J Mater Sci: Mater Electron DOI 10.1007/s10854-017-7495-0

Facile fabrication of a direct Z-scheme ­MoO3/Ag2CrO4 composite photocatalyst with improved visible light photocatalytic performance Wen Li1 · Jinfen Chen1 · Rongting Guo1 · Jiaming Wu1 · Xiaosong Zhou1 · Jin Luo1 

Received: 17 June 2017 / Accepted: 5 July 2017 © Springer Science+Business Media, LLC 2017

Abstract  A direct Z-scheme type M ­ oO3/Ag2CrO4 composite photocatalyst was successfully fabricated using a facile in  situ precipitation method and characterized by X-ray diffraction, scanning electron microscopy (SEM), high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet–visible diffuse reflection spectroscopy (UV–Vis DRS) and photoluminescence spectroscopy (PL). The photocatalytic activity of the as-obtained ­MoO3/Ag2CrO4 composite was evaluated by the photo-degradation of methyl orange (MO) under visible light irradiation (λ > 420  nm). Compared with pure ­Ag2CrO4 and M ­ oO3, ­MoO3/Ag2CrO4 composites presented enhanced photocatalytic activities in the degradation of MO. Significantly, the optimal ­MoO3(4.0  wt%)/Ag2CrO4 sample showed the highest degradation rate of MO, which was approximately 2.5 times higher than that of ­Ag2CrO4 and 13 times larger than that of ­MoO3, respectively. Such an enhancement could be mainly ascribed to the efficient separation of photogenerated charge carriers through a Z-scheme system composed of A ­ g2CrO4 and M ­ oO3. Furthermore, a direct Z-scheme mechanism responsible for the photocatalytic activity enhancement in the ­MoO3/Ag2CrO4 binary photocatalytic system was proposed based on the results of energy band positions, active species trapping experiments and photoluminescence spectroscopy.

* Jin Luo [email protected] 1



School of Chemistry and Chemical Engineering, Institute of Physical Chemistry, Development Center for New Materials Engineering & Technology in Universities of Guangdong, Lingnan Normal University, Zhanjiang 524048, China

1 Introduction In recent years, visible-light-sensitive photocatalysts have sparked considerable attention for their promising applications in the fields of environmental remedy, water splitting, ­CO2 reduction due to the high solar light utilization rate [1–4]. Up to now, a great deal of visible light responsive photocatalysts have been developed, including N doped ­TiO2 [1], CdS [5], g-C3N4 [6, 7], Ag-based semiconductors [8, 9], Bi-based semiconductors [2, 10], and so forth. Among them, silver chromate ­(Ag2CrO4) has been regarded as a promising and attractive visible light responsive photocatalyst for the degradation of organic pollutants under visible light irradiation due to its appropriate band gap (ca.1.8 eV), strong visible-light responsibility, unique electronic structure and crystal structure and high oxidization ability of photogenerated holes [11–13]. Nevertheless, its practical application is still restricted by some severe drawbacks such as the photocorrosion by the photogenerated electrons ­(Ag+ + e­ − ⟶Ago) and the fast recombination rate of photogenerated charge carriers, thus leading to the serious destroy of structure and the decrease of photocatalytic activity and stability [13–15]. To deal with the above mentioned bottlenecks, considerable efforts have been devoted to improving and optimizing the photocatalytic performance of A ­ g2CrO4, such as modulating preparation method [16], coupling with other band structure matching semiconductors [17] or constructing heterojunctions [18, 19] or Z-scheme systems [12, 20], and so on. Excitingly, the construction of artificial direct Z-scheme photocatalytic system by coupling of two semiconductors with matchable band edge position has been proved to be an effective and controllable way to improve the photocatalytic performance under visible light irradiation through promoting the spatial separation of photogenerated charge carriers as

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well as preserving excellent redox ability [21, 22]. To date, unfortunately, there are only a few reports concerning the development of ­Ag2CrO4-based Z-scheme systems, such as ­Ag2CrO4/GO [11], ­Ag2CrO4/g-C3N4 [13, 23], ­WO3/ Ag2CrO4 [24], ­In2O3/Ag2CrO4 [14] and I­n2S3/Ag2CrO4 [12]. These composites can not only effectively promote the separation of photogenerated charges carriers, but also inhibite the photocorrosion of ­Ag2CrO4. Furthermore, molybdenum trioxide ­(MoO3), as a wellknown n-type metal oxide semiconductor with a band gap of 2.8 ~ 3.2 eV with 20 ~ 30% ionic character, has attracted plenty of interest as a promising candidate for UV and visible light driven photocatalytic applications due to its nontoxicity, high chemical stability, unique double-layered planar structure and distinctive electrochemical properties [25–27]. However, to the best of our knowledge, there are no reports on the synthesis and photocatalytic properties of ­ MoO3/Ag2CrO4 composite photocatalysts so far. As a consequence, ­MoO3 was employed to combine with the ­Ag2CrO4 to obtain ­MoO3/Ag2CrO4 hybrid photocatalysts. In consideration of the matching band potentials of ­Ag2CrO4 and M ­ oO3, it is possible to form an efficient Z-scheme type ­MoO3/Ag2CrO4 photocatalyst system with enhanced photocatalytic activity and stability. Herein, we constructed a series of ­ MoO3/Ag2CrO4 composite photocatalysts were synthesized by an in-situ precipitation method. The photocatalytic activities of the as-obtained samples were evaluated by the photocatalytic degradation of methyl orange (MO) in aqueous solution under visible light irradiation (λ >420  nm). It was worth noticing that the as-obtained M ­ oO3/Ag2CrO4 composites showed distinctly enhanced photocatalytic activities than that of pure ­Ag2CrO4 and single ­MoO3. Eventually, a direct Z-scheme mechanism responsible for the photocatalytic activity enhancement in the M ­ oO3/Ag2CrO4 binary photocatalytic system was proposed based on the results of energy band positions, active species trapping experiments and photoluminescence spectroscopy.

2 Experimental All the chemicals used in this work were of analytical reagent grade and used without further purification. Distilled water was used throughout. 2.1 Catalyst preparation MoO3 was prepared by directly calcining ammonium molybdate ((NH4)6Mo7O24·4H2O) in a muffle furnace according to a reported procedure [28]. Typically, 3.0  g ­(NH4)6Mo7O24·4H2O was placed in an alumina crucible

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with a cover. The crucible was heated to 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. After cooling to room temperature, the product was washed several times with distilled water and absolute ethanol and dried in a oven at 80 °C for 12 h. MoO3/Ag2CrO4 composites were fabricated by a facile in situ chemical precipitation method under the dark condition according to a modified reported procedure [29]. In a typical procedure, first of all, a certain amount of the as-prepared M ­ oO3 samples (13.5, 27.6 and 42.3 mg) were dispersed thoroughly into 40  mL distilled water with the assistance of ultrasonication for 30  min. Then, 4  mmol ­AgNO3 was added into the dispersion of ­MoO3 under the vigorous stirring. After stirring for 30  min, 2  mmol ­K2CrO4 dissolved in 40  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 M ­ oO3(x)/Ag2CrO4, where x% stands for the theoretical mass percent of ­MoO3 in the ­MoO3/Ag2CrO4 composites. That is to say, the as-prepared samples were also accordingly denoted as ­ MoO3(2.0  wt%)/Ag2CrO4, ­MoO3(4.0  wt%)/Ag2CrO4 and M ­ oO3(6.0  wt%)/Ag2CrO4. Moreover, for comparison, pure A ­ g2CrO4 nanoparticles were also synthesized following the similar procedure mentioned above in the absence of M ­ oO3. 2.2 Catalyst characterization X-ray diffraction (XRD) patterns of the as-prepared samples were recorded on an X-ray diffractometer (D/ max-IIIA, Japan) using Cu Kα radiation. The surface morphologies and microstructures of the as-prepared samples were observed by a scanning electron microscopy (SEM) (LEO1530VP, LEO Company) and a highresolution transmission electron microscope (HRTEM, JEOL, JEM-2100). The UV–Vis light absorption spectra of the 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. X-ray photoelectron spectroscopy (XPS) analysis was performed with a Krato Axis ultra DLD spectrometer equipped with an Al Kα X-ray source, the binding energy was referenced to C 1s peak at 284.6 eV for calibration. Photoluminescence (PL) spectra were obtained by a Varian Cary Eclipse spectrometer with an excitation wavelength of 270 nm.

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2.3 Photocatalytic activity tests 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, and the average light intensity striking on the surface of reaction solution was about 50  mW  cm−2 as measured using a UV-A radiometer. 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 sonicated for 15 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−At)/A0 × 100%, where A0 and At are the concentration of MO before illumination and after illumination time t. Furthermore, the tests of active species trapping were carried out under the identical procedure mentioned above except for adding 2 mmol tert-butyl alcohol (t-BuOH, a quencher of ·OH), 1 mmol triethanolamine (TEOA, a quencher of ­h+) or 0.2 mmol 1, 4-benzoquinone (BQ, a quencher of ·O−2 ), respectively. In order to determine the reproducibility of the results, duplicate runs were performed for each condition for averaging the results.

3 Results and discussion 3.1 Characterization 3.1.1 XRD analysis The X-ray diffraction (XRD) patterns of all as-obtained samples were depicted in Fig.  1. It could be found that the pure ­Ag2CrO4 exhibited the main diffraction peaks of (031), (211), (002), (240) and (242) facets at 2θ = 31.1°, 31.4°, 32.3°, 45.4° and 55.8°, which can be indexed to the orthorhombic phase of ­Ag2CrO4 according to the standard card (JCPDS Card No. 26-0952) [13, 16, 17]. In the pattern of bare M ­ oO3, the main diffraction peaks appeared at 2θ = 12.8°, 23.3°, 25.7°, 27.3° and 39.0°, which could be perfectly indexed to the (020), (110), (040), (021) and (060) planes of the orthorhombic phase of ­MoO3 according to the standard card (JCPDS Card No. 35-0609) [28, 30]. Remarkably, it could be observed that the M ­ oO3/Ag2CrO4

Fig. 1  XRD patterns of the as-obtained samples

composites exhibited similar XRD patterns to that of pure ­Ag2CrO4. Meanwhile, no characteristic diffraction peaks of ­MoO3 was discerned in the patterns of the M ­ oO3/Ag2CrO4 composites, which was probably due to the fact that the low mass ratio of ­MoO3 in these composites could not be resolved by XRD analysis. To further verify the existence of ­MoO3 in the as-obtained M ­ oO3/Ag2CrO4 composites, the microstructure and composition of the as-obtained samples will be characterized by the following HRTEM, EDS and XPS analysis. Furthermore, it was worth mentioning that there was no change the diffraction peak positions of the ­MoO3/Ag2CrO4 composites in comparison with those of bare A ­ g2CrO4 after the introduction of ­MoO3, indicating that ­MoO3 nanoparticles was not incorporated into the ­Ag2CrO4 lattice. 3.1.2 SEM and HRTEM analysis The morphologies and microstructures of the as-obtained ­Ag2CrO4, ­MoO3 and M ­ oO3(4.0  wt%)/Ag2CrO4 composite were revealed by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), and the results were illustrated in Fig. 2. It could be seen from Fig.  2a that the pure A ­ g2CrO4 showed irregular and stacked particle-like morphology,

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Fig. 2  SEM a–c images of ­Ag2CrO4 (a), ­MoO3 (b) and ­MoO3(4.0 wt%)/Ag2CrO4 (c), and HRTEM image (d) of the ­MoO3(4.0 wt%)/Ag2CrO4 composite

which were obviously agglomerated. From Fig.  2b, it was found that primitive M ­ oO3 exhibited irregular and stacked flake-like morphology, which had obvious edges with relatively smooth surface. In the case of the ­MoO3(4.0  wt%)/Ag2CrO4 composite, it could be clearly observed that some irregular A ­ g2CrO4 nanoparticles were deposited on the surface of M ­ oO3 (Fig.  2c), indicating that ­Ag2CrO4 nanoparticles were gradually grown on the surface of M ­ oO3 flakes during the solution phase reaction and were attached to the support firmly. It was worth mentioning that the in  situ growth should establish inti­ oO3, which could mate contact between A ­ g2CrO4 and M benefit better photogenerated charge carrier transfer and separation in comparison with pure ­Ag2CrO4. In order to further confirm the intimate contact between ­Ag2CrO4 and ­MoO3, HRTEM analysis was performed. As shown in Fig. 2d, obviously, two types of distinct lattice fringes were observed. For one set of the clear fringes, the spacing was ca. 0.204 nm, corresponding to the (240) lattice spacing of the orthorhombic phase of ­Ag2CrO4 [13]. For the other set of the clear fringes, the spacing measured ca. 0.35 nm, which is in accordance with the (040) lattice

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plane of orthorhombic phase M ­ oO3 [30]. As a result, the results revealed that the coexistence of ­ Ag2CrO4 and ­MoO3 in the as-obtained ­MoO3/Ag2CrO4 composite photocatalyst. 3.1.3 EDS and elemental mapping analysis In order to illuminate the chemical compositions and elemental distributions of the as-obtained ­MoO3(4.0  wt%)/ Ag2CrO4 composite, the energy-dispersive X-ray spectrometry (EDS) and EDS elemental mapping were performed and the results were shown in Fig. 3. As displayed in Fig.  3b, it can be unambiguously observed that the detectable elements included Ag, Cr, O and Mo, confirming that the coexistence of ­MoO3 and ­Ag2CrO4 in the asobtained ­MoO3/Ag2CrO4 composites. In addition, as can be seen from Fig. 3d–g, it can be clearly revealed that the Ag, Cr, O and Mo elements were distributed homogeneously in the M ­ oO3(4.0 wt%)/Ag2CrO4 composite, further illustrating that the composite was composed of ­MoO3 and ­Ag2CrO4.

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Fig. 3  SEM images (a and c), EDS pattern (b) and the element mapping images (d–g) of Ag (d), Cr (e), O (f) and Mo (g) elements in the ­MoO3(4.0 wt%)/Ag2CrO4 composite

3.1.4 XPS analysis In order to further confirm the coexistence of M ­ oO3 and ­Ag2CrO4 in the as-obtained composites, X-ray photoelectron spectroscopy (XPS) analysis was carried out to elucidate the chemical states and compositions of the as-obtained ­MoO3(4.0  wt%)/Ag2CrO4 composite, and the typical full survey and high-resolution spectra for the C 1s, Mo 3d, O 1s, Ag 3d and Cr 2p regions of the ­MoO3(4.0  wt%)/Ag2CrO4 composite were illustrated in Fig.  4. As shown in Fig.  4a, the survey XPS spectrum further confirmed the existence of Mo, O, Ag and Cr elements in the ­MoO3(4.0  wt%)/Ag2CrO4 composite with C 1s signal from adventitious carbon-based contaminant from XPS instrument itself, which was well consistent with the chemical compositions of ­MoO3(4.0  wt%)/ Ag2CrO4, and was also in accordance with the aforementioned EDS and HRTEM results. It was worth mentioning that the peak positions in all of the XPS spectra were calibrated with C 1s at 284.6 eV. The C1s peak shown in Fig. 4b could be deconvoluted into three bands at 284.6,

285.6, and 288.2  eV, respectively. The main and strong peak located at 284.6 eV was usually assigned to adventitious carbon, whereas two week peaks centered at 285.6 and 288.2 eV were attributed to C–O and O–C=O bonding, respectively [17, 24, 31, 32]. The high-resolution Mo 3d spectrum in Fig.  4c showed two deconvoluted peaks centered at 232.2 and 235.3 eV, which could be ascribed to the characteristic binding energies of M ­ o3d5/2 and 6+ ­Mo3d3/2, respectively, suggesting that ­Mo was present in the composite [28, 33]. Figure  4d showed the high resolution XPS spectrum of O 1s, and it was clear to see that there exist two main peaks located at 530.1 and 531.8 eV. The former was ascribed to the lattice oxygen in the ­MoO3 and ­Ag2CrO4, while the latter probably corresponded to the external hydroxyl groups adsorbed on the surface of the as-obtained sample [13, 30, 34]. As can be seen in Fig. 4e, there are two strong peaks located at binding energies of 367.7 and 373.7  eV corresponding to the characteristic Ag 3­ d5/2 and Ag ­3d3/2, respectively, which were attributed to ­Ag+ [12, 35, 36]. With respect

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Fig. 4  XPS spectra of the ­MoO3(4.0 wt%)/Ag2CrO4 composite: a survey, b C 1s, c Mo 3d, d O 1s, e Ag 3d and f Cr 2p

to the high resolution XPS spectrum of Cr 2p in Fig. 4f, it could be observed that the two individual peaks with the binding energies of 578.7 and 587.9 eV, which could be assigned to Cr ­2p3/2 and Cr ­2p1/2, respectively [12, 23]. Hence, all of these XPS results gave the insight that the as-obtained ­MoO3/Ag2CrO4 composite were composed of ­MoO3 and A ­ g2CrO4.

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3.1.5 UV–Vis DRS analysis The optical properties of the as-obtained pure ­Ag2CrO4, ­MoO3 and M ­ oO3/Ag2CrO4 composites with varied ­MoO3 contents were investigated by ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) in the wavelength range from 250 to 850  nm and the results were shown in Fig. 5. As shown in Fig. 5a, it was obvious that

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Fig. 5  UV–vis DRS spectra a of the as-obtained samples, and plots of (αhv)1/2 and (αhv)2 versus hv b for the band gap energy of A ­ g2CrO4 and ­MoO3, respectively

the single of M ­ oO3 had narrow absorption in the visible region with an absorption edge of around 450  nm, while the pure A ­ g2CrO4 displayed light absorption over nearly the entire visible light range, and its absorption edge was located around 720  nm. Apparently, in comparison with the single ­Ag2CrO4, the absorption intensities of ­MoO3/ Ag2CrO4 composites had a slight decrease within the visible light region (650 ~ 850 nm) with increasing ­MoO3 contents owing to the weak visible-light absorption ability of ­MoO3. Nevertheless, all of the ­MoO3/Ag2CrO4 composites still had broad absorption in the whole visible light region, indicating that the as-obtained ­MoO3/Ag2CrO4 composites can be used as visible light-driven photocatalysts. Furthermore, to mine more information from the UV–Vis DRS, the band gap energies of ­ MoO3 and ­Ag2CrO4 were calculated by the following formula [37–39]: αhv = A(hv−Eg)n/2, where A, α, v, Eg and h are a constant, absorption coefficient, light frequency, band gap energy and Planck constant, respectively. 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). According to the previous literature, the n value of ­MoO3 was 1 [28, 40], while the n value of ­Ag2CrO4 was 4 [12, 13, 41]. Hence, the Eg values of M ­ oO3 and ­Ag2CrO4 were calculated according to a plot of (αhν)2 versus energy (hν) and plot of (αhν)1/2 versus energy (hν), respectively. By calculation, the band gap energy of ­MoO3 and ­Ag2CrO4 were approximately 2.77 and 1.72 eV, respectively (Fig.  5b). Besides, the valance band (VB) potential and the conduction band (CB) potential of ­MoO3 and ­Ag2CrO4 were also calculated by the following equations [42, 43]: EVB = χ-Ee + 0.5Eg and ECB = EVB−Eg, where EVB and ECB are the valence band and conduction band edge potentials, respectively, χ is the absolute electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. Ee is the energy

of free electrons on the hydrogen scale (about 4.5 eV versus NHE) and Eg is the band gap of the semiconductor. According to previous literatures reported, the values of χ for ­Ag2CrO4 and M ­ oO3 are 5.86 eV [12, 13] and 6.40 eV [28, 44], respectively. As a consequence, according to the above equation, it can be calculated that the EVB of ­Ag2CrO4 and M ­ oO3 were estimated to be 2.22 and 3.29 eV, respectively, and the ECB of A ­ g2CrO4 and M ­ oO3 were also estimated to be 0.50 and 0.52 eV, respectively. 3.1.6 PL analysis In order to provide the evidence on the photogenerated electron–hole pairs transfer and separation efficiency of ­Ag2CrO4 before and after it was coupled with ­MoO3, photoluminescence emission spectra (PL) of the M ­ oO3/ Ag2CrO4 composites were examined in comparison with that of pure ­Ag2CrO4 with an excitation wavelength of 270 nm at room temperature, because the PL analysis was widely applied to estimate the mitigation, transfer and recombination processes of the photogenerated charge carriers in semiconductor photocatalysts since PL emission arises from the recombination of free carriers [36, 45, 46]. It is well acknowledged that the higher PL intensity indicated the fast recombination of the photogenerated charge carriers, meaning that the inferior separation efficiency for photogenerated electron–hole pairs, leading to the lower photocatalytic activity [47, 48]. As shown Fig. 6a, pure ­Ag2CrO4 displayed four apparent characteristic emission peaks centred at about 360, 420, 480 and 530 nm, respectively, which was ascribed to the band gap recombination of electron–hole pairs. After introduction of ­MoO3, nevertheless, all of the ­MoO3/Ag2CrO4 composites exhibited lower PL emission intensities compared with that of pure A ­ g2CrO4, suggesting that the recombination rate of photogenerated electron–hole pairs was

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Fig. 6  PL spectra (a) of the as-obtained samples, photocatalytic activities (b) and first-order kinetic plots (c) for the photodegradation of MO in aqueous solution over the as-obtained samples under vis-

ible light irradiation, and the effect of various scavengers (d) on the photocatalytic activity of the ­MoO3(4.0 wt%)/Ag2CrO4 sample under visible light irradiation

moderately impeded and the cooperative effects between ­Ag2CrO4 and M ­ oO3 contributed to decreasing the recombination of electron–hole pairs effectively and enhancing the charge carriers separation efficiency. Concretely, the PL intensities of the as-obtained samples were ranked as: ­Ag2CrO4 > MoO3(6.0  wt%)/Ag2CrO4 > MoO3(2.0  wt%)/ Ag2CrO4 > MoO3(4.0  wt%)/Ag2CrO4. Notably, it was clearly observed that the M ­ oO3(4.0 wt%)/Ag2CrO4 composite had the lowest PL intensity, suggesting the photogenerated electron–hole pairs possessed the highest separation efficiency in this sample, resulting in the superior photocatalytic activity. Surprisingly, it can be seen that the ­ MoO3(6.0  wt%)/Ag2CrO4 composite exhibited the higher PL intensity than that of the M ­ oO3(4.0 wt%)/ Ag2CrO4 sample, implying that the separation of the photogenerated charge carriers in the former was lower than that of the latter, which may be ascribed to excess ­MoO3 may act as recombination centres or decrease the formation of heterojunctions at the interface between ­MoO3 and ­Ag2CrO4, thereby reducing the efficiency of charge separation.

3.2 Photocatalytic activity and stability

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3.2.1 Photocatalytic performance The photocatalytic performances of the pure A ­ g2CrO4, ­MoO3 and ­ MoO3/Ag2CrO4 composites with different weight percentage of M ­ oO3 were evaluated toward photocatalytic degradation of methyl orange (MO) as a target pollutant in an aqueous solution under visible light irradiation (λ >420  nm) without using any sacrificial reagent at room temperature. Figure  6b expressed the MO degradation rates in the presence of the as-obtained samples, where C is the MO concentration after visible light irradiation, and C0 is the MO concentration after reaching the adsorption–desorption equilibrium between the MO molecules and photocatalysts in the dark. As shown in Fig. 6b, it can be observed that the self-degradation of MO was almost negligible within the test period in the absence of photocatalyst, indicating that MO molecule is quite stable without any self-photodegradation under visible light irradiation and MO is degraded via photocatalytic process.

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The concentration of MO decreased along with the irradiation time if the photocatalyst was introduced. After irradiation for 120  min, there was no obvious degradation of MO in the presence of pristine M ­ oO3, and only 18.1% of MO could be decomposed by pure A ­ g2CrO4 due to their fast recombination rate of photogenerated charge carriers. Excitingly, all the M ­ oO3/Ag2CrO4 composites showed enhanced photocatalytic performances in compared with the individual ­MoO3 and ­Ag2CrO4 under the identical testing conditions. As the content of ­MoO3 increased, the photocatalytic performances of the M ­ oO3/Ag2CrO4 composites were improved gradually and then decreased. For example, the ­MoO3(4.0  wt%)/Ag2CrO4 sample exhibited the highest photocatalytic activity, approximately 38.7% of the MO was decomposed from the solution after 120 min visible light irradiation, which was 2.1 and 10.1 times as high as that of pure A ­ g2CrO4 and M ­ oO3, respectively. Such an enhancement was predominantly ascribed to the appropri­ oO3/Ag2CrO4 composites had ate amount of ­MoO3 in the M a synergistic effect with ­Ag2CrO4, which was beneficial to the fast separation of photogenerated charge carriers and thus enhanced the photocatalytic activity, which was well consistent with the above-mentioned PL analysis. Nevertheless, further increasing the ­MoO3 content to 6.0 wt% in the composite leaded to a decreased photocatalytic performance, indicating that the photocatalytic activities of the as-obtained ­MoO3/Ag2CrO4 composites depended on the mass ratio of ­MoO3 and ­Ag2CrO4. Such an decline may be mainly attribute to the following two factors: one is that the excessive loading of ­MoO3 would impede the light absorption of ­ Ag2CrO4, which was well accordance with the above-mentioned UV–Vis DRS analysis. The other one is that the excessive loading of M ­ oO3 might act as a recombination centre and would cover the partial reactive sites of ­Ag2CrO4, which was not benefit for the separation of photogenerated charge carriers, it was well consistent with the above-mentioned PL analysis. Moreover, in order to go deep into understand the enhanced photocatalytic performances of the M ­ oO3/ Ag2CrO4 composites, the photocatalytic degradation kinetics curves of MO of the as-obtained samples were fitted by a pseudo-first-order model, which was depicted by the following equation [34, 49]: −ln(C/Co) = kt. where Co is the equilibrium concentration of MO after 60  min dark adsorption, C is the MO concentration remaining in the solution at irradiation time t (min), and k is the apparent first-order rate constant. The linear relationships between ln(C/Co) and t of the as-obtained samples were shown in Fig.  6c, which confirming that the photodegradation reaction of this study is indeed pseudo-first-order. It was noteworthy that the values of k were determined from the slops of ln(C/Co) and t. Obviously, the apparent rate constants were calculated to be 0.00030, 0.00158, 0.00303, 0.00390

and 0.00207  min−1 for ­MoO3, ­Ag2CrO4, ­MoO3(2.0  wt%)/ Ag2CrO4, ­MoO3(4.0  wt%)/Ag2CrO4 and M ­ oO3(6.0  wt%)/ Ag2CrO4 respectively. It could be clearly seen that the apparent rate constant of ­MoO3(4.0 wt%)/Ag2CrO4 was the highest among all the samples, which was almost 2.5 times higher than that of A ­ g2CrO4 and 13 times larger than that of ­MoO3, respectively. These results clearly demonstrated that the synergistic effect between ­MoO3 and ­Ag2CrO4 played an important role in enhancing the photocatalytic activity under visible light irradiation, meanwhile, the introduction of ­MoO3 could indeed improve the photocatalytic activity of ­Ag2CrO4. 3.2.2 Photocatalytic stability Taking into account the practical application, the reusability and stability of the M ­ oO3(4.0  wt%)/Ag2CrO4 composite was carried out under identical conditions each time, and the corresponding results were shownin Fig.  7a. It could be found from Fig. 7a, the photocatalytic activity of ­MoO3(4.0 wt%)/Ag2CrO4 composite was retained at about 80% of its original activity after three successive recycling run, indicating that the M ­ oO3(4.0  wt%)/Ag2CrO4 composite had a relatively good photocatalytic stability. The decline of photocatalytic activity may be mainly ascribed to the slight loss of ­MoO3(4.0  wt%)/Ag2CrO4 during the recovery steps or a small quantity of ­Ag2CrO4 are inevitably decomposed to weakly active metallic Ag in the photocatalytic reaction due to A ­ g2CrO4 is easily photocorroded by the photogenerated electrons [11, 13]. Hence, in order to further elucidate the existence of metallic Ag in the used sample, the high resolution XPS spectra of Ag 3d of the M ­ oO3(4.0  wt%)/Ag2CrO4 sample before and after three cycle experiments were measured. As illustrated in Fig. 7b, the peaks located at about 367.7 and 373.7 eV corresponded to the binding energies of Ag ­3d5/2 and Ag ­3d3/2, which belong to the A ­ g+ [12, 35, 36], whereas two weak peaks located at about 368.3 and 374.3 eV appeared in the reused sample were ascribed to the binding energies of Ag ­3d5/2 and Ag 3­ d3/2, which belong to the ­Ago [11, 36], suggesting that a small fraction of A ­ g2CrO4 was indeed reduced to metallic Ag during the photocatalytic reaction. 3.3 Possible photocatalytic mechanism 3.3.1 Detection of active species In order to elucidate the possible photocatalytic degradation mechanism in depth, a serious of active species trapping experiments of the ­MoO3(4.0  wt%)/Ag2CrO4 sample were conducted to investigate the contributing active species during the photocatalytic reaction process. In this work, tert-butyl alcohol (t-BuOH), triethanolamine

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Fig. 7  Recyclability of the ­MoO3(4.0 wt%)/Ag2CrO4 sample for the photodegradation of MO in aqueous solution under visible light irradiation (a), and Ag 3d high resolution XPS spectra (b) of fresh and reused ­MoO3(4.0 wt%)/Ag2CrO4 sample

(TEOA) and 1, 4-benzoquinone (BQ) was employed as the scavenger for hydroxyl radical (·OH), photogenerated holes ­(h+) and superoxide radical (·O2−), respectively [13, 45, 50]. As shown in Fig. 6d, it could be found that different scavengers had different effects on the photocatalytic degradation of MO over ­MoO3(4.0  wt%)/Ag2CrO4 photocatalyst. Detailedly, it was apparent that the degradation rate of MO was slightly decreased by the addition of t-BuOH under visible light irradiation, suggesting that the ·OH played an insignificant contribution in the photodegradation of MO under visible light irradiation. On the contrary, when TEOA or BQ was introduced, the photodegradation rate of MO was greatly suppressed, implying that the ­h+ and ·O2− were the major active oxidizing species in the photocatalytic process. As a result, it was reasonable to conclude that the ­h+ and ·O2− were the dominant reactive oxidation species for the M ­ oO3/Ag2CrO4

composite in photocatalytic degradation of MO under visible light irradiation. 3.3.2 Possible photocatalytic mechanism On the basis of the above experimental results, a possible Z-scheme photocatalytic mechanism for enhanced photocatalytic performance of the ­ MoO3/Ag2CrO4 composite was tentatively proposed and schematically illustrated in Fig. 8b. When the ­MoO3/Ag2CrO4 composite was exposed under visible light irradiation, both ­MoO3 and ­Ag2CrO4 could be excited to produce the photogenerated electron–hole pairs. If the M ­ oO3/Ag2CrO4 composite followed the traditional double charge transfer mechanism (Fig. 8a), the photogenerated electrons would prefer to migrate from the CB of ­Ag2CrO4 to the CB of M ­ oO3, meanwhile, and the photogenerated holes would prefer to migrate from the VB

Fig. 8  Possible schemes for photogenerated electron–hole pairs separation and transport at the ­MoS2/Ag2CrO4 composite interface under visible light irradiation: a double charge transfer mechanism and b Z-scheme mechanism

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of ­MoO3 to the VB of A ­ g2CrO4, because the CB and VB potentials ­Ag2CrO4 were both more negative than those of ­MoO3. In this channel, the accumulation of electrons in the CB of ­Ag2CrO4 could not reduce O ­ 2 to generate ·O−2 due to the CB potential of M ­ oO3 (ECB = 0.52 eV) was more positive than the standard redox potential of ­O2/·O−2 (−0.33 eV versus NHE) [45, 51], which was not consistent with the above-mentioned trapping experiments results that the ·O−2 was the major reactive species in the ­MoO3/Ag2CrO4 photocatalytic system. Accordingly, we hereby conclude that charge transfer in the as-obtained ­ MoO3/Ag2CrO4 composite occurs via a Z-scheme mode under our experimental conditions (Fig. 8b). Under visible light irradiation, the photoinduced electrons from the VB of A ­ g2CrO4 and ­MoO3 could be easily transferred into their corresponding CB of ­Ag2CrO4 and M ­ oO3, respectively. At the same time, the photoinduced electrons in the CB of ­MoO3 tended to recombine with the holes in the VB of ­Ag2CrO4, resulting in an efficient space separation of electrons in the CB of ­Ag2CrO4 and holes in the VB of ­MoO3. The photoinduced 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. It was noteworthy that the metallic Ag nanoparticles inevitably produced by the photocorrosion of A ­ g2CrO4 at the interface between A ­ g2CrO4 and M ­ oO3 under visible light irradiation, it is confirmed by the above-mentioned XPS analysis (Fig. 7b). Subsequently, the transferred electrons on the surface of Ag nanoparticles could react with absorbed oxygen to yield ·O−2 [52, 53], which could further oxidize MO to degradation products, while the holes in the VB of M ­ oO3 could directly oxidize MO to degradation products. In this way, the photogenerated electrons and holes could be efficiently separated by the Z-scheme charge transfer mechanism, it was in accordance with the abovementioned results of the PL spectra. As a consequence, the Z-scheme charge transfer mechanism enabled the efficient separation of photogenerated electron–hole pairs as well as strong redox ability for the enhanced photocatalytic degradation efficiency of MO over the ­ MoO3/Ag2CrO4 composite.

4 Conclusion In conclusion, Z-scheme ­MoO3/Ag2CrO4 composites were successfully synthesized via a simple in-situ precipitation method and applied in the photocatalytic degradation of MO under visible light irradiation (λ > 420 nm). The optimum ­MoO3/Ag2CrO4 composite at a weight content of 4.0% ­MoO3 showed the best photocatalytic activity, the rate constant of which was almost 2.5 times higher than that of ­Ag2CrO4 and 13 times larger than that of ­MoO3,

respectively, which could be ascribed to the formation of the Z-scheme ­MoO3/Ag2CrO4 photocatalytic system which possessed higher separation and transfer efficiencies of photogenerated charge carriers. In addition, the Z-scheme photo-degradation process of the as-obtained ­ MoO3/ Ag2CrO4 composite photocatalyst was evidenced by the radical trapping experiments and photoluminescence spectroscopy analysis. This research will broaden the studies of ­Ag2CrO4-based Z-scheme photocatalytic system with excellent photocorrosion inhibition ability and high photocatalytic activity and stability. Acknowledgements  This research was supported by the Natural Science Foundation of Guangdong Province (No. 2016A030307015, 2015A030310431) and Innovative Undertaking Training Program for University Students (201710579485).

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