Facile Construction Engineering of Pr₆O₁₁@C with Efficient Photocatalytic Activity

Abstract

In this study, facile construction engineering of Pr₆O₁₁@C with efficient photocatalytic activity was established. Taking advantage of the flocculation of Pr³⁺ in the base medium, acid red 14 (AR14) was flocculated together with Pr(OH)₃ precipitate, in which Pr(OH)₃ and AR14 mixed highly uniformly. Calcinated at high temperature in N2, a novel Pr₆O₁₁@C was successfully synthesized. The resulting materials were characterized by XRD, SEM, FT-IR, Raman, and XPS techniques. The results show that the cubic Pr₆O₁₁@C with Fm3m space group, similar to that of Pr₆O₁₁, was obtained. From the results of the photodegradation of AR14, it is found that the photocatalytic efficiency of Pr₆O₁₁@C is higher than that of pure Pr₆O₁₁ due to the formation of abundant carbon bonds and oxygen vacancies. Compared with pure Pr₆O₁₁ and other carbon-based composites, the acid resistance of Pr₆O₁₁@C is greatly improved due to the highly uniform dispersion of Pr₆O₁₁ and C, which lays a solid foundation for the practical application of Pr₆O₁₁@C. Moreover, the role of NH3·H₂O and NaOH used as precipitants for the photocatalytic efficiency of Pr₆O₁₁ was investigated in detail.

1. Introduction

Due to the complete degradation of organic pollutants, photocatalytic technology has received extensive attention [1,2,3]. In general, TiO₂, ZnO, CuS et al. were often chosen as photocatalysts to degrade organic pollutants [4,5,6]. However, many difficulties, such as the large band gap for TiO₂ and ZnO [7,8] and the photocorrosion problem for sulfide [9], should be overcome when using these photocatalysts. Therefore, there is an urgent need to develop other photocatalysts with lower band gaps and stable performance.

Recently, rare earth oxides such as CeO₂ have attracted considerable attention due to their potential photocatalytic activity and their stability during photocatalysis [10,11,12]. Unfortunately, pure CeO₂ has a large band gap of about 3.2 eV, which greatly decreases its absorption of visible light and hence photocatalytic efficiency. Pr₆O₁₁, an n-type semiconductor with a band gap of around 1.77–3.3 eV [13], is very stable among the praseodymium oxide family at ambient temperature and pressure [14]. Consequently, Pr₆O₁₁ was chosen as a photocatalyst to degrade organic pollutants [15,16,17]. In order to reduce the band gap, carbon materials were introduced into Pr₆O₁₁ to prepare the Pr₆O₁₁@C composite. For example, Shende et al. synthesized a new Pr₆O₁₁/g-C₃N₄ heterostructure material by a single-step solvent-free solid-state method and found that the notable increase in the photocatalytic activity of the Pr₆O₁₁/g-C₃N₄ heterostructure was ascribed to the reduced band gap and hence the improved visible light absorption [13]. However, the synthetic process of g-C₃N₄ is relatively complex, and the interaction between g-C₃N₄ and Pr₆O₁₁ is weak. Can g-C₃N₄ be replaced by other carbon materials during the synthesis of Pr₆O₁₁@C with efficient photocatalytic activity?

Besides C₃N₄, activated carbon, carbon nanotube, grapheme, etc., are usually used as carbon sources to prepare carbon-based photocatalysts [18,19,20]. Nevertheless, the cost of carbon sources and the complex synthesis process of carbon-based catalysts hinder the practical application of these carbon materials. Moreover, these carbon-based Pr₆O₁₁ photocatalysts always react with H⁺ in an acid environment, which makes them unstable and limits their application in practice. Acid dyes containing rich carbon elements are always considered organic pollutants and removed by physical and/or chemical methods [21,22]. However, acid dyes are rarely used as carbon sources to synthesize carbon-based photocatalysts. Recently, it has been interesting to find that Pr³⁺ can flocculate acid red 14 (AR14) in the basic medium, which makes us consider the use of acid dyes such as AR14 as carbon sources for the preparation of Pr₆O₁₁@C. In the flocculation process, Pr³⁺ was precipitated into Pr(OH)₃ by mixing with AR14 uniformly to form Pr(OH)₃@AR14. Calcinated at high temperatures, Pr(OH)₃ and AR14 can be changed into Pr₆O₁₁ and carbon, respectively. The synthetic process of Pr₆O₁₁@C based on this design may be illustrated in Scheme 1. To the best of our knowledge, there are no reports about the synthesis of Pr₆O₁₁@C via this facile construction engineering.

Guided by the above idea, Pr₆O₁₁@C was synthesized using acid dye as a carbon source and Pr³⁺ as a flocculatant. The photocatalytic performance of Pr₆O₁₁@C was measured by the degradation of AR14, and the results show that the photocatalytic efficiency of Pr₆O₁₁@C is higher than that of pure Pr₆O₁₁ due to the uniform mixing of carbon with Pr₆O₁₁ and the efficient carbon bond formation rate. Accordingly, the acid resistance of Pr₆O₁₁ was greatly improved. Furthermore, the effect of different precipitants (NH₃·H₂O and NaOH) on the photocatalytic efficiency was also investigated.

2. Experimental

The experimental part is presented in the Supporting Materials.

3. Results and Discussion

Figure 1 shows the powder X-ray diffraction (XRD) patterns of Pr₆O₁₁ and Pr₆O₁₁@C prepared with different precipitants. The diffraction pattern of the synthesized samples can be ascribed to the cubic Pr₆O₁₁ with the Fm3m space group (JCPDS file No. 00-042-1121) [16]. It is clear from Figure 1 that all samples exhibit obvious peaks corresponding to the (111), (200), (220), (311), (222), (400), (331), (420), and (422) planes, indicating that both the precipitant and the introduction of carbon have no effect on the crystal structure of the prepared samples.

The elemental mapping of O, Pr, and C from EDS is presented in Figure 2, and the results show that C was actually introduced into the resulting sample and that all the composited elements were evenly distributed in the sample. Similar phenomena can be found in other samples with different content of carbon, illustrating that AR14 can be changed into carbon in our experimental process.

The photocatalytic efficiency of Pr₆O₁₁-NH₃·H₂O and Pr₆O₁₁-NaOH was evaluated by the degradation of AR14, which is presented in Figure 3. From Figure 3, it is easy to find that the photocatalytic efficiency of Pr₆O₁₁-NH₃·H₂O is much higher than that of Pr₆O₁₁-NaOH because the characteristic absorption peak intensity of AR14 at 516 nm is very low after degradation over Pr₆O₁₁-NH₃·H₂O. From the results of XRD, the structure of Pr₆O₁₁-NH₃·H₂O and Pr₆O₁₁-NaOH is similar. Why are the photocatalytic results so different?

It is well known that the adsorption capability of organic pollutants over the catalyst plays an important role in photocatalytic efficiency [23]. In order to investigate the intrinsic reason for photocatalytic difference, the adsorption of 0.1 mmol/L (mM) AR14 on Pr₆O₁₁-NH₃·H₂O and Pr₆O₁₁-NaOH was carried out, and the results are shown in Figure 4. Compared with Figure 4a,b, it is obvious that the adsorption efficiency of Pr₆O₁₁-NH₃·H₂O is higher than that of Pr₆O₁₁-NaOH and that 30 min is enough for the adsorption equilibrium of AR14 over Pr₆O₁₁-NH₃·H₂O and Pr₆O₁₁-NaOH. From our previous report, it can be seen that the number of amino groups and hydroxyl groups in the sample prepared using NH₃·H₂O as the precipitant is higher than that of the sample synthesized using NaOH as the precipitant, which results in a higher adsorption efficiency [24].

In order to investigate if there are amino groups and hydroxyl groups in Pr₆O₁₁-NaOH and Pr₆O₁₁-NH₃·H₂O, FT-IR was used, and the results are presented in Figure 5. It can be found from Figure 5 that amino groups and hydroxyl groups actually reside in Pr₆O₁₁-NH₃·H₂O because of the peaks at 1630 cm⁻¹ ascribed to the O-H bending vibration [25] and 1467 cm⁻¹ attributed to the N-H bending vibration mode [26]. The peak at 1630 cm⁻¹ is also detected in Pr₆O₁₁-NaOH, but the intensity is lower than that of Pr₆O₁₁-NH₃·H₂O, implying that the hydroxyl content of Pr₆O₁₁-NH₃·H₂O is higher than that of Pr₆O₁₁-NaOH. According to the relevant literature [27,28,29], the peaks at 1489, 1436, and 1384 cm⁻¹ are due to the stretching vibration of the redundant ${\mathrm{NO}}_3^{-}$ on the surface of the prepared samples. From Figure 5, it can be inferred that the number of ${\mathrm{NO}}_3^{-}$ in Pr₆O₁₁-NaOH is higher than that of Pr₆O₁₁-NH₃·H₂O because two peaks representing ${\mathrm{NO}}_3^{-}$ are detected on a curve, while only one characteristic ${\mathrm{NO}}_3^{-}$ peak is found on curve b. Consequently, the reasons for the higher adsorption efficiency of AR14 over Pr₆O₁₁-NH₃·H₂O can be presented as follows. On one hand, there are lots of hydroxyl and amino groups on the surface of Pr₆O₁₁-NH₃·H₂O, resulting in strong interaction between Pr₆O₁₁-NH₃·H₂O and AR14 due to the electrostatic attraction of sulfo groups (-${\mathrm{SO}}_3^{-}$) and -${\mathrm{OH}}_2^{+}$ and ${\mathrm{NH}}_4^{+}$ in the acid system [24]. On the other hand, less content of ${\mathrm{NO}}_3^{-}$ in Pr₆O₁₁-NH₃·H₂O can decrease the repulsive force between ${\mathrm{NO}}_3^{-}$ and -${\mathrm{SO}}_3^{-}$. Moreover, the existence of ${\mathrm{NO}}_3^{-}$ on the surface of the photocatalyst can decrease the content of radicals, such as hydroxyl radicals, due to the capture of ${\mathrm{NO}}_3^{-}$ for radicals [30,31], which can also explain the lower photocatalytic efficiency of Pr₆O₁₁-NaOH.

The light, especially for visible light, absorption efficiency of photocatalysts is another important factor affecting photocatalytic efficiency. From the diffuse reflectance UV-vis spectra of Pr₆O₁₁-NaOH and Pr₆O₁₁-NH₃·H₂O (Figure 6), it can be seen that the absorption efficiency of visible light over Pr₆O₁₁-NH₃·H₂O is much higher than that of Pr₆O₁₁-NaOH, implying that photogenerated electrons and holes can be efficiently produced and, hence, photocatalytic efficiency will be improved when Pr₆O₁₁-NH₃·H₂O is used as a photocatalyst.

As discussed above, Pr³⁺ and Pr⁴⁺ are always detected in Pr₆O₁₁, and the ratio of Pr⁴⁺ to Pr³⁺ can affect the photocatalytic efficiency of Pr₆O₁₁. The content of Pr⁴⁺ and Pr³⁺ in Pr₆O₁₁-NaOH and Pr₆O₁₁-NH₃·H₂O can be defined by the XPS spectra of Pr 3d, which is shown in Figure 7. The Pr 3d spectra in Figure 7a,b can be deconvoluted into five peaks centered at 933, 953, 948, 928, and 956 eV, respectively. The strong peaks at 933 and 953 eV correspond to Pr³⁺, and the other peaks are ascribed to Pr⁴⁺ [32,33]. According to the peak area of Pr⁴⁺ and Pr³⁺ in Figure 7, it can be concluded that the ratio of Pr⁴⁺ to Pr³⁺ in Pr₆O₁₁-NaOH (0.27) is higher than that of Pr₆O₁₁-NH₃·H₂O (0.25), implying less content of Pr⁴⁺ in Pr₆O₁₁-NH₃·H₂O. Therefore, we can conclude that more Pr⁴⁺ ions on the surface of Pr₆O₁₁-NH₃·H₂O were reduced to Pr³⁺, and thus more oxygen vacancies were formed. According to Gregson et al., trivalent lanthanide ions make it easier to form lanthanide carbon bonds than that of tetravalent ones [34]. Generally, the easy formation of carbon bonds in the photocatalyst is helpful for photocatalytic efficiency [35]. Moreover, oxygen vacancies can improve the separation of photogenerated electrons and holes, resulting in more efficient photocatalytic efficiency [36], which can be proved by the results of photocurrent over Pr₆O₁₁-NaOH and Pr₆O₁₁-NH₃·H₂O (Figure 11b). From the analysis of XPS spectra of Pr 3d, the results in Figure 3 can be further understood.

Besides the structural information given by XPS spectra of Pr 3d, XPS spectra of O 1s can also provide useful messages for the structure of Pr₆O₁₁. From XPS spectra of O 1s for Pr₆O₁₁-NaOH and Pr₆O₁₁-NH₃·H₂O (Figure 8), it can be inferred that the strength of the Pr-O bond in Pr₆O₁₁-NH₃·H₂O is higher than that in Pr₆O₁₁-NaOH because the peak area at about 528.30 and the characteristic binding energy of Pr-O [33] in Pr₆O₁₁-NH₃·H₂O are higher than those of Pr₆O₁₁-NaOH. A stronger Pr-O bond can make Pr₆O₁₁-NH₃·H₂O more stable, which is beneficial for its practical application. Moreover, more Pr-O bonds may be helpful for the transition of photogenerated electrons to the surface of the catalyst to combine the adsorbed oxygen and form a superoxide radical to degrade AR14. The peak at about 531 eV in Figure 8 can be attributed to oxygen species in the defects [37] and adsorbed oxygen [38]. Due to the high intensity, the peak at about 531 eV is mainly assigned to the adsorbed oxygen. It is clear from Figure 8 that the ratio of adsorbed oxygen to binding oxygen in Pr₆O₁₁-NaOH is higher than that in Pr₆O₁₁-NH₃·H₂O. It can be concluded from the literature [39] that the high content of adsorbed oxygen can result in poor activity of the catalyst, which can further prove the lower photocatalytic efficiency of Pr₆O₁₁-NaOH.

From the above discussion, it can be concluded that NH₃·H₂O is suitable for the preparation of Pr₆O₁₁ with enhanced photocatalytic efficiency. Therefore, NH₃·H₂O is designated as a precipitant to synthesize Pr₆O₁₁ and Pr₆O₁₁@C in the following context. The photodegradation of 0.3 mM AR14 over Pr₆O₁₁ and Pr₆O₁₁@C prepared via the route of Scheme 1 is illustrated in Figure 9. It is obvious that the introduction of carbon into Pr₆O₁₁ can improve the photocatalytic efficiency of Pr₆O₁₁ under the same experimental conditions. From the dark reaction, it is easy to find that the adsorption capacity of Pr₆O₁₁@C is higher than that of the pure composite, which is good for the enhancement of photocatalytic efficiency. Furthermore, it can be found from Figure 10a that, compared with Pr₆O₁₁, the UV-vis absorption of Pr₆O₁₁@C is more efficient, which is helpful for the improvement in photocatalytic activity. From Figure 10b, we can find that the intensity of peak at 543 nm, associated with Pr³⁺ and oxygen vacancy (positively charged) [40], of Pr₆O₁₁@C is higher than that of Pr₆O₁₁, implying more efficient photocatalysis of Pr₆O₁₁@C because the photogenerated electrons can be easily captured by the positively charged oxygen vacancies [34,36]. The existence of Pr³⁺ and the oxygen vacancy in Pr₆O₁₁@C is further demonstrated by the XPS spectra of Pr 3d for Pr₆O₁₁@C (Figure 10c) because the ratio of Pr⁴⁺ to Pr³⁺ is 0.22, which is lower than that of Pr₆O₁₁ (0.25). A possible reason for this is that the carbon produced by the pyrolysis of AR14 is active [35] and can reduce Pr⁴⁺ to Pr³⁺, resulting in the formation of oxygen vacancies via the replacement of Pr⁴⁺ by Pr³⁺.

From our previous report, the formation of carbon bonds between carbon and photocatalyst can greatly improve photocatalytic efficiency [35]. The carbon bonding in Pr₆O₁₁@C can be investigated by XPS spectra of C 1s, which is presented in Figure 11a. The C1s spectra in Figure 11a can be deconvoluted into four peaks centered at 284.5, 285.9, 288.1, and 289.3 eV corresponding to C=C, C-OH, O=C and O-C=O bonds, respectively [41]. Consequently, it can be concluded that numerous carbon bonds were formed in Pr₆O₁₁@C, which can enhance the separation of photogenerated electrons and holes. In order to test this inference, a photocurrent experiment was carried out, and the results are shown in Figure 11b. It is obvious from Figure 11b that the photocurrent on Pr₆O₁₁@C is higher than that on Pr₆O₁₁, indicating more efficient photoelectron–hole separation efficiency for Pr₆O₁₁@C. It can be concluded from Figure S1 that AR14 was actually degraded over Pr₆O₁₁@C. From the results of Figure S2, it is obvious that 0.075 mM AR14 is the best concentration to synthesize Pr₆O₁₁@C with efficient photocatalytic efficiency. Moreover, Figure S3 proves that ${\mathrm{O}}_2^{•-}$ and OH• are the main oxidative species responsible for the photodegradation of AR14.

According to the above analysis, it can be concluded that lots of oxygen vacancies and carbon bonds were formed in the Pr₆O₁₁@C, and O₂ can be adsorbed on the surface of Pr₆O₁₁@C. Therefore, under visible light irradiation, the electrons can be excited from VB to CB, intermediate energy levels resulting from oxygen vacancies, and transfer carbon bond and reach the surface of Pr₆O₁₁@C to combine with the adsorbed oxygen to form ${\mathrm{O}}_2^{•-}$. On the other hand, the holes formed after the departure of excited electrons can interact with water to form OH• due to their strong oxidation. ${\mathrm{O}}_2^{•-}$ and OH• degrade AR14 together. Therefore, the possible photocatalytic mechanism of AR14 over Pr₆O₁₁@C can be illustrated by the schematic diagram of the energy band of Pr₆O₁₁@C, as shown in Figure 12.

From the viewpoint of practical application, a stable photocatalyst in a medium with different pH values is welcome. The effect of pH on the degradation of 0.3 mM AR14 over Pr₆O₁₁@C is shown in Figure 13. It is clear from Figure 13 that Pr₆O₁₁@C is stable in both acidic and alkaline media and that the photocatalytic efficiency of Pr₆O₁₁@C in an acid medium is higher than that of an alkaline medium. The reason for this may be that the adsorption ability of acid dyes over photocatalysts is increased in an acid medium [40]. It can be seen from Figure 13 that AR14 is almost degraded in the system with a pH of 5, and the degradation rate of AR14 is not obviously increased when the pH value is decreased to 3. From the experimental results, we found that pure Pr₆O₁₁ can be dissolved in the solution with a pH value of 4 because of an acid-base reaction. However, the prepared Pr₆O₁₁@C in this study is stable in the solution with a pH value of 3. In order to investigate the stability of other carbon-based Pr₆O₁₁ in acid solution, we synthesized Pr₆O₁₁@C using C₃N₄, carbon nanotube, and grapheme as carbon source. The preparation process is similar to Pr₆O₁₁@C prepared using AR14 as a carbon source. The AR 14 solution was replaced by a 100 mL solution containing 0.01 g C₃N₄ or carbon nanotube or grapheme. The results show that these carbon-based Pr₆O₁₁ materials are not stable in the solution with a pH value of 4, indicating that using AR14 as a carbon source is helpful for the stability of Pr₆O₁₁@C in the acid medium and that Pr₆O₁₁@C synthesized via Scheme 1 is a potential photocatalyst. A possible reason for the acid resistance of Pr₆O₁₁@C synthesized in this study is that carbon and Pr₆O₁₁ are mixed highly uniformly with each other, which can be seen from Scheme 1.

From the results of Figure 14a, it can be seen that Pr₆O₁₁@C demonstrates good photocatalytic stability because the sixth photocatalytic efficiency is similar to the first one. Moreover, the Pr³⁺ and Pr⁴⁺ are detected in Pr₆O₁₁@C after six cycles (Figure 14b), which is the same as that of the initial sample.

4. Conclusions

Pr₆O₁₁@C with efficient photocatalytic efficiency and acid resistance was prepared via facile construction engineering. As a precipitant, NH₃·H₂O is more suitable for the preparation of Pr₆O₁₁ with enhanced photocatalytic activity than that of NaOH because more amino groups and hydroxyl groups were detected in the sample synthesized using NH₃·H₂O as a precipitant. Moreover, the absorption intensity of visible light and the ratio of Pr³⁺ in Pr₆O₁₁-NH₃·H₂O are higher than those of Pr₆O₁₁-NaOH. Due to the formation of carbon bonds and oxygen vacancies, Pr₆O₁₁@C prepared using NH₃·H₂O as a precipitant has more efficient photocatalytic activity compared with that of the pure composite. The optimum one is 0.075 mM AR14 for synthesizing Pr₆O₁₁@C. ${\mathrm{O}}_2^{•-}$ and OH• are the main oxidative species responsible for the photodegradation of AR14. The use of AR14 as a carbon source is helpful for the stability of Pr₆O₁₁@C in the acid medium. Pr₆O₁₁@C had good photocatalytic stability, indicating that it has the potential application for the removal of organic pollutants.