Formation of TiO2/natural Melanin Nanostructure Hybrids with Enhanced Optical and Thermal Properties

doi:10.21203/rs.3.rs-632614/v1

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Formation of TiO2/natural Melanin Nanostructure Hybrids with Enhanced Optical and Thermal Properties

https://doi.org/10.21203/rs.3.rs-632614/v1

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Recently, with the progress of research in amending and developing the properties of inorganic materials, researchers have become focused on using inexpensive and environmentally friendly materials such as organic materials from natural sources that have proven effective in improving the properties of materials in various applications. In recent years, melanin has become an attractive topic for researchers due to its distinctive structural properties that have been distinguished in various application fields. Here, we report the use of ultrasonication under UV radiation for synthesis. The influence of natural melanin on the structural, optical and thermal properties of TiO₂ nanoparticles was investigated by using Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and UV–Vis spectroscopy. It was observed that incorporating natural melanin on TiO₂ nanoparticles (TiO₂-Mel) occurred at approximately 2.01 eV with low value of Urbach energy around 100 meV indicates improvement in crystalline structure. Thermal results showed that TiO₂-Mel is stable even with temperatures up to 400°C. According to the results obtained by the thermal stability of melanin with titanium dioxide can makes it a good candidate in many application such as solar cells and optoelectronic that require optical efficiency and thermal stability for long periods.

Electronic Materials and Devices

Optical Materials and Devices

Natural Melanin/TiO2

thermal stability

optoelectronic

Nanostructure

UV radiation

One of the research areas that has attracted scientists in recent years is improving the properties of inorganic polymers in electronic, magnetic and optical applications. Organic-inorganic hybrids have received much research attention[1]. Many physicists and chemists researchers have been working to improve the properties of low-cost polymers and environmentally friendly to be used as an alternative to conventional semiconductor.

Titanium dioxide (TiO₂) is a kind of one of non-organic semiconductor in several applications and has unique advantages, for optical and photocatalytic applications[2, 3]. nontoxicity[4],and low cost[5]. Moreover, it can be prepared in various phases, such as thin film[6] pellet[7], or solution phases[8], by several techniques, such as pulsed laser deposition[9], sputtering[10] and chemical vapor deposition[11]. The thin film form is more suitable for photocatalysis because it enables the reuse of the photocatalyst without the need to separate it from the surrounding medium. In addition, oil slicks or spills can be treated easily by using thin films. Titanium dioxide has three polymorphs: anatase, rutile and brookite[12].

In the continuous search for organic–nonorganic hybrid, TiO₂ is one of the most investigated semiconductors. This is due to its distinctive: nontoxic, inexpensive, and photostable material [13]. Moreover, TiO₂ is individual electrochemical and optoelectrical attributes such as a high dielectric permittivity (around 64 at 1MHz frequency) [14] ,very high refractive index and excellent optical transmittance in the visible and IR regions and ,this has led to its common use in thin-film optics. Otherwise, it has a great photocatalytic in industrial applications[3]. Despite these advantages, titanium dioxide has a rather complex chemistry, so we find little research into grafting it with surface of organic polymers. More recently, literature has emerged that reports findings about the importance of titanium dioxide with different polymers in water separation applications[15]. A recent study, reported the preparation and characterization of a polymer nanocomposite composed of TiO₂ in a polypyrrole matrix. The resulted composite might exhibit improved optomechanical properties. [16].

Hybrid nanoscale of organic and non-organic compounds can be synthesized a variety of different ways, providing many diverse characteristics of known compounds or can be synthesized as needed in specific applications.[17]. The nature of organic semiconductor materials is related to the presence of carbon, with hybridization between the s and p orbitals to form three sp2 orbitals with σ-bond. The fourth orbital, pz, is perpendicular to the plane of atoms, which produces an overlapped electronic component, namely, the π-bond[18]. This overlap is related to the mechanism of the electrical conductivity of the material. This is a relatively recent studies, focused on the modulation of the surface chemistry of non-organic nano-composite to improve their functional properties such as extending light absorption spectra[19].

Notably, melanin an organic conductor polymer, possesses unique properties. The oxidation products of 3,4-dihydroxyphenylalanine (DOPA), 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindloe-2-carboxylic acid (DHICA) are known as the main intermediates of melanin[20].However, synthetic melanin has very similar characteristics to natural melanin from herbal source such as Nigella Sativa. Recent studies have demonstrated that the content of DHICA-derived units in synthetic melanin is approximately 40% less than that of natural melanin[21]. Natural melanin is synthesized in vivo through a free radical-mediated process from its precursor, dopaquinone, which is generated by tyrosine, which in turn is produced through a tyrosinase-mediated process[22]. Melanin has a relatively diverse and heterogeneous structure. This is due to the ubiquitous sources of melanin, which leads to its heterogeneity in composition, size, color, and function[23]. It is considered one of the promising polymers due to its chemical properties that contain different functional groups. In recent years, melanin has shown stimulating results, especially in optical applications due to its high and wide optical absorption in the ultraviolet and visible region[24]. It can be considered as a class of soft conductor used in numerous technological applications including: memory devices[25], light emitting diodes and field effect transistors[26]

In this study, the thermostability and UV protective properties of natural melanin as organic materials and TiO₂ as inorganic nanocomposites were thoroughly investigated. This article has increased in importance in light of recent studies on the synthesis of natural eumelanin and TiO₂ nanocomposites while maintaining them original structures with high-temperature effects in thermal measurements. Moreover, and equally important, the study of the energy gap change in photometric measurements shows the possibility of using hybrid melanin and titanium dioxide in bioelectronic applications on a wider range than conventional semiconductors.

2.1 Preparation

Anatase TiO₂ nanoparticles were purchased from Guangzhou Hongwu Material Technology Co., Ltd. The natural melanin powder used in this work was extracted from Nigella sativa (black seeds) using the same method reported in [24].The black seed coats were solubilized in a solution of NaOH for 4 h. Then, the solution was filtered and centrifuged at 5000 rpm for 5 min. The melanin was precipitated from the solution using HCl at pH 2. This treatment was repeated twice to ensure a higher purity of melanin from other herbal metals. The melanin precipitate was thoroughly washed four times with high-purity distilled water, filtered, and dried overnight at 80°C.

For the synthesis of melanin doped with TiO₂ (TiO2-Mel), as shown in Fig. 1, 1g of melanin powder was dissolved in 20 mL of NaOH solution and mixed using ultrasonication for 5 min. One gram of TiO₂ powder was added to the melanin solution and stirred under UV light for seven hours. The color of the melanin solution gradually changed from black to gray and the production of oxygen gas increased due to the interaction between melanin and TiO₂. The resulting powder was dried in an air-drying oven at 80°C for one hour.

2.2 Characterization Techniques.

Fourier transmission infrared (FTIR) spectra for all samples were obtained using a Perkin-Elmer 580B IR spectrometer. Characteristic UV-Vis absorption spectra in the range between 200 to 800 nm were measured by a spectrophotometer (Perkin Elmer Lambda,40). Thermal gravimetry analysis (TGA) and differential scanning calorimetry (DSC) were performed on a TGA/DTA Mettler (Toledo, AG, Analytical CH-8603, Schwerzenbach, Switzerland) at a heating rate of 10°C/min under a flow of nitrogen. Transmission electron microscopy (TEM) images were obtained via FE-TEM (JEM-2100F JEOL, Japan) at an acceleration voltage of 80 kV.

3.1 Fourier transform infrared (FTIR) spectroscopic analysis

Figure 2 shows the FTIR transmission spectra of TiO₂, melanin and TiO₂-Mel in the range of 400–4000 cm^− 1. The FTIR spectrum (A) of melanin shows a broad band at 3430.62 cm^− 1 due to the presence of the catechol (O-H) group, while the two bands at 2924.07 cm^− 1 and 2852.53 cm^− 1 are attributed to the C-H stretching vibration. The signals in the range of 3500 − 2800 cm^− 1 are associated with the stretching vibrations O-H and N-H in indole or pyrrole of the carboxylic acid and phenolic groups of melanin[27]. The band at 1650.58 cm^− 1 is attributed to aromatic C = C and/or carboxylate (COO-) groups[28]. The bands at 1545.41 cm^− 1 and 1411.90 cm^− 1 are attributed to the C-H groups[28]. The band at 1100.96 cm^− 1 is related to the asymmetric stretching of the C-N group.

In the TiO₂ spectrum (B), the broad band at 3428.58 cm^− 1 was due to stretching of the hydroxyl O–H bond, which represents the free surface adsorbed water molecules. Another band at 1633.38 cm^− 1 was attributed to the bending vibration of water and Ti-OH[29]. The weak peaks at 2924.90 cm^− 1 and 1400.67 cm^− 1 can be assigned to the bending in functional groups CH [30]. The strong bands at approximately 447.76-682.84 cm^− 1 correspond to Ti-O stretching.

The spectrum (A) of TiO2-Mel is similar to that of melanin in the range of 4000 − 1700 cm^− 1, but we noticed that it is more complicated nanostructure and has more functional groups. There was an increase in the intensity of some peaks in the range of 1700 ~ 1400 cm^− 1 corresponding to the bending vibration modes of the C = O double bond (COOH) and C = C and C-N bonds of the aromatic system and/or the carboxylate group (COO-). The intense peak at 1455.65 could be associated with COO − stretching vibrations in bridging and chelating bidentate Ti[31].

The C-N group shifted from 1100.96 cm^− 1 to 1169.11 cm^− 1. The new peaks observed at 908.96 cm^− 1 and 866.90 cm^− 1 may be associated with the CH and CH₂ groups, respectively. A Ti-O stretching band was observed at 595.39 cm^− 1 after TiO₂ interacted with melanin molecules. This suggests that Ti was bound to oxygens in the functional groups of melanin (see Fig. 3).

3.2 Transmission Electron Microscopy (TEM)

Transmission electron microscopy (TEM) micrographs were employed to characterize and describe the shape and particle size of the nanoparticles of all samples, and the particle size distribution was measured using particle size analysis software(Image-J). As seen in the micrograph of the melanin sample in Fig. 2(A), melanin granules exhibited a spherical shape, which corresponds to the result previously reported in [32, 33]

In Fig. 2(B), TiO₂ appeared as aggregates with rod-like structures (approximately 12 nm×35 nm in size). This observation is consistent with that reported in the literature[33]. The TEM micrograph in Fig. 2(C) shows that the synthesized TiO2-Mel exists as an aggregate mix of ovoid and rod shapes with many different sizes, and TiO₂ nanoparticles were observed on the surface of melanin. The particle size distribution image shown in Fig. 2(D,E,F) correspond to mean diameters of 33.30 nm, 14.07 nm and 24.28 nm for melanin, TiO₂ and TiO2-Mel, respectively. This means that the hybrid (TiO2-Mel) became smaller in size compared to the melanin samples.

3.3 Thermal gravimetry analysis (TGA)

Figure 5 (A) Thermal gravimetry analysis (TGA) curves of TiO₂, melanin and TiO2-Mel samples. (B) The differential thermal analysis (DTA) curves of TiO₂, melanin and TiO2-Mel samples.

The TGA of TiO₂ nanoparticles shows no significant weight loss over the entire temperature range, which indicates that TiO₂ is thermally stable at high temperatures. Two main endothermic peaks of the DTA curve of TiO₂ were observed. The first temperature of 63°C was attributed to the loss of absorbed water. The second temperature of 765°C was associated with the phase transformation from anatase TiO₂ to rutile TiO₂[34]. The TGA and DTA curves of melanin showed two clear steps of thermal degradation, beginning with a small weight loss of approximately 6.43% of its weight in the 65°C to 215°C range. The first endothermal peak appeared at 85°C, caused by the evaporation of bound water[32, 35]. An exothermal peak appeared immediately after that at 177°C, which was mainly due to the evolution of carbon dioxide[36]. Then, a continuous weight loss up to 66.47% was observed from 230°C to 500°C. An endothermal peak appeared at 273°C. This high percentage of weight loss could be explained by considering melanin degradation because of decarboxylation[36, 37]

The TGA curve of TiO2-Mel showed multiple steps of thermal degradation, with a large weight loss of approximately 13.43% of the sample weight in the 70°C to 120°C range. Then, there was a steady rate of weight loss of approximately 8.7% for each increase of 145°C, whereas the DTA curve confirms that the first endothermal peak that appeared at approximately 110°C was attributed to evaporation of water. We also observe here, as in the melanin curve, an exothermal peak immediately after the first endothermal peak at 150°C. Then, there were two endothermal peaks at 284°C and 470°C due to decarboxylation. After that, several similar endothermic peaks appear at 600°C, 758°C and 934°C. This may be associated with the possibility of titanium decomposition from the melanin molecule.

Table 1

Summary of the DTA endothermal peaks of TiO2-Mel
Assignment Peak	Inset	Offset	Peak
Assignment Peak	T_i (⁰C)	T_f (⁰C)	T(⁰C)
1	70	138	110
2	246	388	284
3	415	515	470
4	524	667	600
5	693	834	758
6	855	999	934

3.4 Differential scanning calorimetry (DSC)

The differential scanning calorimetry of TiO₂, melanin, and TiO2-Mel samples are examined and the heat flow curves are shown in Fig. 6. Measurements were performed over the temperature range 25 to 500°C. Three characteristic endothermic peaks were observed for the TiO2-Mel at 44°C, 91°C and 115°C. These endothermic peaks are associated with the evaporation of water. After the 150°C there is no reaction, which mean there are no crystallization and no melting happening.

The endothermic peak at 60°C for the TiO₂ sample is due to the decomposition of water molecules. A broad peak starts at 320°C ascribed to the transformation of

anatase TiO₂ nanoparticles to rutile. However, melanin DSC curve show four endothermic peaks, The first and larger peak at 100°C is corresponds to the moisture. The other three peaks appeared at 227°C, 299°C and 443°C respectively are mainly due to the decarboxylation as we explain in the TGA result.

3.5 Optical properties.

The optical absorption between 200 and 800 nm was investigated at room temperature. Figure 7 shows the optical absorption spectra of the TiO₂, melanin and melanin-doped TiO₂ (TiO2-Mel) nanoparticles with absorption peaks at 344, 233 and 213 nm, respectively, and the evaluation of their indirect bandgap. The observed absorption increases from 200 to 350 nm upon the incorporation of melanin molecules into the TiO₂ nanoparticles, and this absorption decreases exponentially and gradually in the visible light region in the TiO2-Mel sample, which indicates that the absorption in the ultraviolet region improved, as observed in Fig. 7, possibly due to the association of titanium metal with the -COOH functional groups in melanin. It can thus be concluded that the incorporation of melanin increased the shielding ability of TiO₂[38].

The band gap energy E_g was evaluated by the Tauc plot equation given by[39]

αhν = K(hν - E_g )ⁿ

where h is Planck’s constant, A is a constant, ν is the incident radiation frequency, n is equal to ½ for an indirect band gap material and α is the absorption coefficient, which was calculated from the relation α= 2.303A/d, where d is the pathlength of light and A is the measured absorbance. The band gap energy E_g can be estimated by plotting (αhv)^1/2 versus energy hv and extrapolating the linear portion of the plot to the x-axis. The inset of Fig. 6 (A, B, C) shows that the band gap energies E_g of TiO₂, melanin and TiO2-Mel are 3.19 eV, 2.75 eV and 2.01 eV, respectively (Table 2).

It can be concluded that compared to TiO₂ and melanin, the energy band gap of TiO₂ doped with melanin (TiO₂-Mel) decreases. This behavior has been attributed to the induced energy rearrangement between TiO₂ and melanin due to the formation of C-O-Ti bonds[40]. Narrowing the band gap would improve the absorption of TiO₂-Mel in the visible light range. To understand more the enhanced the optical and thermal properties of melanin and titanium dioxide, Urbach energy have been calculated as seen in Fig. 8 and (listed in Table 2). Urbach energy describe the cavity exponential broadening of absorption edge which related to the thermal and disorder structural model of semiconductor either amorphas (as melanin) or crystalline (as titanium dioxide). According to this concept, the Urbach energy is determined by exponential of an empirical optical gap, which is defined according to the following relation known as Urbach rule[41]:

$$\alpha ={\alpha }_{0} \text{e}\text{x}\text{p}\left(\frac{h\upsilon }{{E}_{u}}\right)$$

where α is the absorption coefficient, α₀ is characteristic parameter depend on material, hv is the incident photon energy and E_u is Urbach energy. It is observed that ( TiO₂-Mel) Urbach energy enhancement comparing with samples of melanin and titanium dioxide, that means the formation of oxygen vacancies decreased after doped. The diminution of oxygen vacancy defects can be explained based on neutral charge concept. These oxygen vacancy defects decrease additional localised defect states which can effectively influence the edge of conduction and valance bands. This indicates an improvement in the structural properties and localized states of the hybrid (TiO₂-Mel).

Table 2

Experimental values for the absorption wavelength ,band gap ,Urbach energy variation of TiO₂, melanin and TiO₂-Mel.
Sample	Absorption wavelength λ (nm)	Bandgap (eV)	Urbach energy(eV)
TiO₂	344	3.19	0.241
Melanin	233	2.75	0.292
TiO₂-Mel.	213	2.01	0.102

The purpose of the current study was to synthesized a nanostructure hybrid of natural melanin/TiO₂ nanoparticles and characterized it by FTIR and SEM. In this paper our motivation was to explore the optical and thermal properties of this nanocomposite. The basic idea was to explore how the natural melanin would be affected by the TiO₂. It turned out that this research has raised many questions in need of further investigation in organic and inorganic polymers. However, natural melanin and TiO₂ played a crucial role in preparing the melanin and TiO₂ composite systems. TiO₂ doped with melanin (TiO2-Mel) proved to be the important factor for improving the optical properties and enhancement in the nature of crystalline of hybrid (TiO2-Mel) depends on the Urbach energy results. One of the most spectacular results was the ability to had much higher absorption with a lower band gap than TiO₂. Another interesting feature was the thermal results, as we saw TGA and DSC provides a subtle means of understanding the thermal denaturation events when TiO2-Mel is heated. It showed that TiO2-Mel nanoparticles is thermally stable until 400°C, and at higher temperatures, titanium will decompose from the melanin molecule. Finally owing to the physical properties of Natural Melanin/ TiO₂ Nanostructure, this provides new routes for hybrid materials for new potential application such as organo-electronic devices, UV shielding and solar cell.

Declaration of Interest Statement

The authors declare that they have no conflict of interest.

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Formation of TiO2/natural Melanin Nanostructure Hybrids with Enhanced Optical and Thermal Properties

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Abstract

Figures

1. Introduction

2. Experimental

2.1 Preparation

2.2 Characterization Techniques.

3. Results And Discussion

3.1 Fourier transform infrared (FTIR) spectroscopic analysis

3.2 Transmission Electron Microscopy (TEM)

3.3 Thermal gravimetry analysis (TGA)

3.4 Differential scanning calorimetry (DSC)

3.5 Optical properties.

4. Conclusion

5. Declarations

6. References

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