Ag-decorated Bi₂O₃ nanospheres with enhanced visible-light-driven photocatalytic activities for water treatment

Abstract

Uniform Ag/Bi₂O₃ nanocomposites with different amounts of Ag were prepared via the deposition precipitation method. Compared with Bi₂O₃ nanosphere supporters, Ag/Bi₂O₃ nanocomposites exhibit highly enhanced visible-light-driven Cr(VI) photoreduction activity and photocatalytic bacteria inactivation. Accordingly, the effects that influence the photocatalytic activity of Ag/Bi₂O₃ nanocomposites were investigated through analyzing its optical properties, photoluminescence, and production of reactive oxygen species (ROS). It was found that the decorated Ag nanoparticles could improve the visible-light adsorption, electron–hole separation efficiency, surface oxygen vacancies and the production of reactive oxygen species of Ag/Bi₂O₃ nanocomposites, resulting in enhanced photocatalytic performance. This study not only demonstrates that Ag/Bi₂O₃ could be a promising visible-light-response photocatalyst for water treatment, but it also provides an ideal strategy for improving photocatalytic performance under visible light irradiation.

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Introduction

With the development of industry, environmental problems arising from wastewater have become a significant social issue. The various dangerous constituents in wastewater are harmful to ecosystems and human health. For example, Cr(VI) is a common highly toxic and carcinogenic contaminant in wastewater, which is produced by industrial processes such as leather tanning, paint making, electroplating, and production of chromate.^1,2 Pathogenic microorganisms in wastewater are also identified as a major pollutant that spread and breed easily, and are resistant to drugs. Therefore, the removal of various pollutants from water is considerably important.^3,4 To date, a variety of traditional methods have been applied to remove pollutants in water treatment such as adsorption, chemical reduction and oxidation, and UV disinfection. However, these traditional technologies often are limited by their drawbacks. For instance, adsorption merely eliminates Cr(VI) ions from the water phase to the surface of absorbent, rather than reducing highly toxic Cr(VI) to non-toxic Cr(III) species. Chemical reduction often requires massive use of harmful and high-cost reducing agents.¹ For pathogenic microorganisms, although traditional chlorination, ozonation and UV disinfection are highly effective for their inactivation, they usually produce potential carcinogenic and mutagenic by-products or obvious resistance to UV irradiation.^5,6

Since the report of photocatalytic H₂ production from water splitting over TiO₂ by Fujishima, photocatalytic degradation of pollutants in water over semiconductors has been recognized as one of the promising clean, low-cost and environmental-friendly strategies in environmental remediation, such as organic dye photodegradation, Cr(VI) photoreduction and photocatalytic inactivation of bacteria.^7–10 From the viewpoint of fully utilizing solar light, high-efficiency visible-light-response photocatalysts are significantly important for the practical application of photocatalysis in environmental remediation.¹¹ However, many semiconductor photocatalysts, including TiO₂, can only absorb UV light, which accounts for only 4% of sunlight.¹² Consequently, considerable efforts have been devoted to improve the visible-light-driven photocatalytic ability of single-component photocatalysts by compositing them with other metals or semiconductors. This produces benefits like widening of the photo-absorption region, mediating the energy band configuration and suppressing recombination of electron–hole pairs, thus leading to the enhancement of photocatalytic activity.^13–15

Bismuth oxide (Bi₂O₃) is an important p-type semiconductor, which demonstrates potential application in photovoltaic cells, optical coating, gas sensing and fuel cells due to its unique physical and chemical properties.^16,17 Besides, Bi₂O₃ nanomaterials have also attracted considerable attention in recent years for photocatalysis owing to the high mobility of lattice oxygen in the material.^18–21 In our previous study, Bi₂O₃ porous nanospheres displayed good photocatalytic activity for Cr(VI) photoreduction and organic dye degradation.²² However, Bi₂O₃ nanospheres mainly exhibit intense absorption in the UV-light region, which remains a huge potential problem for promoting its photocatalytic ability via proper modification. Recently, decorating with noble metal nanoparticles has been recognized as one of the most efficient strategies for improving the visible-light-response photocatalytic ability of photocatalysts. For example, decorated gold nanoparticles on TiO₂–SiO₂ mixed oxide nanocrystals promoted photocatalytic dye-sensitized H₂ production activity.²³ Au/TiO₂ present better photocatalytic performance for Cr(VI) reduction than single-component TiO₂.^24,25 Wong et al. demonstrated that a Ag decorated AgBr/Bi₂WO₆ nanojunction system shows effective visible-light photocatalytic disinfection of E. coli K-12.²⁶ Ag-loaded BiOI also exhibits better photocatalytic disinfection performance upon visible light irradiation.²⁷

In this study, to improve the photocatalytic performance of Bi₂O₃ porous nanospheres, uniform Ag/Bi₂O₃ nanocomposites with different Ag quantities were prepared via the deposition precipitation method. The applications of Ag/Bi₂O₃ nanocomposites for Cr(VI) photoreduction and photocatalytic disinfection in water via visible-light-treatment were reported. Accordingly, the effects that influence the photocatalytic activity of Ag/Bi₂O₃ nanocomposites were investigated through analyzing its optical properties, photoluminescence, and production of reactive oxygen species (ROS). To the best of our knowledge, the visible-light-driven photocatalytic activity of Ag/Bi₂O₃ nanocomposites for Cr(VI) removal and photocatalytic disinfection has not been reported to date.

Experimental

Synthesis

Bi₂O₃ porous nanospheres (S1) were prepared from Bi(NO₃)₃ by a solvothermal method in EG solvent according to our previous study. Ag/Bi₂O₃ nanocomposites with different Ag quantities were synthesized by the deposition–precipitation method. In a typical synthesis, 50 mL aqueous solution containing AgNO₃ (0.5 mL, 50 mmol L⁻¹) and 0.1 g Bi₂O₃ nanospheres (S1) was placed into a round-bottom flask. The mixture was ultrasonicated for about 5 minutes until the solid substance was uniformly dispersed. Then, the mixture was heated to 80 °C, followed by adjusting the pH value to 8 using an NaOH solution (2 M). Subsequently, the mixture was continuously stirred at 80 °C for 4 hours. The final product was collected by centrifugation, washed 5 times with deionized water, and finally dried in a desiccator for a few days for further characterizations (S2, 2.5 wt% Ag/Bi₂O₃). 5.0 wt% Ag/Bi₂O₃ (S3), 10 wt% Ag/Bi₂O₃ (S4) and 15 wt% Ag/Bi₂O₃ (S5) nanocomposites were also prepared using the identical reaction conditions, through the addition of 1, 2 and 3 mL AgNO₃ solution (50 mmol L⁻¹), respectively. In addition, sample S6 was prepared by simply mixing 2.5% Ag nanoparticles (synthesized according to our previous work) and Bi₂O₃ porous nanospheres.²⁸

Characterization

Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) spectra were obtained on a Hitachi S4800 scanning electron microscope operating at 5.0 kV. Transmission electron microscopy (TEM) images and energy dispersive X-ray spectroscopy mapping (EDX mapping) patterns were recorded on a Philips Tecnai 20 electron microscope, using an accelerating voltage of 200 kV. Powder X-ray diffraction (XRD) was carried out on a Bruker axis D8 Discover (Cu Kα = 1.5406 Å). UV-vis diffuse reflectance spectra (DRS) were recorded on a UV-vis spectrometer (Shimadzu UV-2550) using BaSO₄ as a reference, and were converted from reflection to absorbance by the Kubelka–Munk method. X-ray photoelectron spectra (XPS) were recorded on a VG Multilab 2000 spectrometer using Al Kα (1486.6 eV) radiation as the source. Photoluminescence (PL) spectra were recorded on a Shimadzu RF-5301 PC fluorescence spectrophotometer. Electron spin resonance (ESR) spectra were recorded by an ESR spectrometer (EMX-8/2.7, Bruker) at room temperature under or without visible light irradiation using DMPO as the radical trap. Atomic force microscopy (AFM) images of cell surface morphology were recorded at room temperature on equipment monitored by a NanoScope IIIa controller from Veeco Instruments Co. (Veeco, multimode AFM, USA) operated in tapping mode.

Photocatalytic reduction of Cr(VI)

The photocatalytic activities of the as-prepared samples were evaluated by Cr(VI) photoreduction under visible light irradiation by a 500 W Xe lamp with a 400 nm cut-off filter. In a typical experiment, 0.02 g of prepared Ag/Bi₂O₃ sample (S2–S5) was added to a cylindrical glass vessel surrounded by a circulating water jacket containing 25 mL Cr(VI) solution (40 mg L⁻¹). Prior to irradiation, the suspension was stirred in the dark for 1 h to reach the adsorption–desorption equilibrium. Subsequently, the solution was exposed to visible light irradiation under magnetic stirring. At each given time interval, 2 mL of suspension was sampled and centrifuged to remove the solid photocatalyst. The concentration of Cr(VI) during the photoreduction was monitored by colorimetry using a LB-UV2800 UV-vis spectrophotometer. For comparison, the photocatalytic activity of Bi₂O₃ nanospheres (S1) was also evaluated under identical conditions. pH-dependent experiments were carried out under identical conditions by varying the pH value from 1 to 11. All the measurements were carried out at room temperature.

Photocatalytic bacteria inactivation

The photocatalytic inactivation of different bacteria over Bi₂O₃ nanospheres (S1) and Ag/Bi₂O₃ nanocomposites (S2) were conducted under visible light irradiation using a 500 W xenon lamp with a 400 nm cut-off filter as the light source. The bacterial cells were incubated in Luria–Bertani (LB) agar plates at 37 °C for 24 h with shaking at 150 rpm, and then re-suspended in sterilized saline solution (0.9% NaCl) to about 1 × 10⁵ CFU mL⁻¹. In each experiment, 30 mL of bacterial cell suspension and 0.03 g of the as-prepared sample were added into the reactor with magnetic stirring. At different time intervals, aliquots of the mixture were collected and then immediately spread on LB agar plates, which were incubated at 37 °C for 24 h to determine the number of viable cells. For comparison, two control experiments were conducted under identical conditions. The light control was investigated in the absence of a photocatalyst (S1 or S2) under visible light irradiation, and the dark control was investigated in the absence of visible light irradiation. All the experiments were performed in triplicate. All glass apparatuses and materials used in this experiment were autoclaved at 121 °C for 20 min to ensure sterility.

Results and discussion

Ag/Bi₂O₃ nanocomposites with different Ag content were successfully prepared via the deposition–precipitation method, as confirmed by the powder X-ray diffraction (XRD) patterns (Fig. S1, ESI†). With the increase of Ag content introduced, the characteristic peaks of Ag were gradually observed, which could be indexed to Ag (JCPDS 27-0997, a = 4.08 Å). No Ag signal was detected in the characteristic peaks of the 2.5 wt% Ag/Bi₂O₃ nanocomposites (S2), probably due to the low concentration of Ag content of less than 5.0%, resulting in the difficult observation of this pattern.

The morphology and structure of the obtained Ag/Bi₂O₃ nanocomposites (S2–S5) were further characterized by SEM and TEM images. Fig. 1a–e show SEM images of the as-prepared Bi₂O₃ nanospheres (S1) and Ag/Bi₂O₃ nanocomposites (S2–S5), which demonstrate that all the composited products maintain the original sphere-like morphology of Bi₂O₃ nanospheres with an average diameter of 170 nm. With the increase of Ag content in the nanocomposites, it was clearly observed that Ag nanoparticles were successfully decorated on the surface of the Bi₂O₃ nanospheres. The corresponding TEM images (Fig. 1f–g) illustrate the porous structure of Bi₂O₃ nanospheres and uniform dispersion of Ag nanoparticles on Bi₂O₃ surface. More Ag nanoparticles were detected on the surface of Bi₂O₃ nanospheres in the high Ag loading capacity nanocomposites. The chemical composition of different Ag/Bi₂O₃ nanocomposites were also analyzed by energy dispersive X-ray analysis (EDX) spectra (Fig. 1k–o). Signals for elemental Ag could be observed in the EDX spectra of the Ag/Bi₂O₃ nanocomposites (S2–S5). The calculated weight ratio of Ag was about 2.6%, 4.9%, 9.7% and 15.0% for the Ag/Bi₂O₃ samples S2, S3, S4 and S5, respectively, which agreed well with the theoretical value for Ag/Bi₂O₃ nanocomposites. The distribution of Ag, Bi and O species in the Ag/Bi₂O₃ nanocomposites was further characterized by energy dispersive X-ray spectroscopy mapping (EDX mapping). The corresponding scanning transmission electron microscopy (STEM) image and Ag–K, Ag–L, Bi–L, Bi–M and O–K elemental maps demonstrate that Ag nanoparticles were uniformly loaded on the surface of Bi₂O₃ nanospheres (Fig. S2, ESI†).

Fig. 1 SEM, TEM images and EDX patterns of as-prepared Bi₂O₃ nanospheres and Ag/Bi₂O₃ nanocomposites: (a, f, k) S1; (b, g, l) S2; (c, h, m) S3; (d, i, n) S4 and (e, j, o) S5.

Fig. 2a shows the variation of Cr(VI) concentration with irradiation time in the presence of different Ag/Bi₂O₃ nanocomposites (S2–S5) under visible light irradiation. For comparison, a blank control and Cr(VI) photoreduction over Bi₂O₃ nanospheres (S1) were also performed under identical conditions. Most of Ag/Bi₂O₃ nanocomposites exhibited superior photocatalytic activities to Bi₂O₃ nanospheres. Upon visible light irradiation, Cr(VI) was completely removed within 10, 40 and 100 minutes over 2.5 wt% (S2), 5.0 wt% (S3) and 10 wt% Ag/Bi₂O₃ (S4), respectively. However, Bi₂O₃ nanospheres (S1) and 15 wt% Ag/Bi₂O₃ nanocomposites (S5) were only able to remove 70% and 40% of Cr(VI) after 120 min visible light irradiation, respectively. 2.5 wt% Ag/Bi₂O₃ (S2) exhibits the most efficient Cr(VI) photoreduction performance among Ag/Bi₂O₃ nanocomposites. In addition, the mixture of 2.5 wt% Ag nanoparticles and Bi₂O₃ nanospheres (S6) was also used for Cr(VI) photoreduction under identical conditions, which merely exhibited similar photocatalytic ability to single component Bi₂O₃ nanospheres. Fig. 2b shows the kinetics of Cr(VI) photoreduction over different photocatalysts (S1–S6). The obtained kinetic data of Cr(VI) photoreduction over different photocatalysts (S1–S6) fits to a pseudo-first-order model as expressed by ln(C′₀/C_t) = k_Crt, where C′₀ and C_t represent the Cr(VI) concentration before and after irradiation, t is the irradiation time, and k_Cr is the apparent rate constant. The corresponding photocatalytic reaction rate constants (k_Cr) and the linear correlation (R²) are listed in Table S1 (ESI†). Compared with other photocatalysts, 2.5 wt% Ag/Bi₂O₃ (S2) presents the fastest photoreduction rate upon visible-light irradiation.

Fig. 2 Cr(VI) photoreduction performance (a) and kinetics (b) of the as-prepared Bi₂O₃ nanosphere (S1) and Ag/Bi₂O₃ nanocomposite samples (S2–S6).

To verify Cr(VI) ions were completely photoreduced to Cr(III) ions by Ag/Bi₂O₃ nanocomposites, rather than being merely adsorbed on the face of the photocatalyst, the photocatalyst and Cr solution before and after photocatalysis were characterized by XPS and UV-vis spectra, respectively. Fig. 3a shows the UV-vis spectra of the HNO₃ treated solution obtained from Ag/Bi₂O₃ nanocomposites after adsorption (1), adsorption and light irradiation (2) and direct photocatalysis (3). The Cr(VI) absorption peak could only be detected in the spectrum of the HNO₃ treated solution obtained from Ag/Bi₂O₃ nanocomposites after adsorption (1), which indicates that the adsorbed Cr(VI) species have been completely reduced to Cr(III) species upon visible light irradiation. The chemical states of Cr species on the surface of Ag/Bi₂O₃ nanocomposites (S2) after adsorption and photocatalysis were further examined by XPS analysis. Compared with the survey XPS spectra of Ag/Bi₂O₃ nanocomposites (S2), Ag/Bi₂O₃ nanocomposites after adsorption (S2-AA) and Ag/Bi₂O₃ nanocomposites after photocatalysis (S2-AP), Cr signals were only present in the XPS spectra of S2-AA and S2-AP. The high resolution XPS spectra of Cr 2p peaks of S2-AA and S2-AP are depicted in Fig. 3d and f. In the high resolution XPS spectrum of S2-AA, the Cr 2p peaks could be curve-fitted with two components at binding energies of 587.90 and 578.70 eV, which were attributed to Cr(VI). Moreover, two stark peaks at 586.00 and 575.90 eV in the high resolution XPS spectrum of S2-AP correspond to the Cr 2p_1/2 and Cr 2p_3/2 orbits of the Cr(III) species, illustrating the complete reduction of Cr(VI) to Cr(III) after photocatalysis. The results indicate that 2.5% Ag/Bi₂O₃ nanocomposites could be a novel and ideal visible-light-response photocatalyst for Cr(VI) removal in water treatment.

Fig. 3 UV-vis spectra (a) and photograph of HNO₃ treated solution obtained from Ag/Bi₂O₃ nanocomposites after adsorption (1), adsorption and light irradiation (2), and direct photocatalysis adsorption (3). XPS spectra of Ag/Bi₂O₃ nanocomposites (S2), Ag/Bi₂O₃ nanocomposites after adsorption (S2-AA) and after photocatalysis (S2-AP): survey spectra (b, c, e) and Cr 2p spectra (d and f).

It has been well investigated that strong light adsorption, high electron–hole separation efficiency and oxygen vacancies benefit photocatalytic performance. To understand the enhancement of the photocatalytic activities of Ag/Bi₂O₃ nanocomposites, the optical properties of as-prepared Bi₂O₃ and Ag/Bi₂O₃ nanocomposites were characterized using diffuse reflectance spectroscopy (DRS). Fig. 4a displays the UV-vis diffuse reflectance spectra of different photocatalysts. Bi₂O₃ nanospheres (S1) only show weak photoabsorption in the visible light region. However, Ag/Bi₂O₃ nanocomposites (S2–S5) present a wide photoabsorption range in the visible light region after being composited with Ag nanoparticles, indicating improved visible-light-driven photocatalytic ability of Ag/Bi₂O₃ nanocomposites. On the other hand, it was reported that the deposited Ag nanoparticles could capture the photo-induced electrons and reduce the recombination of hole–electron pairs, thus leading to improved photocatalytic activity.²⁴ Therefore, the photoluminescence (PL) emission spectra of different photocatalysts (S1–S5) were obtained, as depicted in Fig. 4b, and are considered to survey the electron–hole separation efficiency. It was found that all the samples demonstrated an intense peak at about 380 nm. However, Ag/Bi₂O₃ nanocomposites exhibit lower photoluminescence intensity than that of Bi₂O₃ nanospheres, and 2.5 wt% Ag/Bi₂O₃ nanocomposites (S2) present the weakest photoluminescence intensity. This indicates that Ag/Bi₂O₃ nanocomposites result in highly efficient prohibition of the recombination of the photogenerated electron–hole pairs, which might be due to the increase of surface oxygen vacancies (V_O). Consequently, electron spin resonance (ESR) spectra of the different photocatalysts (S1–S5) were obtained, which are generally used to characterize the native defects in metal oxides and surface oxygen vacancies (V_O) with unpaired electrons.^29,30 As depicted in Fig. 4c, V_O signals of Ag/Bi₂O₃ nanocomposites (S2–S5) are obviously stronger than that of Bi₂O₃ nanospheres (S1). Because the signals of the five samples were obtained from equal mass, the sample with higher intensity of the signal is associated with more V_O on the surface of the sample. This illustrates that the amount of V_O in the Ag/Bi₂O₃ nanocomposites (S2–S5) increased after Ag nanoparticles were introduced, which is also indicative of their improved electron–hole separation efficiency.

Fig. 4 UV-vis diffuse reflectance spectra (DRS) (a), photoluminescence (PL) spectra (b), ESR-V_O spectra (c) of different photocatalysts (S1–S5) and pH-dependent Cr(VI) photoreduction performance of S2 (d).

To extend its practical application in water treatment, the pH effect on Cr(VI) photoreduction efficiency of Ag/Bi₂O₃ nanocomposites (S2) was also investigated. Fig. 4d shows the Cr(VI) photoreduction efficiency over Ag/Bi₂O₃ nanocomposites (S2) after 10 minutes visible-light irradiation under different pH values (1–11), illustrating that Ag/Bi₂O₃ nanocomposites exhibit excellent photocatalytic performance in a wide pH range (1–9). Although the photoreduction efficiency slightly decreased when the pH value increased to 11, the Cr(VI) removal rate was still over 70% after 10 min irradiation and eventually reached 100% within 30 minutes. Furthermore, Ag/Bi₂O₃ nanocomposites collected after photocatalysis remain in their original chemical composition and morphology, demonstrating that the photocatalyst is stable during the Cr(VI) photoreduction (Fig. S3, ESI†).

In addition to the removal of heavy metal ions, the inactivation of pathogenic microorganisms in water is also of great importance for water treatment. In this work, E. coli, S. typhimurium and B. subtilis were chosen as representative microorganisms to evaluate the photocatalytic disinfection performance of Ag/Bi₂O₃ nanocomposites. Fig. 5 shows the photocatalytic disinfection efficiency of Bi₂O₃ nanospheres (S1) and Ag/Bi₂O₃ nanocomposites (S2) towards E. coli, S. typhimurium and B. subtilis. In the absence of a photocatalyst (light control), the bacterial population had no obvious change even after 3 h of reaction, illustrating that the bacterial cells could not be inhibited upon visible light irradiation alone. In the absence of visible light (dark control), the population of bacterial cells was also nearly unchanged (inhibition rata was less than 10%) over Bi₂O₃ nanospheres (S1). Only 28% E. coli, 27% S. typhimurium and 38% B. subtilis bacteria were inhibited in the presence of Ag/Bi₂O₃ nanocomposites (S2) after 3 h of dark incubation. This suggests that both Bi₂O₃ nanospheres and Ag/Bi₂O₃ nanocomposites display no or poor antibacterial activity towards the three types of bacteria. Upon visible light irradiation, Bi₂O₃ nanospheres (S1) could inhibit 57%, 52% and 47% bacteria growth of E. coli, S. typhimurium and B. subtilis, respectively. More importantly, Ag/Bi₂O₃ nanocomposites (S2) could completely inhibit the growth of E. coli, S. typhimurium and B. subtilis upon visible light irradiation, which demonstrates the excellent photocatalytic bacteria inactivation efficiency of Ag/Bi₂O₃ nanocomposites. The time-dependent visible-light-driven photocatalytic disinfective performance of Bi₂O₃ nanospheres (S1) and Ag/Bi₂O₃ nanocomposites (S2) are depicted in Fig. S4 (ESI†). This result indicates that the decoration of Ag nanoparticles on the surface of Bi₂O₃ nanospheres could highly improve its photocatalytic disinfection efficiency.

Fig. 5 Photocatalytic disinfection of Bi₂O₃ nanospheres (S1) and Ag/Bi₂O₃ nanocomposites (S2) towards different bacteria.

The visible-light-driven photocatalytic inactivation of E. coli over Ag/Bi₂O₃ nanocomposites was also studied by atomic force microscopy (AFM). Fig. 6 illustrates AFM images of E. coli cells treated by Ag/Bi₂O₃ nanocomposites (S2) under visible light irradiation at different times. As shown in Fig. 6a, the untreated E. coli cells had a rod-like morphology and a relatively smooth surface with no ruptures or bulges. After 1 h photocatalytic disinfection over Ag/Bi₂O₃ nanocomposites, the bacteria cells maintain the rod-like morphology; however, the cell surface becomes a little rough, and the height of the treated cells was lower than that of untreated cells (Fig. 6b).

Fig. 6 AFM images of E. coli treated by Ag/Bi₂O₃ nanocomposites (S2) upon visible light irradiation for 0 h (a), 1 h (b), 2 h (c) and 3 h (d).

After 2 h of photocatalytic inactivation treatment, the bacteria cells collapsed and the rod-shaped morphology became plate-like shaped (Fig. 6c). Eventually, the cells were destroyed completely after 3 h of photocatalytic disinfection. The membrane components scattered from their original ordered and close arrangement, and the rod-like morphology of bacteria cell became almost disorganized (Fig. 6d). The result illustrates that the Ag/Bi₂O₃ nanocomposite could heavily damage the structure of the bacteria cells under visible light irradiation, which would be an important factor for the photocatalytic disinfection performance of Ag/Bi₂O₃ nanocomposites.

It is well known that reactive oxygen species (ROS), such as hydroxyl radicals (OH˙), superoxide ions (˙O₂⁻) and singlet oxygen (¹O₂), are often proposed to be the reactive oxidative species responsible for various damages to living organism in photocatalytic disinfection.^31,32 To determine the produced reactive oxygen species (ROS) in the photocatalytic disinfection, the ESR-DMPO method was performed in the dark and under light irradiation. Fig. 7a shows that no obvious signal was detected in dark conditions for all the photocatalysts, illustrating that both Bi₂O₃ nanospheres and Ag/Bi₂O₃ nanocomposites could not produce ROS without light irradiation. However, three major line signals were observed for all the photocatalysts after 2 min visible light irradiation (Fig. 7b), which belong to the products from DMPO oxidation by ¹O₂.³³ This is consistent with the results reported in the literature that noble metals and Bi₂O₃ can produce ¹O₂ in photocatalysis.³⁴ Noticeably, the ESR signals of Ag/Bi₂O₃ nanocomposites are stronger than that of Bi₂O₃ nanospheres, verifying that Ag/Bi₂O₃ nanocomposites promoted the production of ROS, thus leading to enhanced electron–hole separation efficiency.

Fig. 7 ESR-DMPO spectra of different photocatalysts in dark (a) and upon visible-light irradiation (b).

On the basis of the experimental results, a reaction process for Cr(VI) photoreduction and photocatalytic disinfection over Ag/Bi₂O₃ photocatalyst is preliminarily proposed and is schematically illustrated in Scheme 1. Upon visible light irradiation, the oxygen vacancies can promote visible light absorption and the generation of photoexcited electron–hole pairs over the surface of the photocatalyst.³⁵ Subsequently, surfaces decorated with Ag nanoparticles could accept the photogenerated electrons and prolong the lifetime of charge carriers due to the surface plasmon resonance (SPR) effect.^36,37 The charge separation efficiency was improved by Ag/Bi₂O₃ nanocomposites in Cr(VI) photoreduction, thus resulting in the enhancement of photocatalytic activity. The excellent visible-light-driven photocatalytic disinfection over Ag/Bi₂O₃ nanocomposites could be ascribed to the generated reactive oxygen species (ROS), which could lead to bacterial death.

Scheme 1 Schematic of Cr(VI) photoreduction and photocatalytic disinfection process over Ag/Bi₂O₃ nanocomposites.

Conclusion

In summary, uniform Ag/Bi₂O₃ nanocomposites with different Ag concentrations were prepared via the deposition precipitation method. The visible-light-response Cr(VI) photoreduction and photocatalytic disinfection performance of Ag/Bi₂O₃ nanocomposites were investigated. Compared with Bi₂O₃ nanosphere supports, the as-prepared Ag/Bi₂O₃ nanocomposites not only exhibit improved Cr(VI) photoreduction activity upon visible light irradiation, but also display highly enhanced visible-light-driven photocatalytic bacteria inactivation. The decorated Ag nanoparticles could improve the visible-light adsorption, electron–hole separation efficiency, surface oxygen vacancies and reactive oxygen species of Ag/Bi₂O₃ nanocomposites, resulting in the enhanced photocatalytic activity. This study not only demonstrates that Ag/Bi₂O₃ could be a promising visible-light-response photocatalyst in water treatment, but also provides an ideal strategy for improving photocatalytic performance under visible light irradiation.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant 21171136, 21371139 and 51378183), and the High-tech Industry Technology Innovation Team Training Program of Wuhan Science and Technology Bureau (2014070504020243).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12779c

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