1.   Introduction

Based on the difference in wavelength, the ultraviolet (UV) light is divided into three parts: ultraviolet radiation a (UVA), ultraviolet radiation b (UVB), and ultraviolet radiation c (UVC). UVC ranges from 200 to 280 nm and is absorbed by the ozone layer. UVB ranges from 280 nm to 315 nm and can reach the skin epidermis layer leading to suntan. However, most of UVB can be absorbed by the ozone layer and can be blocked by glass, umbrella, and clothes. UVA ranges from 315 nm to 400 nm and is the longest wavelength of UV light. UVA can reach skin dermis and causes skin aging. This kind of UV light cannot be absorbed by the ozone layer. In addition, anti-UV agents that prevent skin aging, skin cancer, and cataract are in demand because of the destruction of the ozone layer [1]. UV-shielding materials are classified into organic and inorganic classes, including cinnamic acid and titanium dioxide [2]. Common inorganic anti-UV agents include zinc oxide (ZnO), titanium dioxide (TiO₂), and cerium dioxide (CeO₂). These materials have similar UV-shielding property, but cerium dioxide has a lower photocatalytic activity [3].

As a UV-shielding reagent, cerium dioxide possesses numerous advantages. This compound can absorb and scatter UV light. In addition, the film coated with cerium dioxide has a high transparency [4]. Several of the excellent activities of cerium dioxide are related to two valence states, namely, Ce³⁺ and Ce⁴⁺ [5−8]; electronic and geometric effects influence the properties of cerium dioxide [9].

Various methods are used to synthesize cerium dioxide, and they are applicable to the preparation of other metal oxides or other types of materials. Zhou et al. [10] synthesized nanometer-size single-crystal CeO₂ by room-temperature homogeneous nucleation. Matijevi and Hsu [11] synthesized spherical composite particles in yttrium cerium salt mixed solution by homogeneous precipitation method. Chen et al. [12] synthesized reactive CeO₂ powders by aging a cerium(III) nitrate solution in the presence of hexamethylenetetramine. Verma1 et al. [13] prepared nanophase CeO₂ films with photoluminescence characteristics by sol-gel process. Chu et al. [14] synthesized monodispersed submicrometer CeO₂ spheres and studied the related sintering processes. Yin et al. [15] synthesized CeO₂ nanoparticles with sonochemistry process by using cerium nitrate and azodicarbonamide as starting materials and ethylenediamine or tetraalkylammonium hydroxide as additives. Zhang et al. [16] synthesized CeO₂ nanorods via ultrasonication assisted by polyethylene glycol. Zhou et al. [17] synthesized nanocrystalline CeO₂ powders by electrochemical synthesis. Yang et al. [18] prepared monodispersed CeO₂ nanocubes via an acrylamide-assisted hydrothermal route. Yu et al. [19] synthesized CeO₂ with flower-like and well-aligned nanorod hierarchical architectures by a phosphate-assisted hydrothermal route. Abbas et al. [20] synthesized CeO₂ nanostructures by a soft chemical method and studied the structural, morphological, Raman, optical, magnetic, and antibacterial characteristics of CeO₂. Wu et al. [21] synthesized well-crystalline CeO₂ nanowires via a surfactant-assisted hydrothermal process. Numerous applications are also utilized for the preparation of materials by hydrothermal process [22−28]. In addition to the above methods, reverse emulsion and microwave synthesis are used to synthesize cerium dioxide [29−30]. The complex-precipitation method we used is simple to operate, has a short processing period, and can be directly used to obtain nanoparticles. Yokota et al. [31] synthesized tetragonal CeO₂ and YO_1.5 powders doped with ZrO₂ at relatively low temperatures by a simple aqueous solution technique utilizing citric acid as a complexing agent. Zhou [32] synthesized ceria by mixing cerium chloride solution and ammonium citrate solution or adding cerium carbonate to citric acid solution.

In this article, cerium dioxide nanoparticles were synthesized through citric acid complex- precipitation route. On this basis, the precursors and final products were characterized. The effects of the molar ratio of citric acid to Ce³⁺ (n) and pH on the composition and proportion of precursors were studied. The effects of composition and proportion of precursors on ceria nanoparticles, including crystal structure, specific surface area, morphology, crystal plane spacing, average transmittance of UVA (TUVA), and that of UVB (TUVB), were discussed in detail. The correlation between various properties and UV transmittance was also studied.

2.   Experimental

2.1   Materials required

Cerium chloride was employed as cerium source. Citric acid was used as a dispersant and complexing agent. NaOH was used to adjust the pH. All the chemicals used were of analytical grade and used as received without further purification.

2.2   Synthesis

Based on different n values (0.25, 0.5, 0.75, and 1), cerium chloride solution and citric acid were mixed in a water system. The reaction was performed at 60°C at a constant stirring rate. The pH was adjusted to 3.5, 4.5, 5.5, or 6.5 with NaOH solution (6 mol/L), and stirring was carried out in an ambient environment for 30 min. After the reaction, the solution was filtered, and the precipitation was washed thrice with water and with ethanol for another three times. At the calcination temperature (T) of 300°C and the calcination time of 30 min, the dried samples were roasted to obtain ceria nanoparticles. Seven groups of samples were prepared, and the effects of n and pH on the properties of the products were investigated.

2.3   Material characterization

Elemental analysis (EA) was performed on a Vario EL cube elemental analyzer. X-ray diffraction (XRD) was determined on a Bruker AXS D8 Advance X-ray diffractometer with Cu K_α1 radiation. The scanning rate was 3°/min in the 2θ range from 20° to 90°. The grain size of the samples was calculated with Scherrer formula. The Brunner–Emmet–Teller (BET) method was used for the specific surface area. Scanning electron microscopy (SEM, voltage 20 kV) was performed on ULTRA PLUS instrument. Transmission electron microscopy (TEM) measurements were performed using a Tecnai G2 F20 S-TWIN field-emission transmission electron microscope. UV−visible (UV−Vis) transmittance spectra were collected under ambient conditions on a UV-2550 spectrophotometer (Shimazu, Japan) equipped with an integrating sphere in the wavelength between 200–800 nm.

3.   Results and discussion

3.1   Characterization of precursors

Two citric acid functions were considered in this work. First, citric acid can be used to form rare-earth citric acid complex precipitation with Ce³⁺; second, it can be used as dispersant to obtain products with uniform particle size and high specific surface area. According to Refs. [32−33], two kinds of complexation forms, which are composed of citric acid and Ce³⁺, exist: Ce(H₂Cit)₃ and CeCit. Ce³⁺ undergoes hydrolysis to produce Ce(OH)_3. On this basis, the precursors were analyzed by EA, and the results are summarized in Table 1. As shown in Table 1, at specific values of n and other conditions, the contents of C, H, and O all showed a downward trend with the increase in pH, whereas the contents of Ce exhibited an upward trend. Given that the pH and other conditions were constant, the contents of C, H, and O increased with the increase in n value, whereas the content of Ce decreased. This finding suggests that pH and n value have the opposite effect on the precursors.

Table  1.  Element composition of precursors

Sample No.	n	pH	C / wt%	H / wt%	O / wt%	Ce / wt%
1	0.25	3.5	18.50	2.41	38.56	40.53
2	0.25	4.5	17.20	2.35	37.60	42.85
3	0.25	5.5	14.40	2.25	36.94	46.41
4	0.25	6.5	11.27	2.18	36.90	49.65
5	0.50	5.5	16.60	2.33	37.18	43.89
6	0.75	5.5	18.81	2.43	38.79	39.97
7	1.00	5.5	19.87	2.48	39.55	38.10

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In addition, the composition of the precursors is calculated by using the result in Table 1. Fig. 1 shows the composition and proportion of the precursors prepared under different conditions. The results show that when pH was 3.5, 4.5, or 5.5, the precursors were composed of Ce(H₂Cit)₃ and Ce(OH)₃, and with the increase in pH, the proportion of Ce(H₂Cit)₃ decreased, whereas that of Ce(OH)₃ increased. When pH reached 6.5, the type of precursor changed, and the precursor mainly consisted of CeCit and Ce(OH)₃. This phenomenon can be explained by the titration curve of mixed solutions shown in Fig. 2. The titration curve is roughly divided into three parts. Curve (1) corresponds to the first-step dissociation of citric acid, second-step dissociation, and the formation of Ce(OH)₃ and Ce(H₂Cit)₃. Moreover, with the increase in sodium hydroxide, the dissociation degree of citric acid increased. Thus, the content of Ce(H₂Cit)₃ showed a downward trend. In curve (2), citric acid was further dissociated to form CeCit, and Ce(OH)₃ was formed throughout the process. In addition, when n value was 0.25, 0.50, 0.75, or 1.00, the precursors included Ce(H₂Cit)₃ and Ce(OH)₃. With the increase in n value, the proportion of Ce(H₂Cit)₃ increased, whereas that of Ce(OH)₃ decreased. The reason for this phenomenon is that the hydrolysis of Ce³⁺ and the dissociation of citric acid inhibited each other. In curve (3), most of the reaction was basically completed, and the consumption of NaOH was small, so the pH rose quickly. Fig. 3 shows the formulas for Ce(H₂Cit)₃ and CeCit.

Figure  1.  Composition and proportion of precursors.

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Figure  2.  Titration curve of mixed solutions.

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Figure  3.  Structural formula of complexes: (a) Ce(H₂Cit)₃; (b) CeCit.

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3.2   Characterization of ceria nanoparticles

Fig. 4 shows the XRD patterns obtained from as-prepared ceria nanoparticles. The peaks of all samples are in good agreement with the standard PDF card 00-004-0593 for face-centered cubic structure. Based on the XRD patterns, the grain sizes of the samples (Table 2) were calculated by Scherrer formula. Fig. 4(a) displays the XRD patterns of samples at different pH values. At pH 6.5, the minimum grain size of 4.8 nm was obtained. Combined with the results of EA, this phenomenon is attributed to the increased degree of hydrolysis of Ce³⁺ and decreased proportion of Ce(H₂Cit)₃ with the increase in pH, which prevented the particle agglomeration caused by excessive heat generated by the decomposition of citric acid during calcination. Fig. 4(b) shows the XRD patterns of samples with different n values. With the increase in n value, the grain sizes increased, with the minimum value of 5.1 nm. With the increase in n value, the proportion of Ce(H₂Cit)₃ increased, resulting in the slightly increased grain size.

Figure  4.  XRD patterns of CeO₂ nanoparticles at (a) different pH values (n = 0.25) and (b) n values (pH 5.5).

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Table  2.  Grain size and specific surface area of ceria nanoparticles

Sample No.	n	pH	T / °C	Grain size / nm	Specific surface area / (m2∙g−1)
1	0.25	3.5	300	5.5	65.33
2	0.25	4.5	300	5.7	76.58
3	0.25	5.5	300	5.1	83.17
4	0.25	6.5	300	4.8	73.70
5	0.50	5.5	300	5.8	72.06
6	0.75	5.5	300	5.9	66.13
7	1.00	5.5	300	6.0	33.57

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Table 2 shows the specific surface area of samples prepared under different conditions. When n was 0.25, the specific surface area increased as pH increased from 3.5 to 5.5, and the maximum was obtained at pH 5.5 (83.17 m²/g). At pH 5.5, the specific surface area decreased with the increase in n value. This finding is attributed to the increased proportion of Ce(H₂Cit)₃, increased atomic heat activation energy, and easy migration during roasting.

Analysis of the samples by SEM (Fig. 5) revealed that the particle size gradually decreased with the increase in pH and at n value of 0.25. Particle sintering occurred, and it diminished with the increase in pH. The particle size enlarged with the agglomeration phenomenon at pH 6.5. The reason for the rule and phenomenon is the different composition and proportion of precursors. At pH 5.5, the particle size gradually increased as the n value increased (Fig. 6). Particle agglomeration was constantly accompanied by this phenomenon.

Figure  5.  SEM images of CeO₂ nanoparticles with different pH values at n = 0.25: (a) 3.5; (b) 4.5; (c) 5.5; (d) 6.5.

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Figure  6.  SEM images of CeO₂ nanoparticles with different n values at pH 5.5: (a) 0.25; (b) 0.50; (c) 0.75; (d) 1.00.

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Several samples were analyzed by TEM to better characterize the role of citric acid in the experiment. Fig. 7 shows the TEM images and the corresponding selected area electron diffraction (SAED) patterns. Given the electron diffraction pattern, the crystal plane spacing of the main crystal faces was calculated, and the settlement results are summarized in Table 3. With the pH values of 3.5, 4.5, 5.5, the crystal plane spacing of each crystal plane increased with the increase in pH. When pH rose to 6.5, the crystal plane spacing of each crystal plane decreased. At n = 0.25 and pH 5.5, the maximum crystal plane spacing was obtained, and the crystal plane spacings of {111}, {200}, {220}, and {311} were 0.321388, 0.278455, 0.195934, and 0.166196 nm, respectively. Thus, the larger the spacing, the denser the arrangement of atoms.

Figure  7.  TEM images of CeO₂ nanoparticles with different pH values at n = 0.25 and corresponding SAED patterns: 3.5; (b) 4.5; (c) 5.5; (d) 6.5.

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Table  3.  Crystal plane spacing of ceria nanoparticles

Sample No.	n	pH	T / °C	d{111} / nm	d{200} / nm	d{220} / nm	d{311} / nm
1	0.25	3.5	300	0.313308	0.271058	0.191086	0.161570
2	0.25	4.5	300	0.320256	0.275539	0.195666	0.166168
3	0.25	5.5	300	0.321388	0.278455	0.195934	0.166196
4	0.25	6.5	300	0.316431	0.271813	0.191534	0.162443

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3.3   UV-shielding property of ceria nanoparticles

Fig. 8 shows the UV−Vis transmittance curves of all samples in the experiment. The values of TUVA and TUVB were calculated based on the UV transmittance curves to compare the UV performance of the samples intuitively (Table 4). TUVA and TUVB refer to the average transmittance of UV light between 315–400 and 280–315 nm, respectively.

Figure  8.  UV−Vis transmittance spectra of ceria nanoparticles.

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Table  4.  TUVA and TUVB of ceria nanoparticles

Sample No.	n	pH	T / °C	TUVA / %	TUVB / %
1	0.25	3.5	300	7.85	2.31
2	0.25	4.5	300	7.14	2.19
3	0.25	5.5	300	4.42	1.56
4	0.25	6.5	300	4.53	1.84
5	0.50	5.5	300	4.70	1.77
6	0.75	5.5	300	7.40	2.58
7	1.00	5.5	300	7.54	2.66

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As shown in Table 4, at constant n value, TUVA and TUVB decreased with the increase in pH, and at pH 5.5, the minimum values were 4.42% and 1.56%, respectively. As the pH increased further, TUVA and TUVB enlarged. At constant pH, TUVA and TUVB increased with the increase in n value. Combined with the previous representation results, the change rule of TUVA and TUVB agrees with the proportion of Ce(H₂Cit)₃ in the precursors. Thus, the higher the proportion of Ce(H₂Cit)₃ in the precursors, the poorer the absorption of UV light by ceria is. However, the change rule of TUVA and TUVB is opposite to that of the specific surface area and crystal plane spacing, that is to say, high specific surface area and large interplanar crystal spacing are beneficial to blocking UV light. This condition is attributed to the composition of Ce(H₂Cit)₃, which consists of one Ce³⁺ and three (H₂Cit)⁻. Thus, the decomposition of (H₂Cit)⁻ provides considerable heat during roasting. Thus, the lower the content of Ce(H₂Cit)₃, the higher the defect content in the formation of ceria nanoparticles. Furthermore, the relation between crystal plane spacing and band gap is inversely proportional. The larger the crystal plane spacing, the smaller the band gap is, and the stronger the absorption capacity of crystal to light is.

4.   Conclusion

Ceria nanoparticles have been successfully synthesized via a simple complex-precipitation route that employs cerium chloride as cerium source and citric acid as precipitant. EA indicated that the precursors were composed of Ce(OH)₃, Ce(H₂Cit)₃, or CeCit. XRD analysis showed that all ceria nanoparticles had face-centered cubic structure. The specific surface area was measured by using BET equipment. The specific surface area achieved the maximum value of 83.17 m²/g at n = 0.25 and pH 5.5. SEM and SAED patterns showed that all samples comprised nanoparticles, and the crystal plane spacing of each low-exponent crystal plane was calculated. The UV−Vis transmittance curve showed that cerium dioxide nanoparticles can absorb UV light and pass through visible light. Among all samples, the minimum TUVA and TUVB reached 4.42% and 1.56%, respectively. The above information suggest that cerium dioxide nanoparticles are good anti-UV agent candidates.

Acknowledgements

This work was financially supported by the Major State Basic Research Development Program of China (No. 2012CBA01205) and the National Natural Science Foundation of China (No. 51274060).

Conflict of Interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.