Microspheres Nanohybrid with SPR-Promoted Visible-Light Phot

Article pubs.acs.org/JPCC

Bi Cocatalyst/Bi2MoO6 Microspheres Nanohybrid with SPR-Promoted Visible-Light Photocatalysis Zaiwang Zhao,† Wendong Zhang,‡ Yanjuan Sun,† Jiayan Yu,∥ Yuxin Zhang,⊥ Hong Wang,† Fan Dong,*,† and Zhongbiao Wu*,§ †

Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environment and Resources, Chongqing Technology and Business University, 400067 Chongqing, China ‡ Department of Scientific Research Management, Chongqing Normal University, Chongqing 401331, China § Department of Environmental Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China ∥ Chongqing Key Laboratory for Urban Atmospheric Environment Integrated Observation & Pollution Prevention and Control, Environmental Monitoring Center of Chongqing, Chongqing 401147, China ⊥ College of Materials Science and Engineering, National Key Laboratory of Fundamental Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing 400044, China ABSTRACT: To develop efficient visible light driven photocatalysts for air purification, we constructed a novel semimetal−semiconductor Bi−Bi2MoO6 (Bi−Mo) nanohybrid via the in situ deposition of Bi nanoparticles on the surface of Bi2MoO6 microspheres. In this strategy, the Bi3+ ions were in situ reduced to metallic Bi particles by glucose during in hydrothermal process. The XRD, XPS, SEM, TEM, UV−vis, DRS, PL spectra, and surface photovoltage were employed to explore the structural and optical properties. The assynthesized Bi−Bi2MoO6 nanohybrid was applied in photocatalytic removal of NO in air. The results indicated that the amount of reductive glucose not only exerted a pivotal effect on the morphological structure but also affected the photocatalytic capability of the Bi−Bi2MoO6 nanohybrid. The optimized Bi−Mo-50 hybrids exhibited exceptionally high visible-light photocatalytic performance with a NO removal ratio up to 68.1%, far outperforming other decent photocatalysts, like BiOBr (21.3%), C-doped TiO2 (21.8%), N-doped TiO2 (36.5%), N-doped (BiO)2CO3 (43.5%), and g-C3N4 (32.7%). This drastically enhanced photocatalytic capability was ascribed to the cocontributions of the enhanced light absorption and the improved separation efficiency of the charge carriers owing to the surface plasmon resonance (SPR) induced by Bi metal. The Bi metal performs as noble metal-like cocatalyst for promoting the photocatalysis efficiency. Based on the DMPO-ESR spin trapping, the active species generated from Bi/Bi2MoO6 under visible light were •OH radicals. The Bi/Bi2MoO6 produced more •OH radicals contributing to strengthen oxidation ability in comparison with that of the pristine Bi2MoO6. In addition, this advanced Bi/Bi2MoO6 nanohybrid also exhibited high photochemical stability under repeated irradiation. This work demonstrated the feasibility of utilizing economical Bi element as a cocatalyst to substitute the precious noble metals to advance the photocatalysis efficiency.

1. INTRODUCTION

photocatalytic reactions. These results indicated that the photocatalytic capability was strongly associated with the morphology and charge separation efficiency. However, the photocatalytic capability of Bi2MoO6 is not efficient enough to meet the application requirement due to the low separation efficiency of the photoexcited electron−hole pairs. Thus, it is highly desirable to develop effective methods to enhance photocatalysis efficiency of Bi2MoO6.

The increasing environmental pollution and energy shortage have stimulated extensive efforts to develop more effective visible-light-driven photocatalysts.1−4 Various metal oxide semiconductors such as Bi2MoO6,5,6 BiVO4,7−9 (BiO)2CO3,10 Bi2O3,11 Fe2O3,12 and WO313 have been investigated as visiblelight driven photocatalysts. Among them, Bi2MoO6 with a band gap of 2.70 eV has been regarded as one of the most promising photocatalysts due to the decent intrinsic merits, such as suitable band gap for visible light excitation and tunable morphology. Therefore, Bi2MoO6 with different morphologies, such as cage-like,14 microtube-like,15 fiber-like,16 and flowerlike,17 have been fabricated and explored for different © XXXX American Chemical Society

Received: February 3, 2016 Revised: May 19, 2016

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DOI: 10.1021/acs.jpcc.6b01188 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

ethylene glycol was further added to the above mixture and stirred for 10 min. Afterward, different masses (0.123, 0.308, 0.616, and 1.232 g) of glucose (Sigma, analytical grade) was added into the above solution with vigorous stirring. After all solids were dissolved absolutely, the aqueous suspension was hydrothermally treated under 160 °C for 20 h. After the hydrothermal reaction, the solid product was collected by filtration, washed with distilled water for two times and ethanol for two times, and dried at 60 °C for 12 h to obtain the final Bi/ Bi2MoO6 products. This different mass ratio of glucose to Bi2MoO6 was 10, 25, 50, and 100%, and the corresponding obtained samples were labeled as Bi−Mo-10, Bi−Mo-25, Bi− Mo-50, and Bi−Mo-100, respectively. For comparison, pure Bi2MoO6 microspheres and Bi nanoparticles were also synthesized as references and labeled as Bi2MoO6 and Bi, respectively.6,27 2.2. Characterization. The X-ray diffraction (XRD) patterns of the samples were obtained using an X-ray diffractometer equipped with intense Cu Kα radiation (Model D/max RA, Rigaku Co., Japan). The morphology, architecture, and chemical composition of the as-prepared products were analyzed using scanning electron microscope (SEM, JEOL model JSM-6490, Japan), transmission electron microscope (TEM, JEM-2010, Japan), and high-resolution transmission electron microscope (HRTEM). The Brunauer− Emmett−Teller (BET) specific surface area (SBET) of the photocatalysts was determined using a nitrogen adsorption apparatus (ASAP 2020, USA) with all samples degassed at 100 °C for 12 h prior to measurements. X-ray photoelectron spectroscopy (XPS) measurement was utilized to explore the surface chemical compositions and states with Al Kα X-ray (hν = 1486.6 eV) radiation source operated at 150 W (Thermo ESCALAB 250, USA). The UV−vis diffuse reflection spectra (UV−vis DRS) were obtained for the dry-pressed disk samples by using a Scan UV−vis spectrophotometer (UV-2450, Shimadzu, Japan) with 100% BaSO4 as the standard sample. Photoluminescence (PL, F-7000, HITACHI, Japan) was utilized to investigate the optical properties of the obtained samples. Electron spin resonance (ESR) signals of radicals spintrapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were recorded on a JES FA200 spectrometer. Samples for ESR measurement were conducted by mixing the samples in a 40 mM DMPO solution tank (aqueous dispersion for DMPO−•OH and methanol dispersion for DMPO−•O2−) and irradiated with visible light. The surface photovoltage (SPV) measurements were taken with a home-built apparatus equipped with a lock-in amplifier (SR830) synchronized with a light chopper (SR540). 2.3. Evaluation of Visible Light Photocatalytic Activity. The photocatalytic activity of the as-synthesized samples was evaluated by removing NO at ppb level in a continuous flow reactor. The reactor was 4.5 L (30 cm × 15 cm × 10 cm), made of polymeric glass, and covered with SaintGlass. A commercial tungsten halogen lamp (150 W) was vertically placed 20 cm above the reactor. A UV cutoff filter (420 nm) was applied to remove UV light for the test of visible light photocatalytic activity. The as-prepared sample (0.20 g) was dispersed in distilled water (60 mL) in a beaker via ultrasonic treatment for 10 min and then coated onto two glass dishes (12.0 cm in diameter). The coated dishes were pretreated at 60 °C to remove water in the suspension and were placed at the center of the reactor after cooling down to room temperature. The NO gas was acquired from a

Construction of noble metal−semiconductor hybrid has been regarded as an effective strategy to improve the photocatalytic performance. The noble metal nanoparticles coupled with the semiconductor photocatalysts could behave as an electron sink and hence accelerate the interfacial charge transfer via the Schottky barriers at the interface of the metal−semiconductor nanocomposites.18,19 To date, noble metal such as Au, Ag, or Pt were deposited on TiO2, ZnO, and (BiO)2CO3 semiconductors in order to strengthen the photocatalytic activity.20−23 Following these works, Wang et al. deposited the ultrafine Ag nanoparticles onto the Bi2MoO6 microspheres and successfully built the Ag/Bi2MoO6 metal−semiconductor heterojunctions. This Ag/Bi2MoO6 hybrid showed enhanced visible light photocatalytic capability in photodegradation of dyes. However, the great demands as well as overexpenditure of these precious metals sharply limit their large-scale applications. Very recently, low-cost base-metal bismuth, has been discovered to exhibit a direct plasmonic photocatalytic ability,24 which may perform as a promising candidate to replace noble metals. Bi metal has been coupled with various photocatalysts, such as BiOCl,25 (BiO)2CO3,26 and g-C3N4.27 Encouragingly, all these Bi coupled nanocomposites demonstrated conspicuously improved photocatalytic performance when compared with that of their individual components alone. Considering the noble-metal-like behavior of Bi metal and the matched band structure of semiconductor Bi2MoO6 photocatalyst, design and synthesis of Bi/Bi2MoO6 nanohybrid to improve photocatalysis efficiency is of significance.7 To the best of our knowledge, the utilization of the SPR effects of Bi metal to advance the photocatalytic capability of Bi2MoO6 has seldom been reported.7 In this work, we developed a one-pot hydrothermal reduction method to in situ deposit Bi metal cocatalyst on Bi2MoO6 microspheres. During the solvent-controlled reaction process, glucose performed as the reductant to in situ reduce Bi3+ to metallic Bi. The removal of low concentration NO (600 ppb) in air were employed to evaluate the photocatalytic capability of Bi/Bi2MoO6 nanohybrid under visible-light illumination. The as-synthesized Bi/Bi2MoO6 nanohybrid showed dramatically enhanced visible-light photocatalytic activity because of the SPR effect, enhanced visible light absorption, and efficiently accelerated electron−hole pair separation induced by the Bi nanoparticles. In addition, the relationship between photocatalytic performance and amount of coupled Bi metal was also revealed. The catalytic behavior of Bi nanoparticles performed as a cocatalyst is similar to that of noble metals (Au, Ag, etc.). Significantly, this Bi/Bi2MoO6 photocatalysts also exhibited excellent photochemical stability, which will pave a way for utilizing the inexpensive base metal to substitute the precious noble metals to enhance the semiconductor photocatalyst for practical applications.

2. EXPERIMENTAL SECTION 2.1. Preparation of Bi/Bi2MoO6 Nanohybrid. All the reagents used were purchased from Sigma-Aldrich. In a typical synthesis, 1.6866 g of bismuth nitrate pentahydrate (Sigma, analytical grade) was first dissolved in 5 mL of ethylene glycol (Sigma, analytical grade) in an autoclave Teflon vessel of 50 mL with vigorous stirring, which is marked as solution A. Then, 0.4210 g of sodium molybdate dihydrate (Sigma, analytical grade) was dissolved in 5 mL of ethylene glycol in a breaker and marked as solution B. After dissolution, the solution B was dropwise added to solution A with stirring. Then 20 mL of B


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Figure 1. Photocatalytic activities of the samples for NO degradation in air under visible light illumination (NO concentration: 600 ppb) (a), and the corresponding reaction kinetics (b).

Figure 2. XRD (a) and the enlarged view (b) of XRD for the pure Bi2MoO6 and the as-prepared Bi/Bi2MoO6 nanocomposites.

irradiation. Figure 1a showed that the photocatalytic activity of the individual Bi2MoO6 is limited with a NO removal ratio of 18.4% after reaching equilibrium in 30 min. After the introduction of Bi nanoparticles on Bi2MoO6, the photocatalytic activity is conspicuously enhanced. The photocatalytic NO removal ratio of Bi−Mo-10, Bi−Mo-25, Bi−Mo-50, and Bi−Mo-100 is about 32.3, 59.6, 68.1, and 11.8%, respectively. Notably, the extremely high performance of Bi−Mo-50 (68.1%) even outperforms that of the decent BiOBr (21.3%), 28 C-doped TiO 2 (21.8%), 29 N-doped TiO 2 (36.5%), 29 (BiO) 2 CO 3 (43.5%), 30 and porous g-C 3 N 4 (32.7%)31 under the same test conditions. Nevertheless, when the Bi content is further increased, the photocatalytic activity is sharply hampered. This phenomenon can be understandable because the excessive Bi particles on the Bi/ Bi2MoO6 nanohybrid can cover the active site of Bi2MoO6 for further charges generation. This phenomenon has also been observed in other metal−semiconductor nanohybrid system.27,32,33 To better understand the reaction kinetics (kapp) of the NO removal catalyzed by Bi/Bi2MoO6 nanocomposites photocatalysts, the photocatalytic activity data were fitted by a firstorder model. Figure 1b gives the values of the rate constants kapp of all the samples, which are 0.371, 0.867, 1.078, 1.204, and 0.174 min−1, respectively. Figure 1 demonstrated that Bi metal is an excellent cocatalyst for promoting the visible light photocatalytic activity of Bi2MoO6.

compressed gas cylinder at a concentration of 100 ppm of NO (N2 balance). The initial concentration of NO was diluted to about 600 ppb via air streaming. The flow rates of the air stream and NO were controlled at 2.4 L min−1 and 15 mL min−1, respectively. The two gas streams were then premixed in a three-way valve. The relative humidity is controlled at 50% in the air stream. When the adsorption−desorption equilibrium was achieved, the lamp was turned on. The concentration of NO was measured every 1 min by using a NOx analyzer (Thermo Scientific, 42i-TL), which also monitored the concentration of NO2 and NOx (NOx represents NO + NO2). The removal ratio (η) of NO was calculated using η (%) = (1 − C/C0) × 100%, where C is the outlet concentration of NO after reaction for time t and C0 represents the inlet concentration after achieving adsorption−desorption equilibrium. The kinetics of photocatalytic NO removal reaction is a pseudo-first-order reaction at low NO concentration as ln(C0/ C) = kappt, where kapp is the apparent rate constant.

3. RESULTS AND DISCUSSION 3.1. Visible Light Photocatalytic Air Purification. To evaluate the photocatalytic capability, the as-synthesized pristine Bi2MoO6 and Bi/Bi2MoO6 samples were applied to the photocatalytic removal of gas phase NO (ppb level) in a continuous reactor under visible-light illumination (Figure 1a). NO is usually stable at room temperature and cannot be removed under visible light illumination without photocatalytic materials or in the presence of photocatalysts without light C


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The Journal of Physical Chemistry C 3.2. Phase Structure. Figure 2a reflects the XRD patterns of the as-synthesized samples. The diffraction peaks of pure Bi2MoO6 are perfectly indexed as the orthorhombic Bi2MoO6 (JCPDS Card No. 21-0102, a = 5.506 Å, b = 16.226 Å, and c = 5.487 Å). After the addition of glucose to the precursors, the peak intensity Bi2MoO6 for all Bi/Bi2MoO6 is altered. As the amount of the glucose increased, the peak intensity of the Bi2MoO6 gradually decreased, which demonstrates that the addiction of glucose results in the decreased crystallinity of the samples. However, the typical (012) characteristic peaks of the rhombohedral phase Bi (JCPDS card no. 44-1246) centered at 27.1 cannot be clearly observed in the XRD patterns, which can be ascribed to the overlapping the dominant diffraction peak of Bi2MoO6 and Bi metal. As can be seen from the enlarged view in Figure 2b, the main diffraction of Bi2MoO6 shifts to lower angles with the increased Bi amounts, which can be ascribed to the overlapping of the dominant diffraction peak of these two components. When further increasing the content of the glucose, the sample is transformed into dominant phase of Bi metal, and the characteristic peaks indexed to elemental Bi can be distinctively observed. This result further demonstrates that elemental Bi metal is generated on the surface of Bi2MoO6. The production of the Bi nanoparticles can be assigned to the in situ reduction of Bi3+ because of the reduction effects of glucose. No peaks of other impurities could be detected, which demonstrated that Bi-deposited Bi2MoO6 composites have been successfully fabricated without other impurities. 3.3. Morphological Structure. The morphology of the assynthesized Bi2MoO6 and Bi/Bi2MoO6 photocatalysts were characterized by SEM. Figure 3a,b reflects that the morphology of the pure Bi2MoO6 are microspheres assembled with lots of capsule-like nanorods. The as-prepared microspheres have an average diameter of about 1.0 μm (Figure 3b) with various porous architectures. The morphology is apparently altered when glucose is introduced into the reaction system. A host of hierarchical nanosheets are generated on the surface of Bi−Mo10 samples (Figure 3c,d). When further increasing the amount of the glucose, some nanoparticles are produced and located at interspace of the capsule-like nanorods (Figure 3e,f). For Bi− Mo-50, the capsule-like nanorod architectures disappeared, and the whole microspheres are mainly assembled with smaller nanoparticles (Figure 3g,h). When more glucose is added to the reaction system, the morphology of the obtained Bi−Mo-100 was transferred to solid microspheres with the diameter of 700−800 nm, and the structures became more dense with few pores on the smooth surface (Figure 3i,j). Obviously, the microspheres are self-assembled with even smaller particles. The morphology transformation of the samples can be ascribed to the addition of the reductive glucose, which could inhibit the anisotropic growth of crystal, resulting in microspheres assembled with small nanoparticles. Figure 4 shows the EDX elemental mapping of the Bi−Mo50 photocatalysts. The EDX mapping further demonstrated that the Bi (b), Mo (c), O (d), and C (e) elements coexisted in Bi−Mo-50 samples and that the Bi elements are well dispersed in the Bi−Mo-50 nanocomposites. Figure 5 depicts a typical TEM images of Bi2MoO6 and Bi− Mo-50. Figure 5a,c further shows that the morphology of Bi2MoO6 and Bi−Mo-50 are in the form of microspheres. Figure 5b,d show the HRTEM images of the as-prepared samples, which are taken on the edge of building blocks of Bi2MoO6 and Bi−Mo-50. The lattice spacings of 0.315 nm (Figure 5b,d) correspond to the (131) crystal plane of the

Figure 3. SEM and the enlarged view of Bi2MoO6 (a,b), Bi−Mo-10 (c,d), Bi−Mo-25 (e,f), Bi−Mo-50 (g,h), and Bi−Mo-100 (i,j).

Figure 4. EDX of Bi−Mo-50 (a) and the corresponding elemental mapping of Bi (b), Mo (c), O (d), and C (e).

orthorhombic phase of Bi2MoO6. Besides, the lattice spacing of 0.280 nm ascribing to the (012) lattice plane of the Bi particles D


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Table 1. SBET, Pore Volume, Peak Diameter, and NO Removal Ratio of Bi2MoO6 and Bi/Bi2MoO6 Samples sample name

SBET (m2/g)

pore volume (cm3/g)

peak diameter (nm)

NO η (%)

Bi2MoO6 Bi−Mo-10 Bi−Mo-25 Bi−Mo-50 Bi−Mo-100

23 52 60 84 50

0.17 0.30 0.12 0.11 0.17

25.1 19.3 3.8/5.9 3.7 3.8

18.4 32.3 59.6 68.1 11.8

were produced (Figure 3i). Because of the high surface energy of the ultrafine particles, these particles tend to assemble together to rebuild more compact architecture with less pores on the surface, which reversely reduces the surface areas. The pore volume of Bi−Mo-50 (0.11 cm3/g) is almost the same with that of Bi2MoO6 (0.17 cm3/g), Bi−Mo-25 (0.12 cm3/g), and Bi−Mo-100 (0.17 cm3/g), and even much smaller than that of Bi−Mo-10 (0.30 cm3/g), which demonstrated that the pore volume is not the main factor for improving the photocatalytic activity. Combining with the photocatalytic experiments results (Figure 1), we can deduce that the high BET surface areas of the Bi−Mo-50 exert significant roles in enhancing the photocatalytic performance. 3.4. Chemical Composition. The surface chemical composition of the Bi2MoO6 and Bi−Mo-50 samples was investigated by XPS spectra (Figure 7). Figure 7a depicts the survey of the samples, which indicates that all the samples consist of Bi, O, Mo, and C elements. The C 1s peak in the spectrum of Bi2MoO6 and Bi−Mo-50 comes from adventitious carbon.34 The high resolution XPS spectra of Bi 4f is displayed in Figure 7b. Two strong peaks located at 158.8 and 164.1 eV for both samples are attributed to Bi 4f7/2 and Bi 4f5/2 of Bi3+.34,35 For Bi−Mo-50 samples, other two peaks centered at 156.7 and 162.1 eV can be observed, which was attributed to the metallic Bi, consistent with EDS (Figure 4b) and HRTEM (Figure 5d). The concentration of Bi on the surface of Bi−Mo50 is determined to be 1.23%. Figure 7c reveals that the binding energies for Mo 3d5/2 and Mo 3d3/2 of Mo6+ in Bi2MoO6 are around 232.4 and 235.5 eV, respectively. The binding energies for Bi−Mo-50 center at 232.1 and 235.2 eV, respectively.36 The O 1s peak (Figure 7d) located at 529.9 and 529.8 eV can be corresponding to the Bi−O bonds in Bi2MoO6 and Bi−Mo-50, respectively.37 The shift of the binding energy for Mo 3d and O 1s reflects the interaction of Bi metal with Bi2MoO6 in the nanocomposites. The molar ratio of Bi deposited on Bi2MOO6

Figure 5. TEM and the enlarged view of Bi2MoO6 (a,b) and Bi−Mo50 (c,d).

could also be observed in Bi−Mo-50 (Figure 5d), which is in accordance with the XRD result. The generation of Bi nanoparticles can be assigned to the reduction of Bi3+ ions via the reductive carboxy group of glucose under high hydrothermal conditions. The nitrogen adsorption−desorption isotherms and corresponding pore size distribution curves of Bi2MoO6 and the Bi/ Bi2MoO6 nanocomposites are displayed in Figure 6. The BET surface areas of the as-prepared Bi2MoO6, Bi−Mo-10, Bi−Mo25, Bi−Mo-50, and Bi−Mo-100 are 23, 52, 65, 87, and 50 m2/g (Table 1), respectively. Notably, the BET surface areas of Bi− Mo-50 (87 m2/g) reaches maximum, about four times higher than that of the pristine Bi2MoO6 (23 m2/g). The enhanced BET areas of Bi−Mo-10 can be ascribed to the hierarchical nanosheets generation. As more glucose was added to the reaction system, the building blocks of Bi−Mo-25 and Bi−Mo50 turned to much smaller particles, which could result in increased BET areas. Nevertheless, when excessive glucose was added, an array of the solid microspheres with smooth surface

Figure 6. N2 adsorption−desorption isotherms (a) and the corresponding pore-size distribution curves (b) of pure Bi2MoO6 and the as-prepared Bi/ Bi2MoO6 nanocomposites. E


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Figure 7. XPS spectra of the Bi2MoO6 and Bi−Mo-50 samples: survey (a), Bi 4f (b), Mo 3d (c), and O 1s (d).

surface is about 5.8% for Bi−Mo-50 samples, according to the XPS dates analysis. 3.5. Optical Absorption and Charge Separation. UV− vis DRS was utilized to investigate the optical absorption properties of the as-systhesized photocatalysts in the range of 200−800 nm. As depicted in Figure 8, pristine Bi2MoO6 reflects photoabsorption from the UV to visible light region with an absorption edge at about 480 nm because of the intrinsic band gap transition. It is apparent that a slight red-shift phenomenon as well as enhanced light absorption in UV−vis region can be observed after Bi nanoparticle deposition. Such

absorption is consistent with the color transformation of the photocatalysts from light white to brown induced by the appearance of dark metallic Bi (inset of Figure 8). Remarkably, a SPR absorption centered around 500 nm can be detected for individual Bi metal. The conspicuously enhanced strong light absorption in the range of 400−600 nm can be easily observed because the SPR effects of Bi nanoparticles. Recently, several previous reports have demonstrated that Bi, as a non-noble metal, exhibits distinct metal-like SPR effects in the nearultraviolet and visible light range.38,39 Wang et al. reported that a SPR absorption of Bi centered at 550 nm was found due to the surface plasmon resonance and light scattering.38 Broad visible absorption bands in the range of 450−600 nm for Bibased nanocomposite were also observed by several groups.9,25,39 Generally, the construction of Bi metal modified semiconductor can obviously improve the visible-light absorption because of the SPR effect, which is pivotal for enhancing the photocatalytic performance. The PL emission is an effective measure method to investigate the separation efficiency of photoexcited electron− hole pairs and helpful in understanding the fate of electron− hole pairs in semiconductor photocatalysts.27,40 In general, a weak PL intensity indicates a low recombination rate of the photodriven electron−hole under light illumination.40 Figure 9 reflects the comparison of the PL spectra of as-synthesized pristine Bi2MoO6 and Bi/Bi2MoO6. The PL spectra of individual Bi2MoO6 displays an emission peak located at ca. 475 nm (consistent with the band gap of Bi2MoO6) under excitation at 330 nm, which is mainly attributing to the

Figure 8. UV−vis DRS of pure Bi, Bi2MoO6, and the Bi/Bi2MoO6 nanocomposites. F


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Figure 9. PL spectra of the pristine Bi2MoO6 and the coupled Bi/ Bi2MoO6 nanocomposites.

Figure 10. SPV spectra of individual Bi2MoO6 and the coupled Bi/ Bi2MoO6 nanocomposites.

emission of the band gap transition. Significantly, the intensity of the PL peaks is sharply quenched after Bi coupling. The more Bi content, the lower the intensity of the PL peaks. This can be assigned to the electron enrichment of metal nanoparticles via the formation of Mott−Schottky barrier.27,41 The diversity of the work function allows the electron flow from the conduction band of Bi2MoO6 to the Bi metal side, building a potential barrier. The backward flow of electrons is prohibited once this energy barrier is constructed.41 Hence, the Bi metal can perform as an electron sink for continuously accepting the photodriven electrons generated from the conduction band of the Bi 2 MoO 6 under visible light illumination, resulting in high separation efficiency of photogenerated electron−hole.25,27,41 This electron transfer process is thermodynamically favorable owing to the fact the conduction band position of Bi2MoO6 is much more negative than the Fermi level of Bi metal. The Bi−Mo-100 samples show low PL intensity but demonstrates low photocatalytic performance, which can be ascribed to the fact that large numbers of Bi particles covered on the surface of the nanocomposites could hamper the light absorption of the Bi2MoO6. The surface photovoltage (SPV) measurement was carried out to investigate the charge transfer process at the interface of the Bi/Bi2MoO6 photocatalysts (Figure 10). In generally, SPV could reflect the electron transition-related separation processes. The stronger the peak intensity, the higher the separation efficiency of the charges.42,43 For the individual Bi2MoO6, a SPV peak can be observed (Figure 10), which proved that some electrons and holes are separated after excitation. As the amount of Bi increased from 0, 10, and 25 to 50%, the SPV peaks first increased distinctly, while they decreased when the amount of Bi further increased to 100%. The diversity of the peaks intensity for all Bi/Bi2MoO6 hybrid photocatalysts was well matched with the photocatalytic performance for all nanohybrids. Notably, the strongest peak for Bi−Mo-50 is well corresponding to the highest photocatalytic activity of Bi−Mo-50. The peak intensity of Bi−Mo-50 is apparently enhanced after coupling with the Bi metal. The conspicuously expedited charge separation in the Bi−Mo-50 metal−semiconductor hybrid can be assigned to the internal electric field of Bi and the electron transfer from Bi2MoO6 to

the Bi metal. This is vitally significant to highly improved photocatalytic performance of Bi−Mo-50 nanocomposites. 3.6. Active Species Trapping and Photocatalysis Mechanism. The spin-trapping ESR is an effective technique to investigate the main reactant accounting for the photocatalytic NO removal process. We measured the DMPO spintrapping ESR spectra of Bi−Mo-50 in aqueous dispersion for DMPO−•OH (Figure 11a) and in methanol dispersion for DMPO−•O2− (Figure 11b). As shown in Figure 11, four characteristic peaks of DMPO−•OH adducts with an intensity ratio of 1:2:2:1 were distinctively detected under light illumination, while the peak intensity of DMPO−•O2− adducts was negligible even after 15 min irradiation. These results demonstrated that the •OH radical is the major active species, which is responsible for NO photooxidation. This highly active • OH radicals can effectively oxidize the NO in gas to final products. Based on the above results and analysis, the schematic elaboration of photocatalysis mechanism upon Bi/Bi2MoO6 for NO removal under visible-light illumination was proposed in Figure 12. First, the light-harvesting of Bi2MoO6 is apparently enhanced in the range of UV to near-infrared light region because of the introduction of Bi particles, which is beneficial for generating more active electrons and holes on the Bi2MoO6 surface (eq 1). Second, the separation of the photogenerated electrons and holes is a key factor in enhancing the photocatalysis efficiency.44−47 This process will be facilitated through a built-in electric field induced by the SPR effect of Bi metal (Figures 9 and 10).27 Third, the tightly coupled Bi nanoparticles could act as electron traps to accelerate the separation of photodriven electron−hole pairs (Figure 12).25,39 The Fermi level (vs NHE) of Bi can be estimated to be about −0.17 eV.27 The conduction band (CB) position of Bi2MoO6 (−0.32 eV) is more negative than that of the Fermi level of Bi (Figure 12),48 which is beneficial for promoting the electron transfer from Bi2MoO6 to Bi (eq 2). Evidently, this potential difference thermodynamically favors the electron transfer from the conduction band of Bi2MoO6 to Bi metal. Afterward, the photogenerated h+ will oxidize OH− into •OH radicals (eq 3) since the potential of the h+ on the VB of Bi2MoO6 (+2.34 eV) is more positive than the redox potential of OH−/•OH (1.99 eV), which is in accordance with the ESR G


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Figure 11. (a) DMPO spin-trapping ESR spectra of Bi−Mo-50 in aqueous dispersion for DMPO−•OH. (b) DMPO spin-trapping ESR spectra of Bi−Mo-50 in methanol dispersion for DMPO−•O2−.

investigated by carrying out the photocatalytic reaction for five runs under repeated irradiation. The testing results are shown in Figure 13, which reflects that the photocatalytic

Figure 12. Visible-light-induced charge separation and proposed photocatalysis mechanism of Bi/Bi2MoO6 for NOx purification.

results (Figure 11a).48 As •OH is a very reactive oxidation species, it will completely oxidize NO to the final products (eqs 4 and 5). The generated electrons cannot further reduce the O2 to •O2− (eq 6) because the redox potential of O2/•O2− (−0.33 eV) is more negative than that of the conduction band of Bi2MoO6 (−0.32 eV) and Fermi level of Bi (−0.17 eV).48 This explanation is in well agreement with ESR result that the DMPO−•O2− adducts were not detected even after 15 min of light illumination (Figure 11b). In addition, the active h+ on the valence band (VB) of Bi2MoO6 can also directly oxidize the NO to the final products because of its strong oxidation ability shown in eq 7.27,29 Bi 2MoO6 + hv → e−(Bi 2MoO6 ) + h+(Bi 2MoO6 ) −

−

Figure 13. Repeated photocatalytic activity of Bi−Mo-50 under visible light irradiation for the removal of NO in air.

performance is slightly decreased after several photocatalytic reactions, demonstrating that this kind of Bi/Bi2 MoO 6 nanocomposites holds decent photocatalytic stability.

(1)

e (Bi 2MoO6 ) → e (Bi)

(2)

h+(Bi 2MoO6 ) + H 2O → •OH + H+

(3)

2•OH + NO → NO2 + H 2O

(4)

NO2 + •OH → NO3− + H+

(5)

4. CONCLUSION In summary, a novel semimetal/semiconductor Bi/Bi2MoO6 nanohybrid photocatalyst with high photocatalytic capability was first fabricated through an in situ hydrothermal reduction strategy. During the hydrothermal process, Bi3+ was in situ reduced to metallic Bi particles by glucose on the surface of Bi2MoO6 microspheres. The amount of reductive glucose exerts a pivotal role on the morphology of the nanocomposites. This Bi/Bi2MoO6 nanohybrid showed exceptionally high visible light photocatalytic performance for NO purification compared with that of the individual Bi2MoO6. The deposited Bi metal nanoparticles perform as a noble-metal like cocatalyst. The improved photocatalytic activity can be ascribed to the cocontributions of the enlarged BET surface areas, remarkably enhanced light absorption, and improved charge separation efficiency because of the SPR effects of Bi metal. Based on the

×

e−(Bi) + O2 → •O2−

(6)

NOx + h+(Bi 2MoO6 ) → NO3−

(7)

3.7. Photochemical Stability of Photocatalyst. The photochemical stability and durability of an ideal photocatalyst is extremely significant for this practical application. The stability experiment of the optimized Bi−Mo-50 was H


Article


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ESR analysis, a SPR-based visible-light photocatalysis mechanism was proposed. In addition, the optimal Bi−Mo-50 photocatalysts also exhibited decent photochemical stability. This work not only provides new insights for in situ synthesis of Bi-based semimetal−semiconductor nanohybrid photocatalysts but also demonstrates the feasibility of utilizing the cheap and abundant Bi metal to replace the precious noble metals (like Au and Ag) as cocatalyst for enhancing photocatalysis efficiency.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel: +86-23-62769785-605. Fax: +86-23-62769785-605. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is financially supported by the National Natural Science Foundation of China (21501016, 51478070, and 538 51108487), and the innovation project from CTBU (153003).

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Microspheres Nanohybrid with SPR-Promoted Visible-Light Phot

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