Urea assisted ceria nanocubes for efficient removal of malachite green organic dye from aqueous system

This study describes a simple, high-yield, rapid, and inexpensive route for the synthesis of cubic shape-like cerium oxide nanocubes (CeO2 NCs) using different urea concentrations (0.5, 1.0, and 2.0 g) by the hydrothermal method. The synthesized nanocubes (NCs) are labeled as CeO2 NCs-0.5, CeO2 NCs-1.0, and CeO2 NCs-2.0, corresponding to 0.5, 1.0, and 2.0 g of urea, respectively. The synthesized NCs were characterized by FT-IR, UV-visible, XRD, XPS, SEM and HR-TEM analysis. The synthesized NCs were cubic in shape with average sizes of 12, 12, and 13 nm for the CeO2 NCs-0.5, CeO2 NCs-1.0, and CeO2 NCs-2.0, respectively, obtained by the XRD analysis. The catalytic activity of the CeO2 NCs was studied for the purpose of obtaining the reduction of malachite green (MG) in the presence of sodium borohydride (NaBH4) at room temperature.

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 CeO 2 NCs (2 mg/mL) and 0.2 mL of (0.2 M) freshly prepared NaBH 4 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 C o / C t and ln C o /C t , where C o is initial concentration and C t is the final concentration of the MG dye. Blank control experiments were performed without catalyst (MG + NaBH 4 ) or NaBH 4 (MG + CeO 2 NCs) to study the catalyst efficiency.
cos MGS 0 94 / (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   www.nature.com/scientificreports www.nature.com/scientificreports/ The dislocation density δ, of the NCs is defined as the length of dislocations lines per unit volume and given by The lattice strain (LS) of the NCs was calculated using the following relation 60,61 .
From Table 2, the MGS increased with the increasing urea concentration, for CeO 2 NCs-0.5 and CeO 2 NCs-1.0 were showing similar value (12 nm), and for CeO 2 NCs-2.0 showing slightly bigger size (13 nm), whereas CeO 2 NCs-0.0 showing much higher than (31 nm) urea mediated CeO 2 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 (CeO 2 NCs-1.0 and CeO 2 NCs-2.0) dislocation density was constant, in CeO 2 NCs-0.0 too low than the urea mediated CeO 2 NCs. The microstrain was constant with increasing urea concertation but without urea CeO 2 NCs showing lower than other CeO 2 NCs, however, lattice strain (LS) was constant for the first two concentrations (CeO 2 NCs-0.5 and CeO 2 NCs-1.0), and for the higher concertation (CeO 2 NCs-2.0), it was slightly lowered compared with those of the lower concentrations, whereas in CeO 2 NCs-0.0 showing very lower values, based on these results urea playing vital role as capping and reducing agent in the formation of CeO 2 NCs.
To identify the chemical composition on the surface of the CeO 2 NCs, we performed the XPS analysis (XPS analysis only for CeO 2 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 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 CeO 2 NCs, not doped in CeO 2 NCs, which are similar to the values in literature 62,[65][66][67] .
The morphology of CeO 2 NCs was obtained by SEM, and the images are revealed in Fig. 5. For CeO 2 NCs-0.5, CeO 2 NCs-1.0, and CeO 2 NCs-2.0, SEM images show that aggregates were formed by the accumulation of nanocubes. SEM images of CeO 2 NCs-0.0 was shown in Fig. S3, the results are indicating that the CeO 2 NCs-0.0 were more in aggregation and size also higher than the urea mediated CeO 2 NCs. The details of CeO 2 NCs were further confirmed by HR-TEM studies. Fig. 6 shows the HR-TEM images of CeO 2 NCs synthesized with different concentrations of urea. The TEM images show that CeO 2 NCs were well dispersed, almost polydispersed, consisting of cubic shapes. The CeO 2 NCs obtained with 0.5 urea-based NCs were smaller in size and less aggregated  www.nature.com/scientificreports www.nature.com/scientificreports/ 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 CeO 2 NCs-0.5, CeO 2 NCs-1.0, and CeO 2 NCs-2.0, respectively. The lattice fringe distance values were similar, 0.307 nm for CeO 2 NCs-0.5, 0.307 nm for CeO 2 NCs-1.0 and 0.306 nm CeO 2 NCs-2.0, and were assigned to the (111) plane of the CeO 2 NCs. The corresponding selected area electron diffraction (SAED) pattern (Fig. 6a-v, b-v and c-v for CeO 2 NCs-0.5, CeO 2 NCs-1.0, and CeO 2 NCs-2.0) shows well-defined diffraction rings that confirm the polycrystalline nature of CeO 2 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.
Catalytic activity. MG dye was selected as a model pollutant to examine the catalytic activity of CeO 2 NCs in the presence of NaBH 4 as the reducing agent under ambient conditions. The results are presented in Fig. 7a to c for CeO 2 NCs-0.5, CeO 2 NCs-1.0, and CeO 2 NCs-2.0, respectively. Fig. 7d shows a comparison of the absorption  www.nature.com/scientificreports www.nature.com/scientificreports/ spectra of the catalytically degraded MG after 21 min in the presences of catalyst and NaBH 4 . 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 C o /C t versus degradation, time is shown in Fig. 8a for all samples, where C o is initial concentration and C t is concentration at time 't' .
Langmuir-Hinshelwood pseudo-first order kinetics was applied to determine the first-order rate constant for dye degradation using the following relation 68 : where 'k' is the rate constant (min −1 ) of the dye degradation reaction, C o is the initial concentration and C t 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 NaBH 4 + CeO 2 NCs and in the second experiments, we performed the reaction between dye (MG) + NaBH 4 , respectively. In both these control experiments, there was no observed degradation, based on these control experimental results we confirmed that the catalyst (CeO 2 NCs) playing a vital role in the dye decolorization, and when both the catalyst + NaBH 4 were together, the degradation materialized rapidly; the results are shown in Fig. 7a 26 . This reaction leads to a reduction in the MG dye, which is generally leuco MG (colorless). The CeO 2 nanocatalysts show slope of −0.127, −0.061 and −0.064 for CeO 2 NCs-0.5, CeO 2 NCs-1.0, and CeO 2 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 CeO 2 NCs-0.5, CeO 2 NCs-1.0, and CeO 2 NCs-2.0, respectively (Fig. 8a,b). CeO 2 NCs-0.5 exhibited the highest dye degradation due to their size and high dispersion. We evaluated the catalytic activity of the CeO 2 NCs-0.0, the rate constant was 0.03 min −1 , as shown in Fig. S3 and it has shown poor performance than CeO 2 NCs-1.0 and 2.0 that of counterparts due to bigger size and aggregation of particles.  www.nature.com/scientificreports www.nature.com/scientificreports/

Conclusion
We developed cost-effective CeO 2 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 CeO 2 NCs-0.5, CeO 2 NCs-1.0, CeO 2 NCs-2.0 and CeO 2 NCs-0.0, respectively. These results were indicating that urea acting as capping and reducing agent. The synthesized CeO 2 NCs also acted as efficient catalysts in the degradation of the MG dye.