Introduction

Over the past decades, semiconducting photocatalytic materials like ZnO, TiO2 are widely applied for degradation and complete mineralization of toxic organic compounds in water, soil, and air (Zeng et al. 2008; Qian et al. 2009). Titania (TiO2) based catalyst is widely applied for decomposition of toxic organic chemicals both in the aqueous and gas phase due to its unique properties like nontoxicity, chemical stability, high photocatalytic activity and low cost. However, its widespread application is still limited because of the high band gap energy of photocatalytically active titania (3.2 eV for anatase TiO2). It requires UV irradiation; solar light contains only a small fraction of (about 4–5 %) UV light, therefore, it is necessary to develop a photocatalyst that could be activated under visible light irradiation in order to use sunlight as a chief energy source.

The photocatalytic activity of titania can be improved by decrease in grain size, increasing surface area, sensitization using dye molecules and doping with metals and non-metals (Zhang et al. 2010; Fu et al. 1996). Doping with transition metal ions like Fe2+, Mn2+, Cu2+, Zn2+, etc. (Kubacka et al. 2009) is the one of the most powerful way to improve the photocatalytic activity of titania. Of all the transition metal ions, Zn2+ causes more effective doping with titania since ionic radii of Zn2+ (0.74 Å) and Ti4+ (0.75 Å) are similar to each other (Rauf et al. 2011). Hence Zn2+ ion can easily substitute Ti4+ ion in TiO2 lattice without destroying the crystal structure, thereby stabilizing the anatase phase. Recently, Zn-doped TiO2 powder was synthesized by various preparative methods like sol-hydrothermal (Yang et al. 2010; Zhu et al. 2012); sol–gel and solid phase reaction (Liu 2005); homogeneous hydrolysis (Österlund et al. 2009); controlled hydrolysis of titanium(IV) butoxide (Mahata et al. 2011); electrospinning (Khajavi and Abbasipour 2012); assembly process based on Ligand exchange reaction (Xu et al. 2005) micro emulsion (Sanchez-Dominguez et al. 2015) and sol–gel method (Seabra et al. 2011; Chauhan et al. 2012).

Sol–gel method is one of the most versatile technique generally adapted for the synthesis of metal-doped nanocrystalline titania; it has some advantages over other conventional methods like good homogeneity, control grain size, particle morphology and porosity. Surfactant-assisted sol–gel method was widely used to synthesize mesoporous materials (Jentys et al. 1996; Beck et al. 1992; Wang et al. 1999; Tanev et al. 1997).With successfully synthesized MCM-like mesoporous material (Liu et al. 2004; Yang et al. 2015), it is believed that nanoparticles of zinc-doped titania could be possible to synthesize using similar surfactant-assisted sol–gel method.

In this article, a very simple, ecofriendly straightforward, catalyst free, surfactant template assisted, single step sol–gel process in alcohol–water mixed solution was reported. A potential technique for nanocrystalline mesoporous undoped and zinc-doped TiO2 nanoparticle synthesis. The photocatalytic activities of as synthesized catalyst were thoroughly studied in relating to its photocatalytic activity of methyl red photodegradation. The satisfactorily high photocatalytic efficiency was attained by doped samples as compared to undoped titania.

Experimental

Chemicals

For the synthesis of pure TiO2 and zinc-doped TiO2 nanoparticles, the materials used were titanium (IV) butoxide [M = 340.76, Ti (OC4H9)4], zinc nitrate [M = 297, Zn (NO3)2·6H2O], obtained from Sigma-Aldrich, cetyltrimethyl ammonium bromide [M = 364.46, C19 H42BrN], Isopropyl Alcohol [M = 60, (CH3)2CHOH], were obtain from SD Fine Chemicals and ethyl alcohol [M = 46, C2H5OH] from local distillery. The titanium (IV) butoxide and Zn (NO3)2·6H2O were used as the precursors for the TiO2 and Zn components, respectively, and deionized water was used as a solvent.

Synthesis

Synthesis of TiO2 nanoparticles

Pure TiO2 nanoparticles were synthesized by sol–gel method. A solution containing 160 ml of ethanol and 20 ml of iso-propanol was mixed slowly with 20 ml (50 mmol) of Titanium (IV) butoxide under sonication. The resultant mixture was slowly added to 100 ml of deionized water containing 2 g of cetyltrimethyl ammonium bromide (CTAB) to form a white precipitate. The system was kept under constant stirring at 80 °C for 4 h. The excess water was removed by evaporation on the water bath with continuous stirring. The resultant precursor was then dried at 110 °C for 12 h and then finally calcined at 500 °C for 4 h in high temperature muffle furnace.

Synthesis of series of Zn-doped TiO2

The Zn-doped TiO2 nanoparticles was synthesized using the same method as that of TiO2, except that the water used for the synthesis (100 ml) contained the required amount of zinc nitrate (1, 3 and 5 mol%).

Evaluation of photocatalytic activity of the samples

In this study, the photocatalytic activity was tested using methyl red (MR) solution as a model pollutant under UV light radiation. The photocatalytic degradation was carried out with 100 ml aqueous methyl red solution (100 ppm) containing 50 mg of catalyst nanoparticles. This mixture was stirred in dark for 30 min to reach adsorption equilibrium. Then, the mixture was placed inside the photoreactor in which the vessel was 3.5–4 cm away from the UV lamp. The quartz vessel and the light source were placed inside a black box to prevent UV leakage. The experiments were performed at room temperature and a pH of about 6.3. Small portions of the mixture were taken at periodic intervals during the irradiation, and after centrifugation, they were analyzed with the UV–Vis spectrophotometer. The % mineralization of dye was determined by COD measurement following the closed reflux volumetric method.

Characterization

The synthesized materials were characterized by various sophisticated Techniques. X-ray diffraction (XRD) patterns were carried out by using Philips X-ray diffractometer with diffraction angle 2θ in between 20° and 80° using Cu-Kα radiation of wavelength 1.542 Å. Surface morphology and elemental analysis of the samples were carried out using scanning electron microscopy with electron dispersion spectroscopy (SEM–EDS) characterization conducted using a JEOL-JED 2300 (LA) instrument. The microscopic nanostructure and particle size was determined using a CM-200 PHILIPS transmission electron microscope (TEM) at 200 kV (L = 600, λ = 0.0025 nm). The surface area of the samples was determined from nitrogen adsorption–desorption isotherms at liquid nitrogen temperature using a Quantachrome ASiQwin1994–2013 instrument with out gas 15 min at 150 °C. The BET method was used for surface area calculation (Brunauer et al. 1938), the pore size distribution (pore diameter and pore volume of the samples) was determined by the BJH method (Barrett et al. 1951). The light absorption by sample was carried out using Varian Carry 5000 (UV–VIS-DRS) spectroscopy in the range 800–200 nm.

Results and discussion

X-Ray diffraction of doped and pure TiO2

Figure 1 shows XRD patterns of the pure TiO2 and Zn-doped TiO2 with different dopant concentrations calcined at 500 °C. All the samples exhibit anatase phase with no evidence of rutile phase and zinc ions. The existence of zinc ion is confirmed by EDX (see Fig. 3).

Fig. 1
figure 1

XRD Pattern of pure TiO2 and zinc doped TiO2 nanoparticles calcined at 500 °C

Full size image

Also, the sharp, intense peak at 2θ = 25.281° corresponds to anatase (101) phase, representing that the synthesized nanomaterials were well crystallized. All the diffraction peaks obtained from XRD agreed with the reported JCPDS card no. 21-1272 for anatase. The crystallite sizes were estimated by the Scherrer’s equation, using the most intense reflection (2θ = 25.281º).When the mol% of the zinc ion doping in TiO2 was increased, the crystallite size decreased. The particle sizes in the samples were in the range of 12.6–18.1 nm when calcined at 500 °C.

Scanning electron microscopy (SEM) analysis

SEM analysis was carried out for two representative samples calcined at 500 °C, named as TiO2 and 5 mol% Zn-doped TiO2, the results are shown in Fig. 2. As it follows from the SEM micrographs, synthesized nanoparticles are spherical in shape. A slight agglomeration was seen in both samples.

Fig. 2
figure 2

SEM micrographs of the synthesized nanoparticles: a Pure TiO2 and b 5 mol Zn-doped TiO2 calcined at 500 °C with CTAB at different magnification

Full size image

Energy dispersive X-ray spectroscopy (EDS) analysis

Energy dispersive X-ray spectroscopy (EDS) analysis was also carried out for 5 mol% Zn-doped TiO2 (Fig. 3) and the result shows that the Ti and Zn atomic % in the obtained materials is in very good agreement with the nominal (expected) composition.

Fig. 3
figure 3

Energy dispersive X-ray analysis of 5 mol% Zn-doped TiO2 nanoparticles calcined at 500 °C with CTAB

Full size image

Transmission electron microscopy (TEM) analysis

By HRTEM it was possible to carry out an assessment of particle size, morphology and crystallinity. Figure 4 shows results for pure TiO2 and 5 mol% Zn-doped TiO2. In Fig. 4a and b, it can be observed that nanoparticles of TiO2 have a spherical shape and a size around 14–18 nm. Figure 4b shows that the particles are crystalline, as evidenced by the lattice fringes observed. Selected Area electron diffraction (SAED) analysis of the nanocrystals in Fig. 4b is shown as inset; the calculated d-spacing was 0.350536 nm which is in agreement with (hkl) (internuclear distance of anatase TiO2 phase (JCPDS 21-1272). 5 mol% Zn-doped TiO2 nanoparticles were also spherical in shape, and the particle size was in the order of 12–14 nm, as shown in Fig. 4c; lattice fringes were also observed for this sample, as shown in Fig. 4d (calculated d-spacing was 0.351802 nm, which is in agreement with (hkl) (101) of anatase TiO2 phase, JCPDS 21-1272).

Fig. 4
figure 4

TEM micrographs of synthesized nanoparticles: a, b Pure TiO2; c, d 5 mol% Zn- doped TiO2. The inset of b and d shows the corresponding SAED pattern, calcined at 500 °C with CTAB at different magnification

Full size image

BET surface area

Figure 5 shows the N2 adsorption–desorption isotherm of two representative synthesized samples i.e. Pure TiO2 and 5 mol% Zn-doped TiO2. According to IUPAC the entire synthesized sample shows a type IV isotherm with hysteresis loop indicates existence of mesopores which provides highest photocatalytic activity towards methyl red. The multipoint BET specific surface area for both studied catalyst was found to be 43.376 and 63.667 m2/g, respectively, these values are higher than non-porous TiO2 materials (2–30 m2/g). Our findings clearly indicates that Zn-doped TiO2 have a larger surface area and a greater N2 adsorption capacity than pure TiO2, which is due to the increase in the pore size. Table 1 shows the physical parameters of nitrogen isotherms, such as BET surface area (S BET), BJH average pore diameter (d p) and the pore volume (V p).

Fig. 5
figure 5

Nitrogen adsorption–desorption of a TiO2, b 5 mol% Zn-doped TiO2

Full size image
Table 1 Band gap energies of the synthesized photocatalyst
Full size table

UV–visible diffusion reflectance spectroscopy (UV-DRS)

The band gap energies (E bg) of the synthesized nanomaterials were calculated by using UV–visible diffusion reflectance spectroscopy (DRS). To determine band gap energy of the synthesized materials, the diffusion reflectance spectra were recorded and cut off wave length at which absorption sharp edge rises were determined by drawing a tangent on this curves (Fig. 6). The band gap energies were calculated using cut-off wavelength and are represented in the following table. The band gap energy is calculated using the following equation (Bhatkhande et al. 2002):

$$E_{\text{bg}} = \frac{1240}{\lambda }$$

where λ is the wavelength in nanometer and E bg is the band gap energy.

Fig. 6
figure 6

UV-DRS image of pure and Zn-doped TiO2 nanoparticles

Full size image

From the results of band gap energy data (Table 2), we can conclude that pure TiO2 had weak visible light response than Zn-doped TiO2, i.e. due doping ability of light absorption of TiO2 is slightly enhanced.

Table 2 Degradation and mineralization results obtained after 2 h for TiO2 and Zn- doped TiO2
Full size table

Photocatalytic activity result

Photocatalytic activities of the synthesized nanomaterials were tested using methyl red as a model pollutant. Methyl red is from synthetic azo dye and according to International Agency for Research on Cancer (IARC) is probably carcinogenic to human beings. Methyl red is a major water pollutant and its degradation to safe levels is not an easy task due to its resistance to chemical and biological degradation.

The photocatalytic activity study of synthesized nanomaterials was carried out under UV light irradiation and the results of photocatalytic performance are shown in the Figs. 7 and 8. From Fig. 7 we can conclude that removal of dye reaches an approximately constant value after 75 min of photocatalytic treatment for all the tested photocatalyst and also methyl red removal achieves values between 86 and 96.99 % after 120 min of irradiation time, presenting the 5 mol% Zn-doped TiO2 the almost photodegradation under UV light (93 and 96.99 % removal of methyl red after 75 and 120 min of irradiation time, respectively) among the studied photocatalyst. Table 2 shows % methyl red photocatalytic removal after 2 h under both UV light irradiation.

Fig. 7
figure 7

Temporal evolution of MR removal during photocatalytic experiments under UV light irradiation

Full size image
Fig. 8
figure 8

Photodegradation graphs of methyl red with a Pure TiO2, b 1 mol% Zn-doped TiO2, c 3 mol% Zn-doped TiO2, and d 5 mol% Zn-doped TiO2

Full size image

It is important to note that the photocatalytic reaction does not take place at all in the absence of a photocatalyst. Additionally, if the mixture of methyl red and the synthesized catalyst are kept in the dark for more than 24 h, no significant degradation of dye takes place. When the reaction mixture containing methyl red and catalyst is kept under visible light, photodegradation takes place, but the reaction takes a very long time compared to the degradation that occurs in the presence of UV light. The higher photocatalytic activity of doped TiO2 under UV irradiation is attributed due to its smaller grain size high surface area narrower band gap energy.

Conclusion

Herein, we report very simple route for synthesis of mesoporous nanocrystalline Zn-doped TiO2 nanoparticles via sol–gel technique using CTAB as surface directing agent. The XRD pattern of all synthesized samples calcined at 500 °C confirms the formation anatase phase with very good crystallinity. From the XRD, SEM-EDX, TEM, BET, and UV–Vis results, it was confirmed that the doping of Zinc in TiO2 decreases the grain size and shifts the absorption to higher wavelengths. The results of photocatalytic study confirm the highest photodegradation of methyl red under UV irradiation for 5 mol% of Zn-doped TiO2 compared to other synthesized catalyst.