1 Introduction

In recent years, the synthesis and characterization of new nanostructure have received much attention due to their significant potential application [1–3]. On the other hand, the specific properties of nanoparticles are closely related to their size and distribution. Regular morphology, highly distributed nanoparticles with small size and narrow size distribution are ideal for various applications, such as high electrocatalyst activity [3, 4], optical properties [5–7] and antibacterial activity [8], owing to large surface-to-volume ratio.

Fabricating CNT-supported nanomaterial is thereby desirable since it combines both the advantages of CNTs and nanomaterial, which may be useful in expanding the applications [3, 9, 10].

In recent years, TiO2 nanoparticles have received much attention due to their excellent biocompatibility, optical properties, unique electronic and catalytic properties [11–13]. The properties of the titanium oxide is highly dependent on their nonstructural characteristic, including phase and crystal size, morphology, aspect ratio and even distribution density [4, 12, 14–17]. Due to the chemically ‘inert’ characteristic and the highly hydrophobic nature of CNTs surface, it is difficult to make attachment with metal precursors [1–3, 9, 10, 18]. Then, the surface modification is required so as to establish efficient tube–nanoparticles interactions are established. The extensive dispersion of various functional groups such as hydroxyl, carboxyl and carbonyl on CNTs surface could provide active sites with metal ions to attach [2, 9, 10, 18]. Different from the previously reported methods, TiO2/CNT composite is produced by modifying CNTs to achieve high dispersion, suitable morphology and better size distribution of TiO2 nanoparticles [5, 9–12, 19]. Therefore, it is desirable to develop a simple and effective synthesis route that provides homogeneously dispersed TiO2 nanoparticles while maintaining some degree of control over particle size and size distribution of CNTs. Modifying CNTs surface with desired functional groups by chemical treatments is a common strategy to get well-dispersed TiO2 nanoparticles on CNTs [9–12, 19]. Moreover, TiO2 nanoparticles prepared in ethanol solution by sol–gel technique, maintained their small particle size and narrow size distribution deposited on the surface of CNTs [14, 20]. Although these strategies are effective, the controllability has not been fully known.

In the present study, the main purpose is to demonstrate a simple and general procedure for the decoration of CNTs with TiO2 nanoparticles. More importantly, the content of TiO2 supported on CNTs can be appropriately controlled by simply tuning the relative ratio of TiO2 to CNTs. We have measured the optical absorption and band gap energy of TiO2 nanoparticles with rutile and anatase structures by UV–Vis spectroscopy. The results indicate that application of CNTs as a support can decrease the value of band gap energy, censurability.

2 Experimental

CNTs were produced by chemical vapor deposition method using acetylene cracking over Co3O4/MgO catalyst-support. The process of synthesis had been described earlier [21]. The raw product contains impurities such as the support, catalyst particles, and a few amorphous carbons. The as-grown CNTs were purified using a two-step purification procedure involving thermal treatment at 490 °C for 2 h under air flow condition and then were sonicated for 30 min in a mixture of concentrated nitric acid/sulfuric acid (5 M and 1:1 by volume) and stirred for 4 h. This suspension was filtered and then washed several times with distilled water to acid-free, and finally dried at 120 °C for 2 h.

TiO2/CNTs nanocomposite was prepared using sol–gel technique. For the decoration of CNTs, in a typical synthesis, 0.2 g of functionalized CNTs were dispersed into ethanol solution by ultrasonication for 10 min. Afterward, a desired amount of titanium tetrachloride (99.5 %, Merck) was slowly added dropwise into ethanol solution containing functionalized CNTs under stirring. The size of TiO2 nanoparticles were controlled by the ratio of TiO2 to CNTs. In order to achieve different weight ratio of TiO2 to CNTs (1:0, 1:1, 1:2, 1:4, 1:8 and 1:12) in nanocomposites, we selected the desired value of TiCl4 in solution. For each synthesis, the ratio of TiCl4 to ethanol was 1:10.

The solution was stirred for 24 h under atmospheric presser and temperature of 24 °C. Each sol–gel solution was vaporized at 80 °C in environmental conditions, until a dry-gel was obtained. The dry-gel was dried overnight in an oven at 120 °C. The TiO2 powders were grounded using mortar and pestle. The as-prepared samples were calcinated at 400°C in a tubular electrical furnace under argon gas atmosphere (at ramping rate of 10 °C/min) for 2 h.

3 Results and discussion

3.1 Characterizations of synthesized materials

The crystal structure of samples was investigated by X-ray diffraction [GBC, Cu (Kα) Spectra, λ = l.54 Å] in 2θ ranging from 10º to 90º. A scanning electron microscope (SEM, Philips, 15 kV, 60 kx), transmission electron microscopy (TEM, Philips, CM10, 100 kV) were used to check the morphology of nanocomposites. FT-IR apparatus (Shimadzu—8400 s) was used to identify the functional groups on the surface of CNTs. UV–Vis spectra of TiO2 nanoparticles were recorded by using of a spectrometer (GBC, Cintera 20). In addition, the sizes of TiO2 nanoparticles were determined by Debby–Scherrer equation.

3.2 Structural and morphology analysis

In order to study the morphology of TiO2/CNT nanocomposites, SEM images prepared from all samples (Fig. 1). As Fig. 1a (1:0 ratio), due to surface activity, TiO2 nanoparticles are seen as dense aggregates and make large particles. It is evident that the aggregated particles have irregular shape. This aggregation is not appropriate for physical and chemical applications and can decrease their unique properties. As SEM images (Fig. 1b–f), by using CNTs as a support, they prevented aggregation of TiO2 nanoparticles. It can be observed that for lower ratio of CNTs (1:1 ratio, Fig. 1b), there is a nonuniform distribution of TiO2 particles with irregular shapes. However, by increasing density of CNTs, the images clearly showed high homogeneity and good morphology of the particles which was strongly dependent on the ratio of TiO2 to CNTs. At large ratio of CNTs to TiO2 (Fig. 1f), the surface of purified CNTs is covered completely with TiO2 nanoparticles and there is not any aggregated TiO2 nanoparticles between interface of CNTs. The well-distributed nanoparticles deposited on surface of the carbon nanotubes demonstrate that the CNTs pretreatment processing was effective, which resulted in many active sites on the carbon nanotubes.

Fig. 1
figure 1

SEM images of TiO2/CNT samples calcinated at 400 °C for 2 h, a 1–0, b 1–1, c 1–2, d 1–4, e 1–8 and f 1–12 ratios. (The magnification of images is 60 kx)

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Figure (2) shows TEM images prepared from purified and decorated CNTs. Figure (2a) shows the morphological structure of CNTs where the crystalline tubular structure of nanotubes are observed. It is observed that the nanotubes have clear inner and outer surfaces. Figure (2b) displays typical TEM images of typical CNTs decorated with TiO2 nanoparticles (with 1:1 ratios of TiO2 to CNTs). It presents the morphology and the size distribution of TiO2 nanoparticles on the sidewalls of the carbon nanotubes (dark spots) and the inner cavity of the tube still clear. It can be seen that the average size of TiO2 nanoparticles is about 8.2 nm.

Fig. 2
figure 2

TEM images of purified CNTs (a) and TiO2/CNT nanocomposites in ratios of 1:1 (b), calcinated at 400 °C for 2 h

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The Structure of TiO2 powder and TiO2/CNTs nanocomposite samples have been investigated by XRD patterns. Figure 3 shows that the XRD patterns of the both TiO2 powder and TiO2/CNT nanocomposite samples were calcinated at 400 °C.

Fig. 3
figure 3

XRD pattern of TiO2/CNT samples calcinated at 400 °C for 2 h, a 1–0, b 1–1, c 1–2, d 1–4, e 1–8 and f 1–12 ratios

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The main diffraction peaks of the TiO2 nanoparticles confirmed the good crystallite, which is well consistent with the results of other studies [14, 20]. In X-ray diffraction (XRD) patterns of TiO2 and TiO2/CNTs the diffraction peak at 2θ of 25.4°, 37.9°, 48.1°, 54.8, 55.3°, 62.9°, 69.3°, 70.5°, 75.2°, 75.6° and 82.5°, which are attributed to anatase TiO2 [20, 21]. There were also no any corresponding peaks to confirm the crystallization of rotail at this temperature for TiO2 nanoparticles in all TiO2/CNT nanocomposites. Nevertheless, in XRD patterns of TiO2/CNT nanoparticles, the additional peak could be well seen at 26.06, which is corresponding to graphite [22]. Moreover, for all samples, observation of spectrum revealed that with the increasing the ratio of CNTs in nanocomposites, the intensity of the TiO2 peaks decreased significantly. The average crystallite size (D) was calculated from maximum peak according to Debby–Scherrer’s formula [23]:

$$ D = 0.9\lambda /\left( {\beta \, cos\theta } \right) $$
(1)

where λ is the wavelength of the X-ray radiation (0.1541 nm), θ is the Bragg’s angle in degree and β is the full width at half maximum (FWHM) of the peak in radian.

The XRD confirms that the highest intensity peaks were at 2θ = 26°, which.

corresponds to the reflection plane (311) and the average particle size was found to be 14.3, 8.6, 6.2, 4.7, 3.4, 2.1 nm for 1:0, 1:1, 1:2, 1:4, 1:8 and 1:12 ratio, respectively. Accordingly, the particle sizes decreased with the increasing of the ratio of CNTs in nanocomposites ranging from 1:0 to 1:12, respectively.

The FTIR spectra of “as-prepared” and oxidized CNTs are compared in Fig. 4. The FT-IR spectroscopy study of oxidized CNTs confirms the defective sites at the surface of CNTs, and the presence of function groups O–C=O (1210/cm), C–O–O (1415/cm), C–O (1475/cm), and C=O (1,025 and 1720/cm) which leads to the hydrophilic nature of CNTs [18]. These functional groups may also act as stimulant in the formation of the composites, and enhance the interfacial bonding of TiO2 nanoparticles with carbon nanotubes.

Fig. 4
figure 4

FT-IR spectra of pristine a and purified CNTs b

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As a result, it is speculated that the TiO2/CNTs nanocomposite powder form the redox reaction between Ti ions with functional groups on the surface of CNTs under sol–gel conditions. Then the bonding C–O–Ti or O=C–O–Ti might be formed between two materials and TiO2–CNTs composites were formed naturally through some physicochemical bonding [18]. Consequently, Ti ions nucleated heterogeneously via sol–gel method because CNTs have prevented TiO2 from growing into aggregated nanoparticles.

The optical properties of the synthesized TiO2 nanoparticles and TiO2/CNTs nanocomposite were characterized by the UV–Vis spectroscopy (Fig. 5). The optical band gap of TiO2 nanoparticles was determined from the absorption spectra by using Tauc equation [24]. The absorption coefficient, α is related to the optical band gap energy (Eg) by the relation:

$$ \left( {\alpha {h}\nu } \right)^{1/n} = {B}\left( {{h}\nu - {Eg}} \right) $$
(2)

where h is the Plank’s constant, ν is the photon frequency of the radiation and B is the parameter of absorption edge width. The exponent n could be determined by the type of electronic transitions causing the optical absorption and it has taken the values of 1/2 and 2 for direct-allowed and indirect-allowed transition, respectively.

Fig. 5
figure 5

Optical absorption spectrum of TiO2 nanoparticles and TiO2/CNTs nanocomposite (with 1:1 weight ratio of TiO2 to CNTs) at different calcination temperatures

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In the present case, the optical band gap for the absorption peak can be obtained by extrapolating the linear portion of the \( (\alpha h\nu )^{2} - {\text{h}}\nu \) curve and the type of electronic transitions indirect-allowed transition (Fig. 6). The value of band gap energy (Eg) of TiO2 nanoparticles and TiO2/CNT nanocomposites are piloted as a function of different calcination temperatures (Fig. 7). As Fig. (7), the band gap energy of TiO2 nanoparticles decrease by using CNTs as support, considerably. It is clearly that its value is alternated by increasing calcination temperature. As these experimental results, they show that the CNTs and different calcination temperature can influence the band gap energies of the TiO2 nanoparticles. By application of CNTs as a support of TiO2 nanoparticles, this effect is probably due to increase in chemical defects or vacancies present in the intergranular regions and also chemical intraction between CNTs and TiO2 nanoparticles, generating a new energy level to reduce the band gap energy [21, 25]. The TiO2/CNT nanocomposite with lower band gap energy are optically active and have been extensively used in the field of photo-oxidation catalysis for environmental cleanup and solar sell.

Fig. 6
figure 6

Extrapolating the linear portion of the \( (\alpha h\nu )^{2} - {\text{h}}\nu \) curve for TiO2 nanoparticles and TiO2/CNTs nanocomposite powders (with 1:1 weight ratio of TiO2 to CNTs) at different calcination temperatures

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Fig. 7
figure 7

Diagram of band gap energy of TiO2 nanoparticles and TiO2/CNT nanocomposites, (with 1:1 weight ratio of TiO2 to CNTs) in terms of different calcination temperatures

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4 Conclusions

The TiO2 nanoparticles and TiO2/CNT nanocomposites have been successfully synthesized by sol–gel technique and their structural and morphology were studied. High efficiency of proposed method was confirmed as well as the possibility of the coating of TiO2 nanoparticles over CNTs, without aggregation of these particles. The size of TiO2 nanoparticles and morphology of powder are varied by changing the weight percent of carbon nanotubes in solution. The results show that application of CNTs as a support was decreased the band gap of TiO2 nanoparticles.