Optimization and detailed stability study on Pb doped ceria nanocubes for enhanced photodegradation of several anionic and cationic organic pollutants

2.1 Materials

Cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O), lead(II) nitrate (Pb(NO₃)₂, hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and sodium hydroxide (NaOH) were purchased from Merck Chemical Co. Terephthalic acid (TA), p-benzoquinone (BQ), isopropyl alcohol (IPA), triethanolamine (TEOA), methylene blue (MB), methylene orange (MO), methylene red (MR), rhodamine B (RhB), reactive blue 160 (RB160), salicylic acid (SA), coumarin and phenol were purchased from Sigma-Aldrich Co. All reagents were analytical grade and used without further purification.

2.2 Synthesis of CeO₂ and Pb doped CeO₂ nanocubes

The CeO₂ and Pb doped CeO₂ nanocubes were synthesized via modified hydrothermal method (Xiang et al., 2011). In this typical method of synthesis, the precursor solution having stoichiometric ratio of Pb/Ce was prepared by dissolving 2.04 g of Ce(NO₃)₃·6H₂O (94 mol%) and 0.066 g of Pb(NO₃)₂ (6 mol%) in 50 ml of DI water. Then, 5 mol NaOH (10 g in 50 mL) solution was added drop wise (20 drops per minute) under mild stirring until the pH of solution becomes 12. The slurry is formed and transferred into Teflon reactor and autoclaved at 140 °C for 14 h. The obtained product was isolated and washed several times using ethanol and DI water through centrifugation and dried at 120 °C for 8 h. The dried powder was calcined at 350 °C for 3 h. The same procedure was carried out for the synthesis of other Pb doped CeO₂ nanocubes with different molar ratios (1–12 mol% of Pb and 88–99 mol% of Ce) of metal precursors. The pure CeO₂ nanocubes were prepared without adding Pb(NO₃)₂ in initial precursor solution.

2.3 Characterization techniques for photocatalyst

The crystallographic structures of as-prepared nanocubes were determined by using powder X-ray diffraction analysis (Rigaku Miniflex X-ray diffractometer equipped with Cu Kα (λ = 1.54 Å) radiation operating at 40 kV with 30 mA). The diffraction patterns were recorded for 30 min in the wide range of 2θ = 20–80° with continual scanning rate of 5° min⁻¹. The chemical state analysis was carried out by X-ray Photoelectron Spectroscopy (XPS) (Krotas analytical Instrument, Shimadzu Corporation, ESCA 3400, Japan). The thermal properties of as-prepared materials were characterized by thermo-gravimetric analyzer (TGA-NETZSCH STA449 F1 Jupiter). The Fourier transform infrared (FT-IR) spectra were recorded to determine the chemical nature of nanocubes using a Brucker Tensor 27 spectrophotometer in the range of 4000–400 cm⁻¹ at KBr phase. The morphology and elemental composition of as-prepared materials were characterized through scanning electron microscopy (SEM, Zeiss18 Evaluation) equipped with energy dispersive X-rays spectroscopy (EDS) and high resolution transmission electron microscopy (HR-TEM) on a Jeol/JEM 2100 high resolution transmission electron microscope operating at 200 kV. The UV–Vis diffuse reflectance spectra (UV–Vis DRS) of synthesized nanocubes were obtained by SHIMADZU-UV 1800 spectrometer in the wavelength range of about 200–800 nm.

2.4 Photocatalytic degradation of dyes and reagents

The photocatalytic activity of synthesized nanocubes were evaluated for the photodegradation of several organic dyes and reagents such as MB, MO, MR, RhB, RB160, SA, coumarin and phenol. The reaction was carried out in a dark box containing 450 W low-pressure quartz mercury lamp. Typically, a 250 mL double layered beaker was placed at 15 cm from the light source containing a mixture of 100 mg of catalyst and 100 mL of a 30 mg/L dye solution at pH = 7. The reaction mixture was kept under constant air-equilibrated atmosphere during photocatalytic process. The degradation of cationic dyes, anionic dyes and reagents were tested at room temperature and atmospheric pressure. Before the photocatalytic reaction, the reaction mixture was stirred in dark for 20 min to ensure complete adsorption/desorption equilibration of organic dye on the photocatalytic surface. The mixture was then irradiated by mercury lamp. At certain time interval, 5 mL of reaction solution was collected followed by centrifugation to remove the photocatalyst suspended on the solution. The collected sample was used to determine the concentration of dye in reaction solution, through UV–Vis spectroscopy. The initial and temporal concentration of organic pollutant in aqueous solution was taken as C₀ and C_t respectively. The degradation of all the above mentioned dyes and reagents was carried out in the same manner for 90 min. To optimize the degradation of MB, the degradation of MB was also studied in different conditions such as initial concentration of MB, catalytic dosage, pH of the reaction solution and temperature of the reaction solution for 40 min. where, the other initial parameters were kept constant as given in typical reaction condition. The photodegradation reaction rate constant of as synthesized nanocubes was found by using kinetics of MB degradation.

2.5 Reactive species trapping experiment and estimation of hydroxyl radicals

The efficiency of hydroxyl radical produced by 6% Pb-CeO₂ were estimated by using terephthalic acid (TA) as a probe molecule. The generation of ^•OH were detected by using PL technique using TA (2 mmol/L) as probe molecule recorded with Perkin Elmer LS-45 fluorescence spectrophotometer. The mechanism implicates the conversion of TA into 2-hydroxyterephthalic acid (a fluorescent molecule) by the reaction of TA with ^•OH radicals that produced by photocatalyst (Jiang et al., 1999). Trapping experiment was carried out for the further confirmation on generation of reactive species. Superoxide radical (O₂^•−), hydroxyl radical (^•OH) and holes (h⁺) were investigated by the addition of BQ (a quencher of O₂^•−), IPA (a quencher of ^•OH) and TEOA (a quencher of h⁺), all the above reagents were taken in the concentration of 1 mmol/L. This method was similar to the above discussed photocatalytic degradation process.

3.1 Structural analysis

The XRD patterns of synthesized pure CeO₂ and Pb doped CeO₂ nanoparticles are shown in Fig. 1a. The characteristic diffraction peaks of pure CeO₂ at 28.55°, 33.08°, 47.47°, 56.33°, 76.70° and 79.07° can be indexed to cubic structure (JCPDS 34-0394) (Ranjith et al., 2014; Oveisi et al., 2010). All the Pb-CeO₂ samples exhibited same crystal structure. Fig. 1b, clearly shows the peak broadening with an introduction of Pb ions indicating the doping of Pb atom has an appreciable effect on the crystal structure of CeO₂ (Santra et al., 2016; Yu et al., 2002). During the formation of nanocubes, at first Ce(OH)₃ nanostructures are formed and then the outer surface is oxidized to form CeO₂. When Pb is introduced during synthesis, the formation of Pb(OH)₂ arises to the replacement of Pb²⁺ ions in CeO₂ sites. With the addition of higher mol% of Pb, the characteristic peaks are getting broaden and showed a little shift towards lower diffraction angle. The XRD patterns in a Fig. 1a shows only the diffraction peaks of ceria. It confirms the formation of single phase to the successful doping of Pb. When Pb is doped into the CeO₂ lattice, the lattice parameter decreases with increasing in Pb concentration. The increase in Pb concentration increases the oxygen vacancies where the effective ionic radius in fluorite lattice decreases with an increase in the number of oxygen ion vacancies. The decrease in effective ionic radius confirms the formation of fluorite-like solid structures when Pb ions are incorporated in ceria lattice (Ye et al., 2012). The lattice parameters, a degree of crystallinity, lattice strain and crystallite size of the respective nanomaterials were calculated and tabulated in Table 1. The average crystallite size of pure and Pb-CeO₂ nanoparticles are calculated by using Debye- Scherrer formula and d-spacing of 6% Pb-CeO₂ nanoparticles were calculated as 0.32 nm from high intense peak (28.55°) in XRD spectra of 6% Pb-CeO₂ by using Bragg’s law (Qi and Yang, 2004). The calculated d-spacing was compared with TEM results of respective nanomaterial.

Close

Fig. 1 (a and b) XRD patterns of prepared CeO₂ and Pb doped CeO₂ nanocubes.

Export to PPT

3.2 Chemical state analysis

The oxidation states and chemical states of 6% Pb-CeO₂ nanocubes are examined using XPS analysis. The XPS survey spectrum of 6% Pb-CeO₂ is shown in Fig. 2a, it confirms the presence of Ce3d, Pb4f, and O1s. Fig. 2b represents the peaks at 883.2, 885.8, 891.4 and 900.4 eV corresponds to Ce3d_5/2. Whereas, photoelectron peaks at 905.0, 910.3, 912.7 and 917.8 eV corresponds to Ce3d_3/2, respectively (Channei et al., 2014). It is the proof that Ce (3d) spectrum is relatively complex due to the presence of Ce in both Ce³⁺ and Ce⁴⁺ oxidation states. The spin orbit doublets for CeO₂, 3d_3/2 (905.3 and 910.4 eV), and 3d_5/2 (883.2 and 885.8 eV) are clearly evident for the presence of both valence states of Ce³⁺ and Ce⁴⁺. The two main peaks of Pb4f in Fig. 2c are observed at 138.5 and 143.6 eV of 4f_7/2 and 4f_5/2, which corresponds to the Pb²⁺ ions (Yu et al., 2002; Taniguchi et al., 2011). The typical peaks of Pb⁴⁺ ions are not found in the XPS spectra that confirms the presence of Pb²⁺ ions instead of Pb⁴⁺. Fig. 2d shows the O1s spectra of the 6% Pb-CeO₂ nanocubes. The peak at 531.2 eV peak is credited to the O²⁻ ions in cubic Ce ion array, which are surrounded by cerium atoms with the complete supplement of nearest-neighbour O²⁻ ions (Ye et al., 2012). Consequently, the 531.2 eV peak of the O1s spectra can be credited to the Ce—O bonds.

Close

Fig. 2 XPS profiles of 6% Pb-CeO₂ nanocubes (a) Survey spectrum (b) Ce3d (c) Pb4f (d) O1s.

Export to PPT

3.3 FT-IR analysis

The chemical structure of pure and Pb doped CeO₂ nanocubes were studied through its respective FT-IR spectra and shown in Fig. 3. The catalytic oxidation of organic dyes are highly affected by the chemical structure on surface of the catalyst. The carbon-oxygen and —OH functional groups predominantly influences the surface behavior and surface characteristics of the catalytic materials. The FT-IR spectra of both pure and Pb doped CeO₂ nanocubes shows a uniform strong peak at 1383 cm⁻¹ and wide region of transmission between 3000 cm⁻¹ and 3750 cm⁻¹ can be attributed to deformation vibration and -OH stretching in chemisorbed H₂O molecules on the surface of catalyst (Krishnakumar et al., 2017). The higher absorption of H₂O molecules is due to the greater porosity. In pure CeO₂, the peak at 503 cm⁻¹ is due to stretching of Ce—O bonding, the peak 1360 cm⁻¹and 1380 cm⁻¹ are due to —CH₃ stretching and the peak at 1056 cm⁻¹ is attributed to stretching mode of aliphatic ether. The characteristic bands at 1270–1350 cm⁻¹ shows the presence of epoxy groups in as prepared catalyst and peak at 720 cm⁻¹ is due to the aliphatic ⚌C—H group (Chen et al., 2016). A very small peak is observed between 2340 and 2360 cm⁻¹ are attributed to the carbon dioxide molecules evolved during the combustion The band in between 1400 cm⁻¹ and 1600 cm⁻¹ can be assigned to stretching vibration mode of C⚌C. The peak at 850 cm⁻¹ consequent to plane bending of ⚌C—H and it can be seen that, transmission intensity of ⚌C—H peak is getting increased by increasing the doping concentration (Yang et al., 2014; Bineesh et al., 2010). From the above results it can be concluded that doping of Pb on CeO₂ is increasing the amount of function groups present in pure CeO₂.

Close

Fig. 3 FT-IR spectra of pure CeO₂ and Pb doped CeO₂ nanocubes.

Export to PPT

3.4 Thermogravimetric analysis

The thermal behavior of CeO₂ and 6% Pb-CeO₂ nanocubes in temperature range of 27–850 °C were investigated through thermogravimetric (TG) analysis and the graphs are shown in Fig. S1. The TG curves of CeO₂ and 6% Pb-CeO₂ nanocubes shows two stages of weight loss in the temperature range of about ∼30–200 °C and ∼200–550 °C. The initial weight loss observed in the temperature range ∼30–200 °C could be attributed to the physically adsorbed water molecules on the surface of catalyst (Ranjith et al., 2014). The second stage of weight loss observed between ∼200 and 550 °C can be ascribed to the removal of chemisorbed water molecules. The organic residues trapped inside the pores on the surface of catalyst, also gets released on the second stage of weight loss (Liu et al., 2013). Above ∼550 °C, no significant weight loss is observed for both samples. It indicates that the phase transition and crystallization was not occurred in this temperature region. The total weight loss observed in prepared CeO₂ and 6% Pb-CeO₂ were ∼3.3% and ∼4.2% respectively, these results reveals that both the catalyst are more stable.

3.5 Morphological characterisation

The surface morphology and particle size of synthesized 6% Pb-CeO₂ nanocubes were revealed through SEM, TEM and HR-TEM shown in Figs. 4 and 5. The TEM images of pure CeO₂, 2% Pb-CeO₂, 4% Pb-CeO₂, 8% Pb-CeO₂, 10% Pb-CeO₂ and 12% Pb-CeO₂ nanoparticles were shown in Fig. S2. All the prepared CeO₂ and Pb-CeO₂ nanostructures exhibits cubic shape. The highly aggregated, non-uniform and irregular grains are observed in both pure CeO₂ and all concentrations of Pb-CeO₂ nanostructure as showed in Figs. S2 and 5a. The morphology of 6% Pb-CeO₂ was densely packed with the irregular grain size due to hydrothermal process. During synthesize process, NaOH was used as a reactant and not any surfactant was used. The interaction and replacement of Pb ions with Ce sites alters the electronic distribution on the surface of grains results to high agglomeration (Yu et al., 2002; Taniguchi et al., 2011). The TEM and HR-TEM images of 6% Pb-CeO₂ showed well-distributed cubic shaped nanoparticles (Fig. 5a and b). An average size of nanocubes obtained through the histogram was predicted between 14 to 18 nm as presented in Fig. 5d and approximately matches with XRD result of respective nanocubes. The highly magnified HR-TEM image is shown in Fig. 5b and it reveals the lattice fringes corresponding to (1 1 1) plane of CeO₂ nanocube (Priyadharsan et al., 2017). Interplanar lattice distance of (1 1 1) plane is found to be 0.32 nm and verified with respective XRD pattern of 6% Pb-CeO₂ at high intense peak (28.55°). The selected area electron diffraction (SEAD) pattern of 6% Pb-CeO₂ was shown in a Fig. 5c. The SEAD pattern exhibits serious of rings with bright spots due to its polycrystalline in nature. The each bright rings are due to the respective (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of CeO₂ nanocubes (Du et al., 2017). The elemental composition of as-prepared catalyst is investigated by EDS (Fig. 4b). EDS shows the formation of highly pure material and confirms the quantity of dopant material as approximately 6 mol% of Pb.

Close

Fig. 4 (a) SEM micrograph (b) EDS pattern of 6% Pb-CeO₂ nanoparticles.

Export to PPT

Close

Fig. 5 (a and b) TEM and HR-TEM images (c) SEAD pattern (d) histrogram graph of 6% Pb-CeO₂.

Export to PPT

3.6 Optical characterization

The optical properties of synthesized pure CeO₂ and Pb doped CeO₂ nanocubes were characterized by using UV–Vis DRS spectroscopy. Fig. 6a shows the absorption spectra of prepared nanomaterials and the optical band gap energies of pure and modified CeO₂ nanocubes were calculated by using Tauc plot (relationship of (αhν)² vs photon energy) and shown in a Fig. 6b (Chen et al., 2016). The obtained band gap values are given in Table 2. The band edges of as-prepared samples were identified by extra plotting the linear region of absorption spectra and the band edges exhibited in the visible region (λ > 400) hence they are active in the visible region of electromagnetic spectra (Efendi and Nurhasanah, 2015). In the previous cases, the band gap energy of CeO₂ nanocubes exhibits much lower than CeO₂ nanospheres (3.2 eV). This is owing to the formation of localized energy states within the band gap of CeO₂ due to increase in a concentration of Ce³⁺ ions and oxygen vacancy. The characteristics bands at 250 and 330 nm for pure CeO₂ nanocubes can be credited due to the charge transfer of Ce³⁺(5d) ← O²⁻(2p) and Ce⁴⁺(5d) ← O²⁻(2p) respectively (Ye et al., 2012). The addition of Pb results to slight red shift when compared to an absorption band of pure CeO₂. The red shift is attributed due to an incorporation of Pb ions with the crystal lattice of CeO₂. In previous reports, it has been shown that the presence of Ce³⁺ ions reduces the band gap energy of CeO₂ nanocubes leading to a red shift. The substitution of Pb ions with Ce⁴⁺ to form Ce³⁺ will increase the concentration of oxygen vacancy in Ce based materials due to the mechanism of charge compensation (Xu et al., 2015). But, when the dopant concentration of Pb is greater than 6% the blue shift occurs in the absorption results to increase in the band gap. This may be due to the partial suppression on the conversion of Ce⁴⁺ to Ce³⁺ ions when cation like Pb ions are doped with ceria. The shift in absorption band from lower wavelength to the higher wavelength is also due to the formation of imperfection in CeO₂ lattice (Chen et al., 2016; Mori et al., 2014). The band gap energies of nanomaterials will also possibly affected due to change in crystallite size of CeO₂ nanoparticles with an addition of Pb as proposed through their respective XRD patterns. Such structural modifications will alter the lattice oxygen vacancies. These modifications in crystal system also influence the band gap edges that lead to decrease and increase in band gap energy of nanomaterials.

Close

Fig. 6 (a) UV–Vis DRS spectra (b) Tauc plot of synthesized nanomaterials.

Export to PPT

3.7 Photocatalytic examination

The photocatalytic activity of CeO₂ and Pb doped CeO₂ are studied against degradation of anionic dye (methylene orange) in a photocatalytic reactor setup for over 90 min. Pb doped CeO₂ nanocubes has higher photocatalytic performance when compared to pure CeO₂. According to Fig. 7a, 6% Pb-CeO₂ exhibited highest photocatalytic performance and displays 30% higher degradation than pure CeO₂ nanocubes and respective degradation spectra of MO is shown in Fig. 7b. When CeO₂ nanocubes are doped with <6% of Pb, they shows a better photocatalytic performance compared to pure CeO₂ nanocubes. On the other hand, when the doping concentration of Pb is further increased the photocatalytic performance goes down rapidly. The amount of oxygen vacancies present in the catalyst is related to electron-hole pair (e⁻ - h⁺) separation and while increasing the concentration of Pb, number of oxygen vacancies also gets increased. According to Mori et al., the possibility of cations sitting in close proximity to each other was increased while more oxygen vacancies are created and therefore, all the existing movable oxygen vacancies combines to form a deep traps and due to the assembling of isolated cations and oxygen vacancies in crystal lattice a little amount of microdomains may be generated (Mori et al., 2002; Kong et al., 2011). The generation of these traps again would restrict (e⁻ – h⁺) recombination resulting to enhanced photocatalytic performance. A similar consequence was perceived in case of TiO₂ and it was found that defects in surface acts as a traps for charge carriers and adsorption sites where the (e⁻ – h⁺) pair recombination can be prevented by transformation of charges to absorbed species. In contrast, bulk defects in the lattice act as a charge carrier traps where the (e⁻ – h⁺) pair recombination occurs (Yu et al., 2015). Hence, at room temperature, the oxygen vacancies at lower doping concentration upholds the photocatalytic performance of CeO₂, and while increasing the doping concentration it gets declined. It was found to be lower concentration of Pb doping is favorable for efficient photocatalysis and approximately 6% Pb in CeO₂ was found to be an optimum doping concentration that provides the higher photocatalytic performance.

Close

Fig. 7 (a) Photocatalytic degradation rate curves of anionic dye (MO) (b) degradation spectral changes of MO with 6% Pb-CeO₂ (reaction condition: catalyst dosage = 100 mg, dye = 30 mg/L, pH = 7 and reaction time = 90 min).

Export to PPT

The cationic dyes like MB and RhB were also taken as a model dyes and the degradation rate curves are shown in Fig. 8a and c. In this photocatalytic study, 6% Pb-CeO₂ showed higher photocatalytic performance than other synthesized photocatalyst and respective degradation spectra is shown in Fig. 8b and d. But the photocatalytic efficiency for all the synthesized catalysts showed comparatively lower photocatalytic performance on degradation of anionic dyes than cationic dyes. In this photocatalytic process, the catalyst surface is positively charged and so anionic dyes has a greater attraction toward the catalyst (Ye et al., 2012). This may be the reason for higher photocatalytic performance of Pb doped CeO₂ nanocubes on anionic dyes. The photocatalytic performance of 6% Pb-CeO₂ towards photodegradation of other cationic/anionic dyes and reagents such as MR, RB160, SA, coumarin and phenol were also carried out for over 90 min and shown in Fig. S3. The degradation of RB160, MB and coumarin follows the degradation pattern of previously studied anionic and cationic dyes. But in case of phenol and SA it shows much lower degradation than other dyes. This may due to the strong binding energy of phenol and SA molecules which is very harder to break by photogenerated radicals (Rochkind et al., 2016; Srivastava, 2013). From the obtained results it can be concluded that 6% Pb-CeO₂ nanocubes are respectable photocatalyst for the degradation of both cationic/anionic dyes and reagents.

Close

Fig. 8 Photocatalytic degradation rate curves of cationic dyes: (a) MB (c) RhB and degradation spectral changes with 6% Pb-CeO₂: (b) MB (d) RhB (reaction condition: catalyst dosage = 100 mg, dye = 30 mg/L, pH = 7 and reaction time = 90 min).

Export to PPT

To go further in understanding the photocatalytic performance of 6% Pb-CeO₂ nanocubes on the degradation of cationic dye, the initial parameters such as pH value of dye solution, reaction temperature, initial dye concentration and catalytic dosages were adjusted and carried out for 40 min to find the optimum condition for better photocatalytic performance and obtained results are shown in Figs. S4 and S5. pH of the dye solution plays important role in the photodegradation of azo dyes. Hydroxyl radical concentration and adsorption/desorption of the dye molecules into the surface of photocatalyst depends upon the pH value of reaction solution (Yao et al., 2013; Li et al., 2011). The pH of the dye solution is adjusted from pH 2 to pH 11 by using HCl and NaOH to examine its effect on degradation of methylene blue. From the Fig. S4, it can be observed that the degradation of methylene blue is comparatively higher in pH = 11 than the lower pH. Increase in degradation is due to the surface of photocatalyst becomes more anionic when pH is increased. Hence, the cationic dyes like MB has a higher affinity towards the photocatalyst. It results to the higher adsorption of dye molecules onto the surface of catalyst and consequently enhanced degradation of MB. The degradation of MB is increased by increasing the reaction temperature as shown in Fig. S4. This is due to the presence of higher oxygen vacancies and enhancement in mobility of oxygen ion at higher temperatures. In previous report, Li et al. showed that, at high temperature the charge carrier separation in mesoporous CeO₂ is higher, results to higher photocatalytic performance (Boobas et al., 2017). Typically the lattice phonon scattering governances the efficiency of charge separation in semiconductor materials. The phonons are the pressure waves that are generated at any temperature due to the vibrations of lattice atoms or ions within crystal. When these generated phonons collides with charge carriers, they get scattered. At higher temperature the lattice ions and atoms vibrates strongly resulting to generation of more phonon waves. Higher scattering of electrons or holes occurs within the lattice leads to lower recombination of photogenerated charge carriers and forms the higher oxygen vacancies (Yao et al., 2013). As results to the increase in oxygen vacancies, the oxygen ion mobility gets increased. The transportation mobility of lattice oxygen ions is highly beneficial for the separation of photogenerated charge careers, which leads to efficient photocatalytic performance.

The effect of initial concentration of dye and catalyst dosage on photodegradation of MB is shown in Fig. S5. The photodegradation of MB was studied in the different catalytic dosages (50, 75, 100, 125, and 150 mg) and all other parameters were kept constant. From Fig. S5 we can observe that degradation rate of MB gets increased by increasing the catalyst dosage. Maximum degradation of MB was observed at a photocatalyst dosage of 1.5 g/L (93%). Where the degradation rate gets decreased at lower dosages as 80% (at 1.25 g/L) to 32% (at 0.25 g/L). The enhancement of dye degradation by increasing photocatalyst dosage may be trigged by enrichment of number of active sites present on photocatalyst for degradation of MB (Li et al., 2011). The degradation rate of MB by varying its Initial concentration is shown in Fig. S5. Initial concentration of MB was varied from 10 mg/L to 50 mg/L and other initial parameters were kept constant. The degradation of dye gets declined with an increase in initial concentration of dye. At lower concentration the degradation rate of MB is higher because of the presence of sufficient amount of active species for the degradation of MB. At dye concentration of 40 mg/L, degradation rate of MB is decreasing much lower when compared to 30 mg/L. This may be due to the insufficient amount of active species for the degradation of higher amount of MB molecules. A penetration of light through the reaction solution also plays a key role in the degradation of dye molecules. Where at higher concentration of dye, dye molecules screens the incident light source reaching the catalyst and results to decline in photocatalytic performance (Jamil et al., 2012).

The photocatalytic degradation efficiency of MB by synthesized CeO₂ and Pb doped CeO₂ nanocubes were determined using the pseudo-first-order kinetics model as follow $-\ln \left(\frac{C_t}{C_o}\right)=-\mathit{kt}$ where C₀ and C_t are concentrations of dye initially and at time t respectively and k is the apparent first order photoreaction rate constant (min⁻¹) (Yang et al., 2014). The results from the kinetic plot shown in Fig. 9, clearly exhibits that the 6% Pb-CeO₂ shows the highest photocatalytic performance with photoreaction rate constant k = 3.93 × 10⁻² min⁻¹ and it was found to be 3.96 times higher than that of pure CeO₂ (0.99 × 10⁻² min⁻¹). The photoreaction rate constant (k) of 6% Pb-CeO₂ nanocubes are comparitively higher than other doping concentrations of Pb in CeO₂ and the rate constants for other synthesized photocatalyst are given in Fig. 9. From this results we can conclude that the photocatalytic activity of CeO₂ can be observably improved in the presence of Pb.

Close

Fig. 9 The pseudo first order kinetics plot of MB by as synthesized pure and Pb doped CeO₂ nanocubes.

Export to PPT

3.8 Recyclability and stability analysis of photocatalyst

The photocatalytic stability is the prime factor for catalyst recyclability based on its importance for practical application. To investigate the photocatalytic stability of as-prepared 6% Pb-CeO₂ nanocubes several photodegradation trials of MB were executed under mercury lamp irradiation, as shown in Fig. 10a. The recycled photocatalyst exhibited good performance and stability. After Over all five cycles, the degradation performance of 6% Pb-CeO₂ remained nearly unchanged compared to the fresh photocatalyst. The phase stability of 6% Pb-CeO₂ nanocubes after five trials of MB degradation was investigated through XRD analysis shown in Fig. 10b. XRD pattern showed alike structure and phase of the catalyst before and after the photocatalytic process. Therefore, 6% Pb-CeO₂ photocatalyst has an excellent stability and can be reprocessed without loss in photocatalytic activity for degradation of organic pollutants (Li et al., 2011). Fig. 10c shows the FT-IR spectrum of 6% Pb-CeO₂ photocatalyst before and after photocatalytic processes. The FT-IR spectrum of MB exhibits a typical aromatic ring stretching vibration peaks at 1465 and 1450 cm⁻¹ but, no characteristic MB peaks were found in FT-IR spectrum of 6% Pb-CeO₂ after degradation process (Ahmed et al., 2016). The absence of characteristics peaks of MB and changes in intensity of peaks in reused 6% Pb-CeO₂ attributed to the almost complete degradation of MB and catalyst stability. The purity of 6% Pb-CeO₂ after photocatalytic process was confirmed by EDS shown in Fig. 10d. These results shows that 6% Pb-CeO₂ nanocubes has excellent stability and reusability for photocatalytic dye degradation application.

Close

Fig. 10 (a) Recycled photodegradation performance of 6% Pb-CeO₂ to MB, (b) XRD pattern (c) FT-IR spectra and (d) EDS pattern of 6% Pb-CeO₂ after 5th cycle of MB degradation.

Export to PPT

3.9 Estimation of hydroxyl radicals

The generation of hydroxyl radicals during photocatalytic process by 6% Pb-CeO₂ was detected by PL technique by using TA as a probe molecule. TA is a well-recognized ^•OH radical scavenger which does not reacts with other radicals like O₂^•− and H₂O₂. The ^•OH radicals converts TA into 2-hydroxyterephthalic acid (HTA) during 6% Pb-CeO₂ photocatalytic process (Jiang et al., 1999). When the solution comprising TA and HTA undergoes PL measurement, HTA molecules emits light at λ = 425 nm, while TA molecules does not produce any emission. The PL spectra of reaction solution is shown in Fig. S6, at 0 min the blank curve indicates no emission from TA solution. When ^•OH radicals tends to react with TA during photocatalytic process, HTA molecule is produced and the respective emission spectra is obtained. The obtained enhancement in the intensity of PL peaks could be credited to the higher production of ^•OH radicals with increasing time.

3.10 Photocatalytic mechanism

The redox reactions on the photocatalytic surface significantly increases the photocatalytic activity of the catalyst. The organic pollutants are ultimately degraded through the photocatalytic oxidation by reactive species like O₂^•−, ^•OH and h⁺. The trapping experiment was conducted during the photocatalytic degradation of MB, for further investigation of reactive species estimation and photocatalytic mechanism of 6% Pb-CeO₂ nanocubes. BQ (1 mmol/L), IPA (1 mmol/L) and TEOA (1 mmol/L) were taken as O₂^•−, ^•OH and h⁺ scavengers, respectively (Ji et al., 2014; Priyadharsan et al., 2017; Xu et al., 2016). As the results shown in Fig. 11, TEOA has predominant effect on the photocatalytic degradation efficiency, while the photodegradation of MB is noticeably affected by the addition of BQ and IPA. This results demonstrates that h⁺ is the prime active species and plays a major roles in the degradation of MB. The reactive species like O₂^•− and ^•OH also play major role in the photocatalytic reaction, but comparatively lesser than h⁺ in the presence of 6% Pb-CeO₂ nanocubes.

Close

Fig. 11 Trapping experiment of active species during the photocatalytic degradation of MB by 6% Pb-CeO₂ (reaction condition: catalyst dosage = 1 g/L, MB = 1 mg/L, pH = 7 and reaction time = 90 min).

Export to PPT

The mechanistic photocatalytic reaction scheme diagram for Pb-CeO₂ photocatalyst is shown in Fig. 12. Generally the absorption of photon energy by photocatalyst and rate of recombination of electron hole pairs in photocatalyst plays major role in the photocatalytic performance (Zhang et al., 2016). The high absorption of photon energy and extension of excitation wavelength by 6% Pb-CeO₂ than pure CeO₂ was verified by above UV–Vis DRS characterization. While comparing 6% Pb-CeO₂ with pure CeO₂, the absorption coefficient for 6% Pb-CeO₂ was raised to some extent in both ultraviolet and visible region. As a result, 6% Pb-CeO₂ shows much stronger light absorption capability and gradual decrease in band gap energy. These factors could be beneficial for the photocatalytic performance. The conduction band (CB) and valance band (VB) edges of 6% Pb-CeO₂ was determined by using the Eqs. (1) and (2):

Close

Fig. 12 Proposed photocatalytic mechanism of 6% Pb-CeO₂.

Export to PPT

The above discussion concludes that the doping of Pb in CeO₂ lattice improves the transfer of photoexcited e⁻–h⁺ pairs and declines the recombination rate of photoinduced charge carriers and facilitate the production of more hydroxyl and superoxide radicals. The highly efficient charge-carrier separation and improved reactive species generation leads to the enhanced photocatalytic activity of Pb-CeO₂.