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. 2023 Oct 17;8(43):40099–40109. doi: 10.1021/acsomega.3c02652

rh-In2O3 Nanoparticles for Efficient Photocatalytic Degradation of Rifampin

Shunxi Tang †, Xinyou Zhang †, Shufeng Li ‡, Chupei Zheng ‡, Huili Li †, Xianbo Xiao †,*
PMCID: PMC10620787  PMID: 37929117

Abstract

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Photodegradation, a widely accepted and promising technology, has gained significant attention for addressing the escalating concerns of environmental deterioration. In this article, rhombohedral corundum-type In2O3 nanocrystals were obtained from the transformation of InOOH via a simple calcining process. Under ultraviolet light irradiation, they showed higher photocatalytic activity in the decomposition of rifampin compared to that of the cubic phase In2O3 and P25–TiO2. Furthermore, the probable pathway and the feasible mechanism for the degradation of rifampin were also deeply explored and discussed.

Introduction

Over the past few decades, antibiotics have gained widespread recognition as remarkable agents for combating various bacterial infections. Rifampin, as a broad-spectrum antimicrobial, is commonly used to treat meningococcal carriers and tuberculosis and even in animal husbandry and aquaculture.1 Unfortunately, due to the excessive or improper use and incomplete metabolism in the body, part of the used rifampin remains active after excretion and then is released into the environment. Because of the stable chemical structure and hydrophobic property of rifampin, it will widely accumulate in aquatic resources, resist biological degradation, and finally get into the human food chain.2 As we know, frequent intake of antibiotics may lead to the development of human drug resistance, increase the risk of intestinal dysbacteriosis, and even cause potentially severe medical problems.1 Currently, antibiotics are perceived as emerging contaminants in the ecological environment, and they have attracted significant attention. Thus, in order to solve the ecological crisis and human health risk, a cost-effective approach is urgently required for the removal of rifampicin (RIF) from wastewater. In recent years, a variety of methods have been used, mainly including physical adsorption, biodegradation, advanced oxidation technologies (such as photocatalysis, sonocatalysis, Fenton, and ozonation), and so forth, for removing varieties of antibiotic pollutants.3−9 However, conventional physical and biological treatment methods have shown limitations in terms of sorption affinity, microbial dissipation, and degradation rates, resulting in the persistence of these recalcitrant antibiotics in the environment.2 In contrast, semiconductor-based photocatalysis has been proven to be a promising technology and widely used to decrease the negative environmental impacts of these harmful antibiotics and other hazardous wastes in aqueous media.7,8,10−13

With the development of nanofabrication techniques, a variety of inorganic metal oxide semiconductor nanostructures have been prepared and considered to be favorable photocatalysts for their large surface area, excellent adsorption, and satisfactory photochemical stability.14−16 Among them, indium oxide (In2O3) has received substantial attention since it has excellent photocatalytic performances and outstanding optical properties.17,18 In2O3 usually crystallizes in the stable cubic phase (c-In2O3) at ambient conditions, whereas the metastable rhombohedral corundum-type phase (rh-In2O3) is under high temperature and pressure conditions (Table S1).19−21 For c-In2O3, numerous morphologies, different synthesis processes, and their applications in various fields have been extensively and deeply researched in recent years.22−24 In contrast, due to the extreme reaction conditions, the syntheses and properties of rh-In2O3 nanostructures have been seldom reported. Previous studies have demonstrated that some of the prepared rh-In2O3 nanostructures exhibited superior photoelectric and gas sensing properties.25−29 However, only limited research suggested that rh-In2O3 should be an efficient photocatalyst for the degradation of environmental pollutants. For instance, Yin and Cao fabricated the hollow rh-In2O3 nanocrystals to photodegrade rhodamine B and methylene blue under ultraviolet (UV) light.30 Wu et al. and Zhou et al. found that the porous flowerlike and urchin-like rh-In2O3 should be better candidates for the degradation of tetracycline and oxytetracycline, respectively.31,32 Yet, as far as we know, it is rare to have a report on the utilization of rh-In2O3 nanostructures as efficient photocatalysts for the photodegradation of rifampin.

Herein, we present the transformation from ultrathin InOOH nanowires to rh-In2O3 and c-In2O3 nanocrystals via a simple calcining process. Their morphological characteristics and optical and photoelectrical properties were carefully checked. Furthermore, the photocatalytic activities of rh-In2O3 were evaluated by degrading rifampin under UV light irradiation with c-In2O3 and P25–TiO2 nanocrystals for comparison. Additionally, the probable pathway and feasible mechanism for degradation were comprehensively discussed.

Experimental Section

Synthesis of rh-In2O3 and c-In2O3 Nanocrystals

Fine powders of InOOH nanowires used as precursors in our experiments were prepared following the steps described in the previous work (Figure S1).33 A quartz crucible containing 0.3 g of InOOH powders was placed in a muffle furnace and heated to various temperatures (200, 300, 400, 500, 600, and 700 °C) at a heating ramp rate of 5 °C/min under an air atmosphere. After maintaining each desired temperature for 5 h, the crucible was allowed to cool naturally to room temperature, resulting in the formation of white target products for further characterization.

Materials Characterization

The crystalline phases of the products annealed at different temperatures were determined by using a Bruker D8 ADVANCE powder X-ray diffractometer equipped with Cu Kα radiation. Further analyses of these ray diffraction (XRD) data were performed with Rietveld refinements using GSAS-II software.34,35 Their morphologies were imaged with a field emission scanning electron microscope (JSM 7001F). Further structural information and elemental distributions were analyzed by a JEM-2200FS transmission electron microscope coupled to an energy-dispersive spectrometer. Additionally, X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi) was employed to determine the elemental compositions, chemical states, and valence band values of the samples. In order to show the optical properties of these samples, ultraviolet–visible (UV–vis) absorption spectra were examined by using a Shimadzu UV-3150 spectrometer. The specific surface areas and pore size distribution were observed on an ASAP 2020 analyzer (using Brunauer–Emmett–Teller and Barrett–Joyner–Halenda models). All of the electron spin resonance (ESR) spectra were recorded by a JEOL JES-FA200 spectrometer at room temperature.

Photoelectrochemical Measurements

The photoelectrochemical performances, such as the Nyquist plots, transient photocurrent–time (I–t) curves, and Mott–Schottky (M–S) plots, were evaluated on a Chenhua CHI660E electrochemical workstation with a standard three-electrode system. The ITO glass covered with the catalyst, Pt wire, Ag/AgCl (saturated KCl), and 0.1 M Na2SO4 solution were utilized as the working electrode, counter electrode, reference electrode, and electrolyte, respectively. In addition, a 300 W Xe lamp was used as the light source.

Photocatalytic Activity Test

The photocatalytic activities of the as-prepared rh-In2O3, c-In2O3, and commercial P25–TiO2 nanocrystals were evaluated by the degradation of RIF under UV irradiation in a photocatalytic reactor (Figure S2 and Table S2). Parallel experiments were conducted, with 25 mg of the catalyst mixed into a 10 mg/L RIF solution (250 mL) for each group. Then, these solutions were transferred to darkness under continuous stirring for establishing sorption equilibrium. 1 hour later, they were allowed to expose to UV light for up to 120 min (magnetically stirred, 25 °C). During this process, 5 mL of the RIF solution was taken out from one of the groups in turn at regular intervals for analyzing the variations of the RIF solution concentrations (using a Shimadzu UV-2600 spectrophotometer). Finally, the possible intermediates of RIF generated during the photocatalytic degradation processes were characterized using ultrahigh-performance liquid chromatography coupled to high-resolution mass spectrometry (UHPLC-HRMS). Chromatographic separation was performed on an UltiMate 3000 liquid chromatograph with a Hypersil GOLD C18 column (2.1 × 100 mm, 3 μm). The column temperature and flow rate were maintained at 25 °C and 0.25 mL/min, respectively. The mobile phase was composed of methanol (A) and 0.1% formic acid (B). The gradient elution conditions were set as follows: 0–2 min (5% A), 2–15 min (5–90% B), 15–20 min (90% A), 20–21 min (90–5% A), and 21–27 min (5% A). Mass spectra (MS) were acquired by using a Thermo Q-Exactive mass spectrometer equipped with a heated electrospray ionization source. The spray voltage was set at 3.2 kV for the positive mode and 3.5 kV for the negative mode, and the resolutions of the full MS and data-dependent MS2 (dd-MS2) scan modes were 70,000 and 17,500, respectively.

Results and Discussion

Characterization of rh-In2O3 Nanocrystals

The calcination of the InOOH nanowires was carefully investigated by a series of time-dependent experiments. The corresponding XRD analysis and SEM observation are shown in Figures S3 and S4 for details. It is clear that these InOOH nanowires undergo a transition to the rh-In2O3 and c-In2O3 nanocrystals successively with increasing temperature. From Figure 1a, XRD examination indicates that the sample is well-crystallized at 400 °C. The Rietveld refinement result reveals that the crystals are rh-In2O3 with the lattice parameters of a = b = 5.4816 (3) Å and c = 14.507(2) Å. Their average crystallite size was estimated by the Williamson–Hall (W–H) equation36

graphic file with name ao3c02652_m001.jpg 1

where βT is the broadening of the diffraction peak (the full width at half-maximum), ε is the strain, K (=0.89) is a constant, and λ, D, and θ are the wavelength of the X-ray, crystallites size, and peak position, respectively. The calculated result of the average crystallite size of the rh-In2O3 nanocrystals is about 30.2 nm (Figure 1b). This value is in good accord with the average particle size (31.4 nm shown in Figure 2c) directly observed from the SEM image (Figure 2a). Transmission electron microscopy (TEM) analysis (Figure 2b) further confirms the transformation of InOOH nanowires into rh-In2O3 nanoparticles with an irregular morphology. According to the high-resolution (HR)TEM image of a random nanoparticle (Figure 2d), it is pretty obvious that these well-defined lattice planes reveal the single-crystalline nature of these nanoparticles. The HRTEM image also allows estimation of the crystalline interplanar spacing, which corresponds to the (012) and (101) lattice faces in the rh-In2O3 crystal structure, as confirmed by the calculated lattice distances (Figure 2e) and the corresponding fast Fourier transform (FFT) pattern (inset of Figure 2d). Moreover, the chemical composition of the annealed rh-In2O3 nanocrystals was seriously checked by energy-dispersive spectrometry (EDS) and XPS analyses. The typical EDS spectrum (Figure 2f) and EDS mapping images (Figure 2g–i) exhibit that In and O elements are uniformly distributed in these nanocrystals with the contents of about 38.6 and 61.4 at. %, respectively. Obviously, these values roughly match the nominal stoichiometry of In2O3.

Figure 1.

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(a) Rietveld refinement of the XRD pattern and (b) Williamson–Hall plot of rh-In2O3 nanocrystals obtained at 400 °C.

Figure 2.

Figure 2

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Typical (a) SEM and (b) TEM images of rh-In2O3 nanocrystals. (c) Distribution histogram of the particle size of the as-prepared rh-In2O3 nanocrystals. (d) HRTEM image and FFT pattern (inset) corresponding to the selected area (green dashed box in the HRTEM image) of a random nanoparticle and (e) screenshot showing the interplanar spacings of the (012) and (202) planes. (f) EDS spectrum of the prepared rh-In2O3 nanocrystals (the Cu element is detected from the supporting mesh). (g–i) EDS mapping images for the distribution of C, In, and O elements, respectively.

As shown in Figure 3, XPS analysis was further performed to determine the elemental composition and valence states of the rh-In2O3 nanocrystals. From Figure 3a, the signals of the In, O, and C elements can be easily found in the XPS full spectrum. The C 1s peak, located at 284.8 eV, is normally attributed to the adventitious contamination and used for calibrating the binding energies. The narrow-scan spectra of In 3d (Figure 3b) exhibit two distinct peaks corresponding to In 3d5/2 and In 3d3/2 at binding energies of 444.4 and 452.1 eV, respectively, confirming the presence of In in the In3+ state.26,37 Also, the high-resolution O 1s spectra with three deconvoluted peaks (marked as OL, OV, and OC, respectively) are shown in Figure 3c. According to previous reports, OL, OV, and OC peaks with binding energies of approximately 529.9, 531.3, and 532.1 eV are normally ascribed to the In–O bonding, oxygen vacancies, and surface-chemisorbed oxygen, respectively.38−40 It is reasonable that the hydroxyl species will release oxygen to form oxygen vacancies during the annealing processes.41 Furthermore, the presence of oxygen vacancies in the rh-In2O3 nanocrystals can also be visualized from the ESR spectrum. As shown in Figure S5, the rh-In2O3 nanocrystals exhibit an obvious ESR signal at g = 2.004, which is usually ascribed to the typical signal of oxygen vacancies.18,42,43

Figure 3.

Figure 3

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XPS spectra for (a) full scan survey, (b) In 3d, and (c) O 1s of rh-In2O3 nanocrystals.

While annealed at 700 °C, it is quite apparent that the final product is entirely composed of c-In2O3 nanocrystals (Figure S6a). The morphology, size distribution, and elemental composition of these c-In2O3 nanoparticles were meticulously examined (Figures S6b,c and S7, respectively). It is apparent that the average diameter of these c-In2O3 nanoparticles (Figure S6b) is around 63.5 nm. XPS analysis indicates that these nanoparticles contain only In and O elements with an atomic ratio of about 1:1.43. This slight deviation from the nominal stoichiometry of In2O3 can be attributed to the presence of oxygen vacancies, as revealed in Figure S7c.

Finally, before evaluating the photocatalytic activity of these rh-In2O3, c-In2O3, and commercial P25–TiO2 nanocrystals, their optical absorptions were essential to be studied. From Figure S8a, it is clear that P25–TiO2 nanocrystals show strong photoabsorption in the UV region with an absorption edge of about 396 nm. In contrast, the rh-In2O3 and c-In2O3 nanocrystals display effective photoabsorption in both the UV and visible light regions up to 442 and 461 nm, respectively. It hints that the as-prepared rh-In2O3 and c-In2O3 nanocrystals can absorb more photons at the initial stage of the photodegradation process.32 Typically, their band gap values can be calculated by the following equation32

graphic file with name ao3c02652_m002.jpg 2

where α, hν, and Eg are the absorption coefficient, incident photon energy, and band gap energy, respectively. In this equation, A is a constant and n = 1/2 or 2 for an indirect or a direct semiconductor, respectively. As shown in Figure S8b, all of the rh-In2O3, c-In2O3, and P25–TiO2 nanocrystals have a direct band gap with values of about 2.92, 2.74, and 3.20 eV, respectively. These calculated results tally with the band gaps reported in previous works and allow for the rh-In2O3, c-In2O3, and P25–TiO2 nanocrystals to be better excited under the UV light region for enhancing their photocatalytic performance.44−49

Photocatalytic Degradation of RIF

The photocatalytic activity of the as-prepared rh-In2O3 nanocrystals for the degradation of RIF under UV irradiation was evaluated by comparing them with c-In2O3 and commercial P25–TiO2 nanocrystals. Initially, all suspensions that contain RIF (10 mg/L) and the photocatalysts (0.1 g/L) were kept in darkness for 1 h to establish the adsorption/desorption equilibrium. From Figure 4, it is clear that the rh-In2O3 and P25–TiO2 nanocrystals possess larger specific surface areas (43.8 and 50.1 m2 g–1 shown in Figure 4a,c, respectively) than c-In2O3 nanocrystals (14.3 m2 g–1 shown in Figure 4b). However, all the three photocatalysts exhibited poor adsorption capacities for RIF, showing no significant difference (Figure S9). By the way, consider that the average particle sizes of rh-In2O3, c-In2O3, and P25–TiO2 nanocrystals are about 30, 63, and 25 nm, respectively; thus, the pores with sizes around 20–100 nm observed in the three samples should be attributed to the interstices among different nanocrystals. During the irradiation process, without photocatalysts, RIF degradation could be almost ignored (Figure 5b). For rh-In2O3 nanocrystals as photocatalysts, the RIF concentration decreased dramatically in the first 20 min. Then, with the extension of irradiation time, the concentration diminished continuously, accompanied by the destruction of the molecular structure of RIF, which can be reflected by the temporal absorption spectral changes of RIF solutions shown in Figure 5a. After irradiation for 120 min, the highest RIF degradation rate reached about 94%, while the degradation rates of c-In2O3 and commercial P25–TiO2 nanocrystals were only 85 and 71%, respectively, under the same conditions.

Figure 4.

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Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curves as the insets of (a) rh-In2O3, (b) c-In2O3, and (c) P25–TiO2 nanocrystals.

Figure 5.

Figure 5

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(a) Variation of UV–vis absorption spectra of RIF over rh-In2O3 nanocrystals (0.1 g/L) under UV irradiation. (b) Time-dependent normalized concentrations of RIF solutions in the presence of different catalysts. (c) Plots of ln(C0/C) against the irradiation time, fitted by using the pseudo-first-order model with the photodegradation rates shown in (d).

Furthermore, in order to compare the photodegradation efficiencies of RIF over different catalysts, the kinetic curves were simulated following the pseudo-first-order kinetic equation50

graphic file with name ao3c02652_m003.jpg 3

where k is the pseudo-first-order rate constant and C0 and C are the concentrations of solution at equilibrium and different irradiation times, respectively. As shown in Figure 5c,d, the linear fitting results demonstrate that these photodegradation processes follow pseudo-first-order behavior, and the corresponding reaction rate constants further confirm that the rh-In2O3 sample (0.0208 min–1) exhibits better photocatalytic performance than the c-In2O3 (0.0148 min–1) and commercial P25–TiO2 (0.0101 min–1) nanocrystals.

The apparent quantum efficiency (ΦAQE), which is a crucial parameter for evaluating the performance of a photocatalyst in a homogeneous system, is commonly defined as51

graphic file with name ao3c02652_m004.jpg 4

where dx/dt is the change rate of the number of pollutant molecules, qλ is the number of photons incident per time interval, and A is the absorbance. In our study, the apparent quantum efficiencies of rh-In2O3, c-In2O3, and P25–TiO2 nanocrystals on the photocatalytic degradation of RIF could be determined as

graphic file with name ao3c02652_m005.jpg 5

where k is the pseudo-first-order rate constant, C0 is the concentration of solution at equilibrium, V is the total liquid volume in the reactor, ε is the molar extinction coefficient, ΔL is the length of the light path, Iinc is the incident light intensity, S is the illuminated area, and Uλ=365 is the energy of 1 mol of photons. According to the parameters shown in Tables S2 and S3 and Figure S10, the results of the apparent quantum efficiencies of rh-In2O3, c-In2O3, and P25–TiO2 nanocrystals on the photocatalytic degradation of RIF were obtained (Figure 6). It is obvious that rh-In2O3 exhibits a higher photonic efficiency and is therefore a more efficient photocatalyst. It is generally accepted that the specific surface area, which is a key factor, can affect the photocatalytic activity of materials. Materials with larger specific surface areas typically demonstrate superior photocatalytic activity due to a greater number of reactive adsorption/desorption sites available during the photocatalytic reactions.33 Interestingly, in our case, P25–TiO2 possesses the largest specific surface area but exhibits the lowest efficiency in the photodegradation of RIF. Therefore, this discrepancy cannot be simply explained by the effect of the specific surface area.

Figure 6.

Figure 6

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Plots of the apparent quantum efficiencies of rh-In2O3, c-In2O3, and P25–TiO2 nanocrystals against irradiation time.

Considering the generation and transfer of photogenerated electrons and holes can also severely impact the photocatalytic performance of rh-In2O3, c-In2O3, and P25–TiO2 nanocrystals; thus, their photoelectrochemical properties were carefully checked.10,41,52−54 Obviously, from the transient photocurrent spectra demonstrated in Figure 7a, both rh-In2O3 and c-In2O3 display significantly enhanced photocurrent compared with P25–TiO2 nanocrystals. rh-In2O3 presents the highest transient photocurrent response, revealing a more efficient separation and transfer process of photogenic electron–hole pairs in the rh-In2O3 nanocrystals.10 This observation is further supported by the electrochemical impedance measurement results illustrated in Figure 7b. It is obvious that each Nyquist plot consists of one depressed semicircle in the high-frequency region and a sloping straight line in the low-frequency region. It is generally accepted that the high-frequency semicircle is related to the charge-transfer resistance at the interface (Rct), while the sloping straight line can be ascribed to the diffusion-controlled process (Warburg impedance).55 To better understand the Nyquist plots, an equivalent circuit (inset of Figure 7b) was proposed, and the impedance parameters were simulated using Zview software. The calculated value of Rct of the rh-In2O3 nanocrystals (71.7 Ω) is significantly smaller than that of c-In2O3 (103.1 Ω) and P25–TiO2 (120.8 Ω) nanocrystals, demonstrating a higher photogenerated electron–hole separation and transfer efficiency in these rh-In2O3 nanocrystals.56 Based on the above-mentioned discussion, we can recognize that rh-In2O3 has only a slightly smaller specific surface area but much better performance on separation and transfer of photogenerated electron–hole pairs compared to P25–TiO2. Therefore, it is hardly surprising that it has the best efficiency in the photodegradation of RIF. In addition, after a four-circle photodegradation process under the same conditions, it is noticeable that its photocatalytic activity and structure exhibit considerable stability (Figures S11–S14).

Figure 7.

Figure 7

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(a) Transient photocurrent curves. (b) Circuit-fitted Nyquist plots. The inset is the electrical equivalent circuit, which consists of electrolyte resistance (Rs), charge-transfer resistance (Rct), double-layer capacitance (CPE), and Warburg impedance (Zw). (c) Mott–Schottky plots. (d) Valence band XPS spectra. (e) Schematic band structures of rh-In2O3, c-In2O3, and P25–TiO2 nanocrystals obtained according to (c,d).

Photocatalytic Degradation Mechanism

In order to understand the photocatalytic degradation process of RIF intuitively, the possible degradation pathways are proposed by identifying the yielded intermediates (Table S4) via UPLC-HRMS (Figures S15 and S16). As depicted in Figure 8, two distinct transformation pathways for RIF photodegradation have been identified. For pathway I, some RIF molecules could be transformed to RIF 1 and RIF 2 successively through the opening of the piperazine ring, hydroxylation, dehydration, and demethylation reactions. Under further oxidation, the intermediates RIF 3 and RIF 4 were obtained. Then, they could be decomposed to the detected compounds RIF 5 and RIF 6, respectively, by the opening of nitrogen-bearing heterocyclic and chain scission. Next, RIF 6 could be directly oxidized to RIF 5 and RIF 7. As for pathway II, RIF was first degraded to RIF 8 via the opening of nitrogen-bearing heterocyclic and chain scission. On successive oxidation, decarboxylation, and hydrolysis, RIF 9 and RIF 10 were produced. Subsequently, RIF 10 could be decomposed to RIF 11 by a series of reactions including cleavage of the piperazine side chain, decarboxylation, oxidation, hydroxylation, and cleavage of the oxygen heterocyclic ring attacked by hydroxyl radicals. Then, further oxidation caused the opening of the aromatic ring, and RIF 12 was obtained. Ultimately, these small molecules might be further mineralized to CO2 and H2O.12

Figure 8.

Figure 8

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Probable photocatalytic degradation pathway of RIF over rh-In2O3 nanocrystals.

As is well known, reactive species, such as electrons (e–), holes (h+), superoxide radicals (•O2–), and hydroxyl radicals (•OH), play vital roles during the photodegradation processes. In addition, their generation is closely relevant to the band structures of the photocatalyst.52,57 Hence, in order to seek the mechanism of the photocatalytic degradation of RIF over rh-In2O3 nanocrystals, their valence band (EVB) and conduction band (ECB) energies were further determined by the VB-XPS spectrum and the M-S plot. As shown in Figure 7c, a positive slope line implies that the rh-In2O3 sample is an n-type semiconductor. Meanwhile, its flat band potential (Vfb) can be estimated to be about −0.18 eV vs Ag/AgCl or 0.02 eV vs NHE. Consequently, it can be inferred that the Fermi level (Ef) of rh-In2O3 is 0.02 V (vs NHE) since Vfb is nearly equal to Ef for n-type semiconductors.4 Simultaneously, the VB-XPS spectrum (Figure 7d) displays that the energy difference between Ef and EVB is approximately 2.43 eV.17,18 Therefore, as shown in Figure 7e, the EVB value of rh-In2O3 is calculated to be 2.45 V vs NHE, and the ECB value can be confirmed to be −0.47 V vs NHE according to a simple formula (ECB = EVB – Eg). The calculated results show that the ECB value is more negative than E°(O2/•O2–) (−0.33 V vs NHE) and EVB is more positive than E°(•OH/OH–) (1.99 V vs NHE). Theoretically, the photogenerated electrons can react with O2 to form •O2–, while OH– can be oxidized by holes to produce •OH.10,50 Furthermore, ESR analysis with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) serving as the trapping agent was employed to identify whether •O2– and •OH radicals were generated during the photodegradation process. As shown in Figure 9a,b, no signal of DMPO-•O2– or DMPO-•OH was detected under the dark condition. However, after 5 min of UV irradiation, four strong characteristic peaks with an intensity ratio of 1:2:2:1 could be detected and ascribed to the signal of DMPO-•OH, while a weaker signal of DMPO-•O2– could also be observed.10,53 Therefore, these experimental results coincide with the theoretical analysis that both •O2– and •OH radicals could be produced during the photodegradation process.

Figure 9.

Figure 9

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ESR spectra of ·OH and •O2– radicals trapped by DMPO.

For c-In2O3 and P25–TiO2 nanocrystals, the theoretical calculation (Figure 7e) and ESR analysis (Figure 9) also show that both the •O2– and •OH radicals were generated during each photodegradation process. Notably, the intensities of DMPO-•OH and DMPO-•O2– signals of irradiated rh-In2O3 nanocrystals are stronger than those of c-In2O3 and P25–TiO2 nanocrystals in the same conditions. In other words, rh-In2O3 nanocrystals as the photocatalyst yield higher amounts of •O2– and •OH radicals.10 This is likely the key factor contributing to the superior efficiency of rh-In2O3 nanocrystals in the photodegradation of RIF, compared to c-In2O3 and P25–TiO2 nanocrystals.

To sum up, a reasonable mechanism for the photocatalytic degradation of rifampin is suggested (Figure 10). First, under the irradiation of UV light, the photogenerated electron–hole pairs were generated and then transferred to the surface of the rh-In2O3 nanocrystals (when hν = Eg). Subsequently, these photogenerated electrons could reduce O2 to produce •O2–. In contrast, the photogenerated holes could oxidize OH–/H2O to form •OH. Ultimately, both the •O2– and •OH radicals would attack and break rifampin into small molecules.

Figure 10.

Figure 10

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Possible mechanism for the photodegradation of RIF over rh-In2O3 nanocrystals.

Conclusions

In summary, rh-In2O3 nanoparticles with an average size of around 30 nm were successfully prepared via a facile hydrothermal method combined with a simple calcining process. Compared with c-In2O3 and P25–TiO2 nanoparticles, rh-In2O3 nanoparticles demonstrated superior efficiency in the degradation of rifampin under UV light irradiation. The enhanced photoactivity is likely due to the more efficient separation and transfer of photogenic electron–hole pairs. Also, the feasible degradation pathways and mechanism of RIF were proposed by UHPLC-HRMS and ESR analyses. Moreover, this work presents a promising and low/no-toxicity photocatalyst for the efficient removal of antibiotics.

Acknowledgments

This work was supported in part by the National Natural Science Foundation of China (grant nos. 12164021 and 11664019), the Natural Science Foundation of Jiangxi Province (grant no. 20212ACB201005), and the Jiangxi University of Traditional Chinese Medicine 1050 youth talent project (grant no. 5142001010).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c02652.

  • Structural parameters of c-In2O3 and rh-In2O3; XRD pattern and TEM image of ultrathin InOOH nanowires; schematic illustration, dimensions, and main characteristics of the photocatalytic reactor; XRD patterns, TEM images, and SEM images of the samples prepared by annealing InOOH nanowires at different temperatures; ESR spectra of the rh-In2O3 nanocrystals; XRD pattern, SEM image, distribution histogram of the particle size, and XPS spectra of the c-In2O3 nanocrystals; typical absorption spectra of the rh-In2O3, rh-In2O3, and P25–TiO2 nanocrystals; adsorption kinetics of rifampin on rh-In2O3, c-In2O3, and P25–TiO2 nanocrystals; cycling runs in photocatalytic degradation of rifampin by rh-In2O3 nanocrystals; XRD patterns, nitrogen adsorption–desorption isotherms, and XPS spectra of rh-In2O3 before and after the photocatalytic reaction; and identified intermediates and their MS/MS spectra from photocatalytic degradation RIF over rh-In2O3 (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c02652_si_001.pdf (2MB, pdf)

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