1 Introduction

In the past few decades, nanomaterials are highly attracted by researchers to exploit their excellent properties for various applications. Among the semiconductor metal oxides, TiO2 is a fascinating and one of the technologically important materials in the field of nanotechnology and it governs the keen interest of scientific community, due to its salient properties such as high chemical stability, wide band gap, good mechanical resistance and high optical transmittance in visible and IR spectral range [1]. TiO2 exists in three polymorphs: rutile (Tetragonal), anatase (Tetragonal) and brookite (Orthorhombic). One dimensional TiO2 nanowire structure has the potential application in dye sensitized solar cells (DSSCs) [2–6] and also TiO2 is being greatly used in many applications such as photo catalysts [7, 8], gas sensors [9], electro chromic devices [10], antibacterial activity [11] and it also finds applications in biomedical sciences such as bone, tissue engineering and in pharmaceutical industries due to its non toxicity [12] and so on. TiO2 nanoparticles (NPs) with different nanostructures have been synthesized by electrospinning method [13], hydrothermal method [14, 15], template method [16] etc.

Aloe barbadensis miller is one of the important medicinal plants and also it is known as the “Lily of the Desert”. The raw pulp of Aloe vera contains approximately 0.5 % solid material consists of a variety of compounds including water soluble and fat soluble vitamins, minerals, enzymes, polysaccharides, phenolic compounds, organic acids and remaining 99.5 % are water [17–19].

Synthesis of gold nano triangles and silver NPs using A. vera plant extract as reducing agent has been reported [20, 21]. Moreover, A. vera plant extract has been used to synthesize metal oxide NPs such as ZnO [22, 23], CuO [24], MFe2O4 (where M = Cu, Ni and Zn) NPs [25], hydroxyapatite (HAp) powders [26], In2O3 [27] and microorganisms have also been reported for the synthesis of metal oxides [28]. NPs synthesized by chemical methods are involved with toxic chemicals and adsorbed on its surface, which causes adverse effect in the medical applications. Synthesis of NPs using bio-capping agents offer non toxic, facile and one step synthesis with easiest protocol, sustainable and easy scale up to the industrial production at lower cost. Moreover, the uses of environmentally benign materials for the synthesis of NPs offer eco-friendliness and compatibility for pharmaceutical and biomedical applications as they do not use toxic chemical for the synthesis protocol [24]. Synthesis of different nanostructures is gaining a great importance in both fundamental scientific research and technological applications owing to their interesting physico-chemical properties. A nanocoral is one of the novel architecture and it has potential application in DSSCs. Synthesis of TiO2 nanocorals is highly difficult and it has been achieved by means of multi step hydrothermal process [29], polymer gel templating procedure [30] etc. These methods involve multistep processes, time, power consuming, toxic chemicals and expensiveness and so on.

Sol–gel method is one of the best methods for the preparation of metal oxides. It offers an easy synthesis procedure to achieve nano scale counterparts by controlling the synthesis conditions. This method provides many advantages and to name a few are room temperature synthesis under atmospheric pressure, purity, homogeneity, to introduce desire amount of dopants, stoichiometry control and financially viable. However, it is very difficult to individually control the three reactions such as hydrolysis, condensation and agglomeration, which occurs simultaneously in sol–gel process. So, a slight change in experimental condition modifies the particle size and morphology [31, 32]. A fresh A. vera plant extract contains many biologically active components, such as polysaccharides vitamins, proteins, lipids, polyphenols, heterocyclic and carbonyl compounds and so on. In the extracellular synthesis of NPs using plants, on one hand, some biomolecules can act as reducing agent, on the other hand, some biological constituents can act as capping agent for the resulting NPs. As a result, the aggregation of resulting NPs could be impeded by means of stabilization due to the protein- nanoparticle interaction and also the surface morphology can be influenced by the shape-directing ability of the carbonyl compound present in the A. vera plant extract. Hence, it is anticipated that this synergistic effect of A. vera plant extract (both stabilization and capping) might influence on the morphology and size of TiO2.

In this communication, the facile one step sol–gel synthesis of novel TiO2 nanocoral architecture using A. vera plant extract as a bio-capping agent is reported. It has been clearly observed that A. vera plant extract proves its efficacy on the thermal, structural and morphological properties of TiO2 NPs as expected. A novel TiO2 nanocoral architecture through simple experimental procedure such as sol gel method using A. vera plant extract as bio-capping agent is demonstrated.

2 Experimental details

Titanium isopropoxide (SD Fine Chemicals), Ethanol, absolute (MERCK) and A. vera plant extract were used as starting materials for the preparation of pure and A. vera capped TiO2 precursor solution. Fresh and matured leaves of A. vera plant were harvested from the local agricultural land. Then, 30 g of thoroughly washed A. vera leaves were finely cut and boiled in 100 ml of deionized water. The resulting extract was used for further experiments [20]. Pure TiO2 precursor solution was prepared by drop wise addition of 3 ml of Titanium isopropoxide in 20 ml of ethanol under magnetic stirring. Similarly, A. vera capped TiO2 precursor solution was prepared by adding 0.5, 0.75 and 1 ml of A. vera plant extract during the preparation of pure TiO2 precursor solution and they were coded as 1AT, 2AT and 3AT respectively. After 3 h under continuous stirring, the pure and A. vera capped TiO2 precursor solutions were subjected to heating (100 °C) under stirring, until the xerogel completely dried and finally cooled to room temperature. By adding titanium isopropoxide in ethanol solution, Ti(OH)4 was formed. Upon subsequent heating at 100 °C, the formation of TiO2 took place due to the condensation process. The relevant chemical reaction process were followed, as given in Eqs. (1a) and (1b).

$$ {\text{Ti}}\left\{ {{\text{OCH}}\left( {{\text{CH}}_{3} } \right)_{2} } \right\}_{4} + 4{\text{C}}_{2} {\text{H}}_{5} {\text{OH}} \longrightarrow{\hbox{Ti}}\left( {\text{OH}} \right)_{4} + 4\left( {{\text{CH}}_{3} } \right)_{2} {-} {\text{CH}} {-} {\text{O}} {-} {\text{C}}_{2} {\text{H}}_{5}$$
(1a)
$$ {\text{Ti}}\left( {\text{OH}}\right)_{4}\mathop{\longrightarrow}\limits^{\Delta \left( {100\;^\circ{\text{C}}} \right)}{\text{ TiO}}_{2} + 2{\text{H}}_{2} {\text{O}} $$
(1b)

The dried precursor was crushed into fine powder using agate mortar and pestle. Finally, the grinded powder was calcined at different temperatures in a tubular furnace (Carbolite, UK) in an ambient atmosphere. Before calcination, the thermal behavior of the as-synthesized TiO2 powder was analyzed by means of Thermo Gravimetric and Differential Thermal Analysis (EXSTAR 6000 TG/DTA) from ambient to 900 °C with the heating rate of 10 °C/min in air atmosphere. XRD pattern was recorded by Cu Kα radiation (1.54060 Å) using PANalytical X-PERT PRO diffractometer system. The morphology was examined by scanning electron microscopy (SEM) (Model: Hitachi S3000 H SEM). The surface area was measured by Brunauer–Emmett–Teller (BET) using MICROMETRICS ASAP 2020 POROSIMETER. FT- Raman spectra were obtained by BRUKER RFS 27 FT-Raman spectrometer. The functional group vibrations of the 2AT and the pure A. vera plant extract powders were analyzed by FTIR analysis (Model: Perkin Elmer Spectrum RX I). Transmission Electron Microscopy (Model: JEOL-200 FXII) was employed to confirm the nanocoral structure and also to measure the size of the NPs.

3 Results and discussion

In the preparation of TiO2 precursor solution, it has been observed that the precursor solution is clear even after the addition of titanium isopropoxide with the ethanol solution, which manifests the complete dissolution of the metal alkoxide in the solvent. This clear solution immediately becomes to slurry with pale green colour followed by the addition of A. vera plant extract and it has been happened due to the rapid reaction of the precursor solution through hydrolysis and condensation caused by the presence of water molecules in A. vera plant extract. The colour change indicates the encapsulation of TiO2 particles by small amount of solid biomolecules (0.5–1 %) contain in the A. vera plant extract and also the pale green colour becomes rich with respect to the increase of A. vera plant extract. The colour of the as-synthesized powder has been changed to pure white, after the calcination process which implies the elimination of biomolecules from the powder.

The TGA/DTA analysis has been performed for the as-synthesized (1AT and 3AT) TiO2 nanopowder. In TGA curve of 1AT sample, the weight loss around 100 °C is attributed to the removal of physically and chemically entrapped water and a further weight loss at 210 °C is attributed to the elimination of organic matrix, as shown in Fig. 1(a). There is no further weight loss after 300 °C and in association with DTA curve, the oxidation process of the powder is initiated at the same temperature (300 °C), which indicates the formation of metal oxide. In the case of 3AT sample, physically and chemically entrapped water removed at 100 °C and the removal of organic matrix takes place around 235 °C, as shown in Fig. 1(b). This small increase in temperature for 3AT is due to the presence of more surface energy offered by the NPs compared to the 1AT sample. Hence it is understood that biological molecules enter in the metal oxide matrix and serve as capping agent. In DTA curve (3AT) a small exothermic region around 300 °C indicates the crystallization of the TiO2 nanopowder followed by the oxidation process. In TGA curve, the further weight loss is observed till 420 °C associated with the strong exothermic peak in DTA curve, which is due to complete removal of the sheath of biomass present over the NPs. In both cases, a small change observed at around 670 °C may be attributed to the phase transformation of TiO2 from anatase to rutile.

Fig. 1
figure 1

TGA/DTA curve of as-synthesized TiO2 nanopowder. (a) 1AT and (b) 3AT

Full size image

As shown in the XRD pattern of 1AT and 2AT samples, the as-synthesized TiO2 nanopowder is amorphous. After the calcination, the predominant peak found out for the 1AT, 2AT and 3AT samples at 25.3° (2θ) with (101) plane and other planes such as (004), (200), (211), (204), (116), (103), (112), (113), (105) are corresponding to anatase phase as shown in Fig. 2(a)–2(c). In the case of 1AT, the rutile phase of TiO2 is also observed only at the calcination temperature of 600 °C and the planes (110), (101), (111), (210), (211), (220), (310) refers to the rutile phase, as shown in Fig. 2(a). All the observed diffraction peak values are closely match with the standard diffraction data (JCPDS File No: 89-4921, 89-4920). The crystallization temperature of as-synthesized TiO2 nanopowder has been observed at 300 °C for 1AT and 3AT samples and this is consistent with the TGA/DTA results. The free energy of rutile phase is always less than that of anatase phase, making the rutile is more stable phase at all temperatures. But the rutile phase has been not observed for 2AT and 3AT even at higher calcination temperature (600 °C) owing to higher concentration (0.75, 1 ml) of A. vera plant extract. The A. vera plant extract (0.75 onwards), which hinders the grain growth, thereby leads to the NPs and also retards the phase transformation. Hence, the phase transformation of TiO2 from anatase to rutile can be controlled by bio-capping agent. Indeed, the improvement in the degree of crystallinity has been clearly observed with the increase of calcination temperature.

Fig. 2
figure 2

XRD patterns of TiO2 nanopowder. (a) 1AT, (b) 2AT and (c) 3AT

Full size image

The crystallite size of the synthesized TiO2 nanopowder has been calculated using scherrer equation (Eq. 2).

$$ D = \frac{0.9\lambda }{\beta \cos \theta } $$
(2)
$$ \frac{1}{{d^{2} }} = \frac{{h^{2} + k^{2} }}{{a^{2} }} + \frac{{l^{2} }}{{c^{{}} }} $$
(3)

where 0.9 is a constant, λ is the wavelength of X-ray source, β is the full width at the half maximum in radians, θ is the Bragg’s diffraction angle. The effect of concentration of A. vera plant extract and the calcination temperature on the crystallite size of the synthesized TiO2 nanopowder is presented in Fig. 3. The lattice constants are calculated using X ray diffraction data from the formula of Tetragonal crystal system (Eq. 3). The calculated lattice constants are presented in Table 1. These values are in close agreement with the standard values of both anatase and rutile phases of TiO2. A small deviation in both phases of calculated lattice constant values with respect to those standard values may be due to the strain of the TiO2 nanopowder caused by the heat treatment (calcination) and/or may be due to the instrumental error.

Fig. 3
figure 3

Crystallite size versus calcination temperature of TiO2 nanopowder

Full size image
Table 1 Lattice constants of TiO2 nanopowder
Full size table

It is well known that TiO2 exists in three polymorphs namely anatase, rutile and brookite. Rutile is thermodynamically stable, whereas anatase and brookite undergo irreversible exothermic transformation to rutile with respect to the range of temperature. Rutile, anatase and brookite phases of TiO2 have 4, 6 and 36 Raman active modes, respectively [33, 34]. Figure 4(a)–4(c) show the FT- Raman spectra of TiO2 nanopowder (3AT) calcined at 400 °C for 1 h. The appearance of strongest E g mode at 144 cm−1 is due to the Ti–O streaching vibration bond which ascertains the presence of anatase phase in the TiO2 nanopowder. The modes located at 144 (E g ), 197 cm−1 (E g ), 396.9 (B 1g ), 515 (A 1g ) and also 637 cm−1(E g ) are responsible for the Raman active modes of anatase phase TiO2 [35] and no other peaks have been detected, which indicates that the TiO2 nanopowder posses anatase phase only. By comparing these spectra, it seems to be clear that the Raman bands are slightly shifted towards the higher wave number with respect to the increase of calcination temperature, which emphasizes the increase of the particle size. Also the increase of peak intensity with the increase of calcination temperature indicates the increase of crystallinity. There is no any Raman modes pertaining to the rutile phase and it suggested the absence of rutile phase and this result is consistent with the XRD results.

Fig. 4
figure 4

FT Raman spectra of TiO2 nanopowder (3AT)

Full size image

FTIR spectra of A. vera plant extract and TiO2 nanoparticle are shown in Fig. 5(a)–5(c). A broad band at 3,423 cm−1 is assigned to hydrogen bonded –OH stretching vibration. A band observed at 1,615 cm−1 is attributed to the amide group vibration, which is a characteristic peak of proteins/enzymes [36]. The bands appeared at 1,421 and 1,080 are associated with carboxylic acid, C–N stretching vibration of amine group respectively, as shown in Fig. 5(a). This clearly indicates the presence of biomolecules and bio constituents of A. vera extract. A band shift occurs from 1,615 to 1,539 cm−1, which indicates the binding of proteins with the surface of TiO2 and thereby it leads to the stabilization of NPs. Furthermore, a band shifts from 1,421 to 1,442 and 1,080 to 1,034 cm−1 infer the contribution of carboxylic acid and amine groups respectively, which are the capping ligands for the encapsulation of TiO2 NPs. There is a band shift from 3,423 to 3,372 cm−1, which may be due to the condensation of Ti–OH group. A band at 659 cm−1 is attributed to the stretching vibration of TiO2, as shown in Fig. 5(b). Finally, it can be noticed that the proteins/enzymes, carboxylic acid and amine groups present in the A. vera plant extract can lead to the formation of TiO2 NPs through stabilization and encapsulation respectively [22]. Some of the bands appeared for as-synthesized sample have disappeared after the calcination, as shown in Fig. 5(c).

Fig. 5
figure 5

FTIR spectra of a dried Aloe vera plant extract, b as synthesized TiO2 nanopowder, c TiO2 nanopowder calcined at 300 °C

Full size image

Figure 6(a) shows the SEM image of pure TiO2 powder, in which the particles are highly agglomerated due to the simultaneous hydrolysis, condensation and aggregation. It is apparently seen from the Fig. 6(b)–6(f) that the impact of A. vera plant extract is clearly observed on the control of agglomeration as well as reduction in particles size of TiO2 nanopowder. It elucidates the decrease of particles size with the increase of A. vera plant extract and also it indicates the spherical shape of the particles. Moreover, the uniformity of the particles (2AT and 3AT) implies the well association of biomolecules with TiO2 NPs during the synthesis process. In order to ascertain the morphology and particle size (3AT), it has been further subjected to TEM analysis.

Fig. 6
figure 6

SEM image of TiO2 nanopowder calcined at 400 °C. (a) Pure TiO2 without Aloe vera plant extract. (b) 1AT, (c) and (d) 2AT, (e) and (f) 3AT

Full size image

The surface area of the TiO2 nanocorals (3AT sample) calcined at 400 °C has been measured using nitrogen gas adsorption by BET analysis and the specific surface area is 27.6238 m2/g.

As shown in the TEM image, it perspicuously depicted the presence of plausible nanocorals Fig. 7(a)–7(d) with the diameter in the range of 80–200 nm. It can be clearly visualize that the nanocorals are decorated with the nanopolyps of titanium oxide (TiO2) having the size in the range between 15 and 30 nm. SAED pattens of the synthesized TiO2 nanopowder show a polycrystalline nature in Fig. 7(e) and 7(f).

Fig. 7
figure 7

TEM image of TiO2 nanocorals calcined at 400 °C. (a), (b) 3AT, (c), (d) 1AT, (e), (f) SAED patterns of 3AT and 1AT

Full size image

The possible growth mechanism for the resultant TiO2 cauliflower morphology is still unclear but it is understood on the basis of observed experimental results presented in Fig. 8. As already discussed, the addition of A. vera extract results the change of TiO2 precursor solution from its clear nature to slurry. This indicates the rapid reaction of A. vera extract with TiO2 precursor solution. The growth mechanism may occur in two stages. Firstly, the protein/enzyme–nanoparticle interaction takes place i.e. the protein of the A. vera extract can bind with the TiO2 surface via electrostatic interaction. This is clearly supported by the band shift from 1,615 cm−1 to 1,539 cm−1 in the FTIR spectrum. In this stage, the aggregation of NPs could be effectively avoided and thus the stabilization of NPs is takes place by proteins, which is supposed to the formation of primary TiO2 nanopolyps. Secondly, due to the shape-directing ability of carbonyl compounds and other such bio constituents of A. vera extract, the capping of TiO2 nanopolyps stabilized by proteins takes place. During the calcination at 400 °C, the removal of biological constituents also takes place and the spontaneous aggregation of primary TiO2 nanopolyps results in the formation of cauliflower-nanocoral morphology. Hence, the formation of TiO2 cauliflower-nanocoral morphology is due to the synergistic effect of A. vera extract.

Fig. 8
figure 8

Schematic illustration of the growth mechaniam of TiO2 cauliflower-nanocoral morpholgy

Full size image

4 Conclusions

A novel TiO2 nanocoral architecture is obtained first time using A. vera plant extract as a bio-capping agent by sol–gel process. The concentration variation of A. vera plant extract enormously changes the particle size of TiO2 nanopowder and thereby it leads to control the particle size. Thermal analysis reveals the crystallization temperature of 1AT and 3AT at 300 °C. The XRD analysis substantiates the presence of TiO2 in anatase phase and also the rutile phase of TiO2 nanopowder (1AT only) at 600 °C for the lower concentration (0.5 ml). Hence the phase transformation of TiO2 nanoparticle depends on the concentration of A. vera plant extract. FT Raman spectra emphasize the presence of TiO2 anatase phase (3AT) and the Raman band shifts with respect to the particle size variation. The biological molecules are responsible for the formation of TiO2 NPs, which is analyzed by FTIR spectroscopy. The surface area is measured by BET analysis and it is found to be 27.6238 m2/g. SEM images depict the homogeneity, formation of TiO2 NPs with spherical shape and also the reduction of particle size with respect to the concentration of A. vera plant extract. Furthermore, TEM images demonstrate the plausible nanocorals decorated with the nanopolyps having the diameter in the range of 15–30 nm.