Synthesis and Analysis of In₂CdO₄/Y₂SmSbO₇ Nanocomposite for the Photocatalytic Degradation of Rhodamine B within Dye Wastewater under Visible Light Irradiation

Abstract

A new photocatalyst In₂CdO₄ was prepared by a solid phase sintering synthesis method at high temperature for the first time in this paper. The In₂CdO₄/Y₂SmSbO₇ heterojunction (IYH) catalyst was prepared by the solvent thermal method for the first time. The Y₂SmSbO₇ compound crystallized in the pyrochlore-type architecture and cubelike crystal system, and the space group of Y₂SmSbO₇ was Fd3m and the crystal cell parameters of Y₂SmSbO₇ was 9.51349 Å. The band gap width of Y₂SmSbO₇ was 2.63 eV. In₂CdO₄ crystallized with a body centered tetragonal lattice structure which was a tetragonal crystal system with a space group of I41/amd. The band gap width of In₂CdO₄ was 2.70 eV. After 110 minutes of visible light irradiation (VLGI-110min) with IYH as the photocatalyst, the removal rate (RR) of rhodamine B (RhB) concentration was 100% and the total organic carbon (TOC) concentration RR was 99.71%. The power mechanics invariable k toward RhB consistency and visible light irradiation (VLGI) time with IYH as the photocatalyzer reached 0.03073 min⁻¹. The power mechanics invariable k which was involved with TOC reached 0.03621 min⁻¹. After VLGI-110min, the RR of RhB with IYH as the photocatalyzer was 1.094, 1.174 or 1.740 times higher than that with In₂CdO₄, Y₂SmSbO₇ or N-doping TiO₂ (N-TO) as the photocatalyzer, respectively. The results showed that the photocatalytic activity of IYH was the highest compared with In₂CdO₄, Y₂SmSbO₇, or N-TO. With appending a trapping agent, the oxidative capability for degrading RhB, which ranged from strong to weak among three oxidative radical groups, was as follows: hydroxyl radicals > superoxide anion > holes. This work provided a scientific basis for the research which resulted in prosperous development of efficient heterojunction compound catalysts.

1. Introduction

Industrial sewage from fabric and yarn dyeing was one of the major environmental polluters and could not be efficaciously eliminated after the conventional treatments. During the past decades, global concerns about the discharged wastewater from textile industries which usually contained a large amount of various dyes, had been intensified. Dyes and pigments were widely used in various industries such as the textile, leather, ceramic, cosmetic and food processing industries. The majority of these dyes were chemically stable, less adapted to biological treatment and were toxic to the environment due to their resistance to aerobic degradation and formation of carcinogenic aromatic amines during anaerobic degradation. Therefore, decomposition and removal of these pollutants from wastewater were necessary steps before the wastewater was discharged into aquatic ecosystems [1]. However, most textile dyes were photolytically stable and refractory towards chemical oxidation [2,3] and these characteristics rendered them resistant towards decomposition by conventional biochemical and physicochemical methods, such as adsorption on activated carbon, coagulation by a chemical agent or reverse osmosis which was applied to such effluents [4,5]. Rhodamine B (RhB), one of the most common xanthene dyes, has been universally used in many commercial manufacture processes and biotechnology applications, for instance in paper dyeing, dye laser production, fluorescence microscopy, flow cytometry and ELISA [6,7]. However, in recent years, RhB was not permitted to be used in foods and toiletries because it had been found to be potentially toxic and carcinogenic [8,9]. Therefore, an efficient method for degrading RhB needed to be urgently proposed. Chemical oxidation processes such as ozonation was effective for the decolorization of organic dyes. However, the use of such oxidants was limited due to the formation of possibly toxic non-degradable by-products. Therefore, beside the decolorization of polluted water, it was important to mineralize the by-products which could be generated during the treatment process because some of those by-products might be more toxic and carcinogenic than the initial pollutant. Hence, it was necessary that the mineralization rate of toxic organic pollutants in wastewater ought to reach the maximum standard for preventing the untreated wastewater from being discharged into aquatic ecosystems [1].

Recently, in order to overcome these limitations and to meet the need for environmentally friendly methods that would not produce residual toxic by-products, advanced oxidation processes (AOPs) have been widely considered. AOPs were recognized as a powerful and effective method for the degradation and mineralization of a wide range of recalcitrant organic contaminants which could not be eliminated by conventional treatment techniques in wastewater. These methods involved the in situ generation of active, unstable and non-selective oxidizing species like hydroxyl radicals (•OH) that could oxidize most of the durable organic contaminants in polluted water [1]. Among the rest, photodegradation by a photocatalyst was considered to be a promising technology because of the low cost and high environmental benefits. Photocatalysis was well reported in the literature for organic pollutant degradation in water environments under visible light irradiation and UV light irradiation. Aziz et al. (2019) used a photocatalytic down membrane reactor to study the degradation pathway of the pharmaceutical drugs diclofenac and ibuprofen in aqueous solution. The experimental results showed that the low mineralization rate of both drugs went through photocatalytic reactions with low-chain carboxylic acids such as formic acid, acetic acid, oxalic acid, acetic acid, malic acid and succinate which were regarded as the main by-products. Thus, the degradation pathways of diclofenac and ibuprofen were proposed [10]. In our work, we also found that the organic dyes such as RhB in the wastewater could also be effectively degraded by the use of a photocatalyst.

Light-activated semiconductor-based photocatalysis represents the most promising green and eco-friendly technique for wastewater treatment [11,12]. It has received a lot of attention due to its high efficiency, low-cost and outstanding stability [10]. Electrons (e⁻) and holes (h⁺) which migrated to the surface of the photocatalyst [13,14,15,16,17,18] were generated under light irradiation. As a result, redox reactions were induced and the reactive free radical groups such as superoxide radical (O²⁻), singlet oxygen (¹O₂), hydroxyl radical (OH•), hydroperoxyl radical (HOO•) and hydroperoxide (H₂O₂) were formed.

Reactive oxygen species with high oxidative and reductive activity could easily degrade organic pollutant molecules and inorganic pollutant molecules. Although many significant advances have been made in photocatalysis, two major bottlenecks have limited the practical applications of pristine photocatalysts under solar light. First, several pure photocatalysts with wide band gaps could only be activated using ultraviolet light [19,20,21,22,23,24,25], which accounts for less than 5% of the sunlight spectrum. Second, photoproduced e⁻ and photoproduced h⁺ pairs which derived from a single semiconductor photocatalyst were generally easily recombined and, as a result, the light quantum efficiencies and the photocatalytic efficiencies were reduced [25,26,27,28]. For the purpose of improving the catalytic activity of the photocatalysts, element doping [29,30,31,32,33], semiconductor coupling, and dye sensitization [33,34,35,36,37] were investigated. Semiconductor coupling is one of the most popular research hotspots for photocatalysis since it allowed for the introduction of heterojunction systems which could effectively improve the separation of e⁻ and h⁺ [38,39,40,41,42,43,44].

In a previously published report, graphitic carbon nitride (g-C₃N₄), an organic layered with a two-dimensional metal-free catalyst, is constituted by a π-conjugated system. As a metal-free photocatalyst, the narrow bandgap energy (Eg) of g-C₃N₄ (~2.7 eV) resulted in a strong response to the visible light wavelength of 460 nm. However, low photocatalytic efficiency occurred in the single-phase g-C₃N₄ because of its high recombination rate of electron–hole (e⁻-h⁺) pairs, lack of surface redox-active sites, and disordered structure/defects. Therefor Saravanakumar et al. (2021) have rationally obtained a new ternary-symmetrical dual-z-type LaFeO₃/g-C₃N₄/BiFeO₃ (LCB) heterojunction nanometer composite by using a wet chemical process. This double Z-scheme photocatalyst showed outstanding photocatalytic performance for ciprofloxacin (CIP) degradation compared to g-C₃N₄, LaFeO₃, BiFeO₃ and their binary nanocomposites. The enhanced photocatalytic activities were primarily derived from the improved of light absorption capability, effective spatial separation and prolonged charge carrier lifetime in the double Z-scheme system. In particular, the scavenger tests and electron spin resonance spectra demonstrated that •O²⁻ and •OH are the primary oxidative radical species in the photocatalytic systems and confirmed the formation of the double Z-scheme structure. This study may provide new insights into the design and synthesis of highly efficient double Z-scheme photocatalysts for environmental decontamination [45]. Additionally, Saravanakumar et al. (2018) previously reported a novel visible-light-driven Ag/CdO photocatalyst, fabricated for the first time via a one-pot hydrothermal method and further applied for the photodegradation of two important exemplar water contaminants, malachite green and acid orange 7. In their experimental work, they found that (5%) of the Ag/CdO nanocomposites showed strong photocatalytic activity and pointed out this result was attributed to the synergistic effect between Ag and CdO. Ag NPs mainly acted as an electron trap site, which could reduce the recombination of the electron-hole and induce visible light absorption. The preparation of heterojunction has also been an effective strategy because it can increase the photo-responsive range and improve the charge separation efficiency of the photocatalyst [46].

According to the previous reports, serial A₂B₂O₇ compounds could be better photocatalysts [39,40,41,42,43]. For example, some scientists have prepared Bi₂MNbO₇ (M = Al³⁺, In³⁺, Fe³⁺, Sm³⁺) via the sol–gel route, and the results showed that the Bi₂FeNbO₇ compound family possesses momentous light catalyst activity. The Bi₂FeNbO₇ prepared using the sol–gel (400 °C) reaction accelerator had high light catalyst activity in the degradation of methylene blue in dye wastewater [44].

In this article, a series of experiments with In₂CdO₄/Y₂SmSbO₇ heterojunction (IYH) as the photocatalyst for the degradation of RhB were designed for the first time. IYH was compared to the previously reported composites for the degradation of RhB which is an emblematic organic pollutant. Wang et al. found that the degradation of RhB in aqueous solution by swirling jet-induced cavitation combined with H₂O₂ was investigated. It was found that there was an obvious synergetic effect between hydrodynamic cavitation and H₂O₂ for the degradation of RhB, while it was found that the degradation of RhB was strongly dependent on the medium’s pH value. The removal of RhB increased with the decrease in medium pH value. When the pH was 10.0, the residual concentration of RhB was 3.56 mg L⁻¹ and the removal ratio was only 64.4% after 180 min treatment. However, the residual concentration was 0.08 mg L⁻¹ or 0.11 mg L⁻¹ at a pH value of 2.0 or 3.0, respectively. It could be found that swirling jet-induced cavitation combined with H₂O₂ showed the highest RhB degradation rate [44,45,46,47,48,49,50]. However, although this improved the degradation rate of RhB, the results were not ideal. Therefore, the new photocatalyst which was studied in this paper possesses higher degradation efficiency in the degradation process of RhB, which made our work more meaningful, reflecting the advantages of the new photocatalyst IYH.

In this investigation, our aim was to prepare a new type of heterogenous junction (HJ) photocatalyst which could remove RhB from dye wastewater under VLGI. The innovative aspect of our work was that a new type of Y₂SmSbO₇ catalyst and IYH photocatalyst were synthesized by a high-temperature, solid-phase sintering synthesis method for the first time. Additionally, we obtained Y₂SmSbO₇ or IYH that was a visible-light-responsive photocatalyst with higher photocatalytic activity for effectively removing RhB. The removal of organic pollutants from dye wastewater using IYH photocatalyst was more efficient and safer.

2. Result and Discussion

2.1. X-Ray Diffractometer (XRD) Analysis

Figure 1 shows the X-Ray powder diffraction (XRD) pattern of Y₂SmSbO₇. The Materials Studio software was applied to obtain the quantitative data based on Rietveld analysis. The results showed that Y₂SmSbO₇ was a single phase and the lattice parameters of the new type of photocatalyst Y₂SmSbO₇ was 9.51349 Å. At the same time, the ultimate refinement result for Y₂SmSbO₇ showed that there was a good consistency between the observed strength and the surveyed strength of the pyrrhite structure, which was a cubelike crystal system with a interspace group of Fd3m (O atoms were included in the mold). Moreover, the total indices of the crystallographic plane for Y₂SmSbO₇ could be successfully indexed according to the Rietveld analysis results which were derived from the XRD pattern of Y₂SmSbO₇.

Table 1. Structural parameters of Y₂SmSbO₇ prepared by solid reaction process.

Atom	x	y	z	Occupation Factor
Y	0	0	0	1
Sm	0.5	0.5	0.5	0.5
Sb	0.5	0.5	0.5	0.5
O (1)	−0.185	0.125	0.125	1
O (2)	0.125	0.125	0.125	1

shows the atom co-ordinates and architecture parameters of Y₂SmSbO₇. We can conclude from Figure 1 that Y₂SmSbO₇ crystallized into a pyrochlore-type structure which possessed a crystal system and space group of Fd3m. Based on the results of the Rietveld refinement, the R_P =15.30% with space group Fd3m was obtained for Y₂SmSbO₇.

It should be noted that the x co-ordinate of the O (1) atom could be regarded as an index of the change of the transistorization architecture by the pyrochlore-type A₂B₂O₇ compounds (cubic, space group Fd3m) and was equivalent to 0.375 when the six A-O (1) bond distances were equal to that of the two A-O (2) bond distances [51]. Therefore, information about the deformation of the MO₆ (M = Fe³⁺ and Sb⁵⁺) octahedra could be obtained from the x value [49]. The x value was shifted by x = 0.375 [49], and therefore the deformation of the MO₆ (M = Fe³⁺ and Sb⁵⁺) octahedra was visible in the crystalloid architecture of Y₂SmSbO₇. Electric charge disassociation was demanded for photocatalytic degradation (PCD) of RhB under VLGI to avert regroupment of the photo-induced electrons and photo-induced cavities. Inoue [52] and Kudo [53] had shown that the partial deformation of the MO₆ octahedra which was gained from several photocatalyzers, for instance, BaTi₄O₉ and Sr₂M₂0₇ (M = Nb⁵⁺ and Ta⁵⁺), was very important for averting the electric charge regroupment and contributed to increased photocatalytic activity. Therefore, the distortion of the MO₆ (M = Fe³⁺ and Sb⁵⁺) octahedra in the crystal structure of Y₂SmSbO₇ may also enhance the photocatalytic activity. Y₂SmSbO₇ included a three-dimensional (3D) meshwork architecture of corner-sharing MO₆ (M = Fe³⁺ and Sb⁵⁺) octahedra. The MO₆ (M = Fe³⁺ and Sb⁵⁺) octahedra were connected into catena by Y³⁺ ions. Two kinds of Y-O bond distances coexisted: the six Y-O (1) bond distances of 2.614 Å were visibly longer than that of the two Y-O (2) bond distances of 2.183 Å. The six M-O (1) (M = Sm³⁺ and Sb⁵⁺) bond distances were 1.97635 Å and the M-O (2) bond distances were 4.459 Å. The M-O-M (M = Sm³⁺ and Sb⁵⁺) bond angles (BDA) were 138.732° in the crystal structure of Y₂SmSbO₇. The Y-M-Y (M = Sm³⁺ and Sb⁵⁺) BDA were 134.500° in the crystal structure of Y₂SmSbO₇. The Y-M-O (M = Sm³⁺ and Sb⁵⁺) BDA were 134.803° in the crystal structure of Y₂SmSbO₇. The research on its luminescence characteristics showed that the closer the M-O-M bond angle was to 180°, the more the excitation state was nonlocalized [51]. It revealed that the angles between the corner sharing MO₆ (M = Sm³⁺ and Sb⁵⁺) octahedra, for instance, the M-O-M BDA for Y₂SmSbO₇ was significant for affecting the photocatalyst activity of Y₂SmSbO₇. If the M-O-M BDA was close to 180 degrees, the mobility of the photo-induced electrons and photo-induced cavities would be large [51]. The mobilities of the photo-induced electrons and photo-induced holes affected the probability of electrons and cavities to reach reaction sites on the catalyzer skin layer which, in the end, led to the influence on the photocatalytic activity [51].

In addition, the Sb–O–Sb bond angle of Y₂SmSbO₇ was larger, and thus, the photocatalytic activity of the Y₂SmSbO₇ catalyst was improved. As to Y₂SmSbO₇, Y is a 4p-block rare-earth (RE) metallic element, Sm is a 4f-block metallic element, and Sb is a 5p-block metallic element. Based on above analysis, the impact of retrograding RhB under VLGI with Y₂SmSbO₇ as the photocatalyst could be chiefly credited to its crystal structure and electronic structure.

Figure 2 shows the XRD pattern of In₂CdO₄. The Materials Studio software was used to obtain the quantitative data based on the Rietveld analysis. The results (Figure 2) showed that In₂CdO₄ was a single phase and the lattice parameters of the new type of photocatalyst In₂CdO₄ was a = 14.9443 nm, b = 14.9443 nm, c = 9.9394 nm, α = 90°, β = 90°, and γ = 90°. At the same time, the ultimate refinement result for In₂CdO₄ showed that there was a good consistency between the observed strength and the surveyed strength of the body-centered tetragonal lattice structure, which was a tetragonal crystal system with a space group of I41/amd (O atoms are included in the mold). Moreover, the total indices of the crystallographic plane for In₂CdO₄ could be successfully indexed according to the Rietveld analysis results which were derived from the XRD pattern of In₂CdO₄. We can conclude from Figure 2 that In₂CdO₄ crystallized into body-centered tetragonal structure which possessed a tetragonal crystal system and space group of I41/amd. Based on the results of the Rietveld refinement, a R_P = 15.30% with space group I41/amd was obtained for In₂CdO₄. Other impurities was not detected in Figure 2.

Figure 3 shows the XRD diffraction pattern of the In₂CdO₄/Y₂SmSbO₇ HJ photocatalyzer. From Figure 3, the presence of the pure single crystal photocatalyzer In₂CdO₄ and single-phase photocatalyzer Y₂SmSbO₇ could be seen. The indices of crystallographic plane of every diffraction peak of In₂CdO₄ and every diffraction peak of Y₂SmSbO₇ was resoundingly labeled and the impurities were not detected in Figure 3. Figure 3 also showed that the XRD image of the In₂CdO₄/Y₂SmSbO₇ HJ had a single phase with perfect crystallinity and the defects were not found inside the crystal. Thus, the photo-induced electrons and the photo-induced holes which were generated during the photocatalytic reaction, did not aggregate and compound at the defects inside the crystal catalyst. Thereby we should prolong the service life of the photo-induced electrons and photo-induced cavities and improve the catalytic activity of the photocatalyst.

2.2. UV-Vis Diffuse Reflectance Spectra

The absorption spectrum of the Y₂SmSbO₇ sample is shown listed in Figure 4a,b. The absorption edge of this new photocatalyst Y₂SmSbO₇ was discovered to be at 458 nm which was in the visible range of the optical spectrum. The band-gap energy (B-GE) of the crystalline semi-conductors could be calculated using the point of crossing between the photon energy hν axis and the line calculated from the linear section of the absorption edge of the so-called Kubelka–Munk function (1) (called re-emission function) [54,55].

$$\frac{{\left[1-R_d\left(h\nu \right)\right]}^2}{2R_d\left(h\nu \right)}=\frac{\alpha \left(h\nu \right)}{S}$$

(1)

where $S$ is the dispersion factor, $R_d$ is the spread reflection, and $\alpha$ is the delegated absorbability modulus of radialization.

The optic absorbability near the energy band edge of a crystalline semiconductor conforms to Equation (2) [56,57]:

$$Ah\nu =A{\left(h\nu -E_g\right)}^n$$

(2)

Here, A, $\alpha$, $E_g$, and $\nu$ represent the ratable numeric constant, absorbability modulus, band gap, and light frequency, respectively. In this equation, $n$ determines the properties of the transitions in the semiconductors. $E_g$ and $n$ could be determined using the lower row process: (1) draw $\ln (\alpha h\nu$) versus $\ln \left(h\nu -E_g\right)$ assuming an approximate value of $E_g$; (2) infer the value of n based on the rate of grade in the Figure; (3) draw ${\left(\alpha h\nu \right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$n$}\right.}$ and $h\nu$ to refine the value of $E_g$ and extrapolating the graph to ${\left(\alpha h\nu \right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$n$}\right.}$=0. According to the above method, the $E_g$ value of Y₂SmSbO₇ was calculated to be 2.63 eV. The estimated value of $n$ was about 2 and indirectly allowed the optical transition of Y₂SmSbO₇. Figure 5a,b shows the UV-Vis diffuse reflectance spectrum of In₂CdO₄. In light of above the sequences and Figure 5a,b, the value of $E_g$ for In₂CdO₄ was calculated to be 2.70 eV. The estimated value of $n$ was about 0.5, which directly allowed the optical lambda transition of In₂CdO₄.

The absorption spectra of the IYH specimen are ranked in Figure 6a,b. The B-GE of Y₂SmSbO₇ was 2.63 eV, the B-GE of In₂CdO₄ was 2.70 eV, the B-GE of IYH was 2.28 eV [58], and the B-GE of co-doping ZnO was 2.39 eV [59]. All the B-GE of these three chemical compounds (CMPDs) were less than 2.87 eV, which implied that these three catalyzer possessed visible light response properties and held blastissimo latent energy for displaying high photocatalyst activity under VLGI.

2.3. Property Characterization of In₂CdO₄/Y₂SmSbO₇ Heterojunction Photocatalyst

To obtain the skin layer chemical constitution and the quantivalency ungerade states u of each constituent of IYHP, the X-ray photoelectron spectrum (XPS) was measured. Figure 7 shows the XPS investigation spectra of IYHP. Figure 8 shows the XPS spectra of O²⁻, In³⁺, Cd²⁺, Y³⁺, Sm³⁺ and Sb⁵⁺ which were derived from IYHP. In light of the XPS full spectrum which is shown in Figure 7, the synthesized IYHP included the elements of O, In and Gd. In light of the XPS analysis effects which are exhibited in Figure 7 and Figure 8, the oxidation state of O, In, and Cd ion was −2, +3 and +2, respectively. Due to the analysis results, it could be summarized that the chemical formula of the new CMPD was In₂CdO₄/Y₂SmSbO₇. It could be seen from Figure 8 that multifarious elemental apices with unique binding energies were obtained. In Figure 8, the O1s peak of O was situated at 529.90 eV. The In_3d3/2 and In_3d5/2 peak of In was situated at 451.70 eV and 445.40 eV, respectively. The Cd_3d5/2 peak of Cd was situated at 411.50 eV. Thus, Figure 7 and Figure 8 exposed the existence of indium (In_3d3/2 and In_3d5/2), cadmium (Cd_3d5/2), and oxygen (O1s) within the prepared specimen. The skin layer elemental analysis results showed that the media atomic rate of In:Cd:O was 198:99:397. The atomic ratio of In:Cd:O and Y:Sm:Sb:O in the IYHP sample was 1.97:1.00:3.96 and 1.98:0.99:1.00:6.95, respectively.

Obviously, neither shoulders nor widening in the XPS apices of IYHP were observed, which meant that the as-prepared compound for IYHP was pure phase.

Figure S1 displays the XPS survey spectrum of the In₂CdO₄. As illustrated in Figure S1, the full XPS spectrum of In₂CdO₄ revealed that the synthesized In₂CdO₄ consisted of the elements of In, O and Cd. As to In₂CdO₄, the oxidation state of In, Cd or O ion was +3, +2 or −2. The C element came from hydrocarbons used in the testing.

Figure 9 shows the TEM image of In₂CdO₄ and the selected area’s electron diffraction pattern of In₂CdO₄. It could be found from Figure 9 that the mean particle size was 297 nm for In₂CdO₄. Figure 9 clearly shows that In₂CdO₄ crystallized with a body-centered tetragonal lattice structure which possessed a tetragonal crystal system and space group of I4/amd, and the lattice parameters for In₂CdO₄ were proved to be a = 14.9443 nm, b = 14.9443 nm, and c = 9.9394 nm. According to the calculation results from Figure 9, the main diffraction peaks which were the same as those which were derived from Figure 2 for In₂CdO₄ could be found. Figure 10 shows the TEM image of In₂CdO₄/Y₂SmSbO₇. Figure 11 shows the EDS elemental mapping of IYHP (In, Cd, O from In₂CdO₄ and Y, Sm, Sb, O from Y₂SmSbO₇). It could be predicated from Figure 11 that IYHP contained indium, cadmium, yttrium, samarium, stibonium, and oxygen.

Figure S2 shows the SEM photograph of In₂CdO₄. The SEM-EDS analysis results which are shown in Figure S2 revealed that there were no other elements in the In₂CdO₄. Figure S3 shows the EDS elemental mapping of In₂CdO₄. In₂CdO₄ contained indium, cadmium, and oxygen.

Figure S4 shows the EDS elemental mapping of Y₂SmSbO₇. Y₂SmSbO₇ contained yttrium, samarium, stibonium, and oxygen.

Figure S5 shows the SEM photograph of N-doped TiO₂. The SEM-EDS analysis results which are shown in Figure S5 revealed that there were no other elements in the N-doped TiO₂. Figure S6 shows the EDS elemental mapping of N-doped TiO₂. N-doped TiO₂ contained nitrogen, titanium, and oxygen.

Figure 12 shows the TG-DSC-DTA of Y₂SmSbO₇. It could be seen from Figure 12 that the thermal weight curve was obviously lost at 440 °C, and there was a significant prominent heat absorption peak, which meant that the high-purity In₂O₃ and CdO powder materials had an obvious absorption peak at 440 °C because it contained crystalline water. We also found from Figure 13 that there was a prominent endothermic peak at 700 °C, and thus the new phase Y₂SmSbO₇ was formed at 700 °C. Similarly, it could be seen from Figure 13 that a new phase of In₂CdO₄ was formed at 720 °C.

2.4. Photocatalytic Activity

Figure 14 shows the concentration change curve of RhB during the PCD of RhB with IYH, In₂CdO₄, Y₂SmSbO₇, or N-TO as the photocatalyzer under VLGI. As could be seen from Figure 14, we first adsorbed RhB in the dark with IYH, In₂CdO₄, Y₂SmSbO₇, or N-TO as the photocatalyzer.

As could be seen from Figure 14, when IYH, In₂CdO₄, Y₂SmSbO₇, or N-TO was used as the photocatalyst to degrade RhB, the concentration of RhB in the drug wastewater increasingly decreased with increasing VLGI time. The effects shown in Figure 14 indicate that the RR of RhB within the dye wastewater reached 100% and the velocity of the reaction was 3.79 × 10⁻⁹ mol/L/s and the photonic efficiency (PE) was 0.03073% with IYHP as the catalyzer after 110 min of VLGI (VLGI-110min). Additionally, all the other experiments also used 110 min of VLGI. When In₂CdO₄ was used as the photocatalyst, the RR of RhB reached 91.44% and the rate of reaction was 3.46 × 10⁻⁹ mol/L/s and the PE was 0.0152%. The RR of RhB within the dye wastewater reached 85.16% and the rate of the reaction was 3.23 mol/L/s and the PE was 0.01078% with BiTiSbO₆ as photocatalyst. Additionally, the RR of RhB reached 57.48% and the velocity of the reaction was 2.18 × 10⁻⁹ mol/L/s and the PE was 0.00514% with N-TO as the photocatalyst.

Moreover, we could summarize from above effects that the photodegradation efficiency (PGE) of RhB by IYHP was the highest, meanwhile, the PGE of RhB with In₂CdO₄ as the photocatalyzer was higher than that with Y₂SmSbO₇ or N-TO, and ultimately, the PGE of RhB with Y₂SmSbO₇ as the photocatalyzer was higher than that with N-TO, indicating that the visible light photocatalyst activity of IYHP was higher compared to that of In₂CdO₄, Y₂SmSbO₇, or N-TO. The above effects showed that the RR of RhB with IYHP was 1.094, 1.174, and 1.740 times higher than that with In₂CdO₄, Y₂SmSbO₇, or N-TO as the photocatalyzer after VLGI-110min.

The size of the photocatalyst particle morphology would affect the specific surface area of the photocatalyst. The larger specific surface area of the catalyst, the more active sites on the catalyst surface, and thus the photocatalytic activity would be stronger. When IYH was used as the catalyst, the removal rate of RhB reached 100% after VLGI-110min. The aim of this research work was to develop visible-light-responsive nano materials with low price, high catalytic activity, and full use of the 43% visible light in the solar spectrum. Ultimately, the toxic organic pollutants from pharmaceutical wastewater could be thoroughly purified and removed; meanwhile, a healthier, safer, cleaner, and pollution-free water environment could be obtained.

Figure S7 shows the error bar of the concentration change curves of rhodamine B during PCD of rhodamine B with the In₂CdO₄/Y₂SmSbO₇ nanocomposite as the photocatalyzer. Figure S8 shows the error bar of the concentration change curves of rhodamine B during PCD of rhodamine B with the In₂CdO₄ nanocomposite as the photocatalyzer. Figure S9 shows the error bar of the concentration change curves of rhodamine B during PCD of rhodamine B with the Y₂SmSbO₇ nanocomposite as the photocatalyzer. Figure S10 shows the error bar of the concentration change curves of rhodamine B during PCD of rhodamine B with the N-doped TiO₂ nanocomposite as the photocatalyzer.

Figure 15 shows the concentration change curves of total organic carbon (TOC) during PCD of RhB within dye wastewater with IYH, In₂CdO₄, Y₂SmSbO₇, or N-TO as the photocatalyzer under VLGI. The concentration of RhB decreased with increasing VLGI time. As can be seen from Figure 15, the RR of TOC within the dye wastewater reached 99.71%, 87.02%, 82.23%, or 55.3% after VLGI-110min when IYHP, In₂CdO₄, Y₂SmSbO₇, or N-TO was used for degrading RhB, respectively. To sum up, the above effects on the RR of TOC during the degradation of RhB using IYHP was higher than that using In₂CdO₄, Y₂SmSbO₇, or N-TO. Above results also indicated that the RR of TOC during RhB degradation using In₂CdO₄ was much higher than that using Y₂SmSbO₇ or N-TO, which signified that IYHP had the highest mineralization percentage during RhB degradation compared with In₂CdO₄, Y₂SmSbO₇, or N-TO.

Figure S11 shows the error bar of the concentration change curves of TOC during PCD of rhodamine B in dye wastewater with the In₂CdO₄/Y₂SmSbO₇ nanocomposite as the photocatalyzer. Figure S12 shows the error bar of the concentration change curves of TOC during PCD of rhodamine B in dye wastewater with the In₂CdO₄ nanomaterial as the photocatalyzer. Figure S13 shows the error bar of the concentration change curves of TOC during PCD of rhodamine B in dye wastewater with the Y₂SmSbO₇ nanomaterial as the photocatalyzer. Figure S14 shows the error bar of the concentration change curves of TOC during PCD of rhodamine B in dye wastewater with the N-doped TiO₂ nanomaterial as the photocatalyzer.

Figure 16 shows the first-order kinetics plots of PCD for RhB under VLGI with IYH, In₂CdO₄, Y₂SmSbO₇, or N-TO as the photocatalyzer. As can be seen from Figure 16, the power mechanics invariable k which was obtained from the dynamical line between RhB concentration and VLGI time using IYH, In₂CdO₄, Y₂SmSbO₇, or N-TO as the photocatalyzer reached 0.03073, 0.0152, 0.01078, or 0.00514 min⁻¹, respectively. Additionally, the kinetic numeric constant k which came from the dynamic curve of the TOC concentration reached 0.03621, 0.01289, 0.00982, or 0.00465 min⁻¹ with IYH, In₂CdO₄, Y₂SmSbO₇, or N-TO as the photocatalyzer. The fact that the value of K_TOC for degrading RhB was lower than the value of K_C for degrading RhB using the same catalyzer illustrates that the photodegradation intermediates of RhB might appear in the PCD of RhB under VLGI. Meanwhile, in contrast to the other three photocatalysts, IYHP had a higher mineralization workpiece ratio for RhB retrogradation.

Figure 17 presents the impact of different free radical scavengers (FRSs), for instance, benzoquinone (BQ), isopropanol (IPA), or ethylenediamine tetraacetic acid (EDTA), on the RR of RhB using IYH as the photocatalyzer under VLGI. At the start of the photocatalytic experiment, different FRSs were added to the RhB solution for determining the active species in the retrogradation process of RhB. We used isopropanol (IPA) to capture hydroxyl radicals (•OH); benzoquinone (BQ) was used to capture superoxide anions (•O₂⁻); and ethylenediaminetetraacetic acid (EDTA) was used to capture holes (h⁺). The IPA, BQ, and EDTA concentrations were 0.15 mmol/L, and the added amount of IPA, BQ, and EDTA was 1 mL. As for the selection of free radical scavenger concentration, five concentrations of 0.05 mmol/L, 0.1 mmol/L, 0.15 mmol/L, 0.2 mmol/L, and 0.25 mmol/L of free radical scavengers were used to participate in the reaction. According to the experimental results, the concentration of capture agent was used as the abscissa and [99.16%—C] was used as the ordinate and ultimately, five corresponding curves were obtained. In the term [100%—C], 100% is the removal rate of RhB at 110 min in the blank experiment, and C is the removal rate of RhB after adding the corresponding concentration of capture agent for 110 min. At this time, the curve had a maximum value, which meant that the corresponding radicals were completely captured by the capture agent. At this time, the highest point was determined as the concentration of the free radical scavenger which was required in the experiment.

As shown in Figure 17, when IPA, BQ, or EDTA was added into the RhB solution, the RR of RhB decreased by 72.56%, 62.44%, or 27.28%, respectively, compared with the RR of RhB from the control group. Thus, this demonstrated that •OH, h⁺, and •O₂⁻ were all active free radicals in the process of RhB retrogradation. It could be seen from Figure 11 that •OH in the RhB solution played a leading position when RhB was degraded using IYH as the photocatalyzer under VLGI. In the experiment with the added capture agent, the hydroxyl radical possessed maximal oxidation removal ability for removing RhB within the dye wastewater compared with superoxide anions or holes. The oxidation removal ability for degrading RhB from high to low among the three oxidation radicals was as follows: hydroxyl radical > superoxide anion > holes.

Nyquist impedance plot measurements is another significant qualitative method which showed the photo-induced electron and photo-induced hole transfer process of the prepared photocatalysts at solid/electrolyte interfaces [60,61]. The smaller the arc radius (ARs) was, the higher the transport efficiency of the photocatalyst was. Figure 18 shows the corresponding Nyquist impedance plots of the prepared IYHP, In₂CdO₄, or Y₂SmSbO₇ photocatalyzers. In Figure 18 the diameters of the ARs were distinct and were in the order Y₂SmSbO₇ > In₂CdO₄ > IYHP. These effects indicated that the prepared IYHP presented a more efficient disassociation of photo-induced electrons and photo-induced holes and faster interfacial charge transfer capability. After 110 min of VLGI, the removal rate of RhB by IYH, In₂CdO₄, or Y₂SmSbO₇ was 100%, 91.44%, or 85.16%, respectively, which was also consistent with the results of the curvature radius of the catalysts in the electrochemical impedance experiment. Therefore, it could be concluded that the smaller curvature radius of the catalyst, the higher the removal rate of RhB and catalytic activity of the catalyst.

In addition, with the In₂CdO₄/Y₂SmSbO₇ heterojunction as the photocatalyst for degradation of RhB, a series of experiments were designed and compared with the previously reported composites for degradation of RhB in order to reflect the novelty of IYH. Guo et al. studied the synthesis of Fe₂O₃ nanoparticles which were doped with In₂O₃. This work aimed to design a kind of photocatalyst based on Fe₂O₃ nanoparticles which were doped with In₂O₃ and above nanocomposite was used to degrade RhB. The degradation performance of above prepared catalyst for RhB was explored under different conditions including the doped amount of In₂O₃, photocatalyst dosage, H₂O₂ content and pH value. The degradation efficiency reached 94% under optimal conditions [62]. However, although above nanocomposite improved the degradation rate of RhB, the results were not ideal. Therefore, the new catalyst In₂CdO₄/Y₂SmSbO₇ heterojunction which was studied in this paper had a higher degradation efficiency during the degradation process of RhB, ultimately, our experimental results made this work more meaningful and reflected the advantages of the new In₂CdO₄/Y₂SmSbO₇ heterojunction catalyst.

Table S1 shows the comparison of In₂CdO₄/Y₂SmSbO₇ with the other photocatalysts.

2.5. Analysis of Possible Degradation Mechanisms

Figure 19 shows the potential PCD mechanism of RhB with the In₂CdO₄/Y₂SmSbO₇ nanocomposite as the photocatalyzer under VLI. The potentials of the VB and CB for a semi-conductor catalyzer could be calculated using Formulas (3) and (4) [63]:

E_CB = X − E^e − 0.5E_g

(3)

E_VB = E_CB + E_g

(4)

where, E_g is the band gap of the semi-conductor, X is the electronegativity of the semi-conductor, and E^e is the energy of free electrons on the hydrogen scale (about 4.5 eV). According to above formulas, the VB potential or the CB potential for Y₂SmSbO₇ were estimated to be 1.94 eV or −0.69 eV, respectively. Additionally, for In₂CdO₄, the VB potential and CB potential were estimated to be 2.87 eV and 0.17 eV, respectively. Both Y₂SmSbO₇ and In₂CdO₄ could absorb visible light and internally generated electron–hole pairs when the In₂CdO₄/Y₂SmSbO₇ nanocomposite was irradiated with visible light. Owing to the redox potential position of the CB of Y₂SmSbO₇ (−0.69 eV) being more subtractive than that of In₂CdO₄ (0.17 eV), the photo-induced electrons on the CB of Y₂SmSbO₇ could migrate to the CB of In₂CdO₄. Additionally, the redox potential position of the VB of In₂CdO₄ (2.87 eV) was more non-negative than that of Y₂SmSbO₇ (1.94 eV), and therefore the photo-induced holes on the VB of In₂CdO₄ could migrate to the VB of Y₂SmSbO₇.

Therefore, combining Y₂SmSbO₇ and In₂CdO₄ to realize a new HJ nanocomposite could significantly lessen the recombination rate of photo-induced electrons and photo-induced holes, reduce the essential resistance, extend the service life of photo-induced electrons and photo-induced holes, and improve the interface charge migration efficiency [64]. Therefore, more oxidative radicals such as •OH or •O₂⁻ could be produced to increase the removal efficiency of RhB. Furthermore, the CB potential of Y₂SmSbO₇ was −0.69 eV which was more subtractive than that of O₂/•O₂⁻ (−0.33 V), indicating that the electrons within the CB of Y₂SmSbO₇ could absorb oxygen to produce •O₂⁻ which could degrade RhB (as shown in path 1 of Figure 19). At the same time, the VB potential of In₂CdO₄ was 2.87 eV which was more positive than that of OH⁻/•OH (2.38 V), indicating that the holes in the VB of In₂CdO₄ could oxidize H₂O or OH⁻ into •OH for degrading RhB, which was shown as path 2 in Figure 19. Ultimately, the photo-induced holes in the VB of Y₂SmSbO₇ or In₂CdO₄ could directly oxidize RhB and degrade RhB owing to the high oxidizing ability of the holes which was shown as path 3 in Figure 19. In conclusion, the In₂CdO₄/Y₂SmSbO₇ nanocomposite had good photocatalytic activity for the degradation of RhB, which was mainly owing to the efficient separation efficiency of electrons and holes which were induced by the In₂CdO₄/Y₂SmSbO₇ nanocomposite.

For the purpose of studying the retrogradation mechanism of RhB, the intermediary products which were generated during the retrogradation process of RhB were checked by LC–MS. The intermediate products which were obtained during PCD of RhB were identified as N-ethyl-Nʹ-ethylrhodamine (m/z = 387), rhodamine (m/z = 331), N-ethylrhodamine (m/z = 359), 3-nitrobenzoic acid (m/z = 167), 3-hydroxybenzoic acid (m/z = 138), acid phthalic anhydride (m/z = 148), ethanedioic acid (m/z = 90), phthalandione (m/z = 166), propandioic acid (m/z = 104), butanedioic acid (m/z = 118), glutaric acid (m/z = 132), hexane diacid (m/z = 146), maleic resin, and terephthalic. According to above intermediate products, we could draw a conclusion that the photodegradation process of RhB was N-demethylation and the destruction of the conjugated structure, indicating that chromophore cleavage, ring opening, and mineralization occurred during RhB degradation. Finally, RhB was transformed into micromolecule organic compounds which were combined with other organic active groups to transform into CO₂ and water. We used an atomic absorption spectrophotometer to examine the leaching elements after treatment within aqueous solution. As a result, we did not observe or detect any poisonous leaching elements such as cadmium or antimony after treatment. Thus, the final products in our experiments were non-toxic to the environment.

Our reaction system was composed of the In₂CdO₄/Y₂SmSbO₇ heterojunction and visible light. This reaction system was mainly used for the efficient removal of toxic and non-biodegradable pollutants within dye wastewater and pharmaceutical wastewater. Moreover, this reaction system would not cause secondary pollution of the environment and showed a very broad and promising prospect without limitations.

3. Experimental Section

3.1. Materials and Reagents

Ethylenediaminetetraacetic acid (EDTA, C₁₀H₁₆N₂O₈, purity = 99.5%) and isopropyl alcohol (IPA, C₃H₈O, purity ≥ 99.7%) were analytical grade. P-benzoquinone (BQ, C₆H₄O₂, purity ≥ 98.0%) was chemical grade. The above chemical reagents were purchased from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). Absolute ethanol (C₂H₅OH, purity ≥ 99.5%) conformed to American Chemical Society Specifications and was obtained from Aladdin Group Chemical Reagent Co., Ltd. (Shanghai, China). RhB (C₂₈H₃₁ClN₂O₃, purity ≥ 99.0%) was gas chromatography grade and was purchased from Tianjin Bodi Chemical Co., Ltd., Tianjin, China, as the model material. Ultra-pure water (18.25 MU cm) was used from start to finish in this work.

3.2. Preparation Method of In₂CdO₄

The newly fashioned photocatalyzer In₂CdO₄ was compounded by the high-temperature, solid-state sintering method. In₂O₃ and CdO (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China) with purity of 99.99% were used as rough stock without further purification. All powders (n(In₂O₃):n(CdO) 2:1) were synthesized after drying at 200 °C for 4 h. In order to prepare In₂CdO₄, the precursor was stoichiometrically blended, then pushed into a small column and fitted into an alumina crucible (Shenyang Crucible Co., LTD, Shenyang, China). After scorifying at 400 °C for 2 h, the raw materials and the small columns were taken out of the electric stove. The compounded materials were ground and then fitted into an electric furnace (KSL 1700X, Hefei Kejing Materials Technology Co., Ltd., Hefei, China). Finally, calcination was carried out in an electric smelter at 1100 °C for 36 h.

A mixture of 0.30 mol/L In(NO₃)₃·5H₂O and 0.15 mol/L Cd(NO₃)₂ were compounded and stirred continuously for 20 h. The solution was transferred to an autoclave lined with polytetrafluoroethylene and heated at 200 °C for 15 h. Subsequently, the obtained powder was scorified at 800 °C for 10 h in a tube furnace at a velocity of 8 °C/min under an atmosphere of N₂. Finally, In₂CdO₄ powder was also obtained by the hydrothermal synthesis method.

3.3. Preparation Method of Y₂SmSbO₇

The newly fashioned photocatalyzer Y₂SmSbO₇ was compounded the by high-temperature, solid-state sintering method. Y₂O₃, Sm₂O₃, and Sb₂O₅ (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China) with purities of 99.99% were used as rough stock without further purification. All powders (n(Y₂O₃):n( Sm₂O₃):n(Sb₂O₅) = 1:2:1) were synthesized after drying at 200 °C for 2 h. In order to prepare Y₂SmSbO₇, the precursor was stoichiometrically blended, then pushed into a small column and fitted into an alumina crucible (Shenyang Crucible Co., Ltd., Shenyang, China). After scorifying at 400 °C for 4 h, the raw material and small cylinder were removed from the electric furnace. The compounded materials were ground and then fitted into an electric furnace (KSL 1700X, Hefei Kejing Materials Technology Co., Ltd., Hefei, China). Finally, calcination was carried out in an electric smelter at 1010 °C for 25 h.

A mixture of 0.15 mol/L Y(NO₃)₃·5H₂O, 0.15 mol/L Sm(NO₃)₃ · 6H₂O, and 0.15 mol/L SbCl₅ were compounded and stirred continuously for 20 h. The solution was transferred into a Teflon-lined autoclave and was scorified at 200 °C for 15 h. Them, the powder was scorified at 780 °C for 10 h in a tube furnace at a velocity of 8 °C/min under an atmosphere of N₂. Thus, the Y₂SmSbO₇ powder was finally obtained.

3.4. Synthesis of N-Doping TiO₂

A Nitrogen-doped titania (N-doping TiO₂) catalyst was prepared by the sol–gel method with tetrabutyl titanate as a precursor and ethanol as solvent. The procedure was as follows: firstly, 17 mL tetrabutyl titanate and 40 mL absolute ethyl alcohol were combined to produce solution A; 40 mL absolute ethyl alcohol, 10 mL glacial acetic acid, and 5 mL double distilled water were blended to produce solution B; then, solution A was added dropwise into solution B under vigorous magnetic stirring, producing a transparent colloidal suspension. Then, aqua ammonia with an N/Ti ratio of 8 mol% was fitted into the transparent colloidal soliquid under magnetic stirring condition for 1 h. Dry xerogel was then generated after 2 days of aging. The xerogels were ground into powder, calcined at 500 degrees for 2 h, then ground into powder and screened by a vibrating screen to obtain the N-TO powder.

3.5. Synthesis of In₂CdO₄/Y₂SmSbO₇ Heterojunction Photocatalyst

The maximal calcination temperature of In₂CdO₄ which was prepared by the solid-state sintering method was 1100 °C and the insulated time was 36 h. The maximal calcination temperature of Y₂SmSbO₇ which was prepared by the solid-state sintering method was 1010 °C and the insulated time was 25 h. The highest calcination temperature of Y₂SmSbO₇ which was prepared by the hydrothermal synthesis method was 780 °C and the insulated time was 10 h. The maximal calcination temperature of In₂CdO₄ which was prepared by the hydrothermal synthesis method was 800 °C and the insulated time was 10 h. One aspect to consider is that the higher the maximal calcination temperature was, the greater the power energy consumption was, which would reduce and consume the service life of the furnace instrument. On the other hand, the longer insulated time and the higher maximal sintering temperature would cause larger particle sizes for Y₂SmSbO₇ or In₂CdO₄, which reduces the specific surface area of Y₂SmSbO₇ or In₂CdO₄ and their photocatalyst activity. For the purpose of increasing the photocatalyst activity, reducing energy consumption, and increasing the instrument life of the high-temperature calciner, we used the hydrothermal synthesis method for preparing Y₂SmSbO₇ and In₂CdO₄ in the process of preparing the HJ.

Y₂SmSbO₇ and In₂CdO₄ were prepared by the hydrothermal synthesis method, which mainly used the dissolution recrystallization mechanism to dissolve Er (NO₃)₃·5H₂O, Fe(NO₃)₃, SbCl₅, Bi(NO₃)₃·5H₂O, and TiCl₄ in hydrothermal medium; then, the above materials entered the solution in the form of ion groups and molecular groups. Strong convection which was caused by the temperature difference in the autoclave would prompt these ions and molecules to transport to the growth area which has a seed crystal, and ultimately, the saturated solution was formed and crystallized.

First, 0.30 mol/L In(NO₃)₃·5H₂O and 0.15 mol/L Cd(NO₃)₂ were compounded and stirred continuously for 20 h. The above solution was transferred into a Teflon-lined autoclave and was scorified at 200 °C for 15 h. Then, the obtained powder was scorified at 800 °C for 10 h in a tube furnace at a velocity of 8 °C/min under an atmosphere of N₂. In₂CdO₄ powder was finally obtained. Second, 0.15 mol/L Y(NO₃)₃·5H₂O and 0.15 mol/L Sm(NO₃)₃ · 6H₂O, 0.15 mol/L SbCl₅ were mixed and stirred continuously for 20 h. The above solution was transferred into a Teflon-lined autoclave and was heated at 200 °C for 15 h. Then, the obtained powder was calcined at 780 °C for 10 h in a tube furnace at a velocity of 8 °C/min under an atmosphere of N₂. Thus, the Y₂SmSbO₇ powder was finally obtained.

The prepared Y₂SmSbO₇ and In₂CdO₄ powders were prepared by the solvothermal method. The powders of Y₂SmSbO₇ or In₂CdO₄ were dissolved in octanol organic solvent in an autoclave. At this time, under liquid phase and supercritical conditions, the reactants would be dispersed in the solution and become more active. Therefore, the reactants were dissolved and dispersed; meanwhile, the reaction occurred, and the product was synthesized slowly.

A handy solvothermal method [65] was used for synthesizing a new IYHP in this paper. IYHP was prepared by compounding 525.36 mg of In₂CdO₄ with 30 wt% (974.64 mg) of Y₂SmSbO₇ in 300 mL of octanol (C₈H₁₈O) and then dispersed in an ultrasonic bath for 1 h. Then, it was tepefied and channeled back at 140 °C for 2 h under acute whisking conditions to increase the adhesion of In₂CdO₄ onto the surface of Y₂SmSbO₇ nanoparticles to form IYHP. After cooling to room temperature, the product was obtained by centrifugation and rinsed several times with a mixture of n-hexane/ethanol. The purified powder was dried in a vacuum oven at 60 °C for 6 h and stored in a dryer until further use. Finally, IYHP was successfully prepared.

3.6. Characterization

In this article, X-ray diffractometer (XRD), transmission electron microscope (TEM), X-ray photoelectron spectrograph (XPS), and UV-Vis diffuse reflectance spectrophotometer (DRS) were used for resolving the anatomical characteristics of pure-phase Y₂SmSbO₇ and pure-phase In₂CdO₄ which were prepared by power mechanics controlment and elevated temperature solid phase fritting methods. Furthermore, the removal rate (RR) of RhB under VLGI with pure-phase Y₂SmSbO₇, pure-phase In₂CdO₄, N-doping TiO₂ (N-TO), or IYH as the photocatalyst was detected.

The pure crystals of the prepared patterns were analyzed by an X-ray diffractometer (XRD, Shimadzu, XRD-6000, Cu Kα radiation, λ = 1.54184 Å, sampling pitch of 0.02°, preset time of 0.3 s step⁻¹, Kyoto, Japan). The morphology and microstructure of the prepared patterns were characterized using a scanning electron microscope (SEM, FEI, Quanta 250) and the elementary composition was captured by energy dispersive spectroscopy (EDS). Diffuse reflectance spectrum of the prepared sample was obtained using an UV-Vis spectrophotometer (UV-Vis DRS, Shimadzu, UV-3600, Kyoto, Japan). The surface chemical composition and states of the prepared sample were analyzed by an X-ray photoelectron spectrograph (XPS, PHI 5000 VersaProbe, UlVAC-PHI, Maoqi city, Japan) with an Al-kα X-ray source.

3.7. Photoelectrochemical Experiments

The electrochemical impedance spectroscopy (EIS) experiment was performed using a CHI660D electrochemical station (Chenhua Instruments Co., Ltd., Shanghai, China) with a standard three-electrode set-up. The three electrodes consisted of a working electrode (as-prepared catalysts), counter electrode (platinum plate), and reference electrode (Ag/AgCl electrode). The electrolyte was a Na₂SO₄ aqueous solution (0.5 mol/L). The light source for the experiment was a 500 W Xe lamp with an UV cut-off filter. The preparation of the working electrode was as follows: we put the pattern (0.03 g) and chitosan (0.01 g) in dimethylformamide (0.45 mL) and then we used an ultrasonic treatment for an hour to obtain a uniform suspension solution. Whereafter, the solutions were trickled onto an indium tin oxide (ITO) conducting glass (10 mm × 20 mm). Finally, the working electrode was dried at 80 °C for 10 min.

3.8. Experimental Setup and Procedure

The experiments were performed in a photocatalytic reactor (XPA-7, Xujiang Electromechanical Plant, Nanjing, China) and the temperature of the reaction system was 20 °C which was controlled by circulating cooling water. Simulated sunlight irradiation was provided by a 500 W xenon lamp with a 420 nm cut-off filter. There were 12 identical quartz tubes among which the volume of a single reaction solution was 40 mL, and the total reaction volume for the pharmaceutical wastewater was 480 mL. The dosage of Er₂FeSbO₇, BiTiSbO₇, or EBHP was 0.75 g/L and the concentration of RhB was 0.025 mmol/L. The amount of ENR was the residual amount after biodegradation of the pharmaceutical wastewater which contained 1.0 mmol/L RhB (Taihu Lake, Wuxi, China). During the reaction, 3 mL of the suspension was withdrawn periodically, whereafter filtration (0.22 μm PES polyethersulfone filter membrane) was used to remove the catalyst; finally, the residual concentration of RhB in the solution was determined by a UV–visible spectrophotometer (UV-2550, Shimadzu Corporation, Kyoto, Japan). The absorption wavelength (detecting wavelength) of RhB was 276 nm. The standard absorbance curve of RhB at different concentrations was accomplished under ultraviolet light irradiation in the region of 220–320 nm with the ultraviolet–visible spectrophotometer. The relationship between the concentration of RhB and the absorbance value at 276 nm was calculated. The absorbance of RhB in the solution was measured at the absorption wavelength of 276 nm, and the calibration line of RhB was drawn and a linear regression method was used for the quantification of RhB. Prior to VLGI, the suspension which contained the photocatalyzer and RhB was magnetically stirred in darkness for 45 min to ensure the establishment of an adsorption/desorption equilibrium between the photocatalyzer, RhB, and atmospheric oxygen. During visible light illumination, the suspension was stirred at 500 rpm.

The mineralization experimental data of RhB within the reaction solution were measured by using a TOC analyzer (TOC-5000 A, shimadzu Corporation, Kyoto, Japan). In order to examine the concentration of TOC during PCD of RhB, potassium acid phthalate (KHC₈H₄O₄) or anhydrous sodium carbonate was used as a standard reagent. Standard solutions of potassium acid phthalate with a known carbon concentration (in the range of 0–100 mg/L) were prepared for calibration purposes. Six samples which contained 45 mL of the reaction solution were used for measuring the TOC concentration every time.

The identification and measurement of RhB and its intermediate retrogradation products were carried out by liquid chromatography–mass spectrometry (LC-MS, Thermo Quest LCQ Duo, Thermo Fisher Scientific Corporation, Waltham, MA, USA. Beta Basic-C18 HPLC column: 150 × 2.1 mm, ID of 5 μm, Thermo Fisher Scientific Corporation, Waltham, MA, USA). Here, 20 μL of solution, which was obtained after the photocatalytic reaction, was automatically injected into the LC–MS system. The mobile phase contained 60% methanol and 40% ultrapure water, and the flow velocity was 0.2 mL/min. The MS conditions included an electrospray ionization interface, capillary temperature of 27 °C with a voltage of 19.00 V, spray voltage of 5000 V, and a constant sheath gas flow velocity. The spectrum was acquired in the negative ion scan mode and the m/z range swept from 50 to 600.

For the purpose of measuring the photon intensity of incident light, the filter which was 7 cm in length and 5 cm in width was chosen to be irradiated by incident single-wavelength of visible light at 420 nm. According to the formula of υ = c/λ and hv which represents the energy of a photon, Avogadro constant N_A, Planck constant h, photonic frequency υ, incident light wave length λ, and light velocity c were used to obtain the mole number of the total photons or the reactive photons which passed through the total area of the filter per unit time. By regulating the distance between the photoreactor and the xenon arc lamp, the incident photon flux on the photoreactor was changed [66].

The incident photon flux Io measured using a radiometer (Model FZ-A, Photoelectric Instrument Factory Beijing Normal University, Beijing, China) was determined to be 4.76 × 10⁻⁶ Einstein L⁻¹ s⁻¹ under VLGI (wavelength range of 400–700 nm). The incident photon flux on the photoreactor was changed by regulating the distance between the photoreactor and the Xe arc lamp [67,68,69,70,71].

The photonic efficiency was calculated with the following Equation (5):

ϕ = R/I_o

(5)

where ϕ is the photonic efficiency (%), and R is the retrogradation velocity of ENR (mol L⁻¹ s⁻¹), and I_o is the incident photon flux (Einstein L⁻¹ s⁻¹).

4. Conclusions

In₂CdO₄ was prepared by a solid-state method. For the first time, IYHP was prepared by a facile solvothermal method. The photophysical properties of the single-phase In₂CdO₄ and IYHP were investigated and confirmed using TEM, XRD, UV-Vis DRS, and XPS tests. The main conclusion showed that the Y₂SmSbO₇ compound was a pure phase which crystallized in a pyrochlore architecture that belonged to a cubic crystal system with the space group Fd3m. In₂CdO₄ crystallized in a body centered tetragonal lattice structure which was a tetragonal crystal system with a space group of I41/amd. IYHP was proven to be an efficient photocatalyst for removing RhB from wastewater. After VLGI-110min, the RR of RhB or total organic carbon reached 100% or 99.71% with IYH as the photocatalyst, respectively. The RR of RhB with IYH as the photocatalyst was 1.094, 1.174, or 1.740 times higher than that with In₂CdO₄, Y₂SmSbO₇, or with N-TO as the catalyzer, respectively. Therefore, it could be concluded that using IYH as the photocatalyst might be a potent method for treating pharmaceutical wastewater which is polluted by RhB. Finally, the intermediary products which were generated during the retrogradation process of RhB were detected.