Introduction

Environmental pollution is one of the most serious problems facing human beings and other life forms due to the increasing population, industrialization, and urbanization1. Dyes are major pollutants that are released from textile industrial effluent2. Malachite green (MG, including aniline green; basic green 4; diamond green B; and victoria green B) is a water-soluble cationic dye that is available in two forms malachite green chloride (C23H25ClN2) and malachite green oxalate (C52H54N4O12) (Table 1). It is used as an industrial dye for the dyeing of leather, wool, silk, cotton, jute, paper and the manufacturing of printing inks and paints3. The extensive usage of MG dye has caused several health problems, including significant effects on the immune and reproductive systems4. MG also has the potential to cause genotoxic and carcinogenic effects and is extremely cytotoxic to mammalian cells5, and hence the suitable treatment of wastewater containing MG dye is highly necessary. Several methods have been developed, such as physical, chemical, biological, and photocatalytic degradation methods for the treatment of industrial effluent3,6,7,8,9. The physical and chemical methods include adsorption10, ion-exchange11, irradiation12, oxidation processing13, chemical precipitation14, photolysis15, coagulation/flocculation16, electrochemical treatment17, ozonisation18, photo-Fenton degradation19,20, reduction and oxidation21. The biological methods include fungal degradation22, bacterial degradation23, and aerobic and anaerobic degradation24. However, these methods are generally expensive, are unable to completely remove the dye, and produce high sludge and by-products that cause secondary pollution25. In recent years photocatalytic degradation of organic dyes has been used effectively, but this process is slow and energy-consuming. Compared with photocatalytic activity, a catalytic reduction is a relatively fast process without high energy requirements26.

Table 1 Chemical properties of malachite green (MG) dye.
Full size table

Ce is a rare earth element with a wide bandgap (~3.2 eV), lanthanide series, and exists as a free metal or oscillates between in the Ce3+ and Ce4+ oxidation states27. CeO2 NCs, also known as nanoceria, have been widely used in catalysis, fast-ion conductors, UV blockers, energy storage, and optical sensors due to its excellent physical and chemical properties28,29,30,31.

CeO2 is an effective catalyst for the removal of organic dyes from effluents because ceria also hops between Ce3+ and Ce4+ valence states containing oxygen vacancies that allow NCs to act as regenerative catalysts32. Various methods have been reported in the literature for the synthesis of ceria33,34,35,36,37, including sol-gel38, microwave combustion39, flame spray pyrolysis40, solvothermal41, microemulsion42, spray drying system43, hydrothermal44, and thermal decomposition45. Among these, the hydrothermal method is simple, which can produce high product yields46.

In this study we synthesized CeO2 NCs using urea (CH4N2O), as capping and reducing agent, urea was also known as carbamide and it is easily soluble in water. It has been used as a nitrogen source for the synthesis of environmentally benign nanoparticles. It has been widely used in many industries such as in agriculture, plastic, drug, soap, and detergent industries. However, excess concentrations urea in soil or in water causes to soil acidification and eutrophication and toxic effects to aquatic organisms, animals and humans47,48. The ceria nanocubes (CeO2 NCs) were synthesized via the one-pot hydrothermal method using different concentrations of urea. The CeO2 NCs are applied as a catalyst for the degradation of MG in the presence of NaBH4 in an aqueous medium using UV-visible spectroscopy.

Materials and Methods

Materials

Analytical grade cerium nitrate hexahydrate and urea were purchased from Alfa-Aesar, South Korea and used directly. Malachite green dye was obtained from Daejung Chemicals, South Korea. Deionized (DI) water was used throughout the study for the synthesis of CeO2 NCs and dye degradation studies.

Synthesis of ceria nanocubes

Cerium nitrate hexahydrate, 0.01 M was prepared at a volume of 70 mL in DI water. Different quantities of urea (0.5, 1.0 and 2.0 g) were added and stirred until dissolved. This reaction mixture was transferred to an autoclave at a temperature of 180 °C for 6 h. The reaction solution was naturally cooled down to room temperature. The reaction mixture was carefully collected by centrifuge and wished with DI and ethanol to remove the unreacted substance, before drying at 80 °C overnight. This was later calcined at 400 °C for 4 h and for comparison we prepared ceria without urea (CeO2 NCs-0.0).

Characterization

The crystallinity was measured by an X-ray diffractometer (PANalytical X-Pert PRO, USA) using a Cu Kα source (λ = 1.5405 Å). FT-IR (Perkin-Elmer, Bruker) was used to identify the vibrational modes by an ATR mode. The morphology of the samples was investigated by scanning electron microscopy (SEM-4800, Hitachi) and high-resolution transmission electron microscopy (TEM, Titan G2 ChemiSTM Cs Probe) with a 200 kV field emission gun in high brightness Schottky mode FEG (X-FEG). UV-visible absorption was measured on a Neogen (NEO-D3117). The elemental composition was quantitatively compared using an X-ray photoelectron spectrometer (K-alpha, Thermo Scientific, USA) with Al Kα radiation (1486.6 eV).

Catalytic activity

The synthesized samples were used to check the catalytic ability using the MG dye. For the catalytic reactions, 2.5 mL of the MG dye, 25 µL of well-dispersed CeO2 NCs (2 mg/mL) and 0.2 mL of (0.2 M) freshly prepared NaBH4 were added to all the samples. The degradation efficiency was monitored by UV-visible spectroscopy (Neogen, NEO-D3117). The degradation efficiency was calculated as a function of time for Co/Ct and ln Co/Ct, where Co is initial concentration and Ct is the final concentration of the MG dye. Blank control experiments were performed without catalyst (MG + NaBH4) or NaBH4 (MG + CeO2 NCs) to study the catalyst efficiency.

Results and Discussion

Characterization

The UV-visible absorbance spectra of the ceria nanocubes are shown in Fig. 1. The synthesized ceria nanocubes exhibited broad absorption peaks below 400 nm. Couple of broad absorption peaks were observed at wavelengths 208 and 311 nm (CeO2 NCs-0.5); 209 and 310 nm (CeO2 NCs-1.0); and 209 and 314 nm (CeO2 NCs-2.0). These peaks correspond to the characteristic absorption peaks of Ce3+ and Ce4+ of the CeO2 NCs. These results are similar to those obtained by Nurhasanah49 et al. who observed absorption peaks at 207 and 303 nm for CeO2 NCs (9 nm, precipitation method).

Figure 1
figure 1

UV-visible spectra of the cerium oxide nanocubes (CeO2 NCs) as a function of urea concentration.

Full size image

Fig. 2 shows the FT-IR spectra of the as-synthesized urea-based ceria nanocubes (CeO2 NCs-0.5, CeO2 NCs-1.0, and CeO2 NCs-2.0). The features near 1540 and 1630 cm−1 related to (ν C=O) and (N-H stretching) bond stretch50,51. The remaining observed peaks are as follows: near 3362 cm−1 (O-H stretching vibrations)52; near 2980 cm−1 (asymmetric C-H stretching)53; sharp band near 1340 cm−1 (N-O stretch due to the presence of nitrate)54; 1071 cm−1 (C-O stretching vibration)55; 850 cm−1 (formed by CeO2, which is typical peak of the Ce-O stretching vibrations)56; and below 700 cm−1 (O-Ce-O stretching vibrations)57.

Figure 2
figure 2

FI-TR spectra of cerium oxide nanocubes (CeO2 NCs) as a function of urea concentrations.

Full size image

Fig. 3 shows the XRD patterns of urea-based 0.5, 1.0 and 2.0 CeO2 NCs. The XRD revealed sharp peaks which located at 2θ = 28.5°, 32.9°, 47.5°, 56.3°, 59.0°, 69.3°, 76.7°, 78.8°, and 88.4°, are corresponding to (111), (200), (220), (311), (222), (400), (331), (420) and (422) planes, respectively. No extra peaks were observed, which supported the assumption that urea-based NCs are pure with cubic fluorite structure (JCPDC: 034–0394), similar peaks were also observed in CeO2 NCs-0.0 and results are shown in Fig. S1. The average mean grain size (MGS) of the NCs was calculated by the Debye- Scherer equation as follows58,59:

$${\rm{MGS}}=0.94\lambda /\beta cos\theta $$
(1)

where β is the broadening in the full-width at half maximum (FWHM), λ is the X-ray wavelength (1.5406 Å), and θ is the Bragg diffraction angle. The microstrain, ε, of the NCs is evaluated by

$$\varepsilon =(\beta \,\cos \,\theta )/4$$
(2)
Figure 3
figure 3

XRD spectra of the cerium oxide nanocubes (CeO2 NCs) as a function of urea concentrations.

Full size image

The dislocation density δ, of the NCs is defined as the length of dislocations lines per unit volume and given by

$$\delta =1/{(MGS)}^{2}$$
(3)

The lattice strain (LS) of the NCs was calculated using the following relation60,61.

$$LS=\beta /4\,\tan \,\theta $$
(4)

From Table 2, the MGS increased with the increasing urea concentration, for CeO2 NCs-0.5 and CeO2 NCs-1.0 were showing similar value (12 nm), and for CeO2 NCs-2.0 showing slightly bigger size (13 nm), whereas CeO2 NCs-0.0 showing much higher than (31 nm) urea mediated CeO2 NCs, this MGS analysis clearly indicating the urea was acted as capping and reducing agent to control the size of the NCs, meanwhile, the dislocation density decreased with increasing urea concentration, but for higher concentrations (CeO2 NCs-1.0 and CeO2 NCs-2.0) dislocation density was constant, in CeO2 NCs-0.0 too low than the urea mediated CeO2 NCs. The microstrain was constant with increasing urea concertation but without urea CeO2 NCs showing lower than other CeO2 NCs, however, lattice strain (LS) was constant for the first two concentrations (CeO2 NCs-0.5 and CeO2 NCs-1.0), and for the higher concertation (CeO2 NCs-2.0), it was slightly lowered compared with those of the lower concentrations, whereas in CeO2 NCs-0.0 showing very lower values, based on these results urea playing vital role as capping and reducing agent in the formation of CeO2 NCs.

Table 2 Calculated XRD parameters of the urea assisted CeO2 NCs.
Full size table

To identify the chemical composition on the surface of the CeO2 NCs, we performed the XPS analysis (XPS analysis only for CeO2 NCs-0.5) and the results are presented in Fig. 4. The survey scan (Fig. S2(a)) clearly shows the presence of Ce and O elements. The high-resolution XPS spectrum of Ce shown in Fig. 4a contains six peaks, revealing that the Ce contained both the Ce3+ and Ce4+ oxidation states. The binding energies (BE) at 882.9, 901.2 and 917.2 eV are attributed to Ce4+, and the peaks at 885.5, 899.1 and 908.3 eV are the characteristic peaks of Ce3+. These results suggest the presence of mixed oxidations and a valence state for the Ce ions in the NCs62,63 and Deshpande64 et al. reported that Ce3+ ions create oxygen vacancies in the CeO2 lattice that are defects. There were three peaks in the high-resolution O 1 s deconvoluted spectrum in Fig. 4b; the O1 peak, O2 peak, and O3 peak, with 529.8, 532.0, and 533.6 eV binding energies, respectively. The O1 peak (529.8 eV) corresponds to the bond Ce-O, O2 peak (532.0 eV) is attributed to the oxygen vacancies, and O3 peak (533.6 eV) belongs to organic C=O bond or surface –OH groups. To get more information we performed XPS analysis for N 1 s, but the results are not showing any significant peak for N 1 s and results are shown in Fig. S2(b). These results are indicating the urea was acting as capping and reduction the formation of CeO2 NCs, not doped in CeO2 NCs, which are similar to the values in literature62,65,66,67.

Figure 4
figure 4

XPS analysis of the CeO2 NCs-0.5; high-resolution spectra of cerium (Ce 3d (a)) and oxygen (O 1 s (b)).

Full size image

The morphology of CeO2 NCs was obtained by SEM, and the images are revealed in Fig. 5. For CeO2 NCs-0.5, CeO2 NCs-1.0, and CeO2 NCs-2.0, SEM images show that aggregates were formed by the accumulation of nanocubes. SEM images of CeO2 NCs-0.0 was shown in Fig. S3, the results are indicating that the CeO2 NCs-0.0 were more in aggregation and size also higher than the urea mediated CeO2 NCs. The details of CeO2 NCs were further confirmed by HR-TEM studies. Fig. 6 shows the HR-TEM images of CeO2 NCs synthesized with different concentrations of urea. The TEM images show that CeO2 NCs were well dispersed, almost polydispersed, consisting of cubic shapes. The CeO2 NCs obtained with 0.5 urea-based NCs were smaller in size and less aggregated compared with the 1.0 and 2.0 urea-based NCs. To confirm the phase formation of the NCs, HR-TEM images were analyzed using the Gatan software, and the results are shown in Fig. 6a-iv–c-iv for CeO2 NCs-0.5, CeO2 NCs-1.0, and CeO2 NCs-2.0, respectively. The lattice fringe distance values were similar, 0.307 nm for CeO2 NCs-0.5, 0.307 nm for CeO2 NCs-1.0 and 0.306 nm CeO2 NCs-2.0, and were assigned to the (111) plane of the CeO2 NCs. The corresponding selected area electron diffraction (SAED) pattern (Fig. 6a–v, b–v and c–v for CeO2 NCs-0.5, CeO2 NCs-1.0, and CeO2 NCs-2.0) shows well-defined diffraction rings that confirm the polycrystalline nature of CeO2 NCs. These results were in good agreement with the XRD results. Therefore, the HR-TEM images revealed a good crystalline nature of the urea assisted NCs.

Figure 5
figure 5

SEM images of the cerium oxide nanocubes as a function of urea concentrations, (a-i,ii) CeO2 NCs-0.5, (b-i,ii) CeO2 NCs-1.0, and (c-i,ii) CeO2 NCs-2.0.

Full size image
Figure 6
figure 6

HR-TEM images of the cerium oxide nanocubes as a function of urea concentrations. The (a) top, (b) middle, and (c) bottom panels correspond to CeO2 NCs-0.5, CeO2 NCs-1.0, and CeO2 NCs-2.0, respectively. The panels labeled i–iii show different magnifications. The respective panels labeled iv shows the d-spacing values and line profiles, and v correspond to the SAED patterns.

Full size image

Catalytic activity

MG dye was selected as a model pollutant to examine the catalytic activity of CeO2 NCs in the presence of NaBH4 as the reducing agent under ambient conditions. The results are presented in Fig. 7a to c for CeO2 NCs-0.5, CeO2 NCs-1.0, and CeO2 NCs-2.0, respectively. Fig. 7d shows a comparison of the absorption spectra of the catalytically degraded MG after 21 min in the presences of catalyst and NaBH4. The MG maximum absorption peak intensity was at~617 nm. The intense green color of the MG dye solution slowly faded becoming colorless, during the catalytic dye degradation process. A plot of Co/Ct versus degradation, time is shown in Fig. 8a for all samples, where Co is initial concentration and Ct is concentration at time ‘t’.

Figure 7
figure 7

Time-dependent UV-visible absorption spectra of MG by NaBH4 in the presence of (a) CeO2 NCs-0.5, (b) CeO2 NCs-1.0, and (c) CeO2 NCs-2.0, and (d) comparison of catalytic activity of the samples at 21 min.

Full size image
Figure 8
figure 8

Catalytic activity (a), and plot of ln (C0⁄Ct) (b) of CeO2 NCs as a function of urea concentrations.

Full size image

Langmuir-Hinshelwood pseudo-first order kinetics was applied to determine the first-order rate constant for dye degradation using the following relation68:

$${\rm{k}}=-\,ln({C}_{o}/{C}_{t})$$
(5)

where ‘k’ is the rate constant (min−1) of the dye degradation reaction, Co is the initial concentration and Ct is the concentration of the dye solution after time ‘t’ in minutes, respectively. The results are shown in Fig. 8b. To identify the role of catalyst, we performed two control experiments. In the first experiment we performed the reaction between NaBH4 + CeO2 NCs and in the second experiments, we performed the reaction between dye (MG) + NaBH4, respectively. In both these control experiments, there was no observed degradation, based on these control experimental results we confirmed that the catalyst (CeO2 NCs) playing a vital role in the dye decolorization, and when both the catalyst + NaBH4 were together, the degradation materialized rapidly; the results are shown in Fig. 7a. The CeO2 NCs-0.5 exhibited an excellent dye degradation ~90%, while the CeO2 NCs-1.0 and CeO2 NCs-2.0 exhibited ~70% MG dye degradation within 21 min. The possible mechanism of dye reduction by the catalyst is explained by the electron relay system. The NCs start the catalytic reduction by relaying electrons from the donor \(B{H}_{4}^{-}\) to the dye molecules (acceptor), where the catalyst (NCs) accepted electrons from nucleophilic \(B{H}_{4}^{-}\) ions and transferred them to electrophilic dyes. Saikia et al. described hydrogen species generated from \(B{H}_{4}^{-}\) ions attack the dye (MG) molecules after electron transfer to nanocubes26. This reaction leads to a reduction in the MG dye, which is generally leuco MG (colorless). The CeO2 nanocatalysts show slope of −0.127, −0.061 and −0.064 for CeO2 NCs-0.5, CeO2 NCs-1.0, and CeO2 NCs-2.0, respectively. The rate constants for dye degradation were 0.127 min−1, 0.061 min−1, and 0.064 min−1 for CeO2 NCs-0.5, CeO2 NCs-1.0, and CeO2 NCs-2.0, respectively (Fig. 8a,b). CeO2 NCs-0.5 exhibited the highest dye degradation due to their size and high dispersion. We evaluated the catalytic activity of the CeO2 NCs-0.0, the rate constant was 0.03 min−1, as shown in Fig. S3 and it has shown poor performance than CeO2 NCs-1.0 and 2.0 that of counterparts due to bigger size and aggregation of particles.

Conclusion

We developed cost-effective CeO2 NCs using carbamide by one-pot hydrothermal method. The HR-TEM images revealed the synthesized NCs was cubic in shape. XRD results show the synthesized nanoparticles MGS of 12, 12, 13 and 31 nm for CeO2 NCs-0.5, CeO2 NCs-1.0, CeO2 NCs-2.0 and CeO2 NCs-0.0, respectively. These results were indicating that urea acting as capping and reducing agent. The synthesized CeO2 NCs also acted as efficient catalysts in the degradation of the MG dye.