Facile fabrication and enhanced visible-light photocatalytic activity of In₂O₃/Ag₂CrO₄ composites

Abstract

A series of visible-light-driven In₂O₃/Ag₂CrO₄ composites were fabricated by a facile chemical precipitation method and characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and photoluminescence (PL) spectroscopy. The In₂O₃/Ag₂CrO₄ composites showed much higher photocatalytic activity and stability than those of individual Ag₂CrO₄ and In₂O₃ for the photodegradation of methyl orange (MO) under visible light irradiation (λ > 420 nm). Excitingly, the optimum degradation rate constant of the In₂O₃/Ag₂CrO₄ composite at a weight content of 4.0% In₂O₃ for the degradation of MO was 8.8 and 3.3 times as high as that of individual In₂O₃ and Ag₂CrO₄, respectively. The enhancement in photocatalytic activity and stability was predominantly attributed to the effective separation and transfer of photogenerated charge carriers as a result of the formation of a Z-scheme system composed of In₂O₃, Ag and Ag₂CrO₄. Furthermore, radical trap experiments depicted that both the holes and superoxide radical anions are main oxidative species of the In₂O₃/Ag₂CrO₄ composite for MO degradation under visible light irradiation. Finally, based on the experimental results, a possible photocatalytic mechanism has been proposed.

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Introduction

With the increasing aggravation of the energy dilemma and environmental issues, semiconductor photocatalysis has been regarded as one of the most promising strategies for dealing with current environmental pollutants and energy problems through photocatalytic water splitting to produce hydrogen and degradation of organic pollutants into CO₂ and H₂O with abundant solar light.¹ Up to now, an increasingly large number of photocatalysts including TiO₂, ZnO, WO₃, CdS, Ag₃VO₄, Bi₂WO₆, Bi₂MoO₆, ZnWO₄, SrTiO₃, BiOX (X = Cl, Br, I) and more, have been intensively investigated.² Among them, it is well known that TiO₂ has excellent photocatalytic activity and stability under ultraviolet light irradiation. Unfortunately, it can only be activated under ultraviolet light irradiation due to its relatively large band gap (approximately 3.2 eV).³ It is noted that the ultraviolet region occupies only approximately 4% of the entire solar spectrum, while about 43% of the energy belongs to the visible light.⁴ Consequently, in order to utilize solar energy more efficiently in the photocatalytic processes, numerous attempts have been strived to the development of visible-light-driven photocatalysts, such as graphitic carbon nitride,⁵ Ag-based photocatalysts,⁶ Bi-based photocatalysts,⁷ etc. Among them, silver chromate (Ag₂CrO₄), as a novel high-efficiency visible-light-driven photocatalyst, has been recognized as one of the most promising photocatalysts for the degradation of organic pollutants under visible light irradiation due to its strong absorption in visible-light region, unique electronic structure and crystal structure.⁸ Specially, the valence band position of Ag₂CrO₄ is raised by the hybridization of Ag 4d and O 2p orbital, and the conduction band position is lowered by Cr element, leading to a narrow band gap of approximately 1.8 eV. Additionally, the crystal structure of Ag₂CrO₄ is beneficial to effectively facilitate the migration of photogenerated electrons and holes due to its long Ag–O bond length and big O–Ag–O bond angle of AgO₆ octahedron.⁹ Nevertheless, similar to other silver-based photocatalysts, Ag₂CrO₄ greatly suffers from poor stability during the photocatalytic reactions, because it is photosensitive and slightly soluble in aqueous solution.¹⁰ As a consequence, it is highly desirable to explore effective strategies to inhibit the photocorrosion of Ag₂CrO₄ for better stability and activity.

Fortunately, combining Ag₂CrO₄ with other semiconductors with matching band potential to design Ag₂CrO₄-based composites, such as AgBr/Ag₂CrO₄,¹⁰ ZnO/Ag₂CrO₄,¹¹ ZnO/Ag₃VO₄/Ag₂CrO₄,¹² C₃N₄/Fe₃O₄/Ag₂CrO₄,¹³ ZnO/AgBr/Ag₂CrO₄,¹⁴ GO/Ag₂CrO₄,^9b has proven to be an effective strategy for improving the photocatalytic activity and stability of Ag₂CrO₄ under visible light irradiation, because matching band potentials of the two components is contributed to the separation of photogenerated electron–hole pairs, prolonging the lifetime of charge carriers, further resulting in the improvement of photocatalytic activity and stability to some degree. Furthermore, In₂O₃, as an important n-type semiconductor with a direct band gap of about 3.6 eV and indirect band gap of about 2.8 eV, has proved to be an efficient sensitizer to extend the absorption spectra of oxide semiconductor photocatalysts from the ultraviolet region into the visible region.¹⁵ Nonetheless, to the best of our knowledge, there is still no report on the study of In₂O₃/Ag₂CrO₄ composites as an effective visible-light-driven photocatalysts by far. Excitingly, by comparing the energy band positions of Ag₂CrO₄ with In₂O₃, it is fortunately to find that their well-matched energy band structures are suitable to separate and transfer of photogenerated charge carriers, and thus enhance photocatalytic activity.

In this work, a facile chemical precipitation method has been successfully developed to synthesize novel In₂O₃/Ag₂CrO₄ composites with different content of In₂O₃ at room temperature without using any surfactants. The photocatalytic activity and stability of the as-prepared samples were evaluated in the photocatalytic degradation of methyl orange (MO) organic dye in an aqueous solution under visible light irradiation (λ > 420 nm). The results demonstrated that the introduction of In₂O₃ can remarkably improve the photocatalytic activity of Ag₂CrO₄. The enhanced activity was mainly attributed to the intimate interfacial contact and the matched energy band structures between Ag₂CrO₄ and In₂O₃, which effectively improved the separation and transfer of photogenerated charge carriers as a result of the formation of a Z-scheme system composed of In₂O₃, Ag and Ag₂CrO₄. Furthermore, according to the results of trapping experiments, the hole (h⁺) and superoxide free radical (˙O₂⁻) were the main active species in the process of photocatalytic degradation of MO. Finally, a tentative transferred and separated behaviors of photoinduced charge carriers and the photocatalytic mechanisms was proposed based on the band structures of Ag₂CrO₄ and In₂O₃ and obtained experimental results. The present work may provide a new insight for the smart design and synthesis of various highly efficient visible-light-driven photocatalysts for environmental and energy applications.

Experimental

Catalysts preparation

In₂O₃ nanoparticles were prepared by a facile one-pot method by directly heating In(NO₃)₃·4.5H₂O at 500 °C in a muffle furnace for 4 h in a semiclosed system at a heating rate of 20 °C min⁻¹ under air condition. The product was washed several times with distilled water and absolute ethanol and dried at 80 °C for 12 h.

In₂O₃/Ag₂CrO₄ composites were fabricated by a facile in situ chemical precipitation method under the dark condition. Typically, first of all, a certain amount of the as-prepared In₂O₃ samples (13.5 mg, 27.6 mg and 57.7 mg) were sonicated thoroughly into 30 mL distilled water for 30 min. Then, 4 mmol AgNO₃ was added into the dispersion of In₂O₃ under the vigorous stirring. After stirring for 30 min, 2 mmol K₂CrO₄ dissolved in 20 mL distilled water was added dropwise into the dispersion with constant stirring, and the mixture was further vigorously stirred at room temperature for 4 h. Finally, the precipitate was collected by centrifugation, washed several times with distilled water and absolute ethanol, and dried at 55 °C in a vacuum oven for 24 h. The obtained products were denoted as In₂O₃(x)/Ag₂CrO₄, where x% stands for the theoretical mass percent of In₂O₃ in the In₂O₃/Ag₂CrO₄ composites. That is to say, the as-prepared samples were also accordingly denoted as In₂O₃ (2.0 wt%)/Ag₂CrO₄, In₂O₃ (4.0 wt%)/Ag₂CrO₄ and In₂O₃ (8.0 wt%)/Ag₂CrO₄. Moreover, for comparison, pure Ag₂CrO₄ nanoparticles were prepared following the similar procedure mentioned above in the absence of In₂O₃.

Catalysts characterization

X-ray diffraction (XRD) patterns of the as-prepared samples were recorded on an X-ray diffractometer (D/max-IIIA, Japan) using Cu K_α radiation. The surface morphology of as-prepared samples was examined by a scanning electron microscopy (SEM) (LEO1530VP, LEO Company) and a high-resolution transmission electron microscope (HRTEM, JEOL, JEM2100). The UV-vis light absorption spectra of as-prepared samples were obtained from a Hitachi UV-3010 spectrophotometer equipped with an integrating sphere assembly and using the diffuse reflection method and BaSO₄ as a reference to measure all the samples. The each of the chemical nature elements in the In₂O₃/Ag₂CrO₄ has been studied using X-ray photoelectron spectroscopy (XPS) in Krato Axis Ultra DLD spectrometer. The binding energy was referenced to C 1s line at 284.6 eV for calibration. Photoluminescence (PL) spectra were measured on an F-7000 Fluorescence spectrophotometer (Hitachi, Japan).

Photocatalytic tests of catalysts

The photocatalytic performance of the as-prepared samples was evaluated through the photodegradation of MO under visible light. A 300 W Xe-arc lamp equipped with a 420 nm cutoff filter was used as a visible light source. In a typical photocatalytic measurement, suspension including the photocatalyst (50 mg) and MO solution (150 mL, 10 mg L⁻¹) was laid in a 250 mL cylindrical quartz reactor equipped with a water circulation facility. Before irradiation, the reaction suspension was ultrasonicated for 5 min and stirred in the dark for 60 min to ensure the equilibrium of adsorption and desorption. During the photocatalytic tests, 5 mL of the suspension was obtained at a given time intervals, followed by centrifugation at 10 000 rpm for 10 min to remove the photocatalyst. The concentration of the remaining MO was measured by its absorbance (A) at 465 nm with a Hitachi UV-3010 spectrophotometer. The degradation ratio of MO can be calculated by X = (A₀ − A_t)/A₀ × 100%, where A₀ and A_t are the concentration of MO before illumination and after illumination time t.

Results and discussion

To identify the crystal structure and phase composition of the as-prepared samples, X-ray diffraction (XRD) analyses were carried out. Fig. 1 displays the XRD patterns of the In₂O₃/Ag₂CrO₄ composites with different weight ratios of In₂O₃, together with those of pure Ag₂CrO₄ and In₂O₃. It can be seen that all of the diffraction peaks are narrow and sharp, indicated that all the as-prepared samples are good crystallinity. Pure Ag₂CrO₄ shows fourteen distinct diffraction peaks at 2θ = 17.62°, 21.73°, 31.14°, 31.43°, 32.30°, 33.71°, 39.26°, 44.26°, 45.40°, 52.07°, 55.84°, 57.07°, 61.93° and 62.60°, which can be perfectly indexed to the (020), (120), (031), (211), (002), (131), (122), (240), (222), (400), (242), (213), (431) and (402) crystal planes of the orthorhombic Ag₂CrO₄ (JCPDS Card no. 26-0952).^8a,16 On the other hand, pure In₂O₃ shows five distinct diffraction peaks at 2θ = 21.50°, 30.58°, 35.47°, 51.04° and 60.68°, which can be perfectly indexed to the (211), (222), (400), (440) and (622) crystal planes of the cubic In₂O₃ (JCPDS Card no. 06-0416).^15c,17 Although the characteristic peaks of In₂O₃ is not observed from the In₂O₃ (2.0 wt%)/Ag₂CrO₄ and In₂O₃ (4.0 wt%)/Ag₂CrO₄ samples, which may be due to the relatively low amounts of In₂O₃, the position of the diffraction peak at 2θ = 30.58° of In₂O₃ is visible for the In₂O₃ (8.0 wt%)/Ag₂CrO₄ sample, as a result of the higher In₂O₃ content. It was worthwhile to note that no impurities peaks are detected in the XRD patterns, suggesting that the as-prepared samples are all pure phase. Hence, it is confirmed the In₂O₃/Ag₂CrO₄ composites are composed of In₂O₃ and Ag₂CrO₄.

Fig. 1 XRD patterns of the as-prepared samples.

The morphologies of pure Ag₂CrO₄, In₂O₃ and In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite were investigated by scanning electron microscopy (SEM) observation, and the results are showed in Fig. 2. It can be seen from Fig. 2a that the morphology of pure Ag₂CrO₄ is composed of stacked particle-like nanostructures with smooth surfaces. Fig. 2b is an image of single In₂O₃, which consists of small and irregular agglomerates of particles. The SEM image of In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite was presented in Fig. 2c, and it can be clearly seen that In₂O₃ particles are tightly attached and well dispersed on the surface of Ag₂CrO₄, resulting in the intimate contact between In₂O₃ and Ag₂CrO₄, which was beneficial to facilitate the transfer of photogenerated charge carriers and thus improve the photocatalytic activity. It can be confirmed further by the elemental mapping images of In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite. As shown in Fig. 2g and h, the In and O elements are homogeneously distributed in the whole host of In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite, indicating that In₂O₃ was not only successfully combined with Ag₂CrO₄, but also well dispersed on the surface of Ag₂CrO₄. In order to further illuminate the formation of In₂O₃/Ag₂CrO₄ composite, which was characterized by high-resolution TEM (HRTEM). Obviously, the junction displayed two types of clear lattice fringes as depicted in Fig. 2d, one set of the clear fringes spacing was ca. 0.29 nm, corresponding to the (222) lattice spacing of the cubic phase of In₂O₃.¹⁸ Another set of the clear fringes spacing measures ca. 0.28 nm, which corresponded to the (211) lattice plane of orthorhombic phase Ag₂CrO₄.^8a These results confirmed that the heterojunctions were well-formed between In₂O₃ and Ag₂CrO₄. Furthermore, in order to investigate elemental composition of as-synthesized samples, the energy dispersive X-ray spectrum (EDS) was further measured, which can be seen in Fig. 2i. It can be unambiguously seen that the coexistence of In, O, Ag and Cr elements and no other element or impurity found in the fabricated sample, demonstrating that the as-prepare In₂O₃/Ag₂CrO₄ composites was only composed of both In₂O₃ and Ag₂CrO₄.

Fig. 2 SEM (a–c) images of Ag₂CrO₄ (a), In₂O₃ (b) and In₂O₃ (4.0 wt%)/Ag₂CrO₄ (c), and HRTEM image (d), elemental mapping images (e–h) and EDS (i) of In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite.

The elemental compositions and chemical states of the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite were further analyzed by X-ray photoelectron spectroscopy (XPS), and the results are displayed in Fig. 3. The fully scanned XPS spectrum (Fig. 3a) showed that elements In, O, Ag, Cr and C existed in the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite, which is consistent with chemical composition of the In₂O₃ (4.0 wt%)/Ag₂CrO₄ photocatalyst. It is noted that the C element could be ascribed to the adventitious carbon-based contaminant, and the binding energy for C 1s peak at 284.6 eV was used as the reference for calibration. The high-resolution XPS spectra of C 1s, O 1s, In 3d, Ag 3d and Cr 2p peaks for In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite are represented in Fig. 3b–f. For the C 1s, the main peak located at 284.6 eV is usually assigned to adventitious carbon, while another weak peak positioned at 285.6 eV is corresponded to the C–OH bonding.^15b,19 For the O 1s, the peak at 529.7 eV is attributed to the oxygen species in the In–O–In of In₂O₃, while another peak of 531.4 eV is ascribed to the lattice oxygen, which is related to the Cr–O chemical bonding in the Ag₂CrO₄.^8a,20 For the In 3d, the two peaks located at 444.3 eV and 451.9 eV are attributed to In 3d_5/2 and In 3d_3/2, respectively,^8b,20a which is the characteristic of In³⁺ species for In₂O₃. For the Ag 3d, the two peaks located at 367.5 eV and 373.5 eV are attributed to Ag 3d_5/2 and Ag 3d_3/2, respectively,^8a,21 suggesting the presence of Ag⁺ species for Ag₂CrO₄. No peak is observed at 368.4 or 374.4 eV, demonstrating that no Ag⁰ is formed during the preparation.^9b,22 For the Cr 2p, the two peaks located at 578.4 eV and 587.6 eV are attributed to Cr 2p_3/2 and Cr 2p_1/2, respectively,²³ demonstrating the presence of Cr⁶⁺ species for Ag₂CrO₄. Consequently, the above XRD, EDS and XPS results distinctly confirm the coexistence of In₂O₃ and Ag₂CrO₄ in the as-prepared In₂O₃/Ag₂CrO₄ composites.

Fig. 3 XPS spectra of In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite: (a) survey scan, (b) C 1s, (c) O 1s, (d) In 3d, (e) Ag 3d and (f) Cr 2p.

It is well known that the photocatalytic activity of the photocatalysts is closely related to their light absorption ability, thus UV-vis diffuse reflectance spectroscopy (UV-vis DRS) is employed to determine the optical absorption properties of the pure Ag₂CrO₄, In₂O₃ and In₂O₃/Ag₂CrO₄ composites, and the results are represented in Fig. 4. As can be seen from Fig. 4a, the pure In₂O₃ shows photoabsorption from ultraviolet light to visible light, with an absorption edge of around 470 nm. On the other hand, the pure Ag₂CrO₄ presents photoabsorption from ultraviolet light to visible light, with an absorption edge of around 720 nm, presented light-absorbing abilities nearly in the entire visible light region. Interestingly, it can be observed that there is a clear red-shift of the absorption of the In₂O₃/Ag₂CrO₄ composites in comparison with that of pure Ag₂CrO₄. All the In₂O₃/Ag₂CrO₄ composites show strong and broader absorption in the visible-light region, and the ability of light absorption and the absorption intensity of the In₂O₃/Ag₂CrO₄ composites increase with increasing In₂O₃ content. It can be attributed to the heterojunction formed between In₂O₃ and Ag₂CrO₄ had changed the optical properties of the photocatalysts due to the synergetic effect between In₂O₃ and Ag₂CrO₄, which decreased the contact barrier, enhanced the electronic coupling of two semiconductors, improved absorption of heterojunctions at long wavelengths and thus resulted in the generation of more photogenerated electron–hole pairs under visible light irradiation, which subsequently leads to a higher photocatalytic activity. It is similar to the previous reported AgCl/Ag₂CO₃, g-C₃N₄/Ag₂CO₃, ZrO₂/g-C₃N₄ and BiPO₄/g-C₃N₄ heterojunction photocatalysts.²⁴ These results implied that the In₂O₃/Ag₂CrO₄ composite has the potential to be efficient visible light-driven photocatalyst. Additionally, based on the theory of optical absorption for direct band gap semiconductors, the band gap energy of the as-prepared samples can be calculated through the equation: αhν = A(hν − E_g)^n/2, where A, α, ν, E_g and h are a constant, absorption coefficient, light frequency, band gap energy and Planck constant, respectively.²⁵ For the value of n, it was determined by the type of optical transition of semiconductors (n = 1 for direct transition and n = 4 for indirect transition). As previous literatures reported, here the value of n is 1 and 4 for In₂O₃,^20b and Ag₂CrO₄,^8a respectively. Accordingly, as shown in Fig. 4b, the E_g of In₂O₃ is estimated to about 2.62 eV according to a plot of (αhν)² versus energy (hν), while the E_g of Ag₂CrO₄ is estimated to about 1.72 eV according to a plot of (αhν)^1/2 versus energy (hν).

Fig. 4 UV-vis DRS (a) of the as-prepared samples, and plots of (αhν)^1/2 and (αhν)² versus energy (b) for the band gap energy of Ag₂CrO₄ and In₂O₃, respectively.

The photoluminescence (PL) emission spectrum is usually employed to investigate the separation and transfer efficiency of the photogenerated electron–hole pairs during photocatalytic process.^24d,26 It is well acknowledged that the stronger PL intensity indicates the faster recombination speed of photogenerated charge carriers, implying that the fewer photoinduced electrons and holes took part in the photocatalytic oxidation and reduction reactions, resulting in the lower photocatalytic activity.²⁷ Fig. 5 exhibits the PL spectra of the as-prepared samples under the excitation wavelength of 300 nm. Obviously, Ag₂CrO₄ shows higher PL intensity than other samples, indicates Ag₂CrO₄ has the highest electrons and holes recombination rate that deteriorated the photodegradation efficiency. However, in the PL spectra of In₂O₃/Ag₂CrO₄ composites, a considerable fluorescence quenching in the same position is detected, indicating that the addition of In₂O₃ significantly inhibited the recombination of photogenerated electrons and holes, leading to superior separation efficiency of photogenerated electrons and holes and better photocatalytic activity. Consequently, the synergetic interaction between In₂O₃ and Ag₂CrO₄ in the In₂O₃/Ag₂CrO₄ composite contributed to the efficient separation and extending lifetime of photogenerated charge carriers and further improving the photodegradation efficiency. However, over-loading In₂O₃ species increased the recombination rate of photogenerated charge carriers, leading to inferior photodegradation efficiency, arising from the excess In₂O₃ species may act as a recombination centre and thereby reducing the efficiency of charge separation.

Fig. 5 PL spectra of the as-prepared samples.

To compare the photocatalytic activities of Ag₂CrO₄ before and after coupling with In₂O₃, a series of photocatalytic degradation experiments were performed using methyl orange (MO) as a model pollutant under the visible light irradiation (λ > 420 nm) without using any sacrificial reagent at room temperature, and the results are represented in Fig. 6. As shown in Fig. 6a, it can be clearly seen that the self-photolysis of MO without any photocatalyst can be neglected under the light irradiation for 120 min. It should be pointed out that pure Ag₂CrO₄ shows higher photocatalytic activity than that of pure In₂O₃, which is attributed to the stronger visible light absorption and the proper band edge potential of Ag₂CrO₄. Significantly, the In₂O₃/Ag₂CrO₄ composites exhibit much higher photocatalytic activities for photodegrading MO than that of pure Ag₂CrO₄ after 120 min of visible light irradiation, indicating that combining Ag₂CrO₄ and In₂O₃ is an efficient route to enhance the photocatalytic performance of Ag₂CrO₄. Concretely, the photocatalytic activity of In₂O₃/Ag₂CrO₄ composite initially increases with the increase of In₂O₃ loading on the surface of Ag₂CrO₄, and then it decreases with the further increase of In₂O₃ amount. Among all of the samples, it obviously can be seen that the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite exhibits the highest photocatalytic activity, whose photodegradation activity is 6.7 and 2.6 times as high as that of pure In₂O₃ and Ag₂CrO₄, respectively. Nevertheless, a further increase in the amount of In₂O₃ results in a decrease in the photodegradation activity under the same conditions, but it is still much superior compared to the photocatalytic activity of pure Ag₂CrO₄. These results clearly indicated that an appropriate content of In₂O₃ is crucial to achieve optimal photocatalytic performance, which is attributed to the following probable reasons: firstly, the introduction of In₂O₃ can effectively enhanced visible light absorption, widened optical absorption range and thus resulted in the generation of more photogenerated electron–hole pairs under visible light irradiation, and it can be confirmed from the above-mentioned UV-vis DRS results. Secondly, a suitable content of In₂O₃ can form a good dispersion on the surface of Ag₂CrO₄, which effectively favored the separation and transfer of the photogenerated charge carriers, and it can be confirmed from the above-mentioned PL results. Thirdly, excessive In₂O₃ may act as a recombination centre, and cover the active sites on the Ag₂CrO₄ surface, leading to reducing the efficiency of the photogenerated charge carriers, and it can be confirmed from the above-mentioned PL results.

Fig. 6 Photocatalytic activities (a) and first-order kinetic plots (b) for the photodegradation of MO in aqueous solution over the as-prepared samples under visible light irradiation.

In order to comprehend the process of the photocatalytic reaction, the photocatalytic degradation kinetics of MO in the case of the as-prepared samples were investigated, and the experimental data were fitted by applying a first-order model as expressed by the formula: −ln(C/C_o) = kt. Where C_o is the equilibrium concentration of MO after 60 min dark adsorption, C is the MO concentration remaining in the solution at the irradiation time, t is the irradiation time and k is the apparent first-order rate constant.²⁸ Fig. 6b shows a linear relationship between −ln(C/C_o) and the irradiation time for the degradation of MO. As can be seen from Fig. 6b, the photocatalytic degradation curves for all samples fit well with pseudo-first-order kinetics, indicating that the photodegradation MO process followed a pseudo-first-order reaction. Furthermore, the degradation rate constant (k) for MO with In₂O₃, Ag₂CrO₄, In₂O₃ (2.0 wt%)/Ag₂CrO₄, In₂O₃ (4.0 wt%)/Ag₂CrO₄ and In₂O₃ (8.0 wt%)/Ag₂CrO₄ were estimated to be 0.0006 min⁻¹, 0.0016 min⁻¹, 0.0043 min⁻¹, 0.0053 min⁻¹ and 0.0035 min⁻¹, respectively. It is worth mentioning that the MO degradation rate constants of the In₂O₃/Ag₂CrO₄ composites are larger than that of Ag₂CrO₄ and In₂O₃, and the largest degradation rate constant of the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite is 3.3 times higher than that of pure Ag₂CrO₄ and 8.8 times as high as that of pure In₂O₃ under the same experimental conditions, clearly demonstrating that the introduction of In₂O₃ could efficiently enhance the photocatalytic activity of Ag₂CrO₄ under the visible light irradiation. According to the above-mentioned DRS and PL results, it is attributed predominantly to widening optical absorption range, enhancing optical absorption intensity and improving photogenerated electrons and holes separation and migration efficiency because of the intimate interfacial contact and the matched energy band structures between Ag₂CrO₄ and In₂O₃. Consequently, the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite photocatalyst is the best performing sample and is selected for the following recycling experiments. It is noted that although the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite exhibited the highest photocatalytic activity, its degradation efficiency was still relatively weak. Thus we prolonged the visible light irradiation time to further improve the photodegradation efficiency of MO over In₂O₃ (4.0 wt%)/Ag₂CrO₄ photocatalyst. As shown in Fig. 7a, it could be observed that almost 92% of MO was degraded by the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite photocatalyst after 240 min of visible light irradiation, indicating that the as-prepared In₂O₃/Ag₂CrO₄ composite had good photocatalytic performance for MO degradation under the visible light irradiation.

Fig. 7 Recyclability (a) of the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite in three successive experiments for the photocatalytic degradation of MO under visible light irradiation, the XRD patterns (b), UV-vis DRS (c) and XPS spectra (d and e) of the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite before and after three consecutive photocatalytic reactions, and effects of various scavengers (f) on the photocatalytic activity of the In₂O₃ (4.0 wt%)/Ag₂CrO₄ photocatalyst under visible light irradiation.

Besides photocatalytic activity, the stability and reusability of the photocatalyst are of crucial significance for its further practical application. To evaluate the stability and reusability of the In₂O₃ (4.0 wt%)/Ag₂CrO₄ photocatalyst, we carried out recycling experiments under identical conditions. After each run, the recycled catalyst was separated from the aqueous suspension by centrifugation, repeatedly washed by deionized water and ethanol and dried in vacuum at 55 °C for 24 h. As can be seen from Fig. 7a, the photodegradation of MO in every cycling runs, and there was slight catalyst deactivation in the third run, but the photocatalytic activity of the In₂O₃ (4.0 wt%)/Ag₂CrO₄ photocatalyst was retained at over 79% of its original activity after three successive experimental runs, arising from the slight loss of photocatalyst during the recovery steps and a small amount of Ag₂CrO₄ in the composite were inevitably reduced into metallic Ag by the photogenerated electrons during the MO degradation process. It is known that Ag₂CrO₄ is easily photocorroded by the photogenerated electrons and decomposed to weakly active metallic Ag, leading to the inferior stability. As a consequence, in order to further elucidate the Ag state changes of In₂O₃ (4.0 wt%)/Ag₂CrO₄ after the third successive experimental runs, XRD, UV-vis DRS and XPS analysis were carried out. As shown in Fig. 7b, it can be seen that the XRD pattern of the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite has no appreciable change after three cycles. It is noteworthy that there was no a new weak peak located at 2θ = 38.1° corresponding to the (111) plane diffraction of Ag (JCPDS Card no. 89-3722),²⁹ it may be attributed to the relatively low amount of Ag. It can be seen from Fig. 7c, the slightly enhanced absorption intensity of used In₂O₃ (4.0 wt%)/Ag₂CrO₄ after 3 recycling runs in the range of 600–800 nm in comparison with fresh In₂O₃ (4.0 wt%)/Ag₂CrO₄, suggesting that a small amount of Ag₂CrO₄ has decomposed to metallic Ag. On the other side, as illustrated in Fig. 7d and e, compared to fresh In₂O₃ (4.0 wt%)/Ag₂CrO₄, the Ag 3d peaks of used In₂O₃ (4.0 wt%)/Ag₂CrO₄ displayed two new and very weak peaks at about 368.4 eV and 374.4 eV corresponding to Ag 3d_5/2 and Ag 3d_3/2, respectively, which belong to the Ag⁰.^9b,22 The XPS results further confirm the existence of tiny Ag⁰ in the used In₂O₃ (4.0 wt%)/Ag₂CrO₄ sample, it can be inferred that the tiny Ag⁰ nanoparticles have been formed on the surface of the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite during the MO degradation process and could serve as excellent acceptors to trap for photogenerated electrons, which effectively inhibited the further formation of more Ag⁰ nanoparticles during the photocatalysis and thereby preventing the photocorrosion of Ag₂CrO₄, it is similar to the previous reported Ag₃PO₄/TiO₂/Fe₃O₄, AgBr/Ag₂CO₃, Co₃O₄/Ag₃PO₄ and Ag₂CrO₄–GO composite photocatalysts.^9b,30 The results clearly demonstrated that the In₂O₃ nanoparticles loaded on the surface of Ag₂CrO₄ can effectively protect it from dissolution in the aqueous solution, which can significantly increase the structural stability of In₂O₃ (4.0 wt%)/Ag₂CrO₄ during the photocatalytic reaction. Consequently, the incorporation of In₂O₃ with Ag₂CrO₄ can not only enhance the visible light photocatalytic performance of Ag₂CrO₄, but also inhibit the photocorrosion of Ag₂CrO₄ in the photocatalytic process.

To disclose the role of reactive species responsible for photocatalytic degradation of MO over the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite, a serious of reactive species trapping experiments were performed under visible light irradiation and the results are shown in Fig. 7d. In this investigation, tert-butyl alcohol (t-BuOH), triethanolamine (TEOA) and 1,4-benzoquinone (BQ) are used as hydroxyl radical (˙OH), hole (h⁺) and superoxide radical (˙O₂⁻) scavenger, respectively.^9b,31 As illustrated in Fig. 7d, it can be clearly seen that the photocatalytic activity of the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite decreases significantly compared to that without scavengers after addition of TEOA or BQ, suggesting that h⁺ and ˙O₂⁻ are the predominant active species that participate in the photodegradation process. On the contrary, adding t-BuOH has little impact on the photocatalytic activity of the In₂O₃ (4.0 wt%)/Ag₂CrO₄ composite, meaning that ˙OH is not the main active species in In₂O₃ (4.0 wt%)/Ag₂CrO₄ photocatalytic systems.

It is well known that the position of conduction band (CB) and valence band (VB) of a semiconductor is one of the important factors that affect the photocatalytic activity. As a sequence, to fully understand the photocatalytic reaction mechanism occurring during the photodegradation of the as-prepared In₂O₃/Ag₂CrO₄ composites, the band edge positions of the valence band and conduction band of both Ag₂CrO₄ and In₂O₃ are necessary to be determined. Although it is difficult to determine the potentials by experiment, the potentials of the conduction band and valence band edges of both Ag₂CrO₄ and In₂O₃ can be obtained according to the Mulliken electronegativity theory:³² E_VB = X − E^C + 0.5E_g and E_CB = E_VB − E_g, where E_VB and E_CB are the valence band and conduction band edge potentials, respectively, X is the absolute electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. According to the previous reports, the values of X for Ag₂CrO₄ and In₂O₃ have been calculated as 5.86 eV (ref. 8a) and 5.28 eV,^15c respectively. E^C is the energy of free electrons on the hydrogen scale (about 4.5 eV vs. NHE) and E_g is the band gap of the semiconductor.³³ Based on the result in the Fig. 4b, the band gap energies of Ag₂CrO₄ and In₂O₃ were estimated at 1.72 eV and 2.62 eV, respectively. Thus given the equations above, the VB and CB of Ag₂CrO₄ were calculated to be 2.22 eV and 0.50 eV, respectively. Similarly, the VB and CB of In₂O₃ were calculated to be 2.09 eV and −0.53 eV, respectively. It is obvious that the CB and VB edge potential positions of In₂O₃ is more negative than that of Ag₂CrO₄.

On the basis of the above-mentioned results and analysis, a plausible mechanism was proposed to explain the enhancement of the photocatalytic properties of the In₂O₃/Ag₂CrO₄ composites. As illustrated in Fig. 8, In₂O₃/Ag₂CrO₄ transforms to In₂O₃/Ag₂CrO₄@Ag in the early stage of the photocatalytic reaction because of the photosensation of In₂O₃/Ag₂CrO₄. The Ag⁰ nanoparticles at the interface of In₂O₃/Ag₂CrO₄ act as the charge separation center to form the visible-light-driven In₂O₃/Ag₂CrO₄ Z-scheme system. Moreover, both the CB bottom and the VB top of Ag₂CrO₄ lay below the CB bottom and VB top of In₂O₃, respectively. Under the irradiation of visible light, both Ag₂CrO₄ and In₂O₃ can be excited and generate the photoinduced electron–hole pairs, the photogenerated electrons in the CB of Ag₂CrO₄ could easily shift into Ag nanoparticles through the Schottky barrier because of the more positive Fermi energy of Ag than the CB level of Ag₂CrO₄. Simultaneously, the holes in the VB of In₂O₃ could also shift into Ag nanoparticles due to the more negative Fermi energy of Ag than the VB level of In₂O₃. In the photocatalytic process, Ag nanoparticles at the interface of In₂O₃/Ag₂CrO₄ can act as a recombination center for the electrons from the CB of Ag₂CrO₄ and holes from the VB of In₂O₃, inhibiting the electron–hole pairs recombination in both Ag₂CrO₄ and In₂O₃, and enhancing the interfacial charge transfer. In this way, the photogenerated electrons and holes can be efficiently separated, resulting in the recombination of photogenerated electron–hole pairs is remarkably hindered, which further leading to the enhanced photocatalytic activity. It can be confirmed by the above-mentioned PL results. Subsequently, the separated electrons accumulated on the CB of In₂O₃ can transfer to the surface and be annihilated by surface absorbed acceptors like O₂ to produce ˙O₂⁻ due to the position (−0.53 eV) of CB of In₂O₃ is more negative than the standard redox potential of O₂/˙O₂⁻ (−0.33 V vs. NHE).³⁴ The produced ˙O₂⁻ can oxidize the MO to degradation products. Similarly, the separated holes accumulated on the VB of Ag₂CrO₄ can transfer to the surface and directly oxidize the MO to degradation products. This is consistent with above hole and free radical trapping experiment results, the MO degradation rate is decreased obviously in the presence of TEOA or BQ. It is noted that the photogenerated electrons in the CB of Ag₂CrO₄ cannot reduce dissolved O₂ to produce ˙O₂⁻ through one-electron reduction due to the CB potential (0.50 eV) of Ag₂CrO₄ is more positive than the standard redox potential of O₂/˙O₂⁻ (−0.33 V vs. NHE).³⁴ Therefore, it can be concluded that the photocatalytic mechanism of In₂O₃/Ag₂CrO₄ composite is not in accordance with the traditional charge separation process but a typical Z-scheme charge transfer. A similar Z-scheme mechanism for AgI/Ag₃PO₄ and g-C₃N₄/Ag₃PO₄ composite photocatalysts have been reported.³⁵

Fig. 8 Schematic diagram of the photogenerated electron–hole pairs transfer pathway of the In₂O₃/Ag₂CrO₄ composite under visible light irradiation.

Conclusions

In conclusion, the novel In₂O₃/Ag₂CrO₄ composites with different weight ratios of In₂O₃ have been fabricated by a facile chemical precipitation method. Under visible light irradiation, the as-prepared In₂O₃/Ag₂CrO₄ composite displayed enhanced photocatalytic activity and stability for the photocatalytic degradation of MO in comparison with the single In₂O₃ and Ag₂CrO₄. In particular, the optimum degradation rate constant of the In₂O₃/Ag₂CrO₄ composite at a weight content of 4.0% In₂O₃ for the degradation of MO was 8.8 and 3.3 times as high as that of pure In₂O₃ and Ag₂CrO₄, respectively. Such an enhanced photocatalytic performance was predominantly ascribed to the formation of Z-scheme system composed of In₂O₃, Ag and Ag₂CrO₄, which effectively improved the separation and transfer of photogenerated charge carriers. Furthermore, the superoxide radical anions and holes were considered as the main reactive species during the photodegradation process, and a possible photocatalytic mechanism is proposed based on the experimental results. The present work may provide a new insight for the smart design and synthesis of various highly efficient visible-light-driven photocatalysts for environmental and energy applications.

Acknowledgements

This work was supported by the Guangdong Provincial Natural Science Foundation (No. 2016A030307015, 2015A030310431, 2015A030313893).

Notes and references

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