Zinc and boron co-doped nanotitania with enhanced photocatalytic degradation of Acid Red 6A under visible light irradiation
Sustainable Environment Research volume 29, Article number: 29 (2019)
The present investigation summarizes the synthesis and characterization of zinc and boron co-doped TiO2 nano catalyst by sol gel method at relatively low temperature and explored the optimum reaction parameters for the degradation of Acid Red 6A (AR 6A) under visible light irradiation. The structural, morphological, surface properties of the synthesized photocatalysts were characterized by X-Ray Diffraction, Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy, Transmission Electron Microscopy, Brunauer-Emmett-Teller, UV-Vis-Diffuse Reflectance Spectroscopy, X-ray Photo Electron Spectroscopy and Fourier Transform-Infra Red Spectroscopy. The findings of the characterization results revealed that Zn2+ ion was substitutionally doped into TiO2 lattice by replacing Ti4+ ion and a part of the boron substituted the oxygen in TiO2 lattice and the remaining amount was converted to B2O3 due to calcinations at 450 °C. The degradation of AR 6A was achieved at optimum reaction parameters, such as pH = 6 with a catalyst dosage 100 mg L−1 for the degradation of 5 mg L−1 of dye solution.
For the degradation of a wide range of organic dye contaminants from textile and food industry, advanced oxidation process using semiconductors was considered as one of the most promising methods. Mijin et al.  reported the use of semiconductors that can be easily controlled with low power consumption. Among all the semiconductors, Titanium dioxide (TiO2) based photocatalysts were used effectively for the degradation of toxic organic pollutants due to its unique properties like bio-compatibility, low cost, water insolubility, chemical stability and non photocorrosive nature . However, its large band gap energy (3.2 eV) limits its activity in visible light region. To overcome these limitations of TiO2, many investigators [3,4,5] developed a promising method to increase the visible light absorption by chemical doping of metal (Bi, Fe, Au, Zn, Cu, Mg, Co, Mn, Ni)] and non-metal (C, S, B, N, P) ions into titania crystal lattice. Further studies of Huang et al. [6,7,8] have demonstrated that the metal ion doping into TiO2 increases separation rate of the electron hole pairs, increases its electrical conductivity and also directly influences the intrinsic properties of the semiconductor, whereas non-metal ion doping reduces the charge recombination rate of photo generated electron-hole pairs. Metal and non-metal co-doped TiO2 based photocatalysts with more specific surface area and narrow pore size distribution are the promising materials used for various applications in photocatalysis studied by Tan et al. . Others [10, 11] attempted metal and non-metal (Cu & S, Fe & B) co-doping into TiO2 lattice which resulted in the improved photocatalytic activity by minimizing the band gap, decreasing particle size, increasing the surface area and improved the trapping-to-recombination rate . Among all the transition metals, zinc is more effective dopant for titania, since ionic radii of Zn2+ (0.74 Ao) is very close to Ti4+ (0.75 Ao) ; thus Ti4+ ion can be easily substituted by Zn2+ ion into TiO2 crystal lattice. Doping of zinc into TiO2 increases the surface area of TiO2 and facilitates the formation of surface bound OH3 on the TiO2 surface . By doping zinc in TiO2 the band gap energy of TiO2 is decreased when compared with undoped TiO2 . Boron doping can effectively inhibit the growth of crystal size . In view of these advantages, it is proposed to synthesize zinc, boron co-doped TiO2 by eco-friendly single step sol-gel process, which is one of the simple techniques, to control the grain size and produce the porous homogeneous powder. Acid Red (AR) 6A, a model dye pollutant was used to determine the photocatalytic efficiency of the prepared catalysts under visible light irradiation. Acid Red 6A (AR 6A) has been selected based on its toxic effects  on skin, kidney, liver, oral cavity and on subcutaneous glands. And also it is highly persistent in environment due to non-biodegradability [18, 19].
Titanium tetra-n-butoxide (E-Merck, Germany), zinc nitrate (E-Merck, Germany), triethyl borate (Sigma Aldrich) were used as precursors for Ti, Zn and B respectively. All the chemicals used were of reagent grade and the solutions prepared in double distilled water without further purification.
Synthesis of zinc, boron co-doped TiO2
Catalyst samples were prepared by varying different weight percentage of dopants (Zn and B) using sol-gel method. In 150 mL Pyrex glass beaker, titanium tetra-n-butoxide (15 mL) was dissolved in absolute alcohol (30 mL) and stirred for 10 min followed by addition of HNO3 (2.1 mL) drop wise under continuous stirring for 30 min (Solution I). In another Pyrex glass beaker, absolute alcohol (30 mL), H2O (4.32 mL), and the dopants, i.e., zinc nitrate and triethyl borate were taken as per the required weight percentages with respect to titanium as shown in the Table 1 (Solution II). At room temperature and under vigorous stirring, the solution II was slowly added to solution I from the burette until the transparent sol was formed, and stirring was continued for 2 h. The sol formed is kept aside for 48 h for aging in dark at room temperature to obtain gel. The gel was dried in an oven at 100 °C and ground. The catalyst powder was calcined at 450 °C in a muffle furnace for 5 h. The prepared catalysts were labelled as ZBT1, ZBT2, ZBT3, ZBT4, ZBT5 and ZBT6 as presented in Table 1.
Characterization of catalysts
Prepared photocatalysts were characterized by using various instrumental techniques. Ultraviolet-Visible Diffuse Reflectance Spectroscopy (UV-Vis-DRS) spectra were recorded using Shimadzu 3600 UV-Vis-DRS NIR spectrophotometer with an integrating sphere diffuse reflectance. X-Ray Diffraction (XRD) patterns were recorded using ultima IV, RIGAKU model with anode Cu-WL1 λ = 1.5406 nm, nickel filter current 40 mA, voltage 40 kV, 2θ scanning range of 5.000-90.9505°, scan rate of 2 min−1. A PHI quantum ESCA microprobe system, using the ALKα line of a 250 W X-ray tube as a radiation source with the energy of 1253.6 eV, 16 mA × 12.5 kV and working under the pressure lower than 10-8 Pa was used to record X-ray Photo Electron Spectroscopy (XPS) spectra. Morphology, chemical composition and particle size of the catalyst particles were determined by Scanning Electron Microscopy (SEM; JSM-6610 LV) equipped with an Energy Dispersive X-ray spectroscopy (EDS) operated at voltage 20 kV and Transmission Electron Microscopy (TEM) (TECNAI FE12) operated at voltage 120 kV. Brunauer-Emmett-Teller (BET) (NOVA 2200 E) system was used to determine pore size, pore volume and surface area of anatase particles from N2 adsorption-desorption isotherms at 77.3 K. Fourier Transform-Infra Red Spectroscopy (FT-IR) spectrometer (Nicolot Avatar-360) was used to record FT-IR spectra of prepared catalyst. UV-Vis-DRS spectrophotometer (Shimadzu 1601) was used to monitor the degradation percentage of AR 6A dye.
Photocatalytic activity of the Zn and B co-doped TiO2 nano material
The detailed description of photo-reactor was given elsewhere  and photocatalytic procedure was carried out as follows. Required amount of catalyst was added to 90 mL of aqueous solution in a Pyrex glass vessel and stirred the solution in dark for 30 min. Then 10 mL of AR 6A dye (10 mg L−1) was added and continued the stirring for 30 min to achieve the adsorption-desorption equilibrium between catalyst surface and dye molecules. The high pressure metal halide lamp (400 W) with UV filter (Oriel 51472) was used as a visible light source with output 436-546 nm, 35000 lm and placed at a distance 20 cm from the reaction vessel. Running water was circulated around the sample container to filter IR radiation and to keep the reaction mixture at room temperature. The pH of the solution was adjusted before exposed to light illumination by the addition of 0.1 N HCl/0.1 N NaOH to get the required pHs. After the illumination, 5 mL aliquots of samples were withdrawn from the reaction mixture using Millipore syringe (0.45 μm) at different time interval and their concentration was determined using UV-Vis spectrophotometer (Milton Roy Spectronic 1201) at maximum absorbance (λmax) 515 nm. The percentage of degradation of the dye (AR 6A) was calculated by using the following Eq. (1).
where A0 is initial absorbance of dye solution before degradation and At is absorbance of dye solution at time t.
Results and discussion
The XRD patterns of undoped and co-doped TiO2 samples are given in Fig. 1 indicating the formation of less dense anatase phase (JCPDS NO: 21–1272)  with corresponding (101) plane of high intensity at 2θ = 25.3°. The other peaks were observed at 2θ of 38.1, 47.9, 75.4 and 83.2° corresponding to (004), (200), (215) and (224) planes, respectively. The less dense anatase phase formation was may be due to more electropositive nature of zinc with loosely held electronic clouds in each TiO2 matrix as reported by Venkatachalam et al. . There are no characteristic peaks observed for zinc oxides in XRD spectrum in all the co-doped samples, which implies that Zn2+ ions are incorporated into the lattice of TiO2 by substituting Ti4+ ion , whereas B3+ ion cannot substitute Ti4+ ion in the TiO2 lattice due to large ionic radii difference between Ti4+ (61 pm) and B3+ (23 pm) ions. On the contrary, a part of the boron ions substitute the oxygen and the remaining small amount of boron ions converted to B2O3 at 450 °C calcinations temperature. But in the XRD of all co-doped samples, B2O3 peak was not observed, due to decrease in the c-axis parameter of ZBT catalysts and its minute quantity was lower than the detection limit of XRD technique . The average crystallite size of undoped and co-doped TiO2 samples was determined by Debye-Scherer equation  shown in Table 2. Results from the table inferred that the crystallite size of co-doped samples were found to be lesser (ranging from 9.1 to 14.0 nm) than that of undoped TiO2 (36.5 nm). Among all co-doped TiO2 samples ZBT3 (0.50 wt% of Zn & 0.50 wt% of B) exhibits the smallest crystallite size due to inhibition of the crystal growth of TiO2. The comparative results of undoped TiO2, zinc & boron single doped TiO2 and Zn, B co-doped TiO2 (ZBT3) crystallite sizes are given in the Table 2.
The surface chemical composition and electronic states of each constituent elements of the TiO2 (ZBT3) were identified as depicted in Fig. 2a-f. The XPS survey spectrum (Fig. 2a) explored the presence of Ti, B, Zn, and O on the surface of ZBT3 catalyst. The oxidation states of these elements are identified as Ti4+, B3+, Zn2+ and O2− based on their binding energies obtained from the magnifying spectra as shown in Fig. 2b-f. Ti4+ state was confirmed by the peaks located at 460.1 and 465.7 eV, which correspond to Ti 2p3/2 and Ti 2p1/2, respectively . The binding energy peak of boron 1 s appears at 193.5 eV sustained the B3+ state. The peaks of Zn 2p3/2 and Zn 2p1/2 located at binding energies of 1023.5 and 1045.9 eV, respectively , confirmed the presence of Zn2+ state in the catalyst. The XPS spectra of O 1 s (Fig. 2d) show two peaks at binding energies of 531.7 and 533.5 eV, which correspond to lattice oxygen (O2−) of TiO2 and adsorbed H2O/OH on the surface, respectively .
FT-IR spectra of undoped and co-doped TiO2 (ZBT3) samples are described in Fig. 3. The peaks at 3400 and 1628 cm−1 corresponding to stretching and bending vibrations of H2O and OH, respectively . Ti-O-Ti stretching frequency band in undoped TiO2 was observed at 512 cm−1 [27, 28] which was shifted to 576 cm−1 in the ZBT3 catalyst. This shift in the frequency of the Ti-O-Ti network is due to the doping of Zn and B ions into TiO2 lattice. This leads to the distortion in octahedral co-ordination around Ti4+ ions by the formation of new interactions Ti-O-Zn and B-O, with stretching vibrations found at 1201 and 434 cm−1, respectively . The asymmetric B-O stretching band was observed at 1392 cm−1  due to interstitial doped B3+ ion forming the Ti-O-B-O-Ti and substitutional B occupying O sites in a meta stable configuration  in the lattice of TiO2 but because of to low concentration of B and high dispersion, the corresponding peaks were absent in XPS. Hence, FT-IR study confirmed that Zn2+ and B3+ ions were substitutionally doped into TiO2 lattice by replacing Ti4+ and O2− ions, respectively.
The DRS of undoped and co-doped TiO2 samples are given in Fig. 4a. It is noticed that the co-doped TiO2 catalysts exhibited extended optical response in visible part of the spectrum. When compared to undoped TiO2, the red shift in co-doped samples indicated the decrease in band gap. This was caused by the extra energy level created above the valance band of TiO2 by Zn 2p  and substitution of B can also narrow the band gap of co-doped TiO2 . Further it was supported by the calculated band gap energies of the all synthesized catalysts using the Kubelka-Monk formalism and Tauc plot method  as shown in Fig. 4b. The undoped TiO2 exhibited the band gap of 3.12 eV but for the co-doped TiO2 samples showing the band gap ranging from 2.49 to 2.92 eV. Among all the co-doped samples ZBT3 exhibiting the lowest band gap energy, i.e., 2.49 eV. The results indicated co-doping of Zn and B made all the catalysts to be visible light active, by which more number of electron-hole pairs is generated by the absorption of visible light and leads to higher photocatalytic activity.
The surface morphology and chemical composition of undoped TiO2 and ZBT3 catalyst were studied by Field Emission SEM (FESEM) and EDX analyses and the results are shown in Fig. 5a-d. The SEM images of undoped TiO2 (Fig. 5a) shows smooth and spherical shaped agglomerated particles with uneven size distribution. Compared to undoped TiO2, ZBT3 exhibits (Fig. 5b) less agglomerated spherical shaped particles with decreased particle size. From these results it can be inferred that agglomeration and particle sizes were reduced greatly in ZBT3 by co-doping of Zn and B, which further confirmed by EDX analysis. Compared to undoped TiO2 (Fig. 5c) the EDX spectrum of ZBT3 (Fig. 5d) also showed the peaks corresponding to co-doped elements Zn and B. Here the carbon element comes from residual carbon in the testing environment.
As shown in Fig. 6a and b, the undoped and co-doped TiO2 nano catalysts are found to have spherical shape. The concentration of the dopants influences the size of the particle . It reveals that the co-doped sample particle sizes are less than that of undoped TiO2. The select area electron diffraction (SAED) pattern of the ZBT3 shown in Fig. 6c illustrated anatase crystalline phase of TiO2. From the high-resolution TEM (HRTEM) image shown in the Fig. 6d, the lattice fringes with inter planar distance of 0.32 and 0.29 nm are observed, which corresponds to the (101) and (001) planes of anatase TiO2, respectively. It indicates a single crystalline structure. The particle size distribution histogram obtained by Guassian fitting method shown in Fig. 6e confirms the average particle size of ZBT3 as 5.6 nm which exerts the decreased particle size of TiO2 due to co-doping of Zn and B.
BET surface area
To investigate the specific surface area and porosity characteristics of undoped TiO2 and co-doped TiO2 (ZBT3), N2 adsorption–desorption isotherms and their corresponding BJH pore size distributions were recorded (Fig. 7a and b). The isotherm (Fig. 7a) shows the type IV pattern with H3 hysteresis loop, implying that the sample has mesoporous nature . This also is confirmed by the corresponding BJH pore size distribution plot (Fig. 7b), where the sharp peaks observed between 2.2 to 8.6 nm represent the mesopores. Among all the co-doped samples ZBT3 showed high surface area of 91 m2 g−1. This characteristic feature favours the adsorption of more dye molecules on the catalyst surface which leads to enhance its degradation efficiency.
Evaluation of visible light activity of ZBT3 on degradation of AR 6A dye
To evaluate the photocatalytic efficiency of synthesized nano catalyst on degradation of AR 6A dye, a series of experiments were conducted under visible light. Initially, trial experiments were performed by taking only dye solution in the presence and absence of light, where there was no significant degradation observed. Other experiments with dye solution along with synthesized nano catalyst in the presence and absence of light were performed. Here there was a drastic decrease in the absorbance at its λmax 515 nm of the dye in presence of light and small change in the absorbance in absence of light was also observed in the dark, this implies that in the absence of light (dark), there is adsorption of dye molecules on the catalyst surface, and it will cause a slight decrease in the absorbance at λmax 515 nm. This results indicate that light and catalyst are interdependent on each other. The obtained data are included in the Table 3. To determine the reaction parameter for complete degradation of AR 6A, the following effects were conducted such as i) effect of dopant concentration, ii) pH effect, iii) effect of catalyst loading, and iv) effect of initial dye concentration.
Studies on effect of dopant concentration
To determine the effect of dopant concentrations of co-doped and undoped TiO2, experiments for co-doped nanocatalysts containing various dopant concentrations were carried out for the photocatalytic degradation of AR 6A with the results presented in Fig. 8a and b and the experimental data presented in the Table 4. All the co-doped samples showed increased photocatalytic rate when compared to undoped TiO2 under visible light irradiation. This may be attributed to the fact that the co-doping of Zn and B into TiO2 lattice catalyst becomes visible light active. Among all the co-doped catalysts, ZBT3 shows highest percentage of degradation due to increased number of charge carriers per particle . At these dopant concentrations, the catalyst particles are having small particle size and high surface area as confirmed by the TEM and BET analysis. Hence, it shows the higher photocatalytic efficiency in degradation of AR 6A. Further increasing the concentration of dopants causes the growth in particle size leads to decrease in surface area and less adsorption of dye molecules on the surface of the catalyst, which limited the rate of degradation. In addition increase in metal dopant concentration creates more electron trap centres  and reduce the •OH radical .
Further increase in metal ion concentrations in TiO2, i.e., 0.75 wt% of the catalyst results in decreases in the photocatalytic degradation of AR 6A. This doping of Zn2+ ion in TiO2 lattice minimizes the electron-hole recombination. However, further increases in dopant concentration are found to be detrimental. This is due to the recombination of the charge carriers which depends on the distance ‘r’ separating e−/h+ pair. The recombination rate increases exponentially with the dopant concentration because the average distance between trap sites decreases with increasing the number of dopants. Therefore 0.5 wt% of the Zn2+ ion and 0.5 wt% of boron were considered to be an optimal dopant concentration and further photocatalytic experimental effects were carried with ZBT3 catalyst.
Studies on effect of pH
According to Chen , the rate of photocatalytic performance of the catalyst can be greatly influenced by pH of the reaction mixture. Experiments were performed by varying the pH from 2 to 8 by keeping the other parameters constant to perceive the pH effect. The experimental data are presented in the Table 5 and the results are shown in Fig. 9a and b, the results indicated that maximum rate was achieved at pH 6 where the surface of TiO2 acquires more positive charge, which facilitate the adsorption of more negatively charged AR 6A dye molecules, consequently it leads to high percentage of degradation. Below the pH 6 the degradation was decreased due to the capturing of photogenerated electrons by the increased number of H+ ions. When the pH increases to basic medium (pH > 7), the catalyst surface changes to negative charge and causes electrostatic repulsion with the similarly charged dye molecules resulting in decreasing rate of degradation .
Studies on effect of catalyst loading
Studies on the effect of catalyst loading on degradation of AR 6A is shown in Fig. 10a and b. Experimental data are presented in the Table 6. Various catalyst loadings of 25, 50, 100, 150 mg L−1 were added to 100 mL of solution containing 5 mg L−1 of dye at pH 6 to investigate the rate of degradation. It is noticed that the rate was increased linearly up to catalyst loading of 100 mg L−1 and then decreased on further loading. The decrease can be explained by the turbidity and agglomeration of the catalyst particles, which restricts the penetration of light necessary for the activation of catalyst particles , and also collisions between active and ground state catalyst particles results in deactivation of the catalyst particles. Hence, 100 mg L−1 of catalyst was found to be the optimum loading.
Studies on effect of initial concentration of dye
Different concentrations of AR 6A dye from 2 to 20 mg L−1 were used at optimum catalyst (ZBT3) loading of 100 mg L−1 and pH 6 to investigate the effect of initial concentration of dye on dye degradation. From Fig. 11a and b, it is observed that the rate of degradation of AR dye is the maximum at 5 mg L−1. Experimental data are included in the Table 7. At higher concentrations of dye, the rate was decreased due to the deactivation of catalyst particles by the blanket effect. Therefore, the optimum initial dye concentration was found to be 5 mg L−1 and also it is not possible to produce corresponding •OH radicals to counteract the increased number of dye molecules as the catalyst dosage was fixed.
Zn and B co-doped TiO2 nano catalysts synthesized by sol-gel method exhibit small particle size, less band gap energy and high surface area. In Zn and B co-doped TiO2, Zn significantly influenced the extension of optical absorption towards the visible light region, whereas doping of boron restricted the charge recombination and acted as charge carrier during the degradation process. Compared to other nanocatalysts, ZBT3 (0.50 wt% of Zn and 0.50 wt% of B) co-doped TiO2 showed high photocatalytic activity under visible light irradiation. Finally, AR 6A (5 mg L−1), a model dye pollutant was degraded successfully by 100 mg L−1 co-doped catalyst (ZBT3) at pH 6.
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Mulpuri, R.K., Tirukkovalluri, S.R., Imandi, M.R. et al. Zinc and boron co-doped nanotitania with enhanced photocatalytic degradation of Acid Red 6A under visible light irradiation. Sustain Environ Res 29, 29 (2019). https://doi.org/10.1186/s42834-019-0031-6
- Zinc and boron co-doped TiO2
- Acid Red 6A
- Sol-gel method