In recent years, the elimination of organic pollutants from the aquatic ecosystem has aroused much interest because of their association with certain environmental risks and their impact on the environment and human health [1-3]. Therefore, it is crucial to find an effective method to eliminate these organic pollutants. Visible light-driven semiconductor photocatalysts emerged as one of the most effective approaches toward solving energy and environmental pollution problems [4-7]. During photocatalytic degradation, molecular oxygen is activated by photogenerated electrons (e-) to generate the superoxide radical (•O2-) and hydrogen peroxide (H2O2), while holes (h+) are trapped by surface-absorbed hydroxyl (OH-) to generate the hydroxyl radical (•OH), and •O2- react with h+ to produce single oxygen (1O2) [8]. These reactive oxygen species (ROSs) are regarded as the key oxidants for the CO oxidation, alcohol oxidation, and degradation of organic pollutants [9-11]. Many studies proved that the photocatalytic activity was dependent upon the absorption of visible light, separation of charge carriers, and surface chemical reactions [12]. However, the fabrication of a single semiconductor that possesses all of the above advantageous properties remained a great challenge.
Although the excitation of molybdenum disulfide (MoS2) to generate charge carriers via visible light is well known, the redox potential of electron/hole in the conduction/valence band is not high, the quantum efficiency of molecular oxygen activation is suppressed, and the number of active sites on the surface of the photocatalyst is insufficient. Vast efforts had been focused on overcoming the above problems, including the construction of hybrid composite structures [13], metal doping [14], metal embedding [15], coupling with other semiconductors to form a heterojunction [16], and morphology control to expose a high-energy surface. However, the growing number of studies only focused on expanding the light absorption range and improving the separation efficiency of charge carriers. Reports on how to improve the activation efficiency of molecular oxygen were rare. It is worth noting that molecular oxygen plays a critical role in the production of ROSs [17]. Numerous semiconductor materials are known to be nonspecific in catalyzing ROSs production. Therefore, materials that specifically promote ROSs production needed to be combined with MoS2 to further promote the photocatalytic activity. For example, He et al. [18] showed that the deposition of metal (Au) nanoclusters onto ZnO could greatly increase the generation of ROSs, resulting in the enhancement of photocatalytic activity, possibly because of the high oxygen active abilities of Au. Normally, the addition of metallic materials to a photocatalytic system improves the activation efficiency of the molecular oxygen. However, we would preferably choose metallic oxide to not only improve the separation efficiency of the photogenerated charge carriers, but also to increase the redox potential of electron/hole in the conduction/valence band, and hence enhance the photocatalytic activity.
Silver vanadium oxide (SVO) materials (AgVO3 [19], Ag2V4O11 [20], Ag3VO4 [21], and Ag4V2O7 [22], etc.) attracted attention because of their widespread application in rechargeable high-energy-density lithium batteries [23], sensors [24], and photocatalysts [25, 26]. However, AgVO3 has a narrow band gap and highly dispersed valence band owing to the hybridized valence bands (V 3d, O 2p, and Ag 4d orbits) [27], which means it shows potential application as a visible-light photocatalyst. These advantages of AgVO3 motivated us to synthesize AgVO3-modified MoS2, which would promote the oxygen molecular reduction reaction and generate a larger number of ROSs to enhance the photocatalytic activity. Although work relating to AgVO3/MoS2 photocatalysts that could enhance the photocatalytic activity toward organic dyes degradation have been reported [28], the specific catalyzing features of AgVO3, in the AgVO3/MoS2 composites, in ROSs production were not discussed. We adopted an effective method using ESR coupled with a POPHA fluorescence detection method and free radical capture experiment to study the mechanism of the ROSs generated in the presence of photocatalyst. We found that AgVO3-modified MoS2 could produce three kinds of ROSs, i.e., •OH, •O2-, and H2O2. We concluded that the AgVO3 in the AgVO3/MoS2 composites could act as a specific component to catalyze ROSs production.
Herein, we report the ability of AgVO3/MoS2 to achieve an efficient photocatalytic degradation of pollutants. The charge transfer and molecular oxygen activation processes in the system were investigated to explore the possible reaction mechanism. Our work demonstrated a promising way to simultaneously increase the separation of charge carriers and the activation of molecular oxygen.
All reagents were directly used as received without further purification. Silver nitrate (AgNO3, AR), p-benzoquinone (99.0%, AR), triethanolamine (TEOA, AR), and isopropanol (IPA, AR) were all purchased from Shanghai Chemical Reagent Co., Ltd., ammonium metavanadate (NH4VO3), sodium molybdate (Na2MoO4, AR), thioacetamide (C2H5NS, AR), p-hydrophenylacetic acid (POPHA, AR), horseradish peroxidase (AR), tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl, AR), ethanol (C2H5OH, 95.0%), and ammonia solution (NH3·H2O, 28%) were all supplied by Sinopharm Chemical Reagent Co., Ltd. TC, CIP, and RhB were analytically pure and used without further purification, and deionized water was used throughout these experiments.
MoS2 was prepared by using a hydrothermal method. In a typical procedure, 1.03 mmol Na2MoO4 was dispersed in 60 mL deionized water under stirring for 30 min. Subsequently, 2.66 mmol C2H5NS was added to the above solution, which was stirred vigorously for 30 min. The solution was transferred to a Teflon-lined stainless-steel autoclave and maintained at 200 ℃ for 16 h. The sample was washed with deionized water and ethanol several times and then dried at 80 ℃ for 12 h.
The AgVO3/MoS2 nanocomposites were synthesized via a hydrothermal method. A certain amount of MoS2 was added into 20 mL of AgNO3 solution (3.6 mmol L–1) under vigorous stirring. After 30 min, 20 mL of NH4VO3 solution (3.64 mmol L–1) was appended to the above mixed solution in a drip. Then, the pH value of the solution was adjusted to approximately 7.0 with ammonia solution under stirring. This solution was subsequently transferred into a Teflon-lined stainless steel autoclave and maintained 180 ℃ for 24 h. Finally, the sample was washed with deionized water and ethanol several times, then dried in a vacuum oven at 80 ℃ for 12 h. The final products were named 1%-AgVO3/MoS2, 3%-AgVO3/MoS2, 5%-AgVO3/MoS2, and 7%-AgVO3/MoS2 according to the mass fraction of AgVO3 in the MoS2 by weight. The preparation process of AgVO3/MoS2 is shown in Scheme 1.
Powder X-ray diffraction (XRD) measurements of the samples were performed with a D/max-RA X-ray diffractometer (Rigaku, Japan) using Cu-Kα radiation in the scanning angle range of 10°–80° at a scanning rate of 5° min–1. Scanning electron microscopy (SEM) was conducted by using Hitachi S-4800 field emission SEM (Hitachi, Japan) to observe the morphology of the as-prepared samples. Transmission electron microscopy (TEM) was performed by using an F20 S-TWIN electron microscope (Tecnai G2, FEI Co.), equipped with a 200-kV accelerating voltage. The optical properties of the samples were carried out by UV-vis diffuse reflectance spectroscopy (DRS), recorded between 200 and 800 nm on a Shimadzu 2450 UV-vis spectrophotometer (Shimazu, Japan) using BaSO4 powder as a reference. The surface electronic states were investigated by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250X) and an electron spectrometer using 150 W Al Kα X-ray sources, and the binding energies were referred to the position of C 1s at 283.6 eV. The photoluminescence (PL) spectra of the solid photocatalysts were recorded on a F4500 (Hitachi, Japan) photoluminescence detector. The specific surface area and particle size of the materials were obtained via the Brunauer-Emmett-Teller (BET) method based on the N2 adsorption-desorption isotherms (NDVA-2000e).
Electrochemical impedance spectroscopy (EIS) was implemented in 0.5 mol L–1 Na2SO4 solution in the frequency range between 0.1 Hz and 10 kHz at 0.3 V. The amplitude of the applied sine wave potential in each case was 5 mV and was measured using a ZENNIUM electrochemical workstation (Zahner Instruments, Germany), and all electrochemical signals were recorded on a CHI660B electrochemical analyzer (Chen Hua Instruments, Shanghai, China).
The photocatalytic activity of the AgVO3/MoS2 nanocomposites was evaluated by observing the degradation of TC, CIP, and RhB under visible light irradiation. Photocatalyst (0.05 g) was added to 100 mL of a pollutant solution (10 mg L–1), and a 350-W Xenon lamp with a cutoff filter (λ > 420 nm) was used as the light source. Light was excluded from the reaction system, which was magnetically stirred for 30 min to attain adsorption-desorption equilibrium before illumination. At certain time intervals, 4 mL aliquots were collected from the reaction process, centrifuged, and analyzed by UV-vis spectrophotometry.
The ESR signals of radicals spin-trapped by 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) were measured to detect the •OH and •O2- on a Bruker model ESR JES-FA200 spectrometer after irradiating the suspension with simulated sunlight (adding 20 μL DMPO) at different time intervals.
The production of H2O2 was determined by another fluorescence route by employing p-hydrophenylacetic acid (POPHA), which could be oxidized to the fluorescence product 5, 5-dicarboxymethyl-2, 2-dihydroxybiphenyl by a peroxidase-catalyzed reaction in the presence of H2O2. Aliquots (0.5 mL) of irradiated catalyst solution were removed and maintained in the dark for 30 min. Subsequently, 0.5 mL of the fluorometric reagent (8 mg of POPHA and 2 mg of horseradish peroxidase dissolved in 50 mL of Tris buffer (0.1 mol L–1, pH 8.8) solution) was added and allowed to react for 30 min. Then, the above solution was centrifuged, and the supernatant was recovered for fluorescence measurement.
The XRD patterns of pure MoS2 and AgVO3/MoS2 samples with different loading amounts of AgVO3 are shown in Fig. 1. The pure MoS2 sample displayed four broad XRD peaks at 2θ = 14°, 33°, 43°, and 58°, corresponding to the (002), (100), (006), and (100) planes of hexagonal MoS2 (JCPDS card No. 75-1539), respectively [29]. With AgVO3 loaded on the surface of MoS2, the composites (1%, 3%, 5%, and 7%) showed diffraction peaks similar to those of pure MoS2, indicating that the introduction of AgVO3 neither changed the phase of MoS2 nor was it incorporated into the lattice of MoS2. Furthermore, an increase in the ratio of AgVO3 to MoS2 from 1% to 7% led to a remarkable reduction in the intensity of the MoS2 diffraction peaks of the (002) and (110) planes, as well as the gradual disappearance of the diffraction peaks of the (002) plane, owing to the partial coverage of the surface of MoS2 by AgVO3. The diffraction peaks assigned to AgVO3 were not observed for the AgVO3/MoS2 composite materials, which might be due to the low content, i.e., the relatively weak diffraction intensity. However, the XPS, TEM, and SEM results adequately proved that AgVO3 and MoS2 were successfully combined.
The morphology and superficial microstructure of the as-prepared AgVO3, MoS2, and the composite material were characterized by TEM. Fig. 2(A) and (B) show the nanorod morphologies of pure AgVO3 with smooth surfaces. Pure MoS2 exhibited a nanoflower morphology (Fig. 2(C) and (D)), clearly showing that the MoS2 nanoflowers consisted of several single layers of ultra-thin nanosheets overlapping each other, and the pure MoS2 was an agglomerate. However, Fig. 2(E) shows that the MoS2 was distributed on both sides of the AgVO3 nanorods to prevent the agglomeration of MoS2. Fig. 2(F) clearly shows that the composite consisted of both MoS2 and AgVO3, which are tightly combined with each other to form a compact hybrid structure. Two different inter-planar spacings (d) were observed to be 0.24 and 0.63 nm, which can be assigned to the (111) planes of AgVO3 and (002) planes of MoS2. Therefore, the as-prepared photocatalyst had good dispersibility and was considered able to promote the exposure of reactive sites. The photocatalyst was therefore conducive to the transport of charge carriers on the interfaces of AgVO3 and MoS2. The as-prepared samples were also investigated by SEM. Fig. S1(A) and (B) shows the SEM images of the AgVO3 nanorods, which are approximately 100–500 nm wide and crossed together. The size of the MoS2 nanoflowers was approximately 400–500 nm with their petals grown together in a disordered interweaved way and pointing towards the unity direction of the sphere to construct the spherical structure in Fig. S1(C) and (D). In the composite structure of AgVO3/MoS2, which was shown in Fig. S1(E) and (F), the AgVO3 nanorods and MoS2 nanoflowers are linked together in accordance with the TEM results.
The surface chemical composition and chemical states of the 3%-AgVO3/MoS2 composite were analyzed by XPS, and the results are shown in Fig. 3. The full-scale XPS pattern of 3%-AgVO3/MoS2 is shown in Fig. 3(A), which indicates the existence of the elements C, Ag, V, O, Mo, and S. This result is consistent with the chemical composition of the as-prepared photocatalyst. The C 1s peak at approximately 283.6 eV could be assigned to the signal from carbon contained in the apparatus and was used for calibration [30]. Fig. 3(B) shows that the two strong peaks at approximately 368.01 and 374.02 eV could be attributed to Ag 3d5/2 and Ag 3d3/2, respectively. It confirmed that the Ag+ existed in the photocatalyst [31, 32]. As shown in Fig. 3(C), the V 2p peaks were observed at binding energies of 517 and 524.5 eV, which correspond to V 2p5/2 and V 2p3/2, respectively. The high-resolution O 1s spectrum shown in Fig. 3(D) indicates that the asymmetrical O 1s peak could be divided into two peaks located at 530.2 and 530.6 eV, which were ascribed to the oxygen ions in the fully oxidized surrounding and the oxygen ions in oxygen-deficient regions, respectively [33]. For pure MoS2 (Fig. 3(E)), it was clearly observed that the peaks at 229.36 and 232.62 eV are those of the Mo 3d5/2 and Mo 3d3/2 of Mo4+, respectively [34]. The peak at a binding energy of 226.49 eV was that of S 2s, consistent with the characteristic of S2−. Obviously, apart from the XPS peaks of Mo4+ and S2−, the peak for Mo 3d at 233.09 eV was observed for the 3%-AgVO3/MoS2 composites, showing the existence of the Mo6+ state, which might be due to the formation of small amounts of surface oxide species during the hydrothermal reaction [35]. In addition, the Mo 3d of Mo4+ in the 3%-AgVO3/MoS2 composite showed negative shifts in contrast to that of MoS2, suggesting that the chemical environment of Ag+ had changed when AgVO3 coupled with MoS2. For the S 2p spectra (Fig. 3(F)), the peak binding energies were centered at 162.25 and 163.55 eV for MoS2, and 161.08 and 162.55 for the 3%-AgVO3/MoS2 composites, respectively. Similarly, negative shifts were observed on the spectra of 3%-AgVO3/MoS2, indicating that the chemical environments of S2- in the MoS2 and the 3%-AgVO3/MoS2 composite were different. The negative shifts in the binding energy of Mo 3d and S 2p in the 3%-AgVO3/MoS2 composites might be attributed to the existence of strong interaction between MoS2 and AgVO3 rather than simple physical adsorption. These results confirmed the successful formation of a heterojunction structure between AgVO3 and MoS2.
The specific surface areas and pore volumes of the prepared samples were further analyzed by nitrogen adsorption-desorption isotherm measurement. The image in the inset in Fig. 4 shows that all the samples had a mesoporous pore size (approximately 25–30 nm). Furthermore, according to the IUPAC classification, both isotherms belong to type Ⅳ physisorption isotherms. The characteristic features of this type of isotherm are its hysteresis loop associated with capillary condensation occurring in the mesoporous structure, and the limiting uptake in the range of high relative pressure [36, 37]. The as-prepared photocatalysts displayed an H3-type hysteresis loop (P/P0 > 0.4), which may have resulted from the MoS2 nanoflowers consisting of several ultra-thin MoS2 nanosheets overlapping each other. The BET surface area measurements of pure MoS2 and 3%-AgVO3/MoS2 were 17.36 and 7.71 m2 g-1, respectively, demonstrating that the introduction of AgVO3 influenced the surface area. The specific surface area and pore volume calculated from the isotherms are summarized in Table 1.
The UV-vis diffuse reflection spectra of the AgVO3, MoS2, and AgVO3/MoS2 composites with different loadings of AgVO3 are also shown in Fig. 5. The pure MoS2 showed significant absorption both in the ultraviolet and visible regions. For the pure AgVO3, strong absorption at wavelengths shorter than approximately 500 nm was attributed to the intrinsic band gap absorption [38, 39]. The optical absorption of the composite material increased after combining AgVO3 with MoS2 compared to that of pure MoS2, suggesting that the composites possessed enhanced visible light photocatalytic properties. In addition, the band gap energy (Eg) of AgVO3 and MoS2 was calculated by:
where α, h, ν, Eg, and A denote the absorption coefficient, Planck's constant, light frequency, band gap energy, and a constant, respectively. Among them, the index n depends on the type of optical transition of a semiconductor (n = 1 and 4 for direct and indirect transitions, respectively) [40, 41]. For MoS2, n was 2, and the corresponding Kubela-Munk transformed reflectance spectra are shown in Fig. S2. The band gap energy could be estimated from the intercept of the tangent to the plot of (αhν)2 vs. the radiation energy (hν)2. The obtained band gap energy for MoS2 and AgVO3 were 1.79 and 2.18 eV, respectively, which are consistent with the previous report [42, 43].
The adsorption capacity is one of the important properties of the photocatalytic degradation of pollutants. Fig. 6 displays the nonlinear fitting curves and linear fitting curves of pure AgVO3, MoS2, and 3%-AgVO3/MoS2 for adsorbing TC molecules, which could be used to study the adsorption performance with different as-prepared samples quantitatively. These adsorption experiments were carried out at room temperature and the adsorption kinetics could be described by the pseudo-second-order model [44]:
where Qe and Qt (mg g−1) are the amount of TC molecules adsorbed on samples at equilibrium and at time t, respectively, k (min−1) is the equilibrium rate constant of pseudo-second-order adsorption (g mg−1 min−1). Here, a fast adsorption process with high rate constant is desired. The k values for these three processes with pure AgVO3, MoS2, and 3%-AgVO3/MoS2 were calculated by fitting the plots of t/qt vs. t. The adsorption of TC obeyed the pseudo-second-order rate equation well because of the favorable agreement between experimental and calculated values of Qe (R2 > 0.99). The relevant data are provided in Table 2. The adsorption rate of MoS2 was greatly improved when the AgVO3 was combined with MoS2. The enhanced adsorption capacity and improved adsorption rate constant might be regarded as one of the reasons for the enhanced photocatalytic degradation process.
The photocatalytic degradation performance of as-prepared samples was evaluated under visible light irradiation (λ > 420 nm) using TC as the model organic contaminant. The degradation results in Fig. 7(A) show that the blank curve provides evidence of negligible degradation of TC, indicating that the TC was stable under visible light irradiation. After visible light irradiation for 180 min, only 32.05% and 48.0% of pure MoS2 and AgVO3, respectively, were degraded. Furthermore, the composite photocatalytic activity first increased to a maximum and then decreased as the AgVO3 loading on the surface of MoS2 increased. The photocatalyst with optimal performance is 3%-AgVO3/MoS2, which indicated that the appropriate modification of MoS2 with AgVO3 played an important role in the improvement of the photocatalytic degradation efficiency. This tendency could possibly be attributed to the poor distribution of excessive AgVO3 on the surface of MoS2, which, together with agglomeration, leads to the weak transmitting ability of photogenerated electrons. The photo-degradation kinetic constants of different samples could be measured by the pseudo-first-order equation:
where k is the apparent kinetic rate constants, t is the irradiation time, and C and C0 are the real-time concentration at t and the initial concentration of TC, respectively. As shown in Fig. 7(B), the apparent rate constant of 3%-AgVO3/MoS2 was 0.0087 min−1, which is 1.71 and 1.76 times higher than those of MoS2 (0.00509 min−1) and AgVO3 (0.00495 min−1), respectively. To obtain improved insight into the catalytic performance of the 3%-AgVO3/MoS2 composites, RhB and CIP were also employed as target contaminants. As shown in Fig. 7(C), the results revealed that during the RhB and CIP photo-degradation process, the 3%-AgVO3/MoS2 also exhibited enhanced photocatalytic activity. The degradation efficiency of RhB and CIP was 83% and 62.4%, respectively, and the apparent rate constants of degradation of RhB and CIP were 0.0139 min−1 and 0.00685 min−1 (Fig. 7(D)), respectively. The improved photocatalytic performance owing to the modification of MoS2 with AgVO3 could facilitate O2 adsorption/activation and thus generated a larger number of ROSs to degrade organic pollutants.
The stability and reusability of the photocatalyst were crucial factors to estimate the quality of photocatalyst and for subsequent practical application. To examine the stability of the 3%-AgVO3/MoS2, the photocatalyst was collected after degradation of TC for reuse. As shown in Fig. 8(A), the photocatalyst activity of 3%-AgVO3/MoS2 remained approximately 73% after four successive experimental cycles and showed almost no deactivation, revealing the improved stability of the photocatalyst during the process of degrading TC. The stability of the as-prepared photocatalyst was further investigated by recording the XRD patterns of the 3%-AgVO3/MoS2 before and after four cycles, as shown in Fig. 8(B). There is almost no difference between the two samples, with the main characteristic diffraction peaks of 3%-AgVO3/MoS2 being in almost the same position on the pattern, indicating that the crystal structure remained unchanged after the cycling tests. All the above results indicated that the 3%-AgVO3/MoS2 composite was an effective and stable photocatalyst for the degradation of contaminations. For the sake of investigating the stability of samples more thoroughly, the TEM images of the 3%-AgVO3/MoS2 before and after the cyclic experiment were acquired and are shown in Fig. 8(C) and (D). It can be observed from Fig. 8(C) that many MoS2 nanoflowers are linked together with the AgVO3 nanorods. However, the MoS2 nanoflowers in direct contact with the AgVO3 nanorods were not dislodged, thereby ensuring that the AgVO3/MoS2 heterojunction existed stably throughout the degradation process. This was also the main reason for the negligibly small reduction in photocatalyst activity after the fourth cycle experiment.
The degradation intermediates of TC over the 3%-AgVO3/MoS2 was investigated by examining the HPLC-MS results. The corresponding changes in the characteristic peak of TC (m/z = 445) became increasingly smaller, and some additional peaks appear in Fig. S3. This illustrates that the TC had been decomposed into other intermediate products and these substances were gradually decomposed to CO2, H2O, and other small molecules. As shown in Fig. 9, the prominent anion with m/z of 445 is the deprotonated tetracycline molecular ion, showing that the tetracycline was fragmented into small molecules during the process of degradation [45]. It was found that the intermediate product A (m/z = 362) was obtained by breaking the naphthalene ring under alkaline conditions [46] to obtain the lactone structure product B (m/z = 361) by the formation of hydroxyl electron-rich groups (negative oxygen ions). This product undergoes a nucleophilic reaction with carbonyl to become product C (m/z = 318) by losing N-methyl groups. Furthermore, another degradation pathway involved the fragmentation of tetracycline into product D (m/z = 345) by losing the leaving groups of –CH3, –N(CH3)2 and –CO(NH2) to become product E (m/z = 329) by losing –OH. This product decomposed into product F (m/z = 274) through ring-opening reactions [47]. Finally, the tetracycline was gradually broken down into small molecules as the photocatalytic reactions progressed.
Generally, the catalytic performance had two key properties: (1) the separation efficiency of charge carriers and (2) the application of an electron-hole mechanism in the production of ROSs [48]. In terms of the first, the separation efficiency of charge carriers can always be reflected by measuring the EIS and PL. The interfacial charge transfer resistance and optical carrier separation efficiency were measured by EIS [49], where a smaller radius indicates higher charge transfer efficiency [50, 51]. Fig. 10(A) shows that the radius of 3%-AgVO3/MoS2 particles is smaller than that of pure MoS2, indicating that adding AgVO3 could improve the separation efficiency of charge carriers.
It is well known that the peak intensity obtained by PL emission spectroscopy indirectly reflects the separation efficiency of charge carriers. The emission of photoluminescence by semiconductor materials originating from the radiative recombination of photo-generated electrons and holes, as well as two major photo-physical processes, could produce photoluminescence signals [52]. The lower PL intensity represented a reduced electron-hole recombination rate and superior photocatalytic activity [53-55]. Fig. 10(B) shows the PL spectra of AgVO3, MoS2, and 3%-AgVO3/MoS2. The emission spectrum of MoS2 has the highest intensity with the peak located at 468 nm. However, the PL intensity of 3%-AgVO3/MoS2 significantly decreased when AgVO3 was added, further demonstrating that the recombination of photogenerated electron-hole pairs was suppressed owing to the addition of AgVO3. In addition, the time-resolved fluorescence decay spectra were also recorded. The fitted time-resolved fluorescence emission decay spectra are shown in Fig. 10(C). The time-resolved fluorescence decay curves of AgVO3, MoS2, and 3%-AgVO3/MoS2 were fitted by the bi-exponential function:
and the average lifetime was calculated by
where I(t) is the PL intensity at a certain delay time, A1 and A2 represent the relative weight of the decay components at t = 0 [56]. Table 3 lists the radiative lifetime of the fitted decay curves of MoS2, AgVO3, and the 3%-AgVO3/MoS2 heterojunction, which were 0.95, 1.30, and 1.32 ns, respectively. The increase in decay lifetime of 3%-AgVO3/MoS2 could be explained by the introduction of AgVO3, which increased the O2 adsorption/activation and accelerated the consumption of photo-excited electrons. The participation of additional electrons in the degradation process could enhance the photocatalytic efficiency.
In terms of the second key property, combining AgVO3 with MoS2 could increase the ROSs production. To provide evidence for this, an electron spin resonance (ESR) spin-trap technique was used for •O2− and •OH detection with 5, 5-dimethyl-1-pyrroline N-oxide (DMPO), and the H2O2 yield was determined by using p-hydrophenylacetic acid (POPHA) [57]. In Fig. 11(A) and (B), none of the characteristic signals of the DMPO-•OH and DMPO-O2•− were observed in the dark; thus, it could be concluded that the formation of O2•− and •OH on the as-prepared samples would require irradiation by light. In Fig. 11(C), the characteristic peaks of the DMPO-•OH adduct were only observed on 3%-AgVO3/MoS2 in the presence of light. However, in Fig. 11(D), the characteristic peaks of the DMPO-O2•− adduct were observed both for the pure MoS2 and 3%-AgVO3/MoS2 composite photocatalysts. Moreover, it was obvious that the intensities of both the DMPO-•OH and DMPO-O2•− signals of the 3%-AgVO3/MoS2 composite were much higher than that of MoS2, respectively. These results reveal that the introduction of AgVO3 could enhance the formation of O2•− and •OH species. In addition to the direct determination of ROSs by ESR tests, indirect analysis by the addition of scavengers was also conducted in this study. Triethanolamine (TEOA), isopropanol (IPA), and benzoquinone (BQ) were used as scavengers for holes (h+), hydroxyl radicals (•OH), and superoxide radicals (•O2-), respectively. As shown in Fig. S4, under visible light irradiation, the degradation effect of TC decreased slightly from 73% to 69.2% after adding BQ. In contrast, the TC degradation rate was largely reduced to 61.7% and 45% upon addition of TEOA and IPA, respectively. The above results enabled us to conclude that the species •O2-, h+, and •OH all took part in the process of photocatalytic degradation.
To further investigate that the addition of AgVO3 could increase the generation of ROSs under UV light irradiation, a POPHA fluorescence method was employed to investigate the generation of H2O2 from different photocatalysts and further examine the pathway of molecular oxygen activation [58]. Fig. 12 shows that significantly more H2O2 was generated over 3%-AgVO3/MoS2 than over MoS2, which was attributed to increased H2O2 generation because of the addition of AgVO3. The amount of H2O2 generated by the 3%-AgVO3/MoS2 composite photocatalyst increased with prolonged exposure to light, but almost no H2O2 was generated in the dark after 30 min; thus, one of the necessary conditions for the production of ROSs was irradiation by light. This result also indicated that H2O2 was generated via a two-electron reduction of O2 during photocatalytic degradation (O2 + 2H+ + 2e− = H2O2, 0.682 V vs. NHE) [59].
The photocatalytic activities depended on the migration, transfer, and separation of photogenerated charge carriers and the band positions of AgVO3 and MoS2 also determined the migration path of electrons and holes. The conduction band edge (ECB) was calculated by the following empirical equation:
where X 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 (4.5 eV), and Eg is the band gap of the semiconductor that can be obtained from Fig. S2, and the valence band edge (EVB) can be determined by EVB = ECB + Eg [60, 61]. As shown in Fig. 13, the densities of the total states of the MoS2 and AgVO3 valence band were measured to be 1.70 and 2.49 eV, respectively. The band gaps of MoS2 and AgVO3 acquired from the DRS analysis were 1.79 and 2.18 eV, respectively. Therefore, the ECB values of MoS2 and AgVO3 were –0.09 and 0.31 eV, respectively. According to the band energy structure, the photogenerated electrons from the CB of MoS2 were transferred to the CB of AgVO3, while the photogenerated holes from the VB of AgVO3 were transferred to the VB of MoS2, which allowed for more effective charge separation.
Based on the observed photocatalytic activity and the characterization presented above, we conclude that a two-electron process and the surface modification by AgVO3 facilitated the O2 adsorption/activation for •OH and H2O2 production, thus enhancing the photocatalytic activity. The mechanistic pathway is illustrated in Fig. 14. According to the DRS results, the band gap of MoS2 was as narrow as 1.79 eV (Fig. S2), which indicated the charge carriers generated by MoS2 underwent facile recombination. Therefore, only few of the electrons photo-generated by the MoS2 could react with the dissolved oxygen and, because of the difficulty of O2 adsorption/activation, these unreacted electrons were often recombining with holes. However, when AgVO3 was deposited on the surface of MoS2, the advantageous O2 adsorption/activation property of the AgVO3 would improve the oxygen reduction reaction on the MoS2 surface. This would enhance the production of ROSs via electronic consumption, leaving a larger number of holes for the degradation of organic pollutants. The reaction process can be represented by Eqs. (8)–(15). The combination of ESR and capture experiments determined (13), (14) to be the main mechanistic pathway.
In summary, we successfully prepared a highly efficient AgVO3/MoS2 composite photocatalyst by a hydrothermal method. The as-prepared 3%-AgVO3/MoS2 composite achieved the best performance toward the degradation of different organic pollutants compared to pure MoS2. The enhanced photocatalytic performance was attributed to the specific catalytic properties of AgVO3, which effectively promote the formation of adsorbed O2 to ROSs on the surface of the 3%-AgVO3/MoS2 composite, thus greatly improving the photocatalytic activity. Simultaneously, the effective consumption of photo-generated electrons on the AgVO3 surface leaves behind more holes for the degradation of pollutants. This study provided new insights into the rational design of composite photocatalysts to promote the generation of ROSs by surface catalysis engineering and may be put into practice in environmental governance.
2018,
Vol. 39 Issue (9): 1470-1483 
PDF
2018,
Vol. 39 Issue (9): 1470-1483 PDF