatalysts in the visible region. The synthesized nanocomposite has been structurally formed from a combination of TiO2 and Co3O4 confirmed by XRD characterization.