In-situ growth of CeO2/Ce(OH)CO3 composites with enhanced photocatalytic activity by the incomplete conversion of CeO2 to Ce(OH)CO3

In this paper, a novel strategy was used to grow CeO2/Ce(OH)CO3 composite in situ by the incomplete conversion of CeO2 to Ce(OH)CO3 using ethylene glycol (EG) as CO3 2− source and solvent. Interestingly, the content of Ce(OH)CO3 in CeO2/Ce(OH)CO3 composite can be controlled by changing the solvent composition. The obtained CeO2/Ce(OH)CO3 composite all exhibited enhanced photocatalytic performance for methylene blue (MB) degradation. The CeO2/Ce(OH)CO3 composite prepared at 200 °C for 24 h with a H2O/EG volume ratio of 0.5 showed the best visible-light activity with a degradation efficiency of 98.84% within 120 min. This work provided a novel method to fabricate basic rare Earth carbonates and their composites for environmental purification.


Results and discussion
The crystal structure of CeO 2 and as-prepared samples were analyzed by x-ray diffraction (XRD) as shown in figure 1. All samples exhibited the well-defined diffraction peaks at 28.7°, 33.3°, 47.7°, and 56.6°, which corresponded to the (111), (200), (220), and (311) planes of CeO 2 (JCPDS no. 34-0394) [18,19]. Interestingly, the samples, prepared under the condition that the volume ratios of H 2 O to EG were 25:5, 20:10, 15:15, and 10:20, showed several new peaks at 17.7°, 24.6°, 30.5°, 35.9°, 43.3°and 44.0°, which were assigned to the (002), (300), (032), (004), (330), and (304) planes of Ce(OH)CO 3 (JCPDS no. 52-0352) [20], indicating the formation of CeO 2 /Ce(OH)CO 3 composite. Besides, the intensity of these peaks increased with the increasing of EG proportion in the solvent, indicating the increasing of Ce(OH)CO 3 content in the CeO 2 /Ce(OH)CO 3 composite. The Ce(OH)CO 3 content in the CeO 2 /Ce(OH)CO 3 composite reached the maximum value at H 2 O/EG ratio of 10:20. Thus, the content of Ce(OH)CO 3 in CeO 2 /Ce(OH)CO 3 composite could be controlled by adjusting the solvent composition. In the range of the volume ratio of H 2 O to EG from 25:5 to 10:20, the smaller the volume ratio of H 2 O to EG, the larger the content of Ce(OH)CO 3 in CeO 2 /Ce(OH)CO 3 composite.
The FT-IR spectra of as-prepared samples were shown in figure S1. The stretching vibration of Ce-O bonds caused a wide band in the range of 500-700 cm −1 [21], and the absorption peak at 3460 cm −1 was the stretching vibration of the hydroxyl group [22]. For the CeO 2 /Ce(OH)CO 3 composites prepared under the condition that the volume ratios of H 2 O to EG were 25:5, 20:10, 15:15, and 10:20, the broad peaks between 1417 and 1496 cm −1 were ascribed to the stretching vibration of the CO 3 2− groups [23]. The sharp absorption peaks at 694 cm −1 , 725 cm −1 and 850 cm −1 were caused by the bending vibration of the CO 3 2− groups [24]. The XRD and FT-IR results indicated that the samples obtained when the volume ratios of H 2 O to EG of 30:0, 5:25, and 0:30 were CeO 2 while at the other ratios were CeO 2 /Ce(OH)CO 3 composites. Figure 2 showed the SEM images of CeO 2 and as-prepared samples under different H 2 O/EG volume ratios. CeO 2 displayed apparent 2D layered structure. When the volume ratios of H 2 O to EG were 30:0 and 25:5, the 2D layered structure was destroyed and a few small particles appeared, which was attributed to the forced hydrolysis of CeO 2 . As the amount of EG in the solvent increased, the destroyed layered structure gradually recovered and thickened, and the CeO 2 /Ce(OH)CO 3 composite showed a pie-like structure with stacked pieces when the volume ratio of H 2 O and EG was 10:20, then the morphology of the sample was consistent with the CeO 2 nanosheets. Figure 3 showed the TEM images of CeO 2 and the CeO 2 /Ce(OH)CO 3 composite prepared at a H 2 O/EG ratio of 10:20. The CeO 2 showed a clear sheet-like structure composed of tiny particles. After the hydrothermal treatment, CeO 2 /Ce(OH)CO 3 composite maintained the sheet-like structure. However, the tiny particles constituted the sheet-like structure was clear larger which was due to the dissolution-recrystallization process.
To investigate the formation process of the composites, series experiments were carried out. Figure S2 are the XRD patterns of the samples prepared at volume ratio of H 2 O to EG for 10:20 with different temperatures. It was clear that the samples just showed the characteristic diffraction peaks of CeO 2 , and Ce(OH)CO 3 cannot be synthesized when the reaction temperature was lower than 200°C. When the reaction temperature was 200°C, the characteristic diffraction peaks of CeO 2 and Ce(OH)CO 3 were obvious, indicating the formation of CeO 2 /Ce(OH)CO 3 composite. The corresponding SEM images were shown in figure S3. When the reaction temperature was lower than 200°C, the morphology of the sample had not been destroyed and it was still a 2D layered structure. The sample obtained at 200°C exhibited a pie-like structure with stacked pieces. Based on the above results, it can be deduced that CeO 2 underwent forced hydrolysis under the conditions of high reaction temperature, and the forced hydrolysis of CeO 2 was a precondition for the formation of CeO 2 /Ce(OH)CO 3 composite in this reaction.
For this synthesis set-up, the reaction process of the formation of Ce(OH)CO 3 can be proposed. It is actually a dissolution-recrystallization process. For CeO 2 , it was forced to hydrolyze at high temperature and released Ce 4+ which was reduced to Ce 3+ by EG. Trivalent Ce 3+ was easily changed into Ce(OH) 2+ groups. As for EG, it was gradually oxidized to oxalic acid [25], then released carbon dioxide. Then carbon dioxide reacted with water to produce CO 3 2− . Finally, Ce(OH) 2+ combined with CO 3 2− to yield Ce(OH)CO 3 . The reaction can be expressed as follows: Thus the forced hydrolysis of CeO 2 (Reaction (1)) and the release of Ce 4+ (Reaction (2)) were crucial for the formation of Ce(OH)CO 3 , which was the reason why the sample prepared under low reaction temperature or with low water content in the solvent cannot produce Ce(OH)CO 3 . Besides, Ce(OH) 4 cannot be completely ionized to  release Ce 4+ (Reaction (2)). Therefore, the conversion of CeO 2 to Ce(OH)CO 3 cannot be fully carried out, resulting the formation of CeO 2 /Ce(OH)CO 3 composite. Due to the similar chemical properties, this method to synthesize CeO 2 /Ce(OH)CO 3 composite can be extended to rare Earth elements with variable valences. Figure 4 showed the UV-vis diffuse reflectance spectra of the samples obtained at 200°C for 24 h with various volume ratios of H 2 O to EG. The absorbance intensity of CeO 2 nanosheets was weak. And there was no response in the visible region for CeO 2 nanosheets. After hydrothermal treatment, the obtained samples all showed a weak response in the visible region at the range of 450∼800 nm. Besides, the UV-vis spectra of the obtained CeO 2 /Ce(OH)CO 3 composites were red shifted with respect to the pure CeO 2 nanosheets. The above results indicated that CeO 2 /Ce(OH)CO 3 composites had better visible-light response which was favorable to photocatalytic activities.
The visible-light catalytic activities of the as-prepared samples were evaluated by the degradation of MB after irradiation for 2 h as shown in figure 5. Obviously, 76.87% of MB was degraded over CeO 2 photocatalyst. After hydrothermal treatment, the obtained samples all showed enhanced MB removal activity, among which the The cycling photocatalytic experiments was examined to confirm the stability of the CeO 2 /Ce(OH)CO 3 composite. As shown in figure 6(a), the photodegradation efficiency of CeO 2 /Ce(OH)CO 3 composite dropped  from 99.8% to 83.9% after 3 cycles, which was ascribed to the inevitable reduction in catalyst mass during collection. From figure 6(b), the diffraction peaks of CeO 2 /Ce(OH)CO 3 composite after the cycling experiment were the same as before, which indicated the fine stability of CeO 2 /Ce(OH)CO 3 composite.

Conclusions
In conclusion, the CeO 2 /Ce(OH)CO 3 composites were successfully synthesized by a facial hydrothermal method. Due to the incomplete conversion of CeO 2 to Ce(OH)CO 3 , the content of Ce(OH)CO 3 in CeO 2 /Ce(OH)CO 3 composite can be controlled only by tuning the solvent composition. The conversion process was also proposed. CeO 2 was forced to hydrolyze, then reduced by EG, generated Ce 3+ and CO 3 2− , growed CeO 2 /Ce(OH)CO 3 composite in situ. After the conversion process of CeO 2 to Ce(OH)CO 3 , all the CeO 2 /Ce(OH)CO 3 composites exhibited enhanced photocatalytic activities compared with pristine CeO 2 . The CeO 2 /Ce(OH)CO 3 10:20). After stirring for 30 min, the above mixture was transferred into a 40 ml Teflon-lined autoclave and maintained at 200°C for 24 h. After being cooled to room temperature naturally, the resulting grey precipitates were collected and washed several times with distilled water and absolute ethanol and dried at 60°C.
In order to explore the synthesis mechanism, two sets of control samples were prepared just by adjusting the volume ratio of H 2 O to EG in the reaction solvent and the reaction temperature, respectively. The volume ratios of H 2 O to EG are 30:0, 25:5, 20:10, 15:15, 5:25, and 0:30. And the reaction temperatures are 140°, 160°and 180°C.
The photocatalytic activity was estimated by the degradation of methylene blue (MB). 10 mg of photocatalyst was dispersed in 50 ml of MB solution (10 mg l −1 ). A 3 00 W Xe lamp equipped with a cutoff filter (<420 nm) was used as the light source, and kept the distance between the light source and the solution at 10 cm. Prior to irradiation, each solution was stirred for 30 min at dark to reach the adsorption equilibrium. About 4 ml solution was taken out and centrifuged after regular interval of 30 min. The concentration of MB was determined by an UV-visible spectrophotometer (UV-vis, Hitachi, Japan, Hitachi-U3301).