Direct Z-Scheme AgBr/β-MnO2 Photocatalysts for Highly Efficient Photocatalytic and Anticancer Activity.

The preparation of visible light-responsive efficient photocatalysts for removing organic contaminants from water and killing cancer cells has gotten a lot of attention due to the growing global concern. In this study, we have successfully fabricated an efficient AgBr/β-MnO2 nanocomposite via a facile deposition and precipitation method at room temperature. Techniques such as XRD, SEM-EDS, TEM, DRS, PL, EIS, ESR, and FTIR were used to determine the crystalline, structural, morphological, optical, and other properties. The SEM and TEM analyses reveal that AgBr NPs are decorated on the surface of β-MnO2, which possesses rods with a sphere-like structure for AgBr/β-MnO2. The EDX analysis confirms the existence of Mn, O, Ag, and Br elements in the nanocomposites without an extra peak, indicating that the synthesized samples are highly pure. The high photocatalytic performance of AgBr/β-MnO2 could be attributed to the formation of Ag NPs and the construction of the Z-scheme heterojunction between AgBr and β-MnO2. This may enhance fast light absorption and efficient photogenerated (e-/h+) pairs, as indicated by EIS and photoluminescence measurements, which in turn achieved high activity for the decomposition of MB (97%, in 12 min), RhB (98.9%, in 9 min), and paracetamol (80%, in 180 min), respectively. The kinetic model study proposed that the first-order model showed a better fit than the zero- and second-order for the photocatalytic decolorization of RhB dye. XRD analysis of 0.2 AgBr/β-MnO2 before and after recycling confirms the high stability of the catalyst. HPLC results showed that no detectable by-products are produced through the decomposition of paracetamol. Interestingly, 0.2 AgBr/β-MnO2 nanocomposites showed visible light-induced anticancer activity against A549 cancer cell lines. The mechanistic degradation pathway has been proposed using the involvement of active species like superoxide radicals (-•O2) and photoinduced holes (h+). The proposed work focuses on synthesizing effective photocatalysts in a less hazardous environment with superior biological activity.

Introduction

Highlights

A highly efficient visible light-responsive photocatalyst, AgBr/β-MnO2 nanocomposite, has been synthesized.

The synthesized material was fully characterized using standard analytical techniques.

The photocatalyst showed enhanced activity toward degradation of organic pollutants (dyes and drugs) and anticancer activity against A549 cancer cell lines.

Water is the most imperative part of our daily life and is used extensively for drinking, industrial purposes, public hygiene, energy, and agriculture. The rapid growth of the world population and the development of industries result in a vehement increase in environmental contamination by producing industrial effluents and waste materials. These toxic materials in the water system harm the ecological system and human health.[1−5] Many industries use organic dyes for paint, leather, textiles, and other items. Ultimately, these colors are released into the environment, which is the primary source of raising the rate of contamination and declining water quality.[6−8] Many dyes and pigments are produced annually in the entire world. Additionally, around 2,000,000 tons of industrial effluents are discharged daily into worldwide water.[8,9] Although these dyes are chemically resistant and carcinogenic, they are still excessively used for different purposes, such as paint, cosmetics, textiles, food, pharmaceuticals, and pulp industries, which are considered the main source of organic dyes.[8,10,11] Removal of dyes from water systems is extremely desirable for a healthy environment.[10] Generally, organic pollutants and toxic contaminants are stable and hard to remove or degrade by traditional biochemical treatment. Recently, various technologies have been proposed for the remediation of pollutants, such as adsorption, oxidation process, photocatalysis, ion exchange process, metal–organic framework (MOF), and biodegradation.[12−17] Qiu et al. studied the advantages and properties of biochar-based catalysts and their applications in the photocatalytic removal of organic contaminants.[18,19] Among them, photocatalysis is considered a green and facile suitable technology for treating contaminated water.[7,10,15,20−29] The synthesis of high-performance heterogeneous photocatalysis has become one of the most popular research areas, as this method is eco-friendly, low-cost, and highly effective.[30−35] In addition, this method may lead to the degradation of organic/inorganic contaminants into harmless components.[8,36] Metal oxide semiconductors based on photocatalysts have shown a promising capacity for the decomposition of organic contaminants.[15,37] Recently, various metal oxide-based semiconductors, such as BiOCl, SnO2, CeO2, NiO2, TiO2, and MnO2, have attracted wide attention due to their good stability and low toxicity. Moreover, Yao et al. reported various modified routes for the synthesis of BiOCl and used them in the photocatalysis of organic contaminates.[38] Furthermore, these materials have better chemical properties and suitable band gaps to stimulate the organic reaction under UV/visible solar irradiation.[39] Among different semiconductors, manganese dioxide (MnO2) is considered a popular and most active photocatalyst for the oxidation of contaminates due to its high chemical stability,[40,41] simple synthesis, low cost, environmental compatibility, low toxicity, and high adsorption. They have a narrow band gap as well as numerous morphologies over a wide range of temperatures up to 1200 °C.[8,10,42−44] Unfortunately, MnO2, due to a narrow band gap, offers low activity under visible light. Therefore, the photocatalytic activity of MnO2 under visible light can be improved by combining it with other semiconductors.[45,46] In addition, low electron transfer and quick recombination of photogenerated electron–hole restricted the photocatalytic efficiency of MnO2.[7,47−49] Hence, research has offered the best way to enhance the use of MnO2 in photocatalysis by rising the lifetime of charge carriers. Wu et al. synthesized the nanostructures of α-MnO2/Mn3O4 through catalytic oxidation via a potential-transformation synthesis process.[50] Zhang et al. reported the synthesis of the CuBi2O4/MnO2 composite for removal of lomefloxacin, ceftiofur, and tetracycline with the loading of 0.3 g·L–1 of catalyst at pH 11 and offered excellent percent rate degradation up to 93.6% within 40 min.[51] The nanocomposite 2D/2D g-C3N4/MnO2 fabricated by Xia et al. studied the decolorization of dye and proposed a direct Z-scheme.[52] Wan et al. investigated the photocatalytic degradation of Congo red, methylene blue, and rhodamine B in visible light with composite core–shell nanospheres of CS@MnO2.[53] The photocatalyst CS@MnO2 established an excellent performance for decolorizing cationic and anionic dyes. Bose et al. reported the coupling of α-MnO2 with h-MoO3 for photocatalysis and as a supercapacitor.[54] Besides, silver bromide (AgBr) is also considered a visible light p-type semiconductor having a low band gap (Eg = 2.69 eV) and is used in the photocatalytic reaction and antibacterial test.[55−58] On the other hand, examples of heterostructures like Ag/AgBr@m-WO3, AgBr/BiVO4, AgBrO3/AgBr, AgBr/La2Ti2O7, and BON-Br-AgBr have demonstrated excellent photocatalytic performance.[59−63] Besides, Hariharan et al. reported superior anticancer activity on A549 cell lines with Ag@TiO2 nanoparticles and compared with TiO2.[64] The present work aims to synthesize a highly efficient nanocomposite (β-MnO2/AgBr) with different molar ratios of AgBr (0.1, 0.2, and 0.3 M) by the co-precipitation route. Various methods will be used to test the morphology, structure, and optical properties of prepared photocatalysts. In addition, the photocatalytic activity will be tested by studying the decomposition of chromophoric dyes, such as MB and RhB, as well as a drug derivative like paracetamol, in an aqueous solution. These model compounds are present in industrial effluents, as they are chemically stable, carcinogenic, and non-biodegradable. The mechanistic investigation will be conducted by a quenching study with different electron acceptors and photoluminescence probe methods. In continuation of our studies, the anticancer activity of the synthesized nanocomposite will be investigated on A549 cell lines using an MTT assay. To the best of our knowledge, this is the first study on the utilization of the β-MnO2/AgBr nanocomposite for the degradation of MB, RhB, and paracetamol, as well as against human lung cancer cell lines.

Experimental Details

Synthesis of Pure AgBr Nanoparticles

1.7 g of AgNO3 was dissolved in 50 mL of distilled water under vigorous stirring for 30 min in the dark to form a solution (A). At the same time, 1.19 g of KBr was prepared in 50 mL of distilled water and labeled as solution (B). Solution (B) was added dropwise to solution (A) under stirring to give a yellow precipitate. The mixed solution was kept under continuous stirring for 1 h in the dark for complete precipitation of AgBr. The product was filtered, washed with water, and dried at 120 °C for 12 h.

Synthesis of Pure β-MnO2 Nanorods

β-MnO2 was synthesized by stirring a mixture of 1.69 g of manganese sulfate monohydrate (MnSO4·H2O) and 2.28 g of ammonium persulfate ((NH4)2S2O8) in 80 mL of water. The dissolved mixture was transferred into an autoclave (100 mL) and heated at 140 °C for 12 h. The obtained product was filtered, washed several times with water and ethanol, and then dried at 70 °C overnight.[65]

Synthesis of the AgBr/β-MnO2 Nanocomposite

0.2 g of as-synthesized β-MnO2 was suspended in 50 mL of water through stirring and sonicating for 45 min. To this mixture, 50 mL of 0.1 M aqueous solution of AgBr was added through stirring in the dark for 30 min. Later on, 50 mL of KBr (0.1 M) was added dropwise to the above mixture and stirred for 80 min for complete precipitation. The obtained product was filtered, washed thoroughly with water, and dried at 80 °C overnight. The product was designated as 0.1 AgBr/β-MnO2. For comparison, a similar procedure was adopted for the preparation of other prepared nanocomposite materials with 0.2 and 0.3 M AgBr onto β-MnO2, and the obtained samples were named 0.2 AgBr/β-MnO2 and 0.3 AgBr/β-MnO2, respectively.

Results and Discussion

Structural Study

The crystal structures of as-prepared samples (β-MnO2 and AgBr) including AgBr/β-MnO2 nanocomposites with the varying molar ratio of AgBr are presented in Figure . As can be seen from the obtained results, different diffraction peaks located at 28.63, 37.30, 40.84, 42.81, 56.60, 59.36, 65.87, 67.43, and 72.36° correspond to the (110), (101), (200), (111), (211), (220), (002), (310), and (301) tetragonal phases of pure photocatalyst β-MnO2.[65−67] The AgBr diffraction peaks centered at 26.68, 31.0, 44.4,52.69, 55.04, 64.2, and 73.36° were attributed to (111), (200), (220), (311), (222), (400), and (420) crystal planes of hexagonal AgBr.[56,58,68] The AgBr/β-MnO2 nanocomposites have the same diffraction peaks as those of AgBr. At the same time, the diffraction peaks of β-MnO2 are so weak that they are difficult to observe in AgBr/β-MnO2 nanocomposites compared with AgBr, which may be due to the small amount of β-MnO2 in the nanocomposite materials, and its existence was later confirmed by the EDS. According to the above results, the prepared nanocomposite photocatalyst is AgBr/β-MnO2.

Figure 1

XRD patterns of bare β-MnO2, AgBr, and AgBr/β-MnO2 nanocomposites with different molar ratios of AgBr.

FT-IR Analysis

FTIR spectroscopy was used to confirm the structural properties of the bare β-MnO2, AgBr, and nanocomposites AgBr/β-MnO2 with the varying molar ratio of AgBr, and their results are shown in Figure . The FTIR spectrum of β-MnO2 showed the bands at 429, 543, and 720 cm–1, corresponding to stretching and bending vibrations of the Mn–O bond in MnO6 octahedral units. The oxygen symmetry stretching modes of Mo=O appear at 1095 cm–1.[7,52,67] The FTIR spectrum of AgBr shows absorption peaks at 3442 and 1617 cm–1, which are caused by H2O molecules (H–O stretching and bending vibration) adsorbed on the surface of AgBr.[55] The typical characteristic peaks of AgBr and β-MnO2 are observed in AgBr/β-MnO2 nanocomposites and combined spectra are shown, which means that there was a covalent bond formed between the β-MnO2 and AgBr nanoparticles in the AgBr/β-MnO2 system as a result of the nanocomposite formation.

Figure 2

FTIR spectra of pure AgBr and β-MnO2 and the AgBr/β-MnO2 nanocomposite with varying concentrations of AgBr.

SEM Analysis

The surface morphology and microstructure of β-MnO2, AgBr, and 0.2 AgBr/β-MnO2 nanocomposites were investigated by SEM analysis at different magnifications, and the obtained images are shown in Figure . As displayed in (Figure a,b), the bare β-MnO2 demonstrated nanorod-shaped morphology. While Figure c,d showed smooth spherical-like particles in pure AgBr, the SEM images of AgBr/β-MnO2 nanocomposites (Figure e–f) showed that the majority of AgBr NPs are deposited on the surface of β-MnO2, which possesses a rod with a sphere-like structure. It is interesting to note that AgBr particles were found to be smaller in the nanocomposite compared to bare AgBr particles. This phenomenon implies that the β-MnO2 can significantly inhibit the growth of AgBr particles. The SEM analysis confirmed that the AgBr particles coupled with β-MnO2 nanorods to form binary AgBr/β-MnO2 hybrid heterostructures.

Figure 3

SEM images of pure β-MnO2(a,b), AgBr (c,d), and 0.2 AgBr/β-MnO2 nanocomposite (e,f) at various magnifications.

Energy Dispersion Spectra (EDS) and Elemental Mapping Analysis

The EDX analysis was carried out to determine the elemental composition of β-MnO2, AgBr, and 0.2 AgBr/β-MnO2 nanocomposites. The EDX confirms the existence of two elements (Mn and O) in the pure β-MnO2 (Figure a), while the bare AgBr clearly shows two absorption peaks related to Ag and Br elements (Figure b). Figure c displays the EDX spectrum of the 0.2 AgBr/β-MnO2 nanocomposite’s existence in all the elements (Mn, Ag, Br, and O), and the absorption peaks corresponding to these elements, which indicate the co-existence of AgBr and β-MnO2. To further investigate the elemental dispersion in the sample, EDS elemental mapping was performed, and the results are shown in Figure S1. The pure β-MnO2 shows uniformly distributed elements on the surface of a sample, while AgBr shows the non-uniform distribution of elements, as displayed in Figure S1a-b and Figure S1c,d, respectively. In addition, Figure S1e–h shows the elemental mapping of 0.2 AgBr/β-MnO2 nanocomposites, confirming that (Ag, Br, Mn, and O) elements are homogeneously distributed and the heterojunction nanocomposite is effectively created.

Figure 4

EDX analysis of pure β-MnO2(a), AgBr (b), and the 0.2 AgBr/β-MnO2 nanocomposite (c).

TEM Analysis

The internal morphology and crystal structure of β-MnO2, AgBr, and 0.2 AgBr/β-MnO2 nanocomposites were further investigated by TEM analysis, and the representative morphology is shown in Figure . Figure a displays the TEM image of β-MnO2, which confirmed the nanorod-shaped structure of the as-synthesized β-MnO2. Figure b illustrates the spherical shape of the AgBr sample. Figure c shows the TEM image of 0.2 AgBr/β-MnO2 nanocomposites, revealing the AgBr nanosphere’s dispersion on the surface of β-MnO2 nanorods and suggesting that the AgBr nanospheres have grown directly on the surface of β-MnO2 nanorods. It is worth noting that the TEM images of β-MnO2, AgBr, and 0.2 AgBr/β-MnO2 nanocomposites found structural morphologies that were similar to the SEM images in Figure .

Figure 5

TEM images of the as-synthesized β-MnO2 nanorod (a), AgBr nanospheres (b), and the 0.2 AgBr/β-MnO2 nanocomposite (c).

The synthesis of 0.2 AgBr/β-MnO2 heterojunctions has been confirmed by the foregoing discussions.

Optical Studies

The UV–vis diffuse reflectance analysis was used to investigate the optical properties of the generated β-MnO2, AgBr, and AgBr/β-MnO2 nanocomposites with different molar ratios of AgBr, and the results are shown in Figure a. The absorption edge for AgBr was found to be around 469 nm, while the UV spectrum for β-MnO2 nanorods showed a broad absorption band in the region of 520 to 800 nm with a peak at 595 nm, which is attributed to the d–d transition of Mn ions.[69] On the other hand, AgBr/β-MnO2 composites with different AgBr molar ratios showed a wide absorption peak from 562 to 800 nm with the absorption edge at 600–635 nm. The results indicate that the absorption of the AgBr/β-MnO2 nanocomposite increased with the loading of AgBr on β-MnO2 materials, which was ascribed to the formation of Ag nanoparticles, which may be beneficial for light absorption and enhanced performance. The band gap energies of β-MnO2, AgBr, and AgBr/β-MnO2 nanocomposites were determined by eq .[36]

Figure 6

(a) UV–vis diffuse reflectance spectra of different materials and (b) their corresponding Tauc’s plot.

Where the absorption coefficient, Planck’s constant, light frequency, and band gap energy are denoted by α, h, υ, and Eg, respectively. For β-MnO2, AgBr, and AgBr/β-MnO2 nanocomposites with varying molar ratios of AgBr, the value of n is 1/2.[7,68,69] According to the plots of (αhν)1/2 versus hν (Figure b), the calculated values of Eg for β-MnO2, AgBr, 0.1 AgBr/β-MnO2, 0.2 AgBr/β-MnO2, and 0.3 AgBr/β-MnO2 were 1.33, 2.64, 1.30, 1.26, and 1.17 eV respectively.

Photoluminescence Study (PL)

PL spectroscopy was used to assess the recombination rate of photogenerated (e–/h+) pairs in the synthesized samples. In general, fast e– and h+ recombination results in a higher PL signal, while lower recombination of e– and h + results in a lower PL signal. Figure shows the PL spectra of β-MnO2, AgBr, and AgBr/β-MnO2 nanocomposites with various molar ratios of AgBr at an excitation of 350 nm. All synthesized samples produced emission peaks at about 680 nm; however, the 0.2 AgBr/β-MnO2 sample showed the weakest emission signal, implying that the heterojunction between AgBr and β-MnO2 could inhibit their recombination rate of photogenerated charge carriers and effectively enhance photocatalytic activity.

Figure 7

Photoluminescence (PL) spectrum-synthesized photocatalysts in DMSO at 350 nm excitation.

Electrochemical Impedance Spectroscopy (EIS) Analysis

Photocatalytic performance is influenced by the efficiency of charge separation of the photogenerated electron–hole pair, and EIS analysis gives the details of this parameter. In general, the lower charge transfer resistance and higher charge transfer impact are represented by the smaller arc radius.[36,62]Figure shows the Nyquist impedance spectra of β-MnO2, AgBr, 0.1 AgBr/β-MnO2, 0.2 AgBr/β-MnO2, and 0.3 AgBr/β-MnO2 photocatalysts. Among all samples, the EIS of 0.2 AgBr/β-MnO2 exhibits the lowest semicircle diameter, signifying the lowest charge transfer resistance and the best transfer efficiency of the photoexcited charge carriers. In addition, the EIS analyses further confirm the formation of AgBr/β-MnO2 heterostructures, enhanced (e–/h+) separation, and interface charge transfer properties.

Figure 8

Nyquist plots of all above-synthesized materials.

Photocatalytic Degradation Study

To test the photocatalytic activity of all the produced catalysts, they were used to study the photo-decolorization of dyes (RhB and MB) in an aqueous solution under a visible light source in the presence of atmospheric air. All photochemical tests were carried out after 30 min of agitation of the aqueous dye solution in the presence of a photocatalyst to equilibrate adsorption–desorption. The UV–vis absorption spectra of an irradiated aqueous solution of MB and RhB at different time intervals in the presence of 0.2 AgBr/β-MnO2 nanocomposites are shown in Figures a and 10a. The absorption intensity diminishes as illumination time increases, leading to a 97% degradation of MB in 12 min and 98.9% of RhB in 9 min, respectively.

Figure 9

(a) Changes in methylene blue (MB) absorbance on irradiation of an aqueous suspension over 0.2 AgBr/β-MnO2 nanocomposites and (b) changes in MB concentration as a function of time in the absence and presence of different catalysts.

Figure 10

(a) Changes in RhB absorbance over 0.2 AgBr/β-MnO2 nanocomposites and (b) changes in RhB concentration as a function of time in the absence and presence of various catalysts under visible light illumination.

Figures b and 10b depict the change in concentration against illumination time for the degradation of MB and RhB under comparable conditions with and without photocatalysts. The figure shows that in the absence of catalysts, dye degradation is negligible, implying that they are extremely stable and cannot be decolorized by direct photolysis. It was also discovered that in the presence of 0.2 AgBr/β-MnO2, both dyes degraded more effectively than in the presence of any other produced catalysts.

To distinguish between direct and indirect photocatalysis, the degradation of a colorless compound such as paracetamol was studied under similar reaction conditions. The change in UV–vis absorption spectra of paracetamol in an aqueous suspension is illuminated over 0.2 AgBr/β-MnO2 nanocomposites under visible light irradiation as shown in Figure a. The results show that the optical intensity diminishes with increasing illumination time, showing 80% degradation after 180 min.

Figure 11

(a) Changes in paracetamol absorbance over 0.2 AgBr/β-MnO2 nanocomposites and (b) changes in paracetamol concentration as a function of time in the absence and presence of produced catalysts under visible light illumination.

In addition, Figure b shows the change in paracetamol concentration as a function of irradiation time with and without various catalysts. Without a catalyst, the photodecomposition of paracetamol is mostly negligible, indicating that paracetamol is stable under direct photolysis. Furthermore, the 0.2 AgBr/β-MnO2 nanocomposite was found to degrade paracetamol faster than any other photocatalyst.

HPLC Measurement

The photocatalytic degradation of paracetamol by the synthesized catalyst (0.2 AgBr/β-MnO2) in the presence of visible light was also carried out by HPLC to analyze the by-products formed during the photo-oxidation reaction. The starting material peak, which occurs at a retention time of R = 7.5 min, continuously drops with illumination time and is eliminated within 180 min, as shown in Figure S2. Furthermore, no by-products are detected as observed in the HPLC spectra.

Kinetics of RhB Degradation

The kinetic models are essential for familiarizing yourself with the behavior of the photocatalytic activity. Herein, we have utilized three kinetic models to determine the kinetics of RhB decolorization with all produced catalysts. Eqs –4 expressed in terms of the decomposition rate with pseudo zero-, first-, and second-order kinetics.

The concentrations of RhB before and after irradiation time t are C0 and C, respectively, and k0, k1, and k2 are the pseudo zero-, first-, and second-order rate constants. Figure a–c show the linear fit for the decolorization of RhB with bare AgBr, β-MnO2, and various nanocomposites (AgBr/β-MnO2) vs. irradiation time. The rate constant (k0, k1, and k2) for the removal of RhB, corresponding to pure and nanocomposite materials determined from Figure , is shown in Table . The table displays the rate constant and the corresponding correlation coefficient (R2) for AgBr, β-MnO2, and nanocomposites (AgBr/β-MnO2) with a varied molar ratio of AgBr (obtained from a linear fit of C, ln(C0/C), and 1/C vs. irradiation time). In comparison to pure materials, all nanocomposite materials showed a better apparent rate constant. Due to more significant dye adsorption on the catalyst’s surface, the photocatalyst 0.2 AgBr/β-MnO2 nanocomposite demonstrated the most significant rate constant within 9 min of irradiation duration. In addition, the results suggest that the first-order model showed a better fit than the zero- and second-order models for the photocatalytic decomposition of RhB dye.

Figure 12

Plots of C(a) – ln C/C0(b) and 1/C(c) at vs. time irradiation along with their linear fits for the decomposition of RhB with and without various photocatalysts under visible light.

Table 1

Pseudo Zero-, First-, and Second-Order Rate Constants, and the Corresponding R2 Values for all the Synthesized Photocatalysts

	kinetic model parameters
	zero-order		first-order		second-order
sample	(kapp) (ppm min–1)	R2	(kapp) (min–1)	R2	(kapp) (ppm–1 min-1)	R2
blank	–0.0114	0.8499	0.0	0.0	0.0033	0.8211
β-MnO2	–0.019	0.9658	0.01345	0.77624	0.0066	0.95526
AgBr	–0.110	0.9438	0.0658	0.92791	0.08741	0.9582
0.1 AgBr/β-MnO2	–0.1664	0.7849	0.2463	0.9881	0.80305	0.94765
0.3 AgBr/β-MnO2	–0.1621	0.7911	0.3233	0.98816	0.4539	0.98726
0.2 AgBr/β-MnO2	–0.1736	0.7027	0.4964	0.99782	2.4057	0.86581

Stability of the Photocatalyst

The recyclability of photocatalysts has always been an essential factor in determining their stability. As a result, for the degradation of RhB dye under comparable conditions, a four-cycle photostability test of 0.2 AgBr/β-MnO2 was performed. The catalyst was collected after each irradiation, filtered, washed, dried, and reused for the next photoreaction experiment. Figure a shows the % degradation of RhB during each cycle of irradiation of an aqueous suspension for 9 min under similar conditions. A minimal decrease in photocatalytic efficacy was seen after four progressive runs, demonstrating the outstanding stability of the 0.2 AgBr/β-MnO2 nanocomposite, which is essential from an application standpoint. In addition, the XRD study of 0.2 AgBr/β-MnO2 before and after the photocatalytic experiment revealed no difference in the crystal structure of the catalyst (Figure b), showing outstanding stability of the produced material. The results clearly indicate that the photocatalyst is easily extracted and recycled for subsequent reactions with no perceptible efficiency loss.

Figure 13

(a) Concentration change of RhB at different cycling runs over 0.2 AgBr/β-MnO2 and (b) XRD of 0.2 AgBr/β-MnO2 before and after the runs of the photolysis experiment.

In Vitro Cell Viability Studies Using the MTT Assay

The nanocomposite AgBr/β-MnO2 (with a 0.2 M concentration of AgBr) performed better photodegradation activity and was chosen to evaluate the anticancer test. In vitro anticancer activity of 0.2 AgBr/β-MnO2 was carried out on human lung cancer cell lines (A549). The viability of the cells was evaluated using an MTT assay by increasing the treatment concentration (25–200 μg/mL) of catalysts under dark and visible light conditions after 48 h. The detailed experimental procedure has been provided in the Supporting Information. The catalyst showed excellent anticancer activity under visible light and was found to inhibit A549 cell proliferation in a dose-dependent manner. Figure indicates that there was no difference in the activity under dark and light conditions up to 100 μg/mL, but an enhancement in the activity was observed at higher concentrations under visible light treatment. For instance, cell viability at 200 μg/mL under dark treatment was approximately 45%, which was reduced to approximately 30% upon exposure to visible light. These observations could be attributed to the fact that photocatalysts increase ROS production, which leads to cell death. Furthermore, Figure clearly shows a reduction in the typical morphology of A549 cells upon treatment with 0.2 AgBr/β-MnO2.

Figure 14

Cell viability of A549 human lung cancer cells treated under dark and visible light conditions with 0.2 AgBr/β-MnO2.

Figure 15

Changes in the morphology of A549 lung cancer cells were observed upon treatment with (a) control and (b) 0.2 AgBr/β-MnO2.

Quenching Experiments and Photocatalytic Mechanisms

The purpose of the trapping studies was to find out the key active species involved in the photodegradation reaction. During RhB photodegradation, quenchers such as benzoquinone (BQ), isopropanol (IPA), and ethylenediaminetetraacetic acid disodium (EDTA-2Na) are used to capture O2, •OH, and h+, respectively. Figure a demonstrates the effect of IPA, BQ, and EDTA-2Na on the photocatalytic decomposition of RhB in the presence of 0.2 AgBr/β-MnO2 in an aqueous solution under visible light. The results imply that O2 and h+ were the primary active species. On the other hand, the introduction of isopropyl alcohol as an •OH scavenger did not affect the photodegradation of RhB.

Figure 16

(a) Effect of different scavengers on photocatalytic degradation of RhB over 0.2 AgBr/β-MnO2 nanocomposites under visible light and (b) ESR signals of DMPO-•O2– for 0.2 AgBr/β-MnO2 in the dark and under influence of light (irradiation time 1 and 2 min).

The ESR study was conducted using 0.2 AgBr/β-MnO2 composites to further demonstrate the formation of the main active substance during the degradation process, and the results obtained are shown in Figure b. As observed from the figure, no DMPO-•O2– signals are produced in the dark; however, DMPO-•O2– peaks can be seen under the influence of light (500 W lamp, 1–2 min irradiation). This result and the trapping experiment imply that •O2– is the primary active species.

In addition, the participation of •OH in the photocatalytic degradation process was investigated using the terephthalic acid photoluminescence probe method. The irradiation of terephthalic acid under an alkaline medium in the presence of a photocatalyst produces 2-hydroxy terephthalic acid at an excitation of 315 nm, giving a fluorescence peak at 425 nm.[70]Figure exposed the fluorescence spectrum during light irradiation of a basic terephthalic acid over the P25 and 0.2 AgBr/β-MnO2. Figure a shows that the peak intensity centered at 425 nm gradually increases with irradiation time in the presence of P25, indicating the participation of the hydroxyl radical to give TA-OH. Nevertheless, no fluorescence signals were found in the presence of 0.2 AgBr/β-MnO2 (Figure b), signifying that the hydroxyl radicals in this photodegradation process are not prominent under prevailing conditions.

Figure 17

Fluorescence spectrum recorded during illumination of a 5 × 10–4 M basic terephthalic acid solution (λexc = 315 nm) with TiO2(a) and 0.2 AgBr/β-MnO2(b).

The positions of band edges for the semiconductor were estimated using the electronegativity concept to investigate the charge transfer mechanism of the 0.2 AgBr/β-MnO2 heterojunctions using the following equations:Where ECB and EVB are the CB and VB edge potentials, respectively, Ee is the free electron energy on the hydrogen scale (4.5 eV), χ is the absolute electronegativity of the compound, and Eg is the semiconductor band gap. The absolute electronegativity was determined from the mean of the first ionization energy (eV) and electron affinity (eV). Thus, the calculated values of χ for β-MnO2 and AgBr were found to be 5.94 and 5.8 eV, respectively. By referring to eqs and 6, the values of the VB and CB for β-MnO2 are found to be 2.1 and 0.77 V, and those for AgBr are 2.62 and −0.02 V, respectively. The measured band gap energies (Eg) of β-MnO2 and AgBr are 1.33 and 2.64 eV, respectively. Based on the aforementioned experimental findings, a potential Z-scheme photocatalytic mechanism for the excellent photocatalytic performance of the AgBr/β-MnO2 nanocomposite has been proposed and is schematically represented in Schemes and 2. It is well known that both β-MnO2 and AgBr can absorb photons and create electron/hole pairs when exposed to light (Scheme ). Because the CB of AgBr is more negative than that of β-MnO2, photogenerated electrons can be transported from the CB of AgBr to the CB of the β-MnO2 semiconductor. In addition, the valence band (VB) of β-MnO2 (2.1 V) is lower than the VB of AgBr (2.62 V), and the photogenerated holes in the VB of AgBr can move to the VB of β-MnO2; meanwhile, the VB potential of β-MnO2 (2.1 V) is lower than the potentials of •OH/OH– (+2.38 V) and •OH/H2O (+2.72 V);[56,71] the photoinduced h+ in the VB of β-MnO2 cannot oxidize OH– or H2O to •OH, which is in agreement with the fluorescence measurement that •OH remains undetected by using β-MnO2/AgBr as the catalyst under light illumination, and the photoinduced h+ in the VB of β-MnO2 can directly oxidize the adsorbed organic compound to give harmless products. Moreover, the accumulated electron in the CB of β-MnO2 could not react with O2 to produce O2•– due to the redox potential of O2/O2•– (−0.33 V vs. NHE) being more negative than the CB energy level of β-MnO2 (0.77 V). This contradicted the results of the above-mentioned trapping studies, which showed that –•O2 was the dominant reactive species in the AgBr/β-MnO2 system. As a result, the Z-scheme system would be closer to the actual photocatalytic mechanism. As shown in Scheme , the photogenerated electrons in the ECB of β-MnO2 tend to transfer and recombine with the photogenerated holes in the EVB of AgBr due to the relative positions of the ECB and EVB, resulting in efficient separation of electrons in the CB of β-MnO2 and holes in the VB of AgBr.

Scheme 1

Possible Schematic Diagram of the Heterojunction Structure of the Charge Separation and Transport in the Photocatalytic Process Under Visible Light Over AgBr/β-MnO2

Scheme 2

Possible Z-Scheme Mechanism of the Charge Separation and Transport in the Photocatalytic Process Under Visible Light Over AgBr/β-MnO2

The photoinduced electrons in the CB of AgBr could easily be shifted to the Fermi level of Ag nanoparticles. Interestingly, the photo-corrosion of AgBr at the interface between AgBr and β-MnO2 under visible light irradiation results in the formation of metallic Ag nanoparticles. As the Fermi level of Ag −0.4 V (vs. NHE)[56,72] shifts from −0.02 V (vs. NHE) to an energy level greater than the usual redox potential of O2/O2•– (−0.33 V vs. NHE), the transferred electrons on the surface of AgBr nanoparticles may then react with absorbed oxygen to produce –•O2,[56,73−75] which might further react directly with the compound to give the degradation products, whereas the holes in the VB of β-MnO2 could directly oxidize the organic compound.

These results are consistent with the trapping experiment, which showed that adding IPA as an •OH scavenger did not affect the degradation process, while adding BQ/EDTA as –•O2 and h + drastically affects the degradation rate (Figure ).

Conclusions

In conclusion, Z-scheme AgBr/β-MnO2 nanocomposites were effectively produced using a simple in-situ precipitation process used in the photocatalytic decomposition of organic compounds in water using visible light (>420 nm). Different techniques were used to investigate the as-made photocatalyst’s structure, morphology, surface, and optical properties. Under visible light irradiation, the synthesized nanocomposite AgBr/β-MnO2 showed better activity for the photodecomposition of MB, RhB, and paracetamol compared with pure AgBr and β-MnO2. Of these, the nanocomposite AgBr/β-MnO2 with 0.2 M AgBr onto β-MnO2 showed the highest activity. Furthermore, radical trapping tests demonstrated the Z-scheme photodegradation of the compounds under investigation using the as-obtained AgBr/β-MnO2 nanocomposite. Besides, the photo-corrosion of AgBr nanoparticles was significantly reduced due to the effective transfer of electrons between AgBr and β-MnO2. This work may give more insights into the understanding of the mechanism for the degradation of organic compounds with AgBr/β-MnO2 composites via Z-scheme heterostructure design. In addition, the visible light-mediated anticancer activity of AgBr/β-MnO2 was carried out on A549 lung cancer cell lines. The MTT assay indicated higher anticancer activity (cell viability = 30%) upon irradiation compared to dark (cell viability = 45%) on treatment with 200 μg/mL of 0.2 AgBr/β-MnO2. This may be due to the generation of a higher level of ROS in vitro over irradiated AgBr/β-MnO2, leading to cell cytotoxicity. A more mechanistic investigation of its potential activity is currently underway in our laboratory. This approach offers several advantages such as the use of visible light, short reaction time, use of a smaller amount of the photocatalyst, high degradation efficiency, utilization of air as the oxidant, use as an anticancer agent, and ease of catalyst recycling without reduction in the degradation efficiency.