Facile Construction Engineering of Pr6O11@C with Efficient Photocatalytic Activity

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.


Introduction
Due to the complete degradation of organic pollutants, photocatalytic technology has received extensive attention [1][2][3].In general, TiO 2 , 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 TiO 2 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 CeO 2 have attracted considerable attention due to their potential photocatalytic activity and their stability during photocatalysis [10][11][12].Unfortunately, pure CeO 2 has a large band gap of about 3.2 eV, which greatly decreases its absorption of visible light and hence photocatalytic efficiency.Pr 6 O 11 , an n-type semiconductor with a band gap of around 1.77-3.3eV [13], is very stable among the praseodymium oxide family at ambient temperature and pressure [14].Consequently, Pr 6 O 11 was chosen as a photocatalyst to degrade organic pollutants [15][16][17].In order to reduce the band gap, carbon materials were introduced into Pr 6 O 11 to prepare the Pr 6 O 11 @C composite.For example, Shende et al. synthesized a new Pr 6 O 11 /g-C 3 N 4 heterostructure material by a single-step solvent-free solid-state method and found that the notable increase in the photocatalytic activity of the Pr 6 O 11 /g-C 3 N 4 heterostructure was ascribed to the reduced band gap and hence the improved visible light absorption [13].However, the synthetic process of g-C 3 N 4 is relatively complex, and the interaction between g-C 3 N 4 and Pr 6 O 11 is weak.Can g-C 3 N 4 be replaced by other carbon materials during the synthesis of Pr 6 O 11 @C with efficient photocatalytic activity?Besides C 3 N 4 , 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 Pr 6 O 11 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 Pr 3+ 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 Pr 6 O 11 @C.In the flocculation process, Pr 3+ 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 Pr 6 O 11 and carbon, respectively.The synthetic process of Pr 6 O 11 @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 Pr 6 O 11 @C via this facile construction engineering.
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 Pr 3+ 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, Pr 3+ 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.Scheme 1.The scheme for the synthesis of Pr6O11@C.
Guided by the above idea, Pr6O11@C was synthesized using acid dye as a carbon source and Pr 3+ 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.

Experimental
The experimental part is presented in the Supporting Materials.
Guided by the above idea, Pr 6 O 11 @C was synthesized using acid dye as a carbon source and Pr 3+ as a flocculatant.The photocatalytic performance of Pr 6 O 11 @C was measured by the degradation of AR14, and the results show that the photocatalytic efficiency of Pr 6 O 11 @C is higher than that of pure Pr 6 O 11 due to the uniform mixing of carbon with Pr 6 O 11 and the efficient carbon bond formation rate.Accordingly, the acid resistance of Pr 6 O 11 was greatly improved.Furthermore, the effect of different precipitants (NH 3 •H 2 O and NaOH) on the photocatalytic efficiency was also investigated.

Experimental
The experimental part is presented in the Supporting Materials.

Results and Discussion
Figure 1 shows the powder X-ray diffraction (XRD) patterns of Pr 6 O 11 and Pr 6 O 11 @C prepared with different precipitants.The diffraction pattern of the synthesized samples can be ascribed to the cubic Pr 6 O 11 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.
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].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.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. From our previous report, it can be seen that the number of amino groups and hydroxyl groups in the sample prepared using NH 3 •H 2 O 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 3 ) and -OH + 2 and NH + 4 in the acid system [24].On the other hand, less content of NO − 3 in Pr 6 O 11 -NH 3 •H 2 O can decrease the repulsive force between NO − 3 and -SO − 3 .Moreover, the existence of NO − 3 on the surface of the photocatalyst can decrease the content of radicals, such as hydroxyl radicals, due to the capture of NO − 3 for radicals [30,31], which can also explain the lower photocatalytic efficiency of Pr 6 O 11 -NaOH.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 NO  3 on the surface of the prepared samples.From Figure 5, it can be inferred that the number of NO 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 UVvis 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.The light, especially for visible light, absorption efficiency of photocatalysts is another important factor affecting photocatalytic efficiency.From the diffuse reflectance UVvis 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, Pr 3+ and Pr 4+ are always detected in Pr6O11, and the ratio of Pr 4+ to Pr 3+ can affect the photocatalytic efficiency of Pr6O11.The content of Pr 4+ and Pr 3+ in Pr6O11-NaOH , 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 Pr 6 O 11 -NaOH and Pr 6 O 11 -NH 3 •H 2 O (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 Pr 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 Pr 6 O 11 -NaOH is higher than that in Pr 6 O 11 -NH 3 •H 2 O.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 Pr 6 O 11 -NaOH.
Pr6O11-NH3•H2O (0.25), implying less content of Pr 4+ in Pr6O11-NH3•H2O.Therefore, we can conclude that more Pr 4+ ions on the surface of Pr6O11-NH3•H2O were reduced to Pr 3+ , 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 From the dark reaction, it is easy to find that the adsorption capacity of Pr 6 O 11 @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 Pr 6 O 11 , the UV-vis absorption of Pr 6 O 11 @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 Pr 3+ and oxygen vacancy (positively charged) [40], of Pr 6 O 11 @C is higher than that of Pr 6 O 11 , implying more efficient photocatalysis of Pr 6 O 11 @C because the photogenerated electrons can be easily captured by the positively charged oxygen vacancies [34,36].The existence of Pr 3+ and the oxygen vacancy in Pr 6 O 11 @C is further demonstrated by the XPS spectra of Pr 3d for Pr 6 O 11 @C (Figure 10c) because the ratio of Pr 4+ to Pr 3+ is 0.22, which is lower than that of Pr 6 O 11 (0.25).A possible reason for this is that the carbon produced by the pyrolysis of AR14 is active [35] and can reduce Pr 4+ to Pr 3+ , resulting in the formation of oxygen vacancies via the replacement of Pr 4+ by Pr 3+ .
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 Pr 3+ 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 vacan- Molecules 2024, 29, x FOR PEER REVIEW 9 of 14 cies [34,36].The existence of Pr 3+ 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 Pr 4+ to Pr 3+ 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 Pr 4+ to Pr 3+ , resulting in the formation of oxygen vacancies via the replacement of Pr 4+ by Pr 3+ .From our previous report, the formation of carbon bonds between carbon and photocatalyst can greatly improve photocatalytic efficiency [35].The carbon bonding in Pr 6 O 11 @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 Pr 6 O 11 @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 Pr 6 O 11 @C is higher than that on Pr 6 O 11 , indicating more efficient photoelectron-hole separation efficiency for Pr 6 O 11 @C.It can be concluded from Figure S1 that AR14 was actually degraded over Pr 6 O 11 @C.From the results of Figure S2, it is obvious that 0.075 mM AR14 is the best concentration to synthesize Pr 6 O 11 @C with efficient photocatalytic efficiency.Moreover, Figure S3 proves that O •− 2 and OH• are the main oxidative species responsible for the photodegradation of AR14.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   Molecules 2024, 29, x FOR PEER REVIEW 1 [41].Consequently, it can be concluded that numerous carbon bonds were form Pr6O11@C, which can enhance the separation of photogenerated electrons and hol order to test this inference, a photocurrent experiment was carried out, and the resul shown in Figure 11b.It is obvious from Figure 11b that the photocurrent on Pr6O11 higher than that on Pr6O11, indicating more efficient photoelectron-hole separation ciency for Pr6O11@C.It can be concluded from Figure S1 that AR14 was actually degr over Pr6O11@C.From the results of Figure S2, it is obvious that 0.075 mM AR14 is the concentration to synthesize Pr6O11@C with efficient photocatalytic efficiency.More  According to the above analysis, it can be concluded that lots of oxygen vaca and carbon bonds were formed in the Pr6O11@C, and O2 can be adsorbed on the surfa Pr6O11@C.Therefore, under visible light irradiation, the electrons can be excited from to CB, intermediate energy levels resulting from oxygen vacancies, and transfer ca bond and reach the surface of Pr6O11@C to combine with the adsorbed oxygen to fo  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 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.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 Pr 6 O 11 @C is shown in Figure 13.It is clear from Figure 13 that Pr 6 O 11 @C is stable in both acidic and alkaline media and that the photocatalytic efficiency of Pr 6 O 11 @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 Pr 6 O 11 can be dissolved in the solution with a pH value of 4 because of an acid-base reaction.However, the prepared Pr 6 O 11 @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 Pr 6 O 11 in acid solution, we synthesized Pr 6 O 11 @C using C 3 N 4 , carbon nanotube, and grapheme as carbon source.The preparation process is similar to Pr 6 O 11 @C prepared using AR14 as a carbon source.The AR 14 solution was replaced by a 100 mL solution containing 0.01 g C 3 N 4 or carbon nanotube or grapheme.The results show that these carbon-based Pr 6 O 11 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 Pr 6 O 11 @C in the acid medium and that Pr 6 O 11 @C synthesized via Scheme 1 is a potential photocatalyst.A possible reason for the acid resistance of Pr 6 O 11 @C synthesized in this study is that carbon and Pr 6 O 11 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 Pr 6 O 11 @C demonstrates good photocatalytic stability because the sixth photocatalytic efficiency is similar to the first one.Moreover, the Pr 3+ and Pr 4+ are detected in Pr 6 O 11 @C after six cycles (Figure 14b), which is the same as that of the initial sample.
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 Pr 3+ and Pr 4+ are detected in Pr6O11@C after six cycles (Figure 14b), which is the same as that of the initial sample.

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 Pr 3+ 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.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.

Figure 1 .
Figure 1.XRD patterns of Pr6O11 and Pr6O11@C prepared with different precipitant.Figure 1. XRD patterns of Pr 6 O 11 and Pr 6 O 11 @C prepared with different precipitant.

Figure 1 .
Figure 1.XRD patterns of Pr6O11 and Pr6O11@C prepared with different precipitant.Figure 1. XRD patterns of Pr 6 O 11 and Pr 6 O 11 @C prepared with different precipitant.

Figure 2 .
Figure 2. Elemental mapping of (a) O, (b) Pr, (c) C and SEM image (d) of Pr6O11@C prepared using 100 mL of AR14 with a concentration of 0.075 mM as carbon source.Figure 2. Elemental mapping of (a) O, (b) Pr, (c) C and SEM image (d) of Pr 6 O 11 @C prepared using 100 mL of AR14 with a concentration of 0.075 mM as carbon source.

Figure 2 .
Figure 2. Elemental mapping of (a) O, (b) Pr, (c) C and SEM image (d) of Pr6O11@C prepared using 100 mL of AR14 with a concentration of 0.075 mM as carbon source.Figure 2. Elemental mapping of (a) O, (b) Pr, (c) C and SEM image (d) of Pr 6 O 11 @C prepared using 100 mL of AR14 with a concentration of 0.075 mM as carbon source.

Figure 2 .
Figure 2. Elemental mapping of (a) O, (b) Pr, (c) C and SEM image (d) of Pr6O11@C prepared using 100 mL of AR14 with a concentration of 0.075 mM as carbon source.
1 mmol/L (mM) AR14 on Pr 6 O 11 -NH 3 •H 2 O and Pr 6 O 11 -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 Pr 6 O 11 -NH 3 •H 2 O is higher than that of Pr 6 O 11 -NaOH and that 30 min is enough for the adsorption equilibrium of AR14 over Pr 6 O 11 -NH 3 •H 2 O and Pr 6 O 11 -NaOH.
Pr 6 O 11 -NaOH and Pr 6 O 11 -NH 3 •H 2 O, 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 Pr 6 O 11 -NH 3 •H 2 O 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 Pr 6 O 11 -NaOH, but the intensity is lower than that of Pr 6 O 11 -NH 3 •H 2 O, implying that the hydroxyl content of Pr 6 O 11 -NH 3 •H 2 O is higher than that of Pr 6 O 11 -NaOH.According to the relevant literature [27-29], the peaks at 1489, 1436, and 1384 cm −1 are due to the stretching vibration of the redundant NO − 3 on the surface of the prepared samples.From Figure 5, it can be inferred that the number of NO − 3 in Pr 6 O 11 -NaOH is higher than that of Pr 6 O 11 -NH 3 •H 2 O because two peaks representing NO − 3 are detected on a curve, while only one characteristic NO − 3 peak is found on curve b.Consequently, the reasons for the higher adsorption efficiency of AR14 over Pr 6 O 11 -NH 3 •H 2 O can be presented as follows.On one hand, there are lots of hydroxyl and amino groups on the surface of Pr 6 O 11 -NH 3 •H 2 O, resulting in strong interaction between Pr 6 O 11 -NH 3 •H 2 O and AR14 due to the electrostatic attraction of sulfo groups (-SO − Molecules 2024, 29, x FOR PEER REVIEW 5 of 14

 3
in Pr6O11-NaOH is higher than that of Pr6O11-NH3•H2O because two peaks representing NO  3 are detected on a curve, while only one characteristic 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 (-SO  3 ) and -OH  2 and NH  4 in the acid system [24].On the other hand, less content of NO  3 in Pr6O11-NH3•H2O can decrease the repulsive force between NO  3 and -SO  3 .Moreover, the existence of NO  3 on the surface of the photocatalyst can decrease the content of radicals, such as hydroxyl radicals, due to the capture of NO 3

Figure 5 .
Figure 5. FTIR spectra of Pr 6 O 11 -NaOH (a) and Pr 6 O 11 -NH 3 •H 2 O (b).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 Pr 6 O 11 -NaOH and Pr 6 O 11 -NH 3 •H 2 O (Figure 6), it can be seen that the absorption efficiency of visible light over Pr 6 O 11 -NH 3 •H 2 O is much higher than that of Pr 6 O 11 -NaOH, implying that photogenerated electrons and holes can be efficiently pro-

Figure 6 .
Figure 6.Diffuse reflectance UV-vis spectra of Pr 6 O 11 -NaOH and Pr 6 O 11 -NH 3 •H 2 O.As discussed above, Pr3+ and Pr 4+ are always detected in Pr 6 O 11 , and the ratio of Pr4+  to Pr 3+ can affect the photocatalytic efficiency of Pr 6 O 11 .The content of Pr 4+ and Pr 3+ in Pr 6 O 11 -NaOH and Pr 6 O 11 -NH 3 •H 2 O can be defined by the XPS spectra of Pr 3d, which is shown in Figure7.The Pr 3d spectra in Figure7a,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 Pr 3+ , and the other peaks are ascribed to Pr 4+[32,33].According to the peak area of Pr 4+ and Pr 3+ in Figure7, it can be concluded that the ratio of Pr 4+ to Pr 3+ in Pr 6 O 11 -NaOH (0.27) is higher than that of Pr 6 O 11 -NH 3 •H 2 O (0.25), implying less content of Pr 4+ in Pr 6 O 11 -NH 3 •H 2 O. Therefore, we can conclude that more Pr 4+ ions on the surface of Pr 6 O 11 -NH 3 •H 2 O were reduced to Pr 3+ , 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 Pr 6 O 11 -NaOH and Pr 6 O 11 -NH 3 •H 2 O (Figure11b).From the analysis of XPS spectra of Pr 3d, the results in Figure3can 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 Pr 6 O 11 .From XPS spectra of O 1s for Pr 6 O 11 -NaOH and Pr 6 O 11 -NH 3 •H 2 O (Figure 8), it can be inferred that the strength of the Pr-O bond in Pr 6 O 11 -NH 3 •H 2 O is higher than that in Pr 6 O 11 -NaOH because the peak area at about 528.30 and the characteristic binding energy of Pr-O [33] in Pr 6 O 11 -NH 3 •H 2 O are higher than those of Pr 6 O 11 -NaOH.A stronger Pr-O bond can make Pr 6 O 11 -NH 3 •H 2 Omore 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 Figure8can 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 Figure8that the ratio of adsorbed oxygen to binding oxygen in Pr 6 O 11 -NaOH is higher than that in Pr 6 O 11 -NH 3 •H 2 O.It can be concluded from 6 O 11 .From XPS spectra of O 1s for Pr 6 O 11 -NaOH and Pr 6 O 11 -NH 3 •H 2 O (Figure 8), it can be inferred that the strength of the Pr-O bond in Pr 6 O 11 -NH 3 •H 2 O is higher than that in Pr 6 O 11 -NaOH because the peak area at about 528.30 and the characteristic binding energy of Pr-O [33] in Pr 6 O 11 -NH 3 •H 2 O are higher than those of Pr 6 O 11 -NaOH.A stronger Pr-O bond can make Pr 6 O 11 -NH 3 •H 2 O

Figure 7 .
Figure 7. XPS spectra of Pr 3d for Pr 6 O 11 -NaOH (a) and Pr 6 O 11 -NH 3 •H 2 O (b).From the above discussion, it can be concluded that NH 3 •H 2 O is suitable for the preparation of Pr 6 O 11 with enhanced photocatalytic efficiency.Therefore, NH 3 •H 2 O is designated as a precipitant to synthesize Pr 6 O 11 and Pr 6 O 11 @C in the following context.The photodegradation of 0.3 mM AR14 over Pr 6 O 11 and Pr 6 O 11 @C prepared via the route of Scheme 1 is illustrated in Figure 9.It is obvious that the introduction of carbon into Pr 6 O 11 can improve the photocatalytic efficiency of Pr 6 O 11 under the same experimental conditions.From the dark reaction, it is easy to find that the adsorption capacity of Pr 6 O 11 @C is higher than that of the pure composite, which is good for the enhancement of photocatalytic efficiency.Furthermore, it can be found from Figure10athat, compared with Pr 6 O 11 , the UV-vis absorption of Pr 6 O 11 @C is more efficient, which is helpful for the improvement in photocatalytic activity.From Figure10b, we can find that the intensity of peak at 543 nm, associated with Pr 3+ and oxygen vacancy (positively charged)[40], of Pr 6 O 11 @C is higher than that of Pr 6 O 11 , implying more efficient photocatalysis of Pr 6 O 11 @C because the photogenerated electrons can be easily captured by the positively charged oxygen vacancies[34,36].The existence of Pr 3+ and the oxygen vacancy in Pr 6 O 11 @C is further demonstrated by the XPS spectra of Pr 3d for Pr 6 O 11 @C (Figure10c) because the ratio of Pr 4+ to Pr 3+ is 0.22, which is lower than that of Pr 6 O 11 (0.25).A possible reason for this is that the carbon produced by the pyrolysis of AR14 is active[35] and can reduce Pr 4+ to Pr 3+ , resulting in the formation of oxygen vacancies via the replacement of Pr 4+ by Pr 3+ .

Figure 9 .
Figure 9.The photodegradation of 0.3 mM AR14 in the presence of Pr6O11 and Pr6O11@C under visible light irradiation.

Figure 9 .
Figure 9.The photodegradation of 0.3 mM AR14 in the presence of Pr 6 O 11 and Pr 6 O 11 @C under visible light irradiation.

Figure 9 .
Figure 9.The photodegradation of 0.3 mM AR14 in the presence of Pr6O11 and Pr6O11@C under visible light irradiation.
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

Figure S3 proves that O   2
Figure S3 proves that O   2 and OH• are the main oxidative species responsible for the todegradation of AR14.

  2 .
On the other hand, the holes formed after the departure of excited electrons can int with water to form OH• due to their strong oxidation.O   2 and OH• degrade AR1 gether.Therefore, the possible photocatalytic mechanism of AR14 over Pr6O11@C ca illustrated by the schematic diagram of the energy band of Pr6O11@C, as shown in F

Figure 11 .
Figure 11.XPS spectra of C 1s for Pr 6 O 11 @C (a), and photocurrent-time profiles of Pr 6 O 11 -NaOH, Pr 6 O 11 -NH 3 •H 2 O, and Pr 6 O 11 @C-NH 3 •H 2 O (b).According to the above analysis, it can be concluded that lots of oxygen vacancies and carbon bonds were formed in the Pr 6 O 11 @C, and O 2 can be adsorbed on the surface of Pr 6 O 11 @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 Pr 6 O 11 @C to combine with the adsorbed oxygen to form O •− 2 .On

Figure 13 .
Figure 13.Effect of pH on the degradation of 0.3 mM AR14 over Pr6O11@C for 180 min.

Figure 13 . 14 Figure 14 .
Figure 13.Effect of pH on the degradation of 0.3 mM AR14 over Pr 6 O 11 @C for 180 min.