Fabrication of Er3+/Yb3+ Co-Doped Bi5O7I Microsphere With Upconversion Luminescence and Enhanced Photocatalytic Activity for Bisphenol A Degradation

Er3+/Yb3+ co-doped Bi5O7I uniform porous microsphere photocatalysts were synthesized by a two-step chemical method, which possesses excellent photocatalytic performance and upconversion luminescence property. The photocatalytic performance of the photocatalysts was studied by degradation of bisphenol A in aqueous solution under visible light and different monochromatic light irradiation. The photocatalytic performance of Er3+/Yb3+ co-doped Bi5O7I sample is better than that of the pristine Bi5O7I and Er3+-doped Bi5O7I samples. Moreover, Er3+/Yb3+ co-doped Bi5O7I possesses photocatalytic ability with a red light monochromatic LED lamp (3 W, λ = 630 nm) and an infrared monochromatic LED lamp (100 W, λ = 940 nm) irradiation whose wavelength is longer than the absorption-limiting wavelength of pristine Bi5O7I sample. This phenomenon further verified that the upconversion property of Er3+ and Yb3+ causes the improved photocatalytic efficiency of Er3+/Yb3+ co-doped Bi5O7I sample.


INTRODUCTION
Upconversion is a particular type of photoluminescence (PL), which converts low-energy excitation light into high-energy emission light through a multiphoton absorption process (Obregón and Colón, 2014a;Chuai et al., 2015;Ma et al., 2015;Fu et al., 2017). For this excellent characteristic, many upconverting materials, such as YF 3 and NaYF 4 , have been used as powerful assistants to combine with semiconductor photocatalysts to improve light utilization recently . For instance, Qin et al. (2010) reported that the graphene-supported NaYF 4 :Yb 3+ , Tm 3+ , and N-doped P25 nanocomposite photocatalysts exhibit outstanding photocatalytic efficiency, because upconverting materials can effectively convert long-wavelength infrared (IR) light into short-wavelength light (such as visible light). The semiconductors in the composite photocatalysts can absorb the converted short-wavelength light to make full use of incident light. However, many up-conversion materials did not have photocatalytic performance because of their large band gap Xu et al., 2013). Therefore, it is important to fabricate single-phase photocatalyst with excellent photocatalytic activity and upconversion property.
GRAPHICAL ABSTRACT | Er 3+ /Yb 3+ co-doped Bi 5 O 7 I porous microsphere photocatalysts show efficiency photocatalytic properties for bisphenol A degradation under visible and near-infrared light irradiation.
Bi 5 O 7 I as a novel semiconductor photocatalytic material with an optical band gap of ∼2.8 eV has received a lot of attention (Zhang et al., 2020a). The lamellar crystallographic structure of Bi 5 O 7 I can form an internal electrostatic field whose direction is vertical to the atom layer. The internal electrostatic field can promote the separation of photo-generated electronhole pairs (Lan et al., 2020). However, the shortcomings of low light absorption and transmission efficiency of carriers still limit its photocatalytic activity. It is well-known that combining Bi 5 O 7 I with other semiconductors to form heterojunction could improve the separation rate of photo-generated charge carriers and show enhanced photocatalytic efficiency for pollutants degradation Zhang et al., 2020b). In addition, our previous report indicated that the doping of Er 3+ into the Bi 5 O 7 I can broaden the photo-response range due to the upconversion effect (Hojamberdiev et al., 2020), but the light conversion is not thorough enough. It can be inferred that the Er 3+ /Yb 3+ codoping would cause more intensive upconversion fluorescence effect (Ding et al., 2016), which can enhance photocatalytic degradation properties for pollutions with full spectral solar light response.
In this work, uniform Er 3+ /Yb 3+ co-doped Bi 5 O 7 I microsphere photocatalysts were prepared by a two-step hydrothermal and thermal-decomposition method. The as-prepared photocatalysts have excellent photocatalytic performance and upconversion luminescence property. From the results of photocatalytic performance tests under the illumination of visible and monochromatic light and trapping experiments, the detailed mechanism of improved photocatalytic activity was also proposed.

EXPERIMENTAL
The synthesis methods of Bi 5 O 7 I and 6%Er 3+ -doped Bi 5 O 7 I samples are detailed in the Supporting Information and the prepared samples were recorded as BOI and 6EBOI, respectively. In addition, the synthesis method of Yb 3+ /Er 3+ co-doped Bi 5 O 7 I samples was similar to that previously reported , except the addition of 2 to 18% Yb(NO 3 ) 3 6H 2 O. The prepared samples were recorded as 2Y6EBOI, 4Y6EBOI, 6Y6EBOI, 12Y6EBOI, and 18YEBOI, respectively. The characterization and photocatalytic test are also described in the Supporting Information.

XRD Analysis
The XRD patterns of 6EBOI and YEBOI samples are shown in Figure 1, and all the prepared samples are crystallized well. As previously reported, the Bi 5 O 7 I synthesized without any doping corresponds with orthorhombic phase Bi 5 O 7 I (JCPDS FIGURE 2 | SEM images of BOI (a,b), 6EBOI (c,d), and 6Y6EBOI (e,f) samples.
Frontiers in Chemistry | www.frontiersin.org 40-0548) (Hojamberdiev et al., 2020). However, the synthesized 6EBOI sample is in accordance with orthorhombic phase Bi 5 O 7 I and monoclinic phase (JCPDS 38-0669) Bi 5 O 7 I. In addition, the phase structure of the samples is completely converted into monoclinic Bi 5 O 7 I after doping with Yb 3+ . Moreover, as the Yb 3+ doping content increases, the width of these peaks broadens and the intensity decreases, especially the (004) peak. This is because the doping of Yb 3+ and Er 3+ limits the growth of Bi 5 O 7 I crystals. According to the previous reports, the existence of Yb 3+ and Er 3+ in the compound can cause phase transition from the orthorhombic phase to the monoclinic phase Obregón and Colón, 2014b;Obregón et al., 2014). Figure 2 displays the scanning electron microscope (SEM) images of the synthesized pure BOI, 6EBOI, and YEBOI samples. As shown in Figure 2a, the BOI sample has a uniform porous spherical morphology with a radius in the range of 1 to 1.5 µm. From Figure 2b, the high-resolution Frontiers in Chemistry | www.frontiersin.org SEM image shows these spheres are stacked by numerous nanosheets. While Figures 2c-f show the SEM images of 6EBOI and 6Y6EBOI samples, respectively. The Yb 3+ /Er 3+ doping has little effect on the morphology, and all the as-prepared samples also have the uniform porous spherical morphology. Energy dispersive spectrometer (EDS) mapping was performed further to analyze the elemental distribution in the 6Y6EBOI sample (Figure 3). It is observed that the

XPS Analysis
In order to investigate the elemental composition, XPS analysis was performed on the 6Y6EBOI sample, and the consequences are presented in Figure 4. The survey spectrum in Figure 4A clearly reveals the compound consists of Bi, O, I, Er, and Yb elements. There are two peaks at ∼164.4 and 158.9 eV (Figure 4B), which are ascribed to Bi 4f 7/2 and Bi 4f 5/2 (Liu et al., 2017), respectively. In Figure 4C, it is observed that the O 1s peak is located at 529.5 and 531.4 eV, which corresponds to FIGURE 5 | UV-vis adsorption spectra of synthesized pure BOI, 6E BOI, and Yb 3+ /6%Er 3+ co-doped Bi 5 O 7 I photocatalysts. the lattice oxygen and surface-adsorbed oxygen in the prepared sample . The peaks located at 619.4 and 630.6 eV ( Figure 4D) correspond with the I 3d 5/2 and I 3d 3/2 . It is also seen that the Er 4p ( Figure 4E) and Yb 4p ( Figure 4F) peaks are located at 321.1 and 346.5 eV, which corresponds with the Er 3+ and Yb 3+ (Hou et al., 2012;Reszczynska et al., 2015), respectively. Thus, the XPS results indicate that the Er 3+ and Yb 3+ were triumphantly doped into the Bi 5 O 7 I sample.

Ultraviolet-vis DRS Analysis
The ultraviolet-visible (UV-vis) absorption spectra of the prepared samples are depicted in Figure 5. The adsorption edge of pure BOI is shorter than 450 nm, indicating that pristine BOI could be excited by the ultraviolet light and a small fraction of visible light. Compared with pure BOI, the visible light absorption of Yb 3+ /Er 3+ -doped BOI samples undergoes a significant redshift. It can be seen that three peaks are located at 522, 655, and 797 nm for 6EBOI. This is attributed to the upconversion effect from the 4 I 15/2 ground state to 2 H 2/11 , 4 F 9/2 , and 4 I 9/2 states of Er 3+ (Rodríguez et al., 2013;Xu et al., 2014). An exception absorption peak at nearly 950 nm for the Yb 3+doped 6EBOI sample is also clearly observed, which is attributed to the upconversion conversion from the 2 F 5/2 ground state to 2 F 7/2 states of Yb 3+ (Wang et al., 2014).
In order to know the cause of these new peaks in the visible and near IR (NIR) light range, the upconversion spectra of YEBOI samples were carried out. Figure 6 exhibits the UC emission spectra (350-800 nm) of YEBOI samples. It shows that there are two green emission bands near 533 and 547 nm, and a red emission band near 654 nm after excitation by an NIR laser (λ = 980 nm). The former between 515 to 538 nm and 540 to 560 nm are ascribed to the 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 transitions (Zhang et al., 2005;Sun et al., 2011;Mahalingam et al., 2013). The latter between 640 and 680 nm corresponds with the transition of 4 F 9/2 to 4 I 15/2 . It is very clear that the intensity of the green and red emission bands increases over Yb 3+ -doped 6EBOI sample. Therefore, the observation results indicate that the new absorption bands appearing in the UVvis DRS spectra are caused by the upconversion radiation of the YEBOI system Bai et al., 2014;Zhou et al., 2015). It is well-known that the lifetime of the upconversion materials exhibits a positive correlation with the upconversion quantum yield (Dai et al., 2013). Thus, the luminescence decay curves of the as-synthesized 6EBOI and 6Y6EBOI are also compared under the excitation light with 650 nm wavelength (Figure 7). The decay curves of 6EBOI and 6Y6EBOI are 184 and 376 µs, respectively. Hence, the lifetime is significantly prolonged after Yb 3+ dopping compared with the 6EBOI sample. It is concluded that the tendency of lifetime variation is consistent with that of upconversion intensity variation.
Apparently, the photocatalytic activity was greatly improved after the doping of Yb 3+ and Er 3+ with Bi 5 O 7 I under visible light irradiation. It is more interesting that the 6Y6EBOI also possesses the best photocatalytic activity under NIR light (940 nm LED light) irradiation. For comparison, BOI and 6EBOI were also used as reference photocatalysts under the same experimental condition. As exhibited in Figures 10A,B, the photodegradation efficiencies of BOI, 6EBOI, and 6Y6EBOI samples are 0.3, 1.8, and 9.4%, respectively. The characteristic peak of BPA does not show any change even when the irradiation time reached 60 min over the BOI sample ( Figure 10C). However, it has an obvious decrease of the peak intensity at 277 nm of BPA with the addition of 6E6YBOI sample as shown in Figure 10D. From the above photocatalytic results, the photocatalytic activity of the 6E6YBOI photocatalyst has excellent photocatalytic performance under visible light and NIR light irradiation.

PL Spectra and I-V Analysis
The transient photocurrent (I-V) and PL are effective tests in displaying the separation ability of photo-generated carriers in photocatalytic research (Chang et al., 2019;Li et al., 2020a). The responses of I-V for BOI, 6EBOI, and 6Y6EBOI were also recorded under visible light irradiation. As shown in Figure 11A, the intensity of photocurrent signal of 6E6FBOI is much stronger than the pristine BOI and 6EBOI, which   suggests the best excellent effective transfer ability of photoinduced charge carriers. The PL spectra were also carried out to probe the recombination of photo-generated charge carriers (Li et al., 2020b;Nie et al., 2020). Compared with BOI and 6EBOI samples, the lowest intensity of 6Y6EBOI suggests that it possesses the lowest recombination rate of photo-generated charge carriers, which is beneficial to improve the photocatalytic activity ( Figure 11B). According to above results, the Er 3+ and Yb 3+ doping into Bi 5 O 7 I samples shows enhancing photocatalytic degradation activities for BPA. Figure 12 illustrates the photocatalytic reaction mechanism of the Yb 3+ /Er 3+ -doped Bi 5 O 7 I photocatalyst. It can be seen that the Yb 3+ /Er 3+ -doped Bi 5 O 7 I sample could absorb low-energy IR light, and then the electrons would be excited from the level of 2 F 7/2 to 2 F 5/2 . Then, the excited electrons would be transferred back to the ground state of 2 F 7/2 , and the energy released in this process is mainly transferred to the active Er 3+ in a non-radiative manner, leading to a population of Er 3+ from 4 I 15/2 to 4 I 11/2 (Wu et al., 2013). Next, a second or more similar photons from excited Yb 3+ may convert to higher 4 F 9/2 , 4 F 7/2 , and 2 I 9/2 energetic levels of Er 3+ . Then, some of the excited electrons will relax non-radiatively to the energy levels of 2 H 11/2 , 4 S 3/2 , 4 F 9/2 etc. energy levels through a fast multiphonon decay process (Lei et al., 2015), leading to a stronger green ( 2 H 11/2 , 4 S 3/2 -4 I 15/2 ) and red emission ( 4 F 9/2 -4 I 15/2 ), especially the latter. Therefore, the improvement in photocatalytic efficiency of the YEBOI samples could be elaborated more clearly in three factors. First, the Yb 3+ /Er 3+ doping in the photocatalyst can cause significant redshift with the absorption of visible light, which would excite more electron-hole pairs. Second, the upconversion process in Yb 3+ /Er 3+ -doped Bi 5 O 7 I sample will take place, and it will produce electron-hole pairs under low-energy IR light irradiation. In this process, the photoactivity of Yb 3+ /Er 3+doped Bi 5 O 7 I sample is evidently enhanced. Third, the Yb 3+ and Er 3+ in the Bi 5 O 7 I would promote the separation of e − /h + pairs, so more photo-induced charge carriers would migrate to the sample surface for photocatalytic reaction .

CONCLUSIONS
In this work, the Yb 3+ /Er 3+ -doped Bi 5 O 7 I microsphere photocatalysts were prepared through combining hydrothermal and heat-treatment method. The Yb 3+ /Er 3+ -doped Bi 5 O 7 I photocatalysts have excellent photocatalytic for BPA under visible light irradiation and upconversion luminescence properties. It is expected that the synthetic method and properties of this catalyst will offer some inspiration and help for the future researchers to improve similar photocatalytic and upconversion luminescence materials.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

AUTHOR CONTRIBUTIONS
GZ designed the project. BC, SG, and LL performed the experiments. BC, SG, and RD performed the data analysis. GZ, YX, and LG contributed to the theoretical analysis. BC, SG, and SZ wrote the paper. All authors contributed to the general discussion, contributed to the article, and approved the submitted version.