Muhammad
Ikram
*a,
Muhammad Ahsan
Ul Haq
b,
Ali
Haider
c,
Junaid
Haider
d,
Anwar
Ul-Hamid
e,
Iram
Shahzadi
f,
Muhammad Ahsaan
Bari
a,
Salamat
Ali
b,
Souraya
Goumri-Said
g and
Mohammed Benali
Kanoun
h
aSolar Cell Applications Research Lab, Department of Physics, Government College University Lahore, Lahore, 54000, Punjab, Pakistan. E-mail: dr.muhammadikram@gcu.edu.pk
bDepartment of Physics, Riphah Institute of Computing and Applied Sciences (RICAS), Riphah International University, 14 Ali Road, Lahore, Pakistan
cDepartment of Clinical Medicine and Surgery, University of Veterinary and Animal Sciences, Lahore 54000, Punjab, Pakistan
dTianjin Institute of Industrial Biotechnology, Chinese Academy of Sciences, Tianjin 300308, China
eCenter for Engineering Research, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran, 31261, Saudi Arabia
fPunjab University College of Pharmacy, University of the Punjab, 54000, Pakistan
gCollege of Science, Physics Department, Alfaisal University, P. O. Box 50927, Riyadh 11533, Saudi Arabia
hDepartment of Physics, College of Science, King Faisal University, P. O. Box 400, Al-Ahsa, 31982, Saudi Arabia
First published on 16th August 2022
Degradation in the presence of visible light is essential for successfully removing dyes from industrial wastewater, which is pivotal for environmental and ecological safety. In recent years, photocatalysis has emerged as a prominent technology for wastewater treatment. This study aimed to improve the photocatalytic efficiency of synthesized TiO2 quantum dots (QDs) under visible light by barium (Ba) doping. For this, different weight ratios (2% and 4%) of Ba-doped TiO2 QDs were synthesized under ambient conditions via a simple and modified chemical co-precipitation approach. The QD crystal structure, functional groups, optical features, charge-carrier recombination, morphological properties, interlayer spacing, and presence of dopants were analyzed. The results showed that for 4% Ba-doped TiO2, the effective photocatalytic activity in the degradation process of methylene blue (MB) dye was 99.5% in an alkaline medium. Density functional theory analysis further corroborated that the band gap energy was reduced when Ba was doped into the TiO2 lattice, implying a considerable redshift of the absorption edge due to in-gap states near the valence band.
Many metal oxide semiconductors (Fe2O3, TiO2, MgO, and WO3) are widely used for photocatalytic dye degradation due to their excellent chemical stabilization, low toxicity, and broad band gap. Among them, titanium dioxide (TiO2) has versatile characteristics such as a large band gap, high physiochemical stability, zero toxicity, high oxidative power, inexpensiveness, effective bactericidal potential, and photocatalytic activity under UV light.15–17 TiO2 naturally exists in three crystalline phases: anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic).18,19 Among these, strong photocatalytic activity, chemical stability, and a negative charge-carrier potential are credited with making the anatase phase one of the most effective photocatalysts.20 The physicochemical parameters, surface-to-volume ratio, grain size, geometry, crystallinity, and surface texture influence the photocatalytic activity.21,22
The development of new mixed metal oxides (MOs) for degrading various organic dyes by photocatalysis has been a major focus in recent years. As a result, doping TiO2 with noble metals, rare-earth metals, and transition metals has been extensively studied to reduce the particle size and improve the photocatalytic and antibacterial properties of TiO2. Doping TiO2 with the alkaline earth metal Ba alters the oxide's optical absorption characteristics, which may affect its antibacterial activity and PCA. By modifying the band gap energy, the Ba dopant may operate as an electron trap, inhibiting the recombination rate of e−/h+ pairs. This enhances the antibacterial action and PCA by promoting the generation of reactive oxygen species.23 According to Mills and Hoffmann's research, the photocatalytic efficiency of a nanocatalyst can be improved by doping the nanocatalyst with an appropriate substance.24 However, few studies are reported in the literature regarding Ba-doped TiO2 synthesized through different methodologies. In a recent study, up to 3% Ba-doped TiO2 was synthesized using the sol–gel method at 80 °C for 5 hours. The product was calcined at 500 °C for 2 hours and a nanoparticle morphology was obtained. The acquired nanoparticles were utilized for the degradation of methylene blue (MB) dye for up to 150 minutes, and the calculated degradation percentage was insufficient.25 In another study, up to 2% barium-doped TiO2 was prepared using a mild hydrothermal method and placed in an oven at 100 °C for 12 hours to complete the chemical reaction. After drying the product at 50 °C, the nanocrystal morphology was determined and used for the degradation of acid red 18, a dye. The degradation efficiency was calculated to be 98.6% under acidic conditions for 90 minutes.26 In this regard, pure TiO2 and Ba-doped TiO2 in the anatase form are particularly interesting among the various photoactive MOs.
Herein, 2% and 4% Ba-doped TiO2 were prepared with a simple co-precipitation technique and a QD morphology was achieved. We determined the maximum doping effect of Ba on TiO2 using structural, optical, and morphological techniques and tested its ability to remove organic pollutants in different media from contaminated water. The obtained results showed maximum degradation in almost all media; however, the most promising results were in the basic medium and were found to be 99.5%. Furthermore, computational density functional theory was employed to investigate the Ba-doped TiO2 systems. We show that the changes in the electronic structure of TiO2 doped with Ba can be explained by combining experimental results with theoretical calculations.
(1) |
(i) Photoexcitation: photocatalysis (PCA) begins with photoexcitation, which occurs when photons have energy equal to or greater than the band gap energy of the material (Eg). These photons urge electrons in the VB to move towards the CB.
TiO2 + hv → TiO2 (eCB− + hVB+) |
(ii) Water ionization: in order to form a hydroxyl radical (OH˙), the positive holes (h+) can oxidize hydroxide ions or water molecules that have been adsorbed on the surface of TiO2.
TiO2 (hVB+) + H2O → TiO2 + OH˙ + H+ |
TiO2 (hVB+) + OH− → TiO2 + OH˙ |
At the photocatalyst surface, these OH˙ radicals act as oxidizing agents that strike adsorbed organic molecules that are very close to the catalyst surface and take part in mineralization to improve decontamination.
(iii) Ionosorption of oxygen: superoxide radicals (O2˙−) are formed when excited electrons react with oxygen. Protons and superoxide radical anions combine to generate hydroperoxyl radicals (HOO˙).
TiO2 (eCB−) + O2 → TiO2 + O2˙− |
O2˙− + H+ → HOO˙ |
(iv) Photosensitized oxidation: in conditions where catalysts are present, the excited state of MB can inject an electron into the conduction band. After this step, the dye is changed into a cationic radical, which subsequently degrades and produces compounds in accordance with the following reactions:
MB+ + TiO2 → MB˙+eCB− (TiO2) |
O2 + e− → O2˙− |
MB˙+ + OH− → MB + OH˙ |
MB˙+ + OH˙ → products |
MB˙+ + O2˙− → products |
When the catalyst absorbs visible light holes in the valence band (VB), electrons are produced in the conduction band (CB). In this way, electron–hole pairs are generated and initiate the redox reaction on the TiO2 surface. OH˙ radicals are produced whenever the holes (hVB+) interact with an adsorbed hydroxyl group. In contrast, the reaction of electrons (eCB−) with oxygen produces the superoxide anion radical O2˙−. The degradation of MB occurs due to a collaborative effort of OH˙ and O2˙− radicals. Lastly, the oxidation of organic molecules produces CO2 and H2O.27
Fig. 3 (a) X-ray diffraction patterns, (b) FTIR spectra and (c–e) SAED patterns of pure and Ba (2%, 4%)-doped TiO2 QDs. |
FTIR analysis was applied for the functional group identification of the doped TiO2 samples. Wavenumbers ranging from 4000 to 500 cm−1 were used to record the spectra (Fig. 3b). The Ti–O–Ti bridging stretching and Ti–O stretching modes were observed at the bands at 527 and 628 cm−1, respectively.29 Bands appeared at 1640 cm−1 and 3200–3400 cm−1, corresponding to the hydroxyl group (OH) bending and stretching vibrations caused by the presence of H2O molecules in the TiO2 catalyst.30 Band ranges at 2150–2360 cm−1 were revealed with higher doping concentrations of Ba, ascribed to the physical adsorption of CO2 and CO.31 The spreading of the adsorption region over 950–400 cm−1 upon Ba doping was attributed to the characteristic M–O bond oscillations (where M = Ti and Ba). Shifting in the band position suggests that Ba2+ ions were successfully doped into TiO2.32
The optical properties of TiO2 are widely known to be highly dependent on the material type (e.g., single-crystal, powder) along with the fabrication methods. The absorbance spectra of undoped and barium-doped TiO2 were measured by UV-vis spectrophotometry. The absorption peak identified for the as-prepared TiO2 at ∼350 nm is attributed to electron transfer from the 2p to 3d region of the VB of O2− to the CB of Ti3+.33 In the absorption spectra, a redshift is observed with Ba (2%, 4%) doping, resulting in decreased band gap energy (Eg). Tauc transformation was used to calculate Eg, as displayed in Fig. 4b. The estimated Eg was 3.25 eV for TiO2,33 and Eg was reduced with a low and high doping concentration of Ba (3.16 eV and 3.12 eV, respectively). The reduction in the band energy gap can be attributed to the increasing crystallite size, as demonstrated by the XRD measurements.
Fig. 4 (a) Absorbance spectra, (b) Eg by Tauc plot, (c) photoluminescence spectra and (d) Raman spectra of the TiO2 and Ba (2%, 4%)-doped TiO2 QDs. |
PL spectroscopy was used to assess the e−–h+ recombination rate, recorded between 380 and 500 nm at an excitation wavelength of 350 nm (Fig. 4c). The TiO2 peak intensity observed at 414 nm demonstrated the high recombination rate of e−–h+ pairs, which influences the TiO2 photocatalytic performance significantly. Furthermore, the peak intensity was reduced, which indicates a lower charge recombination rate upon doping and thus higher photocatalytic activity.34,35 These emission transitions have been credited to unilaterally charged oxygen vacancies in TiO2 and arise when a photogenerated h+ mixes with an e− taking up oxygen vacancies.36 When the concentration of Ba increases, the peak intensity reduces as higher concentrations restrict the inter-nuclear space, and the emission intensity begins to flow towards the energy-killing domains.37–40 Raman spectroscopy was used to investigate the lattice strain, phase transformations, lattice dynamics, and local symmetry of the samples.41,42
Fig. 4d presents the Raman spectra of doped TiO2 between 300 and 700 cm−1. The peaks appearing at 399, 519, and 640 cm−1 for the samples are attributed to distinct LO vibration modes, demonstrating the improved crystallinity of TiO2 with Ba (2%, 4%) doping, having minimum interlayer stress and defect density.43 All the samples exhibited high bands at 399, 519, and 640 cm−1, which most certainly correlate to the permissible Raman modes B1g, B1g, and Eg, respectively, with the space group D4h19.44
The surface morphology and chemical composition of the synthesized QDs were confirmed by FESEM and EDS spectroscopy, as shown in Fig. 5. Aggregated quantum dots (QDs) of different sizes appear (Fig. 5a). Upon Ba doping, small-scale agglomeration was observed (Fig. 5b). A layer of Ba is formed as the concentration increases, and the QDs are dispersed (Fig. 5c). The EDS spectra for the TiO2 QDs are presented in Fig. 5d–f. Prominent peaks of Ti with O reveal the successful preparation of TiO2 QDs. The presence of Ba in the spectra confirms the successful incorporation of dopants, whereas the peaks of Cu and C are attributed to the Cu grid and carbon tape used to mount the samples, respectively (Fig. 5e and f). Furthermore, Au, Zn, and Cl peaks emerged, which were assigned to the coating of gold sprayed over the sample and some contamination. Additionally, a peak of Na was found in each prepared sample, ascribed to the use of NaOH to maintain the pH of the solutions.
The morphology, size, and shape of the TiO2 and Ba (2%, 4%)-doped TiO2 QDs were examined using TEM, as illustrated in Fig. 6a–c. Spherical-shaped small-size QDs ranging from 6 to 9 nm are observed, as shown in Fig. 6a. On doping with Ba, the synthesized QDs appeared to be loosely bound with the TiO2 QDs (Fig. 6b). By increasing the dopant concentration, the QDs seem to become overlapped and highly aggregated, which may result in their decreased size (Fig. 6c). Moreover, HRTEM was utilized to determine the interlayer d-spacing, as shown in Fig. 6d–f. The interlayer d-spacing of TiO2 and Ba (2%, 4%)-doped TiO2 were estimated to be 0.35, 0.34, and 0.34 nm, respectively, assigned to the (101) plane for doped and undoped TiO2, and well agreed with the XRD results. The above optical, structural, and morphological characterizations for doped TiO2 indicate that the Ba-doped TiO2 lattice has an anatase phase with a tetragonal crystalline structure, as confirmed by SAED. The addition of Ba increased the absorption and introduced a redshift as the crystallite size increased, which could capture a wide range of wavelengths. The morphological study confirmed the formation of QDs with agglomeration upon doping.
Fig. 6 (a–c) TEM images of pure and Ba (2%, 4%)-doped TiO2; (d–f) interlayer d-spacing HRTEM images of undoped and Ba-doped TiO2. |
To determine the PCA of the prepared nanocatalyst, UV-vis spectrophotometry was used (Fig. 7). The degradation efficiency was calculated against acidic, basic and neutral media. In a neutral medium (pH 7), the percentage degradation was estimated to be 65.56%, 81.84%, and 82.91% for dopant-free and doped TiO2, respectively, as presented in Fig. 7a. The solution pH influences the photocatalytic mechanism, which may affect the adsorption of the dye on the photocatalyst surface. MB, as a positively charged dye, degrades slowly at lower pH; as a result, the degradation efficiency is enhanced on increasing the pH. Moreover, in an acidic medium (pH 4), the measured degradation results are 47.41%, 49%, and 70.41% for the doped and undoped TiO2, respectively, as demonstrated in Fig. 7b. From the results in the acidic medium, the degradation of MB seems to be ineffective compared to that in the neutral medium, attributed to the positively charged surfaces of the catalysts, which tend to resist the adsorption of the cationic adsorption species. Furthermore, in a basic medium (pH 12), the prepared nanocatalyst exhibits efficient dye degradation results of 92.17%, 97%, and 99.5% for TiO2 and Ba (2%, 4%)-doped TiO2, respectively, as illustrated in Fig. 7c. Increased electrostatic interaction between the positively charged dye and the negatively charged catalyst in the basic dye solution causes the surface charges to become negative, which facilitates dye degradation.45–47 Generally, the PCA is influenced by the surface area of the nanocatalyst, as a large surface area provides more active sites, increases the number of redox reactions on the nanocatalyst surface and results in the degradation of MB.48,49 The results show that the MB dye degrades maximally when treated with Ba-doped TiO2 rather than with the TiO2 QDs. For the Ba-doped TiO2 photocatalysts, the possible charge separation mechanism is described as follows. Charge pairs are formed when visible light interacts with the TiO2 QDs, and electrons in the charge pairs move from the valence band (VB) to the conduction band (CB), leaving a hole in the VB. Most of these electron–hole pairs tend to recombine and lower the degradation capability of TiO2. Incorporating metal ions (Ba) into the TiO2 lattice as electron traps inhibits the recombination of charge pairs, resulting in increased photocatalytic activity. The separation of charges and the transport of these charges play a significant role in enhancing the photocatalytic activity.25 In this regard, 4% Ba-doped TiO2 shows the maximum results in all the media for MB degradation, attributed to the lower recombination rate as evidenced from the PL spectra (Fig. 4c). 4% Ba-doped TiO2 has lower charge recombination that results in enhanced photocatalytic degradation. Additionally, recycling experiments were used to examine the photostability of TiO2 doped with 4% barium, as presented in Fig. 7d. Even after being subjected to six separate cycles of the photocatalytic degradation of MB by visible light, the high-concentration Ba-doped TiO2 QDs exhibited a high level of stability and maintained an exceptional level of activity. A literature comparison with the current study is provided in Table 1.
Fig. 7 Photocatalytic degradation of TiO2 and Ba (2% 4%)-doped TiO2 in (a) neutral, (b) acidic and (c) basic media and (d) the photocatalytic degradation of MB by 4% Ba-doped TiO2. |
The BET surface area and pore size of the as-prepared TiO2 and Ba-doped TiO2 samples were investigated using BET analysis. The N2 adsorption–desorption isotherms of all the samples are shown in Fig. 8. The adsorption–desorption isotherm of TiO2 and Ba-doped TiO2 matched the characteristics of a traditional type IV curve that had an H4 hysteresis loop (Fig. 8). A mesoporous structure was found in both the undoped and doped TiO2 QDs. In addition, the BET surface areas of the prepared QDs increased gradually to 119.183, 132.846, and 138.5 m2 g−1 for TiO2 and Ba (2%, 4%)-doped TiO2, respectively. The average pore size was calculated to be 3.6, 4.5, and 10.8 nm for dopant-free and doped TiO2, respectively. It can be observed that the incorporation of Ba increased the surface area, which is beneficial for enhanced photocatalytic activity.
The crystalline atomic structure of anatase TiO2 was modeled by a 2 × 2 × 2 supercell containing 32 Ti atoms and 64 O atoms, as illustrated in Fig. 9a. The calculated lattice constants for TiO2 (a = b = 3.782 Å and c = 9.552 Å) are very close to the experimental data (a = b = 3.7842 Å, c = 9.5146 Å).60 The Ba-doped structures were prepared by substituting one and two Ti atoms with one and two Ba atoms, affording doping concentrations of 3.125% (1/32) and 6.25% (2/32), respectively (Fig. 9b). When Ba impurities were introduced into the TiO2 structure, the a- and c-lattice values (a = b = 3.790 Å, c = 9.766 Å and, a = b = 3.778 Å c = 10.013 Å for 3.125% and 6.25% Ba-doped TiO2, respectively) were slightly higher than those in the pristine material. The hexagonal lattice deformation due to Ba-doping causes this difference.
The electronic densities of states of the undoped and doped systems were calculated to analyze the effect of Ba doping into the TiO2 lattice from the perspective of the electronic structures (Fig. 10). The calculated pristine TiO2 band gap was found to be 3.17 eV based on the GGA-1/2 method, which is very close to our measured experimental value and previously reported results.61–65 The analysis of the partial DOS of pure TiO2 (Fig. 11a) shows that the upper valence region is mostly dominated by O 2p states, while the lower conduction region is mainly due to the Ti 3d character. Doping Ba at Ti sites modified the electronic structure by generating a slightly asymmetric DOS near the valence band, as illustrated in Fig. 11b. It is observed that in-gap states appear in the band gap, especially in the peak in the DOS of the minority spin close to the upper part of the valence band. It can also be seen that the edge of the valence band advances closer to the Fermi level, indicating an increase in charge-carrier concentration and a modest reduction in the band gap.
Fig. 10 Calculated total of (a) pure, (b) 3.125% Ba- and (c) 6.25% Ba-doped TiO2 using the DFT-1/2 method. The Fermi level is represented by the red vertical dashed line. |
Fig. 11 PDOS of (a) pristine TiO2 and (b) 3.125% Ba-doped TiO2 and (c) the optical absorption spectra. |
The obtained band gap values are found to be 3.07 eV and 3.002 eV for 3.125% Ba- and 6.25% Ba-doped TiO2, respectively. Additionally, the band gap values are shown to slightly decrease with further increase in Ba concentration. The same behavior was observed in our experimental measurements.
Moreover, the states that appear in the band gap can help to stimulate valence electrons to the conduction band, boosting the visible light absorption, which is beneficial to improving the optical absorption efficiency. The in-gap states may lower the threshold for photoexcitation, decrease the energy needed for electron transitions, and thereby broaden the optical absorption spectrum without altering the energy of the electrons or holes. This result gives a good explanation of the redshift. The features of the PDOS of Ba-doped TiO2 systems show that the Ba 5p states are coupled with O 2p, while partially appearing in the band gap. The absorption spectra of the pristine and Ba-doped TiO2 were computed and are illustrated in Fig. 11c. The absorption spectrum analysis displays that the pristine TiO2 only absorbed photons in the ultraviolet range with no photo-absorption activity in visible light. In the case of the Ba-doped TiO2 configurations, there is a considerable redshift of the absorption edge owing to the presence of an in-gap near the valence band, which makes it simpler for electrons to be driven to the conduction band.
This journal is © The Royal Society of Chemistry 2022 |