In-situ fabrication of MoO3 nanobelts decorated with MoO2 nanoparticles and their enhanced photocatalytic performance

Applied Surface Science 480 (2019) 427–437

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In-situ fabrication of MoO3 nanobelts decorated with MoO2 nanoparticles and their enhanced photocatalytic performance

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Qingyang Xia, Jinsong Liua,b, , Zhengying Wuc, Hongfei Bia, Ziquan Lia,d, Kongjun Zhub, Jiajia Zhuanga, Jixun Chena, Songlong Lua, Yanfang Huanga, Guoming Qianb a

Department of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China c Laboratory for Environment Functional Materials, Suzhou University of Science and Technology, Suzhou 215009, China d Institute of Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing 210003, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: In-situ hydrothermal method MoO2/MoO3 nanocomposites Photocatalytic activity Detailed mechanism

Photocatalysis has been rapidly developed as a sustainable technology to decompose contaminants by using photogenerated carriers excited through light irradiation. Electrons for molybdenum trioxide (MoO3) semiconductor with wide band gap can be easily transferred to its conduction band via dye sensitization effect under visible light. However, MoO3 still suffers from poor photocatalytic ability for organic dyes due to the low energy level of the conduction band and the insufficient utilization of the induced electrons. In this study, molybdenum dioxide (MoO2) nanoparticles were decorated on the surface of MoO3 nanobelts without requiring an additional Mo source by using a simple in-situ hydrothermal method. In the reaction process, the partial MoO3 itself was reduced to metallic MoO2 nanoparticles, and the resulting intimate interface between MoO2 and MoO3 could accelerate the transfer of dye sensitization-induced electrons. The as-prepared MoO2/MoO3 nanocomposites exhibited extremely enhanced visible light photocatalytic activity for decomposing rhodamine B (RhB) with the assistance of H2O2. The mechanism for high-efficiency degradation was analyzed and explored by conducting theoretical calculations and designing further experiments. The high-efficiency degradation might be due to the synergistic effect caused by the well-matched energy band structure between dyes and MoO3, and the metallic MoO2 nanoparticles, which can accelerate the production of hydroxyl radical (%OH) from H2O2. %OH is a dominant reactive species for the degradation of RhB under visible light irradiation.

1. Introduction As a green and sustainable technology that uses solar energy for hydrogen production, CO2 emission reduction, and environmental purification, photocatalysis has recently received attention due to its potential applications in energy and environment fields [1–5]. Photogenerated electrons and holes and subsequent free radicals under light irradiation could entirely decompose various organic dyes [6,7], thereby effectively mitigating harm to the environment and human health and promoting the cyclic utilization of water resources [8–10]. Molybdenum trioxide (MoO3) is a highly anticipated photocatalytic semiconductor with abundant reserves, non-toxicity, and high chemical stability. Inadequately, the large band gap (2.7–3.2 eV) of MoO3 usually limits the visible light response [11]. Different nanocomposites based on MoO3, such as AgBr/MoO3 [11], g-C3N4/MoO3 [12], Bi2Mo3O12/ MoO3 [13], and MoS2/MoO3 [14,15], have been explored to harvest

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visible light. Furthermore, the second phases involved make the reaction complex, and some of them may even cause secondary pollution. Actually, the excellent organic dye adsorption of MoO3 may provide another promising way to promote photocatalytic reaction under visible light with the help of dye-sensitized electrons [11,16]. In dye-sensitized photocatalysis system, organic dyes could easily harvest visible light and then transferred the dye-sensitized electrons to the photocatalyst [17,18]. The well-matched energy level between the dyes and the semiconductors is necessary and critical, as reported in our previous research [11], to ensure the successful transfer of dye-sensitized electrons. Furthermore, the efficient use of dye-sensitized transferred electrons is crucial for the photocatalysis process, which determines final photocatalytic efficiency. Maximizing the role of dyesensitized electrons is substantial to completely degrade dyes. Previous studies showed that depositing noble metals, such as Pt [19], Ag [20], and Au [21] could effectively enhance the photocatalytic

Corresponding author at: Department of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China. E-mail address: [email protected] (J. Liu).

https://doi.org/10.1016/j.apsusc.2019.03.009 Received 11 November 2018; Received in revised form 27 February 2019; Accepted 1 March 2019 Available online 02 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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dried in a vacuum at 80 °C for 12 h.

activity of semiconductors with wide band gap similar to TiO2 and MoO3 [22–24] due to surface plasmon resonance, and the fast transfer of electrons to noble metals could depress the recombination of photogenerated carriers. Nevertheless, the expensive cost and potential environmental threat of noble metals limited their application. Molybdenum dioxide (MoO2) possesses high conductivity and chemical stability similar to noble metals [25,26], making it a promising substitute in transferring electrons. Meanwhile, the common Mo element of the MoO3 and MoO2 makes the synthesis of MoO2/MoO3 nanocomposites by a facile in-situ reduction method possible. This in-situ-formed intimate interface may be favor of the interfacial charge transfer and the prolonged lifetime of carriers [27,28]. Therefore, preparing such novel visible-light-driven dye-sensitized photocatalyst of MoO2/MoO3 nanocomposite and understanding detailed mechanism is necessary to enhance photocatalytic efficiency. We synthesized MoO2 nanoparticle-decorated MoO3 nanobelts by partial in-situ reduction using a simple hydrothermal method based on the aforementioned analysis. Abundant adsorption sites of dyes on the surface of MoO3 nanobelts stimulated the dye sensitization effect, in which the dyes responded to visible light and then injected the electrons into the conduction band of MoO3. In addition, the dispersed metallic MoO2 nanoparticles played a similar role as noble metals to accelerate electron transfers, which was further transformed into active radicals with strong redox potential with the help of the electron acceptor H2O2 by the photo-Fenton-like reaction, thereby achieving the degradation of dyes [29–32]. The excellent photocatalytic performance of MoO2/MoO3 composites was confirmed by different experiments and characterizations, and the reason for the enhanced photocatalytic activity was investigated by further trapping experiments and degrading tests for different dyes. Furthermore, the possible mechanism for the photocatalytic process was proposed and discussed in detail. Our study provided a novel way for designing photocatalytic system with rapid charge transfer by dye-sensitization effect.

2.3. Material characterization The crystalline structure of the samples was characterized by X-ray diffraction (XRD) (Rigaku D-max-γA XRD with Cu Kα radiation, λ = 1.54178 Å) at 40 kV and 40 mA in the range of 2θ = 10°–80°. Morphologies of the samples were observed via scanning electron microscopy (SEM) (S-4800, Hitachi, Japan). Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images were examined using TEM (F-20, FEI, USA) at an accelerating voltage of 200 kV. UV–vis diffusion reflectance spectra of the samples were examined at room temperature by UV–vis–NIR spectrophotometer (JASCO V-670), and BaSO4 was used as the reference. Photoluminescence (PL) spectra of the samples were obtained using an Edinburgh FL/FS900 spectrophotometer under 325 nm excitation wavelength. Measurements of electrochemical impedance spectroscopy (EIS) are as follows: Slurry of as-prepared powder (80 wt%), polyvinylidene fluoride (PVDF) binder (10 wt%), and acetylene black (10 wt %) were dissolved in N-methylpyrrolidone (NMP) and then pasted on Al current collector using a medical blade technique. The electrode was then dried under vacuum at 110 °C for 12 h and punched into 12-mm diameter disc. The total mass loading on the current collector was approximately 2 mg. CR2016 coin cells were assembled in an argon-filled glove box. The as-prepared powder, lithium metal, and polypropylene film were used as cathode, anode, and separator, respectively. Electrolytes were 1 mol·L−1 LiPF6 in a volumetric ratio of 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Electrochemical impedance spectrum (EIS) was measured by using a CHI 660E electrochemical workstation in the frequency range of 10−2–10−6 Hz. 2.4. Photocatalytic experiments The photocatalytic performance of different samples were evaluated by degrading RhB or other dyes under light irradiation (250 W Xenon lamp, 400–800 nm) with cooling circulation water, and all experiments were carried out at room temperature ranging from 20 to 25 °C. In a typical photocatalytic experiment, 10 mg of the as-prepared photocatalyst and 100 mL of RhB (10 mg·L−1) or other aqueous dye solutions were added into the photocatalytic reaction device. First, the mixture was magnetically stirred in the dark for 60 min to establish absorption–desorption equilibrium between the dye molecules and the catalysts. Second, H2O2 was added into the mixture as assistant catalyst before irradiation. At given time intervals, approximately 5 mL suspensions were collected and centrifuged (9000 rpm, 2 min) for two times to remove the photocatalyst powder. The UV–Vis spectrum of the supernatant was recorded to monitor the degradation behavior of dyes based on different characteristic absorption peaks (RhB: 553 nm; EY: 517 nm; MB: 664 nm; and MO: 466 nm). Subsequently, the absorption values were transformed to their corresponding concentrations by using the standard curves of RhB and other dyes. For radical trapping experiments, AgNO3, EDTA-2Na, and DMSO were added into the solution beforehand to consume e−, h+, and %OH, respectively. For reused tests, the photocatalyst was separated out from the solution, washed with deionized water, and dried under vacuum after each use. The specific volume of the used dye solution for the next degradation was determined according to the remaining catalyst mass.

2. Experimental section 2.1. Materials All reagents were of analytical grade and used without further purification. Molybdenum powder (Mo, AR), rhodamine B (RhB, C28H31ClN2O3, AR), eosin Y (EY, C20H6Br4Na2O5, AR), acid orange 7 (AO7, C16H11N2NaO4S, AR), methylene blue (MB, C16H18ClN3S, AR), methyl orange (MO, C14H14N3SO3Na, AR), and potassium thiocyanate (KSCN, AR) were used as reagents and supplied by Aladdin. Hydrogen peroxide (H2O2, 30 wt%), absolute ethyl alcohol (C2H5OH, AR), silver nitrate (AgNO3, AR), dimethyl sulfoxide (DMSO, C2H6OS, AR), and edetic acid disodium salt (EDTA-2Na, C10H14N2Na2O8·2H2O, AR), hydrochloric acid (HCl, 36.5–38%), and sodium hydroxide (NaOH, 50%) were purchased from Sinopharm Chemical Reagent (Shanghai, China). Deionized water was produced using a Direct-Q Millipore filtration system with resistivity of 18.2 MΩ·cm. 2.2. Preparation MoO3 nanobelts were synthesized using a method similar to our previous research, except for different heating methods [11]. MoO2/ MoO3 nanocomposites and MoO2 nanoparticles were synthesized via a simple hydrothermal method by using the obtained MoO3 nanobelts as a starting material. In a typical procedure, the MoO3 nanobelts (0.12 g) and KSCN with different mole ratios (mol(Mo/S) = 12:1, 10:1, 8:1, 6:1 for MoO3/MoO2; 1:1 for MoO2) were first dispersed into deionized water (30 mL) under ultrasonication for 15 min. The suspension was then transferred into a 60 mL autoclave with a Teflon liner. The autoclave was sealed and maintained at 220 °C for 24 h and air-cooled to room temperature. The composite powders were filtered and washed with deionized water and anhydrous alcohol three times and finally

3. Results and discussions 3.1. Characterization of the MoO2/MoO3 nanocomposites The crystalline structure of the as-prepared samples was characterized by XRD technique (Fig. 1a). The distinct diffraction peaks indexed to the orthorhombic α-MoO3 (JCPDS card no. 05-0508) 428

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Fig. 1. (a) XRD patterns of the as-prepared MoO3, MoO2, and MoO2/MoO3; (b) Lattice parameters of MoO3, MoO2, and MoO2/MoO3; (c) XPS patterns of MoO2/MoO3 nanocomposite (mol(Mo/S) = 10:1).

surface of the MoO3 due to the introduction of KSCN into the reaction, thereby verifying the partial reduction of MoO3. The amount of MoO2 on the surface was increased with KSCN. For 12:1 and 10:1 of the mole ratio between MoO3 and KSCN (Fig. 2b and c), the MoO2 nanoparticles showed good dispersion on the surface of MoO3 nanobelts. Obvious aggregation of the MoO2 nanoparticles were observed by the continuous increase of the mole ratio to 8:1 and 6:1 (Fig. 2d and e). When the mole ratio reached 1:1, pure MoO2 was obtained and showed microspheric morphology with a diameter that ranges from 0.6 to 1.6 μm (Fig. 2f). TEM images in Fig. 3a–d showed a detailed description of the MoO3 nanobelts and the MoO2/MoO3 nanocomposite (mol(Mo/S) = 10:1) with a distinct MoO2-decorated MoO3 structure. The intimate contact (shown in Fig. 3d) between the MoO2 nanoparticles and the MoO3 surface may facilitate electron transfer. The crystal lattice fringes of 0.371 nm and 0.264 nm can be indexed to the (110) and (111) plane of MoO3, and the spacing with 0.215 nm of the adjacent lattice planes was consistent with the (210) plane of MoO2. Concurrently, the ring-like SAED pattern for the MoO2/MoO3 nanocomposite (the inset of Fig. 3d) proved the dispersion of MoO2 nanoparticles on the surface of MoO3 in

confirmed the successful synthesis of pure MoO3 by using hydrothermal method [33]. After adding KSCN to the subsequent reaction, the diffraction peaks that attribute to monoclinic MoO2 (JCPDS card No. 655787) appeared [34]. With the increase of KSCN, the intensities of the MoO2 peaks that correspond to the planes of (213), (220), (202), and (211) strengthened. The coexistence of MoO2 and MoO3 diffraction peaks proved the formation of MoO2/MoO3 nanocomposites by the partial reduction of MoO3. When the mole ratio between MoO3 and KSCN reached 1:1, MoO3 was completely reduced to MoO2. Lattice parameters of MoO2 and MoO3 in the MoO2/MoO3 nanocomposites were calculated using Prague equation and interplanar spacing formula (Fig. 1b). The deviation of less than 0.227% indicated the stability of the crystalline structure. XPS results in Fig. 1c confirmed the coexistence of Mo4+ and Mo6+ ions in the MoO2/MoO3 nanocomposite (mol(Mo/S) = 10:1). Two peaks at 235.6 eV (Mo6+ 3d3/2) and 232.2 eV (Mo6+ 3d5/2) were from MoO3 [35], and those at 232.6 eV (Mo4+ 3d3/2) and 230.2 eV (Mo4+ 3d5/2) were from MoO2 [36,37]. Morphologies of the samples were investigated by SEM and TEM. As-prepared pure MoO3 exhibited stacked nanobelts with smooth surface and clear edge (Fig. 2a). Granular MoO2 began to appear on the 429

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Fig. 2. SEM images of (a) pure MoO3, (b) 12:1 MoO2/MoO3, (c) 10:1 MoO2/MoO3, (d) 8:1 MoO2/MoO3, (e) 6:1 MoO2/MoO3, (f) pure MoO2.

obvious enhancement of the visible light response, as well as the pure MoO2, which maybe because that the MoO2 nanoparticles can act similarly to noble metals as a good visible light harvester [38,39], with the gradual increase of the MoO3/KSCN mole ratio to 8:1 and 6:1. The estimated band gap of MoO3 by the plot of (αhν)2 vs. hν was 3.2 eV (Fig. 4b), which is identical with other studies [40,41]. By contrast, the band gap energies of the MoO2/MoO3 nanocomposites were reduced by the in-situ transformation from the partial MoO3 to the MoO2, and the value gradually decreased with the increase on KSCN content. The changes may affect the state distribution of the electron orbits, thereby adjusting the photocatalytic activity. Simultaneously, the PL spectra of the MoO2/MoO3 nanocomposites were more or less different from that of the pure MoO3 (Fig. 4c). For the MoO3, the emission peaks at 417 nm and 492 nm were ascribed to the

contrast with the spot pattern (the inset of Fig. 3b) for pure MoO3. The formation of the MoO2/MoO3 composites was attributed to the directional movement of the SCN− anion to the MoO3 surface and the subsequent in-situ reaction (Fig. 3e). After the SCN− ions were adsorbed on the surface of the MoO3 nanobelts with positive charges, the partial MoO3 were reduced to the MoO2 during hydrothermal reaction, thereby spontaneously attaching to the original MoO3 nanobelts. The MoO2/MoO3 nanocomposites displayed different optical properties from pure MoO3 and MoO2, as shown in UV–Vis diffuse reflectance spectra of Fig. 4a. Pure MoO3 exhibited the intrinsic absorption band in the UV region and the weak absorption in visible wavelengths. After in-situ formation of MoO2 on MoO3 surface, and the visible light response of the MoO2/MoO3 nanocomposites were improved to a certain extent. The MoO2/MoO3 composite showed the

Fig. 3. (a) Low TEM images and (b) HRTEM images of MoO3 nanobelts; (c) Low TEM images and (d) HRTEM images of 10:1 MoO2/MoO3 nanocomposite; (e) Schematic illustration of preparation process of the MoO2/MoO3 nanocomposites. 430

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Fig. 4. (a) UV–vis diffuse reflectance spectra of pure MoO3 and MoO2/MoO3 nanocomposites; (b) Plots of (αhv)2 vs (hv) for estimating optical band gaps of pure MoO3 and MoO2/MoO3 nanocomposites; (c) PL spectra of pure MoO3, 10:1 and 6:1 MoO2/MoO3 nanocomposite; (d) Electrochemical impedance spectroscopy Nyquist plots of MoO3, MoO2 and 10:1 MoO2/MoO3 nanocomposite.

light irradiation for 60 min after adsorption equilibrium (Fig. 5a). Apparently, all the MoO2/MoO3 nanocomposites exhibited markedly improved photocatalytic activity in comparison with the pure MoO3 with the assistance of H2O2. The MoO3/KSCN ratio certainly has a distinct effect on the degradation rate. The MoO2/MoO3 nanocomposite with 10:1 MoO3/KSCN ratio showed the most superior photocatalytic activity among all the nanocomposites (Fig. 5c), and its degradation efficiency reached 90.2%, attributing to the highest intuitionistic kapp value of 0.072 min−1, which is more than triple than that of the pure MoO3 (0.019 min−1) (Fig. 5b). The roles of H2O2 and light irradiation could also be confirmed by further designed experiments. Fig. 5d showed that no degradation effect could be observed regardless of the absence of H2O2 or light, implying the synergistic reaction of catalysts and H2O2 under visible radiation. As an excellent electron acceptor, H2O2 could easily react with electrons at low energy level, and stronger mobility of the photogenerated electrons on the surface of MoO2 than MoO3 suggested easier interaction between the electrons and the H2O2 in the MoO2/MoO3 composite [47]. Degradation curves of the 10:1 MoO2/MoO3 nanocomposite were obtained under different H2O2 volumes (Fig. 6a). The degradation efficiency within 30 min was almost the same and does not seem affected by the H2O2 volume, although higher volume accelerated the starting rate. This finding could be attributed to the reduced capture time of the transferred electrons by sufficient H2O2 molecules, further resulting in the rapid participation of the produced radicals into the degradation of RhB (reaction formula: H2O2 + e− → %OH + OH−) [48,49]. Excessive H2O2 was not conducive to the acceleration of the degradation attributed to the formation of weak oxidant HO2% obtained by consuming % OH (H2O2 + %OH → HO + HO2%) [50]. In a word, a small amount of

rapid recombination of the photoinduced electron-hole pairs and low valence Mo5+ ion associated with an oxygen vacancy as the neighbor [42], and other peaks in the range of 450–482 nm might be attributed to the radiative decay of self-trapped excitons or linked to lattice imperfections [43]. Apparently, the PL intensity of the MoO2/MoO3 nanocomposites was lower than that of pure MoO3, indicating the suppressed recombination of photo-induced carriers by the MoO2 nanoparticles [44]. Furthermore, the higher PL intensity of the MoO2/ MoO3 (mol(Mo/S) = 6:1) than the MoO2/MoO3 (mol(Mo/S) = 10:1) revealed that excess MoO2 may act as new recombination centers for consuming photo-induced carriers. Thus, an appropriate amount of MoO2 was vital to accelerate the separation of electrons and holes. EIS measurement was employed to further detect the kinetics of charge transfer of the obtained photocatalysts (Fig. 4d). The radius of the arc on the Nyquist plot implies that the interface layer resistance happened on the surface of the electrode [45,46]. Small arc radius of the pure MoO2 was attributed to its high conductivity, and the very large arc radius of the pure MoO3 indicated the low efficiency of charge transfer. Accordingly, the arc radius of the MoO2/MoO3 (mol(Mo/S) = 10:1) nanocomposite was a bit larger than that of pure MoO2 and much less than that of pure MoO3, suggesting the improved efficiency of the electron transfer by the MoO2 nanoparticles. Such composite will certainly exhibit a preferable possibility in the field of pollutant degradation. 3.2. Photocatalytic performance of the MoO2/MoO3 nanocomposites The photocatalytic performance of the products was investigated by monitoring the photodegradation of RhB (10 mg·L−1) under visible431

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Fig. 5. (a) Photocatalytic activities in RhB degradation for pure MoO3 and MoO2/MoO3 nanocomposites with different contents of MoO2 under visible-light irradiation; (b) The kinetic plots for photocatalytic degradation of different samples; (c) Degradation efficiency of different samples; (d) Comparison of the photocatalytic activities of 10:1 MoO2/MoO3 nanocomposite under different condition.

repulsion which diminished the adsorption and photocatalytic rate. Hence, the photocatalyst in this study showed good stability and high activity under acidic conditions.

H2O2 can achieve good degradation effect of RhB. In this case, photostability and recyclability of the MoO2/MoO3 nanocomposite photocatalyst were also very excellent (Fig. 6b). The degradation efficiency for the MoO2/MoO3 nanocomposite (mol(Mo/S) = 10:1) was still above 80% after four cycling runs despite a slight reduction caused by the diminished absorptivity of photocatalyst during the consecutive recycling assessments, showing the potential in practical applications. Influences of pH values on the photocatalytic degradation rates were investigated. As shown in Fig. 6c, the natural pH of original suspension containing RhB solution (10 mg·L−1) and the MoO2/MoO3 nanocomposite (mol(Mo/S) = 10:1) was measured to be about 6.3. The pH of the solution slightly decreased as increasing irradiation time during the photocatalytic reaction, and finally remained stable at 5.1. Meanwhile, the pH value of the suspension was adjusted to different values (2.0, 4.0, 8.0 and 10.0) by slowly adding a few drops of HCl or NaOH at the beginning of the runs. The results in Fig. 6d showed that both adsorption capacity and photocatalytic activity of the MoO2/MoO3 nanocomposite depended on the pH values. At a low pH, both adsorption efficiency and photocatalytic degradation rate of the catalyst were slightly increased. However, in alkaline suspension, weak adsorption of RhB on the surface of the nanocomposite was observed, and the photocatalytic degradation for the RhB was also deterred remarkably. Especially, at pH = 10, the MoO2/MoO3 nanocomposite (mol(Mo/S) = 10:1) showed no photocatalytic activity. The difference could be ascribed to charge distribution, functional groups of RhB molecules and the surface of the catalyst, resulting in an optimal range of pH value for specific photocatalytic systems [51,52]. In this work, it is presumed that RhB molecules and the catalyst surface were charged by like-charges under alkaline conditions, introducing the coulombic

3.3. Photocatalytic mechanism discussion The detailed process of the photocatalytic degradation of RhB was examined by UV–vis spectra ranging from 200 to 600 nm. During the photocatalytic degradation process, intensities of the main absorbance peak from UV–vis spectra gradually reduced as the increasing degradation time, and the marked increment caused by the intermediate product was observed in UV-region (Fig. 7a). Meanwhile, the maximum absorption peak showed a significant blue-shift from 553 nm to 522 nm coupled with the gradual decolorization of RhB (the inset of Fig. 7a), suggesting that the RhB was degraded into N, N′-Diethyl-Rhodamine in the De-ethylation reaction, and cochromatography on the liquid chromatograph also supported the coincidence of the product in the reported literature [53]. No further change from the adsorption spectra was observed after 30 min of irradiation under visible light, indicating that the photocatalytic reaction finished once the structure of RhB molecular was destroyed. Chemical oxygen demand analysis for product from photodegradation of RhB (ranging from 101 to 97) also verified the results. However, when the visible light was replaced by the UV light, the adsorption intensity in UV-region decreased gradually without any new characteristic peak appeared (Fig. 7b), suggesting that the intermediate product could be further degraded under UV-light irradiation, and it was because that the %OH was generated and completely degraded RhB into CO2 after the photo-induced electrons were excited from the valence band to the conduction band of MoO3. 432

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Fig. 6. (a) Photocatalytic degradation of RhB by using the 10:1 MoO2/MoO3 with various volume of H2O2; (b) Cyclic photodegradation of RhB by 10:1 MoO2/MoO3 photocatalyst. (c) Change of pH of the original suspension; (d) Effect of pH on photocatalytic degradation.

(Fig. 8b), the slight decrease of absorption intensity indicated the negligible photocatalytic degradation of RhB. (3) When AgNO3 was used as a competitor of H2O2 to trap the generated electrons (e−), the absorption intensity weakened in the same degradation time but lesser than that without any scavenger, and the blue shift became unobvious (Fig. 8c). This phenomenon may be because the small number of electrons, which had not been timely reduced by Ag+ ions, still reacted

Specific dominant active species that respond to the degradation of organic dyes could be ensured by trapping experiments (Fig. 8) [54]. (1) The absorption intensity decreased dramatically accompanied with the decolorization of RhB without any scavenger in Fig. 8a, and significant blue shift of the maximum peak at 553 nm was primarily attained from N-deethylation during RhB degradation [55,56]. (2) When adding DMSO to eliminate the hydroxylradical (%OH) in solution

Fig. 7. UV–vis spectra of RhB during the photodegradation under (a) Visible light; (b) UV-light. 433

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Fig. 8. Temporal absorption spectral comparison of RhB during the trapping experiments by adding (a) no scavenger, (b) DMSO, (c) AgNO3 and (d) EDTA-2Na.

with H2O2 to form %OH, thereby leading to the weak degradation of RhB and the restrained blue shift. (4) The addition of hole (h+) scavenger EDTA-2Na almost had no effect on the degradation process (Fig. 8d), suggesting that the %OH cannot be formed by the reaction: H2O + h+ → H+ + %OH. In addition, when the Ar gas was bubbled into the suspension, the degradation rate of RhB showed no obvious change compared with that in the absence of the Ar bubbled, which suggested that neither dissolved oxygen nor superoxide radical (O2−%) was involved into the photocatalytic reaction. Therefore, the hydroxylradical (%OH) should be the main active radical responsible for the deethylated degradation of RhB, as noted in a series of reports on the photocatalytic studies with the help of H2O2 [57–59]. To further detect the photocatalytic degradation mechanism of the MoO2/MoO3 nanocomposite, different types of organic dyes and colorless toxic pollutant were used as simulated pollutants for evaluating photocatalytic effects. EY and AO7 with high lowest unoccupied molecular orbital (LUMO) level were usually used in dye-sensitized photocatalytic reaction [60,61], whereas MB and MO hardly showed sensitization effect due to their low LUMO levels [62]. In Fig. 9, the MoO2/ MoO3 sample showed obvious degradation effects on AO7 and EY under visible light irradiation in spite of long degradation time for AO7 due to its stable molecular structure as an azo dye. This phenomenon is similar to that on RhB, but did not appear for MB and MO contaminants, thereby demonstrating the sensitization effect in the degradation process of RhB. Under visible light irradiation, photo-induced electrons were excited from the highest occupied molecular orbital (HOMO) level to the LUMO level of dye sensitizers, followed by injection into the conduction band of MoO3. This finding was consistent with the weak

Fig. 9. Photocatalytic activities of 10:1 MoO2/MoO3 for different organic dyes under visible light irradiation.

visible light response of the MoO2/MoO3 nanocomposite from UV–Vis diffuse reflectance spectra. Despite all these finding, we observed good degradation on MB and MO by the MoO2/MoO3 nanocomposite under UV light irradiation, which was caused by the excited electronics from the valence band to the conduction band of MoO3. Salicylic acid solution (10 mg·L−1) and mixed solution of salicylic acid (10 mg·L−1) and RhB (10 mg·L−1) were both used as model 434

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Fig. 10. Photocatalytic degradation of pure salicylic acid solution and mixed solution of salicylic acid and RhB: (a) Degradation efficiency; (b) UV–vis absorption spectra.

nanoparticles could easily capture the electrons to form the %OH radicals by the promoting reaction: H2O2 + e− → OH− + %OH. These %OH radicals retained strong oxidability, which is responsible for the outstanding degradation of RhB. Differently, only UV-light illumination could provide enough energy to the electrons to migrate from valence band to the conduction band of the MoO3 with the wide bandgap, and the obtained %OH radicals through the same transferring and reacting processes mentioned above further oxidized the MO and MB without dye-sensitization effects. 4. Conclusions In summary, a dye-sensitized photocatalytic system of MoO2 nanoparticles interspersed MoO3 nanobelts has been successfully constructed through a simple in-situ hydrothermal method. The obtained MoO2/MoO3 composites displayed high degradation efficiency for the RhB dye under visible-light irradiation. Mechanism analysis showed that a well-matched LUMO level of the sensitized-dye insured the excited electrons injected into the conduction band of MoO3. Further experiments suggested that the %OH was generated by the reaction between the H2O2 and the e− and played a dominant role in the photocatalytic process. Our study comprehensively utilized the remarkable adsorption capacity of MoO3 and high conductivity of MoO2, thereby providing a new idea for designing novel photocatalytic system with high efficiency and low cost. We believe that this work has broad prospects in practical application of environmental improvement or energy transformation.

Fig. 11. Schematic drawing of charge-transfer mechanisms for dye-sensitized MoO2/MoO3 photocatalytic system.

pollutants. The MoO2/MoO3 nanocomposite (mol(Mo/S) = 10:1) showed no photocatalytic degradation effect on the pure salicylic acid under visible-light irradiation, while 23.7% of the salicylic acid in mixed solution was degraded within 60 min, as shown in Fig. 10a. Reduction of the main absorbance peak for salicylic acid at 205 nm, 235 nm and 296 nm were observed with the increment around 260 nm caused by by-products during the degradation [63], as shown in Fig. 10b. Furthermore, the photocatalytic degradation of salicylic acid mainly happened in the first 30 min, and then the degradation rates declined dramatically due to small amount of the remained RhB. The result also proved that photo-induced electrons were excited from RhB molecules rather than the nanocomposite itself under visible light irradiation. A possible transfer process of dye-sensitized electronics of the MoO2/MoO3 nanocomposite for enhancing photocatalytic activity under visible light irradiation was proposed and described in Fig. 11 based on the above analysis. Under the illumination of visible light, the electrons from the HOMO levels of the dye-sensitizers were quickly excited to the LUMO levels. The electrons were subsequently injected into the conduction band of MoO3 via the self-sensitization process of the dye due to higher positive calculated conduction band of MoO3 (+0.334 eV) [64,65] than LUMO levels of the dye-sensitizers (RhB: −1.42 eV [20,66], EY: −0.78 eV [60], AO7: −1.32 eV [61]). Then, these photo-induced electrons were immediately transferred to the MoO2 nanoparticles dispersed on the MoO3 nanobelts due to the super conductivity of MoO2. At the same time, H2O2 molecules around MoO2

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